Oral Presentation Abstracts

Talks 301 through 358

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301

Sorption of Hydrophobic Organic Contaminants to Soil Minerals

S. Müller, K.-U. Totsche, I. Kögel-Knabner, Lehrstuhl für Bodenkunde, Department für Ökologie, Technische Universität München, 85350 Freising-Weihenstephan, Germany

In subsurface soil horizons, the sorption of hydrophobic organic contaminants may primarily be controlled by the composition and the properties of the soil minerals and less by the low concentrations of organic carbon. Therefore this study aimed to elucidate the properties and mechanisms which control the sorption and the sorption kinetics of hydrophobic organic contaminants to different inorganic soil constituents. Batch sorption and soil column experiments are conducted with three polycyclic aromatic hydrocarbons (PAHs; phenanthrene, pyrene and benzo(a)pyrene) and with the model minerals quartz sand, quartz sand coated with goethite and a quartz sand - montmorillonite mixture. Batch experiments show a considerable sorption of PAHs to all minerals. The sorption process is rapid and seems to be completed after 4 hours. The affinity of PAHs to soil minerals increases with their KOW values. Sorption capacity is higher for clay minerals and iron oxides than for quartz sand. However, PAHs have a high specific affinity to the goethite-coated quartz sand. Sorption isotherms are best described by a nonlinear isotherm which may indicate different sorption mechanisms for different PAHs and mineral sorbents due to differences in slope and curvature.


302

Interaction of Humic Substances with Cationic Polyelectrolytes

John Gregory, University College London, Gower St., London WC1E 6BT, UK, and Sang-kyu Kam, College of Ocean Sciences, Cheju National University, Cheju 690-756, Korea

The anionic charge of aquatic humic substances plays a large part in their interaction with metal ions and other cationic species. Removal of humics by coagulation and flocculation is closely linked with charge neutralization. In this work, the charge densities of a commercial humic acid and an aquatic humic extract were investigated by studying their interactions with synthetic cationic polyelectrolytes, having different charge densities and molecular weights. Colloid titration and flocculation were used, each by two different techniques. For a given polyelectrolyte all four methods gave consistent charge densities for humic substances. However, there were systematic differences depending on the charge density of the cationic polyelectrolyte used. With low charge density polyelectrolytes, the apparent anionic charge of the humic substances is quite low. With higher polyelectrolyte charge densities the apparent humic charge density increases and reaches a limiting value when the polyelectrolyte charge is greater than about 3 meq/g. Optimum flocculation of humics occurs with less cationic charge in the case of low-charge polyelectrolytes than those with higher charge density. However, the degree of removal was better in the latter case. In all cases, the molecular weight of the cationic polyelectrolytes appeared to have no effect on the results.


303

Sorption-Desorption of Polar and Ionogenic Compounds in Iron-Oxide Rich Soils.

Dharni Vasudevan, Ellen M. Cooper, and Oliver Van Exem, Nicholas School of the Environment, Duke University, Box 90328, Durham, NC, 27708.

This research explores the potential for sorption and desorption of three compounds, 2,4-D, norflurazon, and quinmerac, in two iron-oxide rich soil profiles of the North Carolina Piedmont. Following extensive characterization of the soils, we used batch and continuous flow stirred tank reactors (CFSTRs) and principal component analysis to examine sorption and desorption as a function of soil sample depth, composition, and physical-chemical properties and compound molecular structure and physical-chemical properties. Loss of the two ionogenic compounds, 2,4-D and quinmerac, from soil solution was primarily due to sorption; the loss was strongly correlated with soil surface area, total Al and Fe, and crystalline iron oxide and amorphous aluminum oxide content. Loss of the non-ionic compound, norflurazon, however, was only correlated to total soil carbon. Under the experimental conditions of our CFSTR, we observed that deionized water could either completely or partially desorb the sorbed compounds and that desorption was initially more rapid than sorption. We hypothesize that the mass of 2,4-D and quinmerac involved in reversible sorption was associated with soil iron oxides via favorable electrostatic interactions and/or weak surface complexation, while desorbing norflurazon fraction was associated with soil organic matter. The irreversibly retained compound mass was most likely involved in strong surface complexation or is entrapped within the soil matrix.


304

Adsorption of (Hydroxyamino)Carboxylate Chelating Agents and the Subsequent Dissolution of FeOOH (Goethite)

Charles F. Whitehead and Alan T. Stone, The Johns Hopkins University, DOGEE, 3400 N. Charles St., Baltimore, MD, 21218.

Adsorption and ligand-assisted dissolution depend upon the number, identity and arrangement of Lewis base groups within the chelating agent structure. In solution, hydroxyl, amino and carboxylate groups all facilitate metal ion chelation. With regards to adsorption onto charged (hydr)oxide surfaces however, long range electrostatic interactions play a far greater role. At environmentally relevant pHs (4-8), carboxylate groups are anionic and encourage adsorption to positively charged surfaces whereas amino groups are cationic and discourage adsorption. Our work shows that the extent of tricarboxylate adsorption to FeOOH (goethite) decreases with the addition of amino groups as follows; citrate (0 amino groups) > nitrilotriacetate (NTA, 1 amino group) > hydroxyethylethylenediaminetriacetate (HEDTA, 2 amino groups). However, the rate of FeOOH dissolution promoted by HEDTA is much greater than with either citrate or NTA. HEDTA promoted dissolution rates are not well correlated to the extent of adsorption as a function of pH. This suggests that protonation level of adsorbed HEDTA, or some other aspect of surface speciation, is more significant than the extent of adsorption in determining whether subsequent dissolution of the underlying surface will occur.


305

Manganese Oxide Induced Oxidative Coupling Reactions of Hydroxylated Aromatic Compounds in Natural Sorbents

Michael Keinath, Hildegarde Selig, Roger Pinto, and Walter J. Weber, Jr., Civil and Environmental Engineering, University of Michigan, 108 EWRE, 1351 Beal Avenue, Ann Arbor, MI 48109-2125

A novel approach for contaminant sequestration by natural sorbents using birnessite, a manganese(III/IV) oxide, is being investigated in batch and continuous flow systems. Catalyst addition was expected to increase the non-extractability of phenols by oxidative coupling reactions with reactive groups in the sorbent organic fraction and/or by formation of phenolic polymers with decreased mobility. Experimental results indicate that only a surface soil exhibited natural coupling capacity under abiotic conditions, while peat and shale type sorbents displayed completely reversible sorption equilibrium behavior. Batch systems (both containing sorbent and sorbent-free) indicated that rates of formation and extractability of the coupling products are predominantly controlled by the birnessite concentration in the system. At high concentrations, extraction of phenolic products seems to be controlled by particle-polymer associations. In flow-through systems, birnessite concentration also appears to dominate the nature of the reactions so that: i) at low birnessite concentrations, non-extractability of phenols increased in peat and shale systems but not in soil-free (birnessite only) or surface soil systems, ii) at increasing birnessite concentrations, non-extractability of phenols is decreased in peat and shale systems while no apparent change occurs in the other systems, and iii) at high birnessite concentrations, non-extractability increases dramatically in all systems.


 

306

WITHDRAWN


307

Probing DLVO Forces Using Interparticle Magnetic Forces

Ching-Ju Chin, Sotira Yiacoumi, Georgia Institute of Technology, School of Civil and Environmental Engineering, Atlanta, GA, 30332-0512, USA; Costas Tsouris, Oak Ridge National Laboratory, Chemical Technology Division, P.O. Box 2008, Oak Ridge, TN 37831-6266, USA.

The transition from secondary- to primary-minimum aggregation of superparamagnetic colloidal particles is investigated in this study. An external magnetic force and a suspension of colloidal superparamagnetic particles comprise the model system. The external magnetic force is used as an additional force to manipulate the total interparticle force. The magnetic induction needed for this transition is calculated from the extended DLVO theory, which includes van der Waals, electrostatic, and magnetic-dipole forces, as well as non-DLVO hydrophobic attraction and steric repulsion. The magnetic induction at which the transition from secondary- to primary-minimum aggregation occurs is experimentally determined from visualization of chain formation and breakup. Experimental and theoretical values of transitional magnetic induction show good agreement for small particles in a narrow (50 mm) capillary tube and for large particles in a wide (100 mm) capillary tube, which indicates that the DLVO theory and the existing formulae for interparticle potential calculations are reliable. Effects of surfactant molecules on the transition from secondary- to primary-minimum aggregation are also investigated. Observations show that the transitional magnetic induction increases with increasing surfactant concentration until the particles are completely covered by surfactant molecules.


