Oral Presentation Abstracts

Talks 151 through 200

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151

Determination of the Relaxation Kinetics in a PEP-PEO/DMF Micellar System by Time-Resolved SANS

Lutz Willner, Andreas Poppe, Jürgen Allgaier, Michael Monkenbusch and Dieter Richter, Forschungszentrum Jülich, Institut für Festkörperforschung, D-52425 Jülich, Germany

The static properties of block copolymer micelles has been extensively studied theoretically and experimentally. Less attention has been paid to the kinetics of micelle formation. That is due to the fact that the exchange kinetics under equilibrium conditions is difficult to access by expermintal techniques. Here we report a kinetic investigation using time-resolved SANS-measurements. The basic idea is to mix two micellar solutions made of differently labelled polymers and to study time-dependent the variation of scattered neutron intensity. The advantage of this method is that labelling is restricted to simple H/D isotope exchange. The study was performed on poly(ethylene-alt-propylene)-polyethylene oxide (PEP-PEO in dimethylformamide which is a selective solvent for PEO. It turned out that at elevated temperatures the formation of mixed micelles proceeds on a time-scale which is accessible by SANS. The observed kinetics consists of two well separated processes. From temperature dependent measurements the activation energies could be determined. A comparison with a scaling theory indicates that the fast process can be associated with the exchange of unimers between different micelles while the slow process could not yet be assigned to a microscopic picture. Both experiment and results will be presented.


152

Nanostructural Lithography of Thin-Film Silica Mesophases

Dhaval A. Doshi1, Nicola K. Huesing2, Mengcheng Lu1, Hongyou Fan3, Alan J. Hurd3, C. Jeffrey Brinker1,3, 1Department of Chemical and Nuclear Engineering, University of New Mexico, 1001 University SE, Suite 100, Albuquerque, NM, 87106; 2Institute of Inorganic Chemistry (E-153), Vienna University of Technology, Getreidemarkt 9, A - 1060 Vienna, Austria; 3Sandia National Laboratories, Albuquerque, NM, 87185-1349

Cooperative self-assembly processes of inorganic species and amphiphilic molecules have experienced major advances since their discovery by Mobil researchers. Various pathways have been explored to access a wide spectrum of mesostructured materials with a variety of macro – and microstructures. Recently we reported the ability to optically define and continuously control film location, mesostructure, and properties of photosensitive thin-film silica mesophases. Our procedure exploits the pH sensitivity of both the siloxane condensation rate and silica-surfactant self-assembly by the addition of a photoacid generator (PAG). The procedure starts with a homogeneous solution of silica, surfactant, ethanol, PAG, HCl and water. Preferential ethanol evaporation during dip-coating concentrates the depositing solution in water and non-volatile constituents, thereby promoting self-assembly of a photosensitive, one-dimensional hexagonal (1-dH) silica-surfactant mesophase. Exposure to UV light results in localized photoacid generation. During heat treatment, we observe a hexagonal to tetragonal mesophase transformation for the UV exposed regions. X-ray diffraction, transmission electron microscopy, in-situ thickness and stress measurements are used to characterize and understand the phase transformation behavior. A mechanism to explain the observed structural changes in the silica-surfactant system is proposed. The effect of acid concentration on the phase behavior of silica-surfactant self-assembly will be presented.


153

Evidence for Spinodal Phase Separation in Two-Dimensional Nanocrystal Self-Assembly

Guanglu Ge and Louis E. Brus, Department of Chemistry, Columbia University, New York, NY 10027

The drying of nanocrystal solution films on graphite surface creates a varied range of structured aggregate spatial patterns as a function of surface coverage. These images can be understood as predicted kinetic stages in the fluid-fluid spinodal phase separation of a 2D van der Waals particle system. It is suggested that phase separation occurs due to increased van der Waals nanocrystal-nanocrystal interaction in the presence of air, as compared with the shielded interaction in organic solvents.


154

Transformation of AOT Inverse Micelles to Rigid Organogels –Microstructure Determination Through SAXS, NMR and AFM.

Blake Simmons, Vijay T. John, Sichu Li, Dept. of Chemical Eng., Chad Taylor, Gary L. McPherson, Daniel K. Schwartz, Dept. of Chemistry, Tulane University, New Orleans, LA, 70118, Forrest Landis, Robert Moore, University of Southern Mississippi, Dept. of Polymer Science, Hattiesburg, MS 39401

Dry reverse micelles of the anionic twin-tailed surfactant bis(2-ethylhexyl) sulfosuccinate (AOT) dissolved in nonpolar solvents form an organogel when p-chlorophenol is added in a 1:1 AOT:phenol molar ratio. The proposed microstructure of the gel is based on strands of stacked phenols linked to AOT through hydrogen bonding. Small-angle x-ray scattering (SAXS) spectra of the organogels suggest a characteristic length scale for these phenol-AOT strands that is independent of concentration but dependent on the chemical nature of the non-polar solvent used. Correlation lengths determined from the SAXS spectra indicate that the strands self-assemble into fibers. Direct visualization of the gel in its native state is accomplished by using tapping mode atomic force microscopy (AFM). It is shown that these organogels consist of extended fiber bundle assemblies. The SAXS and AFM data reinforce the theory of a molecular architecture consisting of three length scales – AOT/phenolic strands (ca. 2 nm in diameter) that self-assemble into fibers (ca. 10 nm in diameter), which then aggregate into fiber bundles (ca. 20-100 nm in diameter) and form the organogel. Ferrite nanoparticles can be selectively incorporated into the fiber bundles to create field responsive materials.


155

Dielectrophoretic Assembly of Electrically Functional Metallic Nanowires

Kevin D. Hermanson, Simon O. Lumsdon, Jacob P. Williams, Eric W. Kaler and Orlin D. Velev, Department of Chemical Engineering, University of Delaware, Newark, DE 19716.

We have developed a novel colloidal process for the dielectrophoretic assembly of conductive wires from suspensions of metallic nanoparticles. The fibrillar aggregates are formed in the gaps between planar electrodes. These wires can grow faster than 500 micrometers/s, thereby enabling quick electrical connections between the edges of gaps of few millimeters across. The nanowires grow in the direction of the electric field gradient and thus "automatically" form electrical connections to conductive islands or particles situated between the electrodes. The thickness of the wires can be controlled by changing the electrical frequency. Under certain conditions the nanoparticles cluster to form a "string of beads", and in the presence of latex particles assemble into "insulated wires" that have a metallic core surrounded by a latex shell. The metallic wires have a good Ohmic conductance both in AC and DC modes. The simple assembly process, the ultra high surface-to-volume ratio and the possibility for easy functionalization make these unique structures promising for sensors and bioelectronic circuits.


156

Photochemical Incorporation of Silver Quantum Dots within Monodisperse Silica Colloids for Photonic Crystal Application

Wei Wang and Sanford A. Asher, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260

We report here a novel method to fabricate nanocomposite colloidal particles consisting of monodisperse SiO2 spheres (~100 nm) with homogeneously dispersed Ag quantum dots (2~5 nm). The morphologies of the inclusions are controlled through the timing of the photochemical reduction of silver ions during the hydrolysis of tetraethoxysilane in a microemulsion. Depending on this timing Ag quantum dots can be directed to different annuli within the SiO2 spheres, as well as onto the SiO2 sphere surface. These Ag@SiO particles have significant surface charge and readily self assemble into crystalline colloidal array photonic crystals which Bragg diffract light in the visible region.


