Oral Presentation Abstracts

Talks 51 through 100

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Surface Light Scattering Spectroscopy: Electric Field Effects on Interfaces

Brian Doucet, J. Adin Mann, Jr., Department of Chemical Engineering, Case Western Reserve University, Cleveland OH 44106-7217

The methodology for collecting surface light scattering spectra has greatly improved during the last five years. The new generation of spectrometers developed collaboratively by the group at CWRU and NASA Glenn will be described. The instrumentation and theory was used to measure the surface tension variation with an electric field jump at the surface. Theory of Onuki that amounts to adding the Maxwell tensor jump at the interface while keeping the surface tension constant with field was tested. The fluctuation spectra of 99.999% CF6 was measured as a function of the electric field between 0 and 40 kV/cm and temperature between 25 C and 44 C (just below the critical temperature) in a closed cell. Onuki’s theory was followed at lower temperatures but deviations were observed close to the critical temperature. These results suggest that the surface tension was dependent on electric field for this insulating fluid. A model will be proposed for this behavior.


Light Scattering Characterization of Polystyrene Latex Spheres

Gilbert Min, Michael Bevan, Dennis Prieve and Gary Patterson, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, PA, 15213

It is a common observation that the hydrodynamic radius determined by dynamic light scattering from polystyrene latex spheres exceeds the number average radius determined by TEM. We have characterized a sample of nominal radius 140 nm by both static and dynamic light scattering. The measured particle scattering function S(q) was compared with theoretical scattering functions for a polydisperse sample. Evidence of polydispersity was also obtained from the shape of the dynamic scattering function and from the angular dependence of the average hydrodynamic radius. Theoretical dynamic scattering functions were calculated as a function of polydispersity and compared to the measurements. The measured hydrodynamic radius also slightly exceeded the average radius measured by static light scattering. The additional friction could be accounted for by electroviscous drag and quantitative calculations of this effect were carried out.


Probing Static Structure Factor in Concentrated Colloidal Suspensions Using Frequency Domain Photon Migration

Zhigang Sun, School of Chemical Engineering, Purdue University and Department of Chemical Engineering, Texas A&M University, Eva Sevick-Muraca, Department of Chemical Engineering, Texas A&M University, College Station, TX, 77843-3122

Frequency domain photon migration (FDPM) technique, which is based on multiple scattering, is a useful tool to study particle interaction and structure of concentrated colloid suspensions. In this work, near-infrared FDPM measurements are used to derive isotropic scattering coefficients and angular integrated structure factors in binary and polydisperse polystyrene suspensions at high volume fraction (up to 30%). The experiment data are compared with the values predicted by Mie theory and Percus-Yevick (P-Y) approximation for binary and polydisperse hard sphere systems. Particle interactions of binary systems strongly depend on relative concentrations of different particles, and the interaction between small and large particles is significant. For polydisperse suspensions, Percus-Yevick model is suitable for predicting particle interactions. By combining polydisperse PY model with FDPM measurement, we show that it is possible to recover particle size distribution (PSD) at high volume fractions.


Meaning of the Structure Factor of Smectite-Water Systems as Revealed by Small Angle X-Ray Scattering Measurements

Chao Shang and James A. Rice, Department of Chemistry and Biochemistry, South Dakota State University, Brookings, SD 57007

Small-angle x-ray scattering (SAXS) is a technique widely used to determine the internal and external structure of colloidal particles over a wide range of length scales. The thickness, interparticle spacing in hydrated states, and the influence of interparticle forces fall into its characterization length scale. The scattering intensity (I(q)) of various smectite-water systems was recorded over a range of scattering angles (q = 0.053 to 4.0 nm-1) on the 10-m SAXS facility at Oak Ridge National Laboratory. The experimental structure factors (S(q)), dependent on inter-particle scattering, of these smectite-water systems were obtained from the SAXS data. The angular position (qmax) of the S(q) maximum varies with clay concentration when clay concentration is equal to or greater than 2%, but remains constant for more dilute suspensions. The interpretation of the S(q) maximum in the literature, however, has not been straightforward. There exists a consistent pattern for the discrepancy between the experimental interparticle distance, derived from the S(q) maximum, and the theoretical values which assume a complete dispersion. The results presented here indicate the necessity for providing a more rational explanation for the physical significance of the structure factor.


Effect of Spatial Distribution of Granular Porous Medium Geochemical Heterogeneity on Colloid Transport

Jeffrey Y. Chen, Chun-Han Ko, and Menachem Elimelech, Yale University, PO Box 208286, New Haven, CT 06520-8286.

Geochemically heterogeneous surfaces are important in determining the transport behavior of colloidal particles in subsurface aquatic environments. Previous studies modeled surface heterogeneity as random microscopic sites or patches and dealt primarily with porous media surfaces having constant geochemical heterogeneity. The present study investigates the effect of spatial distribution of geochemical surface heterogeneity on colloid transport in flow through porous media. Colloid transport experiments were performed by injecting a pulse of aqueous colloid particle suspension through a packed quartz sand column. Surface charge heterogeneity was introduced into the porous medium by silanizing a fraction of the quartz sand. Colloid transport experiments at various degrees of surface charge heterogeneity and several spatial distributions of heterogeneity (e.g., uniformly mixed, bottom-layered, and mid-layered) were conducted at different flow rates and electrolyte concentrations. The observed particle breakthrough curve data were fitted to the solution of a one-dimensional advection-dispersion equation with a sink term for colloid deposition, using the Levenberg-Marquadt non-linear least square analysis. The results revealed the particle deposition rate to be independent of the spatial distribution of geochemical heterogeneity. It is the mean value of geochemical heterogeneity rather than its distribution that governs the colloid transport behavior.


Colloid Stability in Corrosion Leachates of Unirradiated Uranium Metal Fuel

C.J. Mertz, S.F. Wolf, J.A. Fortner, M.D. Kaminski and M.M. Goldberg, Argonne National Laboratory, Chemical Technology Division, 9700 S. Cass Ave., Argonne, IL 60439 and C. Shelton-Davis, Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415

Colloids generated during the corrosion of unirradiated uranium metal fuel have been characterized to determine properties of these colloids relevant to stability and transport in a subsurface environment. As colloids have the potential to transport contaminants from a waste storage or disposal site, understanding colloid stability is essential for predicting contaminant release and mobility. Work performed in this study demonstrates that unirradiated uranium metal fuel forms stable colloids under a range of solution compositions including aqueous solutions of silicate, carbonate, and ferric ion, a silicate-saturated groundwater, and deionized water. The resulting colloids were characterized by a variety of techniques including dynamic light scattering, ultrafiltration and inductively coupled plasma-mass spectrometry, transmission electron microscopy, and size exclusion chromatography. This presentation provides analytical results documenting important properties of the colloids that affect stability and transport such as size, concentration, and mineralogical composition.


