Adsorption of benzyldimethyldodecylammonium bromide on silica nanoparticles in water

Adsorption of the cationic surfactant benzyldimethyldodecylammonium bromide (BDDABr) on silica nanoparticles with ~ 12 and 31 nm in size (denoted as S-SiO2 and L-SiO2, respectively) is investigated at various solid dosages (Cs, 10–40 g/L), pH (3–10), and temperature (T, 298–308 K). No Cs-effect is observed in the adsorption. However, it is interestingly found that, besides pH and T, the size of the silica particles has an obvious influence on the adsorption. The adsorption may show the Langmuir type (L-type), S-type, and “double plateau” type (LS-type) isotherms, depending on silica particle sizes and pH. Increasing pH may lead to a change in the isotherm types from S-type through LS-type to L-type. The S-type and LS-type isotherms can be adequately described using the one-step and two-step surface micellization models, respectively. The affinity of the S-SiO2 toward BDDABr is lower than that of the L-SiO2, consistent with the dissociation tendency of their surface hydroxyl groups.