Bulletin of the American Physical Society
APS March Meeting 2016
Volume 61, Number 2
Monday–Friday, March 14–18, 2016; Baltimore, Maryland
Session H36: Soft Matter at Interfaces (Surfactants)Focus
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Sponsoring Units: GSOFT Chair: Jonathan Whitmer, Notre Dame Room: 339 |
Tuesday, March 15, 2016 2:30PM - 2:42PM |
H36.00001: Stretching and ordering of amyloid fibrils at liquid interfaces Emanuela Del Gado, Konrad Schwenke, Sophia Jordens, Ivan Usov, Raffaele Mezzenga We investigate the formation of nematic domains, which might be precursor of plaque formation, in the adsorption of amyloid fibrils at liquid interfaces. Combining experiments and computer simulations we analyse spatial correlations in the nematic order and in apparent persistence length. Non-equilibrium numerical simulations provide new insight into the coupling between those quantities. The emerging scenario is that the out-of-equilibrium adsorption favors the formation of spatial heterogeneities due to the presence of local nematic order that tend to persist upon increasing the surface coverage. Such structural heterogeneities are directly coupled to the apparent straightening of the brils and might affect the density and the mechanical properties of the final self-assembled material. [Preview Abstract] |
Tuesday, March 15, 2016 2:42PM - 2:54PM |
H36.00002: Effect of headgroup-substrate interactions on the thermal behavior of long-chain amphiphiles Saranshu Singla, He Zhu, Ali Dhinojwala The structure of amphiphilic molecules at liquid/solid and solid/solid interfaces is relevant in understanding lubrication, colloid stabilization, chromatography, and nucleation. Here, we characterize the interfacial structures of long chain amphiphilic molecules with different head groups (OH, COOH, NH$_{2})$ using interface-sensitive sum frequency generation (SFG) spectroscopy. The behavior of these self-assembled monolayers (SAMs) on sapphire substrate is recorded in situ as a function of temperature (above and below bulk T$_{m})$ using SFG. Previous studies using synchrotron X-ray reflectivity and SFG show that the melting point of an ordered hexadecanol monolayer is around 30\textdegree C above its bulk T$_{m}$. The thermal stability of the monolayer is explained due to strong hydrogen bonding interactions between the head-group and the sapphire substrate. The strength of these hydrogen-bonding interactions between substrate and different head groups is calculated using the Badger-Bauer equation. Below T$_{m}$, the ordered monolayer influenced the structure of the interfacial crystalline layer, and the transition from monolayer to the bulk crystalline phases. The results with different head groups will be presented. [Preview Abstract] |
Tuesday, March 15, 2016 2:54PM - 3:06PM |
H36.00003: The role of hydrophobic mismatch in tuning lipid membrane dynamics Elizabeth Kelley, Rana Ashkar, Robert Bradbury, Andrea Woodka, Michihiro Nagao, Paul Butler Lipid membranes undergo an array of conformational and dynamic transitions, ranging from individual lipid motions to undulations of micron-sized patches of the membrane. However, the collective dynamics at intermediate length scales are largely unexplored due to experimental challenges in accessing the appropriate length and time scales. Here we use neutron spin echo spectroscopy (NSE) to provide unique insights into these elusive dynamics and measure membrane mechanical properties by probing both bending and thickness fluctuations in model lipid bilayers. We show that hydrophobic mismatch between lipids with different acyl chain lengths tunes the dynamics in a way not achievable in single component systems. For example, the thickness fluctuation amplitude is enhanced in the fluid phase of mixed lipid bilayers, reaching approximately 20{\%} of the bilayer thickness. Combining these experimental results with deformation free energy calculations suggests the mixed bilayers are more compressible than single component bilayers and provides new insights into the role of lipid diversity in controlling the rich dynamics of biomembranes. [Preview Abstract] |
Tuesday, March 15, 2016 3:06PM - 3:18PM |
