Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Z18: Surfaces and Adsorption II |
Hide Abstracts |
Sponsoring Units: DPOLY Chair: Elaine Zhu, University of Notre Dame Room: 319 |
Friday, March 20, 2009 11:15AM - 11:27AM |
Z18.00001: Dynamics of six-member molecular rings adsorbed onto graphite and MgO(100) John Z. Larese, Peter Yaron Molecular dynamic studies of adsorption of six-member molecular rings (cyclohexane and benzene) onto various substrates (like MgO(100) and graphite) have been undertaken using a commercial modeling package (Materials Studio by Accelyris Software Inc.) that employs central force field potentials. These studies indicate that both systems exhibit rotational translational coupling and strong signs that the translational diffusion is lattice-like on graphite. The cyclohexane rotational motion has a distinct out of plane component coupled to the translational diffusion much like the rolling motion of a wobbly wheel with three hydrogen atoms in ``contact'' with the substrate plane, while the benzene is more planar. These studies will be compared to results of neutron scattering investigations of the temperature dependance of the dynamics. [Preview Abstract] |
Friday, March 20, 2009 11:27AM - 11:39AM |
Z18.00002: Heterogeneity of nitrogen multilayers adsorbed on graphite M. Golebiowska, L. Firlej, B. Kuchta, M.W. Roth The low temperature structures and phase transitions in nitrogen multilayers physisorbed on graphite are analyzed using Monte Carlo simulations. The systems that differ in number of layers and their relative packing (hexagonal or cubic) are analyzed. The nitrogen molecules are simulated as rigid, interacting via site-site potential (Etters model). The interaction with graphite includes atomic corrugation (Steele's potential). We show that the temperature and the mechanism of both orientational and melting phase transitions vary with the surface coverage and change from layer to layer. In particular, melting of the bilayer is preceded by compression of the first layer, which has not been observed before. The results are compared with simulations of two similar systems: (i) three nitrogen layers confined in slit graphite pore, and (ii) an adsorbed incommensurate structure that mimics low temperature alpha phase of bulk nitrogen. [Preview Abstract] |
Friday, March 20, 2009 11:39AM - 11:51AM |
Z18.00003: Surface Hopping and Sliding of Single DNA Chains under Electric Field Benxin Jing, Jiang Zhao The motion of non-adsorbing DNA chains under electric field at solid-liquid interfaces was investigated by single molecule fluorescence microscopy at the total internal reflection geometry (TIRF). In-situ observation discovered that the motion of single non-adsorbing lamda-DNA chains was hopping-and-sliding-like along the surface. By varying the surface chemistry of the solid substrates, from the negative-charged hydroxyl group-rich surface to positive-charged amino group-rich surface, as well as hydrophobic surfaces, the dependence of DNA mobility on the surface-DNA interaction was studied. The results show that a well-defined dependence of the mobility of DNA on the surface polarity with respect to DNA itself. The study on different surfaces such as hydroxyl, amide, amino, and methyl-group rich surface show a sequence of DNA mobility of hydroxyl $>$ amide$>$ amino. The mobility of DNA on methyl terminated surface was found to be similar to that in amino surface. [Preview Abstract] |
Friday, March 20, 2009 11:51AM - 12:03PM |
Z18.00004: First Principles study of the formation of molecular junctions: benzenethiolate on Au (111) Yongduo Liu, Vidvuds Ozolins We perform density functional calculations to study the formation mechanism of benzenethiolate molecular junctions on the Au (111) surface. Specifically, we investigate the geometry change and the mechanical properties of the metal-molecule contact when it is under stretching. It is found that by pulling up the thiolate molecule from Au (111), one Au surface atom can be converted to an adatom. And moreover, if the stretching is continued, another Au atom would successively be pulled up to form a two-atom bridge between the Au (111) and the sulfur end group. Based on these findings, we propose a mechanism to the formation of pyramidal molecular junctions: benzenethiolate on Au (111). [Preview Abstract] |
Friday, March 20, 2009 12:03PM - 12:15PM |
