The heat of immersion of modified silica in polar and nonpolar liquids

The influence of the degree of silica surface coverage by trimethylsilyl groups on the heat of immersion in various polar and nonpolar liquids has been investigated using microcalorimetry technique. The decrease of Gibbs free energy gamma as a result of replacement a solid/gas interface with the solid/liquid is accompanied by the emission of heat, which value is determined by the intensity of the interaction of the molecules of liquid with the solid surface and area of interaction. The polarity of surface decreases due to substitution of polar silanol groups for the nonpolar trimethylsilyl (TMS) groups on the SiO2 surface, and consequently, the intensity of the interaction of this surface with polar and nonpolar substances changes. The heats of immersion of modified silica in polar triethylamine (TEA), 2-propanol, acetonitrile and water decrease almost linear with increasing degree of surface modification. The changes of heat of immersion of modified silica in nonpolar liquids are minor and may be associated with some decrease in the specific surface area of the samples owing to the synthesis. The heat of immersion in water is less than in hexane and decane at high degrees of surface modification with TMS groups (0.75 or higher), indicating the hydrophobic nature of the surface. The highest heat of immersion is observed for TEA, what is consistent with the specific interaction of a strong organic base TEA with surface silanol groups.