308

Relating Charge Nonuniformity and Colloidal Stability

Darrell Velegol, Prasanna K. Thwar, and Yuexin Liu, Department of Chemical Engineering, Penn State University, University Park PA 16802

We are investigating how charge nonuniformity over the surface of individual particles affects bulk colloidal stability. We have developed the experimental method of "rotational electrophoresis" to measure the charge nonuniformity on colloidal particles. This technique combines video microscopy measurements of colloidal trajectories in electrophoresis with classical electrokinetic theory to obtain the standard deviation of zeta potential (s z ) over individual particle surfaces. Our data show that 4.3 m m PS latex particles (IDC) in KCl solutions have a significant s z , sometimes greater than 50 mV on a pertinent length scale. For similar experimental systems, we measure the bulk stability ratio (W), again with a video microscopy method in which we manually count doublets and obtain the initial rate of doublet formation. The measured W are much lower than predictions that assume the particles were uniformly-charged. By comparing the stability ratios under various conditions to the measured charge nonuniformity, we determine whether a relationship exists between the two. Finally, using a model we have developed that relates charge nonuniformity to the interactions between particles, we provide a quantitative explanation for our measurements. This model assumes "patch-wise" charging with negligible interactions between patches.


309

Attraction Between a Macroion Pair of Like Charge

R. Behera# and P. Gupta-Bhaya, Department of Chemistry, Indian Institute of Technology, Kanpur, 208016, INDIA

Classical DLVO potential between a macroion pair of like charge is repulsive. Integral equation theory in HNC approximation predicted an attractive minimum, but it has been believed to be an artifact arising from neglected correlations. We calculate b Veff(r), effective pair potential at infinite dilution as defined in Adelman’s theory of solution from renormalized HNC theory. b Veff(r) is shown to reduce to DLVO potential at low concentration of macroion and salt, shows an attractive minimum at high macroion charge in concentrated solutions and does not disappear in self consistent theories that are thought to incorporate correlations neglected in HNC theory. Our result correlate with recent experiments, which detect an attractive potential between a macroion pair located near a charged wall, a situation simulated in a concentrated solution. We parameterize deviations from DLVO potential in terms of quantities that are expressed in terms of short ranged direct correlations functions. The attractive potential is studied as a function of system parameters and is found to be stronger at low salt concentration.

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# Present Address: Department of Chemistry, University of California Davis, Davis, CA-95616


310

The Operation of Hydrophobic Forces in Concentrated Colloidal Dispersions

D. Y. C. Chan, T. W. Healy, J. Stankovich, Particulate Fluids Processing Centre, The University of Melbourne, Parkville 3010 VIC Australia

There is now convincing experimental evidence regarding the physical mechanism of the "hydrophobic force" between hydrophobic colloidal particles with dissolved gases or the vapour phase as being responsible for the magnitude and range of such forces. We build on these findings and propose a quantitative mechanism as to explain how this force may operate in modest to high volume fractions of particles that will give rise to network structures and bulk properties of such systems.


311

Phase Coexistence in Colloidal Suspensions: The Effect of Density-Dependent Macroion Charge

Michael Knott and Ian J. Ford, Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom

It has been observed experimentally that dense and rarefied regions can coexist in a suspension of charged colloids (macroions) and oppositely charged simple ions (microions). This has often been seen as contradicting established theories of the interactions in such systems. Recently, a number of theoretical treatments have been proposed [1,2,3,4] which are qualitatively successful in explaining the phenomenon. Such approaches generally make the simplifying assumption that the surface charge, Z, on a macroion is independent of the macroion configuration. However, a more faithful representation of the system would recognise that, in reality, this charge is expected to depend on the density of the suspension. In this work, we take account of that point. We start from a recently developed model [3] for the free energy of the system under conditions of constant surface charge and zero added salt. We add terms to take account of the effect of varying particle number, and then minimise the free energy with respect to Z at fixed macroion density, in order to determine how Z varies with the macroion density. This allows us to make comparisons with recent experimental results [5] for the macroion surface charge and for the phase behaviour of the suspension.

1. R. van Roij, M. Dijkstra and J.-P. Hansen, Phys. Rev. E 59, 2010 (1999).

2. P. B. Warren, J. Chem. Phys. 112, 4683 (2000).

3. M. Knott and I. J. Ford, Phys. Rev. E, in press.

4. D. Y. C. Chan, P. Linse and S. N. Petris, Langmuir, submitted.

5. H. Yoshida, J. Yamanaka, T. Koga, T. Koga, N. Ise and T. Hashimoto,

Langmuir 15, 2684 (1999).


312

Investigation of Long Range Order in Clay Gels by X-Ray Microscopy

I. Bihannic, L.J. Michot, B.S. Lartiges, D. Vantelon, J. Labille, F. Thomas

Laboratoire Environnement et Minéralurgie, UMR 7569 CNRS-INPL, BP40, 54501 Vandoeuvre lès Nancy Cedex, France, J. Susini, M. Salomé, B. Fayard X-Ray Microscopy Beamline, European Synchrotron Radiation Facility, BP220 , 38043 Grenoble Cedex, FRANCE

Clay particles are anisotropic charged platelets that form thixotropic gels in aqueous suspensions. The formation mechanisms and structure of these gels are still debated and two main models have been proposed to describe the arrangements of colloidal platelets in such phases. The first one assumes a house of card organization due to direct attractive forces between faces and edges with opposite charges. According to the second model, the development of double layer around particles leads to the formation of an electrostatically stabilized gel. Up to now, the arrangement of particles was deduced from a combination of indirect methods, such as scattering techniques, rheological and thermodynamical measurements, and from electron microscopy observations. However, artefacts due to sample preparation can always be suspected. In contrast, X-Ray spectromicroscopy appears as a powerful technique to obtain in situ morphological descriptions as investigations without sample pre-treatment can be performed. A direct visualisation of montmorillonite gels at 50g/l has been obtained at the ESRF ID21 X-ray microscopy beamline by mapping the Silicon distribution in fluorescence yield. Areas of local nematic order separated by orientational defects were evidenced, in agreement with previous birefringence studies.


313

Interaction Force at Very Close Separations between Dissimilar Particles: Implications for Colloidal Deposition and Stabiltiy Calculations

Anders O. Wiström and Armik Khachatourian, Department of Chemical and Environmental Engineering, Bourns Hall, University of California, Riverside, CA 92521

As new materials with complex microstructures are being developed based on fabrication by self-assembly there is a corresponding need for analytic and numerical methods to predict the interaction force between particles at very close separations. The framework for solving the interaction force problem is obtained from a generalization of the screened Coulomb force for distributed charges. The charge density distribution is cast in the form of the Fredholm integral equation. A mathematical identity permits transformation of the boundary value integral equation into a closed form series solution. The transformation permits the evaluation of the interaction for all Debye lengths and separation distances which distinguishes this work from all other methods. We present simulation results for colloid stability as a function of size/charge polydispersity, and discuss the concept of electrostatic saturation which allows a seamless treatment of particle deposition in all regimes. In closing, we will discuss how the theoretical framework for the pair-interaction can be generalized for an arbitrary number of charged surfaces and present simulation results for the pair interaction in the presence of a third, charged, surface.


314

Light Induced Destabilization of Giant Vesicles

Daniel T. Chen, Keng-Hui Lin, Arjun G. Yodh The Department of Physics and Astronomy, The University of Pennsylvania, 209 S. 33rd Street, Philadelphia, PA. 19104-6396

We have investigated the photochemical destabilization of large vesicles. PhotofrinÒ (QLT Inc.), a photosensitizer used in photodynamic therapy of tumors, was added to a suspension of vesicles wherein it intercalated into the bilayer membrane of giant (20-40 m m) vesicles. We irradiated the samples with light at specific wavelengths. The light excited the hematoporphyrin component of the Photofrin, which in turn induced the excitation of oxygen gas molecules from an unreactive triplet ground state to a highly reactive singlet state. The reactive oxygen photooxidatively cleaved hydrocarbon chains of the surrounding membrane. Using optical microscopy, we have observed bilayer destabilization and lysis resulting in loss of encapsulated contents when these vesicles are irradiated with visible light (488-546 nm). We present the results of our experimental investigations with regard to applications in drug delivery schemes. This work is supported by NSF (DMR 99-71226) and MRSEC (DMR 00-79909).


315

Langmuir and LB films of Organophosphorous Acid Anhydrolase

Sarita V. Mello and Roger M. Leblanc University of Miami, Chemistry Department, 1301 Memorial Drive, 33146, Coral Gables, Florida - USA..