Z18.00005: Molecular Dynamics Simulation of friction in contact-mode Atomic Force Microscopy of alkane films and nanoparticles F.Y. Hansen, P. Soza, H. Taub, U. Volkmann In addition to sample topography, contact-mode Atomic Force Microscopy (AFM) can yield the lateral frictional force experienced by the AFM tip as it moves across a surface. This frictional force is measured by the torsional angle of the microscope's cantilever arm, which, in the case of a surface composed of highly anisotropic alkane molecules, can depend on the molecular orientation. We have conducted molecular dynamics simulations of an AFM tip moving over films and nanoparticles of $\rm C_{24}H_{50}$ (C24) in the contact mode. For films in which the long axis of the C24 molecules is oriented parallel to the surface, we find a smaller frictional force in a scan direction perpendicular to the long axis than parallel to it. On surfaces where the alkane molecules are oriented perpendicular to the interface, we find that in all scan directions the frictional force is less than when the long molecular axis is parallel to the interface. All of these findings are consistent with experimental observations \footnote{S. Trogisch {\it et al.}, J. Chem. Phys, {\bf 123}, 154703 (2005).} [Preview Abstract] |
Friday, March 20, 2009 12:15PM - 12:27PM |
Z18.00006: The Measurement of Surface Rheological and Surface Adhesive Properties of a PDMS Rubber using Micro- and Nano-Particle Embedment Stephen Hutcheson, Gregory McKenna In previous work, we used particle embedment data to determine the rheological response of the surfaces of a polystyrene film, a phase separated copolymer and a commercially available polydimethylsiloxane (PDMS) rubber through the application of a viscoelastic contact mechanics model. The goal of the current research is to build off this analysis and use micro- and nano-sphere embedment experiments to probe the surface rheological behavior of PDMS in the rubbery state. The work includes measurements made with different particle diameters and chemistries. An atomic force microscope (AFM) is used to measure the embedment depth as nanoparticles are pulled into the surface by the thermodynamic work of adhesion. Present results show that silica probes of different sizes (500 nm and 300 nm) give different results for the surface adhesion properties and the surface rheological properties determined from the particle embedment data and at scales much larger than the nanometer size scale where one might expect such deviations. Possible water entrapment and effects of particle surface composition on the results will be discussed. [Preview Abstract] |
Friday, March 20, 2009 12:27PM - 12:39PM |
Z18.00007: Test of the universality of the scaling energy in alkanes based on the melting transition of monolayers adsorbed on graphite L. Firlej, B. Kuchta, M. Roth, C. Wexler The quality of the results of computational treatment of complex systems depends crucially on the quality of the interaction potentials used. When modeling non-rigid molecules an essential difficulty resides in the correct accounting for all energies related to internal degrees of freedom. Of particular importance is to avoid over- or under-counting non-bonded intramolecular van der Waals and electrostatic interactions between close neighbors that are chemically bonded. In many force fields (e.g. CHARMM) 1-4 non-bonded interactions are scaled down by the use of the ``scaling factor'' (SF). Typically, this SF is assumed to be universal (and $\sim 0.5$). In this talk, we study this universality hypothesis by comparing computational and experimental melting temperatures of alkane monolayers adsorbed on a solid graphite surface. Three alkanes (C6, C12, C24) of different lengths (7-30 $\AA$) have been analyzed using an all-atom representation, standard CHARMM parameters, and various SF's. We show that the scaling parameter has no universal value but depends on the molecular length. [Preview Abstract] |
Friday, March 20, 2009 12:39PM - 12:51PM |
