Oral Presentation Abstracts

Talks 101 through 150

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101

Modeling Shear Thickening in Pipe Flow of Wormlike Micellar Solutions

J:E. Puig1, J.F.A. Soltero1, F: Bautista1, E. Macias1, O. Manero2 and P. Attané3, 1Departamento de Ingeniería Química, Universidad de Guadalajara, Guadalajara, jal. MÉXICO, 2Instituto de Investigaciones en materiales, UNAM, México, D.F., MÉXICO, 3Labororatorie de Rhéologie, Universite J. Fourier, Grenoble, FRANCE

Dilute solutions of wormlike micelles exhibit shear thickening due to changes in microstructure induced by shear flow. Here, we examine by flow visualization with a PIV apparatus, shear thickening in pipe flow of wormlike micellar solutions. A simple model consisting of the Codeformational Maxwell constitutive equation coupled to a kinetic equation that accounts for the breaking and reformation of the micelles, is used to reproduce the experimental data. The model is capable to predict the main features of the shear stress-shear rate flow curve in simple shear as well as the instability region where the shear-induced microstructures, responsible for the shear thickening, form. Moreover, the model predicts an induction time for the development of shear thickening, in the sense that shear thickening is only observed for tube lengths many times its diameter. For short L/D ratios, the model predicts Newtonian behavior, in agreement with experimental results.


102

Viscoelastic Fluids Formed by Synergistic Self-Assembly in Surfactant Mixtures

Srinivasa R. Raghavan and Eric W. Kaler, Department of Chemical Engineering, University of Delaware, Newark, DE 19716

The self-assembling character of a surfactant mixture is enhanced when there is a net attractive interaction between the surfactants. Due to these synergistic interactions, highly structured fluids can form at low total surfactant concentration, and such fluids often show interesting viscoelastic character. This will be illustrated for two classes of surfactant mixtures: (a) anionic/cationic; and (b) anionic/nonionic. In the first case, attractive interactions between the oppositely charged headgroups promote the growth of long wormlike micelles in the mixtures. A crucial factor is the size of the hydrophobic moiety in the two surfactants - by rational choice of hydrophobe size, self-assembly can be promoted while minimizing undesired effects such as phase separation. Synergistic effects can also occur for mixtures of anionic and nonionic surfactants, leading to isotropic gel-like materials in some cases at ~ 5% total surfactant. These materials display a yield stress, and their rheology is very different from that of wormlike micelles. The microstructure in these materials is investigated using a combination of rheology, scattering, and microscopy.


103

On the Molecular Transport of Volume

Howard Brenner, Department of Chemical Engineering, MIT, Cambridge, MA 02139-4307

Molecular transport and production of volume within flowing liquids is addressed within the context of traditional transport phenomena balance equations. Specifically, a novel convective/diffusive/'creation'-type continuum-mechanical balance equation is written for the transport of the (generally non-conserved) extensive property of volume, in much the same manner as one writes standard balance equations for mass, chemical species, momentum, and energy. Appearing in this classical-type transport phenomena equation for volume are constitutive expressions for the local molecular flux density vector and rate of production of volume, each being intensive field variables defined at every point of the flowing fluid continuum. Closely related to this notion of a volumetric molecular flux density is the concept of a volume-based velocity, different from the usual mass-based (barycentric) velocity ubiquitous to fluid mechanics. Applications are made to volume transport in multicomponent and nonisothermal systems. It is speculated (at least as of the date of this abstract deadline) that the notions advanced here regarding volumetric transport phenomena may contribute to understanding the origin of slip boundary conditions existing at solid surfaces.


104

Molecular Transport Across Phase Boundaries in Oil-Water-Polymer Systems

E.E. Meyer, B.J. Martin, M.F. Islam, L.A. Hough, H.D. Ou-Yang, Lehigh University, Physics Dept., Bethlehem, PA 18015, W. Lau, Research Laboratories, Rohm and Haas Company, Spring House, PA 19477

Molecular transport across phase boundaries in oil-water-polymer triphasic systems dictates conversion efficiency in emulsion polymerization. A new concept that uses cyclodextrin molecules as catalytic agents to facilitate the boundary crossing transport has shown great potential in extending the capability of emulsion polymerization to produce hydrophobic polymers. The intrinsically heterogeneous nature of the triphasic system hindered previous attempts to examine the molecular transport mechanisms quantitatively. The application of optical tweezers to isolate individual monomer droplets and polymer particles in suspensions, and the use of optical methods to monitor the catalytically assisted reactions in these isolated femtoliter reactors have made it possible for us to study quantitatively the molecular transport mechanisms in great detail. In this talk we will report data obtained from (1) the inclusion complexation reactions of methy-beta-cyclodextrin with alkane chains and alcohols confined in the monomer droplets, and (2) the transport of these hydrophobic monomers by cyclodextrin to the polymer particles.


105

Adsolubilization of Styrene by Nonionic Surfactants on Silica

Yongqiang Tan, John O'Haver, University of Mississippi, Dept. of Chemical Engineering, Anderson Hall, University, MS, 38677

Adsolubilization, solubilization of sparingly soluble compounds in adsorbed surfactant aggregates has demonstrated applications in many fields including composite materials, pharmaceutics, and separation processes. The adsolubilization of styrene by Polyethyoxylated alkylphenols(Triton X) series surfactants on amorphous precipitated silica was investigated. The Triton X surfactants used vary from 8.5 ethyoxylated(EO) groups to 12.5 ethyoxylated(EO) groups. The results show that at concentrations below the critical micelle concentration (CMC), adsolubilizations decreased with increasing numbers of EO groups, especially at the higher styrene loading levels. With the addition of lipophilic linkers (long chain linear alcohols) the adsolubilization of styrene increased with increasing alcohol concentration. Lipophilic linkers increase the interaction between the styrene and the hydrophobic group of the surfactants, increasing the solubilizing capacity of the adsorbed surfactant aggregates.


106

Comparison Study of the Aggregation Behavior of Sodium Dodecyl Sulfate in O/W Emulsion Using The Equilibrium Distribution Rate Constant and Reaction Rate Constant of Benzyl Acetate Hydrolysis.

Hassan Almoazen and Anthony P. Simonelli, Long Island University. Dept. Of Pharmaceutics, Brooklyn, NY 11201

We studied the hydrolysis of benzyl acetate in multiphasic system of O/W emulsion. The reaction rate constant was applied towards determining the aggregation behavior of SDS at

40 o C and pH 9. On the other hand separation of the emulsion into the aqueous and oil layer enable us to determine the equilibrium distribution rate constant between oil and the aqueous layer. As more surfactant is added into the system, it tends to adsorb at the interface of the oil droplets and the aqueous bulk till it completely cover the surface, after that the surfactant monomers tend to aggregate forming micelles or bi layers, at this point the reaction rate constant of hydrolysis of benzyl acetate will begin to change as a function of surfactant. Also, the distribution rate constant of benzyl acetate between oil and aqueous will change as the chemical potential of equilibrium in the system has changed.


107

Mass Transport Phenomena in Emulsions Containing Surfactant Micelles –Adsorption Kinetics, Solubilization and Ostwald Ripening

Jochen Weiss, University of Tennessee, Dept. of Food Science and Technology, 2605 River Road, Knoxville, TN 37996-1071; D. Julian McClements, University of Massachusetts, Dept. of Food Science, Chenoweth Lab, Amherst, MA 01003

Emulsions are thermodynamically unstable systems that tend to break down over time. Surfactants are therefore added to enhance droplet stability. Surfactants adsorb at liquid-liquid interfaces and induce repulsive forces i.e. steric or electrostatic interactions. We present experimental evidence that the introduction of surfactant monomers and aggregates in systems containing emulsion droplets that consist of low molecular weight compounds initiates a number of mass transport mechanisms such as surfactant adsorption, solubilization, Ostwald ripening and/or compositional ripening. Some of these mechanisms proceed sequentially, others proceed simultaneously, i.e. Ostwald ripening is accelerated by swollen surfactant micelles and is therefore influenced by solubilization. We highlight commonalities between these mass transport phenomena by presenting adsorption kinetic, solubilization kinetic and Ostwald ripening data for n-decane-in-water emulsion stabilized by either sodiumdodecylsulfate or polyoxyethylene (20) sorbitan monolaureate above and below the critical micellar concentration. The results indicate that the nature of the dispersed phase, the molecular properties and concentration of the surfactant monomers, the size, number and nature of the micellar aggregates and the presence of colsolvents to the continuous phase are the primary parameters that determine the extent of each mass transport process. The research confirms theoretical predictions developed by Kabalnov and coworkers.