157

Modeling Electrokinetic Flow through Microchannels with Heterogeneous Surface Properties

David Erickson & Dongqing Li, Department of Mechanical & Industrial Engineering, University of Toronto, 5 Kings College Road, Toronto, Ontario, Canada, M5S 3G8

A model of the electrokinetic flow of liquids through microchannels with heterogeneous surface properties has been developed. Flow simulations have been conducted in channels of rectangular and slit geometry to provide qualitative insight into the response of the streaming potential, Es, the streaming current, Is, and the volume flow rate, Q, to changes in the degree of surface heterogeneity, G , and the difference in z -potential between the solid surface and the heterogeneous regions, D z . The classical Smoluchowski equation predicts a linear relation between G and both Es and Is, however the model shows that in cases where the electro-viscous effect is significant, this relation no longer holds for Es. Thus when the streaming potential technique is used as a method of determining G , in protein adsorption studies for example, the classical equation can lead to a significant underestimation of G . Since Is is not affected by the electro-viscous effect it is shown to follow the classical linear relationship. While all quantities, including Q, are strongly dependent on both G and D z , it is shown that in cases where double layer overlap is small, the location of the heterogeneity is not significant.


158

Microfluidic Motion in Electrokinetic Separation Processes

James C. Baygents and Thomas L. Sounart, Department of Chemical and Environmental Engineering, The University of Arizona, Tucson, AZ 85721

Electrokinetic separation schemes are naturally suited to microfluidics channels, as they involve the resolution of mixtures of ionic solutes into discernible sub-populations along the channel axis. Inherent to the separation process is the development of electroosmotic motion, which couples to stream-wise variations in the electrical conductivity, pH, electric field, etc. Superimposed on the electroosmosis, are pressure-driven flows that stem from the need to conserve mass and electrical charge. The resultant fluid motion can cause the mixing or dispersion of analyte materials, and can dramatically affect the evolving spatial gradients and resolution of the separation process. Examples of scenarios where such flows arise include: sample transport in high-ionic-strength plugs on bioMEMS devices[1], and analyte stacking in zone electrophoresis[2]. In our presentation we will discuss the fully-coupled, two-dimensional simulations and experiments on the spatio-temporal development of several prototype flows and their effect on electrokinetic separations. In addition, we will present a semi-analytic lubrication approximations for the flows and assess the use Taylor-Aris dispersion relations as a means to develop effective, one-dimensional macrotransport representations of the separation/analyte transport process.

[1] L. Bousse, et al., Annu. Rev. Biophys. Biomol. Struct. 29 (2000) 155.

[2] D.S. Burgi & R.-L. Chien, Anal. Chem. 63 (1991) 2042.


 

159

DNA Sequencing and Genotyping Matrices Tailored for Use in Microfluidic Devices

Annelise E. Barron, Methal N. Albarghouthi, Brett A. Buchholz, Erin A.S. Doherty, Igor V. Kourkine, Northwestern University, Dept. of Chemical Engineering, 2145 Sheridan Rd., Rm. E136, Evanston, IL 60208

Given the speed and efficiency with which electrokinetic DNA separations are accomplished in miniaturized channels, genetic analysis are an important application of microfluidic devices. While DNA separations on chips have been demonstrated with conventional polyacrylamide and agarose DNA separation matrices, for practical implementation it will be necessary to develop replaceable DNA separation matrices that are tailored for use in miniaturized microfluidic devices. Separation matrices based on conventional water-soluble polymers demand a compromise in their design: whereas highly-entangled solutions of high molar mass polymers provide optimal DNA separation, their high-viscosity solutions require high-pressure channel loading, a step difficult to automate on microfluidic devices. Here, we demonstrate reproducible synthesis, precise characterization, and excellent sequencing performance (> 465 bases in 78 minutes) of thermoresponsive poly(N,N-dialkylacrylamide) matrices exhibiting a reversible, temperature-controlled ‘viscosity switch’ from high-viscosity solutions at 25°C to low-viscosity, microphase-separated colloidal dispersions at an elevated temperature. The viscosity switch decouples matrix loading and sieving properties, accelerating microchannel loading by three orders of magnitude. These matrices have the additional advantage of being "self-coating," i.e., preventing electroosmosis and analyte adsorption through formation of a passivating layer of adsorbed polymer on the microchannel walls. Thermoresponsive, self-coating matrices will be applicable to use in temperature-controlled microfluidic devices.


160

Mass Transfer and Mixing Process in Electro-Osmotic Flow in Micro-Channel

Liqing Ren & Dongqing Li , University of Toronto, 5 King’s College Road, Toronto, Ontario, M5S 3G8, Canada

Mixing of two or more solutions in a micro-channel is a key requirement for many lab-on-a-chip devices. The objective of this paper is to predict mixing process of two solutions in capillary. One unsteady, convection and diffusion model is developed to predict the concentration distribution in capillary. According to the concentration distribution, the capillary is divided into three parts. One part is filled with low concentration solution C1, another part is filled with high concentration solution C2 and the third part is called mixing zone filled with the mixing solution Cmix whose concentration is between high and low concentrations. The electrical resistance in whole capillary can be determined after the length and concentration of different part is known. Then the total current in capillary can be calculated at a given time, so does the electrical field strength for the three parts. The velocity, in turn, is calculated with time. Hence, the concentration, the total current and the time required for high solution to completely replace the low solution can be determined with time. The experiments for current are compared with the predictions of the model, and a good agreement was found for KCl solution with different concentrations and electrical fields.


161

Microbead Utilization in Microfluidic Devices

J. Oakey and D.W.M. Marr, Chemical Engineering Department, Colorado School of Mines, Golden CO 80401

Microfluidic systems have been traditionally hampered by size constraints imposed by conventional fluid control and actuation schemes. Even electroosmotic pumping, while employing a minimum of restrictive hardware, imposes operational constraints upon the nature of fluids that may be handled. We have developed and demonstrated feasibility of a novel microfluidic control scheme using microbeads isolated in specially designed channels as a direct means of flow control. We have exploited the versatility and size of microbeads to create fluid handling devices that are of comparable dimension to the microsystems into which they are integrated. To fabricate these devices we combine conventional rapid prototyping and soft lithography techniques with a novel optical trapping approach. This technique holds the potential for the creation of fluid handling devices that may be seamlessly integrated into micro total analysis systems.


162

Analytical Greens Function Approach to Transient Behavior of Electroosmotic Flow in Rectangular Microchannels

Chun Yang, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798

Electroosmotic pumping has extensively been used in the Lab-on-a chip technology for transporting liquid flow in microchannel networks. A fundamental understanding of the time-dependent electroosmotic flow in mcirochannels is important to the design and control of relevant processes where the transient characteristics of the electroosmotic flow behavior cross-section cannot be neglected. To this end, this work presents an analysis of the transient behavior of the electroosmotic flow in rectangular microchannels that represent the real cross-section shape of microfluidic channels fabricated from modern micromachining technologies. Based on the Debye-Hϋckel approximation, a linear solution to a 2D Poisson-Boltzmann equation governing the electrical potential distribution was obtained. By using the Greens function approach, the Navier-Stokes equation describing the transient electroosmotic flow in rectangular microchannels was solved analytically. Parametric analyses were conducted to demonstrate the influences of the channel geometry, electrolyte concentration, zeta potential, and applied electric filed strength on the fluid velocity distribution and volumetric flow rate.


