Wetting and dewetting of narrow hydrophobic channels by orthogonal electric fields: Structure, free energy, and dynamics for different water models.

Abstract

Wetting and dewetting of a (6,6) carbon nanotube in presence of an orthogonal electric field of varying strengths are studied by means of molecular dynamics simulations using seven different models of water. We have looked at filling of the channel, occupancy and structure of water inside it, associated free energy profiles, and also dynamical properties like the time scales of collective dipole flipping and residence dynamics. For the current systems where the entire simulation box is under the electric field, the nanotube is found to undergo electrodrying, i.e., transition from filled to empty states on increase of the electric field. The free energy calculations show that the empty state is the most stable one at higher electric field as it raptures the hydrogen bond environment inside the carbon nanotube by reorienting water molecules to its direction leading to a depletion of water molecules inside the channel. We investigated the collective flipping of water dipoles inside the channel and found that it follows a fast stepwise mechanism. On the dynamical side, the dipole flipping is found to occur at a faster rate with increase of the electric field. Also, the rate of water flow is found to decrease dramatically as the field strength is increased. The residence time of water molecules inside the channel is also found to decrease with increasing electric field. Although the effects of electric field on different water models are found to be qualitatively similar, the quantitative details can be different for different models. In particular, the dynamics of water molecules inside the channel can vary significantly for different water models. However, the general behavior of wetting and dewetting transitions, enhanced dipole flips, and shorter residence times on application of an orthogonal electric field hold true for all water models considered in the current work.