Structure of ultra-thin confined alkane films from Monte Carlo simulations

Abstract

Grand canonical Monte Carlo simulations are reported of thin films of n-alkane confined fluids (1,4, 10 carbons) and i-decane under thermodynamic conditions away from the freezing line. All simulated solvation pressure profiles exhibit damped oscillations with a main period of oscillation equal roughly to the size of a methane, methyl or methylene bead (4-4.5 Å). This reflects an arrangement of the beads in layers parallel with the confining walls, which causes an oscillation of the local density across the film. However, the alkane molecules themselves are not arranged in perfect layers. The oscillations do not vanish for long chain and/or branched molecules: they are only attenuated. Also n-decane films have been simulated under thermodynamic conditions closer to surface force apparatus experiments. In contrast with an earlier molecular dynamics study, an overall continuous variation in the average number of decane molecules with the walls separation was found, consistent with the smooth transition towards slower relaxation and increasing rigidity with decreasing film thickness observed in some experiments. The possible origin of the discrepancy between the different simulation findings is discussed.