163

Microfluidic Transport Processes Driven by Electrohydrodynamic Flows

Costas Tsouris, David W. DePaoli, Valmor F. de Almeida, Chemical Technology Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN, 37831-6224; Christopher T. Culbertson, Stephen C. Jacobson, J. Michael Ramsey, Chemical & Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, 37831-6142 National Laboratory, P.O. Box 2008, Oak Ridge, TN, 37831-6142.

Interest in microfabricated fluidics has grown considerably over the past decade primarily because these miniature devices provide a platform for applications ranging from inkjets to intensified chemical reactors to sensing and analysis. A common method to transport and mix samples and reagents on these microfluidic devices is electrokinetic transport, which includes electroosmosis for fluid pumping through microchannels and electrophoresis for the separation of the components of a liquid mixture. Electrokinetic transport, however, has limitations dictated by the physical properties of the fluids. For instance, it cannot be used efficiently for nonpolar organic solvents. Also, fluid mixing is generally limited to miscible aqueous systems and depends on relatively slow diffusional mechanisms. A technique that is complementary to electrokinetic transport for fluid pumping and mixing is electrohydrodynamic (EHD) transport. For fluids of relatively low conductivity and moderate to high dielectric constant, EHD transport appears to be a more efficient process than electrokinetic transport. Results on EHD mixing of organic solvents will be discussed in this presentation.


164

Hydrodynamics of Dip-Coated Thin Film in the Presence of Evaporation

Dan Qu, Stephen Garoff, Dept. of Physics, Center for Complex Fluids Engineering, Carnegie Mellon University, Enriqué Ramé, NASA Glenn Research Center

A dip-coated thin film of evaporative liquid demonstrates different hydrodynamics than that of non-evaporative liquid. The film reaches a quasi-steady state with a fixed film length when fully developed, and has a moving contact line with respect to the substrate. We experimentally investigated the film profile of several low molecular weight, evaporative silicone oils with very different evaporation rate as the substrate is retreating from the bulk liquid at different speeds. We find that the film length, cross-sectional area and characteristic thickness all have power law dependence on the withdrawing speed of the substrate. We will discuss the roles of gravity, viscous drag, capillary force and evaporation in the mass transportation mechanism and in forming the pressure field inside the film. We find a new scaling for the dimensions and shape of evaporative films which differs from the conventional scaling for non-evaporative films. We will explain quantitatively the origin of the new scaling.


165

Direct Visualization of Surfactant Distribution on a Spreading and Evaporating Drop

Van X. Nguyen and Kathleen J. Stebe, Department of Chemical Engineering, Johns Hopkins University, 3400 North Charles St, Baltimore, MD 21218

In this presentation, the distribution of surfactants on droplets placed on substrates with contact angles varying from strongly wetting to non-wetting conditions is studied. The droplets are initially formed as pendant droplets in air. They are coated with an insoluble surfactant/ dye mixture. The dye is a fluorescently labeled probe that allows the surface phases of an insoluble surfactant (pentadecanoic acid) to be directly visualized as the droplet interface evolves. The initial surface tension is recorded using the pendant drop technique. The droplet is then placed on a substrate with well-defined wetting conditions; as the drop evolves, the mean surface tension and contact angle are recorded using the sessile drop technique. Concomitant images of the surfactant distribution on the droplet interface are recorded. Control over the hydrodynamic behavior of the droplets depending upon the surface phase of surfactant formed at the interface as well as the dynamic contact angle is demonstrated for both spreading and evaporating drops.


166

Morphological Transitions of Coating Films in Cylindrical Pores: From Bumps to Liquid Bridges

Konstantin G. Kornev and Alexander V. Neimark, TRI/Princeton, 601 Prospect Avenue, Princeton, New Jersey 08542-0625

We suggest a rigorous formulation of the problem of morphological classification of the films coating the cylindrical pores. Competition between the capillary and adhesion forces drives the film patterning. Using the method of dynamic systems we list all admissible configurations of the film profile. Particular attention is paid to the analysis of the bridging process when the film passes through a sequence of configurations such as bumps and menisci. We show that the generic configuration in description of the heterogeneous nucleation in confinements is a bump. The theory of nucleation based on an analysis of the energetic barriers for bumps and lens formation is suggested. The outlined findings allow us to provide a novel vision of the theory of capillary condensation/evaporation.


167

Diffusive Growth of a Precursing Film Extending from a Droplet of Pb on Cu(111)

Jaehyun Moon, Jeniffer Lowekamp, Paul Wynblatt, Stephen Garoff*, and Robert Suter*, Department of Materials Science and Engineering, *Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, U.S.A

The diffusion of Pb away from partially wetting particles the Cu(111) surface to form so-called precursing film has been studied by means of Auger electron spectroscopy at temperature range 353 to 373K. These films achieve a maximum thickness of about 0.85 monolayers(0.85ML) at partilce triple line. The surface concentration profiles of Pb in the vicinity of particles have been analyzed to extract the diffusion coefficient (D) as a function of coverage of various temperatures. D exhibits high values both at low (<0.5ML) and high (<0.5ML) Pb coverages, and a minimum near 0.5ML. This dependence of D on coverages is consistent with the surface structure known to be present on Cu (111) at various coverages. In particular, the low value of D at intermediate coverage is associated with the incorporation of Pb into the Cu surface in the form of surface alloy. The time dependence for growth of the precursing film is very similar to the kinetics of precursing film in much more complex molecular systems. Our observations of a monolayer precursing film in quasi-equilibrium with a partially wetting particle consistent with models of the relationship between precursing films and contact angles.


168

Evaluation of PS and PMMA Probes for the Estimation of Polar and Dispersive Surface Energy Parameters of Polymers Using the Imbedded Fiber Retraction Method1

G. Biresaw and C. J. Carriere, Biomaterials Processing Research Unit, National Center for Agricultural Utilization Research, Agricultural Research Service, United States Department of Agriculture, 1815 N. University Street, Peoria, IL 61604, USA.

Polymers whose dispersive (g D ) and polar (g P ) surface energy parameters are known can be used as probes to estimate the surface energy parameters of other polymers from the interfacial tensions of the probes with the polymer. Such method of estimating surface energy parameters has been used rarely in the past mainly due to difficulties with direct measurement of the interfacial tension of high molecular weight and high viscosity polymer blends using current equilibrium methods such as sessile or pendant drop. This problem can be overcome by using the imbedded-fiber retraction (IFR) method, which is a dynamic method particularly suitable for direct measurement of the interfacial tensions of high viscosity polymer blends in a relatively short period of time. In this work, PS and PMMA were used as probes to estimate the surface energy parameters of several polar and non-polar polymers. The IFR method was used to directly measure the interfacial tensions of the polymers with PS and PMMA. The surface energy parameters of the polymers were then estimated from the reported g D , g P values of PS and PMMA as well as the IFR measured interfacial tensions. Correlations were observed between the g P of the probe polymers and the IFR measured interfacial tensions. The implications of these correlations will be discussed.


