Dependence of surface properties of silylated silica on the length of silane arms

Amorphous precipitated Zeosil 1165 MP silica was silylated with low grafting degrees of organosilicons bearing different alkoxy and hydrocarbon tails, like monomethoxy(dimethyl)octadecylsilane (DMODMS), monomethoxytrimethylsilane (TMMS), trimethoxymercaptopropylsilane (MPTS), and 3-octanoylthio-1-propyltriethoxysilane (NXT®). Thermogravimetry and Elemental Analysis were used to determine the degree of silane grafting and the final number of free silanol OH groups/nm2 on the modified Zeosil surface. Free energy, enthalpy and entropy of adsorption of hydrocarbon probes were determined by Inverse Gas Chromatography at infinite dilution and dispersive component, ? d s , and specific interaction parameter, I sp, of the surface tension of the silica surface were calculated. Silylation changes the hydrophilic character of Zeosil silica to the hydrophobic one, on increasing the grafting degree and, mainly, the length of hydrocarbon tail of the silane molecule (DMODMS and NXT®). The long hydrocarbon tails practically shield the silica particle surface and the adsorbed probes preferentially interact with them. In the case of TMMS-Zeosil the adsorbed probes practically interact with the silica surface, with loss of entropy well above that of the bare silica, while being equal the values of the enthalpy of adsorption. All the other modified silicas show loss of entropy lower than that of bare silica. Steric hindrance, played by the presence of methyl groups of TMMS, is suggested to reduce the freedom of translational and rotational movements of the adsorbed probe.