Molecular dynamics simulations are used to examine the diffusion of acetonitrile within ∼2.4 nm diameter amorphous silica pores with a focus on the mechanism. The role of the pore surface chemistry is examined by comparison of a hydrophilic, −OH terminated, silica pore with one that has hydrogen-bonding turned off and with an effectively hydrophobic pore obtained by setting all pore charges to zero. The anisotropy of diffusion, along and perpendicular to the pore axis, is examined through the mean-squared displacements. The origins of the anisotropy are investigated through the dependence on the acetonitrile position within the pore. The effect of hydrogen bonding of acetonitrile molecules to the hydrophilic pore surface is also probed. The simulations show that acetonitrile molecules do not diffuse axially next to the pore surface. Rather, axial diffusion is preceded by radial diffusion away from the pore surface. The same mechanism is observed for molecules independent of their hydrogen-bonding status to surface silanols though hydrogen-bonded molecules diffuse more slowly.

Molecular dynamics simulations are used to investigate the reorientation dynamics of liquid acetonitrile confined within a nanoscale, hydrophilic silica pore. The dynamics are strongly modified relative to the bulk liquid-the time scale for reorientation is increased by orders-of-magnitude and the dynamics become nonexponential-and these effects are examined at the molecular level. In particular, commonly invoked two-state (or core-shell) models, with and without consideration of exchange of molecules between the states, are applied and discussed. A rigorous decomposition of the acetonitrile reorientational correlation function is introduced that permits the approximations implicit in the two-state models to be identified and tested systematically. The results show that exchange is an important component of the nanoconfined acetonitrile reorientation dynamics and a two-state model with exchange can accurately describe the correlation. However, the faithfulness of the model is related to the separation of time scales in the two states, which exists for a wide range of definitions of the two states. This suggests that caution should be exercised when inferring molecular-level details from application of two-state models.