169

Spontaneous Spreading of Liquids Driven by Marangoni Flow

A. Chengara, A.D. Nikolov and D.T. Wasan, Illinois Institute of Technology, Department of Chemical and Environmental Engineering, 10 W. 33rd Street, Chicago, IL 60616

K. Koczo and G.A. Policello, OSI Specialties, Crompton Corp., Tarrytown, NY 15091

In this paper, we suggest that the spreading of trisiloxane ethoxylates is controlled by a surface tension gradient, which forms when a drop of surfactant solution is placed on a solid surface. A surface tension gradient is artificially imposed by localizing surfactant on the surface of a water drop. Our experiments show that the higher the surface tension gradient, the faster the spreading. The experimental spreading velocity can be predicted by a balance of the kinetic energy of spreading and the surface energy that results from surfactant addition.


170

Dynamic Wetting of Shear-Thinning Fluids

Yue Suo, Stephen Garoff, Center for Complex Fluids Engineering, Dept. of Physics, Carnegie Mellon University; Pittsburgh, PA 15213, Enrique Rame, National Center for Microgravity, NASA, Cleveland, OH 44135

We study the effect of shear-thinning property on dynamic wetting by observing the liquid-air interface shape near the contact line of an aqueous xanthan-gum solution (0.15 wt%) moving across a Pyrex glass surface. The liquid viscosity has a lower critical shear rate of ~0.1s-1 and roughly a power-law shear-thinning behavior. For our system, deviations of the measured interface shape from the Newtonian theory are observed and analyzed as a function of the contact line speed. As the speed increases, the size of the shear-thinning region near the contact line increases. A model based on the lubrication approximation is used to interpret the data. The dependence of the dynamic contact angle on Ca is examined and compared with model for Newtonian and non-Newtonian fluids.


171

Rheology of Alkyl Polyglucoside-Alkyl Sulfate Surfactant Mixtures

Beth A. Schubert, Belgin Baser, Eric W. Kaler, and Norman J. Wagner, Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark DE 19716, USA

Control of the rheological properties of mixtures of alkyl polyglucosides is critical for applications. Alkyl polyglucosides (CmGn) are a novel class of nonionic surfactants which have a hydrophobic carbon chain containing "m" carbons and a hydrophilic head group containing "n" glucose units. Alkyl polyglucosides have significant advantages in consumer products because of their low skin irritation and biodegradability. The addition of small amounts of ionic surfactant to certain glucosides yields an isotropic, one-phase region with high viscosity.

Here we report on the rheology of mixtures of alkyl polyglucosides with alkyl sulfates as functions of the total surfactant concentration and the surfactant composition. The effect of surfactant chain length on the rheological properties is explored by comparing results for mixtures of alkyl sulfates with C10G1 and C12G1. To complement the rheology, flow birefringence and flow small angle neutron scattering will be used to investigate the microstructure under shear. This combination of methods allows measurement of all the relevant length scales in solution. The rheological properties and resulting microstructure can be "tuned" by changing the total surfactant concentration or by altering the charge density by changing the concentration of ionic surfactant.


172

Micellar Structure Changes in Aqueous Mixtures of Nonionic Surfactants: A Study by Rheology and Small-Angle Neutron Scattering

Liang Guo and Ralph H. Colby, Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA, Min Lin, NCNR, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA, Gregory P. Dado, Air Products and Chemicals, Inc., Allentown, PA 18195, USA

Rheology and small-angle neutron scattering are used to probe the structure of nonionic surfactant mixtures in water. Small amounts of a C14 diol (Surfynol 104) cause enormous structural and rheological changes when added to aqueous solutions of an ethylene oxide-propylene oxide-ethylene oxide triblock copolymer (Pluronic P105). Surfynol 104 is only soluble up to 0.1wt% in pure water, but can be added in large quantities to Pluronic P105 solutions. The hydrophobic Surfynol 104 incorporates into the existing Pluronic P105 micelles and causes a cascade of changes in micelle structure, with resultant changes in rheology. Particularly striking is the spherical to wormlike micelle transition, where the viscosity changes by a factor of more than104.


173

Microstructural Changes in Anionic Surfactant Solutions Induced by Hydrotropic Salts

P. A. Hassan, Srinivasa R. Raghavan and Eric. W. Kaler, Department of Chemical Engineering,

University of Delaware, Newark, DE-19716

It is well known that cationic surfactants can self-assemble into wormlike micelles in the presence of low concentrations of aromatic salts such as sodium salicylate. This is the first report of analogous effects for anionic surfactants. Micelle growth under these conditions is thought to reflect the adsorption of such "hydrotropic" salts at the micellar interface, which leads to enhanced electrostatic screening of the charged headgroups and facilitates micellar growth. For anionic surfactants the hydrotropes are typically aromatic salts, e.g., p-toluidine hydrochloride(PTHC), which feature a hydrophobic cation that can bind strongly to anionic micelles. Addition of PTHC to anionic surfactants such as SDS promotes micellar growth at significantly lower than equimolar ratios of salt to surfactant. A variety of techniques, including rheology, light and neutron scattering are used to investigate these mixed micelles. The non-monotonic behavior of the zero-shear viscosity as a function of salt content mirrors the behavior of cationic surfactants.


 

174

Dynamics of Micelles of Surfactant Dimers and of Amphiphilic Block Copolymers in Aqueous Solution

Raoul Zana,* Gilles Waton,** Bernard Michels** and Walter Ulbricht+ (*Institut C. Sadron, CNRS, Strasbourg, France; **Laboratoire de Dynamique des Phases Condensées, CNRS, Strasbourg, France; + Universitat Bayreuth, Bayreuth, Germany).

This talk first reviews the dynamics of micellar solutions of conventional surfactants. It then examine the dynamics of micelles of surfactant dimers, made up of two amphiphilic moieties connected close to the head groups by a polymethylene spacer group. The main differences with conventional surfactants are that the entry of a surfactant dimer in its micelles is slower (factor 50) than for a diffusion-controlled process and that the residence time of a dimer in its micelle is much longer. Micelle formation-breakup proceeds via entry-exit of one surfactant dimer at a time into-from its micelles. For the amphiphilic PEO-PPO-PEO copolymers the entry of a copolymer into its micelles is also slower than for a diffusion-controlled process, except for low molecular weight copolymers but the micelle formation-breakup proceeds via fragmentation-coagulation reactions. Our results do not confirm the existence of a third relaxation process that would be due copolymer micelle clustering at the approach of the cloud temperature. They show rather that the amplitude of the relaxation due to micelle formation-breakup goes from negative to positive as the temperature is increased


175

Self-Assembled Monolayers as Templates for Liquid Crystals

Nicholas L. Abbott, Department of Chemical Engineering, University of Wisconsin, 1415 Engineering Drive, Madison, WI 53706

The orientations assumed by liquid crystals supported on the surfaces of solids are influenced by the presence of structure within these surfaces on a wide range of length scales (0.1-1000nm). Self-assembled monolayers formed from organosulfur compounds on textured metallic films make possible control of the structure of surfaces over this spectrum of length scales. This talk will address the delicate balance of forces that act between liquid crystalline materials and self-assembled monolayers supported on obliquely deposited gold films. Self-assembled surfaces can be designed with multiple levels of structure so as to create antagonistic interactions between liquid crystals and surfaces. These antagonistic interactions can be exploited to place liquid crystals on the edge of orientational transitions, thus making them exquisitely sensitive to changes in the structure of the surfaces. This talk will illustrate the use of these orientational transitions to detect the presence of parts-per-billion levels of targeted low molecular weight compounds (e.g., organophosphonates).