108

Component partitioning and structural changes in alpha-lactalbumin/brine/AOT/isooctane Winsor II systems

Justin W. Shimek, Dept. of Food Science & Technology, Catherine M. Rohloff, Dept. of Chemical Engineering and Materials Science, Stephanie R. Dungan, Dept. of Food Science & Technology and Dept. of Chemical Engineering and Materials Science, University of California, Davis, Davis, CA 95616 USA

We have studied the influence of the protein alpha-lactalbumin on the phase behavior of sodium bis(ethylhexyl) sulfosuccinate (AOT)/brine/isooctane Winsor II systems. Partitioning of protein, water, oil, and surfactant between the microemulsion and excess aqueous phase were studied as a function of total protein, surfactant, and salt concentration in the system. Dynamic light scattering provided information about the size and structure of the self-assembled aggregates in each phase. Partitioning data combined with size information allowed calculations of the composition and number density of the aggregates. Concomitant structural changes in the protein in different regions of the microemulsion phase diagram were investigated using circular dichroism and fluorescence spectroscopy. We present the partitioning behavior for components of the system at varying ionic strengths. Examining the phase behavior of the system at varying ionic strengths will help to clarify the importance of both curvature effects and electrostatic interactions. By combining knowledge of the size and character of the reversed micelles and aqueous phase complexes with structural information, we can develop a conceptual picture of the structure of the system and how the protein fits into the varying types of assemblies formed.


109

Colloidal Mimetics with the Adhesive Properties of White Blood Cells

Daniel A. Hammer, Departments of Bioengineering and Chemical Engineering, and Institute of Medicine and Engineering, 120 Hayden Hall, University of Pennsylvania, Philadelphia, PA 19104

Leukocytes must adhere to blood vessel walls under flow to enter tissues and mediate immunological processes. To elucidate the fundamental mechanisms by which leukocytes adhere to blood vessel walls, we have engineered colloidal mimetics of leukocytes that have similar adhesive properties. Functional determinants of leukocyte receptors that bind to ligands found on blood vessel walls are made synthetically or recombinantly and are attached to polymeric microspheres. The dynamics of adhesion between these polymeric microspheres and surface-immobilized ligands are measured in flow chambers. The mimetics display dynamics of adhesion which are remarkably similar to those displayed by leukocytes. We use this "cell-free" colloidal mimetic to identify ligands that mediate leukocyte adhesion. Our work is supported by computer simulations, in which we can simulate the dynamics of adhesion for a single cell or particle, as well as collections of cells that may be interacting both hydrodynamically and chemically, and understand the relationship between receptor-ligand properties and the dynamics of adhesion. Furthermore, we illustrate how these colloidal mimetics can be used technologically, such as for drug delivery (by making porous polymeric microspheres that carry drug and adhere like leukocytes) and novel materials design (crosslinked colloidal materials with novel optical and rheological properties).


110

Collective and Single-Molecule Forces Between a5b1 Integrins and Peptide-Amphiphiles

Efrosini Kokkoli and Matthew Tirrell, Department of Chemical Engineering, University of California, Santa Barbara, Santa Barbara, CA 93106

This study involves the use of a model biomimetic system that allows us to investigate collective unbinding processes between a5b1 receptor- GRGDSP ligand pairs and single-molecule forces that reveal details of the molecular recognition mechanisms of individual ligand-receptor pairs. An Atomic Force Microscope (AFM) is used to provide high resolution images and direct adhesion measurements at the piconewton level. In this work, bioartificial membranes that mimic the tenth type III module of the extracellular matrix protein fibronectin (GRGDSP) are constructed from peptide-amphiphiles. GRGDSP is the primary recognition site for a5b1 and peptide-amphiphiles containing the GRGDSP sequence are deposited on a surface by the Langmuir-Blodgett technique. The receptor of choice is the a5b1 integrin. Two different antibodies have been used to activate and immobilize isolated a5b1 integrins on a polystyrene sphere that we glue onto the AFM cantilever, or directly onto the AFM tip. The effect of different parameters such as different ions, synergistic effects from other peptides and different peptide conformations have been investigated on the dynamics of the a5b1-GRGDSP interaction using the AFM. The implications of these findings in designing better biomaterials with specific functionalities will be discussed.


111

Structure-Function Relationships of Adhesion Proteins Determined by Direct Force Measurement

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

The surface force apparatus has been used to quantify themolecular forces that mediate biological interactions. These measurements provide simultaneous information concerning the distance-dependence and magnitudes of intersurface forces, deformations in the interacting materials, and the real-time dynamic changes in the interactions and material separations. Recent SFA studies of cell adhesion proteins have elucidated both the mechanisms by which the proteins both bind and detach, and the quantified parameters that control the adhesion strength. This talk will focus on three aspects of cell adhesion that we have investigated with this approach. First, the precise force-distance measurements provided unique information concerning the range at which adhesion proteins bind, and relationships between protein structure and adhesive function. Second, in situ measurements of the real-time dynamics of protein detachment revealed an unusual, three-stage unbinding mechanism of the multidomain protein cadherin. Third, time dependent measurements and the control of lipid fluidity demonstrated directly the importance of lateral mobility in adhesion, particularly for weakly binding proteins. These studies reveal a rich complexity of adhesion protein behavior that extends well beyond simple bond strength measurements.


112

Receptor Ligand Measurements Using Surface Techniques

David Tareste, Eric Perez, Frederic Pincet, Christine Gourier, Luc Lebeau, Charles Mioskowski, Hinrich Gronemeyer, Laboratoire de Physique Statistique, E.N.S. Paris, Paris, France.

Many biological processes and medical applications involve the interaction between ligands and their receptors. We have focused our attention on the retinoïc acid and its receptors. In the course of their biological function, the receptors do not only bind to their retinoïc acid ligands but they also bind to each other in order to form dimers. To quantify these interactions, we have used two experimental techniques: the Surface Force Apparatus (S.F.A.) and the micromanipulated vesicles technique. In order to couple the biological molecules to these systems, Nickel chelating lipids (NTA-Ni lipids) were used. In the S.F.A. technique, bilayers of these lipids were deposited on mica surfaces by the Langmuir-Blodgett method; in the micropipet aspiration technique, the lipids were incorporated in the membrane of the vesicles. The retinoïc receptors bearing a poly-Histidine tag were anchored to these surfaces by adsorption from solution. First we will present the interaction between NTA-Ni lipids layers. Then we will present results concerning the retinoïc acid and its receptors.


