Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F30: Membranes, Micelles, Vesicles, Gels and Complex Fluids |
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Sponsoring Units: DCMP Chair: Elizabeth Mann, Kent State University Room: 338 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F30.00001: For a Safe Diamide Extraction Process, Elucidated by Atomistic Simulations Baofu Qiao, Ross J. Ellis, Monica Olvera de la Cruz The diamide extraction process has been successfully employed in separating trivalent actinides from used nuclear fuels. The extractant, which is an amphiphilic molecule with a metal-binding polar headgroup and hydrophobic tail, binds the actinides, thus extracting them from the aqueous phase into the oil phase. However, the oil phase will split into two phases, once a critical concentration of actinide is reached. This phase splitting is suspected to have caused the Red Oil events, which can decompose explosively. Therefore, it is extremely important for an extractant to have a high extraction efficiency, on one hand, and resist phase splitting, on the other. In comparison with DMDBTDMA, DMDOHEMA has both higher extraction efficiency and phase stability, which we suspect stem from the supramolecular aggregated structures influenced by the different extractant tails. To test our hypothesis, atomistic molecular dynamics simulations were performed on DMDBTDMA in bulk oil system and DMDOHEMA in bulk oil system. Our preliminary results indicate that DMBTDMA is more disposed toward formation of chain-like aggregates, especially at lower water concentration, in comparison with the branched structures observed in DMDOHEMA. [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F30.00002: Stochastic nature of clathrin-coated pit assembly Anand Banerjee, Alexander Berezhkovskii, Ralph Nossal Clathrin-mediated endocytosis is a complex process through which eukaryotic cells internalize various macromolecules (cargo). The process occurs via the formation of invaginations on the cell membrane, called clathrin-coated pits (CCPs). The dynamics of CCP formation shows remarkable variability. After initiation, a fraction of CCPs, called ``productive pits", bind to cargo and then grow and mature into clathrin-coated vesicles (CCVs). In contrast, a large fraction of CCPs, called ``abortive pits", fail to bind to cargo, grow only up to intermediate sizes and then disassemble. There is notable heterogeneity in the lifetimes of both productive and abortive pits. We propose a stochastic model of CCP dynamics to explain these experimental observations. Our model includes a kinetic scheme for CCP assembly and a related functional form for the dependence of free energy of a CCP on its size. Using this model, we calculate the lifetime distribution of abortive pits (via Monte Carlo simulation) and show that the distribution fits experimental data very well. By fitting the data we determine the free energy of CCP formation and show that CCPs without cargo are energetically unstable. We also suggest a mechanism by which cargo binding stabilizes CCPs and facilitates their growth. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F30.00003: Budding transition of a self-avoiding polymer confined by a soft membrane adhering onto a flat wall Yu-Cheng Su, Jeff Z. Y. Chen The Monte Carlo simulation is used to study the structural properties of the system consisting of a self-avoiding polymer chain confined between a fluid membrane and a flat hard surface. As the adhesion between the soft membrane and the hard-wall surface increases, the polymer is subject to a strong confinement; the state containing a pancake-shaped polymer conformation eventually yields to a bud state, through an abrupt, first-order phase transition. We explore the scaling behavior of the physical properties of the system as functions of the polymer's size, the membrane's surface tension, and the adhesion energy, for both pancake and bud states, in terms of Monte Carlo data and analytic scaling theories. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F30.00004: Why square lattices are not seen on curved ionic membranes Creighton Thomas, Monica Olvera de la Cruz Ionic crystalline membranes on curved surfaces are ubiquitous in nature, appearing for example on the membranes of halophilic organisms. Even when these membranes buckle into polyhedra with square or rectangular sides, the crystalline structure is seen to have hexagonal symmetry. Here, we theoretically and numerically investigate the effects of curvature on square lattices. Our model system consists of both positive and negative ions with a 1:1 charge ratio adsorbed onto the surface of a sphere. In flat space, the lowest-energy configuration of this system can be a square lattice. This bipartite arrangement is favored because there are two types of ions. It leads to a fundamentally different defect structure than what has been seen when triangular lattices are favored. We classify these defects and find that curvature disrupts long-range square symmetry in a crystal. Through numerical simulations, we see that small square regions are possible in some cases, but this phase coexists with other structures, limiting the scale of these square-lattice microstructures. Thus, at large length scales, curvature leads to triangular structures. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:00AM |