176

Resonantly Enhanced Sum Frequency Vibrational Spectroscopy of Silanised Mica Surfaces.

Alex. G. Lambert, David J. Neivandt and Paul B. Davies, Department of Chemistry, The University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom.

The non-linear optical technique of Sum Frequency Vibrational Spectroscopy is now recognised as a valuable surface specific probe of molecular conformation and orientation. Molecules adsorbed in comparatively disordered conformations generate only weak sum frequency signals which can typically only be studied with the high surface electric fields associated with a picosecond or faster laser system. We have recently developed a technique allowing nanosecond spectrometer systems to study the weak SF signals generated from molecules adsorbed to a hydrophilic surface. The method involves coherent interference on a gold backed mica substrate and was successfully calibrated by studying the self assembly of octadecyltrichlorosilane. The calibration procedure necessitated the development of a reproducible protocol for the silanisation reaction. The essential pre-requisites for reproducibility were found to be fresh octadecyltrichlorosilane, a clean reaction environment, controlled surface hydration, and high purity anhydrous solvents. During the reaction itself the well recognised parameters of reaction temperature and the degree of bulk hydration required strict regulation. The Sum Frequency Spectroscopy results were corroborated by AFM, contact angle and linear infrared data.


177

Second Harmonic Generation in Self-Assembled Thin Polymeric Films

M.T. Guzy, S. Shah, R.M. Davis, K.E. Van Cott, Department of Chemical Engineering; P. Neyman, J.R. Heflin, Department of Physics; H. Wang, H.W. Gibson, Department of Chemistry, Virginia Tech, Blacksburg VA 24061

Materials exhibiting second-order harmonic generation (SHG) comprise a class of nonlinear optical (NLO) materials that are useful for applications in electro-optic modulators and for spectroscopy. This research concerns polymer-based NLO materials in the form of thin films made using ionically self-assembled monolayers to incorporate NLO-active chromophores into the film with controlled orientation. SHG in the films depends on the chromophore's intrinsic hyperpolarizability, its number concentration in the film, and its orientation in the film. We compare the SHG of films made with polymeric and monomeric chromophores to probe the effect of chromophore architecture on film structure. Films were also made with a polyanionic chromophore and three polycations to study the effect of polymer charge density on chromophore deposition and orientation. These films are formed via a solution phase dipping process. The effects of deposition conditions – pH, ionic strength, polyion concentration - on film thickness and chromophore content were characterized by ellipsometry and spectrophotometry.


178

Chain Segment Order in Adsorbed Polymer Thin Films: A Deuterium NMR Study

S.V. Primak,+ T. Jin,+ A.C. Dagger,# E.K. Mann,+ Kent State University, Department of Physics, Kent, OH, USA; #The University of York, Department of Chemistry, Heslington, York, UK

The orientational order in monomerically thin self-assembled polymer films was investigated by DNMR. The technique probes alignment of CD3 groups in the linear polymer polydimethylsiloxane (PDMS), which also reflects the backbone orientation. Surface induced order was demonstrated directly by the angular dependence of the DNMR spectra with respect to an external magnetic field B. All spectra have two different contributions: (i) liquid film showing up as an isotropic central peak, (ii) oriented film where the quadrupole splitting depends on orientation with respect to the magnetic field B. The oriented film contribution saturates below surface concentrations corresponding to a single dense monomer layer, but evolve dramatically as the surface concentration increases above that level. Experimental NMR spectra were compared with simulated ones to test conformational order and dynamics in the macromolecular film. An overall mechanism of polymer adsorption was suggested.

This work was supported by NSF grant DMR-9984304


179

Watching Configurational Changes During Self Assembled Monolayer Growth Using a Molecular Triangulation Method

John T. Yates, Jr., Jae-Gook Lee and Joachim Ahner, Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260

We have devised a method to measure chemical bond directions in complex adsorbed molecules during their assembly into ordered monolayers. Using hydrogen and deuterium labeling in selected molecular sites of the molecule, we use electron stimulated desorption to measure specific C-H and D-H bond directions by measuring the H+ and D+ ejection direction. These directions closely match the direction of the chemical bond being ruptured. The method has been applied to acetate layers, pyridine layers, and 4-picoline layers on the Cu(110) surface. These measured bond directions provide incisive geometrical information about the conformation of the molecule and changes in conformation as the layer assembles. In addition, internal rotations of CH3 groups have been witnessed using this method which is called Electron Stimulated Desorption – Molecular Triangulation (ESD-MT).


180

The Growth Mechanism of Self-Assembled OTS Monolayers on Fused Silica Surfaces Probed by Sum-Frequency Generation and Lateral Force Microscopy

Yi Liu, Matthew C. Henry, and Marie C. Messmer, Dept. of Chemistry, Lehigh University, Bethlehem, PA 18015.

The adsorption and conformational changes of n-octadecyltrichlorosilane (OTS) self-assembled monolayer are monitored by a nonlinear optical technique, sum-frequency generation (SFG), and lateral force microscopy (LFM). The effect of a small amount of water in the OTS deposition solution on the conformational changes of the resulting OTS monolayers is also investigated. Results show that at initial stage the partial OTS monolayers are disordered even though island structures are observed by LFM. This indicates that the growth of OTS monolayers has two competing processes: islands formation and uniform growth. The addition of a small amount of water seems to favor island formation. SFG has also been used to probe the kinetic effect of a small amount of water in solvent on the conformational changes of the resulted OTS monolayers. The correlation of LFM, contact angles, and SFG measurements is discussed.


181

Evaluating Optical Techniques for Determining Film Structure: Optical Invariants for Anisotropic Dielectric Thin Films

E. K. Mann, Dept. of Physics, Kent State University, Kent, OH 44242-0001

A model-free method, based on optical invariants, of analyzing the reflectivity of thin adsorbed dielectric films is extended to dielectric waveguide techniques such as OWLS. The reliability of film parameters determined from three general types of techniques (OWLS, scanning angle reflectometry and scanning angle ellipsometry) is compared in the context of uniaxially anisotropic adsorbed films, with major axis perpendicular to the film. Expressions for the invariants for stratified anisotropic films in terms of simple moments of the layer optical distribution are presented. These general methods hold out the promise of more extended analysis of the optical response of thin films adsorbing on dielectric waveguides, while avoiding the pitfalls of specific optical models.


182

Non-Natural Polypeptoid Mimics of the Helical Lung Surfactant Proteins SP-B and SP-C

Annelise E. Barron, Cindy W. Wu, Tracy J. Sanborn, Northwestern University, Dept. of Chemical Engineering, 2145 Sheridan Rd., Rm. E136, Evanston, IL 60208

Delivery of a functional lung surfactant (LS) replacement is necessary for rescue of premature infants in respiratory distress. While LS is composed primarily of phospholipids, small amounts of the surface-active proteins SP-B and SP-C are also required for biophysical functioning. At present, these helical proteins are extracted from animal lungs for use in human patients, introducing a risk of viral transmission and potential for immune response. We have synthesized, purified, and performed in vitro testing of a new class of biomimetic spreading agents for use in LS replacements, based on sequence-specific N-substituted glycine polymers (polypeptoids). Despite a close similarity to polypeptides, polypeptoids are invulnerable to protease degradation, stable in vivo, and less prone to immune system recognition. We show by circular dichroism spectroscopy that certain polypeptoid sequences adopt stable helices in aqueous and organic solution. In vitro experiments in which we have tested SP mimics using a pulsating bubble surfactometer and a Langmuir-Wilhelmy surface balance show that peptoid-based spreading agents have promising surface activity both alone and in combination with biomimetic phospholipid mixtures. Helical polypeptoids are promising as functional, biomimetic spreading agents based on that will be safe, bioavailable, and cost-effective as additives to a synthetic phospholipid-based lung surfactant replacement.