113

Study of Controlled Release of Model Drugs from the Polymer Matrices Of 2-Hydroxyethyl Methacrylate and with Its Co-Polymers of Tetrahydrofurfuryl Methacrylate

Mohammad A. Chowdhury, D.J.T. Hill, & A.K. Whittaker*, Department of Chemistry, Centre for Magnetic Resonance*, The University of Queensland, Brisbane, QLD 4072, Australia

The project involves with the study of the controlled release of model drugs from a range of glassy polymer systems. In the drug delivery systems, body fluids, which are principally water, can diffuse into the glassy matrices, which then become rubbery, and so release the encapsulated drugs. Objectives include to prepare homo and co-polymers in a cylindrical form by free radical polymerisation and by ensuring the complete conversion of monomers to polymers; to incorporate the model drugs into the polymer matrices at a range of loadings of the drugs and to investigate the effect of the drug on the initial properties of the glassy cylinders; to study the rate of diffusion of water into the polymer matrices and the release rates of model drugs with different molecular size and to correlate the release rate with the rate of water uptake; to apply N.M.R. and D.S.C. methodologies to examine the effect of the incorporation of the model drugs and their loading on the nature of water diffusion front, structure and behaviour of water in these systems; to apply UV-Visible and N.M.R. techniques to measure the bulk rate of diffusion of the drugs from the polymer; and to study the effect of polymer matrix composition on the diffusion co-efficients for drug release system. A study of the glass transition temperature, N.M.R. imaging and diffusion co-efficients of water at different temperatures for two homopolymer and two co-polymer systems of full composition range, loaded by two model drugs of Aspirin and Vitamin B12 indicate two different types of interactions between the drugs and polymer systems.


114

Interfacial Phenomena of Chitosan-Based Gene Delivery Process

Vincent Chan*, Hai-Quan Mao# and Kam W. Leong#

(*) School of Mechanical and Production Engineering, Nanyang Technological University, Singapore 639798; (#) Johns Hopkins University School of Medicine, Department of Biomedical Engineering, Baltimore, MD 21205, USA and Johns Hopkins Singapore Pte. Ltd., Singapore 117597.

Chitosan, a positively charged polysaccharide under acidic condition, can be used to condense DNA molecules to form nanoparticles. These chitosan-DNA nanoparticles provided a novel vesicle for delivering genes to cells or tissues and was shown efficacious in vitro and in vivo. Cellular uptake of these nanoparticles or escape from the endolysosomal pathway are the two possible rate-limiting steps in the gene transfer process. We therefore studied the fundamental interactions between chitosan and an artificial membrane (DPPC) bilayers with differential scanning calorimetry (DSC) and Fourier Transform (FT)-Raman spectroscopy. Direct addition of chitosan to DPPC and subsequent hydration in PBS solution at 55 ° C significantly decreased the enthalpy of gel-liquid crystalline transition of DPPC bilayers. The highest chitosan concentration (15 % w/w) reduced the gel-crystalline transition enthalpy to 4.4 kcal/mole from 9.1 kcal/mole in pure DPPC bilayers. On the other hand, direct chitosan addition to DPPC had no effect on the phase transition temperature (Tm) from gel to liquid crystalline state. FT-Raman spectroscopy measured the dynamics of the DPPC acyl chains when chitosan molecules interact with the membrane bilayer. This study provided new insights into the physical mechanisms governing the delivery of chitosan-based nanoparticles to cells.


115

Investigations of the Use Ethylene Oxide-Propylene Oxide Block Copolymeric Surfactants for the Removal of Coal Tars from Manufactured Gas Works’ Soils

Jingfeng Dong, Iain Paterson, and Stephen Leharne, School of Earth and Environmental Sciences, University of Greenwich, Pembroke, Chatham, Maritime, Kent ME4 4TB, UK,

Babur Chowdhry, School of Chemical and Life Sciences, University of Greenwich, Wellington Street, London, SE18 6PF, UK

Ethylene oxide-propylene oxide block copolymeric surfactants have been investigated as being of potential benefit for the removal of coal tars and associated polyaromatic hydrocarbons (PAHs) from soils and rocks. Successful soil remediation may be achieved either by solubilisation in surfactant micelles or by mobilization of the coal tar through reduction of interfacial tensions and middle phase formation. Surfactant adsorption at the aqueous mineral interface reduces the efficiency of solubilisation but may aid removal of the coal tar through changes in mineral wettability. In our investigations we have examined:

  1. solubilisation of exemplar PAHs as well as PAH removal from an authentic manufactured gas work’s soil;
  2. surfactant adsorption at aqueous, soil and model mineral interfaces;
  3. interfacial tensions and wetting angles for coal tar- aqueous surfactant systems and
  4. the impact of surfactants upon capillary pressure water saturation relationships for saturated porous media

Finally these issues have been investigated as a function of changing surfactant molecular structure thereby providing some indication of how the changing balance in hydrophobic and hydrophilic block lengths affect observed behaviour.


116

Radioactive Colloids from Degradation of Metallic Uranium Reactor Fuel

J. A. Fortner, C. J. Mertz, and M. M. Goldberg, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, and Colleen Shelton-Davis, Idaho National Engineering and Environmental Laboratory, P.O. Box 1625, Idaho Falls, ID 83415

The corrosion of spent metallic uranium fuel from the Hanford N-Reactor in contact with silicate-saturated groundwater and humid air at 90°C has been found to generate copious solution-born colloids. These colloids may be of concern in the environment of a geologic repository, such as the proposed Yucca Mountain site. Waste-generated colloids may provide a transport mechanism for radioisotopes that is independent of solubility and sorption limits. We will discuss possible mechanisms for colloid generation, including a spallation process that arises from intrinsic mechanical strain generated by the large volume expansion and dense nucleation of UO2 crystallites as the metal oxidizes.


117

Role of Morphological Surface Heterogeneity in Deposition of Colloidal Particles onto Semi-Permeable Polymeric Membrane Surfaces

Eric M. Vrijenhoek, Subir Bhattacharjee, and Menachem Elimelech; Yale University, Department of Chemical Engineering, P.O. Box 208286, New Haven, CT 06520-8286

Membrane filtration is one example of a class of deposition processes whereby colloidal particles deposit onto semi-permeable surfaces under the influence of normal suction forces resulting from permeation flow. In such processes, the initial rate of particle deposition is largely controlled by the coupled influence of permeation drag and particle-membrane colloidal interactions. The particle-membrane interactions are subject to immense local variations due to the inherent morphological heterogeneity (roughness) of the membranes. Consequently, the extent of particle deposition varies considerably with membrane roughness, even under similar physicochemical conditions. This phenomenon became evident when attempts to qualitatively predict deposition rates from the electrokinetic properties of silica particles and polymeric membrane surfaces – based on the assumption of perfectly smooth surfaces – failed. It appears that membrane surface roughness plays a critical role in determining the rate and extent of colloidal particle deposition. Atomic force microscope (AFM) images clearly show that, during the initial stage of particle deposition, more particles deposit on rough membranes than on smooth membranes and the particles preferentially accumulate in the "valleys" of rough membranes. AFM is used to measure interaction forces between particle and membrane surfaces, and model calculations are used to mechanistically explain experimental results.


118

A Coupled Model for Transport of Multi-component Ionic Species through Nanofiltration Membranes: Implications for Arsenic Removal

Jim C. Chen, Subir Bhattacharjee, and Menachem Elimelech, Yale University, PO Box 208286, Yale University, New Haven, CT 06520

A coupled model of concentration polarization and transport of multi-component ionic species in crossflow nanofiltration is presented. The model rigorously predicts local variations of concentrations, flux, and individual ion rejections along a rectangular membrane channel by a coupled solution of the extended Nernst-Planck and convective-diffusion equations. Mechanisms governing transport through the membrane pores are investigated by use of a stand-alone pore transport model. Generalization and delineation of the system operating regimes to describe the extent of Donnan (charge) and steric exclusion effects, influence of hydrodynamic parameters, and effects of salt mixtures in controlling transport through the membrane pores are achieved through analyses by dimensionless parameters. The coupled model provides complete mechanistic details of the system’s local responses to combined influences of concentration polarization and pore transport mechanisms. Channel averaging of local concentrations and rejections provides a qualitative global view of the system behavior characteristics that are attributable to responses at the local level. Model simulations compared favorably well with experimental data of nanofiltration of salt mixtures (NaCl/NaSO4 and NaCl/CaCl2) and arsenic (As(V) species). It is concluded that negatively charged, "loose" nanofiltration membranes can achieve high rejection of As(V) species due to the paramount role of charge (Donnan) exclusion.