Z18.00008: The role of local domain formation in the melting of hexane adlayers on graphite M.W. Roth, L. Firlej, B. Kuchta, C. Wexler Hexane is the shortest alkane [CH$_3$-(CH$_2$)$_{n-2}$-CH$_3$] whose flexibility has any considerable impact on its dynamics. When adsorbed on graphite, a monolayer of hexane melts at a temperature of approximately 175 K. To understand the mechanisms of this transition we have performed large scale molecular dynamics simulations (several runs over 100 ns, total computation time $\sim$ 10 cpu-years), using the most realistic model of the system (a fully atomistic representation of hexane, explicit site-by-site interaction with graphite carbons and CHARMM force field with carefully chosen adjustable parameters of interactions). We show that the melting of the low temperature herringbone solid phase starts with the formation of gauche defects at the ends of neighboring molecules, followed by molecular reorientation within a lamellae, without perturbing the overall structure of the adsorbed film. The melted phase has a domain-type structure with domains' orientation that reflects the 6-fold symmetry of graphite. The size of domains decreases progressively when the temperature increases and the deformation of molecules to quasi globular shape is driven by progressive formation of gauche defects. [Preview Abstract] |
Friday, March 20, 2009 12:51PM - 1:03PM |
Z18.00009: Chain-length dependence in surface stresses of alkanethiolate-covered Au(111) V. Srinivasan, G. Cicero, J. C. Grossman We have recently shown~\footnote{Phys. Rev. Lett. {\bf 101}, 185504 (2008)} that adsorption-induced stresses in alkanethiolate-covered Au(111) contribute significantly to the stress-response in nano-mechanical cantilever sensors. In particular, we proposed a local stress relief (LSR) mechanism whereby charge removal by the Au-S bond from the Au surface promotes a stress reducing rearrangement of surface Au atoms. Since LSR depends on the nature of the Au-S bond it was unclear how the contribution to the stress- response would depend on the alkanethiolate structure. We present a first-principles study of the chain-length dependence in surface stresses of alkanethiolate-covered Au(111). We find that the surface stress upon adsorption is anisotropic and tensile, increasing in magnitude with the chain-length. We analyze this trend in the context of the LSR mechanism and inter-adsorbate interactions. [Preview Abstract] |
Friday, March 20, 2009 1:03PM - 1:15PM |
Z18.00010: All-atom Molecular Dynamics simulations of partial pentane and hexane films on graphite M. Kaspar, M.W. Roth, Carlos Wexler, L. Firlej, B. Kuchta We compare the self-assembly patterns of pentane (C$_{5}$H$_{12})$ and hexane (C$_{6}$H$_{14})$ adlayers physisorbed onto graphite at various coverages using the results of molecular dynamics simulations. Near monolayer coverage, the solid low temperature structure of the pentane film is nematic-like, and that of hexane-herringbone-like. At submonolayer coverages both systems exhibit three distinct topological regimes: vacancy patches at higher densities, percolating networks at intermediate densities and ultimately individual patches. The systems' orientational behavior and melting dynamics is discussed with respect to its unique density-dependent topology. The simulations explicitly include hydrogens of pentane and hexane and the graphite is modeled as a six-layer all atom structure. [Preview Abstract] |
Friday, March 20, 2009 1:15PM - 1:27PM |
Z18.00011: Temperature Dependent Adsorption Dynamics of Binary Mixtures of Halomethanes on Graphite and $\alpha$-quartz Surfaces Jonathan Nehring, G. Leuty, Mesfin Tsige Using atomistic molecular dynamics simulations, we have investigated the structure and dynamics of binary mixtures of halomethanes ($CF_4$, $CF_3Cl$, and $CF_3Br$) as a function of temperature on two structurally and chemically different surfaces. The initial distribution of the binary mixtures is either they are uniformly mixed or a layer or layers of one component is placed on top of a layer or layers of the other component. As a function of temperature (below and above the melting temperature of $CF_4$) and the two surface types, we observed a marked change on the concentration, dynamics, orientation, and structure of each of the components in the first layer of the binary mixtures next to the surfaces. [Preview Abstract] |
Friday, March 20, 2009 1:27PM - 1:39PM |
Z18.00012: Liquid Chromatography at Critical Conditions: Balancing size exclusion and adsorption in nanopores Asem Abdulahad, Jeffrey Amos, Chang Ryu Liquid chromatography at critical condition (LCCC) is a measure to identify thermodynamic conditions, in which polymers elute independently of molar mass during high performance liquid chromatography. Under these critical conditions the entropic exclusions that dominate size exclusion chromatography (SEC) and the enthalpic adsorption that governs adsorption-based interaction chromatography (IC) are said to negate one another resulting in simultaneous elution of the polymer of different molecular weights. Using multiple C18-bonded silica columns with different average nanopore sizes (from 5 nm to 30 nm), we will study the LCCC conditions of PS in methylene chloride/acetonitrile solvent mixture at different temperature. In addition, we will show that the separation of polystyrene can be fine tuned using a refined temperature gradient interaction chromatography (TGIC) that employs multiple columns of varying pore size in sequence. [Preview Abstract] |
