Effect of substrate hydrophobicity on surface-aggregate geometry: Zwitterionic and nonionic surfactants

The zwitterionic surfactant, dodecyldimethylammoniopropanesulfonate (DDAPS), and the nonionic surfactant, penta(oxyethylene) n-decyl ether (C10E5), both form aggregates (hemimicelles) at the interface between aqueous solution and several covalent solids. DDAPS forms spherical micelles on (hydrophilic) silicon nitride, spherical micelles on (hydrophilic) mica, and hemicylindrical micelles on (hydrophobic) graphite. C10E5 forms globular micelles on silicon nitride and flat sheets on graphite. Thus, both surfactants form lower curvature aggregates on a more hydrophobic substrate. This behavior is interpreted in terms of a lower free energy for the hydrophobic-substrate system when there is minimal contact between water and the hydrophobic substrate. Since the surfactant headgroups have a higher affinity for the hydrophilic substrates, there is also a correlation between the surfactant curvature and the affinity of the headgroup for the solid substrate. The aggregate morphology is not a function of surfactant concentration above the critical micelle concentration, but the repeat distance of the aggregate unit is a weak function of concentration. This behavior is interpreted in terms of the distance dependence of the force between the adsorbed aggregates. The effect of substrate hydophobicity has been investigated for net-uncharged surfactants in order to minimize the effect of the strong monopolar charge-charge interactions that occur for anionic or cationic surfactants.