119

In-Line Coagulation with Micron Filtration

Kevin Choi, Brian A. Dempsey, Civil and Environmental Engineering, 212 Sackett, Penn State University, University Park, PA 16802

The objective of this research was to determine the effect of coagulant type and dose on the performance of micron filters. The long-term goal is development of coagulation strategies that are appropriate when micron filters operating at sub-critical flux (20 to 40 gfd) are used for separation of solids. The raw water was taken from the Susquehanna River just downstream from confluence with the Juniata River. The raw water had total organic carbon (TOC) from 2 to 4 mg/L, turbidity from 5 to 50 NTU, pH from 7.8 to 8.1, and alkalinity from 70 to 90 mg/L as CaCO3. Bench-scale hollow-fiber modules with outside to inside flow were used. Commercial alum, ferric chloride, 70% neutralized poly-aluminum chloride (PAC70), and aluminum chlorohydrate (ACH) were tested under conditions that resulted in a range of zeta potentials. KMnO4 was dosed ahead of the coagulant, with the dose stoichiometrially determined based on dissolved Mn. Performance of the coagulants was ACH>PAC70>alum>ferric chloride. Removal of TOC was a function of the coagulant dose, and over 35% removal could be achieved without increases in trans-membrane pressure. Mn was reduced from about 0.4 mg/L in the raw water to <0.05 mg/L in the filtered water. Pilot-scale tests are in progress, based on results from these bench-scale experiments.


120

Electrical Double-Layer Formation in Porous Nanostructured Materials

Kun-Lin Yang, Tung-Yu Ying, Sotira Yiacoumi, Georgia Institute of Technology, School of Civil and Environmental Engineering, Atlanta, GA 30332-0512, USA, Costas Tsouris, Oak Ridge National Laboratory, Chemical Technology Division, P.O. Box 2008, Oak Ridge, TN 37831-6266, USA.

Porous nanostructured materials, which are often charged in a water environment, are encountered in natural systems and also employed in several environmental applications. A grand canonical Monte Carlo method is developed in this work to describe the formation of the electrical double layer on the surface of a charged porous material. Information from the Monte Carlo method is subsequently used to predict the adsorption capacity of the material. Monte Carlo results are also compared with those obtained using the classical Gouy-Chapman theory. The porous nanostructure of the material requires that the formation of electrical double layers within pores of different sizes be considered, as well as the overlapping of the electrical double layer occurring in smaller-size pores. It is found that when the pore size is smaller than a specific value, the electrical double layers are vanished because of the overlapping effect and therefore these pores do not contribute to the adsorption capacity. Application of the model is demonstrated using carbon aerogel, a novel adsorbent material, employed in the removal of ionic pollutants from aqueous solutions and in energy storage devices. The overall adsorption capacity is estimated by considering the pore-size distribution of the material obtained from characterization experiments. Comparison of modeling results with experimental data shows that without including the overlapping effect, the model overestimates significantly the overall adsorption capacity.


121

Removal of Heavy Metals Using Impregnated Activated Carbons

Ronald L. Vaughan, Jr. and Brian E. Reed, University of Missouri-Columbia, Department of Civil and Environmental Engineering, E3502 Engineering Building East, Columbia, MO 65211-2200

The objective of this research was to determine the efficacy of using impregnated activated carbons for the removal of various heavy metals from aqueous solutions. Two impregnates (iron oxide and a silicate-based material) were impregnated onto two different commercial activated carbons (denoted as FeAC and SiAC, respectively). In the first phase of research, acid-base behaviors of the non-impregnated and impregnated carbons were characterized at three ionic strengths by acidimetric-alkalimetric titrations and the pHzpc of the carbons were determined. The impregnated carbons required more acid/base to alter the pH, indicating that there are more surface sites available for metal removal in comparison with non-impregnated carbons. In the second phase, pH-adsorption edges (% metal removed versus pH) were developed for the impregnated and non-impregnated carbons using 1 mg/L solutions of As(III), As(V), Cu(II), Cd(II), Hg(II), Pb(II), and Zn(II). Adsorption for both SiAC and FeAC was a strong function of pH. The impregnated activated carbons removed significantly more adsorbate than their non-impregnated counterparts at all pH values, excluding Hg(II). Freundlich isotherm relationships were developed as a function of pH. It was concluded that adsorption onto impregnated activated carbons is a viable method for removal of heavy metals from most aqueous systems.


122

The Preparation of Composite Particles and Their Use in Removal Flow Cell Studies

Laura Harnett and Brian Vincent, School of Chemistry, University of Bristol, Cantocks Close, Bristol, BS8 1TS

The aim of this project is to investigate the effects of added polymers and surfactants on the removal of silica particles from modified glass surfaces. The study of particle detachment from a substrate may be achieved using a parallel plate flow cell. This allows particle detachment to be studied as a function of flow rate of a suitable wash solution. The number of particles at the surface of the substrate is monitored optically using an epi-illuminating microscope attached to a video camera and PC based image analysis equipment. It was later discovered that it would not be possible to synthesise monodisperse silica particles large enough to be seen in this experimental set up. Thus large composite particles have been prepared consisting of a cationic polystyrene core surrounded by a layer of small silica particles. The synthesis of large cationic polystyrene particles (2.70 mm) was achieved using a dispersion polymerisation technique whilst the small silica particles (170 nm) were prepared using the Stöber method. The composites were then prepared by controlled heterocoagulation of the two. Some preliminary experiments have now been carried out using these composite particles in the parallel plate flow cell.


123

Mechanisms of Coagulation with Polyaluminum Chloride

John Gregory, University College London, Gower St., London WC1E 6BT, UK, and Dongsheng Wang, SKLEAC, Chinese Academy of Sciences, P.O.B. 2871, Beijing 100085, China

A range of polyaluminum chloride coagulants was prepared by controlled neutralization of AlCl3 solutions to give values of the parameter B (= OH/Al) from 0 to 2.5. These were used to coagulate dilute clay suspensions under controlled conditions. As well as conventional testing (by measuring residual turbidity after a standard ‘jar test’ procedure), the coagulation was followed dynamically, using an optical monitor. Electrophoretic mobilities of the particles after coagulant addition were also determined. The variables investigated were coagulant type and concentration, pH and sulfate concentration. At low dosages the predominant mechanism is charge neutralization, with the pre-hydrolyzed products being effective at lower concentrations. However, at the optimum dosages, there were no significant differences in coagulation rate or residual turbidity. At higher dosages, where hydroxide precipitation is expected, AlCl3 gave very effective coagulation over the pH range 7 – 8, but the pre-hydrolysed samples were much less effective, giving only limited turbidity removal at pH values above about 8.5. It is likely that hydroxide precipitation is inhibited with these products. However the presence of moderate amounts of sulfate can change their behavior dramatically, especially for high B values, probably by promoting aggregation of hydrolysed species and hence giving significantly enhanced coagulation.