Monolayers of an organophosphorous acid anhydrolase - OPAA (EC 3.1.8.2) were studied at air-liquid interface. This enzyme is known to degrade organophosphorous (OP) compounds as di-isopropylphosphoro-fluoridate (DFP) and G-type warfares. The investigation of surface chemistry properties of OPAA molecules by Langmuir-Blodgett technique was the first step to the design a detection system for OP compounds, which can be used as a biosensor. Phosphate and ammonium carbonate buffers were tested as subphase to perform Langmuir monolayer measurements. Characterization of Langmuir films was performed by surface pressure and potential isotherms, and UV-Vis absorption in situ. The salting out effect was studied with addition of KCl in the subphase at several concentrations. OPAA was labeled with a fluorescent probe (FITC) and characterized by fluorescence spectroscopy in situ. The activity of OPAA was tested against two organophosphorous compounds, diethyl p-nitrophenyl phosphate (Paraoxon) and DFP. Langmuir-Blodgett films were investigated by infrared spectroscopy (ATR-FTIR) and surface probe microscopy (SPM).


316

Enzyme Adsorption and Reaction Kinetics on Substrate Surface

Joon-Hyung Kim, Shaunak Roy, Alice P. Gast, Channing R. Robertson, Department of Chemical Engineering, Stanford University, Stanford, CA 94305-5025

We study the adsorption and reaction kinetics of subtilisin (enzyme) on bovine serum albumin (substrate) surface by a combination of the surface plasmon resonance (SPR) and surface plasmon enhanced fluorescence (SPEF) techniques. The adsorbed enzyme concentration and the enzyme reaction rates were directly measured in situ. The effect of environment such as ionic strength on the enzyme adsorption and the reaction were studied for several enzyme variants (by point mutation). Enzyme reaction rates decreased when the enzyme was very strongly adsorbed to the surface. Based on these measurements, we propose an enzyme adsorption and reaction model on the surface.


317

Fibrinogen Relaxation on C16 Self-Assembled Monolayers

Christian F. Wertz, and Maria M. Santore, Department of Chemical Engineering, Lehigh University, 111 Research Drive, Bethlehem, PA 18015

Single-species fibrinogen adsorption runs have a characteristic adsorption time determined by the bulk solution concentration and wall shear rate. Analysis of these runs reveal spreading rates that are constant initially, however, lateral interactions between neighboring molecules stunt spreading before an ultimate protein footprint can be achieved. Therefore, to measure the spreading rate during the entire relaxation process, we report kinetic runs on unsaturated fibrinogen layers using fibrinogen as a self-probe to measure the consumption of free surface space as the preadsorbed fibrinogen layers unfold. Analysis of these runs reveals a constant spreading rate initially, which begins to level off after 30 minutes and reaches a maximum footprint of 400 nm2/molecule after 1 hour. Because fibrinogen is a complex molecule that undergoes many structural changes upon adsorption, we use a more structurally stable protein probe, lysozyme, as a second measure of the spreading rate. Lysozyme also reveals a constant spreading rate initially, however, the maximum footprint is slightly larger due to the structural stability of lysozyme. Comparison of these probe runs to the single-species runs shows that the initial protein footprint (100 nm2/molecule) and the constant, initial spreading rate (0.14 nm2/molecule/s) are nearly identical in all three cases.


318

Design Criteria for Polymer-Based Protein-Resistant Coatings

Deborah Leckband and Nadya Efremova, Dept. of Chemical Engineering, Univ. of Illinois, Urbana-Champaign, IL 61801

Nonspecific protein adsorption to surfaces is of significant technological interest in fields as diverse as medical sensors and paints. In many applications, water soluble polymer coatings are used to reduce or eliminate nonspecific adsorption. What is needed, however, are design parameters, which can guide the a priori development of effective nonfouling polymer coatings. In the work described, we use force measurements to quantify directly the ranges and magnitudes of the interfacial forces due to grafted chains and their impact on protein deposition. With grafted poly ethylene oxide films, we show that the range and magnitude of the resultant repulsive surface barriers are consistent with theoretical predictions for simple, end-grafted polymers. Surface plasmon resonance measurements further demonstrated how these barriers affect protein adsorption. The latter studies show good qualitative agreement with behavior predicted by scaling analysis. What emerges from these studies are a number of molecular design criteria that may be used to guide the fabrication of polymer-based, protein-resistant surface coatings.


319

Adsorption of Anionic Protein-Polysaccharides to Titanium and to Stainless Steel Surfaces

M.M. Klinger (1), F. Rahemtulla (2), C.W. Prince (3), and L. C. Lucas (1), Departments of (1) Biomedical Engineering, (2) Oral Biology, and (3) Nutrition Science, University of Alabama at Birmingham, Hoehn Bldg., Rm. 370, 1075 13th St.. S., Birmingham, AL 35294-4440

A narrow zone of protein-polysaccharide appears to strengthen the attachment of bone tissue to implanted metallic surfaces. Differences in the thickness and/or composition of this zone may explain the extremely stable integration of bone to titanium implants as compared to those of stainless steel. We have isolated radioactively-labeled proteoglycans (PGs) from the cell culture medium of a clonal line of osteoblastic cells and compared their adsorption to titanium (cpTi) and to stainles steel (316LSS) powders. In the presence of 2 mM calcium, approximately 98% and 74% of the total PGs adsorb to the cpTi and 316Lss, respectively; the absence of calcium significantly decreases PG binding to cpTi (87%) but not to 316Lss (73%). Analysis of the adsorbed PGs by polyacrylamide gel electrophoresis indicates that the differences are qualitative as well as quantitative; certain subpopulations of the PGs, distinguishable by size and carbohydrate composition (proportions of heparan sulfate and chondroitin sulfate), have a greater affinity for the cpTi than for the 316LSS powder. This adsorption is inhibited to varying degrees by the addition of certain proteins for by sulfated polysaccharides. Based on these results, we suggest a model for the interaction of these complex biomolecules with titanium and stainless steel surfaces.


320

SANS Studies of a Surfactant Based Rigid Mesophase

Blake Simmons, Vijay T. John, Dept. of Chemical Eng, Gary L. McPherson, Dept. of Chemistry, Tulane University, New Orleans, LA, 70118, Arijit Bose, Vivek Agarwal, University of Rhode Island, Dept. of Chemical Eng., Kingston, R.I., 02881, Nitash Balsara, UC-Berkeley, Dept. of Chemical Eng., Berkeley, CA, 94706, Boualem Hammouda, NIST, NCNR, Gaithersburg, MD, 20883

The addition of lecithin (phosphatidylcholine) to AOT water-in-oil microemulsions leads to a dramatic increase in viscosity and the formation of an isotropic rigid gel state as the water content is increased above a specific threshold. Characterization of this phase transition behavior through electrical conductivity measurements, NMR, and rheology indicates a shift to a percolating aqueous network upon the formation of the rigid mesophase. Small angle neutron scattering is used to show evolution of structure from the AOT reversed micelle state to the gel state upon addition of lecithin. SANS experiments show a pronounced peak in the gel state, with a characteristic d-spacing that is a function of the system water content. At elevated temperatures these rigid mesophases undergo a phase transition back to the liquid phase and are thermoreversible in nature. The SANS profiles demonstrate that the rigid mesophases possess a high degree of ordering with pronounced domain sizes varying with system water content. The scattering patterns can be modeled through Teubner-Strey analysis and indicate an ordered bicontinuous microstructure. Of specific interest is the variation in scattering patterns during templated materials synthesis in these mesophases.


321

Pyramid Shaped Droplets in Temperature-Induced Cubic Phase-Containing Emulsions

Matthew Lynch and Kelly Kochvar, Corporate Research Division, the Procter & Gamble Company, Miami Valley Laboratories, 11810 E. Miami River Road, Route 27, Cincinnati, Ohio, 45061.

Temperature-induced cubic phase-containing emulsions are created by adjusting the temperature of a C12E2-water mixture from the cubic phase region into a miscibility gap in which there is coexistence of the cubic phase and a L1 phase. The L1 phase forms pyramidal-shaped droplets presumably reflecting the structure of the cubic phase. It is evident the droplets nucleate at the silica surfaces of the cell containing the mixture creating two layers: one at the top silica surface and the other at the bottom, extending about fifteen microns from the cell wall. Also striking is the observation that the droplets seemingly ‘influence’ each other. All droplets on a given surface point in the same direction, although not necessarily perpendicular to the channel edge. Droplets on the opposing face point in the opposite direction. Two reasonable possibilities are that convection created by small differences in cell heating bias the alignment or that long-range order in the cubic phase forces the alignment.


