Silane modified fumed silicas and their behaviours in water: Influence of grafting ratio and temperature

The interaction of water with pyrogenic silica samples, modified by a controlled partial silylation with dimethyldichlorosilane, was studied by microcalorimetry, at 20 °C, 25 °C and 30 °C. Whatever the coverage ratio was, the heat of immersion in water does not vary significantly with the temperature, except one sample corresponding to a high grafting ratio that exhibit a high temperature dependence of the immersion heat.Two samples of quite the same molecular characteristics were examined by Si29 NMR solid-state spectroscopy, which shows that the temperature dependence is related with a higher mobility of the grafted polydimethylsiloxane chains (PDMS). The temperature susceptibility could be attributed to the competition between adsorption of the grafts onto the silica surface and their association in the water phase favoured by the hydrophobic forces, a competition which is measurable only in case of long chain grafts.