Local Microphase Separation of a Binary Liquid under Nanoscale Confinement

Abstract

The structural and diffusive properties of an ethanol-water mixture under hydrophilic nanoscale confinement are investigated by means of molecular dynamics simulations based on the CHARMM force field. The resulting density profiles illustrate that demixing of solvents occurs at the pore wall region, which is composed of silanol molecules in our case. Ethanol molecules are more likely to attach to the wall via hydrogen bonds than water molecules. A noticeable O-H bond orientation is observed for the ethanol molecules in this region, which can be explained by the formation of two specific hydrogen bonds between ethanol and silanol. Water, in contrast, resides mostly outside the interfacial region and is in favor of forming small hydrogen bonded strings and clusters with other water molecules. This phenomenon is corroborated by both the orientation of ethanol hydroxyl groups and the radial distribution functions of the solvent oxygen atom to the silanol hydrogen atom. Ethanol selectively attaches to the wall and forms a layer close to the wall. The hydrophobic headgroups of these ethanol molecules lead to an internal hydrophobic interface layer, which in turn yields cluster structures in the adjacent water. The self-diffusion of water in the confined ethanol-water mixture at the center of the pore is faster than that of water in the bulk ethanol-water mixture; ethanol, on the other hand, diffuses slower when it is confined.