Effect of Chemical Perturbations on the Mobilization of Colloids in a Ferric Oxyhydroxide-Coated Sand Aquifer: Field Experiments

Rebecca Bunn, Robin D. Magelky, Joseph N. Ryan, University of Colorado, 428 UCB, Boulder, CO 80309-0428, Menachem Elimelech, Yale University, 9 Hillhouse Avenue, PO Box 208286, New Haven, CT 06520-8286

We performed small-scale field experiments (2 m transport) to assess the effect of pH, ionic strength, and surfactant and reductant concentration on colloid mobilization in a ferric oxyhydroxide-coated sand aquifer. Injections of sodium hydroxide (pH 11.0 and 12.5), deionized water, sodium dodecylbenzene sulfonate (0.6 mM), and ascorbic acid (1 mM) were made in two zones of the aquifer – an overlying uncontaminated zone and (2) an underlying zone contaminated by secondary sewage treatment effluent. The mobilized colloids were mainly kaolinite. The pH increases mobilized the greatest mass of colloids. The amount of mobilized colloids was proportional to the increase in pH in the aquifer. Dodecylbenzene sulfonate and ascorbic acid mobilized about the same mass of colloids. Deionized water mobilized only a small amount of colloids owing to the presence of positively charged ferric oxyhydroxide coatings. Colloid mobilization in the uncontaminated zone always exceeded that in the contaminated zone. These trends agreed well with qualitative interpretations of electrostatic interactions between negatively-charged colloids and electrostatically heterogeneous porous media. The appearance of mobilized colloids always lagged the breakthrough of the injected perturbation, indicating that mobilized colloids can migrate no faster than the chemical perturbation responsible for their mobilization.


Permeability Changes Induced by Specific Surfactant-Clay Interactions during Surfactant Flushing

Kevin Gardner, Christopher Berg, Philip Gidley, Department of Civil Engineering, University of New Hampshire, 123 Nesmith Hall, Durham, NH 03824, and Miguel Arias, Dept. of Civil, Environmental, and Architectural Engineering, University of Colorado, Boulder CO 80309

A major concern with the use of surfactant flushing to mobilize non-aqueous phase liquids in aquifers is specific mineral-surfactant interactions which may effect significant permeability changes in the soil formation. Soils comprised of Ottawa sand mixed with small percentages of bentonite (0-5%) that had moderate initial hydraulic conductivity (>10-6 m/s) were investigated for loss of permeability upon flushing with solution containing a nonionic surfactant (polyoxyethylene sorbitan monooleate). Columns containing 0, 1, 2, 3, 4, and 5% clay had permeability reductions of 1, 5, 13, 44, 49, and 69%, respectively. Surfactant / clay interactions were further investigated as the cause of the permeability reductions. Some transport of clay through the column was apparent and a transient permeability change was correlated with the colloid transport in the column. Clay swelling was postulated as the primary mechanism for the permeability reductions, and a model was developed using x-ray diffraction measurements of basal layer spacings. Recent work has focused on permeability changes due to migration of non-swelling components of aquifer matrices, and on predictive modeling of permeability reductions.



Evaluation of Surfactant-Contaminant Interaction in Inert Porous Media

Lutful Khan, Mark Tumeo, Nilufer Dural, Pavan Kumar, Civil and Environmental Engineering Department, Cleveland State University, Cleveland, OH 44145

This paper presents the results of a preliminary research conducted to evaluate the interactions of organic contaminants with non-ionic and anionic surfactants. This research is part of a larger, NSF-funded project being conducted at Cleveland State University and the University of New Hampshire to investigate the controlling factors that produce a change of in-situ hydraulic conductivity in soil when surfactants are used. Non-ionic and anionic surfactant solutions of varying concentrations above the CMC were introduced into a matrix of inert glass beads contaminated with motor oil under constant head conditions and the change in the coefficient of permeability (k) of glass bead/ oil system measured. It was observed that the non-ionic surfactant had mild interaction with the contaminant and the k values reduced slightly compared to that of clean water. The anionic solutions, however, produced a drastic reduction of the k values with the 25% contamination level. The data indicate the formation of a macro-emulsion above a certain oil/surfactant ratio. The reduction in permeability demonstrates that in some cases, contaminated soils may become increasingly more difficult to remediate when anionic surfactants are used.


Soil Organic Matter Mobilization and Enhanced PAH Desorption Using Chelating Agents

Kavitha Subramaniam and Domenico Grasso, Picker Engineering Program, Smith College, Northampton, MA 01063, Barth F. Smets, Environmental Engineering Program, University of Connecticut, Storrs, CT 06269, Joseph J. Pignatello, The Connecticut Agricultural Experiment Station, New Haven, CT 06504

Successful remediation of several contaminated sites has been hindered by the slow desorption of toxic hydrophobic organic compounds such as polynuclear aromatic hydrocarbons (PAHs). Chelating agents are often used to treat soils, sediments and wastes contaminated with toxic metals. This research explores the use of complexing agents such as EDTA, citrate, oxalate, salicylate and pyrophosphate in enhancing the desorption of 16 PAH compounds in soil collected from a former manufactured gas plant site in Winsted, CT. From equilibrium and kinetic experiments, it is evident that both the rate and extent of soil organic matter (SOM) and PAH desorption can be significantly enhanced using chelating agents. The efficacy of the desorption process is shown to depend on a variety of factors such as the nature of the PAH, type of complexant, pH, ionic strength and time allowed for reaction. It is hypothesized that the chelating agents act by disrupting humic-metal ion-mineral linkages in the soil resulting in the mobilization of SOM and adsorbed PAH molecules into the aqueous phase. Chelators may also operate by reducing the degree of cross-linking in the SOM phase, thereby accelerating PAH diffusion.


Influence of Polymer Induced Depletion Forces on the Nucleation Kinetics of Protein Solutions

Amit Kulkarni and Charles Zukoski, University of Illinois, Urbana-Champaign, Department of Chemical Engineering, 114 RAL, Box C-3, 600 S. Mathews Ave., Urbana, IL 61801

We investigate the influence of polymer on the strength of protein interactions and protein crystal nucleation kinetics. The strength of protein interactions induced by the presence of polymer is characterized through the protein solution second virial coefficient, B2. Working with model proteins lysozyme and bovine serum albumin (BSA) and non-adsorbing polymer poly (ethylene glycol) (PEG), we observe significant decrease in the protein solution B2 as a function of polymer concentration indicating strong depletion attractions. The magnitude of these interactions is largely set by the polymer concentration while the extent of the attraction is determined by the concentration correlation length (Kulkarni et. al. J. Chem. Phys. 113, 9863, 2000). Protein crystal nucleation kinetics are characterized through nucleation induction time measurements performed using a micro-scintillation setup (A. Kulkarni & C. Zukoski, J. Cryst. Growth, in press, 2000). Protein solution supersaturation, S = C/Ceq(T), is varied in two ways: by changing concentration C and by varying temperature. It is observed that the induction time is a function of supersaturation only. The experimental results are well captured by classical nucleation theory, which is then used to convert induction times to nucleation rates. Significant enhancement in nucleation rates is observed in presence of PEG. In addition to PEG, we also investigate the effect of solution variables such as salt type, ionic strength and additives (glycerol, MPD etc…) on nucleation induction times. Substantial differences are observed as the strength of protein interactions is altered by using various solution conditions. We present a detailed investigation linking the strength of protein interactions to nucleation kinetics.


Effects of Interparticle Interactions on the Adsorption of C12TAB to Silica in Electrolyte Solutions

William J. Lokar, William Ducker, Virginia Tech Department of Chemistry, Blacksburg, VA 24061

Adsorption of surfactant molecules to colloidal particles is important in determining the stability of colloidal suspensions. In particular, changes in adsorption with particle separation cause large changes in interparticle forces. Atomic Force Microscopy was used to measure the force between a silica particle and a flat silica plate at constant surfactant chemical potential. Force curves were obtained in electrolyte solution as a function of separation and chemical potential. Application of a thermodynamic relation allows force as a function of chemical potential to be converted to adsorption as a function of particle separation. The mechanism of adsorption of dodecyltrimethylammonium bromide (C12TAB) during interparticle interaction is discussed.


