Bidirectional mapping between self-consistent field theory and molecular dynamics

Copyright (2007) AIP Publishing. This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. The following article appeared in Journal of Chemical Physics 127 and may be found at http://dx.doi.org.proxy.lib.uwaterloo.ca/10.1063/1.2776261A bidirectional mapping scheme that bridges particle-based and field-based descriptions for polymers is presented. Initial application is made to immiscible homopolymer blends. The forward mapping (upscaling) approach is based on the use of molecular dynamics simulations to calculate interfacial density profiles for polymer molecular weights that can be readily relaxed using standard simulation methods. These profiles are used to determine the optimal, effective interaction parameter that appears in the one-parameter self-consistent field theory treatment employed in the present work. Reverse mapping from a field representation to a particle-based description is accomplished by the application of a density-biased Monte Carlo method that generates representative chain configurations in the blend using statistical weights derived from fields obtained from self-consistent field theory.This work was supported by the Los Alamos National Laboratory Exploratory Research component of the Laboratory Directed Research and Development program. This work was carried out under the auspices of the National Nuclear Security Administration of the U.S. Department of Energy at Los Alamos National Laboratory under Contract No. DE-AC52–06NA25396

Molecular Dynamics Simulation of Alkylthiol Self-Assembled Monolayers on Liquid Mercury

We report computer simulation of the self-assembly of alkylthiols on the surface of liquid mercury. Here we focus mainly on the alkylthiol behavior on mercury as a function of the surfactant surface coverage, which we study by means of large-scale molecular dynamics simulations of the equilibrium structure at room temperature. The majority of the presented results are obtained for octa- and dodecanethiol surfactants. This topic is particularly interesting because the properties of the alkylthiol self-assembled monolayers on liquid mercury are relevant for practical applications (e.g., in organic electronics) and can be controlled by mechanically manipulating the monolayer, i.e., by changing its structure. Our computer simulation results shed additional light on the alkylthiol self-assembly on liquid mercury by revealing the coexistence of a dense agglomerated laying-down alkylthiols with a very dilute 2D vapor on mercury surface rather than a single vapor phase in the low surface coverage regime. In the regimes of the high surface coverage we observe the coexistence of the laying-down liquid phase and crystalline phases with alkylthiols standing tilted at a sharp angle to the surface normal, which agrees with the phase behavior previously seen in X-ray and tensiometry experiments. We also discuss the influence of finite-size effects, which one inevitably encounters in molecular simulations. Our findings agree well with the general predictions of the condensation/evaporation theory for finite systems. The temperature dependence of the stability of the crystalline alkylthiol phases and details of the surfactant chemical binding to the surface are discussed. The equilibrium structure of the crystalline phase is investigated in detail for the alkylthiols of various tail lengths

Surface tension of liquid mercury: a comparison of density-dependent and density-independent force fields

Motivated by growing interest in interfacial properties of liquid mercury we investigate by atomistic Molecular Dynamics simulation the ability of density-independent, empiric density-dependent, and recently proposed embedded-atom force fields to predict the surface tension and coexistence density of liquid mercury at room temperature, 293 K. The effect of the density dependence of the studied models on the liquid-vapor coexistence and surface tension is discussed in detail and our results are corroborated by Monte Carlo simulations and semi-analytic liquid-state theory. The latter approach is particularly useful to identify and rationalize artifacts that arise from an ad-hoc generalization of density-independent potentials by introducing density-dependent coefficients. In view of computational efficiency and thermodynamic robustness of density-independent model we optimize its functional form to obtain higher surface tension values in order to improve agreement with experiment