Adsorption of an aqueous mixture of surfactants on silica

The behavior of mixtures of a nonionic surfactant, pentaethylene glycol mon odecyl ether (C10E5), and an anionic surfactant, sodium dodecyl sulfate (SD S), was studied at the water-silica interface. In contrast to the C10E5 sur factant, which is adsorbed at the silica-water interface at pH similar or e qual to 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, DeltaG(ads), of C10E5 and the free energy of mixed micellization, DeltaG(mix.mic), of SDS and C10E5 Flow microcalori metry (FMC) results showed that the addition of C10E5/SDS mixtures to silic a 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 coefficien ts, and the regular solution theory led to the conclusion that the main dri ving force for the C10E5 desorption was the formation of mixed micelles by unimeric SDS and the released nonionic surfactant.