Adsorption of surfactants on kaolinite

Abstract

Jan Magne Garnes Norsk Hydro Research Centre N-5001 Bergen, Norway Abstract: The adsorption isotherms of two anionic surfactants (sodium dodecyl sulfate, SDS and sodium dodecanoate, SDC), one cationic (dodecyl trimethylammonium bromide, DTAB) and two nonionics (nonylphenol polyoxyethylene, NPEOH, and dodecyl polyoxyethylene, DEOH) on kaolinite have been determined at 35°C. The ionic strength was 0.05 M NaC1 for all the isotherms. By selecting surfactants with similar hydrophobic structure, we can compare the effect of the hydrophilic head-group on the adsorption properties. In addition to the static adsorption results, electrophoretic mobility of the kaolinite particles, with and without surfactants adsorbed, was measured as a function of pH by the Laser-Doppler technique. The kaolinite particles have a net negative surface charge at neutral pH. The mobility data combined with XRD, SEM and BET surface area measurements suggest that the particles can regarded as cylindrical disks with a positive charge on the edges, covering 20 % of the total surface area. All the isotherms with exception of DEOH show a maximum adsorption level at CMC. For dodecyl polyoxyethylene the adsorption density increases above the cmc, probably due to reorientation of the adsorbed layer. DTAB show a Langmuirian type of isotherm. However, there is a high positive surface charge of the kaolinite/DTAB from the electrophoretic mobility data, and together with the level of adsorption density, DTAB must exceed monolayer adsorption. The isotherm for the other nonylphenol nonionic is also a Langmuirtype, but deviates from Langmuirian at very low concentrations. Calculation based on the plateau level adsorption show that both DTAB and NPEOH exceed monolayers. The anionic surfactants SDS and SDC both approximates monolayers, while the nonionic, DEOH, has a plateau level adsorption which is below that of monolayer adsorption. The electrophoretic mobilities show that the nonionics preferably adsorb on the negative sites. Comparing the nonionics, the nonylphenols adsorb more strongly than dodecyl polyoxyethylenes. The higher adsorption of the nonylphenol cannot explained by a difference in ethoxylation. SDS adsorb primarily on the positive edge surface of the kaolinite particles. The isotherm of sodium dodecanoate is measured at the natural pH of the carboxylates. Abvoe pH 7.8 the edge surface on the kaolinite will have a net negative charge. The high adsorption density and the electrophoretic mobility of kaolinite/dodecanoate indicate that SDC adsorb on the negatively charged surface or there must a strong ion exchange with hydroxyl ions on the edge surface. The level of adsorption density for anionic, cationic and nonionic surfactants having the same hydrophobic structure can ranked by the adsorption maximum: Cationic > Anionic > Nonionic. The solution concentration of the surfactants at the plateau level are different, and if the adsorption is ranked as a fraction of the solution concentration, the order would be Nonionic > Cationic > Anionic. In addition to the static adsorption, we find that the electrophoretic mobility data improves the possibilities of interpreting the adsorption isotherms.