Molecular dynamics studies of the effects of chain branching on the properties of confined alkanes

Abstract

Using a recently developed approach, we perform molecular-dynamics simulations to probe the influence of chain branching on the properties of fluids confined between two solid surfaces. Two decane isomers are investigated: n-decane and 2,2-dimethyloctane. Under confinement, n-decane forms a layered structure consistent with structures observed for n-alkanes in previous simulation studies. In contrast, 2,2-dimethyloctane forms a “pillared-layered” structure consisting of a mix of molecules oriented parallel and perpendicular to the confining surfaces. Although both molecules exhibit solvation forces that oscillate between attractive and repulsive as a function of surface separation, pronounced differences are observed between the two isomers. The number of confined n-decane molecules changes in a stepwise manner as the surface separation is varied, while the number of 2,2-dimethyloctane molecules varies in a smooth fashion due to perpendicular “pillar” molecules gradually switching between parallel and perpendicular orientations. In addition, chain branching reduces the densities and structural changes in the adjacent layers, causing solvation forces and force oscillations to be less pronounced than those of linear chains. It also weakens the influence of pressure on the structure and properties. At separations corresponding to well-ordered films, the translational diffusivity of n-decane is the lowest, while at separations corresponding to disordered films, we recover the bulk diffusivity for n-decane. The diffusivity of 2,2-dimethyloctane is lower and its variation with surface separation is less than that seen for n-decane.