Langmuir, Vol.29, No.47, 14527-14535, 2013
Connection between Thermodynamics and Dynamics of Simple Fluids in Highly Attractive Pores
Using molecular simulations, we investigate the structural and diffusive dynamics properties of a model fluid in highly absorptive cylindrical pores. At subcritical temperatures, self-diffusivity displays three distinct regimes as a function of average pore density rho: (1) a decrease in self-diffusivity with increasing rho at low rho, (2) constant self-diffusivity with respect to varying rho at moderate density, and (3) a decrease in self-diffusivity with increasing rho at high density. These regimes are closely linked to the thermodynamic properties of the fluid in the pore, specifically, the adsorption isotherm, isosteric heat of adsorption, and the density profile. We show that these three diffusivity regimes qualitatively correspond to three distinct adsorption regimes: monolayer formation, multilayer adsorption, and pore filling, respectively. In addition, we find that the self-diffusivity is a universal function of the local film density in the monolayer formation regime at subcritical temperatures. The results of this work suggest a potential means to estimate the self-diffusivity over a broad pressure range using a limited number of experiments.