Grand Canonical Monte Carlo Simulations of Acetonitrile Filling of Silica Pores of Varying Hydrophilicity/Hydrophobicity

Grand canonical Monte Carlo simulations have been used to determine the equilibrium density of acetonitrile in model amorphous silica pores with varying radius and surface chemistry. Pores of diameter similar to 2-4 nm were considered with different ratios of surface -OH moieties to -OC(CH3)(3) groups. The calculations found that the acetonitrile density in the interior of all the pores is essentially identical with that of the bulk liquid. On the other hand, a slightly elevated liquid density is observed near the pore Surface for pores with only -OH surface moieties. Replacement of surface -OH groups with -OC(CH3)(3) units lengthens the liquid/pore interfacial region as acetonitrile molecules can insert themselves between the -OC(CH3)(3) units. The results indicate that the major effect of changing the surface functionality comes from the differences in excluded volume rather than hydrogen-bonding effects. Finally, the choice of the acetonitrile potential can qualitatively change the results,