183

Monitoring DNA Complex Size, Density and Formation Kinetics with Light Scattering

Eva Lai and John H. van Zanten, Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905

Gene delivery via nonviral vectors is an attractive potential treatment for genetic diseases. However, very little is known about nonviral gene delivery vector formation kinetics. Nonviral vector formation, which simply involves DNA complexation with various condensing agents, is a self-assembly process driven primarily by electrostatic interactions. The DNA complexation kinetics influence two physical parameters that have a direct affect on gene delivery and expression efficiency: DNA complex size and density. In this study we demonstrate the utility of time resolved multiangle laser light scattering (TR-MALLS) for probing DNA complexation kinetics and determining DNA complex size and density in real time. Parameters such as condensing agent (poly-l-lysine, cationic liposomes, spermidine, etc.), charge ratio and solution ionic strength are considered. In addition, cell transfection studies are carried out with well-characterized vectors and the measured gene transfer efficiency is analyzed with regards to DNA complex size and density. The ability to accurately measure DNA complex size and density may lead to improvements in the design and control of nonviral gene delivery vectors and facilitate the determination of optimal formulations.


184

Polynucleotide-Nucleolipid Interactions Investigated by SANS

Piero Baglioni, Debora Berti, Emiliano Fratini, Department of Chemistry, University of Florence, via G. Capponi 9, I-50121 Florence, Italy, Sow-Hsin Chen, Department of Nuclear Engineering, 24-209, Massachusetts Institute of Technology, Cambridge, MA 02139 USA.

We synthesized some nucleoside-functionalized phospholipids. These compounds, can be prepared from naturally occurring precursors (lecithins and nucleosides) thanks to an enzymatic pathway. The enzymatic reaction scheme is flexible and allows the design of a class of compounds according to the desired physico-chemical properties. While the length and the degree of unsaturation of the acyl chain modulate and drive the aggregational properties, bases covalently attached to the polar head region can interact through stacking and Watson-Crick H-bonding. The cooperative effect for base-base recognition, occurring upon surfactant aggregation, is particularly interesting in view of the fact that it resembles nucleic acid behavior. We report a SANS structural characterization of single-strand polynucleotide-nucleolipid mixtures. While the behavior of surfactant-polymer mixtures is a subject of current interest in soft-matter science, our case possesses a peculiar relevance in a biomimetical respect and in view of possible applications of these nucleolipid derivatives in gene delivery. We focused our attention on the system PolyUridine/diC8P-Adenosine: the structure of surfactant-polymer interactions have been investigated as a function of lipid concentration in water to follow the structural evolution of the system from below the cac to above the cmc of the nucleolipid surfactant.


185

Surface Modification for Optical Biosensor Platforms: Beyond Thiols and Silanes

Nicholas D. Spencer, Laboratory for Surface Science and Technology, Department of Materials, Swiss Federal Institute of Technology, NO H64, ETH-Zurich, CH-8092 Zurich, Switzerland

Although thiol-gold, and silane-silica self-assembled monolayers are much-studied systems with interesting technological applications, their use in functionalizing surfaces for optical biosensors entails certain problems, such as the opacity of gold in waveguide applications and the tendency of silane chemistry to lead to multilayer formation. We have focussed on two alternative classes of surface modification (alkyl phosphates and polylysine-based graft copolymers) that lend themselves to the functionalization and patterning of a range of oxide surfaces, allowing both highly targeted adsorption of biomolecules as well as the suppression of non-specific adsorption. The surface chemistry of adsorption and the stability of the systems, as well as their use as biosensor platforms will be discussed.


186

Design of Novel Peptide-Based Chemosensors for Copper Ions

Yujun Zheng, Yuqiu Ma, Qun Huo, Xihui Cao, Sarita V.Mello, Roger M. Leblanc, Department of Chemistry, Fotios M. Andreopoulos, Department of Surgery and School of Medicine, Department of Biomedical Engineering, University of Miami, Coral Gables, FL 33124

The development of chemical sensors for metal ions of biological significance has been extensively investigated in recent years. It is known that the complex formed between tripeptide glycyl-histidyl-lysine (GHK) and copper ion has special functions of promoting wound healing and tissue regeneration. In this work we report the interaction of GHK and copper ion (II) by using Langmuir-Blodgett (LB) technique. In order to prepare amphiphilic compounds, we synthesized, by using solid phase synthesis method, GHK- lipid molecules with hydrophobic chains, suitable to form monolayers at air-water interface. Langmuir and Langmuir-Blodgett films are constructed as the model of the chemosensor. The detecting ability and selectivity of the chemosensor for copper ions will be evaluated with the analytical techniques such as UV/Vis absorption and electrochemistry. Besides the possible technological application, this work also aims to study the mechanism of GHK-Cu in wound healing.


187

Separation of Proteins Using Magnetic Fluids

Seyda Bucak, Sylvain Vauthey, Kaitie Roach, Paul E. Laibinis and T. Alan Hatton, Department of Chemical Engineering, Massachusettes Institute of Technology, 77 Massachusettes Avenue, 66-321, Cambridge, MA 02139

Magnetic fluids consisting of stable colloidal suspensions of magnetic nanoparticles dispersed in water have been used effectively as protein separation aids for a range of protein mixtures. The single domain magnetic nanoparticles (~10nm in size) were coated with an outer layer of phospholipids which provided not only electrostatic stabilization of the suspension, but also an affinity handle for selective protein interactions. The protein-laden magnetic nanoparticles were recovered from the suspension by magnetic filtration, and the non-adhering protein collected in the effluent stream. Complete recovery of the magnetic particles was achieved on removal of the magnetic field from the filter column. The principles of this new separation approach will be discussed and elucidated.


188

Interfacial and DNA-Binding Properties of Peptide-Nucleic-Acid Amphiphiles

James Vernille, David Tucker, Cindy Echevarria, and James Schneider, Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh PA 15213-3890

Peptide nucleic acid (PNA), a synthetic polyamide with pendant nucleobases, has remarkable DNA binding properties. Because the PNA backbone is uncharged, PNA-DNA duplexes are often more stable than their DNA-DNA counterparts. Through "strand invasion," PNA binding can identify short sequences of double-stranded DNA through the formation of a local recognition triplex, without fully separating the two strands. By conjugating PNA to amphiphilic molecules, we can functionalize surfactant microstructures with these DNA-binding molecules for a number of applications in biotechnology, including bioseparations, gene therapy, and controlled aggregation of liposomes for drug delivery. These "PNA amphiphiles" are created by a flexible synthetic technique based on solid-phase peptide synthesis methods, affording control of tail architecture and PNA sequence, and enabling molecular-level design of their DNA binding properties and surfactancy. Since hydrophobic and steric interactions between nucleobases and nonpolar tails can modulate the stability and specificity of PNA-DNA assemblies, we must establish the impact of the nonpolar tail on the binding and interfacial properties of these molecules before their potential can be realized. We present self-assembly and monolayer measurements on PNA amphiphiles of various architectures (and in the presence of a cosurfactant), along with DNA binding data obtained by circular dichroism (CD) spectroscopy.