322

A Novel Process for Producing Cubic Liquid Crystalline Nanoparticles (Cubosomes)

Patrick T. Spicer and Kristin L. Hayden, The Procter & Gamble Company, Corporate Engineering, 8256 Union Centre Boulevard, West Chester, OH. 45069, Matthew L. Lynch, Akua Ofori-Boateng, and Janet L. Burns, The Procter & Gamble Company, Corporate Research, 11810 East Miami River Road, Ross, OH. 45061

A novel process for producing cubic liquid crystalline nanoparticles (cubosomes) has been developed. The process entails simple mixing of two water-like solutions with minimal input of energy. The key to this process is the inclusion of hydrotrope. Most lipids, such as monoolein, used to form cubic liquid crystals are essentially insoluble in water. The hydrotrope dissolves the lipid to create a water-like solution. Water is added to the hydrotrope solution, resulting in a precipitous decrease of lipid solubility. Provided the dilution trajectory falls into a cubic phase-water miscibility gap, nanometer-scale cubic liquid crystalline particles form spontaneously, presumably from a homogenous nucleation mechanism. The process is versatile enough to accommodate any lipid and hydrotrope combination that forms cubic liquid crystalline material upon dilution. Actives and stabilizers can be formulated into either of the two solutions, allowing the production of colloidally stabilized, controlled-release dispersions. The phase diagram of the monoolein-ethanol-water system is determined to assess appropriate formulation of solutions and develop dilution trajectories. This process replaces current processes that require long hold times, processing of solid-like materials, and very high energy inputs to create cubosome nanoparticle dispersions. This process produces smaller, more stable cubosomes than by conventional bulk dispersion techniques.


323

Optimal Size and Size Distribution of Vesicles Formed with Lipid and Polymer-Lipid Mixtures

Montse Rovira-Bru, David H. Thompson, Igal Szleifer, Dept. of Chemistry, Purdue University, 1393 Brown Bldg West Lafayette, IN-47907

Solvated grafted-polymer layers at the interface of lipid bilayers is attracting increasing attention for their ability to stabilize liposomes with desirable properties for drug delivery such as long circulating time in the blood stream. However, a theoretical description of the equilibrium size distribution and composition that provides a deeper understanding of the observed behavior and the ability to predict the properties of such liposomes still needs further development. In the present study, a molecular mean-field theory was applied to generate the phase diagram of spontaneous forming liposomes in mixtures of lipid and lipid-poly(ethylene) oxide (PEO) molecules. The theory predicts the optimal size of the aggregates and their size distribution at the thermodynamic equilibrium as a function of the PEO loading and molecular weight for several lipid chain lengths. The theory also provides information about the distribution of the components of the liposomes in both monolayers of the lipid bilayer. The predictions of the theory have been tested with experimental observations and there is very good agreement between them.


324

Temporal Evolution of Microstuctures in Mixed Surfactant Solutions Using Time-Resolved Cryo-TEM and Dynamic Light Scattering

Arijit Bose, Yashen Xia, Department of Chemical Engineering, Paul Johnson, Electron Microscope and Imaging Facility University of Rhode Island, Kingston, RI 02881, T. Alan Hatton, Department of Chemical Engineering, MIT, Cambridge, MA 02139

Using time-resolved cryogenic transmission electron microscopy and dynamic light scattering, we have examined the temporal evolution (over 2hrs) of aggregate structures for the micelle-vesicle phase transition in solutions containing CTAB, SOS and water as well as CTAB, HDBS and water. When starting with single surfactant micelles solutions (1.5%CTAB, 3.4% SOS), the microstructures eveolve from disks to small vesicles to large vesicles, and the mean hydrodynamic diameter changes from 15nm to about 35nm over two hours. The short time aggregates when the initial states are mixed micelles (same final state) are wormlike micelles. The presence of the non-equilibrium disks is confirmed by stage-tilting experiments. The transition in the CTAB/HDBS system is extremely rapid, and no disks are observed. The DLS data shows a rapid increase in the hydrodynamic diameter to its final value. We propose various models to explain the difference in the rates and pathways for self-assembly in these systems.


325

Polymer-like Behavior of Micellar Solutions of Dilauroylphosphonucleosides

Piero Baglioni, Debora Berti, Francesca Baldelli Bombellli, Department of Chemistry, University of Florence, via G. Capponi 9, I-50121, Florence, Italy

Phosphatidylnucleosides are lipids bearing a nucleoside moiety on their polar head that can be synthesized from natural compounds like phosphatidylcholines and nucleosides. They show in water aggregational properties modulated by the acyl chain length. The phase behavior is enriched by the presence of a polar head capable of selective attractive interactions. Self-aggregation of phospholiponucleosides induces molecular recognition similar to that observed in nucleic acids between complementary bases. Static and dynamic properties (SANS and QELS) of dilauroylphosphonucleoside aqueous solutions have been investigated as a function of ionic strength and surfactant concentration. A micellar growth in the axial direction to form wormlike aggregates is observed as the surfactant concentration is increased, until the solution show a distinct viscoelastic behavior resembling polymer solutions in the semidilute regime. The onset of a viscoelastic behavior is investigated by light scattering and small angle neutron scattering. The information obtained has been compared to the dynamic properties of the micellar network, as inferred by rheological tecniques.


326

Micellization and Spontaneous Vesicle Formation in Aqueous Mixtures of Cationic/Anionic Surfactants

Hong-Un Kim, Kyung-Hee Lim, Department of Chemical Engineering, 221 Huksuk-dong Dongjak-gu, Seoul 156-756, Korea, Kye-Hong Kang, Chung-Ang University, and R&D Center, Pacific Corp. and 314-1 Bora-ri Kiheung-eup, Yongin-si, Kyunggi-do, Korea

We have examined the phase behavior for the mixed aqueous surfactant systems of cationic octadecyl trimethyl ammonium chloride (OTAC)/anionic ammonium dodecyl sulfate (ADS)/water at high water contents. These surfactants are well used in the cosmetics and toiletries industry, but the properties and the behavior of the mixed systems of these surfactants have been little known. Below the total surfactant concentrations of 1.5 m molal, mixed micelles are formed and the mixed CMC’s (critical micelle concentrations) at different ratios of two surfactants behave according to the pattern predicted by the regular solution model with the interaction parameter being –5 to 1. At 1.5 m molal, there appears a region where mixed micelles and vesicles coexist. As the surfactant concentration increases, the systems look very turbid and much more vesicles are observed. Vesicles are characterized by structure (or size), surface charge, and stability. These factors were examined by electron microscopy, dye encapsulation, zeta potentials, and UV spectroscopy. The vesicles are of 250-400 nm, unilamellar, and of 17 % of encapsulation. The zeta potentials of the vesicles are 40-60 mV in absolute values. These large zeta potentials played a positive role for vesicle stability. UV absorption spectra of vesicles remained almost the same for two months, implying that the spontaneously formed vesicles were quite stable.


327

Organization of Fine Particles and Structural Transitions in Two-Dimensional Systems

D.T. Wasan and A. D. Nikolov, Department of Chemical and Environmental Engineering

Illinois Institute of Technology, Chicago, IL 60616, A. Trokhymchuk and D. Henderson, Department of Chemistry, Brigham Young University, Provo, UT 84602

The phenomenon of local ordering of both monodispersed and binary hard sphere mixtures near a planar wall has been examined using the integral equation method of statistical mechanics. The theoretical results explain the experimental observations of the wall-induced phase separation of colloidal particles and the formation of crystallites especially for highly asymmetric confined mixtures. We also report on a novel ordering in a two-dimensional system of macroscopic-length scales using millimeter-sized metal balls on a dielectric surface. The liquid- like order transforms to a gas-like order through co-existence upon decreasing the area fraction of the balls. The measured pair interaction of like-charged balls exhibits a long-range attraction analogous to that in charged colloids.


328

Template-Directed Convective Assembly of Colloidal Crystals

Jian Zhang, S. Sanyal, K.H. Lin, and A.G. Yodh Physics Department, University of Pennsylvania

We investigate the use of geometric surface templates to enhance the convective assembly of colloidal crystals[1,2]. In our experiment, we immerse one- and two- dimensional grating templates as substrates into ~5 volume percent monodisperse colloidal suspension at 50 ° C. Assembly occurs as the solvent evaporates[2]. SEM characterization revealed ~ 50 micron single domain size, [100]-oriented fcc crystals. Additional experiments are underway to investigate the effect of the added depletents on the assembly and crystal quality.

Funded by NSF DMR 99-71226, MRSEC DMR 00-79909, NASA NAG3-2172.

[1] K.H. Lin etal, Phys. Rev. Lett, 85, 1770(2000).

