Phase transitions occur when the chemical potentials (including bulk and interfacial contributions) of two phases are equal. For systems with large interfaces, like those typically found in colloid and surface science, interfacial properties strongly affect the phase behavior. Although being important in fundamental and applied science, many details are still poorly understood owing to the scarcity of quantitative experimental data. 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 (1). The domains have no sharp melting point. Instead, they melt gradually below the (sharp) 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. With a newly developed optical interference enhanced microscopy (2) we can easily quantify the thermodynamics of the phase transition and in addition obtain details of the intermolecular interactions within the adjacent film hitherto not accessible with such accuracy. The experimental approach is quite universal and works with many other (rod-like) molecules. 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.

Figure1: Partial melting of C40H82 domains (bright areas) on stepwise increase of the temperature. The frames show the equilibrium situation, that is, neither domain shape nor area change if the temperature is kept constant at the indicated value.

(1) H. Riegler and R. Koehler, Nature Physics 754 (2007)

(2) R. Koehler, P. Lazar, and H. Riegler, Applied Physics Lett. 89, 241906 (2006)