Effect of the measurement temperature on the dispersive component of the surface free energy of a heat treated SiO2 xerogel

The surface free energy of a monolithic silica xerogel treated at 1000 degr ees C has been measured by inverse gas chromatography in the temperature ra nge 25-150 degrees C using n-alkanes. Values of the dispersive component, g amma(S)(D), vary from 49.07 mJ.m(-2) at 25 degrees C to 17.20 mJ.m(-2) at 1 50 degrees C. The gamma(S)(D) value obtained at 25 degrees C is lower than that found for amorphous and crystalline silicas but higher than that found for glass fibres meaning that the heat treatment at 1000 degrees C changes drastically the structure of the silica xerogel showing a surface similar to a glass. However, the higher value of gamma(S)(D) in comparison to glass fibres can be attributed to the mesoporous structure present in the silica xerogel. In the temperature range of 60-90 degrees C there exists an abrup t change of the gamma(S)(D) values as well as in the dispersive component o f the surface enthalpy, h(S)(D). Such abrupt change can be attributed to an entropic contribution of the surface free energy.