[2] P. Jiang etal, Chem. Mater., Vol. 11, No. 8, 2132(1999).


329

The Structure of Self-Assembled Fe and FePt Arrays

Saeki Yamamuro, Dorothy Farrell, Keith Humfeld, and Sara Majetich, Carnegie Mellon University, Dept. of Physics, Pittsburgh, PA 15213

Self-assembled mono- and multilayer arrays can be prepared from suspensions of 4 nm FePt or 8 nm Fe nanoparticles synthesized by solution chemistry methods. The particles are surfactant coated and dispersed in a high vapor pressure solvent. The array formation is associated with the evaporation of the solvent, and is controllable by adjusting the drying conditions. Relevant adjustments include centrifuging the samples and changing the solvent or the pressure under which the samples are dried. The structure of the arrays has been studied by transmission electron microscopy (TEM). All observed monolayers showed hexagonal packing, but multilayers displayed both hexagonal and square lattice structures. TEM image simulations based on the charge-density approximation identified hexagonal close packed stacking of layers in thin superlattice regions. A striking result is the preferred growth of mono- and trilayers in these regions. Lattice spacing measurements of the square arrays found in the thicker Fe superlattices indicated a face centered cubic (fcc) structure, with the (100) plane oriented parallel to the substrate surface. Studies are continuing on the FePt multilayers to determine whether the observed square arrays are fcc or body centered cubic.


330

Epitaxial Assembly in Aged Colloids

R. Lee Penn, David R. Velben, Department of Earth and Planetary Sciences, Gerko Oskam, Peter C. Searson, Department of Materials Science and Engineering, Alan T. Stone, Timothy Strathmann, Department of Geography and Environmental Engineering, Johns Hopkins University, Baltimore, MD 21218

Self-assembly of nanoparticles is an important mechanism of particle growth in the solution phase synthesis of oxides and oxyhydroxides. In this work, particle growth in aqueous colloidal suspensions of anatase (TiO2), hematite (Fe2O3), feroxyhite (FeOOH), and heterogenite (CoOOH) was observed to occur both by coarsening and by topotactic self-assembly. The hematite nanocrystals are nominally equidimensional crystals that are commonly assembled from more than 10 primary building blocks. The heterogenite particles are hexagonal plates that are, on average, 0.7 µm across and 20 – 30 nm thick. These plates are porous and are assemblies of hundreds of oriented nanocrystalline building blocks. The feroxyhite nanocrystals assemble to form ~30 nm porous flakes that are several nanometers thick. The anatase nanocrystals assemble to form elongated, bent, or nominally equidimensional single crystals with ultimate morphologies that frequently violate crystal-symmetry rules. Kinetic experiments, utilizing anatase particles, show that the number of isolated primary particles decreases with time and that the assembly order, which reflects the average number of primary particles per secondary particle, increases with time. Growth by oriented aggregation is highly dependent on solution chemistry and may provide a means by which intricate assemblies can be achieved without the use of organic intermediaries.


331

Controlled Assembly of Freely Suspended Two-Dimensional Colloidal Crystals

Q. -H. Wei, D. M. Cupid, and X. L. Wu, Department of Physics, University of Pittsburgh

Under the influence of capillary forces, colloidal particles embedded in a soap film self-organize to form polycrystalline monolayers when the film is withdrawn from a concentrated suspension. A driven capillary wave, caused by vertical vibrations of the suspension, unexpectedly eliminates grain boundaries, producing large single crystals in a controllable fashion. This capillary-wave-assisted assembly process (CAP) is scalable and paves the way for systematic production of 2D ordered structures for technological applications.


332

Ordered Macroporous Materials Produced by Crystallization of Breath Figures

Mohan Srinivasarao, Jung Ok Park, School of Textile and Fiber Engineering, Georgia Institute of Technology, Atlanta, David Collings, Department of Biological Sciences, Purdue University, West Lafayette IN 27907, Alan Philips, Department of Polymer Science,The University of Southern Mississippi, Hattiesburg, MS, Sanjay Patel, Bell Labs, Lucent Technologies, Murray Hill, NJ

We report the formation of a three dimensionally ordered array of air bubbles of monodisperse pore size in a polymer film through a templating mechanism unlike most published in the literature. Dilute solutions of a simple, coil-like polymer in a volatile solvent are cast on a glass slide in the presence of moist air flowing across the surface. Evaporative cooling, and the generation of an ordered array of breath figures, forms multilayers of hexagonally packed water droplets that are preserved in the final, solid polymer film as spherical air bubbles. The dimensions of these bubbles can be simply controlled by changing the velocity of the airflow across the surface. Such three dimensionally ordered macroporous materials with dimensions comparable to visible light are predicted to have unique optical properties such as photonic bandgaps, and optical stop-bands.


333

Adsorption of Aqueous Madix Polymers onto Silica Using a Novel Reflectometer/Streaming Potential Apparatus

O. Théodoly, L. Cascao Pereira, C.J. Radke, Department of Chemical Engineering, University of California, Berkeley, CA 94720-1460 USA

We have developed a new instrument coupling in-situ z -potential and reflectometry measurements. Since measured z -potentials are related to numerous factors, including the adsorbed amount, the local charge fraction, and the conformation of the adsorbed material, it is essential to obtain additional information about the surface layers in order to understand the mechanisms of adsorption. Reflectometry provides this information. Our apparatus consists of a two-arm reflectometer, one supporting the arc lamp light source and the other supporting a detection system (a chopper, a polarizing beamsplitter, two silicon photodetectorsand lock-in amplifiers). The measurement flow cell is a sandwich of two flat surfaces (a glass plate and a silicon wafer) separated by a thin latex film, with ports for Ag/AgCl electrodes and pressure tranducers. We present new results for the adsorption of copolymers of styrene, which is a hydrophobic monomer, and acrylic acid, whose charge is controlled by pH. We compare the behaviors of these polyelectrolytes at solid interfaces (bare silica or hydrophobized silica) upon changing their structure and their chemical properties. We also investigate the reversibility of the adsorption by washout experiments. The new z -potential /reflectometer provides a unique and important tool for elucidating adsorption mechanisms at the solid/water interface.


334

Aggregation of Colloidal Particles on Electrodes in AC Fields

Junhyung Kim, Stephen Garoff, John L. Anderson, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

Negatively charged colloidal particles (9.7 m m) deposited on a tin-doped indium oxide electrode aggregate or separate when an ac electric field is applied. At lower frequencies such as 50, 100, and 200 Hz, two particles approach each other to form a doublet, while the pair separates at 1000 Hz. To investigate the mechanism of the aggregation or separation, we video taped the trajectories of pairs of latex spheres at different frequencies and field strength. For 4.8 m m radius particles, the particles approached each other at frequencies below 500 Hz until the pair stabilized at a steady state surface-to-surface separation of about one-half the particle radius. This separation appeared to be independent of the frequency. In this talk we discuss these experiments and consider the results in the context of a theory based on electrokinetic aggregation in dc fields.


335

Electrophoretic Mobility And Dielectric Response of Fuzzy Colloidal Particles in Steady and Oscillating Electric Fields.

Reghan J. Hill, Dudley A. Saville and William B. Russel. Department of Chemical Engineering, Princeton University, Princeton, NJ, 08544, USA.

We report on numerical computations on the response of a spherical colloidal particle with a layer of polyelectrolyte polymer attached to its surface. This generalizes extant models for smooth spherical colloidal particles (e.g., Mangelsdorf and White, 1997), and so-called fuzzy particles with thin double layers (Saville, 2000), to include the effects of charge and permeability throughout the polymer layer. Using finite-difference methods, the unsteady Stokes, ion-transport, and Poisson-Boltzmann equations are solved over a wide range of the parameter space. Effects of the polymer layer on the mobility and dielectric response of a dilute suspension are examined and comparisons made with available experimental measurements.


 

336

Electrophoretic Forces on Polystyrene Spheres Near an Electrode Surface

Edward Atman and Dennis C. Prieve, Carnegie Mellon University, 5000 Forbes Ave., Pgh., Pa, 15213

The technique of total internal reflection microscopy is used to measure the colloidal and electrophoretic forces exerted on a charged polystyrene sphere in the presence of an electric field very near to the surface of an indium doped tin oxide electrode. The data supports the theoretical model of Keh and Lien (1989) regarding the hindered electrophoretic mobility of the sphere near the electrode surface. The zeta potential of the sphere may be estimated from the fit of the theoretical model to the Brownian motion of the sphere in a constant electric field.


