Adsorption of an aqueous mixture of surfactants on silica

The behavior of mixtures of a nonionic surfactant, pentaethylene glycol monodecyl ether (C10E5), and an anionic surfactant, sodium dodecyl sulfate (SDS), was studied at the water−silica interface. In contrast to the C10E5 surfactant, which is adsorbed at the silica−water interface at pH 6, the adsorption of the anionic surfactant is not observed. In the presence of SDS, the adsorption of C10E5 is severely restricted as result of the formation of mixed micelles in solution. This behavior is consistent with the adsorption free energy, ΔGads, of C10E5 and the free energy of mixed micellization, ΔGmix.mic, of SDS and C10E5. Flow microcalorimetry (FMC) results showed that the addition of C10E5/SDS mixtures to silica precoated with C10E5 resulted in the release of the preadsorbed nonionic surfactant; the whole phenomenon is exothermic. Combination of calorimetric data (FMC and isothermal titration calorimetry), self-diffusion coefficients, and the regular solution theory led to the conclusion that the main driving force for the C10E5 desorption was the formation of mixed micelles by unimeric SDS and the released nonionic surfactant.