Effects of surface structure and solvophilicity on the crystallization of confined liquids

Abstract

The crystallization of liquids confined at the nanoscale plays an important role in scientific and engineering applications, yet crystallization at the nanoscale is not well understood. Using molecular dynamics simulations, we study the crystallization of a confined liquid characterized by isotropic pair interactions with water-like properties. We focus on the effects of (i) confining surface structure (structureless vs. amorphous), (ii) surface–liquid interaction strengths (solvophobic vs. solvophilic), and (iii) confining dimensions on the crystallization of a liquid confined between two parallel walls. We find that crystallization under severe confinement is very different from crystallization processes in bulk liquids. Specifically, in all cases studied, crystallization involves two steps, a rapid arrangement of liquid particles into layers, followed by a slower increase of order within the layers. For the system under study, the crystallization mechanism depends on the surface structure but it is independent of the solvophobicity/solvophilicity of the surfaces. Specifically, in the case of structureless surfaces, crystallization initiates at the surface and then propagates towards the rest of the liquid, while for amorphous surfaces, crystallization occurs rather uniformly within the confined volume. In addition, crystallization is favored, i.e. crystallization times are shorter, when the surfaces are structureless than in the case of amorphous surfaces. Interestingly, the effect of surface solvophobicity/solvophilicity is opposite for structureless and amorphous surfaces. It is found that increasing the liquid–surface interaction strength favors crystallization for the case of structureless surfaces, while it tends to suppress crystallization for the case of amorphous surfaces. The present results reflect the complexity of crystallization processes at the nanoscale and in particular, its sensitivity to the confining surface properties.