The Effect of Charge Regulation on the Interactions between Adsorbed Polyelectrolyte Layers

Nily Dan, Department of Chemical Engineering, Drexel University, Philadelphia, PA 19104

The properties of adsorbed polyelectrolyte layers have been investigated extensively, both theoretically and experimentally. Yet, little is known regarding the interactions between layers of weakly charged polymers whose charge density varies with system parameters, i.e., the electrostatic potential. In this study we use a simple mean field model to examine the interactions between adsorbed layers of a weak polyelectrolyte. To eliminate the effect of bridging interactions and/or variations in the surface coverage we focus on polymer layers adsorbed onto uncharged surfaces, and on distances that are larger (or equal to) twice the layer thickness. We find that, as the surfaces approach each other, the net polymer charge decreases. This decrease leads, in certain cases, to an effective attraction between the two surfaces. This attraction is not due to bridging (Dahlgren, et al 1993) or to overlap between the adsorbed layers (Borukhov, et al 1999) but to charge regulation, induced by the confinement of the polymer counterions.


Stability Study of Colloidal Particles in the Presence of Oppositely Charged Polyelectrolytes

Wei Li Yu, Frédéric Bouyer, Michal Borkovec, Department of Chemistry, PB. 5814, Clarkson University, Potsdam, NY, 13699

The flocculation of colloidal particles with adsorbed polyelectrolytes is an essential step in many important industrial and environmental processes. Despite the interplay between these materials has been studied for a long time, aggregation of colloidal particles in the presence of oppositely charged polymers is not yet properly understood. The mechanism of the interaction between colloidal particles with adsorbed polymers seems to be influenced by many factors, including the structure, the dosage and the charge density of the adsorbed polymers. The results of the present study reveal that the mechanism of the interaction between the polymer adsorbed colloidal particles may vary from charge neutralization model to bridging model. Meanwhile, the surface charge heterogeneity also plays a very important role.


Temperature Dependence of Carbon Black Aggregation In Oil

S. Meeker,1 V. Trappe,1,2 N. Diggs,3 J. Emert3 and D. A. Weitz1, 1Dept. of Physics and Division of Engineering and Applied Science, Harvard University, Cambridge, MA 02138, USA, 2Current address: Dept. of Physics, University of Fribourg, Fribourg, Switzerland, 3Infineum USA LP, Linden, NJ USA

Suspensions of carbon black in oil, sterically-stabilized with adsorbed dispersant, are commonly used as model systems for investigating the soot-handling characteristics of motor oils. They also serve as model systems for studying colloidal aggregation. The structure of the carbon black aggregates, and consequently the suspension rheology, changes dramatically as the temperature of the sample is increased. At low temperatures the clusters are compact and separate, and a fluid-like rheology is observed. At high temperatures the clusters become more tenuous, forming a sample-spanning network with a solid-like rheology. Clearly the interparticle attractions increase as the temperature is raised. In order to better understand this behaviour, we investigate the effect of temperature on the repulsive portion of the interparticle interaction potential due to the stabilizing effect of the dispersant. We measure the amount of dispersant adsorbed on the surface of carbon black particles, as a function of temperature, by filtration and FTIR spectroscopy. We also investigate possible changes in the thickness of the stabilizing layer, caused as the solvent quality for the dispersant tails change with temperature. Other temperature-dependent contributions to the interparticle potential are discussed.


The Effect of Polymer Chain Architecture on the Adsorption and Dispersion Properties of Functionalised Oligomers

Robin Mogford, Brian Vincent, School of Chemistry, University of Bristol, Cantock’s Close, Bristol, BS8 1TS, UK., Steve Harley, BP Chemicals, BP Naperville Complex, Mail Code C5, 150 West Warrenville Rd, Naperville IL 60563, USA., Dave Moreton, Lubrizol International Laboratories, PO BOX 88, Belper, Derby, DE56 1QN, UK.

The internal combustion engine is a tough and unforgiving environment. Carbonaceous particulates form in the lubricating oil as by products of the combustion process. These particulates can be dispersed within the oil by the addition of derivatised polyisobutylenes. The adsorbed polymer molecules are believed to anchor to the surface of the carbon particles through their polar moieties, with the hydrocarbon chains extending into the solution. Previous research has suggested that mechanism of stabilisation is through a combination of steric and electrostatic forces. In this work we have gained a better understanding of the dispersion process, which allowed us to design better dispersants in the form of hydrophilic end-capped oligomers consisting of polyisobutylene, polyethylene, atactic polypropylene and polydecene. The adsorption kinetics and layer thickness of these functionalised oligomers have been investigated by ellipsometry at the carbon/n-heptane interface and the surface charge characteristics studied by phase analysis light scattering.


Modification of Metal Carbide Tribology Through Reactive Adsorption of Small Molecules

Scott S. Perry, B. I. Kim, S. Lee, R. Guenard, L.C. Fernandez-Torres, Department of Chemistry, University of Houston, Houston, Texas 77240-5641, Peter Frantz, Stephen V. Didziulis, Materials Science Department, Space Materials Laboratory, The Aerospace Corporation, El Segundo, California 90245

The frictional properties of the (100) face of vanadium carbide have been measured with atomic force microscopy (AFM) as a function of ethanol exposure under ultrahigh vacuum conditions. Although exhibiting a small sticking coefficient, ethanol reacts upon adsorption at room temperature, producing a thin chemisorbed layer. This reaction has been characterized through high resolution electron energy loss spectroscopy and temperature programmed desorption. These studies demonstrate the formation of a partially dehydrated species involving bonds to the surface through both oxygen and carbon atoms. The growth of this reaction layer film has been followed with X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and scanning tunneling microscopy. The results of these surface analytical measurements have been correlated with the frictional and adhesion properties measured with AFM. An approximate 40% reduction in frictional forces is observed upon the formation of a complete monolayer of a hydrocarbon reaction layer. Little change in the interfacial friction was detected at higher or lower exposures of ethanol. The origin of the friction reduction, which cannot be directly related to changes in interfacial adhesion, is assigned to a change in interfacial shear strength as a result of the growth of the ethanol decomposition species.


Interplay of Interfacial Behavior and System Dynamics in Stick-Slip Friction

Robert T. Schumacher, Stephen Garoff, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213

We investigate stick-slip friction in a system composed of two sliding surfaces separated by an intervening layer of adhesive material. We seek to understand the roles of system dynamics, interfacial energies, and mechanical properties of the intervening layer in this frictional behavior. The dynamical system is a string under high tension, excited by a transverse force applied by moving perpendicular to the string a 3 mm radius glass rod that has been coated with a layer of abietic acid (AA). The layer thickness is varied from about 100 nanometers to a few micrometers. We also vary the normal force and the position of the rod on the string. The velocity and frictional force at the bowing point are reconstructed from the signals from force transducers at the string's terminations. In addition, the wear track on the rod has been viewed with optical interference and fluorescence microscopy, and by AFM and SEM. At present we have established that the frictional force at the stick to slip transition depends most strongly on the AA layer thickness and the normal force. There is some evidence of brittle failure at the AA layer when the string releases from its sticking position on the rod, and melting during the slipping phase.