337

Simultaneous Determination of the Zeta (z )-Potential and Surface Conductance Using the Improved Slope Intercept Technique

David Erickson & Dongqing Li, Department of Mechanical & Industrial Engineering, University of Toronto, 5 Kings College Road, Toronto, Ontario, Canada, M5S 3G8, Carsten Werner, Institute for Polymer Research Dresden, Dresden, Germany

An improved "slope-intercept" technique for determining the surface conductance and z -potential from streaming potential measurements made in slit microchannels is proposed. In the classical, method the relationship between the pressure drop, D P, and the streaming potential, Es, is measured experimentally at a number of different channel heights, h. The parameter (ereoDP/mEslb) is then plotted as a function of 1/h and linear regression is performed. The y-intercept of the regressed line is then related to the z -potential and its slope to the surface conductance. This classical technique however ignores the electrokinetic effects on the liquid flow thus its use is limited to cases where the channel height is relatively large, the electrical double layer is relatively thin and the streaming potential is sufficiently small such that the electro-viscous effect is negligible. The improved technique retains the simplicity of the classical method while accounting for the electro-viscous effect through the introduction of two analytically determined correction factors. When applied to experimental results a comparison shows that the classical method will always overestimate both the z -potential and surface conductance and that significant errors will occur when the classical method is applied to systems with small channel heights and relatively thick double layers.


338

Determination of Isoelectric Points for Perovskite Compounds in Aqueous Suspension

J.H. Adair, NSF Particulate Materials Center, Penn State University, 217 Materials Research Laboratory, University Park, PA, 16802

The colloidal behavior of complex metal oxides and perovskites, in particular, is critically important to manufacture of electronic devices based on a colloidal processing technique known as tape casting. Perovskite materials have the general chemical formula, ABO3 (where A=K, Na, Ca, Ba, Pb, Mg, etc. and B is Ti, Nb, Zr, etc.). Perovskites are composed of a cation at the A site which is generally soluble in water and a cation at the B site that is only sparingly soluble over most of the pH range of interest from pH 2 to pH 11. As a consequence, the A site cation undergoes incongruent dissolution leaving a surface relatively rich in the B site cation interfacing with the solution phase. The concentration of leachable cation depends on both the solution pH and the solids loading. Thus, determination of colloidal features such as the isoelectric point of perovskite materials can not be determined simply by continuous titration of stock suspensions at a single solid concentration. A protocol is developed and proposed for the determination of the IEP of perovskite materials in aqueous suspension that overcomes the challenges posed by the incongruent dissolution of the A site cation to reproducibly provide IEP values.


339

Polymer Mobility in 2-nm wide Slit Pores: Effect of Interfacial Interactions

E. Manias, V. Kuppa, Materials Science & Engineering Department, Pennsylvania State University, University Park, PA 16802.

The motion of confined polymers is measured experimentally between parallel, mica-type solid surfaces, separated by 2 nm. In particular, polymer mobility is monitored during the intercalation of monodispersed polystyrene in alkylammonium modified 2:1 silicates, using X-ray diffraction. The kinetics of the neat polymers and their functionalized derivatives were measured as functions of molecular weight, extent of functionalization, and silicate surface organic modification, at various temperatures. The emphasis is on revealing the dependence of the polymer diffusion on the interaction strength at the inorganic/polymer interface. The kinetic data are interpreted in terms of an effective diffusion coefficient (Deff) of the polymer, which undergoes a dramatic decrease with stronger silicate surface-polymer interaction. This interaction is varied by either changing the silicate surface modification or by increasing the extent of functionalization of the polystyrene chains. Furthermore, the diffusion coefficient exhibits an inverse dependence on chain length (N), i.e., Deff µ N-1 , for chains up to 900 000 molecular weight, in agreement with recent theoretical predictions by Muthukumar et. al..


 

340

Rheological Properties Of Interfaces

Komunjer Ljepsa, UTC University of Technology Compiègne, Ballerat Karine, UTC, Dalmazzone, Christine, IFP Institut Français du Petrole, Noik, Christine, IFP

One of the key parameters governing the coalescence of the drops in an emulsion is the elasticity of the liquid-liquid interface. These phenomena are of paramount importance in crude oil exploitation, but they determine also the rate of transport of active substances in pharmaceutical and cosmetic formulations. We propose to compare the modulus of elasticity, E, obtained from the monolayer isothermes by means of Langmuir through and the results obtained by "pulsing drop" technique. In the first part of the communication we shall examine the liquid-liquid interface while in the second we will present the results obtained with the specially designed Langmuir balance for two liquids. Experiments were performed both on model paraffinic oil and petroleum oil with addition of different types of surfactants molecules.


341

Experimental Study of Foams for Environmental Remediation

Jiann Gwo Rong, Howard M. Liljestrand, Department of Civil Engineering, Seung Ihl Kam, William R. Rossen, Department of Petroleum and Geosystems Engineering

The University of Texas at Austin, Austin, TX 78712-1076

The properties of foams in porous media have long been studied in connection with applications in the petroleum industry. This same technology can enhance the efficiency of environmental-remediation processes. Previous studies on foams for petroleum applications find that they exhibit two sharply different flow regimes, at high and low gas fractions (qualities), and build up pressure gradients too high for environmental remediation in the shallow subsurface. Using foams for environmental remediation requires extending the technology to much higher permeabilities and lower pressures and finding weaker foams that build lower pressure gradients. Laboratory studies reported here extend previous work to much higher permeabilities and lower pressures. The two flow regimes again appear and follow trends in behavior with permeability and surfactant concentration reported previously. In preliminary studies, reducing surfactant concentration in the search for a weaker foam appears to have little effect until an abrupt drop in foam strength is observed. Therefore it may be difficult to formulate a foam with intermediate strength for environmental remediation based on steady-state behavior. Instead, a promising strategy is to design the process so foam generation does not go to completion, so that steady-state behavior is not attained.


342

SNAP: Solution Behavior of Nanoscale Silicon Sol-Gel Particles

A.J. Vreugdenhil, University of Dayton Research Institute, 300 College Park, Dayton OH 45469-0168; V.N. Balbyshev, Universal Technology Corporation, 1270 N. Fairfield Rd., Dayton, OH 45432; M.S. Donley, AFRL-MLBT, 2941 P Street, WPAFB, OH 45433

A coating system using nanoscale silicon sol-gel colloids has been shown to cross-link and form an excellent pretreatment layer for aluminum alloys (Vreugdenhil et al, J.Coatings Tech., 2001). This system, known as Self-assembling Nanoscale Particles (SNAP), utilizes the co-condensation of two silane precursors: tetramethoxysilane (TMOS) and 3-glycidoxypropyltrimethoxysilane (GPTMS). The solution processes involved in forming these silicon colloids have been investigated using infrared, Raman and NMR spectroscopy as well as dynamic light scattering particle size analysis. The particle size, coagulation and degree of internal condensation were explored as a function of aging and solution composition. Particle growth and solution gel time were shown to be driven primarily by the molar ratio of the two silanes. Relative rates of hydrolysis and condensation are used to explain the solution behavior of the SNAP material.


343

Dispersion and Characterization of Carbon Nanotubes

M. F. Islam, J. Zhang, B. Mei, A. T. Johnson and A. G. Yodh Dept. of Physics & Astronomy, University of Pennsylvania, Philadelphia, PA19104

We investigate dispersions of carbon nanotubes using quasi-elastic light scattering. Surfactants and ultrasonication were used to break apart bundles of nanotubes in aqueous or organic solvents. We determined the lengths and diameters of the resulting tubes as a function of surfactant concentrations, sonication power, and sonication time. Our results suggest that we have created stable dispersions of single-wall carbon nanotubes of varying lengths. We compare the light scattering results to Atomic Force Microscopy measurements. We will also discuss the phase behavior of these dispersions of carbon nanotubes. We gratefully acknowledge support from NSF (DMR99-71226) and MRSEC (DMR00-79909).


344

A Study of Nanodroplet Formation in Supersonic Expansions: Controlling Nucleation and Growth of the Droplets.

Kiril A. Streletzky, Worcester Polytechnic Institute, Worcester, MA 01609, Barbara E. Wyslouzil Worcester Polytechnic Institute, Worcester, MA 01609

Nanodroplet aerosols form in supersonic expansions of condensible vapors. In conventional continuously expanding nozzles, aerosol nucleation and growth occur simultaneously over a wide range of temperatures and supersaturations. To better understand the physics of nanodroplet formation one needs to decouple nucleation and growth and estimate the supersaturation and temperature of nucleation. Here we present the first results of our efforts to decouple the two processes using custom nozzle shapes. Nanodroplet formation in nozzles is usually studied by measuring pressure traces. These experiments only yield the conditions at the onset of condensation, velocity and temperature of droplets, and fraction of material condensed, but little about the aerosol size distribution. Small angle neutron scattering (SANS) on aerosols, pioneered in our laboratory, permits in situ measurement of nanodroplet size distribution. Combining the thermodynamic state measurements, SANS, and light scattering with careful nozzle design allows us to find conditions under which nucleation can be separated from subsequent droplet growth. Under these conditions we observe fewer but considerably larger and more monodisperse particles than under similar conditions in conventional nozzles. Analysis of our results and our first estimates of nucleation rate deepen our understanding of aerosol formation under highly supersaturated conditions.


