Surface melting of octamethylcyclotetrasiloxane confined in controlled pore glasses: curvature effects observed by 1H NMR

We have measured the thickness of the pre-molten surface layer that appears at the interface of octamethylcyclotetrasiloxane (OMCTS) to the matrix in controlled pore glasses with pore diameters ranging 7.5-73 nm. Except for the glass with the largest pores, the layer thickness data for different pore diameters fall on a single master curve when plotted versus Tm - T, where Tm is the size-dependent volume melting point of the pore-confined OMCTS. Hence, at a single temperature, the surface layer thickness depends strongly on the curvature of the pore wall and therefore that of the solid-liquid interface. For temperatures where it exceeds two monolayers, the layer thickness depends logarithmically on Tm - T; for the glass with the largest pores, this turns into a power law with the exponent -1/2. The results are interpreted in terms of a continuous model of the solid-liquid interface with an arbitrary curvature. Because OMCTS is a weakly polar molecule with close to spherical shape, our data also lend themselves to Lennard-Jones type simulations.