Simulation Studies of Dry Sliding Metallic Interfaces

J.E.Hammerberg, T.C.Germann, Los Alamos National Laboratory, Applied Physics Division, MS-D413, Los Alamos, NM 87545, B.L.Holian, P.S. Lomdahl, Los Alamos National Laboratory, Theoretical Division, MS-B268, Los Alamos, NM 87545

Large scale atomistic simulations of well-characterized interfaces can elucidate important physical mechanisms for dissipation at the nano- and meso-scale. Our interest has been in the velocity and density dependence of the frictional force for dry metallic interfaces. We shall discuss the results of large-scale molecular dynamics simulations for interfaces characterized by Embedded Atom Method and Lennard-Jones interactions. These simulations have shown the importance of a range of non-linear phenomena in describing dissipation. Important among these are dislocation nucleation and dynamics, nano/micro-structure formation, and material mechanical mixing, all of which introduce physics beyond anharmonic phonons. The high velocity behavior we have observed has a generic character leading, in general, to a decreasing frictional force with increasing velocity, which in certain cases is well described by an inverse power law in sliding velocity.

This work supported by the Department of Energy under contract W-7405-ENG-36


MD Simulation of Energy Transfer Behavior Between Two Sliding Hydroxylated Alumina Surfaces

Hongwei Xie, Tianying Yan, William L. Hase, Chemistry Department, Wayne State University, Detroit, MI48201, USA

Molecular dynamics (MD) simulations are used to investigate the friction behavior between two sliding hydroxylated alumina surfaces. The energy transfer dynamics of surface interface during the sliding process is studied by observing the changes of surface energy and temperature as the sliding proceeds. The simulations indicate that part of sliding friction work transfers into the inner energy of surface and affects surface temperature dynamically. The chemically attached hydroxylated groups reduce the surface sliding friction force. The temperature of sublayers of surfaces is calculated to investigate the mechanism of friction heat transfer from the surface to the bulk.


Molecular Dynamics Simulations of Kinetic Friction Due to Adsorbed Layers

Gang He and Mark O. Robbins, Johns Hopkins University, Department of Physics and Astronomy, 3400 N. Charles St., Baltimore, MD, 21218

Molecular dynamics simulations were used to study kinetic friction between surfaces separated by a layer of adsorbed molecules as a function of pressure, temperature, relative velocity v, concentration and surface orientation. The adsorbed molecules (third bodies) lead to a friction force per unit area t that rises linearly with pressure P, t=t0+aP, explaining both Amontons’ laws and common exceptions to them. As in earlier studies of static friction, factors such as concentration and surface orientation that are not controlled in experiment have little effect on friction. A weak logarithmic dependence of kinetic friction coefficient on v was found. Similar logarithmic variations are seen in a wide variety of experiments. We show that the logarithmic velocity dependence results from thermally activated motion of adsorbed atoms out of nearly unstable configuration, and establish a close relationship between the low velocity kinetic friction and static friction. A transition to normal viscous lubrication that occurs at very low pressures in some cases is also discussed.


Frictional Processes in Langmuir-Blodgett Films

Tim Alig, Joseph Zasadzinski, Dawn Takamoto, Department of Chemical Engineering, University of California at Santa Barbara, Santa Barbara, CA 93106

Langmuir-Blodgett (LB) films of fatty acid salts provide an ideal system to study the frictional properties of surfaces. These surfaces are chemically consistent and molecularly flat on scales greater than 100nm. Furthermore, the orientation of the surface molecules can be directly imaged by Atomic Force Microscopy (AFM). Varying the counterion and number of layers result in surfaces that differ solely in their molecular packing. Using this ideal system the frictional effects of variables such as molecular packing, tilt, and tip velocity can be explored. Before the variables of interest can be evaluated, a standard measuring technique must be established to account for differences in tip area and cantilever force constants. The defects present in these LB films have provided a novel way to approximate tip area. Furthermore, tip area has been shown to solely affect the adhesion force between tip and surface and not the inherent frictional properties of the surface-tip contact. Currently, an in situ calibration technique is being developed to account for the variation and possible asymmetry of the cantilever lateral force constant. The effects of tip velocity and wear are also being explored.


Characterization of Amphiphilic Dendrimers Thin Films by Fluorescence and Surface Probe Microscopy

Sarita V. Mello, Goudong Sui, Yuqiu Ma, Xihui Cao, Roger M. Leblanc, University of Miami, Department of Chemistry, 1301 Memorial Drive, Coral Gables, Florida 33146, USA.

Dendrimers are highly branched three-dimensional structures. They have possible application in molecular recognition, building blocks in self-assembled superstructures and biomimetic systems. A new series of amphiphilic dendrimers was synthesized. Hydrophobic chains with hydrophilic end-groups were attached to the first generation of poly(amino amide) (PAMAM) dendrimer core. The structure of the compounds has been modeled as disklike molecules with a flat hydrophobic dendrimer core surrounded by amphiphilic chains. This configuration confers a host feature, which can be used for encapsulation of medicines. Langmuir monolayers of dendrimer molecules were characterized by surface pressure and potential isotherms. Rhodamine G, a fluorecent probe, was added to dendrimer solution prior to the Langmuir monolayer formation. The fluorescent monolayer was visualized by fluorescence microscopy and spectroscopy in situ. Langmuir-Blodgett and Langmuir Schaeffer films were characterized by surface probe microscopy.


Determination of Surfactant Monolayer Structure Using X-Ray Reflectivity

Barry B. Luokkala, Stephen Garoff, Department of Physics and Center for Complex Fluids Engineering; Robert M. Suter, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213

We have developed a procedure for analyzing x-ray reflectivity data which yields reliable and objective structural information for monolayer systems. Our approach combines dynamically optimized Monte Carlo and simulated annealing techniques to fit the data by the method of least squares. For monolayer systems in which the features are not all well resolved and the signal-to-noise ratio is comparatively low, the c 2 hypersurface contains multiple statistically equivalent minima, corresponding to different physically plausible structural models. A single good fit to the data is therefore insufficient to construct a reliable model of monolayer structure. Instead, the interpretation of the data must be derived from the average and standard deviation of fitting parameters over an ensemble of statistically equivalent good fits. For high signal-to-noise data there may be a single, sharp global minimum in the c 2 hypersurface which is difficult to locate in a finite amount of computing time. In such situations the addition of noise, as an intermediate step, will broaden the c 2 minimum, making it easier to find a good fit. The added noise is then removed, and fits are performed, using the previously obtained parameter sets as starting points, to see if good fits are still obtained. The details of our method will be presented, including a number of case studies of reflectivity data for surfactant monolayers at the solid-vapor interface.


Phase Transition Behavior and Structure of Adsorbed Alcohol and Fluoroalcohol Monolayers at the Water-Hexane Interface

Mark L. Schlossman, Departments of Physics and Chemistry, Aleksey Tikhonov, Ming Li, Department of Physics, University of Illinois at Chicago, 845 W. Taylor St., Chicago, IL 60607

X-ray surface scattering and interfacial tension measurements are used to study the solid to gas phase transition in soluble monolayers of F(CF2)8(CH2)2OH and F(CF2)10(CH2)2OH adsorbed at the water-hexane interface. The monolayer breaks up into solid domains separated by a gas phase. The line tension for these solid domains is 10-9 – 10-10 N, larger than line tensions previously reported for liquid monolayer domains. Near the transition, the temperature dependence of the coverage of solid domains can be analyzed by a functional form consistent with a critical transition proposed by theory. Recent measurements on long chain n-alcohol monolayers at the water-hexane interface will also be discussed.