H36.00004: Interplay Between Hydrophobic Effect and Dipole Interactions in Peptide Aggregation Sai Ganesan, Silvina Matysiak In the past decade, the development of various coarse-grained models for proteins have provided key insights into the driving forces in folding and aggregation.We recently developed a low resolution Water Explicit Polarizable PROtein coarse-grained Model by adding oppositely charged dummy particles inside protein backbone beads.With this model,we were able to achieve significant $\alpha$/$\beta$ secondary structure content,without any added bias.We now extend the model to study peptide aggregation at hydrophobic-hydrophilic interface using elastin-like octapeptides (GV)4 as a model system.A condensation-ordering mechanism of aggregation is observed in water.Our results suggest that backbone interpeptide dipolar interactions,not hydrophobicity,plays a more significant role in fibril-like peptide aggregation.We observe a cooperative effect in hydrogen bonding or dipolar interactions, with increase in aggregate size in water and interface.Based on this cooperative effect, we provide a potential explanation for the observed nucleus size in peptide aggregation pathways.Without dipolar particles,peptide aggregation is not observed at the hydrophilic-hydrophobic interface.Thus,the presence of dipoles,not hydrophobicity plays a key role in aggregation observed at hydrophobic interfaces. [Preview Abstract] |
Tuesday, March 15, 2016 3:18PM - 3:30PM |
H36.00005: Theory of Kinetics of Registration and Anti-Registration in Lipid Bilayers Peter Olmsted, John Williamson Lipid bilayer leaflets are often treated as if they are coupled; i.e., that the two leaflets undergo simultaneous transitions between phases, and that domains involve both leaflets together in a registered fashion. We present theory and simulation showing how interleaflet couplings and hydrophobic mismatch can lead to a complex phase diagram with multiple metastable two-phase and three-phase states. Many of these states can be discerned in the experimental literature, and are expected in the early stages of coarsening when domains are sub-micron (and thus perhaps of significance to lipid rafts). We present different kinetic scenarios for transitions between these state, and show how lipid flip flop can surprisingly lead to non-symmetric anti-registered patterns. [Preview Abstract] |
Tuesday, March 15, 2016 3:30PM - 3:42PM |
H36.00006: ABSTRACT WITHDRAWN |
Tuesday, March 15, 2016 3:42PM - 3:54PM |
H36.00007: Dynamic surface tension measurements with maximum bubble pressure tensiometry Norman Moreno, Theodore Walker, Adam Burshan, Vivek Sharma Dynamic surface tension refers to the time dependent variation in surface tension, and is intimately linked with the rate of mass transfer of a surfactant from liquid sub-phase to the interface. The diffusion- or adsorption-limited kinetics of mass transfer to interfaces is said to impact the so-called foamability and the Gibbs-Marangoni elasticity of surfaces. Dynamic surface tension measurements carried out with conventional methods like pendant drop analysis, Wilhelmy plate, etc are limited in their temporal resolution (\textgreater 50 ms). In this study, we describe design and application of maximum bubble pressure tensiometry for the measurement of dynamic surface tension effects at extremely short (1-50 ms) timescales. Using experiments and theory, we discuss the challenges and experimental constraints related with the maximum bubble pressure tensiometry measurement. [Preview Abstract] |
Tuesday, March 15, 2016 3:54PM - 4:06PM |
H36.00008: Compression-Induced Fusion of Glassy Core Polymer Micelles at the Air-Water Interface Hyun Chang Kim, You-Yeon Won The surface mechanical and morphological properties of glassy core polymer micelles at the air-water interface were investigated. Asymmetric PS-PEG and PtBMA-PEG block copolymers with PEG weight fractions larger than 0.5 were formulated in the form of aqueous micelles and spread onto water. Compressed films of PS-PEG and PtBMA-PEG micelles reach high dynamic surface pressures. On the detailed level, however, PS-PEG and PtBMA-PEG micelles exhibit different surface pressure-area profiles. The PtBMA-PEG isotherm shows a transition to a plateau around a surface pressure of 24 mN/m, which is attributed to the PtBMA block as it forms a continuous film; this interpretation is supported by the fact that the surface pressure at the plateau transition is identical to the value of the spreading coefficient for PtBMA. This presents evidence that the core domains of PtBMA-PEG micelles melt and merge into a film when the micellar monolayer is laterally compressed. Such behavior was not observed with PS-PEG micelles. We suspect that under lateral compression, PtBMA-PEG micelles undergo fusion into a continuous film because PtBMA has the natural tendency to spread on the water surface, whereas PS-PEG micelles does not because the dewetting tendency of PS preventing formation of a uniform layer. [Preview Abstract] |
Tuesday, March 15, 2016 4:06PM - 4:18PM |
