ucsb che's 4th amgen-clorox grad student symposium poster...

UCSB ChE's 4th Amgen-Clorox Grad Student Symposium Poster Presentation Abstracts Biological Engineering

Bradley Spatola: Serum Antibody Profiling of Celiac Disease Using Bacterial Display for in vitro Diagnostics

Ian Shieh: Confocal Microscopy and Lung Surfactant: Slicing Through the Interface

Jennifer Getz: Protease-Resistant Peptide Ligands from a Cysteine-Rich Scaffold Library

Serra Elliot: Discovery and Characterization of Antibody Biomarkers of Autoimmune Diseases and Reagents for Their Detection

Natalie Forbes: Photothermally Triggered Drug Release from Temperature Sensitive Liposomes

Biomedical Applications

Joel Paustian: High-Pressure Nonlinear Electrokinetic Micropumps for Portable Microfluidic Device

Justin Lee: Robust Performance of Closed-loop Control Design Based on Insulin Pharmacokinetics / Pharmacodynamics

Peter St. John: Multi-parameter Continuation and Higher-order Sensitivities for Quantifying Nonlinear Effects in Circadian Models

Biomolecular Mechanisms

Joohyun Jeon: Charge Effects on the Fibril-forming Peptide KTVIIE: a Two-dimensional Replica Exchange Simulation Study

Ryan Mullen: Solvent Dynamics in Ion Pair Dissociation

Scott Carmichael: A New Multi-scale Platform for the Investigation of Peptide Self-assembly

Stephen Donaldson: Direct Measurement of Light-Modulated Intra- and Inter- Aggregate Forces

Sunyia Hussain: Filming Seven-Transmembrane Protein Activation with Magnetic Resonance: Conformational Dynamics of an α-helical Cytoplasmic Loop Segment

Catalytic Materials

Alan Derk: Structural and Catalytic Properties of Cerium Oxide Nanoparticles Doped with Ruthenium

Anthony Fong: Computational Mechanistic Investigation of the Phillips Polymerization Catalyst

Ting Ann Siaw: Probing the Molecular Environment of Hydrogenated Amorphous Silicon Using 1H Solid State DNP at 7 Tesla

Louis Jones: Synthesis, Characterization, and Testing of Shaped PtAg Nanoparticle Catalysts

Preshit Dandekar: A Mechanistic Growth Model for Ionic Crystals

Travis Koh: Single-Step, Plasma-Based Synthesis of Alloy and Supported Nanoparticle Catalysts

Polymeric Systems

Shamon Walker: Characterization of Multi-domain Polymers by Dynamic Nuclear Polarization Amplified Spin Dynamics Measurements at 7 Tesla

Zoltan Mester: Macro- and Microphase Separation in Multifunctional Supramolecular Polymer Networks

Transport and Interfacial Phenomena

Aviel Chaimovich: Multiscale Studies of Hydrophobic Association

Brian Giera: Molecular Dynamic Studies of Mean-Field Theories for the Electric Double Layer

Chia-Chun Fu: A Coarse-graining Procedure for Mapping Atomistic Models

John Frostad: Cantilevered-Capillary Force Apparatus for Measuring Multiphase-Fluid Interactions

Mansi Seth: Microstructural Transformations in Concentrated Charged Vesicle Suspensions: The Crowding Hypothesis

Zachary Zell: Interfacial Properties of Copolymer Monolayers

Biological Engineering

Serum Antibody Profiling of Celiac Disease Using Bacterial Display for in vitro Diagnostics Bradley N. Spatola and Patrick S. Daugherty

Department of Chemical Engineering, University of California, Santa Barbara

Celiac disease (CD) is an autoimmune disease of the small intestine affecting genetically susceptible individuals following the ingestion of gluten, a protein found in wheat. Although the combination of a serological test with a small intestinal biopsy is a highly effective method for CD diagnosis, there are still critical questions that need to be addressed with regards to early disease detection, identifying additional trigger antigens, and discovering ways to distinguish patients that will not improve with a gluten-free diet. A new serum antibody profiling strategy has been optimized to selectively detect a pool of patients’ antibodies that test positive for two established serological tests for CD and not detect antibodies from a control patient set. A group of 18 unique mimotopes, or peptides that mimic antigenic determinants, were isolated after 5 rounds of enrichment by fluorescent activated cell sorting (FACS) of a bacterial cell display library of random 15-mer peptides. In order to determine which mimotopes had the highest predictive value for CD, the cross-reactivity of these mimotopes was tested on an individual patient basis with the original discovery patient cohort as well as a preliminary training set. A panel of 8 clones correctly classified 17 out of 20 patients as Celiac, while only misclassifying 3 out of 20 controls. Additional CD patient cohorts have been acquired from the clinics of leading CD investigators and our refined bacterial display antibody profiling strategy will be applied to each set of serum samples. After a sufficiently large group of candidate mimotopes are obtained from repeated screenings, the bacterial cells will be printed on glass slides in a novel microarray assay that has been developed in parallel. Bacterial cell microarrays will provide a high-throughput method to characterize the sensitivity and specificity of the CD detector during the training and validation phases of the study and establish the detector is of clinical relevance.

Confocal Microscopy and Lung Surfactant: Slicing Through the Interface Ian C. Shieh(a) and Joseph A. Zasadzinski(b)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Department of Chemical Engineering and Materials Science, University of Minnesota

Lung surfactant is a mixture of lipids and proteins that lines the air-liquid interface of the alveolar walls and plays an integral role in normal respiration. It modulates the otherwise high surface tension in the lungs to reduce the mechanical work of breathing and prevent alveolar collapse. The disruption of this surface tension modulation is characteristic in both neonatal and acute respiratory distress syndromes, caused either by the deficiency and/or dysfunction of lung surfactant. A better understanding of the adsorption, spreading, and other interfacial characteristics of lung surfactant allows for intelligent design of exogenous surfactant therapies for treating these diseases. Here, we present a new methodology for visualizing interfacial phenomena in a Langmuir trough using confocal microscopy. Confocal microscopy offers two distinct advantages over traditionally used widefield fluorescence microscopy: shallow depth of field and easy integration of multiple acquisition channels. We have overcome many of the difficulties and optical aberrations present when imaging an air-liquid interface, therefore providing three dimensional, in situ, real time, and nondestructive visualization of multiple components of our lung surfactant system. As a result, we have more thoroughly examined the mechanisms of lung surfactant dysfunction during acute respiratory distress syndrome, as well as ways to reverse it. In general, our approach is easily adapted for visualizing the behavior of any number of surfactant systems at not only air-liquid interfaces, but other fluid-fluid interfaces as well.

