A zeta-potential model for ionic surfactant adsorption on an ionogenic hydrophobic surface

Abstract

Adsorption of ionic surfactants on a hydrophobic surface (i.e., Nuclepore track-etched polycarbonate membranes, Nuclepore Corp., Pleasanton, CA 94566) is investigated using electrokinetics. Zeta potentials computed from streaming-potential measurements are used in conjunction with an ionogenic surface charge model to characterize, for the first time, the electrical nature of these membranes. The polycarbonate surface exhibits a low, negative surface charge density (ca. a maximum magnitude of 0.2 μC/cm 2) and is ionogenic in nature in that the charge develops by the hydrolysis of discrete carboxylic surface groups with an average p K s of 3.92. Electron spectroscopy for chemical analysis (ESCA) confirms the presence of carboxylic groups on the surface. Streaming-potential measurements are also made for the adsorption of cationic surfactants (alkyltrimethylammonium bromides) and an anionic surfactant (sodium dodecyl sulfate) as a function of solution pH, ionic strength, and surfactant concentration. The ionogenic model for the membrane surface is extended to include Langmuir specific adsorption of the surfactants in the submonolayer region. Agreement between the proposed surfactant adsorption model and the new zeta-potential data is excellent over the range of pH and surfactant concentration studied. The standard free energy of adsorption for dodecyltrimethylammonium bromide is found to be −7.9 RT with a maximum adsorption site density of 4.9 × 10 16 sites/m 2. Likewise, the standard free energy for adsorption for sodium dodecyl sulfate is −8.6 RT with a maximum adsorption density of 1.3 × 10 16 sites/m 2. These results, derived from electrokinetic measurements, are in good accord with accepted values (1–4).