Mesoporous Silica Colloids: Wetting, Surface Diffusion, and Cationic Surfactant Adsorption

We have investigated the wetting and surface diffusion of mesoporous colloidal silica particles at the water surface and the adsorption of cationic cetyltrimethylammonium (CTA(+)) surfactant on these particles. Porous silica colloids diffuse at the surface of water and in the volume, interacting with cationic surfactants that can adsorb inside the pores of the particles. We observed that surfactant adsorption on mesoporous silica depends dramatically not only on the particle pore size but also on specific counterion effects. We measured striking differences both on a macroscopic property of the interface, i.e., surface tension, and also at a single particle level by evaluating the translational diffusion of partially wetted particles at the fluid interface. We varied the pore size from 2 to 7 nm and explored the effects of ions possessing different hydration number and kosmotropic/chaotropic character. At concentrations lower than the critical micellar concentration, we evidence that cationic surfactants adsorb on silica as surface micelles and surfactant adsorption inside the pores occurs only if the pore diameter is larger than the size of surface micelles. With a view to understand the surprising different adsorption behavior of CTA(+)OH(-) and CTA(+)Br(-) on porous silica particles, we investigated the effect of counterions on the surfactant adsorption on porous silica colloids by tuning the pH and the counterion properties.