HYDROPHOBIZATION OF SILICA PARTICLES WITH VARIOUS CATIONIC SURFACTANTS

Abstract

Silica particles with a radius of 3-7 nm (Ludox and Aerosil) and 270 nm (synthesized by the Stöber method), modified with cetyltrimethylammonium bromide (CTAB) and hexylamine, were used to stabilize emulsions. The hysteresis angles θ of the particle selective wetting were measured by the sessile droplet method on the vertical surface or by the pulling a ball method at using a glass substrate modified by contact coagulation of silica. The contact angle at the boundary between the aqueous phase and the saturated hydrocarbon (octane, decane) reached the values θrec = 53 ± 2 ° and θadv = 116 ± 4 ° (the aqueous phase receding and advancing angle, respectively) with an increase in the initial concentration of long-chain CTAB to (1.4–9.5)·10–2 mmol/g of silica. With a further increase in CTAB concentration, a second reoriented layer was formed, which lowered the contact angle. Accordingly, using CTAB, only oil-in-water emulsions were obtained with the oil phase volume fraction Øoil = 0.5. The amount of short-chain hexylamine required to start stabilizing the emulsions turned out to be 2-3 orders of magnitude higher than the amount of CTAB. At using hexylamine, it was possible to increase the contact angle up to the values θrec ≈ θadv = 163 ± 12 ° at a concentration of 7-21 mmol/g. This is due to the fact that hexylamine is not characterized by formation of micelle or reoriented layers. Apparently, the adsorption of hexylamine is possible not only on dissociated silanol groups Si–OH, but also on siloxane groups Si–O–Si≡, which makes it possible to make the silica surface superhydrophobic. The measured contact angles were correlated with the region of stable oil-in-water and water-in-oil emulsions.