Ethanol promotes dewetting transition at low concentrations

Recent studies have suggested important roles for nanoscale dewetting in the stability and self-assembly dynamics of both physical and biological systems. Less known is the cosolvent (such as ethanol) effect on nanoscale dewetting. Here, we use molecular dynamics simulations to investigate the dewetting behavior in-between two hydrophobic plates immersed in ethanol aqueous solutions, particularly at low concentrations. Unexpectedly, the existence of a small amount of ethanol molecules promotes the dewetting transition in the inter-plate region at a greater separation that is otherwise non-existent in pure water or pure ethanol. We find that a competition for ethanol molecules at equilibrium among the inter-plate region, the outer-surfaces of the plates and the bulk solution results in a depletion of ethanol molecules in the inter-plate region. Meanwhile, the preferred inward orientations of the ethanol ethyl groups at the liquid-vapor interface located at the edge of the plates make the inter-plate core more hydrophobic so that water molecules are more favored to be expelled, thus resulting in an enhancement of the dewetting. These findings provide a deeper understanding of the effects of cosolvents on the hydrophobic interaction.