Adsorption and dynamics of sodium alkylbenzenesulfonates on alumina

Deuterium nuclear magnetic resonance (2H NMR) spectroscopy was used to probe the molecular motion of sodium alkylbenzenesulfonates on γ-alumina in aqueous solutions. Two homologs with a single and a double chain were employed to compare the adsorption characteristics at various coverages. Sodium decylbenzenesulfonate-d4 (SDBS-d4) was chosen as a single-tailed surfactant. Motionally narrowed (single) resonances were observed from this single-chained homolog on the surface and no discrete quadrupolar splitting was found in any of the concentration ranges investigated. The spectra indicated relatively restricted molecular motion at low coverages, but relatively free motion as the adsorption amounts increased. A double-tailed homolog, sodium 4-(1′-heptylnonyl)benzenesulfonate-d4 (SHBS-d4), also showed a single resonance at low coverage on alumina. However, at full coverage, a discrete powder pattern overlapped a single resonance, indicating the formation of a liquid crystal-like molecular environment at the alumina/water interface. In this report, we summarize our recent investigations of the dynamics of the surface-adsorbed surfactants and discuss the differences in the adsorption behavior of the single- and double-chained homologs.