Oral Presentation Abstracts

Talks 201 through 250

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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.


201

Interface Dynamics in Bicontinuous Microemulsions Investigated by Neutron Spin Echo Spectroscopy

M. Monkenbusch , M. Mihailescu, J. Allgaier, D. Richter, IFF, FZ-Juelich, D-52425 Juelich, Germany, B. Jakobs, T. Sottmann, Inst. Phys. Chem., Univ. Koeln, D-50939 Koeln, Germany, B. Farago, Inst. Laue Langevin, 38042 Grenoble Cedex 9, France

Water-decane microemulsions with the non-ionic surfactant C10E4 in the bicontinuous phase in the vicinity of the "fish-tail point" or optimal point of the phase diagram have been investigated by neutron scattering techniques. Neutron spin-echo spectroscopy allows for the observation of fluctuations on the scale of a few up to about 100 Angstroems. The effect of added PEP-PEO blockcopolymers that are long chain analoga to C10E4, have been investigated. These polymers strongly enhance the emulsification power of the surfactant, i.e. they shift the "fish-tail point" drastically in the direction of lower surfactant concentration. To separate concentration effects, i.e effects of structural dimensions, from the polymer effect on interface properties, samples with different surfactant concentrations have been investigated. The results at high scattering wavenumbers, Q, qualitativly follow the model of Zilman and Granek that predicts a streched exponential behaviour with streching exponent 2/3 and a Q^3 proportionality of the rate. The validity of this model and the role of the bending elasticity k are discussed.


202

Adsorption of Diblock Copolymers at the Solid/Liquid Interface

Grant Webber, Erica Wanless, Simon Biggs, Chemistry Dept., The University of Newcastle, University Drive, Callaghan, NSW 2308 Australia

The adsorption of micelles of the amphiphilic diblock copolymer poly(2-(dimethyl amino)ethyl methacrylate-b-methyl methacrylate) [DMA-MMA] at the mica-aqueous interface has been studied using in-situ atomic force microscopy (AFM). The copolymer micelles will have a positive corona charge whilst the surface is negatively charged. At low solution pH, the images indicate an adsorbed layer consisting of discrete aggregates with large inter-aggregate spacings.. As the pH of the solution increases, the charge of the coronal chains decreases and the charge of the surface increases. Initially, this results simply in an increase in the number of adsorbed micelles. At a pH where the charge on the polymer decreases close to zero, the adsorbed layer is seen to consist of close packed spheres. The observed AFM images have been correlated with solution data for micelle size and charge to develop a model for the adsorption process.


203

X-ray Reflectivity Study of the Nanostrucuture of Amphiphilic Diblock Copolymer Monolayer on Water Surface

Hideki Matsuoka, Emiko Mouri, Mihoko Matsutani, Kozo Matsumoto, Department of Polymer Chemistry, Kyoto University, Kyoto 606-8501 Japan

Diblock copolymers, which has a Si segment as a backbone of hydrophobic chain and a water soluble segment as a hydrophilic chain, were sythesized by living anionic polymerization. The nanostructure of monolayer of these polymers on water was investigated by X-ray reflectivity (XR) technique. Si containing chain is so flexible that it forms very smooth and uniform hydrophobic monolayer on water, which results in observatioin of many Kiessig fringes in XR profiles. Hence the nanostructure of the polymer "brush" under the water surface was well studied. The thickness of hydrophilic layer of ionic segment was surprizingly thinner comparing to that of non-ionic segment. The effects on nanostructure of the degree of polymerizaion and the surface pressure were duly investigated.


204

Tuning the Solubility of Amphiphilic Diblock Copolymers : Interfacial Behavior

M. Sun, K. Krishana, O. Anthony, R. Farhoosh and M. Joanicot, Rhodia Inc., 259 Prospect Plains Road, Cranbury NJ 08512-7500

The self assembly behavior of polystyrene-polyacrylic acid (hydrophobic-hydrophilic) diblock copolymers in solution has been tuned by varying the length and composition of the two blocks thanks to a proprietary polymerization technique at Rhodia, Inc. We have investigated the ability of these diblocks to make hydrophobic surfaces hydrophilic. Several hydrophobic surfaces were treated with different diblock solutions and surface modification was quantified with advancing and receding contact angle measurements. Only specific diblock compositions were successful in making the surfaces hydrophilic. A direct correlation was observed with the self assembly behavior in solution (investigated using light scattering). Further insight into the mechanism of adsorption and contact angle hysteresis was obtained by imaging the surfaces with AFM. To quantify the relative amounts of diblock polymers at the surface we used Reflectometry. We shall present these results along with the measurements of kinetics of adsorption and the influence of ionic strength.


205

AFM Study of Surfactant Adsorption on Hydrophobic Surfaces

Aysen Tulpar and William Ducker, Chemistry Department, Virginia Tech, Blacksburg, VA 24061

Surfactants form aggregates when they adsorb on surfaces above a critical surface aggregation concentration (cac) in aqueous solutions. The shape of the aggregates depends on the type of the surfactant and the nature of the surface. The aim of this work is to study the adsorption of surfactants on hydrophobic surfaces as a function of surfactant concentration and headgroup in the range of cac and cmc. The hydrophobic surfaces are self-assembled monolayers of alkyl thiols on gold. By using thiols of different alkyl chain lengths (8 and 16), the effect of the surface crystallinity is also studied. Atomic Force Microscopy is used to monitor the shapes of the aggregates of Sodium Dodecyl Sulfate (SDS), Hexadecyltrimethyl-ammoniumbromide (C16TAB), Dodecyldimethylammoniumpropanesulfonate (DDAPS) and Dodecyloctaethoxyethyl Alcohol (C12E8).


 

206

Anomalous Slow Adsorption of Cationic Surfactants At the Silica-Aqueous Interface

Rob Atkin, Erica Wanless, Simon Biggs, Chemistry Dept., The University of Newcastle, University Drive, Callaghan, NSW 2308 Australia, Vince Craig, RSPhysSE, Australian National University, Canberra, ACT 2001

The adsorption of cetylpyridinium bromide (CPBr) at the silica aqueous interface has been studied using optical reflectometry (OR) and atomic force microscopy (AFM). The effects of pH, electrolyte and surface preparation on the surface excess and adsorption kinetics are reported. AFM imaging above the critical surface aggregation concentration (CSAC) indicates the presence of spherical aggregates in the absence of electrolyte and elongated cylindrical structures when electrolyte is added. At concentrations close to the CSAC in the absence of added electrolyte, adsorption is seen to take up to 24h to reach equilibrium. At all other concentrations, or in the presence of electrolyte, the adsorption process is complete within a few minutes. This slow adsorption region (SAR) appears to be related to slow structural rearrangements at the interface. A possible model to account for the slow adsorption region in the adsorption isotherm will be presented and discussed.


207

Simulations of Liquid Crystal Hydrodynamics

Colin Denniston, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218; E. Orlandini, INFM-Dipartimento di Fisica, Universita di Padova-I-35131 Padova, Italy; J.M. Yeomans, Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP, U.K.

