Molecular simulation of the phase behavior of water confined in silica nanopores

Molecular dynamics simulations and grand canonical Monte Carlo simulations of water molecules in 1.04, 1.96, and 2.88 nm diameter silica pores (pores S, M, and L, respectively) were conducted at 300 K to reveal possible phase states of water in each pore and clarify the effect of pore diameter and hydration level on the structural and dynamical properties of water molecules confined in nanopores. Three types of phases appear in nanopores. In the first phase, which appears in pores S, M, and L, gas-phase water adsorption at the pore surface is below monolayer coverage. In the second phase, which appears in pores S and L, there is a condensed water monolayer at the pore surface. In the third phase, which appears in pores M and L, the pore is completely filled with water. Water molecules interact with silanol groups mainly via hydrogen bonds at low hydration when the number of water molecules is much smaller than that of silanol groups. With increasing number of water molecules, the number ratio of water molecules that adsorb on silanol groups via non-hydrogen-bonding interactions increases. Near full hydration, the translational mobility of water in the first adsorption layer is much smaller than that of bulk liquid water in all three pores, while those in the pore center are about 30, 80, and 100% of its bulk liquid value in pores S, M, and L, respectively.