Adsorption and Self-Assembly of Cationic Surfactants at Silica Surfaces

The equilibrium self-assembly of cationic surfactants in bulk solution and at negatively-charged silica substrates is investigated experimentally and theoretically. The surfactants used are quaternary ammonium salts, both monovalent (DoTAB, TTAB, CTAB) and divalent (DoPPDAC). The adsorption of DoPPDAC to silica, in the absence or presence of added electrolyte, is studied with the technique of null ellipsometry, providing both the adsorbed amount and film thickness. In order to determine the structure of the bound surfactant aggregates, a thermodynamic model based on the Poisson-Boltzmann cell construction, is developed to quantify the free energy of adsorbed monodisperse spheres. The theory is found to capture the observed trends in the DoPPDAC isotherms, and in particular, the increasing cooperativity with added salt. This agreement indicates that the adsorbed surfactant associates as discrete micelles, rather than a complete bilayer. The micellization is further characterized by the technique of time-resolved fluorescence quenching for determination of aggregation numbers. The use of alkylpyridinium chloride quenchers, with hydrocarbon chain length tailored to the quaternary ammonium surfactants, naturally facilitates the calculation of their equilibrium distribution in the micelles, again using the Poisson-Boltzmann cell model. For all four surfactants, the aggregation numbers are determined in bulk solution, just above the critical micelle concentration, and on adsorption to silica gels.