Protease-Resistant Peptide Ligands from a Cysteine-Rich Scaffold Library Jennifer A. Getz and Patrick S. Daugherty

Department of Chemical Engineering, Institute for Collaborative Biotechnologies, University of California, Santa Barbara

Peptides within the knottin family have been shown to possess inherent stability, making them attractive scaffolds for the development of therapeutic and diagnostic agents. Given its remarkable stability to gastrointestinal digestive proteases, the cysteine-rich, cyclic peptide kalata B1 was employed as a scaffold to create a large knottin library displayed on the surface of E. coli. A library exceeding 109 variants was constructed by randomizing seven amino acids within a loop of the kalata B1 scaffold and screened using fluorescence-activated cell sorting to identify peptide ligands specific for two distinct targets: human thrombin and human neuropilin-1. Thrombin is an enzyme involved in the formation of blood clots and thrombin inhibitors are useful for treating numerous conditions. Neuropilin-1 is a coreceptor for VEGF and ligands binding to this protein could be used to improve both cancer detection and treatment. Refolded thrombin binders were extensively characterized and exhibited high nanomolar affinities in solution, slow dissociation rates, and were able to inhibit thrombin’s enzymatic activity. Importantly, 80% of a knottin-based thrombin inhibitor remained intact after a two hour incubation both with trypsin and with chymotrypsin, demonstrating that modifying the kalata B1 sequence did not compromise its stability properties. In addition, an identified knottin-based thrombin inhibitor mediated 20-fold enhanced affinity, when compared to the same seven residue binding epitope constrained by a single disulfide bond. We are currently engineering the scaffold to improve the folding of kalata B1 peptides produced in bacteria in addition to affinity maturing the first-generation ligands. Our results indicate that peptide libraries derived from the kalata B1 scaffold can yield ligands that retain the protease resistance associated with the parent peptide. More generally, this strategy may prove useful in the development of stable peptide ligands suitable for in vivo applications.

Discovery and Characterization of Antibody Biomarkers of Autoimmune Diseases and Reagents for Their Detection

Serra E. Elliott,(a) Alex R. Soffici,(b) and Patrick S. Daugherty(a) (a) Department of Chemical Engineering, University of California, Santa Barbara

(b) Maternal-Fetal Medicine Unit, Cottage Health System, Santa Barbara

Bacteria-displayed peptide libraries paired with Fluorescence Activated Cell Sorting (FACS) yields a quantitative method to isolate unique binding ligands to a variety of protein targets. By using antibody repertoires isolated from diseased patients as our target, we seek to utilize this method to discover and characterize peptides specific to a diseased state [1]. In autoimmune diseases, where an aberrant immune response leads to antibodies and/or lympocytes that target human proteins, discovery of these unique peptides can lead to biomarker establishment for improved diagnosis and understanding of disease pathogenesis. As testing grounds for methodology development, we will focus on the advancement of molecular diagnostics for pre-eclampsia, a disease that affects approximately 5% of pregnancies [2] but remains poorly understood. Previous studies have reported that patients develop autoantibodies against the angiotensin II AT1 receptor [3]. These antibodies, upon injection into pregnant mice, induce the symptoms of pre-eclampsia [2]. Based upon these findings, we have expressed the seven amino acid antibody binding region, epitope, on the surface of E. coli for further analysis with a new patient cohort of both pre-eclamptic and “normal” pregnant women. We also screened a bacteria-displayed peptide library with this same patient set, using two pools of labeled disease and one pool of unlabeled “normal” antibodies (Figure 1). Among the isolated peptide mimics, nine unique peptides have demonstrated significantly higher reactivity with pre-eclampsia patients’ samples than “normal” pregnancies. Additionally, we investigated whether some peptide properties help confer disease specificity. Not only can we construct a novel diagnostic array for pre-eclampsia from these peptide reagents, but the methods developed in this work can be generally applied to other diseases for which clear biomarkers remain to be discovered.

References: [1] Hall, S.S.; Daugherty, P.S. Protein Sci. 2009 18, 1926-1934. [2] Zhou, C. C. Zhang, Y. Irani, R. A. Zhang, H. Mi, T. Popek, E. J. Hicks, M. J. Ramin, S. M. Kellems, R. E.; Xia,

Y. Nat. Med. 2008, 14, 855-862. [3] Wallukat, G. Homuth, V. Fischer, T. Lindschau, C. Horstkamp, B. Jupner, A. Baur, E. Nissen, E. Vetter, K.

Neichel, D. Dudenhausen, J. W. Haller, H.; Luft, F. J. Clin. Invest. 1999, 103, 945-952.

5

4

3

Figure 1: Schematic Outlining the Screening Strategy Used to Isolate Unique Peptides that Bind Disease Antibodies (IgG). Figure adapted from [1].

Sequencing and Specificity Analysis

Repeat Sorting to Enrich

Display Library 1

Unlabeled “Normal” IgG

Non-Library Expressing Bacteria

Disease IgG 1

Disease IgG 2

2

Photothermally Triggered Drug Release from Temperature Sensitive Liposomes Natalie Forbes(a) and Joseph A. Zasadzinski(b)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Department of Chemical Engineering and Materials Science, University of Minnesota, Twin Cities

A drug delivery system with rapid contents release under continuous-wave near-infrared (NIR) light will enable precise spatial and temporal control of drug release. The drug is encapsulated within temperature sensitive liposomes, and rapid drug release is triggered by the photothermal heating of hollow gold nanoshells coupled to the liposome surface. The nanoshells locally heat the liposome membrane to the phase transition temperature and thereby increase membrane permeability. The liposome membrane consists of dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC), and monopalmitoylphosphatidylcholine (MPPC). By varying the ratio of these three phospholipids, the temperature at which release was initiated was adjusted between 39-45°C. Rapid release of 60-70% of the encapsulated contents was observed at physiological temperatures under irradiation with continuous-wave NIR light due to localized photothermal heating by the nanoshells. In contrast, no release of encapsulated contents was observed at physiological temperatures without continuous-wave NIR irradiation. This system could be used for cancer therapy to deliver anticancer agents directly to a tumor site.

Biomedical Applications

High-Pressure Nonlinear Electrokinetic Micropumps for Portable Microfluidic Devices Joel S. Paustian and Todd M. Squires

Department of Chemical Engineering, University of California at Santa Barbara E-mail: [email protected]

Microfluidic devices (e.g. Labs on a Chip) are becoming useful scientific and medical tools for automating chemical and biological lab work. Various impediments prevent complex microfluidic devices from being easily removed from a laboratory setting, limiting their utility for day-to-day applications like in-the-field medical diagnostics and drug delivery. The development of portable and integrable high-pressure pumping techniques will be necessary step for truly portable, complex microfluidic devices. Microfluidic pumps based on the nonlinear electrokinetic phenomenon of Induced-Charge Electroosmosis (ICEO) could potentially fill this role. We describe ICEO flow and present a simple idea for a low-voltage, high-pressure micropump. We give simple scaling arguments, and a detailed theory, for its expected performance, and describe the design, fabrication, testing and characterization of a functional ICEO micropump. Our results validate the central idea, are consistent with our theoretical expectations, and suggest routes for the optimization and eventual use of the pump.