345

Mixed Nanoparticles as Catalytic Supports for Chemical Warfare Agent Decomposition

Mark B. Mitchell, Viktor N. Sheinker, and Aron Tesfamichael Department of Chemistry, Clark Atlanta University, Atlanta, GA 30314

The decomposition reaction of dimethyl methylphosphonate (DMMP) has been investigated on a series of metal oxide catalysts, in which the support was formed from a designed mixture of nano-sized alumina, silica, and titania particles. In situ diffuse reflectance infrared spectroscopy has been used to examine the surface reaction, while microreactor experiments have been used to investigate the kinetics and product yield from the different mixed metal oxides. Methanol is the primary product observed from the reaction at temperatures near room temperature, while dimethyl ether is observed in smaller amounts. As the reaction temperature increases, the amount of dimethyl ether formed increases. The reactions are virtually always stoichiometric at temperatures less than 200 oC, with sustained reaction observed at temperatures in excess of 300oC.


346

Rheological Behavior of Heteroflocculating Suspensions: Control of Interparticle Interactions through Electrosteric Replusion

Stacy G. Bike and H. Joon Kong, The University of Michigan, Chemical Engineering Department, Ann Arbor, MI 48109

Heteroflocculating suspensions are good model systems to replicate the behavior of many inorganic dispersions. In this work, we designed a model heteroflocculating system of silica and alumina particles that mimics the behavior of more complex cement dispersions. Cement particles bear positively and negatively charged surface sites which complicates the interpretation of interparticle interactions and the resulting microstructure and macroscopic flow behavior. Through pH control, the model system is designed such that the silica particles are negatively charged and the alumina particles are positively charged; small changes in the pH are shown to have profound effects on the low amplitude oscillatory shear and steady shear behavior. At a given pH, the interactions between the particles are modified through electrosteric repulsion, which involves the adsorption of both a polyelectrolyte and a non-ionic polymer. Subtle alterations in the relative amounts of the polymers are shown to have large effects on both the microstructure, as illustrated by measurements of the moduli and viscosity. Combining these rheological studies with zeta potential, adsorption, and steric layer thickness measurements has allowed us to quantify the changes in suspension microstructure with changes in interparticle interactions and to relate these changes to the macroscopic flow behavior of cement suspensions.


347

Colloidal Gelation and Non-Ergodicity Transitions in Colloid-Polymer Mixtures

J. Bergenholtz, Department of Physical Chemistry, Goeteborg University, 41296 Goeteborg, Sweden, and M. Fuchs, Department of Physics, University of Edinburgh, Mayfield Road, Edinburgh EH9 3JZ, UK

Recent attempts at developing an appropriate theory for the non-equilibrium gel transitions that occur in systems of colloidal particles interacting via short-range attractions are presented. The theory rests upon the application of the idealized mode coupling theory, previously used in the context of the liquid-glass transition, to dilute and semi-dilute suspensions of particles interacting via model potentials (of square-well and Yukawa forms) and the Asakura-Oosawa depletion potential. When the short-range attraction becomes sufficiently strong the long-time translational dynamics arrests due to particle cluster formation. The structural arrest occurs along a well-defined, so-called gel transition. The most recent advancement has been an asymptotic analysis of the theory that yields simple, analytical results for the location of the gel transition in the phase diagrams, as well as analytical results for some of the properties of the gels, such as the mean square displacement and the low-frequency shear modulus. The theoretical predictions are free of adjustable parameters.


348

Weakly Interacting Depletion Aggregates and the Glass Transition

V. Prasad, D. A. Weitz, Harvard University, Cambridge, MA 02138, V. Trappe, University of Fribourg, Fribourg Switzerland, P. N. Segre, NASA, Marshall Space Flight Center, Huntsville, AL 35812

Hard sphere PMMA particles are suspended in a density and index matching solvent mixture (decalin and cycloheptyl bromide). Polystyrene polymer is added to induce attraction through the depletion potential. The clusters formed are free to diffuse around, and motion of these fluid-like clusters slows down critically near a transition as the volume fraction is increased. This is characteristic of the glass transition where the fluid-like clusters replace the hard spheres. As the volume fraction is increased, the clusters percolate to form a space-spanning network. This kinetic transition is well described by a model that takes into account the competition between thermal fluctuations and attractive interactions. An intriguing scaling relation is observed in the moduli of the solid network, as the volume fraction or the energy of interaction of the constituent particles is changed. Finally, the effects of changing the size ratio of the polymer to the particle are also described.


349

Structural Dynamics in Colloidal Gels of Adhesive Spheres

Michael J. Solomon and Priya Varadan, Department of Chemical Engineering, University of Michigan Ann Arbor 48109

The interesting rheological properties of colloidal particulate gels such as solid-like linear viscoelasticity arise due to their distinctive dynamic structure. We have used dynamic light scattering to characterize the cooperative dynamics of colloidal suspensions that have undergone gelation. The particles are colloidal silica (a = 40 nm) grafted with octadecyl aliphatic chains dispersed in the solvent hexadecane. The suspensions undergo a reversible gelation transition at a critical temperature in the volume fraction range 0.01 – 0.10. It is thought that the interaction of the colloids can be described by Baxter’s model of adhesive spheres [Grant and Russel, Phys. Rev. E 47 2606(1993)]. By varying the temperature, the divergence of characteristic times in the vicinity of the gel point has been quantified. A recent model of the dynamics of fractal clusters [Krall and Weitz, Phys. Rev. Lett. 80 778 (1998)] is shown to describe the initial decay of density fluctuations of the gel. At long times an additional relaxation mechanism is observed. We hypothesize that this long-time decay of the dynamic structure factor is due to rare bond disaggregation processes that occur due to the relatively weak interaction between the adhesive spheres of the thermoreversible gel.


350

Sol-Gel Transition of Colloidal Suspensions of Positively Charged Particles of Aluminum Magnesium Hydroxide

Dejun Sun, Wanguo Hou, Chunguang Zhang, Key Laboratory for Colloid and Interface Chemistry of the Ministry of Education, Shandong University, Jinan 250100, P. R. China

It is well established that the suspensions of Laponite, a synthetic Hectorite clay composed of negatively charged, monodisperse, disk-shaped particles, form a gel above a specific volume fraction. Addition of salt lowers the particle volume fraction at which the gel phase appears. In this work, we investigate the colloidal suspensions of positively charged sheet-like particles of aluminum magnesium hydroxide by continuous and oscillatory shear measurements. The results show that although aluminum magnesium hydroxide is oppositely charged to Laponite, the suspensions of aluminum magnesium hydroxide also display a sol-gel transition without macroscopic phase separation as the particle concentration increases, phenomenologically similar to that of Laponite suspensions. In addition, the sol-gel transition for suspensions of aluminum magnesium hydroxide is shifted to lower particle concentrations when ionic strength increases, also similar to behavior observed for Laponite suspensions. These similarities suggest that, for the two kinds of colloidal suspensions of oppositely charged particles, the underlying mechanism for the sol-gel transition is comparable.


351

Microstructure and Rheology of Thermo-Responsive Polymer Colloid Suspensions

Prashant Mullick and Charles Zukoski, University of Illinois at Urbana Champaign, 114 Roger Adams Lab, Box C3, 600 South Mathews Avenue, Urbana IL 61801

Stimuli-responsive particles have received increasing attention in various applications like separation of bio-molecules, drug delivery systems and in fundamental studies of thermo-responsive gel systems. Controlling the stimuli-response of these systems requires an understanding of the underlying links between pair interactions and the resulting phase behavior. The equilibrium phase behavior and rheological properties of such thermo-responsive colloidal systems are investigated here. The systems studied are microgels of cross-linked poly(N-isopropylacrylamide) and cross-linked core-shell particles prepared by emulsion polymerization. The core-shell particles have a polystyrene core grafted with a shell of poly(N-isopropylacrylamide). The poly(N-isopropylacrylamide) undergoes a coil-globule transition in close proximity of its lower critical solution temperature (LCST). As the temperature is increased the microgel particles show a distinct decrease in hydrodynamic radius, radius of gyration and sharp changes in the particle-particle interactions. The inter-particle interactions are characterized by measuring thermodynamic properties such as osmotic compressibility and the second virial coefficient, B2. In the case of the core-shell particles, the poly(N-isopropylacrylamide) on the surface undergoes a similar size change. These effects result in changes in the microstructure and phase behavior of the system and hence in the rheological behavior.


