Surface modification to produce hydrophobic nano-silica particles using sodium dodecyl sulfate as a modifier

Abstract

Hydrophobic silica particles were prepared using the surfactant sodium dodecyl sulfate (SDS) as a modifier by a new route comprising three processes, namely, aqueous mixing, spray drying and thermal treatment. Since SDS dissolves in water, this route is free of an organic solvent and gave a perfect dispersion of SDS, that is, there was excellent contact between SDS and silica particles in the modification reaction. The hydrophobicity of the modified surface was verified by the contact angle of the nano-sized silica particles, which was 107°. The SDS grafting density reached 1.82nm−2, which is near the highest value in the literature. The optimal parameters of the SDS/SiO2 ratio in the aqueous phase, process temperature and time of thermal treatment were determined to be 20%, 200°C and 30min, respectively. The grafting mechanism was studied by comparing the modification with that on same sized TiO2 particles, which indicated that the protons of the Brønsted acid sites on the surface of SiO2 reacted with SDS to give a carbocation which then formed a Si–O–C structure. This work showed that the hydrophilic surface of silica can be modified to be a hydrophobic surface by using a water soluble modifier SDS in a new modification route.