(a) (b)

Figure 1: (a) Conceptual image of ICEO micropump, viewed from above. The pump consists of an array of rectangular photoresist features patterned on a nonconducting substrate. One face of the features has been coated with metal. These features are contained in a microfluidic channel filled with electrolyte. When an AC field is applied through the electrolyte, an ICEO flow is created over the metal, resulting in a net flow to the right. To satisfy mass conservation, nature imposes a pressure driven flow in the opposite direction. Theory predicts that the pressure difference across the pump grows dramatically as the gap size, s, decreases, ΔP~1/s3. (b) Completed microfluidic device containing the pump. The 2 cm long array of 50x10x10 micron rectangular photoresist features is encased in a PDMS microfluidic channel. An AC field is applied across the array via the electrical contacts protruding from the PDMS. This results in a pressure difference along the length of the pump.

Robust Performance of Closed-loop Control Design Based on Insulin Pharmacokinetics / Pharmacodynamics

Justin Lee,(a,c) Eyal Dassau,(a,c) Stuart Weinzimer,(b) Howard Zisser,(a,c) William Tamborlane,(b) and Francis Doyle III(a,c)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) School of Medicine, Yale University (c) Sansum Diabetes Research Institute

An individual with type 1 diabetes mellitus (T1DM) is required to receive insulin therapy due to insufficient insulin production from the pancreatic β-cells. However, manual insulin therapy is a burdensome task with multiple steps. The development of a closed-loop artificial pancreas (AP) started nearly four decades ago to improve blood glucose (BG) regulation and the quality of life of people with T1DM. Currently, a number of insulin formulations with different pharmacokinetic (PK) and pharmacodynamic (PD) characteristics are available, and others are in developmental stages. The PK/PD of a given insulin formulation plays a major role in the tradeoff between performance (e.g. BG peak or duration in the hyperglycemia region, > 180 mg/dL) and robustness (e.g. oscillation of BG or hypoglycemia, < 60 mg/dL) of the closed-loop AP. Thus, a research platform that addresses the performance/robustness tradeoff based on different PK/PD of insulin formulation will be highly beneficial for controller designs and new insulin formulation development. We have developed a research platform to investigate the performance/robustness tradeoff of closed-loop systems based on different PK/PD of insulin formulations. A PK model of different insulin formulations was characterized by one adjustable parameter (τPK) and incorporated into the intravenous (IV) port of the UVa/Padova FDA-accepted metabolic simulator. Then, a Proportional-Integral-Derivative (PID) controller with one adjustable parameter (λ, controller time constant) was calculated based on the PK/PD model of insulin formulation and the IV port, using the Internal Model Control (IMC) methodology. The performance/robustness of the controller was analyzed for a range of insulin formulations (25 min < τPK < 100 min) on 10 in silico subjects. An unannounced 75 g-carbohydrate meal protocol was used for the evaluation. Result analysis shows that the AP with a faster acting insulin formulation performs better, but it may cause a damped oscillation of BG concentration to a subject of high insulin sensitivity (e.g. subject 4 from the UVa/Padova simulator) with aggressive controller tuning (λ = 25 min). The average max BG concentration decreased by 11 mg/dL per 25 min decrease of τPK and 4 mg/dL per 50 min decrease of λ. The average duration in the hyperglycemia region decreased by 0.6 h per 25 min decrease of τPK and 0.4 h per 50 min decrease of λ. No hypoglycemia or instability of BG (e.g. divergence) was observed for different insulin formulations.

Multi-Parameter Continuation and Higher-Order Sensitivities for Quantifying Nonlinear Effects in Circadian Models

Peter St. John and Francis J. Doyle III Department of Chemical Engineering, University of California Santa Barbara

The ubiquitous nature of circadian rhythms is a strong indication to their significance in biological organization. For countless millennia, organisms have used internal biochemistry both to aid in predicting changes in their external environment and to synchronize group behavior. A molecular understanding of the circadian clock permits the formulation of quantitative models, which – if accurate – can be used to quickly assess methods of re-entraining the clock using light or pharmacological action. Because biological systems typically employ redundant pathways for robustness, the inhibition of targets along one pathway is not as significant as the inhibition of targets across redundant pathways. The sensitivity of the system to parameter perturbations is defined as the changes induced in the systems behavior as a result of small changes in parameter values. Using a deterministic mathematical model of the mammalian circadian clock, we have investigated the responses of the system to various changes. Recent experimental work has begun to knock down multiple reaction rates simultaneously in the circadian loop, providing a wealth of data about the nonlinear interactions of key clock components. [1] In order to investigate these effects in silico, we used higher order sensitivity matrices to identify parameter interactions which result in cooperative or competitive interactions. Once found, we further explore these interactions by solving for stable limit cycles over an area of parameter values to generate plots of predicted period, phase, or amplitude. Because this can be a computationally intensive task, we have developed a basic method of multi-dimensional continuation, shown in figure 1.

Figure 1: Preliminary results for multi-parameter continuation for predicting the effects of simultaneous parameter perturbations. At each iteration, a two-dimensional spline is fit and used to predict the next set of solutions. References: [1] Zhang, E., Liu, A., Hirota, T., Miraglia, L., Welch, G., Pongsawakul, P., Liu, X., Atwood, A., Huss, J., Janes, J.,

Su, A., Hogenesch, J. Kay, S. Cell 2009, 139, 199-210.

Biomolecular Mechanisms

Charge Effects on the Fibril-forming Peptide KTVIIE: a Two-dimensional Replica Exchange Simulation Study

Joohyun Jeon and M. Scott Shell Department of Chemical Engineering, University of California at Santa Barbara

The assembly of peptides into ordered nanostructures is increasingly recognized as both a bioengineering tool for generating new materials as well as a critical aspect of aggregation processes that underlie neurological diseases such as Alzheimer’s, Parkinson’s and Huntington’s disease.[1] To understand the detailed microscopic driving forces of self-assembly, we developed a novel methodology to quantify and compare the propensity of different peptide sequences to form small oligomers during early self-assembly stages. This Umbrella Sampling Replica Exchange Molecular Dynamics (UREMD) method [2] performs a REMD simulation along peptide association reaction coordinates using umbrella restraints. The model systems that we study are a set of sequence-similar peptides that differ in net charge: K+TVIIE-, K+TVIIE, and +K+TVIIE. Interestingly, experiments show that only the monovalent peptide, K+TVIIE, forms fibrils, while the others do not.[3] We study dimer, trimer, and tetramer formation processes of these peptides, and compute high-accuracy potential of mean forces (PMFs) association curves. The PMFs recapitulate a higher stability and equilibrium constant of the fibril-forming peptide, similar to experiment, but reveal that entropic contributions to association free energies can play a surprisingly significant role. The simulations also show behavior reminiscent of experimental aggregate polymorphism, revealed in multiple stable conformational states.

