Gas-Expanded Liquids under Confinement: Phase Equilibrium and Transport Properties of Ethylene-expanded Methanol in Mesoporous Silica*

Using Grand Canonical Monte Carlo and molecular-dynamics simulation, we examine the phase equilibrium and transport of a gas-expanded liquid under confinement. The system chosen is ethylene-expanded methanol confined in model silica mesopores, but in equilibrium with the bulk mixture -- a system that has received recent interest as a reaction medium, for example, for epoxidation of ethylene. This system was studied at 20^○C and pressures ranging from 5 to 55 bar. In addition, two pore surface chemistries were examined: a hydrophilic pore, in which the silica dangling bonds were terminated by -OH groups, and a model ``hydrophobic'' pore, in which the charges on the pore atoms (including the -OH groups) were turned off. We find that the pressure significantly affects the ethylene mole fraction in the confined mixture. Poresurface chemistry has a significant effect on the composition and transport properties of the confined ethylene-methanol mixture, relative to the bulk. In addition, there are significant qualitative differences between the hydrophilic and hydrophobic pores with regard to the spatial distributions of the confined ethylene and methanol.