H36.00009: Temperature Dependent Rotational Correlation in Lipids Christina Othon, Neda Dadashvand, Eduardo Vega Lozada The lateral heterogeneity of lipid dynamics is explored in free standing lipid monolayers. As the temperature is lowered the lipids exhibit increasingly broad and heterogeneous rotational correlation. This increase in heterogeneity appears to exhibit a critical onset, similar to those observed for glass forming fluids. We explore this heterogeneous relaxation by measuring the rotational diffusion of a fluorescent probe (NBD-PC) using wide-field time-resolved fluorescence anisotropy microscopy, in single constituent lipid monolayer of DMPC. The observed relaxation exhibits a narrow, liquid-like distribution at high temperatures ($\tau $ \textasciitilde 2.4 ns), consistent with previous experimental measures by different methods. However, as the temperature is quenched, the distribution broadens, and we observe the appearance of a long relaxation population (16.5 ns). This demonstrates that the nanoscale diffusion and reorganization in lipid structures can be significantly complex, even in the simplest unstructured architectures. This result can have a significant impact on the organization, permeability and energetics of natural membrane structures. [Preview Abstract] |
Tuesday, March 15, 2016 4:18PM - 4:30PM |
H36.00010: Interfacial microrheology study of layer formation by staphylococcal nuclease protein and its disordered variant Bilyana Tzolova, Daniel Allan, Daniel Firester, Bertrand Garcia-Moreno, Daniel Reich, Robert Leheny We study the formation of layers of staphylococcal nuclease protein adsorbing at the air-water interface. In a series of experiments, we follow the evolution of the rheological response of the layer using an active microrheology technique that involves tracking the rotational motion of magnetic nanowires at the interface in response to time-dependent external magnetic fields. At early stages of layer formation, the wire mobility can be interpreted using a model for viscous drag with an interfacial viscosity that increases rapidly with layer age; however, at later ages deviations from a simple viscous response indicating non-Newtonian behavior are observed. We compare the evolution in microrheology of layers forming from wild-type protein that assumes a folded conformation in solution with a variant that is disordered due to substitution of a single amino acid, thereby gaining a perspective on the impact of initial protein state on the layer formation and rheology. [Preview Abstract] |
Tuesday, March 15, 2016 4:30PM - 4:42PM |
H36.00011: Interaction of water with melittin inserted in a single-supported lipid bilayer Zachary Buck, Mengjun Bai, James Torres, Helmut Kaiser, Haskell Taub, Flemming Y. Hansen, Andrew Miskowiec, Madhusudan Tyagi The insertion mechanism, conformation, and the function of transmembrane proteins are strongly influenced by both the lipid molecules and the hydration water of a cell membrane. Previously, we have fabricated samples of single-supported lipid bilayers of zwitterionic DMPC and studied extensively their influence on the freezing behavior and diffusion of water in their vicinity [2]. We have recently extended these studies to a more biologically relevant system by depositing melittin proteins onto single-supported DMPC bilayers. By monitoring the elastically-scattered neutron intensity as a function of temperature from such samples, we observe an abrupt freezing transition of the associated water not seen in the bare membrane case. Moreover, the change in elastic intensity of this freezing step increases proportionally with melittin concentration. For a particular peptide concentration, a small increase of the elastically-scattered neutron intensity is measured while annealing the sample at 328 K. We tentatively interpret this increase of the elastic intensity to anchoring and/or insertion of the melittin peptides within the membrane. $^{\mathrm{2}}$M. Bai \textit{et al}., Europhys. Lett. \textbf{98}, 48006 (2012). [Preview Abstract] |
Tuesday, March 15, 2016 4:42PM - 4:54PM |
H36.00012: Formation, disruption and mechanical properties of a rigid hydrophobin film at an air-water interface Lynn Walker, Stephanie Kirby, Shelley Anna Hydrophobins are small, globular proteins with distinct hydrophilic and hydrophobic regions that make them extremely surface active. The behavior of hydrophobins at surfaces has raised interest in their potential industrial applications, including use in surface coatings, food foams and emulsions, and as dispersants. Practical use of hydrophobins requires an improved understanding of the interfacial behavior of these proteins, both individually and in the presence of surfactants. Cerato-ulmin (CU) is a hydrophobin that has been shown to strongly stabilize air bubbles and