We present recent work using a lattice Boltzmann algorithm fro the hydrodynamics of liquid crystals in the isotropic and nematic phases. The coupling between the tensor order parameter and the flow is treated consistently allowing a wide range of non-Newtonian flow behavior. We will present results on the kinetics of phase ordering and on the occurrence of shear banding


208

Dynamic Self-Consistent Field Studies of Rheology and Microstructure in Sheared Polymer Fluids

Maja Mihailovic and Yitzhak Shnidman, Polytechnic University, 6 MetroTech Center, Brooklyn, NY 11201, USA

A novel dynamic self-consistent field (DSCF) theory couples, within a dynamic lattice model of polymer fluids, a convective-diffusive transport of local chain segments and momenta with a matrix generator method for chain conformation statistics. For a derivation of DSCF time evolution equations see the companion presentation by Shnidman and Mihailovic at this meeting. Here we use a simplified version of DSCF that assumes translation invariance symmetry within parallel layers. We apply it to melts and solutions of homo- and block copolymers (below entanglement molecular weight) that are sheared between parallel plates. We present here the results of systematic studies of the dependence of the rheology, chain conformation statistics, and the morphology of such systems on time, shear rate, the distance between the plates, molecular weight, temperature, and segmental interaction parameters. These relations, arising from solutions of DSCF equations, are presented in a dimensionless form allowing comparison with experimental results. This is achieved by rescaling all simulation length, mass, and time units pertaining to series of simulations or experiments by certain characteristic values.


 

209

Screening Silica Zeolites for Membrane-based Gas Separations Using Atomistic Simulations

David S. Sholl, Anastasios I. Skoulidas, Amy K. Lin, E. Demet Akten, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

Membranes made from zeolites offer a number of advantages over traditional membranes for gas separation processes. Because of the ordered, sub-nanometer pores present in zeolites, characteristics of molecular adsorption and diffusion in these materials are strongly influenced by atomic-scale details of the zeolite pore structure. We have developed methods for connecting atomistic simulations of molecules adsorbed in zeolite pores with the macroscopic performance of zeolitic membranes. We will show how these methods are being used to screen a family of zeolites with identical chemical compositions but varying pore structures for use as gas separation membranes. Examples of non-polar adsorbates such as noble gases and quadrupolar species such as CO2 will be discussed.


210

Thermodynamics of Polydisperse Colloids

Shubho Banerjee, Bob Griffiths and Michael Widom, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213

We examine the thermodynamic limit of fluids of hard core particles that are polydisperse in size and shape. In addition, particles may interact magnetically. The free energy is a random variable because it depends on the choice of particles. We prove that a thermodynamic limit exists with probability 1, and is independent of the choice of particles. To ensure existence of a limit, we impose a "packing condition" on the particle shape and size distribution that guarantees any choice of particles fits inside a container with high probability. Our proof applies to polydisperse hard-sphere fluids, colloids and ferrofluids. The existence of a thermodynamic limit implies system shape and size independence of thermodynamic properties of a system.


211

Theories of Phase Separation in Charged Colloidal Dispersions

D. Y. C. Chan, S. N. Petris, Particulate Fluids Processing Centre, Department of Mathematics and Statistics, The University of Melbourne, Parkville 3010 VIC Australia

Gas-liquid and liquid-solid phase separations observed in charged colloidal dispersions appear to contradict the theory of electrical double layer interactions which is one of the foundation elements of the Deryaguin-Landau-Verway- Overbeek (DLVO) theory of colloid stability. The treatment of Coulombic interactions in colloidal systems developed separately from the theory of electrolytes because of the large charge asymmetry between the constituents. Recently the deployment of modern theoretical approaches based on field theoretical techniques of physics or density functional theories of solid state theory appear to have yielded new insight into the problem. We demonstrate that the long-ranged nature of the Coulombic potential require careful consideration in order to match the results of more recent approaches to what has been establised intuition in the field and to give a proper physical interpretation to the results.


212

Colloidal Crystallization Kinetics - Links between Models and Experiments

Narendra M. Dixit and Charles F. Zukoski, Department of Chemical Engineering, University of Illinois at Urbana Champaign, 600 S. Mathews Ave, 114 RAL, Urbana, IL-61820.

The ability to control the rate of colloidal crystallization has important applications in the determination of protein structures and in the manufacturing of novel materials such as photonic crystals. Optimal solution conditions are identified from predictions of crystal nucleation rates using classical nucleation theory. Direct measurements of nucleation rates are inherently difficult. Hence, indirect measures such as induction times or crystal growth velocities are used instead. Links between these indirect and direct measures are semi-empirical and result in conclusions with large uncertainties. We have developed a new model that establishes precise links between measurable induction times and predicted nucleation rates. In this model, cluster formation and growth occurs via single particle aggregation and dissociation. With independent calculations of the rates of these competing processes, a population balance is used to determine the evolution of cluster size distributions with time. This allows the prediction of induction times as measured via light scattering experiments. Links between these induction times and predictions from classical nucleation theory will be described and compared with recent experiments on nucleation rates and induction times of hard-sphere and attractive colloidal crystals.


 

213

Condensation, Crystallization and Swelling of Randomly Charged Polymers

Motohiko Tanaka, National Institute for Fusion Science, 322-6 Oroshi-cho, Toki 509-5292, Japan; Toyoichi Tanaka, Massachusetts Institute of Technology

The behavior of randomly charged polymers (polyampholytes) agaist a wide range of the Bjerrum length lB is studied by molecular dynamics simulations with the electrostatic and Lennard-Jones forces [1]. Neutral polyampholyte collapses for lB>a (a: bond length) where large volume changes are due to multichain effects unlike single-chain cases. Charged chains reptate significantly in a formed globule. Neutral polyampholyte condenses to an imperfectly ordered glass or a BCC crystal depending on the polymer bond strength. Non-neutral polyampholyte whose charge offset exceeds N^(1/2) behaves as polyelectrolyte; it consists of nonovelapped chains for lB>a, and shrinks like noncharged polymers for lB<a (N is the number of charged monomers). Condensed counterions on polyampholyte screen the electric field, making non-neutral polyampholyte close to the neutral one. Added salt of comparable charge density as that of the polyampholyte further compactifies it. However, the addition of more salt results in weakening of the polyampholyte nature and in reentrant swelling of non-neutral polyampholyte. (Reference [1] M.Tanaka and T.Tanaka, Phys.Rev.E62, 3803 (2000)).


214

Entropically Driven Colloidal Crystallization on Patterned Surfaces

Keng-hui Lin, T. C. Lubensky, Arjun G. Yodh, Dept. of Physics and Astronomy, U. of Pennsylvania, Philadelphia, PA 19104, John C. Crocker, Applied Physics Dept, California Institute of Technology, Pasadena, CA 91125, Andrew Schofield, Dept. of Physics and Astronomy, The University of Edinburgh, Edinburgh, Scotland EH9 3JZ, David A. Weitz, Physics Department and DEAS, Harvard University, Cambridge, MA 02138

We investigate the entropic self-assembly of colloidal spheres on periodic patterned templates, induced by the depletion force of a non-adsorbing polymer. A variety of two-dimensional structures with quasi-long-range order are observed to form on templates with one- and two-dimensional periodical structure. On a template commensurate with an FCC(100) plane, the same entropic forces form an FCC crystal more than 30 layers thick without stacking defects. Together, these approaches provide a thermodynamic route for the directed self-assemnbly of mesoscopic periodic structures. We gratefully acknowledge support from the NSF (DMR-9631279, DMR-9623441), MRSEC (DMR-9632598) and NASA (NAG3-2172).