Adsorption of Nonionic Sugar-based Surfactants and Anionic Sodium Dodecylsulfate Mixtures on Alumina

L. Zhang, 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

Adsorption of sugar-based surfactants (n-dodecyl--D-maltoside and dodecyl polyglucoside) and sodium dodecylsulfate mixtures on alumina was investigated under various pH conditions. It was observed that at pH 6 where alumina is positively charged, there was strong synergistic effects between these surfactants, especially in the region where hydrophobic chain-chain interaction dominates the adsorption process as long as the surface is not saturated. At this pH, surfactants promote adsorption of each other and there exists mainly synergism. The strongest synergism was found when the mixing ratio was 1:1. On the other hand, at pH 11where alumina is negatively charged, the adsorption of mixtures is less than those of the individual surfactants alone. Although the anionic surfactant adsorption itself is increased due to the hydrophobic interaction with sugar-based surfactants, the presence of the anionic surfactant in the system reduces the sugar-based surfactants adsorption except in the rising part. At this pH, generally there is mainly antagonistic effect between surfactants. Practical implications of this study in processes that warrants control of adsorption, such as in enhanced oil recovery, are to be noted. The mechanism of synergism/antagonism between these surfactants under various conditions will be discussed.


Hydrophobically Modified Cellulose Polymers and SDS Interaction at the Solid-Liquid Interface

K. Derek Berglund, Todd M. Przybycien, Robert D. Tilton, Department of Chemical Engineering, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh PA 15213

The prevalence of polymer-surfactant mixtures in industrial and natural complex fluids motivates fundamental studies of multicomponent effects on interfacial phenomena. In the current work, we examine the extent and reversibility of co-adsorption for hydroxypropyl cellulose (HPC) with sodium dodecyl sulfate (SDS) and for hydrophobically modified hydroxyethyl cellulose (hm-HEC) with SDS on hydrophilic silica as well as hydrophobic polydimethylsiloxane (PDMS) surfaces. We use optical reflectometry to measure the total amount of adsorption, i.e., the amount of polymer plus surfactant at the interface. Zeta-potential measurements confirm the presence of SDS in the layers. To interpret the adsorption phenomena, we measure the critical aggregation concentration and the saturation concentration for polymer-surfactant complexation in bulk solution, using fluorescence spectroscopy and ultraviolet spectrophotometry with solubilized pyrene probes. Either in the absence or the presence of SDS, hm-HEC adsorbs more tenaciously than HEC, even on the hydrophilic silica surface. The degree of complexation between the polymer and surfactant, the processing history of the adsorbed layer and the surface energy all influence the extent of adsorption.


The Structure and Composition of Mixed Surfactants at Interfaces and in Micelles

J. Penfold, ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, UK, E. Staples, I. Tucker, Unilever Research, Port Sunlight Laboratory, Quarry Road East, Bebington, Wirral, UK, R. K. Thomas, Oxford University, Physical and Theoretical Chemistry, South Parks Road, Oxford, UK

Specular neutron reflectivity and small angle neutron scattering have been used to investigate the nature of surfactant mixing at the air-water, liquid-solid, oil-water interfaces, and in micelles. Recent experimental results for the composition of non-ionic / anionic surfactant mixture of hexaethylene monododecyl ether (C12E6) and sodium dodecyl suphate (SDS) at the air-aqueous solution (1), aqueous solution-solid (2), hexadecane-aqueous solution (3) interfaces and in micelles (4) are compared with theory over a wide range of solution composition and concentration. Structural information on the adsorbed mixed surfactant layer at interfaces is presented. The variation in the aggregation number and structure of mixed surfactant micelles with composition and concentration is compared with recent theories, and the modification of the structure by the solubilisation of hexadecane is discussed.

(1) J Penfold, E Staples, L Thompson, I Tucker, J Hines, R K Thomas, J R Lu, Langmuir 11 (1995) 2496

(2) J Penfold, E Staples, I Tucker, L Thompson, R K Thomas, Int J Thermophys 20 (1999) 19

(3) E Staples, J Penfold, I Tucker, J Phys Chem B (2000) 104 606

(4) J Penfold, E Staples, L Thompson, I Tucker, J Hines, R K Thomas, J R Lu, N Warren, J Phys Chem B (1999) 103 5204


Synergistic Adsorption at the Hydrophobic Solid/Liquid Interface from Mixed Polyelectrolyte/Surfactant Solutions: A Sum Frequency Spectroscopic Study

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

The ordering of the anionic surfactant sodium dodecyl sulfate (SDS) on adsorption at the hydrophobic solid/aqueous solution interface was studied as a function of the addition, both individually and in combination, of the cationic polyelectrolyte poly(ethylene imine) (PEI) and the univalent electrolyte sodium chloride (NaCl) to solution. The non-linear vibrational technique of Sum Frequency Spectroscopy was employed to determine the degree of alkyl chain ordering of the adsorbed surfactant through comparison of the strengths of the symmetric methyl and methylene stretching modes. The electrolyte was found to promote the onset of ordered SDS adsorption at lower bulk concentrations and to result in a more highly ordered layer at any given SDS concentration than occurred in its absence. Introduction of PEI to electrolyte free SDS solutions was found to promote synergistic SDS adsorption through polyelectrolyte/surfactant complexation. Adsorption occurred at lower surfactant concentrations and resulted in more highly ordered adsorbed surfactant than did the addition of NaCl at all surfactant concentrations. The addition of NaCl and PEI in combination resulted in highly co-operative complexation of SDS and PEI and produced maximal ordering of the SDS which was independent of the surfactant concentration and the concentration of added electrolyte.


Synaptic Pattern Formation During Cellular Recognition

Arup K. Chakraborty, University of California, Berkeley, 201 Gilman Hall #1462, Berkeley, CA 94720-1462

Cell-cell recognition is ubiquitous in biology, and often requires the formation of a highly organized pattern of receptor proteins (a synapse) in the intercellular junction. Recent experiments vividly demonstrate a complex evolution of cell shape and spatial receptor-ligand patterns (several microns in size) in the intercellular junction during immunological synapse formation. The current view is that this dynamic rearrangement of proteins into organized supramolecular activation clusters in the intercellular junction is driven primarily by active cytoskeletal processes. Aided by the first statistical mechanical analysis of the relevant physico-chemical processes, we demonstrate that the essential characteristics of synaptic patterns observed in living cells can result from spontaneous self-assembly processes. Active cellular interventions are superimposed on these self-organizing tendencies. We show that the protein binding/dissociation characteristics, protein mobilities, and membrane constraints measured in the cellular environment are delicately balanced such that the length and time scales of spontaneously evolving patterns are in near-quantitative agreement with experimental observations for synapse formation between T-cells and supported membranes. The framework we present provides a common way of analyzing synapse formation in disparate systems (e.g., T-cells, NK-cells, etc.).


Monte Carlo Simulation of Interfacial Effects on Surfactant Mesophases

S.E. Rankin, University of Kentucky, Chemical and Materials Engineering, 177 Anderson Hall, Lexington, KY 40506-0046, A.P. Malanoski, University of New Mexico, Chemical and Nuclear Engineering Department, Advanced Materials Laboratory, 1001 University Blvd. SE, Suite 100, Albuquerque, NM 87106, F. van Swol, Sandia National Labs / University of New Mexico, Advanced Materials Laboratory, 1001 University Blvd. SE, Suite 100, Albuquerque, NM 87106

In work inspired by surfactant templating of nanostructured ceramics, we examine some features of evaporation-driven self-assembly. Our lattice Monte Carlo simulations of ternary amphiphile-solvent mixtures are part of a software package integrating Monte Carlo and density functional techniques. Our simulations allow not only simulations of bulk surfactant mesophases (e.g., lamellae and hexagonal cylinders), but also allow us to approximate interfacial and nonequilibrium phenomena. The amphiphile chains are modeled with a fluctuating bond model, with chain kink, reptation and biased regrowth moves. Both fixed boundaries (walls) and free boundaries (vapor-liquid interfaces) may exist, allowing us to examine mesophase formation in thin film, fiber, and aerosol particle geometries. A novel feature of the simulations is the imposition of composition and chemical potential gradients by defining three contacting control volumes - two open and a third open only to the other two volumes. Both attractive interfaces and concentration gradients orient mesophases (such as lamellar sheets) parallel to themselves. Inert surfaces and free interfaces orient mesophases perpendicular to themselves. Both types of interfaces can either accelerate or delay mesophase nucleation and transitions. If time permits, we will discuss how structured or curved (as in colloidal crystal templates) surfaces influence mesophase formation and orientation.


