Molecular Dynamics Simulations and Theory of Interfaces of Oligomers Jonathan G. Harris and Yantse Wang
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MOLECULAR DYNAMICS SIMULATIONS AND THEORY OF INTERFACES OF OLIGOMERS JONATHAN G. HARRIS AND YANTSE WANG Massachusetts Institute of Technology, Department of Chemical Engineering, Room 66-450, 25 Ames Street, Cambridge, Massachusetts 02139
ABSTRACT These proceedings summarize recent work in our group studying the structure of interfaces involving molecular fluids. Two types of systems are discussed. First, we summarize simulations of the structure and surface tension of liquid-vapor interfaces of the alkanes eicosane and decane. Then, we describe the results of simulations of the confined films studied in surface force apparatus and atomic force microscopy experiments. Our simulations show that in both films of normal and branched alkanes, the formation of a layered structure is observed. The branching inhibits this layering, especially in the narrowest pores. In addition an examination of the molecular motions indicates that a transition to a solid or glassy state is not a prerequisite for layering or oscillating solvation forces.
INTRODUCTION Interfacial structure and thermodynamics are important to understanding adhesion and lubrication at a molecular level. Surface tension measurements, neutron and x-ray diffraction, ellipsometry, surface force apparatus measurements, and atomic force microscopy have provided us with a macroscopic understanding along with some molecular level details of surfaces. Molecular simulations can provide more finely resolved detail for carefully constructed model systems. This paper summarizes molecular dynamics simulations of the liquid vapor interface of alkanes and the structure of thin confined films of linear and branched octane. More detailed results on the liquid-vapor interface have been reported[l] and a more detailed description of the confined films is in preparation[2]. METHODS
Liouid-Vapor Interface of Eicosane and Decane The alkanes were modeled using the OPLS model for liquid hydrocarbons [3], but with flexible bond angles having a harmonic potential. The CH 2 and CH 3 groups are treated as united atoms with the Lennard-Jones parameters from Jorgensen's OPLS model. All interactions are truncated at about 2.5 Lennard-Jones core diameters (10 A). The liquid-vapor interfaces of decane (C10 H22 ) and eicosane (C20 H42 ) were studied using slab geometries. Slabs of 400 decane and 150-300 eicosane molecules are formed by placing the molecules in the all trans conformations but with random positions and orientations in cubic boxes. The molecular dynamics are carried out at constant temperature and pressure to form a liquid sample at about 1 atm. The boundaries in one direction (z) are then moved to 2-3 times the width of the slab (x and y directions) so that the two faces in the z direction cannot interact. This forms the liquid-vapor interface. During the sampling period (1.0 - 2.0 ns) constant temperature is maintained by massive stochastic collisions which reassign all velocities from the Boltzmann distribution every 7 ps[4]. All equations of motion are integrated using RA
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