189

Complex Environmental Separation Through Use of Magnetically Active Polymeric Particles (MAPPs)

David Leun, Arup K. SenGupta, Department of Civil and Environmental Engineering, Lehigh University, Bethlehem, PA 18015, USA

Polymeric sorbent particles can be made magnetically active by dispersing submicron magnetite crystals under controlled redox conditions within the pores of the polymer network. Magnetic activity, thus imparted, is irreversible and can be greatly enhanced by partial functionalization of the polymeric sorbents with sulfonic acid groups. Environmental separation often involves removals of target toxic compounds from the background of complex matrices such as sludge, sediment, viscous or readioactive liquid, biomass slurry, and others. Existing magnetic technologies aided by magnetically active polymeric particles (MAPPs) may greatly enhance the efficiency of these separation processes. Examples to this effect will be presented.


190

The Effect of Dodecyldiethylenetriamine and pH on Selectivity during Ion Flotation of Cu2+, Ni2+ and Co2+

Zhendong Liu and Fiona M. Doyle, University of California at Berkeley, Department of Materials Science and Engineering, 551 Evans Hall # 1760, Berkeley, CA 94720-1760

In ion flotation a non-surface active ion, co-sorbed with an ionic surfactant at a solution-vapor interface, is separated from the solution by gas bubbling and foaming. The process shows promise for removing toxic heavy metal ions from dilute aqueous solutions, provided that the selectivity for individual ions can be manipulated. When sodium dodecylsulfate is used to float simple aquo cations, there is little selectivity between Cu2+, Ni2+ and Co2+. However, it was possible to achieve considerable selectivity for Ni2+ or Co2+ over Cu2+ by using a surface active, ionic chelating agent, dodecyldiethylenetriamine (Ddien) at pH near 9. At pH near 6.5, none of these metals was removed appreciably by Ddien. Speciation diagrams for the H2O-Dien-Ni2+-Cu2+ and H2O-Dien-Co2+-Cu2+ systems showed that at pH near 9, Ni(Dien)22+ and Co(Dien)22+ predominated, whereas little Cu(Dien)22+ was present. At pH near 6.5, all metal ions formed the MDien2+ species. Assuming that the stabilities of Ddien complexes are similar to those of Dien complexes, this suggests that the selectivity for Ni2+ and Co2+ over Cu2+ is due to the M(Ddien)2+ species being much more surface active than the MDdien2+species. Surface tension measurements at various pH values corroborated this hypothesis.


191

The Effect of Triethylenetetraamine on the Ion Flotation Kinetics of Cu2+ and Ni2+ Using Dodecylsulfate

Zhendong Liu and Fiona M. Doyle, University of California at Berkeley, Department of Materials Science and Engineering, 551 Evans Hall # 1760, Berkeley, CA 94720-1760

Ion flotation is a separation process involving the adsorption of surfactant and counter ions at a vapor/solution interface. It shows promise for removing toxic heavy metal ions from dilute aqueous solutions. The kinetics of removal of the Cu2+ aquo ion with alkylsulfates have been modeled successfully using adsorption densities calculated from surface tension data. The removal rates of Cu2+ and Ni2+ with sodium dodecylsulfate (SDS) were found to be much faster in the presence of the chelating agent, triethylenetetraamine (Trien) than for the simple aquo ions. Surface tension data show that these faster rates correlate well with higher adsorption densities. This enhanced surface activity could be due to the fact that the large radius of the metal-trien complex lowers the dehydration component of the Gibbs free energy for the interaction of metal ions with dodecylsulfate ions adsorbed at the solution-vapor interface. The surface activity might also reflect the enhanced hydrophobicity of the metal-Trien complexes.


192

Interfacial Processes in Biological Transformations of Manganese Oxides

A. Belz, Dept. of Geological and Planetary Sciences, C. Ahn, Dept. of Materials Sciences, California Institute of Technology, Pasadena, CA 91125, M. Andrews, and K. Nealson, Jet Propulsion Laboratory, Mail Code 183-301, 4800 Oak Grove Dr., Pasadena, CA 91109-8099

Bacteria strongly influence biogeochemical cycling of manganese directly and through secondary processes. Both reduction and precipitation of manganese oxides are two of the most important biologically mediated processes in which manganese is not assimilated internally in the cellular structure. With a goal of understanding how the aqueous environment affects these processes, we have determined the effects of elevated phosphate and sulfate on the reduction of manganese (IV) oxides by an anerobic manganese-reducing bacterium. We have also studied the phase distribution of manganese oxides precipitated by a manganese oxidizer. Analytical chemistry techniques were used to perform time-resolved studies, while electron energy-loss spectroscopy was used in conjunction with transmission electron microscopy to identify the oxidation state of the poorly crystallized minerals. These investigations will elucidate how aquatic chemistry affects biologic managese cycling in natural systems.


193

Adsorption of Aqueous Heavy Metals onto Biological Substrates

Russell J. Crawford, Elise Kavanagh, Tamna Woolcock and Greg T. Lonergan, Centre for Applied Colloid and BioColloid Science, Swinburne University of Technology, P.O.Box 218, Hawthorn, 3122, Australia

Adsorption of aqueous heavy metals onto colloidal substrates has been shown to reduce the pH at which these metals can be removed from solution. Recently, biological substrates such as fungi and bacteria have been investigated as potential adsorbents, however rigorous studies have not been performed from a colloid science perspective. Biological substrates have the advantage that they are renewable, and are available at low cost as by-products from the fermentation industry. In this study, adsorption of the metals Cr(III), Ni(II) and Zn(II) onto biological substrates of fungal and bacterial origin were performed. The results have been compared to those obtained from adsorption studies using a range of substrates (hydrous oxides, coal) under similar conditions. It was found that Ni(II) and Zn(II) were efficiently adsorbed onto the biological substrates, lowering the removal in some cases by up two pH units compared to that observed for direct precipitation. It was also found that the presence of the biological substrates had little or no effect on the removal characteristics for Cr(III). In the case of the adsorption onto fungal substrates it was found that adsorption increased as the level of fungal cell wall chitin increased, highlighting its contribution to the overall adsorption process.


 

194

Sorption and Biodegradation of Chloroaromatic Pollutants – Practical and Colloid-Chemical Aspects

Barbara Witthuhn, Erwin Klumpp, Peter Klauth, Agnes Csiszar, Attila Bota+, Hans-Dieter Narres, Joost Groeneweg, Research Center Juelich, D-52425 Juelich, Germany, +Technical University Budapest, H-1111 Budapest, Hungary

The bioavailability of pollutants in an aquifer depends on their physical and chemical interactions with colloidal soil or sediment particles. Up to now influence of sorption on biodegradation and impact of both processes on each other are not fully understood. Thus gaining fundamental knowledge on these interactions is an important task, e.g. for predicting successful application of soil bioremediation and a better understanding of microbial ecology. The experiments were carried out in a model system with organo-clays as adsorbing matrix, 2,4-dichlorophenol as contaminant and Ralstonia eutropha as microorganism capable of mineralizing 2,4-dichlorophenol. Ralstonia eutropha was characterized with regard to growth and yield rates with 2,4-dichlorophenol as substrate. For direct counting of bacteria in the presence of clay particles an improved method based on fluorescence microscopy technique was developed. Furthermore sorption behaviour of 2,4-dichlorophenol at different tetra-alkyl-ammonium-exchanged montmorillonites was compared. Increased surface hydrophobicity lead to increased affinity for 2,4-dichlorophenol adsorption. By means of X-ray diffraction measurements chlorophenol intercalation in the interlayer of the clay mineral could be established. Kinetic investigations demonstrated that the desorption is fast compared to biodegradation. It could be proved that in the presence of organo-clay biodegradation of dichlorophenol at originally toxic concentrations takes place.


