Molecular Dynamics Simulations of the Melting of a Hexane Monolayer:  Isotropic versus Anisotropic Force Fields

Abstract

Molecular dynamics simulations of a hexane monolayer on the basal-plane surface of graphite have been performed to investigate the effect of an anisotropic force field on the melting process. In two sets of simulations, which are carried out for a number of temperatures ranging from the solid to the fluid state of the monolayer, the molecules are described either by a skeletal model, where the interaction sites are represented by a "united atom" model, or by a model where the interaction sites are shifted away from the mass site (the "anisotropic united atom" model). Independent of the model, the low temperature configuration is an orientationally ordered herringbone structure, which on heating undergoes an orientational phase transition to a rectangular-centered structure where the molecules tend to align along one direction. The solid subsequently melts at approximately 175 K. However, the anisotropic force field promotes a perpendicular orientation of the backbone of the hexane molecules and, in contrast to the "united atom" model, molecules exhibit a smaller tilt and a lower percentage of gauche defects at melting. This is compensated by increased librational motion in the plane of the substrate.

Melting of Hexane Monolayers Adsorbed on Graphite: The Role of Domains and Defect Formation

“…The pair correlation function (not shown) also confirms that the system undergoes a sharp melting transition. It is noteworthy that the behavior of all the order parameters in Figure 1 illustrate that the system melts directly from a herringbone to the fluid, in contrast to all previous computer simulations where a nematic mesophase is observed [7,8,[10][11][12]. Figure 2 shows typical configurations of the layer at various temperatures.…”

“…The initial low-temperature configuration has an important departure from the previous ones [6][7][8][9][10][11][12]. There are N = 104 hexane molecules in a herringbone arrangement atop a six-layer 68.16 Å x 68.88 Å graphite structure.…”

“…First, the issue of simulating a formally complete monolayer at zero spreading pressure has never been investigated. Although believed to be modest in previous simulations [6][7][8]10,12] any planar stress present in phase transition simulations can, and in fact does, dramatically affect the system dynamics [11]. Second, molecular flexibility was not properly accounted for in previous studies.…”

“…Even with a considerable body of computational work [4][5][6][7][8][9][10][11][12][13], there remain poorly understood elements of this interesting system. In particular, the detailed mechanism of the melting transition and the effect that molecular stiffness has on it has not been elucidated.…”

Simulated effects of odd-alkane impurities in a hexane monolayer on graphite

“…The UA model has been widely used in previous simulations [5][6][7][8][9][10][11] and previous work 8 that studies the difference between the UA and AUA models for hexane monolayers indicates little difference in the phase behavior observed, with the only differences referring to primarily the amount of rolling and tilting of the molecules with respect to the substrate, both of which do not seem to affect the phase transitions significantly.…”