Electrostatics in Many-Body Systems: Charge Density and Point-Wise Force Calculations

A. Khachatourian and Anders Wiström, Department of Chemical and Environmental Engineering, University of California, Riverside, CA 92521

An important probem in colloidal research concerns the determination of electrostatic fields in concentrated dispersions consisting of randomly distributed particles suspended in a solvent of uniform composition. Important quantities which can be obtained from the electrostatic calculations include the electrical transport properties of the composite as well as point-wise values of stress fields in the bulk and at grain boundaries. Time and space resolved structural information would complement thermodynamic information for producing as complete picture of colloidal systems as possible. Despite methodological advances there are virtually no accurate large-scale numerical simulations of dense random dispersions in three dimensions. We report on the analytical formulation of the surface charge density distribution of a particle suspended in a random assembly of charged particles which may be close-to-touching. The framework for the mathematical description of the many-body problem that allows for pointwise evaluation of local properties is provided in a form making the numerical evaluation of concerned many-body systems tractable. We present results from charge density distribution calculations in both ordered and random systems and discuss the resulting interparticle forces taken as the point-wise estimate of the local stress.


Structure and Energetics of DNA in a Viral Capsid

James T. Kindt and William M. Gelbart, Department of Chemistry and Biochemistry, UCLA, Box 951569, Los Angeles, CA 90036-1569.

Simulations of a semiflexible bead-and-spring polymer loaded into a small spherical cavity have been performed to gain insight into the mechanics of DNA loading and packaging in the protein shell of bacterial viruses. The influence on the structure and topology of the confined chain of the length of chain loaded, the effective interchain attractions (as brought about by condensing agents), and attraction to the inner surface of the capsid have been studied. The results are compared with available experimental data on the structure of DNA in bacteriophages, and predictions are made for future experiments aimed at measuring the force necessary to load the chain and the pressure exerted on the capsid interior wall.


Dynamic Self-Consistent Field Theory of Lattice Models of Polymer Fluids

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

As external stresses drive polymer fluids out of thermodynamic equilibrium, equilibrium models based on minimization of free energies become inadequate. We present here a derivation of dynamic self-consistent field (DSCF) evolution equations for segmental volume fractions and local momenta in a lattice model of polymer fluids below entanglement molecular weight. Similarly to the convective-diffusive lattice-gas (CDLG) model of dynamics in simple fluids (Khan and Shnidman, Prog. Coll. Polym. Sci. 103, 251 (1997)), DSCF describes transport of chain segments subject to local conservation of mass, species and momentum. However, DSCF couples chain conformation statistics to the segmental transport equations using a matrix generator similar to that originally introduced in the equilibrium self-consistent field (SCF) lattice theory of Scheutjens and Fleer, J. Phys. Chem. 83, 1619 (1979)). DSCF reproduces SCF results under equilibrium conditions, and reduces to CDLG when for chains composed of a single segment. Time-temperature superposition is recovered by assuming a Vogel-Fulcher-Tammans-Hesse relaxation of the free volume. A systematic DSCF study of rheology, segmental statistics, and morphology in homo- and block co-polymer fluids possessing translation symmetry within layers parallel to the plates, can be found in a companion presentation by Mihailovic and Shnidman at this meeting.


Monte Carlo Simulation of Mixed Solutions of Silanes and Surfactants in the Presence of a Charged Polysilicon Surface

Vivek Kapila, Janna M. Harris, Pierre A. Deymier, Srini Raghavan, University of Arizona, Department of Materials Science & Engineering, Tucson, AZ

The stiction of microstructures is a critical problem in the fabrication of silicon based microelectromechanical structures (MEMS) made by surface micro machining techniques. This problem is often overcome by the deposition of a hydrophobic film on the polysilicon surfaces. The current technology requires these hydrophobic films be deposited from organic solvents, causing environmental concerns. Recently, attempts have been made to develop techniques for the deposition of hydrophobic films from aqueous solutions of silanes and surfactants to avoid the organic wastes. The development of efficient coating methods based on aqueous chemistries depends on the molecular level understanding of the coating solution. An attempt has, therefore, been made to model and simulate these solutions using Monte Carlo techniques. A two dimensional, square lattice model has been used and appropriate interaction parameters between different chemical groups have been determined empirically. The Monte Carlo simulations have been used to study the evolution of structures of different species in the solutions of organosilanes and cationic surfactants and interaction of these chemical species with a charged polysilicon surface has been investigated. Also, the effect of the nature of surface (rough or smooth), and the distribution of charge on the surface (random or homogeneous) on the structure of films forming on the polysilicon surfaces is studied.


Macromolecular Surfactants

Frank S. Bates, University of Minnesota, Minneapolis MN 55455

Block copolymers belong to a broad class of amphiphilic compounds that includes lipids, soaps, and nonionic surfactants. A macromolecular architecture affords certain unique advantages over conventional low molecular weight amphiphiles in controlling the phase behavior, structure and properties of two and three component mixtures. These features will be illustrated in aqueous and non-aqueous systems, with applications ranging from drug delivery to toughening of thermosetting epoxy.


Control of the Phase Behavior and Structure of Amphiphilic Block Copolymers by the Addition of Cosolvents or Cosolutes

Paschalis Alexandridis, Lin Yang, Yining Lin, Bradley Songui, Aristides Docoslis Department of Chemical Engineering, University at Buffalo - SUNY, Buffalo, NY 14260-4200.

A case study on solvent quality effects on the self-assembly of amphiphiles is presented for polyether [poly(ethylene oxide)-poly(propylene oxide): PEO-PPO] block copolymers in mixtures of water and cosolvents such as ethanol, formamide, or glycerol, or cosolutes such as sodium halides. Our study (i) probes the block copolymer organization in both micellar solutions and lyotropic liquid crystals, (ii) is concerned with both the type of structure formed and its characteristic dimensions, and (iii) combines macroscopic observations (e.g., composition-temperature phase boundaries) with microscopic (i.e., small-angle neutron and X-ray scattering) measurements. The various findings are discussed in terms of preferential adsorption (or depletion) of the cosolvent or cosolute in the water-swollen block copolymer chains, and are rationalized in terms of the solvent polarity relative to that of water and of the two blocks, PEO and PPO, of the polyether copolymer.


Self-Assembly of Block Copolymer Solutions Across the Complete Concentration Range

Timothy P. Lodge and Kenneth J. Hanley, Department of Chemistry and Department of Chemical Engineering & Materials Science, University of Minnesota, 207 Pleasant Street SE, Minneapolis, MN 55455-0431

We have investigated the self-assembly of block copolymer solutions as a function of concentration, copolymer molecular weight and composition, temperature, and solvent selectivity. The systems examined contain poly(styrene-b-isoprene) diblock copolymers diluted with dialkyl phthalates. These solvents provide a convenient means to tune the selectivity; for example, dioctyl phthalate is thermodynamically neutral at all temperatures, whereas di-ethyl phthalate is strongly styrene-selective at room temperature, but effectively neutral above 150 oC. The resulting phase prisms show a rich polymorphism, including lamellar, hexagonal, gyroid, bcc, fcc, micellar, and free chain solutions. The main features of the phase prisms can be understood via well-established block copolymer principles, but some unanticipated results still await a quantitative explanation.