215

Field-Induced Colloidal Ordering in Confined Geometries

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

A novel approach has been developed that allows the controlled ordering of colloidal particles in confined geometries. In this, a strong dipole-dipole interaction is used to induce two-dimensional colloidal crystallization. Because particle density is relatively low and the interaction of long range, colloidal mobility is high enough to allow rapid crystallization upon application of the field. The observed phase transitions are fully reversible and can be readily controlled by adjusting either field strength or particle confinement. Small angle scattering is used to illustrate that large regions of colloidal order can be created with this approach. Because of their optical activity, such switchable colloidal crystals may find application as diffraction gratings, waveguides, or photonic bandgap materials.


216

Templated Materials Synthesis in a Rigid Surfactant Based Mesophase

Limin Liu, Sichu Li, Tulane, Blake Simmons, Vijay T. John, Selvam Thangaraj, Dept. of Chemical Eng., Gary L. McPherson, Dept. of Chemistry, Tulane University, New Orleans, LA, 70118, Arijit Bose, Vivek Agarwal, Paul Johnson, University of Rhode Island, Dept. of Chemical Eng., Kingston, R.I., 02881, Boualem Hammouda, NIST, NCNR, Gaithersburg, MD, 20883

A clear rigid bicontinuous mesophase is formed by the addition of lecithin (phosphatidylcholine) to an AOT water-in-oil microemulsion. The novel property of this gel is that it has equal volumes of an aqueous microphase and an organic microphase at the percolation threshold. The rigid gel environment indicates a medium where aqueous phase synthesis can be combined with organic phase synthesis, leading to novel nanostructured composite materials, materials with structured porosities, or materials synthesized at the rigidified oil-water interface. The synthesis of polymer-polymer and polymer-ceramic nanocomposites are reported. The correlation of materials morphology evolution with mesophase structural evolution is the underlying objective of the research and neutron scattering is used to correlate the evolution of template microstructure with the structure of materials synthesized therein.


217

Phase Behavior and Microstructure of Alkyl Polyglucoside Mixtures

Belgin Baser, Eric W. Kaler, Hakan Edlund, Beth A. Schubert, Center for Molecular Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716

Alkyl polyglucosides (CmGn) are a new class of nonionic surfactants consisting of a hydrophobic alkyl chain of "m" number of carbons and a hydrophilic headgroup of "n" number of glucose units. Alkyl polyglucosides are biodegradable, nontoxic, and synthesized from renewable resources. Nonetheless, their properties alone in water are not well suited to many applications, but both their phase behavior and microstructure can be tuned with addition of small amounts of ionic surfactant. We report here the phase behavior and microstructure of binary mixtures of C10G1 or C12G1 and alkyl sulfates in aqueous solution. Remarkably small amounts of ionic surfactant (e.g., 1/1000 mole ratio of ionic to glucoside) can shrink the CmGn – water miscibility gap dramatically, and higher levels can make the gap disappear altogether. In some mixtures there is a complex succession of phases within a small window of composition at low surfactant concentration. There is also a region of high viscosity in the clear, isotropic single-phase region outside the miscibility gap. The microstructure determined by small angle neutron scattering (SANS) ranges from lamellar to L3 to micellar as a function of composition.


218

High-pressure Induced Formation of Novel Surfactant Structures

Dobrin P. Bossev and Michael E. Paulaitis, Johns Hopkins University, Department of Chemical Engineering, 3400 N. Charles Street, Baltimore, MD 21218-2689

We have studied the effect of pressure on surfactant microstructures in solution of nonionic surfactant in water. Small-angle neutron scattering results indicate no significant change in the length or diameter of the threadlike micelles for pressures up to 2400 bar and temperature of 20 ºC. However, further increase in pressure to 2550 bar leads to the appearance of a peak in the scattering pattern which is indicative of highly ordered system similar to La lamellar or H1 hexagonal phases. Visual observations also confirm a reversible transition from completely transparent solution at low pressures to an opaque solution at higher pressures. The pressure where this transition occurs is studied against temperature. This p-T curve resembles the p-T diagrams of freezing of linear alkanes which implies that the hydrophobic interior of the surfactant structures is not in a liquid but in a frozen state at high pressures. The possible mechanism of this pressure induced phase transition is discussed.


219

Self-Assembly, Structure and Solvation in Wet and Dry Octanol

Bin Chen. Department of Chemistry, University of Minnesota, Minneapolis, MN 55455-0431, USA, and Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104-6323, USA, and J. Ilja Siepmann, Departments of Chemistry and of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455-0431, USA,

Octanol-water partition coefficients are extraordinarily successful for correlating and predicting numerous processes of pharmacological, environmental, or technological importance. The reason why the octanol phase is apparently able to mimic the complexities of many different environments ranging from biomembranes to soil, however, is less well understood. Configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to investigate the self-assembly of the amphiphilic octanol molecules. It is demonstrated that water saturation substantially alters the structure of the octanol phase, but a diverse spectrum of hydrogen-bonded aggregates is observed for neat and water -saturated octanol. These simulation results allow us to reconcile the conflicting views of the octanol structure inferred from thermodynamic arguments, spectroscopic measurements, and diffraction experiments. Partition constants were calculated to establish the influence of water saturation on the solubility power of the octanol phase. Analysis of the local environment around a given solute molecule allows us to rationalize the changes in partition coefficients caused by water saturation.


220

Molecular Weight Effects on the Phase Behavior of Silicone Oil Microemulsions

James A. Silas, Eric W. Kaler, University of Delaware, Department of Chemical Engineering, Newark DE 19716, Randal M. Hill, Dow Corning, 220 West Salzburg, Midland, MI 48686

For alkane oils, microemulsions with increasing molecular weight show an increase in the phase inversion temperature and an increase in the amount of surfactant needed to form the microemulsion. However, alkane chain lengths cannot be increased beyond the point of about 14 to 16 carbons without forming crystals that obscure further changes in phase behavior due to increasing molecular weight. Silicone oils, on the other hand, show no such limitation. High molecular weight silicone oils remain fluid and so allow a thorough study of the effects of molecular weight on microemulsion formation. Phase diagrams showing the effect of molecular weight on the formation of silicone oil microemulsions will be presented along with attempts to form the most efficient microemulsions possible using mixtures of cationic and nonionic surfactants.


