Local surface dynamics in the adsorbed film of pentane isomers on graphite

Surface diffusion due to molecular motion within adsorbed fluid films can be a dominant contributor to mass transfer in porous materials. In micro- and mesopores, surface diffusion is classified into three regimes: the monolayer, multilayer, and condensed phase. Although the dominant molecular motion is restricted to the in-plane direction for sub-monolayer coverage, it is more complicated within the multilayer adsorbed phase, especially at the interface between the vapour phase and the adsorbed phase. We have performed molecular dynamic simulations to study the surface self-diffusion and the collective diffusion of pentane isomers on graphite surface. In the bulk liquid, linear n-pentane has higher self-diffusivity than neopentane at the same temperature. The contrary is observed in an adsorption system where neopentane consistently shows a greater lateral self-diffusion coefficient. In the multilayer regime, we show that the contribution from the vapour phase to the global diffusion is significant. In agreement with the trend observed for self-diffusivities, the collective diffusivities of both isomers show distinct behaviours, particularly in the multilayer regimes. This study highlights the complicated diffusion behaviour of multilayered molecules and the significant contribution from the vapour phase to the dynamics at high pressures.