Amphiphilic Block Copolymer Frozen Micelles Dispersed in Water: Controllable Anisotropy and Switchable Morphologies

Denis Bendejacq, Virginie Ponsinet, Mathieu Joanicot, CNRS-Rhodia, Complex Fluids Laboratory, 259 Prospect Plains Road, Cranbury, NJ 08512, USA, Jyotsana Lal, IPNS, Argonne National Laboratories, 9700 S. Cass Avenue, Argonne, IL 60439, USA

The morphological phase diagram of poly(styrene -b -acrylic acid) is found to present spherical, cylindrical and lamellar structures depending on the composition of the diblock. When the hydrophobic block is the minority component, we show with small-angle X-ray and Neutron scattering experiments that the organized spherical, cylindrical or lamellar systems can be swollen in water, keeping a long-distance order on a wide range of concentration and with no morphological transition at room temperature. We obtain dispersions of colloidal objects, called frozen micelles, composed of a glassy core of controlled shape, surrounded by a swollen polyelectrolyte brush. These objects are visualized by transmission electron microscopy. Upon heating above the glass transition temperature of the polystyrene core, the system is allowed to relax towards its equilibrium morphologyin water, which is spherical at low enough concentration for all the diblock compositions studied in our experiments. The dispersions are therefore switchable with temperature from shape-monitored metastable to equilibrium colloids. We will discuss the evolutions of the shape and size of the cores and of the surface density of the brushes, in the frozen and the at-equilibrium states, when the length and the composition of the copolymers are varied.


Nanostructure in Dendritic/Linear Copolymers

B. M. Tande & N.J. Wagner, Center for Molecular and Engineering Thermodynamics, Chemical Engineering, University of Delaware, Newark, DE 19716 (USA), M. Jeong & M.E. Mackay, Stevens Institute of Technology, Hoboken, NJ 07030 (USA) R. Vestberg & C.J. Hawker, IBM Alamaden Research Center, 650 Harry Rd., San Jose, CA 95120 (USA)

We examine the phase behavior and structure of a unique class of copolymers consisting of dendritic polybenzyl ether head groups grafted to linear chains of deuterated polystyrene. A series of hybrids with varying head group generation (4-6) and tail molecular weights (deuterated) were synthesized and blended with hydrogenated polystryrenes of varying Mw. Small angle neutron scattering (SANS) studies were carried out to determine the structure of these molecules both in solution and blended with polystyrene in the melt. The deuterated tails’ radius of gyration in hydrogenated polystyrene for the hybrids with 4th and 5th generation dendritic head groups are observed to be nearly Gaussian. A "macro-surfactant" effect is observed with 6th generation hybrids where the dendritic head groups self assemble into domains with relatively sharp interfaces. The resultant microstructure depends on the molecular weights of both the polystyrene tail and of the matrix polystyrene. Interestingly, when the hybrids are dissolved in the solvent THF, a more compact molecular structure is observed for all generation dendrimers. Also, no evidence of phase separation is seen for the conditions used. The SANS structural data is used to determine the parameters controlling the formation of the microphase domains. Thermodynamic modeling is employed within the context of surfactant theories and compared to the experimental results.


Polyelectrolyte Mediated Interaction Between Similarly Charged Surfaces: Role of Divalent Counterions in Tuning the Surface Forces.

T. Abraham*, A. Kumplainen#, Z. Xu* and J. Masliyah*, *Department of Chemical & Materials Engineering, University of Alberta, Edmonton, Canada and #Royal Institute of Technology and Institute of Surface Chemistry, Stockholm, Sweden

The surface forces apparatus was used to study the role of divalent counterions (Ca and Mg) in promoting the adsorption of weakly charged polyelectrolyte (Polyacrylic acid, PAA, Mw~250,000g/mol) on mica surfaces. In the presence of 3 mM Mg at pH 9.5 (system I), only short range hydration forces (extending up to ~40 Å) was measured, suggesting a complete charge neutralization of mica surfaces. In con2trast, 3mM Ca at pH 9.5 (system II) induced a weak long range repulsive interaction (extending up to ~300 Å) and a short ranged attractive force. Addition of 10ppm PAA to system I induced a long range bridging force (~300 Å) and a short range steric force. It appears that the weak adsorption of PAA on mica surfaces presumably driven by low surface potential and partial screening of PAA chains by Mg++ ions resulted in a loop and tail conformation that gives rise to strong intersurface bridging forces. At a higher PAA concentration (50ppm), the force profile became purely repulsive, extending up to 400 Å due to the build-up of polymer chains on the surfaces. On the contrary, the addition of 10 ppm PAA to system II showed a combination of force profiles: long ranged steric forces extending up to 400 Å, short ranged bridging forces and shorter ranged steric forces. The long ranged steric forces can be attributed to strong adsorption of screened polyelectrolyte chains on mica surfaces presumably due to excess Ca++ (and therefore +ve charges) on the mica surfaces. The short ranged bridging forces can be ascribed to Ca++ assisted bridging between the adsorbed polyelectrolyte chains. It is also interesting to note the dynamic effects of the system, indicating various relaxation processes associated with adsorbed large polymer chains. In conclusion our study showed that divalent counter ions (Mg++and Ca++) exhibit significantly different behavior in respect of promoting the PAA adsorption on mica surfaces, modifying and controlling various surface interactions.


Characterization of Surface Structure on Bitumen Droplets in Water

Alex Wu, Jan Czarnecki: Syncrude Canada Ltd., Edmonton Research Center, 9421 - 17 Ave., Edmonton, Alberta, Canada T6N 1H4, Isabelle Laroche, Jacob Masliyah: Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada T6G 2G2

Cryo-SEM study revealed that the surface of micron-sized bitumen (a heavy crude oil) droplets in water is extremely rough. The origin of the roughness was investigated by comparing the SEM images of deasphalted bitumen surface with whole bitumen surface. Topography of the rough bitumen surface was characterized based on colloidal force data determined directly using hydrodynamic force balance technique. The force data were interpreted using a multiple spherical cap model. The height of the roughness was found to be in the range of 1 - 60 nm.


The Effect of Surface Structure on the Properties of Simple Fluids Confined between Clay Mineral Surfaces

Joan E. Curry and Zhen Su, Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85721

Monte Carlo and molecular dynamics simulations are used to study simple fluids such as cyclohexane and octamethylcyclotetrasiloxane (OMCTS) confined between model clay mineral surfaces.


The Effects of Thin Films on Dynamic Wetting

Xia Chen, Stephen Garoff, Dept. of Physics, Center for Complex Fluids Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, Enriqué Ramé, National Center for Microgravity, NASA, Cleveland, OH 44135

The effects of thin fluid films on the hydrodynamics of macroscopic fluid spreading over solid surfaces are studied. We measure the interface shape within microns of moving contact lines and compare the measurements to two models in the limit of small capillary number, Ca. One model is based on Landau and Levich’s theory and has an asymptotically matched solution. The other model has a numerical solution of the Young-Laplace equation augmented to account for viscous stresses. Both models describe the film as Newtonian. In the experiments, we advance a contact line over pre-existing films with thicknesses from 7Å to 10 m m. For films from 0.8 m m to 10 m m, interface shapes fit the theories in the small Ca limit. Furthermore, the interface shapes depend only on Ca and not on the materials. Thus, for this thickness range, molecular properties of the film are unimportant in the spreading process. We also explore the wetting behavior of the fluid advancing over films of much thinner films where molecular properties may become important and probe the transition from a dry surface to a thick hydrodynamic film.