352

Rheology of Thermosensitive Latex Particles Including the High-Frequency Limit

I. Deike, M. Ballauff, Polymer-Institut, Universität Karlsruhe, Germany, Kaiserstrasse 12, 76128 Karlsruhe, N. Willenbacher, A. Weiss, Polymer Research Division, BASF-AG, Ludwigshafen, Germany

The flow properties of aqueous suspensions of thermosensitive latex particles are investigated as a function of volume fraction and temperature. The particles consist of a solid poly(styrene) core and a shell composed of crosslinked poly(N-isopropylacrylamide) (PNIPA) chains. The PNIPA-network shrinks with increasing temperature leading to a denser layer of polymeric chains on the surface of the core particles. The shear viscosity obtained from suspensions of these particles at low shear is compared to the viscosity measured in the high-frequency limit. In the limit of dilute suspensions the viscosity is modeled in terms of an effective hydrodynamic radius RH. It is shown that RH of highly swollen particles depends markedly on frequency. The data indicate that the swollen network on the surface of the particles is partially drained at high frequencies. For shrunken networks RH measured in the low and high frequency limit coincides again. The high frequency shear modulus G¥ measured at high volume fractions demonstrates that the thermosensitive particles may be regarded as soft spheres. The repulsive interaction may be modeled in terms of a power law with an exponent of 9.


353

Imaging and Spectroscopy at the Single Molecule Level with Carbon Nanotube Probes

Charles M. Lieber, Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138.

Scanning probe microscopies, such as atomic force microscopy, represent important tools for investigating phenomena at the nanometer scale, although the resolution and sensitivity are limited by the nature of the probe tip. Carbon nanotubes are materials that can overcome the limitations of conventional probe tips and thus offer great potential for the future. In this presentation the fabrication, properties and applications of carbon nanotube probes will be reviewed. First, chemical vapor deposition approaches will be described which allows simple and reproducible fabrication of individual single-walled nanotube tips with diameters less than 1 nm. The mechanical properties and resolution of these tips will be discussed. Second, application of single-walled nanotube tips for high resolution imaging of complex soft matter, such as large proteins and protein nucleic acid complex will be presented. Third, highly selective strategies for the modification of nanotube probe ends have been developed and used to probe intermolecular forces in a detailed manner. The use of modified nanotube tips for mapping spatial variations in chemical functionality down to the molecular scale will be described. Future directions and challenges with nanotube probe tips will be discussed.


354

Protein Adsorption Properties of Helical and Disordered Oligo(ethylene oxide)-Terminated Self-Assembled Monolayers

David J. Vanderah, and Curtis W. Meuse, NIST, 100 Bureau Dr., Stop 8313, Gaithersburg, MD 20899

Self-assembled monolayers (SAMs) of methyl 1-thiahexa(ethylene oxide) [HS(CH2CH2O)6CH3] of varying degrees of order have been prepared on gold. Highly ordered SAMs are obtained by assembly in either 95% or 100% ethanol. Reflection-adsorption infrared spectroscopy (RAIRS) data indicated that, in these SAMs, the 1-thiahexa(ethylene oxide) segment adopts the 7/2 helical conformation of the folded-chain crystal polymorph of poly(ethylene oxide). Less ordered SAMs are obtained by assembly from tetrahydrofuran. RAIRS data indicated that these SAMs are mixtures of the 7/2 helical and non-helical conformations. Exposure of these surfaces to bovine serum albumin and lysozyme showed that the less ordered HS(CH2CH2O)6CH3 SAMs exhibit better inhibition of protein adsorption. Thus the inhibition of protein adsorption cannot be attributed to highly ordered, helical conformations.


355

Atomic Force Microscopy Study of Electronically Conductive Polymer Film Electrodes and Ionic Exchange Film/Liquid Interface

Jian Wang, Allen J. Bard, Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, TX 78712

Although electrodes of electronically conductive polymers, like polythiophenes or polypyrroles, are often described as showing metallic behavior, in fact there have been few studies that address the question of electrode surface charge as a function of potential. While the conductivity of the polymer films is clearly electronic over a given potential range, one can also consider how the charge is distributed in the film and at the film/solution interface. We used atomic force microscopy (AFM) force measuring technique to study electronically conductive polymer film electrodes (poly(3-methylthiophene) and polypyrrole films). We found that there is a complete charge compensation in conducting polymer film electrode. This total compensation suggests that there is no diffuse double layer at the conductive polymer/solution interface, a situation distinctly different than that at metal or semiconductor electrodes. For ionic exchange film (like Nafion film), there is a qualitative difference comparing with conducting polymer film electrode.


356

Enzyme-Assisted Nanolithography Using Atomic Force Microscopy

Chang-Hyun Jang and William Ducker, Chemistry Department, Virginia Tech, Blacksburg, VA 24061

Two Nanofabrication methods are studied within self-assembled monolayers using AFM. An AFM with tip-bound enzymes is used as a tool for nanometer-scale lithography. Lipase enzymes are attached to a silica particle via a covalent bond. Lipase enzyme gives specific catalytic action to the hydrolysis of an ester to a carboxylic acid. Writing and reading tips alternately sit above the same part of the sample by switching the sample back and forth under the probes. In the second method, an in situ nanografting process is studied using an AFM tip without enzymes. Nanopatterns terminated with different functional groups are produced. Patterned SAMs are used for subsequent fabrication processes with nanoparticles or proteins.


357

Monolayers on Silicon Prepared by Scribing Silicon in the Presence of Reactive Species

Mathew R. Linford, Travis Niederhauser, Guilin Jiang, Yit-Yian Lua, David A.Berges, Department of Chemistry and Biochemistry, Michael Dorff, Department of Mathematics, Brigham Young University, Provo, UT 84602

We describe a novel preparation of monolayers on silicon, which consists of mechanically removing silicon’s native oxide by scribing it in the presence of reactive species. We present XPS, TOF-SIMS, and wetting data that show that monolayers on silicon can be prepared in this manner from alkenes, alkynes, alkyl halides, alcohols, and reactive monomers. We believe that highly reactive surface species such as silicon free radicals and strained silicon-silicon pi bonds (as in the Si(100) 2x1 reconstruction) react with these molecules to covalently link them to the silicon surface. XPS shows monolayer coverage of reactive molecules and that there are approximately 2 oxygen atoms per alkyl chain on the surface in the case of scribing silicon in the presence of 1-alkenes and 1-alkynes. TOF-SIMS of monolayers prepared from alkyl chlorides, bromides, and iodides shows the presence of SiCl+, SiBr+, and SiI+ in their appropriate isotopic abundances. TOF-SIMS of monolayers prepared from 1-alkenes and 1-alkynes shows the presence of numerous SiCnHm+ species. We show that this method of preparing monolayers on silicon can be used to pattern silicon with functionalized lines to make hydrophobic enclosures, or corrals and further demonstrate that the interiors of these hydrophobic corrals can be derivatized.


358

Site Switching for CO2 Formation and Structural Transformation of (1x2)(1x1) in CO Oxidation on Pt(110)

Md. G. Moula, A. P. Mishra, I. Rzeznicka, Y. Ohno, T. Matsushima, Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan; Catalysis Research Center, Hokkaido University, Sapporo 060-0811, Japan.

The orientation of reaction sites for CO2 formation in steady-state CO oxidation on Pt(110) was studied in a wide range of surface temperatures and reactant pressures through analysis of angular and velocity distributions of desorbing product CO2. The formation site switched sharply from declining terraces to surface parallel ones at a threshold CO pressure, where the surface structure was transformed from (1x2) to (1x1) form. In the active region where the CO2 formation rate was first order in CO, desorbing product CO2 mostly collimated in a two-directional way at ±25° in a plane in the [001] direction. This indicates CO2 formation on the declining terrace of the (1x2) structure. On the other hand, at a certain CO pressure in the inhibited region, desorbing product CO2 mostly collimated along the surface normal direction, showing the formation on the (1x1) form. At high oxygen coverages in the active region, these two desorption channels were operative. The translational temperature of desorbing product molecules was maximized to around 1800 K at ±25°. The normally directed component showed different translational energies in the active region and the inhibited region.


Talks 251 through 300

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