F30.00005: Amphiphilic lipids in solution: a simulational study of lipid bilayer formation Thomas Vogel, David P. Landau, Lili Gai, Katie A. Maerzke, Christopher R. Iacovella, Clare M. McCabe, Peter T. Cummings Amphiphilic molecules consisting of hydrophilic head and hydrophobic tail groups self-assemble into a wide variety of structures, such as bilayers (membranes), micelles, or vesicles (liposomes) when mixed with a suitable solvent. The understanding of this lipid self-assembly is essential for industrial, biological, or medical applications, but computer simulations are generally challenging due to the complex structure of the energy landscape. We show results for the lipid bilayer formation process obtained by newly developed parallel Wang--Landau Monte Carlo and statistical temperature molecular dynamics simulations. By applying those methods to a generic coarse-grained model for amphiphilic molecules in solution, we were able to obtain the thermodynamical data over the whole relevant temperature and energy range and to unravel the membrane formation process including all structural sub-transitions between different fluid and gel-phase \hbox{bilayers}. [Preview Abstract] |
Tuesday, March 19, 2013 9:00AM - 9:12AM |
F30.00006: Study of vesicle size distribution dependence on pH value based on nanopore resistive pulse method Yuqing Lin, Yauheni Rudzevich, Adam Wearne, Daniel Lumpkin, Joselyn Morales, Kathleen Nemec, Suren Tatulian, Oleg Lupan, Lee Chow Vesicles are low-micron to sub-micron spheres formed by a lipid bilayer shell and serve as potential vehicles for drug delivery. The size of vesicle is proposed to be one of the instrumental variables affecting delivery efficiency since the size is correlated to factors like circulation and residence time in blood, the rate for cell endocytosis, and efficiency in cell targeting. In this work, we demonstrate accessible and reliable detection and size distribution measurement employing a glass nanopore device based on the resistive pulse method. This novel method enables us to investigate the size distribution dependence of pH difference across the membrane of vesicles with very small sample volume and rapid speed. This provides useful information for optimizing the efficiency of drug delivery in a pH sensitive environment. [Preview Abstract] |
Tuesday, March 19, 2013 9:12AM - 9:24AM |
F30.00007: Spontaneous Thermoreversible Formation of Cationic Vesicles in a Protic Ionic Liquid Dongcui Li, Carlos Lopez-Barron, Leo DeRita, Madivala Basavaraj, Norman Wagner The search for stable vesicular structures is a long-standing topic of research because of the usefulness of these structures and the scarcity of surfactant systems that spontaneously form vesicles in true thermodynamic equilibrium. We report the first experimental evidence of spontaneous formation of vesicles for a cationic double tail surfactant (didodecyldimethylammonium bromide) in a protic ionic liquid (ethylammonium nitrate) [1-2]. Using small and ultra-small angle neutron scattering, rheology and bright field microscopy, we identify the coexistence of two vesicle containing phases in compositions ranging from 2 to 68 wt \%. A low density highly viscous solution containing giant vesicles and a sponge phase coexists with a dilute high density phase containing large vesicles. Vesicles form spontaneously via different thermodynamic routes, with the same size distribution, which strongly supports that they exist in a true thermodynamic equilibrium. The formation of equilibrium vesicles and the L3 phase is facilitated by ion exchange between the cationic surfactant and the ionic liquid, as well as the strength of the solvophobic effect in the protic ionic liquid.\\[4pt] [1] Lopez-Barron et al., J. Phys. Chem. B 116, 813 (2012).\\[0pt] [2] Lopez-Barron et al., J. Am. Chem. Soc., Accepted. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F30.00008: Phase separation in a DMPC/Dchol mixed Langmuir Film: A combined Brewster Angle, Fluorescence and Light Scattering Microscopy study Pritam Mandal, Fanindra Bhatta, Arne Gericke, Edgar Kooijman, David Allender, Elizabeth Mann Fluorescence microscopy (FM) is one of the most direct imaging techniques for in situ observation of morphology and phase-separation at the macroscopic scale [1] in lipid mono- or bi-layers. However, the presence of fluorescent dye-molecules can affect the system. In Brewster Angle Microscopy (BAM), one can image monomolecular Langmuir films without probes. Here, using a composite set-up of BAM, FM and Light Scattering Microscopy (LSM), we present a comparative study of the three techniques on a binary lipid mixture in the presence of two different probes. In most cases, all three techniques show precisely the same domains. However, depending on conditions, some domain types were more evident in one technique than the others. This established, we can directly test the influence of probe on the domain structure. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F30.00009: Mesoscopic Membrane Morphology Regulated by Molecular Crystallization Cheuk-Yui Leung, Liam Palmer, Bao Fu Qiao, Sumit Kewalramani, Rastko Sknepnek, Christina Newcomb, Megan Greenfield, Graziano Vernizzi, Samuel Stupp, Michael Bedzyk, Monica Olvera de la Cruz A grand challenge in self-assembly of multi-component systems is to control the crystal symmetries and the resulting geometries of co-assembled molecular structures. We generate here crystalline ionic bilayers in a large variety of geometries, which~resemble unusual cellular shell shapes, by mixing $+$3 and -1 ionic amphiphiles. To structurally characterize the co-assembly from the mesoscopic to nanometer scale, we combine electron microscopy with small and wide angle x-ray scattering. We use pH to control the degree of ionization of the amphiphiles and hence their intermolecular electrostatic interactions. At low and high pH, closed faceted vesicles with 2D hexagonal molecular arrangements were observed, while at intermediate pH ribbons with rectangular-C packing of the amphiphiles were observed. Thus pH acts as a switch to control the morphology of the ionic bilayers via transitions in the crystalline lattice. This work promotes the design of nanocontainers for various applications and improves our understanding of the origin of polyhedral shells in nature. [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F30.00010: Thermodynamics of protein driven self assembly in membranes Ramakrishnan Natesan, Richard Tourdot, Ryan Bradley, Ravi Radhakrishnan Recent experimental evidences strongly point to the role of proteins and other membrane binding macromolecules in reshaping biological membranes, at length scales of the molecule and the structure enclosed by the membrane. In this work, we investigate the interplay between the membrane curvature induced at the molecular scale, mainly due to peripheral membrane proteins, and the resulting membrane morphologies, of varying complexity, observed at the mesoscale. The biological membrane, in our approach, is represented by a dynamically triangulated surface while the proteins are modeled as curvature fields on the membrane, which can either be isotropic or anisotropic. Thermal undulations in the membrane and cooperativity in the curvature field, due to the stabilization of a nematic phase, drives the membrane into conformations that resembles those in experiments in vivo and vitro. The stability of these structures are examined by two approaches to compute the free energy of the system: (i) Widom insertion technique to compute excess chemical potentials and (ii) thermodynamic integration using the Kirkwood coupling parameter to compute absolute free energies. Building on these methods, we propose a hybrid scheeme that couples both the approaches for computing free energies. [Preview Abstract] |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F30.00011: Morphology and Performance of PLLA Based Porous Membranes by Phase Separation Control Qian Xing, Xia Dong, Rongbo Li, Charles C. Han, Dujin Wang Poly (L-lactic acid) (PLLA) porous membranes with different morphologies and properties were prepared through immersion precipitation method. It has been proved that the rate and level of phase separation between PLLA/dioxane solution and coagulation baths were the original drive force for the ultimate structure and corresponding performance of PLLA membranes. The equilibrium thermodynamic phase diagram of PLLA/solvent/nonsolvent and the kinetic diffusion rate between solvent and nonsolvent were systematically investigated. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F30.00012: Correlating bulk properties and nanoscale rearrangement during UV-initiated gelation of hybrid nanoparticle/ block copolymer systems K. Anne Juggernauth, Soenke Seifert, Brian Love We use rheology and Small Angle X-Ray Scattering (SAXS) to investigate UV initiated gel formation in aqueous dispersions of clay nanoparticles in the presence of poly(ethyleneoxide-b-propyleneoxide-b-ethyleneoxide) block copolymer surfactants (Pluronics\textregistered ) and small amounts of a photoacid generator (PAG). This material system demonstrates stable liquid-like behavior in the absence of UV but undergoes bulk gelation upon UV exposure. Rheology was used to monitor the bulk properties of a series of samples undergoing UV exposure and confirm bulk gel formation. We further probe nanoparticle rearrangement using time resolved synchrotron SAXS with simultaneous UV exposure. Time dependent SAXS indicate an absence of long range order and crystallinity while changes in the scattering profile are related to short range interparticle interactions leading to a stable or arrested structure. Finally, we compare the time scales for structural rearrangement of nanoparticles with the bulk gelation behavior. Our results show that the kinetics for local structural changes between particles and bulk gelation from UV exposure are strongly correlated. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F30.00013: Cooperative Processes in Restructuring Gel Networks Jader Colombo, Asaph Widmer-Cooper, Emanuela Del Gado Colloidal gel networks are disordered elastic solids that can form even in extremely dilute particle suspensions. Similarly to other network-forming soft materials, including many with important biological function or technological potential, they can locally restructure via breaking and reforming interparticle bonds. Although controlling the link between local restructuring and mechanical response bears enormous potential for designing smart nanocomposites, there is at present little understanding of how local bond changes affect the dynamics of the gel network and the stress transmission through it. Here, using numerical simulations of a model system and a space-resolved analysis of dynamical heterogeneities, we show that bond breaking has non-local consequences and induces cooperative relaxation further away along the network. This provides explicit microscopic insight into why non-local constitutive relations are required to rationalize the non-trivial mechanical response of colloidal gels. [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F30.00014: Elimination of branching in self assembled beta-hairpin based peptide hydrogels Sameer Sathaye, Darrin Pochan Hydrophobic collapse of amphiphilic $\beta $-hairpin peptides (e.g. MAX1 VKVKVKVKV$^{\mathrm{D}}$PPTKVKVKVKV-NH$_{\mathrm{2}})$ into fibrils and their hierarchical assembly into branched, hydrogel networks has been extensively studied. A physically crosslinked hydrogel network is formed due to fibrillar entanglement and branched defects in hydrophobic collapse during fibril formation. Alternating valine residues with side chains of the same size are responsible for the hydrophobic collapse of the molecule into a b-hairpin and fibril nanostructure with branching. In a new sequence LNK1 (LNK1 (Nal)K(Nal)KAKAKV$^{\mathrm{D}}$PPTKAKAK(Nal)K(Nal)-NH$_{\mathrm{2}})$ the non-beta turn valines were replaced with Napthylalanine and alanine amino acid residues, with hydrophobic side chains of larger and smaller volume, respectively, than valine. Thus, formation of a `lock and key' type structure was attempted in the hydrophobic core of the peptide fibrils that would eliminate fibril branching. ~The folding and network formation of LNK1 has been studied by Circular Dichroism spectroscopy (CD), Transmission Electron Microscopy (TEM) and Oscillatory Rheology. Preliminary rheological characterization suggests the elimination of branching in the fibrils and also a possibility that LNK1 networks, unlike MAX1, are just nanofibrillar suspensions rather than permanently physically crosslinked hydrogels. [Preview Abstract] |
Tuesday, March 19, 2013 10:48AM - 11:00AM |
F30.00015: Microstructure and rheology of a thermoreversible gel under large amplitude oscillatory shear (LAOS) deformation using time-resolved oscillatory rheo-small-angle neutron scattering (tOr-SANS) Jung Min Kim, A. Kate Gurnon, Norman Wagner, Aaron Eberle Large amplitude oscillatory shear (LAOS) rheology is an effective way of studying the nonlinear dynamics of complex fluids. Here, we present a new method for a direct, quantitative study of the microstructure under LAOS deformation in the framework of the alignment factor, \textit{Af.} We use a model thermoreversible adhesive hard-sphere system composed of octadecyl-coated silica particles suspended in $n$-tetradecane. With temperature the particle potential is controlled and the system is shifted from behaving as a near hard-sphere to an adhesive hard-sphere system leading to aggregation and ultimately a dynamical arrest transition to macroscopic gelation. Time-resolved oscillatory rheo-small-angle neutron scattering (tOr-SANS) measurements in the 1-3 plane are performed by stroboscopically probing the structural evolution as a function of time during LAOS. Under strong shear, the 2D scattering pattern of the system in the gelled state exhibits a strong anisotropy commonly known as a ``butterfly'' pattern, which corresponds to the stretching of the microstructure along the flow direction. The first structure-Lissajous plots of this model system are presented in terms of an order parameter and \textit{Af} as a function of instantaneous strain and strain rate. This new analysis demonstrates a novel method for simultaneously measuring the rheology and microstructure during a time-dependent deformation (LAOS). [Preview Abstract] |
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