221

Electro-Viscous Effects on Liquid Flow in Microchannels

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

The electrical double layer near a solid-liquid interface results in the electro-viscous effect on pressure-driven liquid flow through microchannels. The objective of this paper is to examine the magnitude of the additional flow resistance caused by the electrokinetic effect in microchannels. Pure water, 10-4 and 10-2M KCl solutions, 10-4M AlCl3 solution, and 10-4M LiCl solution were used as the testing liquids. Carefully designed flow measurements were conducted in three silicon microchannels with a height of 14.1, 28.2 and 40.5 mm, respectively. The measured for pure water, 10-4M KCl solution and 10-4M LiCl solution was found to be significantly higher than the prediction of the conventional laminar flow theory at the same Reynolds number. Such a high flow resistance and the resulting high apparent viscosity strongly depend on the channel’s height, the ionic valence, and the concentration of the liquids. The zeta potentials for the liquid-solid systems were calculated by using the measured streaming potential data. The experimentally determined ~relationships were compared with the predictions of a theoretical model, and a good agreement was found for pure water, 10-4M KCl solution, and 10-4M AlCl3 solution systems. The present electrokinetic flow model cannot interpret the flow characteristics of the LiCl solution.


222

Thermocapillary Flow on Patterned Surfaces: A Design Concept for Microfluidics

A. A. Darhuber, S. M. Troian, J. M. Davis, Dept. of Chemical Eng., W. W. Reisner, Dept. of Physics, and S. Wagner, Dept. of Electrical Eng., Princeton University, Princeton, NJ 08544

The integration of multiple functionality for chemical analysis and synthesis into a handheld device requires efficient methods for transporting ultrasmall volumes of liquid through networked arrays. The majority of devices combine micromechanical and electric field driven methods for controlling flow in closed channels. We recently introduced a non-electronic means of flow control that lends itself to the construction of a chemical reactor on the surface of an integrated circuit. The design concept relies on thermocapillary transport of liquid streams on a surface of mixed wettability produced by micropatterning a self-assembled monolayer. The chemical patterning confines the flowing liquid to selected pathways bearing a streamwise thermal gradient. A distinct advantage of this approach is that micropatterned thermal fields can be used both to route liquid along selected pathways and to induce or quench chemical reactions at electronically addressable sites. We show that an applied thermal gradient can be used to stream liquid along hydrophilic microstripes patterned on a hydrophobic glass or silicon support. Studies of the liquid speed and interface shape as a function of the stripe width, applied thermal gradient and volume of liquid deposited compare favorably with theoretical predictions and numerical simulations of thermocapillary flow on narrow hydrophilic stripes.


223

Charging in Low Dielectric Liquids

Ralph E. Kornbrekke, The Lubrizol Corporation, 29400 Lakeland Blvd., Wickliffe, OH 44092

Particles suspended in low dielectric liquids can become charged, and this will affect the colloidal properties of the suspension. The mechanism of charging in low dielectric liquids is different from that in high dielectric liquids like water. The differences in dielectric properties also affect the manner in which charging influences colloidal properties. The charge on these particles can stabilize the suspensions, or it can cause them to form ordered structures, which have a yield stress. I will discuss the nature of charging in these types of systems, and I will show how the physical properties of the liquid affect charging. The mechanism for charging will also be discussed, and the influence on colloidal properties will be assessed.


224

Simulations of Moving Droplets

Dhaval Doshi, Anthony Malanoski , Chemical and Nuclear Engineering Department, University of New Mexico, Albuquerque, New Mexico, U.S.A and Frank van Swol, Sandia National Laboratories and Chemical and Nuclear Engineering Department University of New Mexico

Albuquerque, New Mexico, U.S.A

We have recently embarked on the modeling of drops on hetereogenous surfaces with a powerful lattice-gas modeling approach that combines both equilibrium and non-equilibrium techniques (i.e. Monte Carlo and density functional theory) that can include chemical reactions and be applied in both open and closed ensembles. The program is aimed at understanding the problem of steering fluids on a very small length scale, ranging from nanons to microns, which is an important aspect of small devices, including MEMS, NEMS and (bio)chem-labs on chips. On larger length scales fluids are typically moved by pumping or gravitational action. On small length scales, fluids can be transported by exploiting another driving force: spatial surface free energy gradients. A fluid droplet will experience such a gradient when the surface with which it is in contact exhibits a spatial variation in wettability ("hydrophilicity"). We present simulation results on the unbalanced Young equation using droplets equilibrated on gradient surfaces in the presence of gravity and we will discuss the effects of contact angle hysteresis. We will report on calculations involving moving droplets on gradient surfaces and, in addition, we will investigate the effects of having present a component that reacts with the surface to vary the wettability.


225

High Frequency Electrokinetics in Low Dielectric Fluids

Lee R. White, Matthew Preston, Chemical Engineering, Carnegie Mellon University, Pittsburgh PA15213-3890, Ralph Kornbrekke, Lubrizol Corporation, 29400 Lakeland Boulevard, Wickliffe OH 44092-2298

Low dielectric fluids provide media for particle electrophoresis that is characterised by long Debye screening lengths relative to particle size. The application of electroaccoustic techniques (which have the potentiality to allow simultaneous measurement of particle size and charge) to these low ka systems is limited due to the dependence of those techniques on the conversion of magnitude and phase of the observed mobility to charge and particle size using theoretical formulae derived for the situation of large ka.We develop approximate analytic results for low dielectric media and demonstrate the effectiveness of those formulae in extracting simultaneous size and charge information.


226

WITHDRAWN


227

Electrokinetic Potential of Gas Bubbles in Aqueous Solutions

Chun Yang, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Tadeusz Dabros, CANMET, Western Research Centre, P.O. Bog 1280, Devon, AB T0C 1E0, Canada, Dongqing Li, Jacob H. Masliyah, Dept. of Chemical and Materials Engineering Dept. of Mechanical Engineering, University of Alberta, Edmonton, AB T6G 2G8, Canada, Jan Czarnecki, Syncrude Canada Ltd., Edmonton Research Centre, Edmonton, AB T6N 1H4, Canada

The electrophoresis method is used to measure the electrokinetic potential of fine bubbles dispersed in different aqueous solutions. An improved apparatus of microelectrophoretic type is developed for the bubble electrokinetic potential measurements. Electrodes for generating either oxygen or hydrogen gas bubbles are designed in such a way that micron-sized bubbles can be produced over the entire cross-section of the electrophoresis cell. Consequently, a bubble can be easily chosen at the so-called stationary level, where the bubble electrophoresis measurements are conducted. Furthermore, a motorized vertical translation stage controlled by a computer is implemented. When bubbles rise due to gravity, the electrophoresis cell mounted on the translation stage is made to move downward so that the bubbles can be kept in the view of the microscope. As a result, the movement of bubbles with diameters up to 80 can be readily followed, and bubble trajectory can be traced for 4 – 8 seconds. The method and apparatus are used to study effects of water chemistry including solution concentration, pH, and type of metal ions on the bubble electrokinetic potential. The measured results for the electrokinetic bubble potential are consistent with data available in the literature.


