Solid-fluid equilibrium in molecular models of n -alkanes

Abstract

Determination of fluid-solid transitions in model protein solutions using the histogram reweighting method and expanded ensemble simulationsThe fluid-solid equilibrium for a charged hard sphere model revisitedWe present a study of the solid-fluid phase equilibrium for flexible hard sphere site united atom models of n-alkanes using Monte Carlo computer simulation. We have considered models with different torsional potentials to examine the effect this has on the phase diagram. Extensive calculations of the fluid and solid phase equations of state have been made and solid phase free energies have also been determined. The initial solid phase structure used for each system was that which allows the chains to reach the highest density at close packing. The data for hard core chain models have been used as a reference system in a generalized van der Waals or mean field calculation of the n-alkane phase diagrams. This theory reproduces trends in the triple point temperature seen in experimental data. These trends are interpreted in terms of the changes in the close packed densities of the solids with chain length and the effect of the torsional energy on the relative stability of the fluid and solid phases.

Lattice-switching Monte Carlo method for crystals of flexible molecules

“…In a pair of papers [23,24], Monson et al have introduced a family of hard-sphere united atom models for normal alkanes. Molecular chains consist of hard sphere beads, diameter d, linked by bonds of fixed length 0.4 d. All bond angles are constrained at 109.47 • .…”

“…Note that this differs from the unit system used in Ref. [23] where the unit of length is taken as the diameter of a sphere with equal volume to that of the molecule.…”

“…The structure considered in the solid-fluid equilibria explored by Malanoski and Monson [23] corresponds to phase III, in which the long axes of all molecules are aligned.…”

Molecular Modeling of Solid Fluid Phase Behavior

“…We studied a flexible united atom hard sphere chain model of nalkanes. The simplicity of this model made it possible for a study of the role of chain length, flexibility and packing on the solid-fluid phase behavior and provided an explanation for the odd-even effect in the n-alkane melting points [18,19]. The work was extended to n-alkane mixtures [20,22].…”

Molecular thermodynamics of solid‐fluid and solid‐solid equilibria

“…This point is illustrated by work on the n-alkanes, where the repulsive force contribution to the free energy was modeled using systems of flexible hard sphere chains. 9 In fact, these ideas are already evident in the organic crystallography literature in the maximum close packing principle of Kitaigorodsky, 10 summarized as: the crystal structure of an organic compound will be that which gives the closest packing of the molecules. We reach the limit of the van der Waals picture when we encounter systems with strongly directional intermolecular forces, such as hydrogen bonds, that can lead to socalled anomalies in SFE behavior (e.g., expansion on freezing).…”