195

The Colloid Chemistry of Oligoaniline Threaded Rotaxanes

J. Adin Mann, Jr.1,Stuart J. Rowan2, Massood Tabib-Azar3, Jerome B. Lando2, Departments of Chemical Engineering1, Macromolecular Science2, Electrical Engineering3, Case Western Reserve University, Cleveland OH 44106-7217

The ultimate goal is the assembly of conducting oligomer based logic arrays of nanometer scale that can be trained to function as computers. The immediate goals include the synthesis of a series of rotaxane systems formed by the threading of certain conducting oligomers through the ring and stoppered by porphyrins. We designate these structures as Conducting Oligomer Threaded Rotaxanes or COTR. The COTR are expected to act as nonlinear elements, enabling the creation of a new "knot" gate-based logic. Prototype systems have been synthesized and characterized at CWRU. This paper will consider the colloid stability and dynamics of self assembly and Langmuir Blodgett assembly of these nanoparticles into logic arrays. Preliminary experiments will be reported and the theory of their operation will be discussed.


196

Holographic Optical Tweezer Arrays: A New Class of Techniques for Manipulating and Measuring Colloidal Dynamics

David G. Grier, Dept. of Physics, James Franck Institute and Institute for Biophysical Dynamics, The University of Chicago, 5640 S. Ellis Ave., Chicago, IL 60637

Optical traps formed by tightly focused laser beams have become increasingly important tools for studying the interactions and dynamics of colloidal particles. Holographic techniques significantly extend the capabilities of laser tweezing, creating extensive trapping patterns for manipulating large numbers of particles and volumes of soft matter. We describe practical methods for creating arbitrary configurations of independently steerable optical tweezers using computer-generated diffractive optical elements. Early applications of dynamic holographic optical tweezer arrays to problems in colloidal interactions and many-body dynamics will be introduced.


197

Selective Heteroaggregation of Sterically Stabilized, Oppositely Charged Colloids

Anthony Y. Kim and John C. Berg, University of Washington, Chemical Engineering, Box 351750, Seattle, WA 98195

The preparation of composite materials often requires uniform mixing of at least two distinct colloidal species. Selective heteroaggregation of oppositely charged colloids, where interactions between unlike species are favored over interactions between like species, is an attractive method for optimizing component distribution. Highly branched (fractal) flocs may form under such conditions, and the aggregation kinetics may influence the degree of branching (fractal dimension). In this work, the kinetics of aggregation for oppositely charged polystyrene spheres are quantified by dynamic light scattering. Resulting floc structures are evaluated by static light scattering. Electrostatic interactions are systematically varied through changes in surface charge densities and pH. A relationship between heteroaggregation kinetics and heterofloc structure is identified. The effects of a steric stabilizer (tri-block copolymer) are investigated with the aim of improving control over heterofloc structure.


198

Heteroaggregation of Microgels

Alexander Routh and Brian Vincent, School of Chemistry, Cantocks Close, Bristol BS8 3HZ England

Poly vinyl pyridine (pvp) microgels are positively charged and swell in acidic conditions. Poly n-iso propyl acrylamide (pnipam) microgels are negatively charged and swell at temperatures below 32 oC. Heteroaggregation of these microgels is followed with a number of techniques. We use small angle light scattering to obtain fractal dimensions and PCS or UV to follow kinetics. Swelling one of the particles eliminates the Van der Waals attraction and results in a system only dependent on electrostatics. The kinetics of aggregation is found to slow considerably at salt concentrations of 3-4 mM NaCl. This is rationalized by considering free polymer chains at the edge of particles, due to the cross-linking agent. These chains compress during aggregation and introduce a soft particle potential. When this is on a length scale comparable to the Debye length, a slowdown in the dynamics is observed. There is no effect of salt concentration on the fractal dimension. Homoaggregation of pnipam is likewise controlled by temperature, collapsing the particles and introducing a Van der Waals attraction. This leads to a large decrease in the fractal dimension as the particle softness decreases. We compare our results for these soft particles with hard polystyrene particles.


199

Electrolyte-Induced Aggregation of Acrylic Latex

Leo H. Hanus, Clarient Corporation, 4331 Chesapeake Dr. Charlotte, NC 28216, Robert U. Hartzler and Norman J. Wagner, Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, DE 19716

We measure the aggregation kinetics of model, aqueous polymer latices of varying particle size as a function of added NaCl using dynamic light scattering to evaluate the available theoretical models for predicting the aggregation rate (stability ratio) and critical coagulation concentration (CCC). Our focus is on dilute colloidal suspensions of fixed surface chemistry, but varying particle size. A master curve for the growth of the aggregate size is observed, with an intermediate regime that follows predictions for diffusion limited cluster-cluster aggregation. Theoretical predictions based on DLVO theory are found to be in quantitative agreement for all but the largest particle size, when the particle surface potentials are determined by matching the experimentally determined CCCs. Thus, we conclude that for sufficiently smooth, nearly monodisperse particles such as those investigated here, DLVO theory can provide accurate predictions of colloidal stability for the range of parameters explored here down to truly atomic dimensions. The particle potential determined from phase analysis light scattering measurements of the zeta potential over predicted colloidal stability, but can be brought into agreement by assigning a Stern layer thickness equal to the hydrodynamic size of the counterion.


200

Thermodynamics and Kinetics of Adsorption of Surfactant Mixtures

R. Miller1, V.B. Fainerman2 and E.V. Aksenenko3, 1MPI of Colloids and Interfaces, D-14424 Potsdam, Germany; 2 IMPC, University of Donetsk, Ukraine, 3Ukraine Academy of Science, Institute of Colloid and Water Chemistry, Kiev, Ukraine

A rigorous theoretical model is presented which describes the equilibrium behaviour of a surfactant mixtures at liquid/fluid interfaces. The theory describes mixtures of surfactants with different molar areas and accounts for the non-ideality of the surface layer. The theoretical results are in good agreement with experimental data and supports the idea of additivity of the interaction parameters in the surface layer. The rigorous equation of state can be transformed into simple relationships for the description of the adsorption behaviour of mixed surfactant systems. The model requires surface tensions of the single surfactant systems or the adsorption isotherms to construct the isotherm of the mixture while no extra interaction parameters between the different compounds is assumed. The model is tested with a number of literature data, such as mixed sodium alkyl sulphates, mixtures of betaine homologues BHB12 with BHB16, non-ionic surfactant mixtures, and anionic-nonionic mixtures (1-butanol with BHB12, and oxethylated decanol (C10EO5) with sodium dodecyl sulphate). The agreement between experimental data and the theoretical calculations is excellent. This approach can be especially important for practical applications of surfactant mixtures for which experimental data are scarce.


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