oil droplets through the formation of a persistent protein film at the interface. In this work, we characterize the adsorption behavior of CU at air/water interfaces by measuring the surface tension and interfacial rheology as a function of adsorption time. CU is found to strongly, irreversibly adsorb at air/water interfaces; the magnitude of the dilatational modulus increases with adsorption time and surface pressure, until the CU eventually forms a rigid film. The persistence of this film is tested through the addition of SDS, a strong surfactant, to the bulk. SDS is found to co-adsorb to interfaces pre-coated with a CU film. At high concentrations, the addition of SDS significantly decreases the dilatational modulus, indicating disruption and displacement of CU. These results lend insight into the complex interfacial interactions between hydrophobins and surfactants.~ [Preview Abstract] |
Tuesday, March 15, 2016 4:54PM - 5:06PM |
H36.00013: Boundary condition in liquid thin films revealed through the thermal fluctuations of their free surfaces Basile Pottier, Laurence Talini, Christian Frétigny We investigate the properties of liquids confined at nanometric scales from a solid wall with a new noninvasive technique. The optical technique used consists of measuring the height of fluctuations of the free surface, using the reflection of a laser beam on that surface. We hence measure the spontaneous thermal fluctuations of the free surfaces of liquids to probe their hydrodynamic boundary condition at a solid wall. The surface fluctuations of a silicon oil film could be described with a no-slip boundary condition for film thicknesses down to 20 nm. Oppositely, a 4 nm negative slip length had to be introduced to describe the behavior of n-hexadecane, consistently with previous surface force apparatus data on the same system. Our results demonstrate that at vanishing flow, a nanometric solid-like layer close to the wall may exist according to the nature of the liquid. [Preview Abstract] |
Tuesday, March 15, 2016 5:06PM - 5:18PM |
H36.00014: Charge Effects on Surfactant Membrane Thickness Fluctuations Robert Bradbury, Michihiro Nagao The mechanical properties of surfactant bilayer membranes have been measured over a range of surface charge densities using small-angle neutron scattering and neutron spin echo spectroscopy. An increase in the surface charge density leads to a stiffening of the membrane, which is consistent with classical theory of charge effects on membranes. The fluctuations in the membrane thickness, however, become slower with increasing charge density, which can be explained by an increase in the membrane viscosity as predicted by Bingham $\textit{et al}$. We suggest that an increase in the repulsive interactions between the charged headgroups is responsible for this increased membrane viscosity. Furthermore, the amplitude of the thickness fluctuations is observed to remain almost constant with variation in surface charge density which suggests almost constant values for the total compressibility modulus of the bilayer and the optimum fluctuation wavelength. This indicates that the time scale and amplitude of membrane thickness fluctuations are controlled by different membrane effects. This work demonstrates that charge stabilization of lamellar bilayers is not merely affected by inter-membrane interactions but that intra-membrane dynamics also have a significant contribution. [Preview Abstract] |
Tuesday, March 15, 2016 5:18PM - 5:30PM |
H36.00015: Branching mechanisms in surfactant micelles. Subas Dhakal, Radhakrishna Sureshkumar The mechanisms of branch formation in surfactant micelles of cetyltrimethylammonium chloride (CTAC) in presence of sodium salicylate (NaSal) counter ions in water are studied using molecular dynamics simulations. The curvature energy associated with the formation of micelle branches and the effect of branching on the solution viscosity are quantified. Highly curved surfaces are energetically stabilized by a higher density of binding counter ions near the branch points. Simulations show that micellar branches result in a significant reduction in the solution viscosity as observed in experiments [Dhakal {\&} Sureshkumar, J. Chem. Phys. 143, 024905~(2015)]. This reduction in viscosity has long been attributed to the sliding motion of micelle branches across the main chain. However, to date, such dynamics of micelle branches have never been visualized in either experiments or simulations. Here, we explicitly illustrate and quantify, for the first time, how branches slide along the micelle contour to facilitate stress relaxation. [Preview Abstract] |
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