124

Inorganic Coagulant Species Formed from Clusters of Keggin Polycations: New Insights from Combining Ga EXAFS, 71Ga NMR, and SAXS

Emmanuelle Montargès-Pelletier 1– Bruno S. Lartiges1 – Laurent J. Michot1 – Jean-Baptiste d'Espinose de la Caillerie2 – Valérie Briois 3– Virginie Ponsinet4 – Raymond Ober4, 1-Laboratoire Environnement et Minéralurgie, UMR 7569 CNRS-INPL-ENSG, 15 avenue du Charmois, 54500 VANDOEUVRE, France., 2-Laboratoire de Physique Quantique ESA 7069 CNRS, ESPCI, 10 rue Vauquelin, 75231 PARIS Cedex 05, France., 3-LURE, UMR130, BP34, 91898 ORSAY Cedex, France. 4-Laboratoire Matière Condensée, Collège de France,11 place Marcelin Berthelot 75005 PARIS, FRANCE

Aluminum salts are widely used as coagulants in drinking water treatment. Upon addition to the raw water, the aluminum salts undergo dissociation, hydrolysis, and polymerization, to yield the actual coagulant species. In the case of base-hydrolyzed aluminum chloride solutions, a Keggin-type polycation with 13 aluminum atoms is formed, and upon further neutralization, these Al13 polycations aggregate to yield fractal clusters. Many details about the formation of aluminum-based coagulant species remain unclear: the nature of the precursors leading to the polycation is still a matter of controversy and the binding of Al13 units within coagulant species is still unknown. Using gallium as a structural analogue for aluminum, we followed the base hydrolysis of gallium chloride and gallium nitrate solutions by combining 71Ga NMR, Ga K-edge X-ray absorption spectroscopies, and Small Angle X-ray Scattering. Using this experimental approach, we demonstrated the existence of various trimeric and tetrameric species in the early hydrolytic stages, identified as Ga13 precursors. Results analysis for high hydrolysis ratios (R=OH/Ga>2.5 for chloride salts) revealed the formation of single corner sharing bondings between Ga13 units. The resulting aggregates present an increasing fractal dimension with increasing hydrolysis ratio. Higher hydrolysis ratios (R=OH/Ga>3.0 for chloride salts) yield the formation of lamellar hydroxyde.


125

The Effect of the Acid-Base Chemistry of Lactose on its Adhesion to Gelatin Capsules

Frank M. Etzler and Richard Deanne, Boehringer-Ingelheim Pharmaceuticals, Inc., Ridgefield, CT 06877, Tonia Burk, Taleb Ibrahim, Gerold Willing and Ronald D. Neuman, Department of Chemical Engineering, Auburn University, Auburn, AL 36849

Dry powder inhalation formulations may be used for drug delivery to the lungs. The performance of dry powder formulations depends upon the adhesion strength between each component of the formulation. Here we explore the adhesion of lactose to gelatin capsule materials found in dry powder inhalers. The surface characterization of each of these materials has been performed using a variety of techniques. Some of the experimental techniques used include angles, IGC, FMC, and XPS (ESCA) as well as SPM. Each of the pharmaceutical grade materials used in the study was found to have varying degress of surface contamination. From our experimental data it is clear that acid-base interactions contribute significantly to the adhesion of these materials. The relation between fundamental adhesion forces to powder disperse ability and ultimate product performance is stressed.


126

Stable, Near Hard Sphere Silica-in-Silicone Dispersions for ER Applications

Sudhir Shenoy, Norman J. Wagner, CMET, Dept. of Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE

A dispersion of silica particles in a non-polar medium, such as silicone oil, is developed as a model, near-hard-sphere system for use in electro-rheological (ER) studies. Stöber silica particles are synthesized and dispersed in both, index-matching and normal silicone oils. Uncoated silica particles are hydrophilic and thus, do not disperse well in silicone oil, a hydrophobic medium. This has been attributed to the non-wettability of the silica surface by the silicone oil, which in turn induces an attractive potential between particle surfaces by allowing hydrogen bonding. To make the particles dispersible, we coat the surface with silane coupling agents of varied chemistry. The type and extent of surface modification is shown to have a dramatic effect on the rheological properties and stability of silica-silicone dispersions. The effect of the change in inter-particle interaction (due to our surface modification efforts) on ER is also illustrated.


127

Phase Behavior of Ferrofluid and Colloidal Spheres and Rods in a Magnetic Field

M. F. Islam, D. Lacoste, T. C. Lubensky and A. G. Yodh, Dept. of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA19104

We have studied theoretically and experimentally different phases formed by aqueous ferrofluid only, or by mixtures of this ferrofluid with colloidal latex spheres and rods. The ferrofluid is an aqueous suspension of magnetite Fe3O4 particles, of average diameter 20nm. The latex spheres are PMMA particles of diameter 42nm. The rods are fd-virus. In the presence of a magnetic field applied perpendicular to a thin sample layer, a pure ferrofluid forms disordered columns. When latex spheres or rods are added, the mixture shows a transition from a disordered phase of columns to a hexagonal phase of columns. The columns merge into sheets of ferrofluid at higher magnetic field. Theoretically, we regard this phenomenon as a kind of liquid-solid phase transition, and we study the formation and the stability of these phases by a simple mean field theory. We gratefully acknowledge support from NSF (DMR-9623441), MRSEC (DMR-9632598) and a grant from the French Ministry of Foreign Affairs.


128

The Role of Interparticle Potential in Controlling the Morphology of Spray-Dried Powders from Nanoparticle Sols

J.R. Bartlett1, S. Lyonnard 2, E. Sizgek 1, K.S. Finnie 1, and Th. Zemb 2, 1Materials Division, Private Mail Bag 1 Menai NSW 2234 Australia, 2 Service de Chimie Moléculaire, C.E. A. / Saclay, Bat. 125, F-91191 Gif-sur-Yvette cedex, France

The effect of ionic strength on the structure and morphology of titania powders obtained by spray-drying concentrated nanoparticle sols has been investigated using combined small angle and ultra-small angle X-ray scattering (SAXS/USAXS), electron microscopy and N2 adsorption. In the absence of added salt, fragmentation of the atomised droplets occurs during drying, leading to the formation of fine powder, in addition to gel particles with distorted shapes (e.g. donuts). In contrast, addition of appropriate salts results in the formation of well-defined spherical particles. The porosity of the gel particles is controlled by the ionic strength and the corresponding inter-particle potential. It is shown that the effect of ionic strength on the morphology of the spray-dried gel particle can be attributed to the relative magnitudes of the Laplace pressure and osmotic pressure immediately before the sol-to-gel transition. If the Laplace pressure exceeds the osmotic pressure, then well defined spherical particles are obtained. Conversely, distorted particles are formed when the osmotic pressure exceeds the Laplace pressure. For a given droplet with rapidly increasing osmotic pressure during the fast process of drying, the relative magnitudes of the Laplace and osmotic pressures before the sol-to-gel transition thus determines whether compression or fragmentation is favoured.


129

Molecular Dynamics of Adsorbed Monolayer Properties and Sliding Friction Modification

Michael L. Greenfield and Hiroko Ohtani, Ford Motor Company, Chemistry Dept., P.O. Box 2053, mail drop 3083/SRL, Dearborn, MI 48121

The surfactant-like "friction modifier" (FM) additive in automatic transmission fluid decreases static friction and causes low-speed dynamic friction to increase with sliding velocity, providing precise control of the friction performance of wet clutches in the boundary lubrication regime (slow sliding speeds). We are using molecular dynamics to investigate how different molecular surface film structures, formed on model surfaces by FM additives, affect sliding friction. We find the same average film structure at different slow sliding speeds, and correlation functions indicate a mix of solid-like and liquid-like properties in the film. The velocity gradient between surfaces occurs at the interface between opposing monolayers and across molecules trapped between the monolayers. The ratio of tangential and normal forces on each wall yields the friction coefficient. Preliminary results suggest that the friction coefficient increases with sliding speed, in agreement with experimental data for model systems and fully formulated fluid. In ongoing work, we are investigating which molecular factors most strongly influence this sliding friction coefficient.