Figure 1: PMF curves for dimerization (left), trimerization (middle) and tetramerization (left) processes. Statistical errors of each curve around the binding regime are represented as error bars in each plot. At the tetramer level, (p) indicates the association of two parallel dimers, while (ap) that of antiparallel dimers. References: [1] Walsh, D. M.; Selkoe, D. J. J. of Neurochemistry. 2007, 101, 1172-1184 [2] Gee, J.; Shell, M. S. J. Chem. Phys. 2011, 134, 064112 [3] Lopez de la Paz, M.; Goldie, K.; Zurdo, J.; Lacroix, E.; Dobson, C.; Hoenger, A.; Serrano, L. Proc. Nati. Acad.

Sci. 2002, 99, 16052-16057

Solvent Dynamics in Ion Pair Dissociation Ryan Gotchy Mullen,(a) Joan-Emma Shea,(b,c) and Baron Peters(a,b)

(a) Deptartment of Chemical Engineering, University of California, Santa Barbara (b) Deptartment of Chemisty & Biochemistry, University of California, Santa Barbara

(c) Deptartment of Physics, University of California, Santa Barbara Having an accurate reaction coordinate provides mechanistic insight, facilitates rate constant calculations, and provides free energy landscapes that are kinetically meaningful. Reaction coordinates for reactions that involve the solvent are notoriously difficult to identify. The aqueous dissociation of NaCl would seem simple because there is only one solute coordinate, the ionic distance. However, many studies have shown that ionic distance is not a good coordinate. By using the aimless shooting version of transition path sampling and likelihood maximization, we have identified a reaction coordinate for this system. The coordinate combines a localized density of water molecules with the ionic distance. We also investigate the relationship between the transmission coefficient and reaction coordinate error. We discuss possible implications for simulations of ionic crystal growth by kink site insertion and peptide aggregation.

A New Multi-scale Platform for the Investigation of Peptide Self-assembly Scott Carmichael and M. Scott Shell


Peptide aggregation plays a role in a number of neurodegenerative diseases, such as Alzheimer's, Huntington’s, and Parkinson’s. Here we aim to develop an accurate molecular-scale picture of the aggregation process using a multi-scale computational approach. Recently, Shell [1] developed a coarse-graining methodology that is based on a thermodynamic quantity called the relative entropy; a measure of how different two molecular ensembles behave. By minimizing the relative entropy between a coarse-grained (CG) system and an all-atom (AA) system an optimized coarse-grain model can be obtained. Here we develop a corresponding numerical strategy for optimizing CG models of arbitrary peptide sequences, and apply it to a model polyalanine molecule (Ala)15. Specifically, we optimize the CG force-field parameters that control the behavior of a two bead per amino acid model (mapping shown in Fig. 1.a) of (Ala)15. Simulations of the optimized CG model peptide are performed, and histograms of the structural correlations (eg. bond length, bond angle, etc.) are extracted and compared to an AA system. We obtain excellent agreement for all of the optimized force-fields (Fig 1.b shows one such histogram and its associated force-field) indicating the CG model was able to accurately reproduce the AA system’s behavior. Our current work focuses on using these CG models to simulate the self-assembly of large-scale systems that are too computationally demanding to study using conventional AA simulations. Simulations containing many CG (Ala)15 peptides formed the correct, experimentally observed [2], β-sheet structure typical of alanine rich peptides (Fig 1.c). These large-scale simulations provide the detailed picture of the self-assembly process that is necessary for elucidating precise mechanistic attributes of it. a) b) c)

Figure 1: a) Coarse-graining groups for two bead model of an alanine residue. The backbone oxygen is grouped into one bead (O), and all of the other atoms are grouped into another bead (C). b) A representative pair distance histogram and its respective force-field for a CG model of (Ala)15. c) Twenty-five double-bead (Ala)15 CG models forming a β-sheet like structure, inset shows two such peptides stacked and in register. References: [1] Shell, M. S. J. Chem. Phys. 2008, 129, 144108-7. [2] Forood, et al., Biochem. and Biophys. Research Comm. 1995, 211, 7-13.

Direct Measurement of Light-Modulated Intra- and Inter- Aggregate Forces Stephen H. Donaldson Jr.,(a) C. Ted Lee Jr.,(b) Jacob Israelachvili,(a) and Bradley F. Chmelka(a)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Department of Chemical Engineering and Materials Science, University of Southern California

Correlations are established among the molecular structures, interaction forces, and physical processes associated with light-responsive self-assembled surfactant monolayers or bilayers at interfaces. Using the surface forces apparatus (SFA), the interaction forces between adsorbed monolayers and bilayers of an azobenzene-functionalized surfactant can be drastically and controllably altered by light-induced conversion of trans and cis molecular conformations. These reversible conformation changes affect significantly the shape of the molecules, especially in the hydrophobic region, which in turn induces dramatic transformations of molecular packing in self-assembled structures, causing corresponding modulation of electrostatic double layer, steric hydration, and hydrophobic interactions. For bilayers the isomerization from trans to cis exposes more hydrophobic groups, making the cis bilayers more hydrophobic, which lowers the activation energy barrier for (hemi)fusion. A quantitative and general model is derived for the interaction potential of charged bilayers that includes the electrostatic double-layer force of the DLVO theory, attractive hydrophobic interactions, and repulsive steric-hydration forces. The model quantitatively accounts for the elastic strains, deformations, long-range forces, energy maxima, adhesion minima, as well as the instability (when it exists) as two bilayers breakthrough and (hemi)fuse.

