Influence of molecular structure on the properties of confined fluids by molecular dynamics simulation

We recently developed a new molecular dynamics (MD) simulation approach for studying fluids confined between two solid substrates and in equilibrium with bulk fluids. Using this method, we perform MD simulations to investigate the influence of molecular structure on the properties of confined Lennard-Jones (LJ), n-decane, and 2,2-dimethyloctane fluids. Under confinement, spherical LJ particles and linear n-decane form layers parallel to the surfaces, while asymmetric 2,2-dimethyloctane forms a 'pillared-layered' structure consisting of both parallel and perpendicular molecules. As surface separation is varied, the number of spherical LJ and symmetric n-decane molecules changes in a step-wise manner, while the number of 2,2-methyloctane molecules varies in a smooth fashion due to pillar molecules gradually switching between parallel and perpendicular orientations. Concomitant configurational transitions cause oscillatory solvation forces, with force maxima corresponding to well-layered configurations. The double branches in 2,2-dimethyloctane reduce the densities and structural changes in the layers adjacent to the surfaces, causing solvation forces and force oscillations to be less pronounced than those of linear chains. Concerning dynamical properties, the translational diffusivity, computed with Einstein relation, and the shear viscosity, computed with Green-Kubo method, both oscillate, the former out of phase and the latter in phase with respect to force oscillations. Better-ordered films having higher densities exhibit lower translational diffusivities but higher shear viscosities. At disordered states, bulk diffusivities and viscosities are recovered. Asymmetric, branched 2,2-dimethyloctane has lower diffusivities due to its bulky t-butyl group, and weaker diffusivity and viscosity oscillations due to its reduced ordering and configurational differences.