Streaming potential studies on the adsorption of amphoteric alkyldimethylamine and alkyldimethylphosphine oxides on mesoporous silica from aqueous solution

We have used streaming potential measurements to determine the zeta-potential of amorphous, hydrophilic, mesoporous silica gel in aqueous nonionic-cationic solutions of the amphoteric alkyldimethylamine oxide surfactants (CnDAO with n = 8, 10, and 12) and alkyldimethylphosphine oxide surfactants (CnDPO with n = 10 and 12). The effect of different mixtures of the nonionic and cationic (protonated) forms of the amine oxide surfactant on the zeta-potential was studied at natural pH and at pH = 4. The nonionic phosphine oxide homologues were chosen as references. The surfactant contents ranged from dilute solutions to well above the cmc. The zeta-potential isotherms of the alkyldimethylamine oxides featured break points at the critical surface aggregation concentration (CSAC), recognized as the onset of counterion association with formed surface aggregates. The surface aggregates of all alkyldimethylamine oxide surfactants studied are charged at the cmc. At natural pH evidence of zeta-potential reversal was found as a result of the onset of formation of C12DAO hemimicelles induced by the surface with the adsorbed surfactant species. At pH = 4 the fractional counterion binding to surface aggregates, composed of almost only cationic-nonionic C12DAO dimers, varies exponentially with the surfactant content in solution, starting from zero at about the CSAC and reaching a plateau at the cmc. As expected, the adsorption of alkyldimethylphosphine oxides studied did not change the zeta-potential of silica. Nonionic C12DAO forms probably ellipsoidal aggregates; meanwhile cationic C10DAO and C12DAO are likely to form spherical surface micelles at the aqueous solution/SiO2 interface. The adsorption mechanism of nonionic C12DAO is different from that of cationic C12DAO, and consequently depends on the degree of protonation of the surfactant.