Filming Seven-Transmembrane Protein Activation with Magnetic Resonance: Conformational Dynamics of an α-helical Cytoplasmic Loop Segment

Sunyia Hussain,(a) Maia Kinnebrew,(b) and Song-I Han(a,c) (a) Department of Chemical Engineering, University of California, Santa Barbara

(b) Department of Biology, University of California, Santa Barbara (c) Department of Chemistry and Biochemistry, University of California, Santa Barbara

Seven-transmembrane (7TM) proteins have diverse and important functions, ranging from signaling receptors to ion pumps. They share a remarkable reversible switching property, epitomized by the solar-powered microbial proton pump Proteorhodopsin (PR), which uses light energy to facilitate the transport of a proton across the cell membrane due to a conformational “switch”. Here, we use PR as a model system to capture the elusive details of activation necessary for the function of physiologically important membrane proteins. Our work finds an interesting structural feature of PR’s third cytoplasmic (E-F) loop: a short α-helical segment that experiences conformational change upon photoactivation, as observed from the unique magnetic resonance techniques of electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP). These methods provide insight into the protein segment mobility and local hydration water dynamics of an amino acid residue spin-labeled with nitroxide-based radicals. Interestingly, the internal α-helix of PR’s E-F loop is a common motif to the G-protein coupled receptor bovine rhodopsin (Rh) [1], where it functions as a docking point for the G-protein signaling molecule. Towards probing the possible link between structure and conformational dynamics between these two non-homologous proteins, we have developed a PR-Rh chimera by replacing the E-F loop of PR with the corresponding loop of Rh. The chimera has been successfully expressed in bacteria and maintains its optical properties. We evaluate its capability to activate the G-protein transducin, and apply the techniques of EPR and DNP to obtain unique information about the biophysics of receptor/G-protein interactions.

GTP GDP

E

GK

SAN

C

T A α-helical E-F loop

α-helical loop 3 of Rh

intracellular

extracellular

PR

H+

light-triggered conformational changes

G-protein transducin

αβγ

PR-Rhchimera

Figure 1: Schematic of the engineered PR-Rh chimera and the properties of structure and function that can be probed by measurements of dynamics via EPR and DNP References: [1] Yeagle, P.L; Alderfer, J.L; Albert, A.D, Biochemistry 1995, 34, 14621-14625.

Catalytic Materials

Structural and Catalytic Properties of Cerium Oxide Nanoparticles Doped with Ruthenium Alan Derk,(a) Eric McFarland,(a) and Horia Metiu(b)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Department of Chemistry and Biochemistry, University of California, Santa Barbara

Ruthenium with cerium oxide (CeO2) has been shown to be a moderate-temperature, stable gas-phase heterogeneous catalyst for methane reforming with implications for energy conversion, especially hydrocarbon processing. In this work, ruthenium oxide was both supported-on and doped-into cerium oxide nanoparticles using wet impregnation, deposition-precipitation and self-propgating high-temperature synthesis (combustion synthesis). These several synthesis methods were used to contrast the differences between ruthenium when supported vs. doped into cerium oxide. The catalytic nanoparticles were structurally characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and nitrogen adsorption. Catalytic testing was performed in a quartz-tube packed bed reactor with effluent analysis via gas chromatograph and mass spectroscopy. With regard to dry reforming of methane (CO2+CH4->CO+H2) and partial oxidation of methane (O2+2CH4->2CO+4H2), the catalysts with ruthenium doped into cerium oxide have better catalytic activity with respect to ruthenium supported on cerium oxide. Future work will include investigating different industrially-relevant reactions and optimization of catalysts and parameters for chemical looping reforming.

Computational Mechanistic Investigation of the Phillips Polymerization Catalyst Anthony Fong, Baron Peters, and Susannah Scott

Department of Chemical Engineering, University of California Santa Barbara

The Phillips chromium catalyst polymerizes ethylene by an unknown mechanism [1]. Investigating the mechanism with experimental techniques is challenging because only a small fraction of chromium atoms participate in catalysis. Somehow a tiny portion of the inorganic precursor, chromium oxide supported on amorphous silica, transforms into the catalytically active organometallic species. Many initiation, propagation, and termination mechanisms have been proposed along with various proposals for the active site geometry. We used density functional theory to evaluate the feasibility of the previously proposed mechanisms. Our calculations rule out propagation by a growing metallacycle chain [2] (see figure 1), propagation by an alternating alkylidene-metallacycle mechanism [3], and a vinylchromium initiation pathway [1]. The only propagation mechanism with theoretical kinetics that are comparable to the experimental kinetics is the Cossee-Arlman mechanism [4], but the initiation step remains unknown.

Figure 1: Insertion of ethylene into a chromacycloheptane (left) to form chromacyclononane (right). The transition state in the middle shows the bonds breaking and forming. Relative free energies are in kcal/mol. Key: red = oxygen, dark gray = carbon, white = hydrogen, light gray = silicon, purple = chromium References: [1] M.P. McDaniel, Advances in Catalysis, 1985, 33, 47-98 [2] G. Ghiotti, E. Garrone, A. Zecchina, Journal of Molecular Catalysis, 1988, 46, 61-77 [3] M. Kantcheva, I.G. Dalla Lana, J.A. Szymura, Journal of Catalysis, 1995, 154, 329-334 [4] J.P. Hogan, Journal of Polymer Science Part A-1: Polymer Chemistry, 1970, 8, 2637-2652

Probing the Molecular Environment of Hydrogenated Amorphous Silicon Using 1H Solid State DNP at 7 Tesla

Ting Ann Siaw,(a) David C. Bobela,(b) Brandon D. Armstrong,(c) Paul Stradins,(b) and Song-I Han(a,d)

(a) Department of Chemical Engineering, University of California Santa Barbara (b) National Renewable Energy Laboratory, Golden, CO

(c) Department of Physics, Harvard University (d) Department of Chemistry and Biochemistry, University of California Santa Barbara

Paramagnetic defects are often central or detrimental to materials function. Dangling bond defects (DBs) in hydrogenated amorphous silicon (a-Si:H) are no exception as they are intimately linked to the photodegradation (through the Staebler-Wronski effect) of an otherwise competitive energy material. We present a novel nuclear magnetic resonance (NMR) approach for achieving selective dynamic nuclear polarization (DNP) of nuclei only within sub-nanometer distances of intrinsic paramagnetic sites. DNP selectively amplifies the clustered Si-H hydrogen signal of the characteristic two-component 1H-NMR spectrum, while the two-component T1n relaxation rates are both found to be insensitive to overall DB density. We provide the first conclusive evidence of inhomogeneously distributed DBs localized near hydrogens in Si-divacancies, taking side in a long-standing debate on a-Si:H microstructures and providing prospects towards solving the detrimental Staebler-Wronski problem.

Figure 1: Pictorial representation of the two components of the NMR line. The red and blue dotted lines correspond to the narrow Lorentzian and broad Gaussian fits to the NMR line (black line), respectively.

