Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z38: Surfaces, Interfaces, and Colloids II |
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Sponsoring Units: DCP Chair: Sunil Sainis, Rowland Institute, Harvard University Room: 410 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z38.00001: Understanding surface energies of transition metals with density-functional theory Aloysius Soon, Martin Fuchs, Matthias Scheffler Determining index-specific surface energies of metals is, to date, still a non-trivial task, both experimentally and theoretically. Density-functional theory (DFT) calculations within the local-density approximation (LDA) for exchange-correlation (XC) have provided understanding of qualitative trends. Yet, absolute surface energies, in particular of $d$-metals still exhibit significant uncertainties related to the description of XC: gradient-corrected functionals (GGA) which improve over the LDA for other properties often predict less accurate surface energies. This calls for a careful analysis of XC effects on surface energies, including non-local exchange and/or correlation. Here we analyze the surface energies of $4d$-metals with modern GGA functionals (PBEsol, AM05, developed to better describe bulk solids and (jellium) surfaces than the LDA and previous GGAs), using the all-electron FHI-aims code [1]. Relating the bulk cohesive energy and surface energy via a bond-cutting model we find modern GGAs can indeed correct the poorer results of the usual PBE-GGA but worsen the bulk cohesive energies of $4d$-metals. In addition, we consider hybrid XC functionals, using a cluster correction scheme [2], and discuss the effects of including exact exchange on the calculated surface energies.\\[0pt] [1] \textit{http://www.fhi-berlin.mpg.de/aims/};\\[0pt] [2] Q.-M. Hu \textit{et al.}, Phys. Rev. Lett. \textbf{98}, 176103 (2007); \textbf{99}, 169903(E). [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z38.00002: The initial stages of NaCl dissolution: Ion or ion pair solvation? Jiri Klimes, Angelos Michaelides The interaction of water with rock salt (NaCl) is important in a wide variety of natural processes and human activities. A lot is known about NaCl dissolution at the macroscopic level but we do not yet have a detailed atomic scale picture of how salt crystals dissolve. Here we report an extensive series of density functional theory, forcefield and molecular dynamics studies of water clusters at flat and defective NaCl surfaces and NaCl clusters. The focus is on answering seemingly elementary questions such as how many water molecules are needed before it becomes favorable to extract an ion or a pair of ions from the crystal or the cluster. It turns out, however, that the answers to these questions are not so straightforward: below a certain number of water molecules ($\sim$ 12) solvation of individual ions is less costly and above this number solvation of ion pairs is favored. These results reveal a hitherto unknown complexity in the NaCl dissolution process born out of a subtle interplay between water-water and water-ion interactions. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z38.00003: Accelerated Molecular Dynamics of Temperature-Programmed Desorption Kristen Fichthorn, Kelly Becker, Maria Mignogna The most widely used experimental method for quantifying thermal desorption is temperature-programmed desorption (TPD). Despite its extensive use, interpretation of this experiment can still be controversial. A significant difficulty with interpreting TPD is that this macroscopic experiment offers a limited picture of the underlying microscopic kinetic events. In this work, we use accelerated molecular-dynamics to simulate TPD of $n$-pentane from the basal plane of graphite, in the first atomistic simulations to probe TPD over laboratory time scales. Although the simulated TPD spectra agree with experiment, a detailed analysis reveals underlying kinetic phenomena that contrast the standard experimental interpretation and opens new possibilities for understanding molecular kinetics at solid surfaces. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z38.00004: Charge Interactions of Unilamellar Vesicles in Aqueous Suspensions Seongmin Park, Joseph Junio, Mahn-Won Kim, H.D. Ou-Yang This project reports the results of an experimental optical trapping study of the charge interactions between phosphor-lipid unilamellar vesicles. A 1064nm laser coupled into a high NA objective lens provided the optical trap. Using fluorescently labeled vesicles, we were able to monitor the particle number density by using a 532nm excitation beam aligned to be parfocal with the trapping beam through the same objective. Fluorescent signals from the focal region common to both beams were band-passed to a pinhole for confocal detection. Using the number density of the vesicles in the focal spot as a function of trapping intensity and a force balance model, we were able to calculate the effective trapping energy per vesicle as well as the osmotic virial coefficients for a system of lipid vesicles prepared with DOPG, cholesterol, and DiI. We measured the compressibility of these vesicle suspensions as a function of surface charge and ionic strength of the suspending medium. Compared to conventional scattering methods, this optical trapping method is advantageous, since it can be used for concentrated suspensions, yielding an in situ measurement of colloidal interactions. [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z38.00005: Probing colloidal physics on the nanometer length scale Sunil Sainis, Frank Vollmer The sharp spectral features associated with ultra-high Q microresonator