130

Tribological Experiments on A Two-Dimension Friction Force Apparatus: Hexadecane Confined Between Two Mica Surfaces

Linmao Qian, Magali Charlot, Eric Perez, Denis Douillet, Dept. Physique, Laboratoire de Physique Statistique. Ecole Normale Superieure; 24, rue Lhomond; 75231 Paris CEDEX 05, France, Gustavo Luengo, L’OREAL Research, 90 rue Général Roguet, 92583 Clichy Cedex, France

The friction is a phenomenon poorly understood although it is an important source of wear and of energy dissipation. We have constructed a two-dimensional Friction Force Apparatus (FFA) and used it for tribological measurements of thin fluid films in two directions. This home made apparatus shares some common features, already in use for this type of technique developped by Israelachvili, the Surface Forces Apparatus or SFA. The bimorph slider is a displacement transducer allowing for steady, sinusoidal, ot triangular motion of the low surface over a large range of distances (up to 0.8mm) and frequencies. The movements of bimorph can be detected by strain gauges with a high precision of 0.1µm. Due to the high resolution of two capacitance sensors mounted on the upper surface, the friction force can be measured with 10µN accuracy. A long chain alkane, hexadecane, has been studied with this FFA. The normal force curve showed that we can obtain 2-3 layer of molecular film betwen two mica sheets. In the load range 0~100mN, the friction force shows a linear trend with load. The effect of speed on friction and the pinning force for different levels of confinement will be presented showing a rich and complex shearing dynamics.


131

Physical and Mechanical Properties of Dendrimer-Metal Nanocomposites

S.C. Street, A. Rar, M. Curry, G. Wei, and J. A. Barnard, Materials for Information Technology Center, The University of Alabama, Tuscaloosa, AL 35487

Ultrathin dendrimer-metal nanocomposites have been formed in a two-stage process on the native surface of Si wafers. Amine-terminated poly(amidoamine) (PAMAM) dendrimers are first deposited using a simple solution phase chemistry, followed by vapor phase deposition of metal overlayers using various methods. The resulting nanocomposites have been studies using XPS, AFM, XRR (x-ray reflectivity) and nanoindentation techniques. Inert overlayer metals, such as Au, penetrate the dendrimer adlayer leading to improvements in metal film smoothness and, interestingly, mechanical hardness. Penetration is apparently less pronounced for active metals such as Al, Co, Cr, and Cu. Observations of mechanical and physical properties are related to the chemical interaction of the metal with the amine terminal groups of the dendrimer, as a function of dendrimer generation number. It appears that the more active metals form a bilayer, yeilding a mechanically softer film as a result of dendrimer compression during nanonindentation.


132

Entropically Driven Attraction Between Telechelic Brushes

A.G. Zilman and S.A. Safran, Dept. of Materials and Interfaces, Weizmann Inst. of Science, 76100 Rehovot, Israel

We discuss the structure and the interaction of telechelic brushes. We show that the association of functionalized chain ends is capable of giving rise to attractive interactions between telechelic brush-covered surfaces, in contrast to conventional repulsion. Our predictions for the interaction free energy are in agreement with experimental data.


133

Nanomechanical Properties of Adsorbed Polymer Layers Studied Using Atomic Force Microscopy

Simon Biggs, Shannon Notley, Chemistry Dept., The University of Newcastle, University Drive, Callaghan, NSW 2308 Australia, Vince Craig, RSPhysSE, Australian National University, Canberra, ACT 2001

The nanomechanical properties of adsorbed polymer layers at the solid-aqueous interface are being investigated using a modified commercial AFM. Standard colloid probe techniques are being utilised to collect force-distance data between two surfaces in the presence of an adsorbing polymer solution. In addition, by utilising small controlled oscillatory motions on one surface we are able to detect differences in phase angle and amplitude between the drive and detection signals. These are used to infer nanoscale mechanical information about the adsorbed polymer layer. Data for the interaction of two silica surfaces in the presence of poly(2-vinyl pyridine) will be presented here. Variables investigated include pH, polymer concentration and salt concentration. Information about the properties of the polymer chains during detachment of the surfaces will also be presented.


134

Molecular Modeling on Diffusion and Surface Interactions in Siloxane Network

Peter C. Qian, Bhukan Parbhoo, C041D1, CO41D1, Interface Expertise Center, Central Research and Development, Dow Corning Corporation, Midland, MI 48686

Small molecules are often incorporated in the formulation of commercial chemical products or are formed as by-products during application or aging processes. This is also true for silicones. In relation with the cure, adhesion, or surface properties influenced by migrating species to the silicone/air interface, inhibitor or adhesion promotion molecules play significant role. Molecular dynamic properties like translational diffusion, orientation of the molecule within the silicone substrate interface, its concentration gradient within the matrix or within the interface are important factors to take into account in order to understand and design high performance silicone systems. We have studied the diffusion coefficient of two adhesion promoters and two inhibitors in siloxane networks in a system-D cured process using a molecular modeling method to test the validity of the molecular modeling simulation method. This study shows that the temperature can markedly affect the diffusion process of small molecules through polymeric networks thereby influencing processes like cure control or adhesion promotion. We have also used a molecular modeling method to quantify the interaction energies of adhesion promoters, inhibitors, siloxane networks with various substrate surfaces. The simulation results indicate that the nature of the substrate surface is the most important factor in determining the interaction energy between a studied molecule and the substrate surface, and therefore the physical adhesion strength.


135

Meniscus Instability in Thin Elastic Film

Animangsu Ghatak, Manoj K. Chaudhury, Department of Chemical Engineering, Lehigh University, Bethlehem, Pennsylvania

A new kind of meniscus instability leading to the formation of stationary fingers with a well-defined spacing has been observed in experiments with elastomeric films confined between a plane rigid glass and a thin curved glass plate. The remarkable feature of this instability is that its wavelength is independent of the material properties, e.g., softness and the surface tension of the rubber; it simply increases with the thickness of the film. A new theory by Shenoy and Sharma shows that the undulations in the elastic film ensue once the gradient of the intermolecular attraction overcomes the elastic spring constant of the thin film. However, neither the precise magnitude of the force nor its origin affects the wavelength of the undulations. This observation resembles the Rayleigh instability in liquid cylinders, the wavelength of which is also independent of the detailed properties of the liquid except the geometrical features of the system. This interfacial instability may have profound implications in understanding adhesion and friction at soft surfaces.


136

Ordered Structures Formed by Polymer-Surfactant Mixtures at the Air-Water Interface: Studied by Specular Neutron Reflection

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

Specular neutron reflectivity has been used to study the structure and composition of polymer/surfactant mixtures at the air-water interface. Some recent results on a range of mixtures, showing different but related behavior, will be presented and discussed. For the cationic polymer, poly-dimethyldialyl ammonium chloride, with the anionic surfactant sodium dodecyl suphate, SDS, and with the anionic-nonionic surfactant mixture of SDS and hexaethylene glycol monododecylether, C12E6, a complex pattern of surface tension behavior is observed for solutions rich in SDS. The surface tension exhibits a sharp maximum at a finite concentration below the cmc, which is associated with changes in the amount of polymer and surfactant adsorbed at the interface, due to the competition between complex formation in bulk and at the interface. In contrast, the polymer/surfactant mixtures of poly-styrene sulphonate, PSS, and the cationic surfactant dodecyl trimethyl ammonium bromide, C12TAB, and of poly-vinyl pyrrolidone, PVP, and SDS exhibit marked changes in their organization at the interface which are not associated with significant changes in surface tension. At a concentration between the cac and cmc highly ordered structures are obtained at the air-water interface, in marked contrast to the more commonly observed monolayers.


137

The Effect of Counterion Condensation on Poly (dimethylaminoethyl Methacrylate) Adsorption Onto Colloidal Silica Surfaces by Means Of NMR, Electrophoresis, pH, and Conductometric Titrations

YongWoo Shin, Maria M. Santore, Department of Chemical Engineering, James E. Roberts, Department of Chemistry, Lehigh University, Bethlehem, PA 18015

The adsorption of Poly (dimethylaminoethyl methacrylate) (DMAEMA) polymers on silica surfaces was studied to understand how the role of charges on the polyelectrolyte ultimately influences polymer adsorption. We have measured adsorbed amounts of up to 0.8-1.0 mg/m2 at pH 8.0 for DMAEMA on colloidal silica particles with NMR. In order to interpret the adsorption phenomena as a function of pH, the NMR experiments, acid-base and conductometric titrations, and electrophoresis data for 12 nm and 200 nm silica particles bearing adsorbed DMAEMA were combined. A maximum adsorption arises at pH 8.5-9.5, corresponding to 3-10 % protonation of the tertiary amine in DMAEMA, and an electrophoretic mobility of 0.5-1.0 10-8 m2/V sec. High molecular weight DMAEMA behaves similarly to the low molecular weight polymer, except the adsorption plateau for the high molecular weight sample extends down to very dilute solutions, higher coverages are observed at pH 9-10, as expected for a stronger polymer-surface interaction adsorption. Information about the interfacial conformation of adsorbed DMAEMA was obtained from NMR solvent relaxation measurements. Low molecular weight DMAEMA has a higher train fraction than the high molecular sample.