Synthesis, Characterization, and Testing of Shaped PtAg Nanoparticle Catalysts Louis Chin Jones, Zachary Buras, and Mike Gordon


Heterogeneous catalysts play a vital role in many industries such as commodity chemical production, pollution control, fine chemical synthesis, and pharmaceutical manufacturing. In these venues, chemical reactions take place on surfaces that typically consist of supported metal and metal oxide clusters or nanoparticles. It is also well known that dramatic changes in catalytic activity can occur due to finite size effects (i.e., unsaturated atoms at surfaces, edges, kinks, and corners), preferred morphologies, or modification of the nanoparticle’s electronic structure by the support. Thus, understanding the functionality of these catalysts requires correlating their local electronic, chemical, and morphological structure to catalytic activity and selectivity. Unfortunately, many traditional catalyst preparation methods (e.g. precipitation and impregnation) can result in unknown particle size and shape distributions that make it difficult to establish connections between reactivity and morphology.\ In this work, we utilize colloidal synthesis to realize Ag on Pt nanoparticle catalysts with well-defined morphology to evaluate the influence of shape (e.g. cubes, cuboctahedra, and octahedra) and surface composition (Pt vs. Ag) on C2 hydrogenation activity and selectivity. Ag on Pt nanoparticles were synthesized using polyvinylpyrrolidone (PVP) and AgNO3 as growth promoters; nanoparticles were subsequently supported on colloidal silica, redox cycled to remove PVP, chemically etched to remove Ag, and evaluated for hydrogenation activity. IR and NMR were used throughout the synthesis process to understand how different adsorbates (CO, C2H2, C2H4, and PVP) associate with specific particle facets as a function of Ag content. This data was used in conjunction with continuous flow and batch reactor experiments to rationalize how shape and surface composition affect catalyst behavior. In particular, it was found that (1) surface-segregated Ag on Pt dramatically decreases hydrogenation activity while increasing selectivity for C2H4, and (2) Ag removal produces a highly-active, yet non-selective hydrogenation catalyst. The catalytic behavior of Ag on Pt nanoparticles was also compared with a reverse catalyst of Pt on Ag (nanocubes) synthesized by galvanic exchange. Nanoparticle synthesis, spectroscopic investigations, and catalytic testing results will be highlighted.

A Mechanistic Growth Model for Ionic Crystals Preshit P. Dandekar and Michael F. Doherty


The relative surface areas and shapes of the various faces of ionic crystals significantly affect their functionality in areas like catalysis, photovoltaics, zeolites, etc. The layered growth rates of the prominent crystal faces determine the steady-state shape of a crystal. The growth rate depends on the rates of attachment and detachment of solute units on to and from the crystal surfaces[1]. For crystals grown from solutions, these rates are determined by the solid-solid as well as the solid-solvent interactions. A mechanistic understanding of these interactions is instrumental in engineering the shapes of ionic crystals to achieve better functionality. The Periodic Bond Chain (PBC) theory[2] has been used here to study the solid-state interactions in ionic crystals by identifying directions of strong intermolecular interactions. The PBCs are parallel to the edges of the growth spirals and their identification provides insight into the spiral growth mechanism of crystals. A general modeling framework is proposed to identify bond chain networks in ionic crystals that are stoichiometric and do not have any perpendicular component of dipole moment. This framework is applied to various crystal systems like anatase (TiO2), calcite (CaCO3) and zinc oxide (ZnO) crystals using their crystallography data as input. The intermolecular interaction energies along the PBC directions in these ionic crystals have been calculated and the lattice energies have been accurately predicted. The kink and edge energies will be calculated to study the growth of spirals on these crystal faces (Figure 1) to estimate the relative growth rates and eventually predict steady-state shapes of ionic crystals.

Figure 1: AFM image of a growth spiral on the {10-14} surface of calcite[3]. The image size is 3 μm × 3 μm.

References: [1] Markov, I. V. Crystal Growth for Beginners, Fundamentals of Nucleation, Crystal Growth and Epitaxy; World

Scientific: Singapore 2003. [2] Hartman, P.; Perdok, W. G. Acta. Cryst. 1955, 8, 49-52. [3] Davis, K. J.; Dove, P. M.; Wasylenki, L. E.; De Yoreo, J. J. Am. Mineral. 2004, 89, 714-720.

Single-Step, Plasma-Based Synthesis of Alloy and Supported Nanoparticle Catalysts Travis Koh and Michael Gordon


Many established and future technologies rely on the unique chemical and opto-electronic properties of nanometer-scale particles. For example, metal, metal oxide, and semiconductor nanoparticles find application in heterogeneous catalysis, photocatalysis, and photovoltaics. In light of such needs, a robust synthesis route for size and composition-controlled nanoparticles would be highly desirable. In this work, we present a single-step, plasma-based synthesis route to produce a wide range of surfactant-free nanoparticles (e.g. metals, alloys, metal oxides, oxide-supported metals, and semiconductors) in the 1-20 nm size range that can be used in catalytic applications. A hydrodynamically-stabilized, atmospheric pressure, micro-hollow cathode plasma discharge with high density and temperature is used to pyrolyze volatile organometallics and semiconductor precursors to nucleate small crystalline nanoparticles. Particle growth is quenched at small, uniform size because the residence time in the plasma is very short (~microseconds). The end result is an aerosol of surfactant-free nanoparticles that can be spray-deposited on a variety of surfaces. This poster will focus on the synthesis and characterization of metallic (Pd, Fe, Ni), alloy (Pd-Ag, Fe-Ni), and oxide-supported metallic nanoparticles (Pd/SiO2) realized via microplasma deposition. In-situ FTIR and catalytic testing of Pd and Pd/SiO2 for selective hydrogenation and CO oxidation were performed to probe activity and metal-support interactions. With respect to hydrogenation, both Pd catalysts exhibited low temperature light-off (< 70 deg C) and high ethylene selectivity at high conversion. Pd catalysts spray-deposited on ITO and graphitic supports were also tested for electrochemical oxidation of methanol and formic acid. Finally, we will highlight how microplasmas can be used to realize a variety of nanostructured materials for various technological applications.

Polymeric Systems

Characterization of Multi-domain Polymers by Dynamic Nuclear Polarization-Amplified Spin Dynamics Measurements at 7 Tesla

S.A. Walker,(a) J. Ortony,(b) J. Hunt,(c) J. Spruell,(c) N. Lynd,(b,c) T.A. Siaw,(a) C.J. Hawker,(b,c) and S. Han(a,b)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Department of Chemistry and Biochemistry, University of California, Santa Barbara