228

Local and Non-Local Microrheology of Complex Fluids

L.A. Hough and H.D. Ou-Yang, Lehigh University, Dept. of Physics, 16 Memorial Dr. East, Bethlehem PA Lehigh University, Dept. of Physics, 16 Memorial Dr. East, Bethlehem PA 18015

Viscoelastic properties of complex fluids can be measured at the microscopic level, for example, by probing them with colloidal particles. The properties measured are often different from those measured by bulk rheometers. To gain insights into the physics governing the microscopic mechanical behavior of complex fluids we have employed an application of single and dual optical tweezers to determine the local and non-local microscopic mechanical responses in complex fluids. In the application of dual optical tweezers, we measure the mechanical response of a colloidal particle (probe) to forces generated by a remote particle. As proof of principle, we use dual optical tweezers for trapping two micron-sized beads suspended in water. One of the tweezers sets one particle in a forced motion and the second tweezers measures the motion of the probe particle, a few bead diameters away. The theory, based on hydrodynamic coupling, agrees well with the data. The theory and experiment are readily extended to both homogeneous and inhomogeneous viscoelastic media. From the displacement and the phase of the probe particle, the non-local rheological properties are determined directly. In this talk, we present data obtained from viscous, as well as, homogeneous and inhomogeneous viscoelastic media for several interparticle separations.


 

229

Brownian Motion in CTAB/NaSal Solutions

Samiul Amin , Stephen Dees , and John H. van Zanten, North Carolina State University, Department of Chemical Engineering, Raleigh, North Carolina 27695-7905, Ryan M.van Zanten, University of California-Santa Barbara , Department of Chemical Engineering, Santa Barbara, California 93106-5080, Thomas Kermis, Johns Hopkins University, Department of Chemical

Engineering, Baltimore, Maryland 21218

The static and dynamic properties of wormlike micellar systems has generated considerable interest. A large number of rheological and dynamic light scattering studies have been conducted on such systems. However, these methods are limited by the length scales and frequencies that are probed. Diffusing Wave Spectroscopy (DWS) is a relatively new, noninvasive method that can be utilized for characterizing the viscoelastic/dynamic properties with the main advantages being that much wider frequency range is probed using extremely small strains. In this report an extensive study of probe Brownian motion in CTAB/NaSal solutions is presented. The viscoelastic moduli obtained from microrheology are compared with those obtained from bulk rheology measurements. These observations are also interpreted within the context of static and dynamic light scattering measurements of the same systems with a view towards elucidating the physics underlying Brownian motion in viscoelastic materials.


230

Two-Dimensional Diffusion of Colloidal Particles in Polymer Solutions

Marc Robert, Rice Quantum Institute, Center of Nanoscale Science and Technology, and Department of Chemical Engineering, Dana Pilaski, Tai-Chou Lee, Trinh Vo, Jiun-Ting Lee Department of Chemical Engineering Rice University, 6100 S. Main St. Houston, TX. 77005, USA

We study the two-dimensional diffusion of spherical polystyrene colloidal particles in poly(ethyleneoxide) (PEO) and Lambda solutions. The diameters of the colloidal particles range from 1 to 2.5 micrometers. PEO and Lambda DNA have gyration radii of 0.139 to 0.372 and 0.5 micrometers respectively. Lambda DNA offers the advantage over other polymers to be monodispersed. Two-dimensional cells are prepared for different colloid sizes and polymer concentrations. The trajectories of the colloidal particles are obtained in real time by using video microscopy and image analysis. The diffusivity is determined as a function of polymer concentrations.


231

Optical Rheology Studies of the Formation and Aging Behavior of Soft Glassy Clay Particle Suspensions

Virgile Viasnoff, Laboratoire de Physico-Chimie Macromoleculaire, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex, FRANCE, Laura D. Collins, United States Coast Guard Marine Safety Center, 400 7th street SW, Washington, D.C. 20590, James L. Harden, Department of Chemical Engineering, Johns Hopkins University, 3400 N. Charles St., Baltimore, MD 21218

Studies of the collective dynamics and microrheology of aqueous suspensions of Laponite, a synthetic Hectorite clay composed of charged, nanoscopic discoid particles, will be presented. Laponite suspensions form soft transparent colloidal glasses at low particle concentrations. In the glassy phase, Laponite suspensions have novel rheological and aging properties. Diffusing wave spectroscopy (DWS), dynamic light scattering in the highly multiple-scattering limit, was used to deduce the sol-glass transition and the linear viscoelastic properties of these colloidal suspensions as a function of clay concentration and sample age by monitoring the thermally-induced fluctuations of added tracer particles. The formation and aging of the soft glassy phase occur in two distinct stages. The viscoelastic behavior of the glassy state is characterized by novel power-law creep behavior, J(t)~ta with an exponent a that approaches zero with increasing concentration or sample age, followed by a long-time collective relaxation. The creep exponent a for samples of different age or Laponite concentration can be rescaled onto a single mastercurve in both the formation and aging regimes, indicating universal behavior for glassy dynamics in this system. These results will be compared with recent models of the rheology and aging of soft glassy materials.


232

Microrheometry and Cytomechanics in Tissue-Like Collagen Matrices

G. Steven Vanni, B. Christoffer Lagerholm, Matthew Vieta, Allison Marciszyn, D. Lansing Taylor and Frederick Lanni (Carnegie Mellon University, Pittsburgh PA), Darrell Velegol (Penn State University, University Park, PA), and Carol A. Otey (University of North Carolina, Chapel Hill NC).

In order to more fully understand the structure, function, and regulation of the molecular machinery within single cells, we utilize the elastic extracellular matrix (ECM) as an in-situ strain gauge. In our studies, 3T3 mouse fibroblasts contract and remodel a type-I collagen gel over a period of 3-7 days. By use of time-lapse light microscopy, we image (1) fluorescently-labeled cytoskeletal components within a cell, and (2) deformations in the ECM caused by the mechanical activity of the cell. Independently, we utilize a near-IR laser optical trap and colloidal microspheres embedded in model ECMs to determine the modulus of the gel on a scale (1-100 um) relevant to cellular activity. From the deformation images, we derive a quasi-static strain field in the plane-strain approximation. Our immediate aim is to combine the strain field and modulus information to estimate the stress field set up by individual cells, then to match this with the cytoskeletal structures identified in the corresponding fluorescence image sets. This will allow us to identify mechanical functions of the cytoskeleton, and understand the rules of engagement between a cell and its native tissue matrix. Supported by NIH grant R37 AR-32461 and NSF grant MCB-8920118.


233

Non-Thermal Fluctuations in Semi-Flexible Polymer Gels Activated by Molecular Motors

E. M. Furst 1, L. Le Goff 1, P. Chaussepied 2, F. Amblard 1; 1 Laboratoire de Physico-Chimie, Insitut Curie, 11 rue Pierre et Marie Curie, Paris, 75005 France. 2 Université de Montpellier, Montpellier, France

The mechanical and dynamical behavior of biological semi-flexible polymers, such as filaments formed by the self-assembly of actin proteins, are fundamental for understanding the mechanical and rheological properties of cells. The interaction between actin and the molecular motor myosin represents a source of non-thermal noise coupled to ATP hydrolysis. In order to understand these non-thermal fluctuations of F-actin, semi-dilute solutions are reconstituted in the presence of scallop S1-myosin fragments. By measuring the mean-squared displacement of tracer particles embedded in the gel using diffusing wave spectroscopy (DWS), dynamic light scattering in the highly multiply-scattered regime, we measure actin gel dynamics for small displacements (0.1-10nm) at short times (10-6 - 0.1s). We observe two dynamical states of acto-myosin gels: active and rigor. During the active state, tracer motion departs from the diffusion exponent measured for actin-only gels, exhibiting faster dynamics that increase with myosin concentration. As the ATP resevoir is exhausted, tracer motion exhibits a rapid transient from the steady-state active to the rigor state, characterized by the actin diffusion exponent; however, the rigor state exhibits a higher modulus than gels comprised of actin alone.


