Role of chain length and type on the adsorption behavior of cationic surfactants and the silica floatability

Abstract

In this study, we investigated the adsorption isotherms of cationic surfactants (i.e., distearyl dimethyl ammonium chloride (DDAC), behenyl trimethyl ammonium chloride (BTAC), and stearyl trimethyl ammonium chloride (STAC)) onto silica surface and the corresponding silica floatability according to chain type and length of cationic surfactants. The results for the adsorption isotherms of cationic surfactants showed that the adsorptive amounts increased with increasing surfactant concentration, but differences in the position of the adsorption isotherms existed between DDAC, BTAC, and STAC. Specifically, in the high range of initial surfactant concentrations (i.e., 10-510-3M), the adsorptive amounts increased with increasing hydrocarbon chain length. In addition, in the presence of DDAC, the adsorptive amounts were much greater than those in the presence of BTAC and STAC. The results were attributed to the fact that the increase in the hydrocarbon chain group of surfactants decreased the Gibbs free energy of the system, resulting in a shift of hemimicelle concentration toward lower concentration. Meanwhile, in the low range of initial surfactant concentrations (i.e., 5 10-7-510-6M), the adsorptive amounts were comparable regardless of surfactant type, which was due to the fact that the interaction between the cationic surfactants and the silica surface was mainly governed by electrostatic attractive force. The results from silica flotation and hydrophobicity tests showed that silica floatability increased with increasing hydrocarbon chain group of surfactants (i.e., floatability was in the order of DDAC > BTAC > STAC) under the conditions where the adsorptive amounts of the surfactants were comparable and that floatability results were consistent with those for the hydrophobicity of silica surface. Overall, the findings from the present study suggested that the hydrocarbon chain length and type of cationic surfactants play a significant role in both the adsorption behavior of surfactants and the silica floatability.