Adsorption of an Aqueous Mixture of Surfactants on Silica

The behavior of mixtures of a nonionic surfactant, pentaethylene glycol monodecyl ether (C 10 E 5 ), and an anionic surfactant, sodium dodecyl sulfate (SDS), was studied at the water−silica interface. In contrast to the C 10 E 5 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 C 10 E 5 is severely restricted as result of the formation of mixed micelles in solution. This behavior is consistent with the adsorption free energy, Δ G ads, of C 10 E 5 and the free energy of mixed micellization, Δ G mix.mic, of SDS and C 10 E 5 . Flow microcalorimetry (FMC) results showed that the addition of C 10 E 5 /SDS mixtures to silica precoated with C 10 E 5 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 C 10 E 5 desorption was the formation of mixed micelles by unimeric SDS and the released nonionic surfactant.