Electrokinetic study of the adsorption of dodecyl ammonium amine surfactants at the muscovite mica-water interface

The adsorption of dodecyl single chain surfactants with primary, secondary, tertiary, and quaternary ammonium headgroups at the muscovite mica basal plane-water interface has been investigated using a flat plate streaming potential apparatus and atomic force microscopy (AFM). Both a hemi-micelle concentration (HMC) and a point of zeta potential reversal (PZR) were observed as the surfactant adsorbed at the muscovite mica-water interface. The HMC and PZR were shifted to higher surfactant concentration as the headgroup was changed from the primary to the quaternary ammonium group. AFM images of surfactant patches adsorbed at the mica-water interface indicate that the surfactant patch size becomes smaller and distributes more uniformly with change from the primary to quaternary ammonium headgroups. The hydrophobic force contribution of the hydrocarbon chain to the adsorption of the C ₁₂ surfactant, estimated by subtracting other contributions from the total adsorption free energy, was calculated to decrease as the headgroup was changed from the primary to the quaternary amine.