Friday, March 20, 2009 1:39PM - 1:51PM |
Z18.00013: Using Amphiphilic Copolymers and Nanoparticles to Organize Charged Biopolymers Jung Hyun Park, Marla McConnell, Yujie Sun, Yale Goldman, Russell Composto Nanoparticles (NPs) on amphiphilic random copolymers control filamentous actin (F-actin) attachment. 3-aminopropyltriethoxysilane (APTES) coated silica NPs are selectively bonded to acrylic acid groups on the surface of a poly(styrene-r-acrylic acid) (PS-r-PAA) film. By changing the concentration of NPs in the medium, the surface density of positively charged anchors is tuned. Using total internal reflection fluorescence (TIRF) microscopy, immobilization of F-actin is observed via electrostatic interaction with NPs at high NP coverages. Below a critical coverage, F-actin is weakly attached and undergoes thermal fluctuations near the surface. Another method to tune F-actin attachment is to use APTES to cross-link and create positive charge in PAA films. Here, the surface coverage of F-actin decreases as APTES concentration increases. This observation is attributed to an increase in surface roughness and hydrophobicity that reduces the effective surface sites that attract F-actin. In addition, in-situ G-actin polymerization to F-actin is observed on both the NP and cross-linked PAA templates. [Preview Abstract] |
Friday, March 20, 2009 1:51PM - 2:03PM |
Z18.00014: Non-translational Molecular Diffusive Motion on Two Different Time Scales in Alkane Nanoparticles S.-K. Wang, M. Bai, H. Taub, E. Mamontov, K.W. Herwig, F.Y. Hansen, J.R.D. Copley, T. Jenkins, M. Tyagi, U.G. Volkmann Using quasielastic neutron scattering, we have investigated molecular diffusive motion in $n$-C$_{32}$H$_{66}$ nanoparticles whose structure and phase transitions have been studied previously.$^{2}$ The spectra reveal non-translational (dispersionless) diffusive motion occurring simultaneously on time scales of $\sim $1 ns and $\sim $40 ps. The onset of the faster motion occurs in the crystalline phase at least 15 K below the melting point and is tentatively identified with rotation about the long molecular axis. Similarly, we suggest that the slower motion involves molecular conformational changes whose onset appears to coincide with the abrupt transition to the bulk rotator phase about 3 K below melting. These two types of diffusive motion bear a strong resemblance to those observed previously in C24 monolayers adsorbed on a graphite surface.$^{3}$ $^{2}$M. Bai \textit{et al.}, Europhys. Lett. \textbf{79}, 26003 (2007). $^{3}$F. Y. Hansen \textit{et al}., Phys. Rev. Lett. \textbf{92}, 046103 (2004)]. [Preview Abstract] |
Friday, March 20, 2009 2:03PM - 2:15PM |
Z18.00015: Synergetic binding and lateral segregation in polymer decorated micelles and nanoparticles Igal Szleifer, Rikkert Nap Nanocarriers show great potential as drug delivery devices or as imaging agents. Experimental relevant examples of nanocarriers involve micelles made of low molecular weight polyethylene glycol and phospholipids. An important feature of these 'nano' micelles is that the polymers are mobile. A fundamental question is how different polymeric coatings result in optimal nanoparticle-surface interactions. We used a molecular theory to investigate the effect of the conformational entropy, specific interactions and lateral mobility on the structure of the polymer coatings and the binding of the nanocarrier to a cell surface. In micelles that contain chains of different molecular weights, the long and short polymer chains segregate upon approaching the surface, as a result of competing entropic forces. Nanocarriers made of mixtures of weak polyelectrolytes with ligands at their free ends and neutral polymers can bind to charged surfaces or through specific ligand-receptor interactions. We show that under appropriate conditions there is a dramatic synergetic effect between electrostatic and ligand-receptor binding. The synergetic effect is due to the optimal compensation between charge regulation, specific binding and counterion release. The potential use of these carriers for cancer drug delivery will be discussed. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700