(c) Materials Department, University of California, Santa Barbara

It is demonstrated that naturally abundant 13C dynamic nuclear polarization(DNP)-enhanced solid state nuclear magnetic resonance (NMR) spectroscopy with strategically placed paramagnetic spin labels can be used to reveal nano-phase separation in a new class of fascinating multi-domain copolymer hydrogels. While interest and development of high field DNP-enhanced NMR has surged in the past 5-6 years, studies have been largely focusing on biological macromolecules in the solid state. Rarely has DNP-NMR been used to study materials properties such as interfaces, surfaces, or dilute phenomena in general. Beyond utilizing the orders of magnitude enhancements from DNP, we employ strategically positioned free electron spin labels to induce local polarization hot spots and attempt to track the propagation of nuclear polarization as a function of microwave (MW) irradiation time to gain insight on the presence and size of domains in the sample of interest. The novel technique in development aims to comparatively elucidate nano-scale heterogeneities in polyelectrolyte hydrogels by tracking the naturally abundant 13C DNP-enhanced polarization emanating from a site specific paramagnetic spin label and moving across domain boundaries from one distinct spin reservoir to another. Under certain conditions the characteristics and rate of the buildup of DNP-amplified NMR signal can depend on material morphology and domain boundaries. Spatially propagating polarization in heterogeneous vs. homogeneous copolymer samples is revealed in the characteristic DNP-NMR build-up curves (3.5 K and 7 T) with a 4-Carboxy-TEMPO spin label– the former showing non-exponential two-regime behavior and the latter showing single exponential behavior. A coacervated domain diameter of ~70 Å vs 120 Å is predicted by DNP-NMR and small angle neutron scattering experiments, respectively. The heterogeneous sample is known to contain two separate domains and it is concluded that DNP-enhanced spin dynamics measurements can successfully probe and reveal these nano-scale heterogeneities. High field solid-state DNP spin dynamics measurements have shown much promise in revealing and quantifying nano-scale heterogeneities in two-domain polyelectrolyte triblock copolymers.

Macro- and Microphase Separation in Multifunctional Supramolecular Polymer Networks Zoltan Mester,(a) Aruna Mohan,(b) and Glenn H. Fredrickson(a,c)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Exxon Mobil

(c) Department of Materials and Materials Research Laboratory, University of California, Santa Barbara

We present a field-based model for the phase separation and gelation of a binary melt of multifunctional monomeric units that can reversibly bond to form copolymer networks. The mean-field phase separation behavior of several model networks with heterogeneous bonding is calculated via the random phase approximation (RPA). The extent of bonding between polymers is controlled by specified bond energies. The phase boundary calculated via RPA is the stability limit of the homogeneous disordered phase to coexisting homogeneous macrophases, for low bond strengths, and to microphases, for high bond strengths. Higher functionality and more favorable bonding energy suppresses macroscopic phase separation due to greater connectivity between unlike species. Gelation occurs with sufficiently high connectivity of tri- or higher functional monomeric units and microphase separation of the copolymer network occurs preceding and after the gel point. Eutectic-like behavior of the spinodal is seen in highly connected networks due to excess loosely connected material in systems having non-stoichiometric ratios of the two components.

Transport and Interfacial Phenomena

Multiscale Studies of Hydrophobic Association Aviel Chaimovich, Scott Shell, and Jacob Israelachvili


Life's self-assembly processes are governed by the hydrophobic force, i.e., the water-mediated association of nonpolar groups. Yet, a comprehensive understanding of hydrophobicity remains lacking, particularly regarding its multiscale behavior; for example, the free energy of hydration changes from volume to area scaling as the hydrophobe’s size increases. In this work, we examine hydrophobic association with the use of molecular simulations that leverage all-atom and coarse-grained water models, being able to probe multiple scales. Our initial investigation examines an isotropic (single-site) water model. We optimize this model using the so-called relative entropy approach, ensuring ideal reproduction of bulk water structural correlations [1]. We show that such a simplified description effectively manifests water’s hydrogen bonding behavior via the variation in state space of the optimized models [2]. These features in turn give rise to signatures of water’s unique thermophysical properties (temperature of maximum density, diffusivity increase upon compression, etc.) [2]. By examining the association between a pair of nonpolar solutes immersed in the spherically-symmetric solvent, we also demonstrate that this model can explain various aspects of the hydrophobic force [3]. Particularly, we investigate the expected manifestation of the crossover in the scaling of the free energy by varying the size of the hydrophobes. Furthermore, we examine the infinite limit of this scenario, the force between two planar hydrophobic surfaces. Corresponding experimental measurements suggest that the (pure) hydrophobic force has two separate regimes (in separation distance): a short-range spanning ~ 0.1-1 nm and a long-range spanning ~ 1-10 nm [3]. We attempt to resolve this unexplained phenomenon, addressing several possible origins of these multiscale effects.

O

H

H

H2 O

Figure 1: The relative entropy optimization procedure for a spherically-symmetric water model References: [1] Shell, M. S. J. Chem. Phys. 2008, 129, 144108. [2] Chaimovich, A.; Shell, M. S. Phys. Chem. Chem. Phys. 2009, 11, 1901-1915. [3] Hammer, M. U.; Anderson T. H.; Chaimovich, A.; Shell, M. S.; Israelachvili J. Faraday Discuss. 2010, 146, 299-308.

Molecular Dynamic Studies of Mean-Field Theories for the Electric Double Layer Brian Giera,(a) Ed Kober,(b) Scott Shell,(a) and Todd Squires(a)

(a) Department of Chemical Engineering, University of California, Santa Barbara (b) Institute for Multiscale Materials Studies, Los Alamos National Laboratory, Los Alamos, New Mexico

In this work, we use a non-dimensionalized molecular dynamics (MD) model to examine mean-field theories that give predictions for the voltage and density profiles of soft-core ions in an implicit solvent between two hard flat electrodes that bound the electrolyte in the z-dimension. These ions form a diffuse nanoscale cloud termed the electric double layer (EDL) that screens the electrode's surface charge and plays a central role in electric double-layer capacitors (EDLCs, or supercapacitors). EDLCs store energy electrochemically across the EDL and can be used to regulate power supply to unbalanced electrical grids, recapture the braking energy of light rail cars, busses, and elevators, and store intermittent energy from solar, wind, or tidal energy sources. Continuum models used to describe EDL structure and formation relate ion density distributions to the electrostatic potential via Poisson's equation by replacing the N-body interaction potential between ions with an effective 1-body mean-field potential. As such, mean-field theories often fail in describing real EDLC systems that operate at large potentials where electrostatic and excluded volume correlations arise that contribute to the excess chemical potential that ideal Poisson-Boltzmann theory assumes is negligible. We determine these components of the excess chemical potential from simulation using a modified Widom insertion technique to assess the validity of mean-field theories that address ion-size effects as well as provide insight into developing more sophisticated models that also capture electrostatic non-idealities as necessary for rational design and engineering of EDLCs.