138

Sum Frequency Spectroscopy of Surfactant-Coated Gold Nanoparticles

Takeshi Kawai*, David J. Neivandt and Paul B. Davies, Department of Chemistry, The University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, United Kingdom. * Present Address: Department of Industrial Chemistry, Science University of Tokyo, Kagurazaka 1-3, Shinjuku-ku, Tokyo, Japan, 162-8601.

The second order non-linear optical technique of Sum Frequency Vibrational Spectroscopy (SFS) has been widely applied over the last decade to the study of the polar orientation and degree of conformational ordering of surfactants at a variety of planar macroscopic interfaces. In a highly novel proof of principle experiment we have recently shown that it is possible to extend the technique to the study of surfactants adsorbed at nanoparticle surfaces. A composite gold nanoparticle/DODAC surfactant film was formed at the air/water interface of a Langmuir trough through electrostatic interactions and studied by SFS both in situ at the air/water interface, and ex situ after immobilisation on a silicon substrate. The sum frequency spectra recorded were markedly different to those typically observed for surfactants adsorbed on either macroscopic planar gold or dielectric substrates. Specifically, an increase in the strengths of the resonances attributed to the terminal methyl groups of the surfactant alkyl chain were observed, without corresponding enhancement of the methylene resonances. It is anticipated that full characterisation of the phenomenon of SF generation in nanoparticle systems will enable the application of the technique to the determination of the polar orientation and conformation of surfactants adsorbed on a wide range of noble metal and semi-conductor nanoparticles.


139

Scanning Transmission X-ray Microspectroscopy of Colloidal Materials

Lisa M. Croll, Ivylo Koprinarov, Adam P. Hitchcock, Harald D.H. Stöver, Department of Chemistry, McMaster University, Hamilton, ON, Canada, L8S 4M1

Structural characterization of polymer colloids and microcapsules usually relies strongly on scanning electron microscopy (SEM) and transmission electron microscopy (TEM). While these techniques give high resolution images of surfaces and cross sections, they provide little chemical information, especially on sub-micron scales. Synchrotron-based Scanning Transmission X-ray Microspectroscopy (STXM) is a fairly new technique that permits detailed chemical mapping of thin polymer sections at better than 100 nm spatial resolution. The use of STXM to analyze polymeric materials will be illustrated. As well, the application of STXM to study sub-micron interfacial polyurea membranes incorporating a compositional gradient will be described.


140

Adsorption of Polymeric Siloxane Surfactants on Hydrophobic Particles

Paschalis Alexandridis and Yining Lin, Department of Chemical Engineering, University at Buffalo - SUNY, Buffalo, NY 14260-4200; Thomas W. Smith, Xerox Corp. Webster NY 14580.

The adsorption isotherm of polymeric siloxane surfactants on hydrophobic carbon black (CB) particles was determined in water and in mixtures of water with cosolvents such as ethanol, formamide, or glycerol. The Langmuir isotherm was obeyed at low filtrate concentrations (and was used to estimate parameters such as interfacial area per surfactant molecule), but the adsorbed amount increased significantly at surfactant concentrations above the CMC. Measurements of the adsorbed layer thickness by means of viscometry and dynamic light scattering indicated a monolayer thickness below the CMC, while well above the CMC the thickness of the adsorbed layer was comparable to the diameter of the siloxane copolymer micelles. The size distribution of the copolymer-coated CB particles became broader above the CMC, suggesting transient particle aggregates. Information, obtained by ESCA, on the chemical composition of the surface of dried surfactant-coated CB particles confirmed the presence of adsorbed siloxane surfactant.


141

In-situ, Fast, Simple Analytical Technique Probing Molecular Adsorption and Orientation at Carbon Black Powder Surfaces in Aqueous Solution

Hong-fei Wang, Laboratory of Molecular Reaction Dynamics, Center for Molecular Sciences, Institute of Chemistry, the Chinese Academy of Sciences, Beijing, P.R.China 100080

Second order non-linear optical techniques, such as Second Harmonic Generation (SHG) and Sum Frequency Generation (SFG) selectively probe the interface region between centro-symmetric bulk media. Progresses and new techniques using these nonlinear optical methods to probe chemical and physical processes at colloidal and micro-particle interfaces will be described. This work demonstrates an important application using these new techniques as in-situ, fast, simple analytical technique to probe molecular adsorption and molecular orientation at carbon black (CB) powder surfaces in aqueous solution. Quantitative result shows that the surface oxygen to carbon ratio (O/C ratio) of CB powder has significant influence on the molecular adsorption free energy and molecular orientation of adsorbed molecules at the CB surfaces. This new technique is readily applicable to other colloidal and micro-particle systems in aqueous solution. Also discussed are the implications and perspectives of these new techniques in surface sciences and powder technology.


142

Spatial Correlation of Spherical Polyelectrolyte Brushes in Salt-free Solution as Observed by Small-Angle X-Ray Scattering

Q. de Robillard, X. Guo, M. Ballauff, Polymer-Institut, Universität Karlsruhe, Kaiserstrasse 12, 76128 Karlsruhe, Germany, T. Narayanan, ESRF, BP 220, F-38043 Grenoble Cedex, France, G. Goerigk, Institut für Festkörperforschung, Forschungszentrum Jülich, Postfach 1913, 52425 Jülich, Germany

We report on the observation of spatial correlation of linear polyelectrolyte chains attached to latex particles. The particles are dispersed in water and consist of a solid poly(styrene) core and a shell of poly(acrylic acid) (PAA). At low pH the PAA-chains are virtually uncharged but full ionization can be reached for pH>10. Small-angle X-ray scattering (SAXS) reveals an additional peak in the region of intermediate scattering angles when the chains are fully charged whereas no signal is seen in the uncharged state (low pH). Also, the peak is only seen if the polyelectrolyte chains attached to different particles overlap sufficiently. The position of the maximum scales with the squareroot of the particle concentration. The peak vanished when the ionic strength is raised by adding salt to the suspension. We assigned this signal to the weak maximum of the scattering intensity seen by small-angle scattering experiments in solutions of free linear polyelectrolytes ("polyelectrolyte peak"). The present investigation demonstrates that the same spatial correlation may occur when the polyelectrolyte chains are attached to the surface of colloidal particles. Further investigations by anomalous small-angle X-ray scattering ASAXS allow to discern the signal of Rubidium counterions as opposed to the scattering contribution of the macroion. The "polyelectrolyte peak" is clearly visible in the ASAXS-signal as well. This demonstrates that the observed partial ordering is directly connected to the counterions within the brush.