Effect of Hydrosoluble Polymer on the Entrained Film Thickness of Rigid Soap Films

Eric A. Adelizzi and Sandra M. Troian, Dept. of Chemical Eng., Princeton Univ. 08544-5263

Frankel’s Law (1959) predicts the thickness of a rigid soap film drawn from a reservoir at constant speed according to the relation ho=1.89d(Ca)2/3. Here, d denotes the capillary length and Ca the capillary number based on the speed of film pullout. This relation, akin to the Landau-Levich result for film entrainment on a wetting substrate, predicates a Newtonian liquid strictly draining under capillary and viscous forces within the lubrication regime. Recent light reflectivity measurements indicate, however, that the addition of low concentration, high molecular weight polymer can drastically change the dependence on the capillary number. The cause of this deviation is not yet understood. Exploratory hydrodynamic models of film thinning including power law or viscoelastic behavior have also been unsuccessful in reproducing experimental results. We present a systematic study of the entrained film thickness as a function of surfactant concentration, polymer concentration and molecular weight. The addition of hydrosoluble polymer appears to decrease the entrained film thickness but prolong film lifetime. We review various constitutive models for non-Newtonian films in an effort to incorporate the effect of polymer-surfactant complexation.


Wetting Behavior of Silicone Oils on Low Energy Solid Substrates Submerged in Aqueous Media.

T. F. Svitova and C. J. Radke, Chemical Engineering Department, University of California, Berkeley, CA 94720-1462

Contact angles of silicone oils (PDMS) have been studied on low-energy substrates, immersed in aqueous media. Silicone oils in air spread completely on all of the substrates studied. When immersed in aqueous media, silicone oils do not spread completely on these substrates, but form droplets with finite contact angles that strongly depend on the substrate polarity. The influence of 1-1 (KCl) and 1-2 (CaCl2) electrolytes on contact angles of silicone oil droplet on solid substrates immersed in aqueous media has been investigated. KCl does not produce noticeable effects on wetting behavior of silicone oils, whereas the presence of CaCl2 in general increases oil contact angle, with the exception of the Chitin-coated silicon substrate. In this case the oil contact angle decreases with CaCl2 concentration, indicating that there is some specific interaction between polymer and Ca-ions, influencing the wettability of this substrate by silicone oil. The detailed investigation of pH influence on wetting behavior of silicone oil on solid substrates has shown that Bartell-Osterhof equation rather accurately predicts interfacial wetting behavior of oil on solid substrates of different polarity and functionality, provided with accurate measurements of aqueous phase static receding angles and interfacial tensions.


Impact of Head Group and Counterion Structure on Flexibility and Dynamics of Wormlike Micelles in Aqueous Solution

L.J. Magid and Z. Li, Dept. of Chemistry, Univ. of Tennessee, Knoxville, TN 37996-1600; P. Butler, Neutron Scattering Section, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6393.

Micellar morphology and solution rheology in aqueous micellar solutions of cationic surfactants such as cetyltrimethylammonium (CTA+,X-) or cetylpyridinium (CPy+,X-) surfactants can be elegantly manipulated by changing the counterions, X-. Some counterions that penetrate the micellar interface, especially substituted chloro- and hydroxybenzoates and tosylate, produce viscoelastic aqueous micellar solutions containing giant semi-flexible wormlike micelles that overlap and entangle. Small-angle neutron scattering (SANS) is used to study the changing micellar structure and interactions on multiple length scales, including contour lengths, persistence lengths (bending moduli), cross-sectional radii and areas per head group at the micellar surface. Neutron spin echo (NSE) spectroscopy is used to study local dynamics of the micellar mesh in semidilute solution. The role of counterions, head group identity, surfactant chain length and ionic strength in determining micellar flexibility and dynamics is discussed.


Neutron Spin Echo Measurements of Micellar Motions

Steven R. Kline, NIST Center for Neutron Research, 100 Bureau Drive, Stop 8562, Gaithersburg, MD 20899-8562.

The surfactant CTVB (cetyltrimethylammonium 4-vinylbenzoate) forms long entangled rodlike micelles in solution (ca. 1 - 5 wt % in D2O) which display many useful physical properties (viscosity, elasticity, etc). The timescales of these motions are expected to be similar to that of traditional polymer systems. The 4-vinylbenzoate counterion can be polymerized on the micelle, producing a much shorter micelle (approximately 60 nm long) with superior stability. SANS measurements confirm the cylindrical structure of the polymerized micelles, which remain isotropic up to concentrations of 20 wt % of micelles. Polymerized and unpolymerized micellar diffusion was measured using Neutron Spin Echo (NSE) over a q-range of 0.045 A-1 < q < 0.15 A-1 and a time range of 0.1 ns to 40 ns. The motion of the unpolymerized micelles obeys a stretched exponential, analogous to a semi-flexible polymer. The polymerized micelle behaves as an isolated particle, appears to be much "stiffer" than the unpolymerized version and displays a distinct power-law scaling of the effective q-dependent diffusion constant as a function of micelle volume fraction.


Shear-Induced Micellar Growth in Dilute Surfactant Solutions

J.-F. Berret, Complex Fluids Laboratory, CNRS, Cranbury Research, F. Molino, Unité Mixte de Recherche CNRS / Université de Montpellier II n° 5581, Groupe de Dynamique des Phases Condensées F-34095 Montpellier Cedex 05 FRANCE, and P. Lindner, Institut Laue-Langevin,

BP 156, F-38042 Grenoble cedex 9 FRANCE

The shear-thickening transition observed in aqueous solutions of cetyltrimethylammonium tosylate (CTAT) is investigated using rheology and small-angle neutron scattering under shear. Above a critical shear rate, the increase of the apparent shear viscosity is due to the formation of a shear-induced phase. Using a Porod representation to analyze the aggregate local morphology under shear, we first demonstrate that the shear-induced viscous state consists of cylindrical micelles strongly aligned in the flow. We also investigate the shear rate and surfactant concentration dependencies of the structrure factor peak originating from electrostatic interactions between cylindrical micelles. In dilute solutions of short rod-like micelles, the systematic shift of the structure factor peak to lower wave-vectors is interpreted in terms of the growth of the micellar aggregates.


Rheology and Dynamics of Mixed Systems of Wormlike Micelles and Nonionic Polymer

M. T. Truong, L. M. Walker, Center for Complex Fluids Engineering, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213

The rheology of wormlike micelles is extremely rich and has led to many proposed applications of these materials. Unfortunately, this rich rheological behavior is not fully understood. One of the goals of this work is to utilize the physicochemical properties of an added polymer to effectively control the rheology of the wormlike micellar system of cetyltrimethylammonium p-toluenesulfonate (CTAT). Our goal is to separate the influence of polymer-surfactant binding from polymer-micellar interactions. The coupling between macroscopic behavior and local micellar structure is quantified through a combination of rheology and small angle neutron scattering (SANS). We have demonstrated that subtle changes in the structure and hydrophobicity of the added nonionic polymer can lead to significant changes in macroscopic rheology. The hypothesis that drives our present research is that the position of hydrophobic moieties on the backbone of a nonionic polymer alters local micellar structure and interactions and leads to changes in rheology.

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