How pre-melting on surrounding interfaces broadens solid–liquid phase transitions

Abstract

Phase transitions occur when the chemical potentials (including bulk and interfacial contributions) of two phases are equal. For systems with large interfaces, such as thin films or small particles, their interfacial properties strongly affect the phase behaviour. This dependence is important in fundamental and applied science (sintering, nucleation) but many details are still poorly understood owing to the scarcity of quantitative experimental data. Here, we use solid alkane domains that are one monolayer thick and deposited on a substrate to investigate and quantify the influence of the interface on the solid–liquid phase transition. The domains melt gradually below the bulk melting point of alkane. They coexist with a ‘pre-molten’ liquid-like film of variable temperature-dependent thickness. Molecules interchange reversibly between film and domains, thereby converting transition enthalpy into interfacial energy. (Thus, we obtain details of the intermolecular interactions within the adjacent film.) We show that irrespective of its dimensionality or confinement, every solid will melt (partially) and spread out at temperatures below its bulk melting point if its liquid wets the adjoining interface.