A Coarse-graining Procedure for Mapping Atomistic Models Chia-Chun Fu, Pandurang M. Kulkarni, M. Scott Shell, and L. Gary Leal


It is known that the continuum assumption breaks down when the length scale of the flow domain approaches the order of a few nanometers. In this case, the conventional boundary conditions such as no-slip are inadequate to provide a correct description of the flow in this region, and the Navier-Stokes equations are no longer valid. Molecular dynamics (MD) simulations, in principle, are capable of resolving such physical problems and have provided accurate flow information near the boundaries. However, due to computational limits, MD can only provide information over extremely short length and time scales (a few nanometers and nanoseconds), which makes it impossible to compare the results against experiments. For these reasons, it is desirable to develop a multi-scale method that combines a molecular description near the boundaries and a continuum based solver in the rest of the domain. A coarse-grained (CG) model is developed to represent an intermediate transition region between the atomistic and continuum scales. The new CG model is constructed to reduce the computational cost relative to using a full atomistic simulation and preserve both the thermodynamic and the transport properties of the underlying atomistic system. The iterative Boltzmann inversion method is employed to determine a CG potential for a (1- )N “particle” system ( is the degree of coarse-graining and 0 ≤ < 1 ) that reproduces the radial distribution function (RDF) of an N “particle” system. The CG potential is then modified to match both RDF and pressure of the full atomistic system by performing an optimization procedure. Dynamical properties, such as diffusion and viscosity, can also be recovered by tuning the friction coefficient, γ, in the dissipative particle dynamics (DPD) thermostat. The properties of the CG force fields and the transport behaviors are presented for a wide range of with various ranges of interaction and γ.

Cantilevered-Capillary Force Apparatus for Measuring Multiphase-Fluid Interactions John Frostad and Gary Leal


Multiphase-fluid interactions are intrinsic to numerous, important industrial processes. The dynamics of droplet coalescence, for example, determines the texture and mouthfeel of many food products and the mechanical properties of blended-polymeric materials. The physical properties of vesicle systems determine their suitability for drug delivery vehicles as well as the quality of cosmetics, laundry detergents and other personal care products. A Cantilevered-Capillary Force Apparatus (CCFA) has been designed and built to increase the understanding of multiphase-fluid interactions. The CCFA consists of two capillaries in which one of capillaries is bent at a 90 degree angle so that it is compliant as shown in Figure 1. A rectangular mirror attached to the cantilevered-capillary is used with a laser to measure the deflection and therefore interaction force. The current setup is capable of measuring interaction forces in the 10 nN – 100 μN range. In this work the CCFA is used to measure interfacial tension and study coalescence. The method for measuring interfacial tension is validated using a bubble of air in water (see Figure 2) for which the value is well known. The interfacial tension of several liquid-liquid systems of interest to our lab is also measured. The CCFA is useful for measuring interfacial tension due to its capability for a precision of ± 0.01 mN/m or better with a wide range of fluid-fluid systems. In addition to measuring interfacial tension, coalescence is studied while applying a constant compressive force between two droplets. When the drops are close enough to interact and deform they are still separated by a thin film of suspending fluid. The time required for this thin film of fluid to drain and then rupture leading to coalescence is measured. This so called drainage time is an important timescale for the stability of an emulsion. The experimental results are compared to the predictions of a scaling theory. The theory accounts for the relevant length scale of the pressure gradient in the thin film that drives the drainage of the thin film. Future work is aimed at measuring the force of adhesion between two deformable vesicles.

Figure1: Schematic of Cantilevered-Capillary Force Apparatus. Figure 2: Stretching a bubble to measure interfacial tension.

Microstructural Transformations in Concentrated Charged Vesicle Suspensions: The Crowding Hypothesis

Mansi Seth, Arun Ramachandran, and L. Gary Leal Department of Chemical Engineering, University of California, Santa Barbara

It is known that vesicles deflate and thereby undergo a transition from a unilamellar to a bilamellar state, when subjected to hyper-osmotic salt gradients [1]. The lamellarity distribution of vesicles in a concentrated suspension plays an important role in its stability and rheology, since it fixes the volume fraction of the suspension at a given concentration of surfactant. We propose that deflation can also occur due to ‘crowding’ of charged vesicles in a concentrated suspension, under purely repelling solution conditions. The repulsive electrostatic pressure between charged vesicles in such suspensions can cause them to deflate and thus undergo a microstructural transformation. We use CryoTEM imaging to study the time evolution of charged vesicle suspensions of different surfactant concentrations. We find that at low concentrations, the microstructure of the suspension is unchanged over long times (~months); while at higher concentrations, it exhibits a unilamellar to bilamellar transformation within a few days [Fig.1]. This is consistent with our hypothesis that there exists a critical volume fraction above which the microstructure of a charged vesicle suspension undergoes a unilamellar to bilamellar transformation.

Figure 1: CryoTEM images of a 35 mg/mL suspension of cationic vesicles. At time t=0, a large number of deflated and curled up vesicles are seen. After 3 days, there is a reduction in the number of deflated/curled up vesicles and an increase in the number of double-walled vesicles.

t = 0 t = 3 days

References: [1] Saveyn, P.; Cocquyt, J.; Bomans, P.; Frederick,P:, Cuyper, M.; Van der Meeren, P. Langmuir. 2007, 23(9),

4775-4781

Interfacial Properties of Copolymer Monolayers Zachary A. Zell, L. Gary Leal, and Todd M. Squires


The stability and flow behavior of multiphase materials can be greatly affected by the presence of surfactants. An understanding of how the interfacial rheology is influenced by surfactants is therefore essential for materials such as foams and emulsions. Typically, the compressional or dilatational rheological properties of surfactants play a larger role than surface shear properties; however, shear rheological measurements can be used as a tool for understanding polymer configurations and relaxation mechanisms at the interface [1]. In this work, we study spread poly(ethylene glycol) (PEO) homopolymer and polystyrene-b-poly(ethylene glycol) (PS-PEO)copolymer monolayers at various air-liquid interfaces in order to understand the role solvent quality has on the equilibrium structure and interfacial shear properties. Surface pressure isotherms of PEO and PS-PEO at the interface of air and water/ ethylene glycol mixtures show similar trends as the volume percent of ethylene glycol is increased. The ethylene glycol appears to reduce the surface activity of the PEO chains at the interface, requiring higher surface concentrations to increase the surface pressure. Preliminary measurements of the surface shear viscoelastic moduli of PS-PEO at the air water interface indicate an elastic dominant behavior. This is similar to what has been reported for a comparable block copolymer [2].

Fig 1. Effect of Ethylene Glycol on Isotherms of PEO Fig 2. Surface Shear Moduli GS’, GS’’ of PS-PEO at air- and PS-PEO water interface vs surface pressure References: [1] Langevin, D.; Monroy, F. Current Opinion in Colloids and Interface Science 2010, 15, 283-293. [2] Kim, Y.; Pyun, J.; Frechet, J.M.J.; Haweker,C.J.; Frank, C.W. Langmuir 2005, 21(23), 10444-10458

ucsb che's 4th amgen-clorox grad student symposium poster...

Documents