234

Shear-Induced Fluid Fracture in Telechelic Polymer Networks

J.-F. Berret, Complex Fluids Laboratory, CNRS, Cranbury Research Center Rhodia Inc., 259 Prospect Plain Road CN 7500, Cranbury NJ 08512 USA

We report on the nonlinear rheology of aqueous solutions of telechelic associating polymers in the network regime. Different telechelic polymers with a poly(ethylene oxide) middle chain and semi perfluorinated end-caps C8F17(CH2)11 were investigated. At a telechelic concentration of 4 %, the aqueous solutions studied are highly viscoelastic and close to Maxwellian fluids. The steady shear stress versus shear rate curves exhibit two main and stable branches separated by a discontinuity. The first branch of the flow curve is Newtonian and shear-thickening, whereas the second branch is shear-thinning. Using particle tracking experiments, we show that in the shear-thinning regime, the fluid undergoes a transition toward an inhomogeneous flow. In the shearing cell, two main regions of different shear rates can be distinguished. They are separated by a thin zone that is described in terms of fracture. Through transient start-up measurements, this shear-induced fracture of the fluid can be related to the discontinuity of the shear stress curve mentioned above.


235

Dip-Pen Nanolithography: A Tool for Generating Organic and Biological Surface Architectures with 5 nm Resolution

Chad A. Mirken, Northwestern University, Chemistry Department and institute for Nanotechnology, 2145 Sheridan Road, Evanston, IL 60208

A new type of ultrahigh resolution soft-lithography that is interfaceable with biomolecules and biofuntionalized building blocks will be presented. This soft lithography allows one to pattern structures with 5 nm spatial and 10 nm linewidth resolution with near-perfect alignment. Implications in materials synthesis, electronics, and biodiagnostics will be discussed.


236

Microcontact Printing of Surfactants on Mica: Effects of Lateral Diffusion and Substrate Registry

Richard K. Workman, Dept of Materials Science, Srinivas Manne, Dept of Physics, University of Arizona, 1118 E. 4th St, Tucson, AZ 85721

Microcontact printing is typically used to stamp molecules which chemisorb to the substrate. This work investigates stamping of molecules which can only physisorb to the substrate. These types of molecules can laterally diffuse across the substrate after stamping, and some have been seen to diffuse preferentially along the substrate symmetry axes. We will show results of stamping amphiphilic molecules onto mica substrates using poly(dimethylsiloxane) (PDMS) stamps. Atomic force microscopy (AFM) images reveal spreading of the ink along substrate symmetry axes under certain conditions. Such spreading sets limits on the fidelity of features, but also affords a new method for pattern formation on mesoscopic length scales. By tailoring the molecule-substrate interactions (e.g. temperature, humidity, pressure, pH, etc) one can pattern features much smaller than the stamp geometry.


237

Model Chromatographic Surfaces : Solvent Induced Changes in Monolayer Structure Studied by Sum Frequency Generation Spectroscopy

Matthew C. Henry, Marie C. Messmer, Lauren K. Wolf, Lehigh University, 7 Asa Drive, Bethlehem, PA 18015

Instability of chromatographic octadecylsilane (ODS, or C18) stationary phases with respect to changing mobile phase composition has been ascribed to a "brush collapse" of the surface chains with increasing water content of the mobile phase. Using sum frequency generation spectroscopy, the degree of disorder in a mixed monolayer alkane( C1 and C18) model surface was studied. This is analogous to a chromatographic surface of C-18 chains, with C-1 groups use to cap the unreacted silanols on the surface. The degree of disorder induced in the monolayer was studied for air, D2O, isooctane-d18, acetonitrile-d3, and acetonitrile/water solutions. The disorder was also observed as a function of C18 content of the monolayer and for partial monolayers with no C1 endcapping. The surface was found to be resistant to solvent induced disorder when it was composed of pure C-18, but susceptible to disorder when C-1 chains are present on the surface or when pure C-18 is present at submonolayer coverages.


238

Measurement of Molecular Surface Structure

Curtis. W. Meuse, David. J. Vanderah, Joseph. B. Hubbard, Raymond. D. Mountain, Chemical Science and Technology Laboratory, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8313.

We are developing internal reflection infrared spectroscopy methods for the characterization of the molecular structures and processes involved in self-assembly and protein surface interactions. To illustrate, we have characterized octadecanethiol and methyl 1-thiahexa(ethylene oxide) monolayers through thin gold layers. The internal reflection geometry permits the in situ observation of both the s- and p-polarized spectra of the monolayer in contrast to the typical external reflection geometry in which the weaker s-polarized spectrum is not experimentally accessible. From this additional information, we are able to deduce the surface concentration and orientations of the functional groups of the monolayers and the proteins. Our studies of ordered and disordered self-assembled monolayers of methyl 1-thiahexa(ethylene oxide) on gold reveal that disordered monolayers exhibit better inhibition of protein adsorption.


239

Photoinduced Processes in Self Assembled Monolayers on Semiconductor Oxide Surfaces

Tao Ye, Eric McArthur and Eric Borguet, Department of Chemistry and Surface Science Center, University of Pittsburgh, Pittsburgh PA 15260, USA

Recent experiments offer convincing evidence that ozonolysis is the primary cause of UV photodegradation of alkanethiol SAM in air. A combination of contact angle, FTIR and AFM measurements provide strong evidence, however, that oxygen radical species are the active agent in Octadecylsilane (ODS) SAM degradation under UV illumination in air. Degradation does not occur in air in the absence of UV. Nor does UV illumination in the absence of O2 induce degradation. Furthermore, ozone alone, in the absence of UV, does not cause significant degradation. The reactive species either is created directly by O2 photocleavage near the surface, or by photoinduced release from the ozone which serves as a relatively inert carrier of highly reactive atomic oxygen. AFM results suggest a microscopic mechanism for ODS SAM photreactivity. These results have implications for photoresist micropatterning and nanotechnology applications.


240

X-ray Mediated Chemical and Physical Modification of Self-Assembled Monolayers

Howard Fairbrother, Anthony Wagner, Steve Carlo, Chad Vecitis, Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218.