modes are sensitive to changes in the local environment and surface of the resonator [1]. Microresonator cavities have been used to detect the binding of single molecules [2] and viruses in an aqueous medium. We report on recent experiments that use microresonators to access colloidal physics on the nanometer length scale. We examine shifts in the resonator as a function of bulk ionic strengths and surface adsorption in a colloid. \\[3pt] [1] S. Arnold et al., Nature Methods {\bf 5}, 591 - 596 (2008)\\[0pt] [2] A. M. Armani, et al. Science {\bf 317}, 783-787 (2007). [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z38.00006: Effect of boundary condition on the viscosity of olefins: A molecular dynamics study Ling Ti Kong, Colin Denniston, Martin Muser, Yue Qi The viscosity of hexene was examined by means of non-equilibrium molecular dynamics simulations under different wall-liquid boundary conditions, namely over-smooth wall, more or less realistic wall, and over-adhesive wall. It is found that the wall-liquid interaction plays an important role in the ordering/layering of liquid, and consequently affects the behavior of olefins upon different normal pressures. With the same moving speeds of walls, the shear-rate (the slope of velocity profile) in the liquid is found to decrease with the increasing of normal pressure under over-smooth wall condition, while it is found to increase under the over-adhesive wall condition. The viscosity, in turn, shows a linear dependence on the normal pressure under over-smooth condition while exhibits an exponential dependence under the over-adhesive wall condition. The underlying mechanism of these observations will be presented and discussed in this talk. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z38.00007: Suppressing and Amplifying Depletion Attractions between Surfaces Roughened by Asperities Kun Zhao, Thomas Mason Motivated by recent experiments on roughness-controlled depletion attractions, we study the effect of roughness on depletion attractions between flat surfaces decorated using hemispherical and hemispheroidal asperities. Our calculations show that the depletion attraction between rough surfaces can be either dramatically reduced or amplified depending on the details of the surface morphologies. This model also explains the observed self-assembly of rough Janus platelets into dimers and provides quantitative predictions of roughness-controlled depletion attractions for conditions that have not yet been explored experimentally. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z38.00008: The Dynamics of Charging of Muscovite Mica: Measurement and Modeling Paul J. Sides, Danish Faruqui, Andrew J. Gellman The advent of a new method for measuring the zeta potential of planar surfaces, the rotating disk, allowed the investigation of the charging process of mica after immersion in water. The zeta potential of freshly-cleaved muscovite mica was recorded within seconds of immersion of the sample and in fractions of a second thereafter. The zeta potential of mica in water at pH = 5.6 with no added potassium changed by 40 -- 50 mV over approximately a minute. A model of adsorption and desorption of potassium ions and protons captured this behavior and provided a framework for determination of surface reaction rate constants. The charging of mica in alkaline KCl solutions of arbitrary concentration, however, was too fast for observation. The equilibrium zeta potential depended on the logarithm of salt concentration, in agreement with a model based on ion exchange reactions. The average values of the proton adsorption, proton desorption, potassium adsorption, and potassium desorption rate coefficients were 45 liter/s $\pm $ 15, 0.0014/s $\pm $ 0.0006, 58 liter/s $\pm $ 5, and 0.14/s $\pm $ 0.03, respectively. Web Page: http://zetaspin.com [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z38.00009: Effects of Embedded Dipoles on the Electrical Response of Self Assembled Monolayers P.P. Zhang, O.M. Cabarcos, T.A. Daniel, P.S. Weiss, D.L. Allara There has been recent interest in the use of polar molecules assembled at electrodes for tuning work functions and engineering charge injection barriers in organic electronic devices. With this in mind we have been investigating the electrostatic properties of simple model systems prepared from self-assembled alkanethiolate monolayers on Au{\{}111{\}} with the incorporation of an embedded ester moiety [-(CO$_{2})$- = E] in the adsorbate molecules. The intrinsic static dipole moment of the ester moiety of $\sim $1 Debye magnitude leads to the formation of a strong, highly organized, planar electric dipole layer in the SAM. From our previous X-ray photoelectron spectroscopy data we observe a consistent shift of the C 1s photoelectron kinetic energies between the top and bottom alkyl segments, defined as -(CH$_{2})_{m}-$E-(CH$_{2})_{n}$CH$_{3}$, regardless of the relative lengths $m$ and $n$. This shift correlates well with the value of the electrostatic potential across the E layer. Our recent surface potential AFM measurements, however, reveal an apparently anomalous strong dependence of surface potential on the sizes and ratios of $m$ and $n$, in contrast to the constant electrostatic potential observed in XPS measurements. Mechanisms underlying these effects will be discussed, with possible implications for the electrostatic behavior of more complicated organic and biological systems. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z38.00010: Interfacial Composition of the Ionic Aqueous Solution Studied by the Adsorption of the Cationic Molecules Jinsuk Song, Mahn Kim Knowing the interfacial composition of the ionic aqueous solution is important not only for understanding many atmospheric and environmental chemistry processes$^{1}$ but also for understanding many biological processes because the interaction between biomaterials happens often at the interfacial region such as water-vesicle interface in ionic aqueous solution. In this study, the surface anion density is estimated by measuring the surface density and adsorption angle of the cationic molecule, Malachite Green(MG) adsorbed at the air-ionic aqueous solution interface using the second harmonic generation technique. The anion number density at the interface increases with the increasing concentration of the ions and with the increasing size of the anions for spherical ions. It is consistent with other experimental measurements and simulation results$^{2,3}$. However, it seems that the anion density depends not only on the anion but also on the cation and shape and chemical composition of the ions. $^{1}$ E. Knipping et al., Science \textbf{288}, 301 (2000) $^{2}$ S. Ghosal et al., Science \textbf{307}, 563 (2005) $^{3}$ P. Jungwirth et al., J. Phys. Chem. B \textbf{105}, 10468 (2001) [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z38.00011: Coverage Dependent X-ray Photoemission on Halogenated Benzene on Graphite Keisuke Fukutani, Ning Wu, Peter Dowben We studied the adsorption of isomers of halogenated benzene on graphite. We found difference in the behavior of three different symmetry types, (1,2), (1,3), and (1,4), of diiodobenzene (C$_{6}$H$_{4}$I$_{2}$) and 1,4-bromoiodobenzene (C$_{6}$H$_{4}$IBr) adsorbed on graphite surface at 95K by X-ray photoemission spectroscopy. Although the molecules are expected to be similar in their electronic structure, the sticking coefficients and the strength of screening effects are considerably different for the different isomers. We find evidence for different intermolecular interactions both in the initial state and in the final state as well. Symmetry, not simply the the chemical constituents, play a role in adsorbate chemistry. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z38.00012: Retardation of ice crystallization by short peptides Jun Soo Kim, Arun Yethiraj The effect of short peptides on the growth of ice crystals is studied using molecular dynamics simulations. The simulations focus on two sequences (Gly-Pro-Ala-Gly and Gly-Gly-Ala-Gly) that are found in collagen hydrolysate, a substance that is known to retard crystal growth. In the absence of peptides, the growth of ice crystal in the solution with the ice/water interface is observed in at a rate comparable to the experimental data. When peptides are present in the liquid phase, the crystal growth is retarded to a significant extent compared to the pure water. It is found that Gly-Pro-Ala-Gly is more effective (crystallization is up to 5 times slower than in its absence) than Gly-Gly-Ala-Gly (up to 3 times slower) implying that the role of the proline residue is important. The mechanism can be understood in the nature of binding of the peptides to the growing crystal. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z38.00013: AC Electrowetting of Polymer Aqueous Drops on Parallel Electrodes Lu Zhang, Nishant Chetwani, Hsueh-Chia Chang, Yingxi Elaine Zhu We have recently observed the strong field dependence of AC-electrowetting of simple electrolyte aqueous drops on parallel gold electrodes, yet the detailed dynamic process of AC-field induced surface wetting remains unclear. In this work, we use fluorescence labeled DNA aqueous solution as a model system to directly visualize the wetting process of aqueous drops under varied AC electric fields by using combined fluorescence microscopy and contact angle goniometer. The electrowetting behavior of DNA aqueous drops is observed at AC-field frequency greater than the reciprocal of the RC time scale for electrode screening. And the onset of AC electrowetting is accompanied by the observed oscillation in drop contour shape and contact line. In addition, the ejection of nanodrops from the parent aqueous drop is observed when the threshold AC-field amplitude is exceeded. A scaling theory based on electrode interfacial screening is developed to quantify the AC-electrowetting behavior with the dependence of AC-field frequency, strength and medium conductivity. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z38.00014: ABSTRACT WITHDRAWN |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z38.00015: Diverging Tolman's length and asymmetric interfacial density profiles in near-critical fluids Mikhail Anisimov, Heather St. Pierre, Deepa Subramanian The surface tension of a curved surface behaves differently than that of a planar surface; however, the curvature correction to the surface tension - known as Tolman's length - is commonly ignored in practice. We show that asymmetric fluid phase equilibria result in diverging Tolman's length at the critical point with the amplitude of the divergence depending on the degree of asymmetry in fluid phase coexistence. The divergence of Tolman's length originates from the diverging critical fluctuations and does not exist in mean-field theories. However, the amplitude of this divergence (``intrinsic asymmetry'') can be obtained from an appropriate mean-field model. The asymmetry in phase equilibria is especially pronounced in ionic fluids and polymer solutions. In particular, Tolman's length in polymer solutions may become as large as the thickness of the interface, thus playing a significant role in behavior of micro droplets and confined polymer fluids. [Preview Abstract] |
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