Adsorption of cationic surfactants on silica. Surface charge effects

The surface charge of Aerosil OX50 in an aqueous dispersion has been studied in the presence of cationic surfactants (alkylpyridinium chlorides and trimethylammonium bromide) as a function of pH and salt concentration. The measurements were complemented with surfactant isotherms. To investigate the effect of the aliphatic surfactant tail, the surface charge-pH curves have also been measured in the presence of tetramethylammonium bromide (TMAB). At low salt concentration the surfactant isotherms show a two-step behavior. In the first step the surface charge follows the surfactant adsorption, indicating that head-on adsorption occurs. In the second step the surface charge increases only slightly. Comparison of the results for the surfactants and TMAB shows that the surface charge adaptation is strongly affected by the hydrocarbon tail and that the kind of head group is of secondary importance. Up to the first pseudoplateau, the molecules adsorb with their head groups and tails on the surface. For the second part of the isotherm, surfactant self assembly occurs. At high salt concentration the initial surface charge is relatively large and the surface charge adjustment is small. Due to the relatively large surface charge the surface is less hydrophobic and hydrophobic interactions with the surface become less probable. Instead hydrophobic tail-tail interactions become important also in the lower part of the isotherm. Surfactant self assembly occurs both before and after the surface charge is compensated by the surfactant charge.