143

The Interaction of Oxygen Atoms with Single Walled Carbon Nanotubes: A Theoretical Study Using Model Potentials and Density Functional Theory

J. A. Steckel, K. Karapetian and K. D. Jordan, Department of Chemistry and Center for Molecular and Materials Simulations, University of Pittsburgh, Pittsburgh, Pennsylvania, 15260,

Ph. Avouris, IBM Research Center,Yorktown Heights, New York, 10598

Carbon nanotubes are readily oxidized and there is evidence that oxidation may occur either at defect sites or at the caps of closed nanotubes1. Current standard cleaning treatments to prepare carbon nanotube samples often include ozonation, oxidation by acids or both. Moreover, the electronic properties of nanotubes are extremely sensitive to the presence of oxygen2. As a result, in order to make use of the remarkable properties of carbon nanotubes, it will be imperative to thoroughly understand and control the reactions of oxygen with carbon nanotubes. In our theoretical calculations, we use periodic boundary conditions and model potentials to characterize the reaction of oxygen atoms with ideal single-walled carbon nanotubes as well as defect-containing single-walled carbon nanotubes. We also use density functional theory (DFT) to characterize the reaction of oxygen atoms with circumtrindene, a C36H12 geodesic dome with an arrangement of five- and six-membered rings that would be expected to appear in the caps of (5,5) armchair single-walled carbon nanotubes.

1 A. Kuznetsova and J.T. Yates, Jr., personal communication.

2 P.G Collins, K. Bradley, M. Ishigami and A. Zettl, Science 287, 1801 (2000).


144

First Principles Studies of Chiral Metal and Metal Oxide Surfaces

Aravind Asthagiri, Timothy D. Power, and David S. Sholl, Dept. of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA

Intrinsically chiral solid surfaces provide an intriguing opportunity to create robust heterogeneous adsorbents with enantiospecific properties. Solid surfaces of materials with simple bulk structures can be chiral due to the existence of atomic-scale surface steps. We have used Density Functional Theory (DFT) to examine several issues related to the performance and synthesis of these materials. We will first discuss the use of DFT to predict the surface structures that can arise on pure metal surfaces after the disruption of surface steps by thermal roughening. By combining our DFT calculations with atomistic studies of molecular adsorption on stepped surfaces, we have developed a description of the enantiospecific adsorption of chiral hydrocarbons on thermally roughened chiral metal surfaces. We have also used DFT to examine the structure of chiral metal oxide surfaces and metal oxide/metal interfaces. We will describe how these results may be used to aid in the synthesis of practical quantities of intrinsically chiral surfaces.


145

Application of Nudged Elastic Band for Locating Transition States of Chemical Reactions on Surfaces.

D.R. Alfonso, J.A. Steckel and K.D. Jordan, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15206 USA

The Nudged Elastic Band (NEB) [1] algorithm shows promise as a viable approach for mapping out minimum energy pathways using only first derivative information. We have implemented a flexible version of this algorithm, used in conjunction with Gaussian 98 [2] and DMOL [3] electronic structure codes. We apply the algorithm to map out rearrangements in water clusters as well as reactions on carbon nanotube surfaces.

[1] G. Henkelman and H. Jonnson, to appear in Phys. Rev. Lett (2000).

[2] M. Frisch et al. (Gaussian Inc., Pittsburgh, PA).

[3] B. Delley, J. Chem. Phys. 113, 7756 (2000).


146

Brownian Dynamic Simulation of Bead Rod Chains under Shear Flow

S. Liu, B. Ashok, Physics Department, University of Massachusetts, Amherst, M. Muthukumar, Polymer Science and Engineering, University of Massachusetts, Amherst.

Employing parallel computational techniques, we have performed a Brownian dynamics simulation of the bead-rod model of a polymer chain of up to 60 beads under shear flow. Excluded volume and hydrodynamic interactions were included. The material functions of the system were calculated, as were configurational quantities and the dynamic structure factor. These results will be presented in detail.


147

Local Friction in Polyolefins - A Small-Scale Simulation Approach

Jutta Luettmer-Strathmann, Department of Physics, University of Akron, Akron, OH 44325-4001

Processes on different length scales affect the dynamics of chain molecules. A convenient measure for small-scale effects is the (monomeric) friction coefficient, which is inversely proportional to the mobility of individual chain segments. Local friction in polymers depends on small-scale chain properties such as local architecture and flexibility and on the local environment of the chain segments. In polymer melts, the density is the important environmental variable. In mixtures of polymers, the local concentration will also play a role. In this work, we investigate local friction in dense polymeric fluids with the aid of a small-scale simulation approach. By evaluating exact enumeration results for two short chain sections on a lattice in conjunction with an equation of state, we are able to make predictions about the variation of segmental mobility with local chain architecture, flexibility, and thermodynamic state (temperature, pressure, composition). We apply the approach here to polyolefins and compare our predictions with experimental data.


148

Segmental Dynamics of Polymers Confined in 1-2 nm Wide Slit Pores

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

Molecular Dynamics simulations are used to explore the structure and dynamics of polystyrene confined in 2-nm wide slit pores and poly(ethylene-oxide) in an 1-nm slit. The systems simulated resemble experimentally studied intercalated nanocomposites, where polymers are inserted between 2:1 silicate layers. The molecular modeling perspective complements the experimental findings and provides insight into the nature of polymers in nanoscopic confinements, especially into the molecular origins of their macroscopic behavior. Namely, in accord with the experiments, simulations show a distribution of relaxations, ranging from extremely faster and to much slower segmental motions than the ones found in the corresponding bulk polymer at the same temperature. The origins of these dynamical inhomogeneities are traced to the confinement induced density modulations inside the 1-2 nm slits. For PS, fast relaxing phenyl and backbone moieties are found in low density regions across the film, and preferentially in the center, whereas slow relaxing moieties are concentrated in the denser regions, especially in the immediate vicinity of the confining surfaces. A comparison of the simulation findings will be made with NMR, dielectric spectroscopy, and DSC experiments.


149

MC and DFT Studies of Metastable States and Hysteresis in Confined Fluids

Alexander V. Neimark, Peter I. Ravikovitch, and Aleksey Vishnyakov, TRI/Princeton, 601 Prospect Av., Princeton, NJ 08542

Confined fluids are characterized by multiple metastable states and associated hysteresis behavior. We have recently developed two novel complementary techniques [1-3], the canonical ensemble density functional theory and the gauge cell MC method, to study metastable states up to the true limits of their stability and calculate the free energy barriers between the metastable and stable states. In this talk, we consider sorption and capillary condensation of a LJ fluid in cylindrical nanopores. We demonstrate that depending on the pore size and temperature the four regimes of fluid behavior should be distinguished: Supercritical sorption without phase separation; Reversible capillary condensation; Capillary condensation with developing hysteresis; Capillary condensation with developed hysteresis. The results are shown to be in agreement with reference experiments on argon and nitrogen capillary condensation on mesoporous molecular sieves. The results obtained provide a new insight into the hysteresis phenomena and specifics of phase transitions in nanoconfinements.

  1. Neimark A.V., Ravikovitch P.I.. In Microscopic Simulations of Interfacial Phenomena in Solids and Liquids, Eds. P. Bristowe, S. Phillpot, J. Smith, D. Stroud, MRS Symposium Proceedings Series, v.492, p.27-33, 1998.
  2. A. V. Neimark, P.I. Ravikovitch, and A. Vishnyakov. Phys. Rev. E, 2000, V.62, p.p.R1493-R1496.
  3. A. V. Neimark and A. Vishnyakov . Phys. Rev. E, 2000, V.62, p.p.4611-4622

150

Self-Consistent Field Theory for Particle/Diblock Composite System

R. B. Thompson, V. V. Ginzburg, A. C. Balazs, University of Pittsburgh, Pittsburgh, PA 15261, M. W. Matsen, University of Reading, Reading, England

Self-consistent field theory has been remarkably successful, both qualitatively and quantitatively, in describing block copolymer systems and blends of copolymer with homopolymer or solvent. In this talk, a self-consistent field theory will be presented for composite systems of diblock copolymer and spherical filler particles. The theory is implemented using a combinatorial screening algorithm so that there is no need to assume phases a priori. It can predict the morphologies of systems with a wide range of segregations, diblock compositions, particle volume fractions, and particle sizes. We show that the morphologies of these filled systems can be engineered by adjusting these parameters.


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Talks 151 through 200

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