Self assembled monolayers (SAMs) are frequently characterized by X-ray and electron based spectroscopic probes. However, these techniques have been shown to initiate changes in both chemical and physical properties of C16H33S, alkanethiol (AT) and CF3(CF2)7(CH2)2S, semi-fluorinated (SF) based SAMs adsorbed on Au and Cu substrates. Electron beam modification of both SAM structures involves Au/Cu-S and C-C bond cleavage including desorption of intact chains. In both AT and SF-SAMs new C-S-C linkages were produced at short irradiation times as the result of reactions involving RS∙ although the extent and distribution of these species within the SAM were found to be sensitive to the SAM’s chemical composition. For SAMs adsorbed on copper X-ray irradiation was detrimental to their ability to act as an oxygen diffusion barriers. In the case of SF-SAMs the X-ray induced defluorination has been shown to be first order with respect to both the fluorine content and X-ray flux. Detailed kinetic modeling indicates that the molecular level reaction mechanism is consistent with a stochastic model of delfluorination involving a series of consecutive C-F bond breaking events (CF3 ® CF2 ® CF ® C), highlighting the ability to use SAMs as models to understand orgnaic surface modification processes.


241

Evidence of Lateral Phase Separation in Lecithin-Cholesterol Membranes

S.P. Wrenn and G. Troup, Drexel University, Chemical Engineering Dept, 3141 Chestnut St. St. Philadelphia, PA 19104

Multi-lamellar egg lecithin vesicles containing 0 mole% to 65 mole% cholesterol were labelled with danyslated lecithin, in which the dansylated lecithin fraction was fixed at 4.7 mole% of the lecithin species. Initial fluorescence spectra were recorded at room temperature (28° C), where the dansyl emission maximum shifted steadily to shorter wavelengths with increased cholesterol loading for all cholesterol loadings exceeding 30 mole%. All samples were heated (to 47° C) and cooled (to 28° C) in a cyclic fashion, and spectra were recorded at both temperatures for several heating/cooling cycles. Upon heating, the dansyl emission maximum shifted to longer wavelengths in the range 30-55 mole%, but heating produced no effects for cholesterol loadings outside this composition range. The initial (room temperature) spectra were recovered upon cooling, and nearly identical spectra were observed at both temperatures for three heating/cooling cycles. We interpret the initial blue shifts as evidence that cholesterol-rich domains begin to form within the membrane once the cholesterol loading exceeds ~30 mole%. The ensuing red shifts that result from heating suggest that the domains can be "melted" if the cholesterol loading does not exceed 55 mole%. Melting is either not possible above 55 mole% cholesterol or requires a temperature greater than 47° C.


242

New X-ray Method to Determine Lipid Bilayer Structure and Interactions

Y. Liu, Y. Lyatskaya, S. Tristam-Nagle, and John Nagle, Carnegie Mellon University, Physics Dept., Pittsburgh, PA 15213

Lipid molecules have short polymeric hydrocarbon chains and they form bilayers which interface with water to compartmentalize cells. A new x-ray methodology is being developed to pursue our work that focuses on the structure of bilayers and their interactions with each other. Precise calculations are made of scattering intensity I(q) from an oriented stack of lipid bilayers using a realistic model of fluctuations. Quantities of interest include the bilayer bending modulus KC, the interbilayer interaction modulus B, and bilayer structure through the form factor F(q). Both KC and B may be obtained from data at large qz where fluctuations dominate and good estimates of F(qz)can be made over wide ranges of qz by using I(q) in q regions away from the peaks. Results will be presented for data taken on fully hydrated, oriented bilayers in the L sub alpha phase that illustrate the advantages of oriented samples compared to powder samples.


243

Effects of Proteins on Liposome Solubilization

Namita Deo, P.Somasundaran, NSF Industry/University Cooperative Center in Novel Surfactants, Henry Krumb School of Mines, 500 W120th. Street, 911 S. W. Mudd, Columbia University, New York, NY 10027, K.Subramanyan, K.P.Anathapadmanabhan, Unilever Research US, Edgewater, NJ 07020

Lipid bilayers and liposomes have been used as experimental models for investigating the behavior of biomembranes. Biological membranes are formed from the supra-molecular aggregates of lipids, cholesterols and proteins and other components like carbohydrates can attach to them covalently. The mechanisms of interactions of surfactants with simpler biomembranes are first studied and different components are added to make them realistic. An objective of this work is to understand the effects of zein protein on liposome solubilization processes. Sodium dodecyl sulfate has been used as a model surfactant for this purpose. From the preliminary results it is clear that a minute quantity of protein can enhance the liposome solubilization processes. Also, cholesterol makes the liposomes more resistant towards the surfactants. The mechanisms of interactions studied using a multi-pronged approach involving techniques such as UV-visible spectroscopy, surface tension, electron spin resonance spectroscopy, atomic force microscopy and mass spectroscopy, will be reviewed.


244

Measurements of Cholesterol Nucleation from Lecithin-Cholesterol Vesicles in Model Bile Systems Comprising Ursodeoxycholic Acid

S.P. Wrenn, M. Gudheti, Drexel University, Chemical Engineering Dept, 3141 Chestnut St., Philadelphia, PA 19104

Nucleation of cholesterol was measured in model bile systems comprising lecithin-cholesterol vesicles and the bile salt, sodium taurursodeoxycholate. Vesicles were labeled with dehydroergosterol and dansylated lecithin, fluorescent analogs of the native lipids, cholesterol and lecithin, respectively. Energy transfer from dehydroergosterol to dansylated lecithin decreased in a dose-dependent manner upon exposure of the fluorescently-labeled vesicles to the bile salt during the first 24 hours after mixing. At intermediate bile salt loadings, the initial alleviation of energy transfer was followed by a lag period lasting several days. Beyond the lag period, and before the appearance of microscopic cholesterol crystals, a secondary alleviation of energy transfer occurred. The onset and rate of the secondary alleviation of energy transfer correlates with cholesterol loading and likely results from cholesterol nucleation from vesicles. Energy transfer from dehydroergosterol to dansylated lecithin is thus a potential methodology for characterizing the putative pro- and anti-nucleating factors that affect gallstone formation in bile.


 

245

Formation and Properties of a Supported Lipid Monolayer

John Woodward, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899

A monolayer of phospholipid will adsorb on an alkanethiol self-assembled monolayer (SAM) exposed to a vesicle solution. We have studied the formation and structure of a dimyristoyl phosphatidylcholine layer supported on an octadecanethiol SAM using impedance analysis and atomic force microscopy (AFM). The combination of a local probe (the AFM) and a non-local probe (impedance analysis) gives insights that neither alone can provide. Impedance analysis gives kinetic information with a relatively fast and reliable probe of the average thickness of the lipid layer. The AFM provides local information about the heterogeneity of the lipid layer as well as the presence of over layers. This information can then be used to better model the process of vesicle adsorption and layer formation. Additionally, the AFM was used to probe the fluidity of the completed lipid layer as a function of temperature.


246

Metal Complexation with Langmuir Monolayer of Mixed Peptide Lipids

Qun Huo, Guodong Sui, Yujun Zheng, Peter Kele, Roger M. Leblanc. Department of Chemistry, University of Miami, Coral Gables, FL 33124, Takeshi Hasegawa, Jujiro Nishijo. Kobe Pharmaceutical University, Motoyama-kita, Higashinada-ku, Kobe, 658-8558, Japan., Junzo Umemura. Institute for Chemical Research, Kyoto University, Uji, 611-0011, Japan.

Metal-protein complexation plays a crucial role in the function and activity of proteins and enzymes. Model systems which can mimic the structure and function of metalloproteins are of primary interest to bioinorganic chemists. We previously proposed a novel "combinatorial surface chemistry" approach to make artificial proteins and enzymes. We herein report our study of using this approach to mimic metal-protein complexation. Three peptide lipids with a histidine amino acid incorporated in the middle were synthesized and the mixed peptide lipids were spread at the air-water interface to form a Langmuir monolayer. This mixed peptide monolayer exhibits different binding activity towards hard metal cations such as K+ and Mg++ from soft metal cations such as Zn++, Cu++ and Cd++. Dramatic fluorescence change was observed when Cu++ binds to the peptide monolayer or Langmuir-Blodgett film. This study also shows that although both Cu++ and Zn++ exhibits binding activity to the monolayer, their binding motifs are different from each other, as suggested from the fluorescence and circular dichroism studies.


247

Peptide-Based Polymeric Surfactants

J.L. Streeter, D.Henderson, M. Lindsay, K.E. Van Cott, and R.M. Davis, Department of Chemical Engineering; W. Ducker, Dept. of Chemistry, Virginia Tech, Blacksburg, VA 24061

C. Russell, Research Genetics Inc., 2130 Memorial Pkwy. SW, Huntsville, AL 35801

There is a need for improved dispersants for aqueous colloidal suspensions due to increasingly strict environmental regulations. It is particularly difficult to control the state of aggregation of metal oxide particles at high volume fractions due to strong van der Waals forces. Block copolymers that form self-assembled, nonionic brush layers have proven to be particularly effective steric stabilizers that are relatively insensitive to pH and ionic strength effects. In this work, biosynthetic techniques were used to make a well-defined, water-soluble diblock copolymer comprised of peptides in which the anchor block was anionic and the tail block was nonionic. The adsorption of this copolymer on alumina was studied by atomic force microscopy, the solution depletion method, and by electrophoresis. The solution properties were studied by dynamic and static light scattering. The anchor and tail block peptide chemistries were chosen based on competitive adsorption isotherm measurements made with binary mixtures of polypeptide homopolymers.


248

Defects and Disorder: Probing the Surface Chemistry of Heterogenite (CoOOH) by Dissolution Using Hydroquinone and Iminodiacetic Acid

R. Lee Penn, David R. Velben, Department of Earth and Planetary Sciences, Alan T. Stone, Department of Geography and Environmental Engineering, Johns Hopkins University, Baltimore, MD 21218

Dissolution, using hydroquinone (H2Q) and iminodiacetic acid (IDA), of synthesized heterogenite particles (CoOOH) was examined in order to evaluate morphology evolution as the particles dissolved. The synthesized heterogenite particles are self-assembled micron-sized hexagonal plates (aspect ratio ~ 1/30) constructed of crystallographically oriented ~ 5-nm primary particles or are ~ 21-nm or ~10-nm heterogenite platelets (aspect ratio ~1/7 and ~1/3, respectively). In experiments utilizing the micron-sized hexagonal plates, two dominant modes of dissolution were observed: non-specific dissolution that dissolved primary building blocks at all locations equally and pathway specific dissolution that occurred along boundaries of misorientation between primary building blocks. Both mechanisms occurred independent of the dissolution agent used. Finally, dissolution by IDA produced two dissolved isomers, u-fac Co[IDA]2¯ and s-fac Co[IDA]2¯. Experiments utilizing identical solution conditions show that dissolution of the micron-sized plates favors the production of the u-fac isomer, while dissolution of the 21-nm platelets favors the production of the s-fac isomer. Comparing particle dimensions before and after dissolution shows dissolution to occur primarily at edge-type surfaces. Stacking disorder in the heterogenite produces unique surface sites on the edge-type surfaces of the 21-nm particles and is proposed as the cause of favored production of the s-fac isomer during dissolution.


249

Effects of Organic Ligands on Dissolution of Barium Sulfate: A Scanning Force Microscopy Study

Kang-Shi Wang1, Roland Resch2, Sheffer Meltzer2, Yongchun Tang3, Bruce E. Koel2, and Teh Fu Yen1, 1Department of Civil and Environmental Engineering, University of Southern California, 3620 S. Vermont Ave., Los Angeles, CA 90089, 2Department of Chemistry, University of Southern California, 920 W. 37th St., Los Angeles, CA 90089, 3Petroleum Energy and Environmental Research Laboratory, California Institute of Technology, 20970 Currier Road, Walnut, CA 91789

Scanning force microscopy (SFM) was carried out to study the dissolution of a barite (BaSO4) (001) surface in aqueous solutions of nitrilotriacetic acid and oxalic acid at pH 12. SFM observations show clearly that organic ligands can control the growth of etch pits and retreat of monolayer steps in the course of dissolution process. Etch pits grew that were elongated along the crystallographic b axis and deeper along the c axis in a layer-by-layer dissolution process. Ex-situ imaging revealed that triangular etch pits in both solutions were most frequently bounded with step walls aligned along the < 010> and < 120> directions. These shallow pits were one-monolayer (one-half of the unit cell) deep and pointed in opposite directions between any two consecutive layers. In addition, deep etch pits with a trapezohedral morphology were found in solutions with higher etchant concentrations. In-situ imaging showed that monolayer-steps parallel to the < 120> directions were the most reactive. These steps retreat faster along the [100] than [00] direction because of anisotropic dissolution behavior. The combination of the in-situ and ex-situ SFM imaging results provide complementary information on microtopographic features that are useful for elucidation of mineral dissolution processes.


250

Hematite Dissolution Promoted by Carboxylate Ligands

Scot T. Martin, Harvard University, Division of Engineering and Applied Sciences, Pierce Hall, Cambridge, MA 02138

The accuracy of quantitative models of groundwater pH and Eh are presently limited by uncertainties in the dissolution mechanisms, pathways, and rates of minerals. One important pathway is assistance by oxygen- and nitrogen-bearing ligands, which chelate the surficial hydroxyl groups of some minerals. In the current work, ligand-promoted dissolution rates of hematite are measured for a homologous series of dicarboxylate ligands (viz. oxalate through adipate). Investigated experimental controls include pH, aqueous ligand concentration, and ligand chain length. At pH = 5 and 10 mM ligand concentration, the relative dissolution rates for the ligands proceeds as follows: oxalate > glutarate > malonate >> {succinate, adipate, proton-promoted control}. The relative dissolution rates are hypothesized to depend upon the structures formed between the organic ligand and surface hydroxyl groups of the minerals. To test the hypothesis, the infrared spectra of dicarboxylate ligands sorbed to hematite are collected by attenuated total reflectance (ATR) spectroscopy. For each ligand, spectra are recorded at several concentrations at pH = 5.0. Spectral interpretation, through analogy to literature metal-ligand complexes, leads to the assignment of specific surface complexation species coincident with observed reactivity: oxalate, glutarate, and malonate form bidentate mononuclear surface complexes while succinate and adipate form monodentate structures.


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