Effects of catanionic surfactant mixture adsorption on the wettability of PTFE and PMMA

Abstract

Surfactants can regulate the wettability of a liquid on a solid substrate as they adsorb to solid-liquid and gas-liquid interfaces. In the present work, a catanionic surfactant mixture, sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (C12TAB) mixed at a molar ratio of 2:1, showed outstanding synergistic effect in improving the wettability of low energy substrates (PTFE and PMMA). The adsorption behaviors of the catanionic surfactant mixture on PTFE and PMMA substrates were studied. The surface tensions and contact angles of aqueous solutions of SDS, C12TAB, and their mixture on PTFE and PMMA were measured, and the adhesion tension and adhesion work were calculated. Compared to individual SDS and C12TAB, the SDS-C12TAB mixture formed a compactly arranged layer at the PTFE-aqueous interface, thereby leading to a more significant hydrophilic modification of PTFE. In the PMMA system, the SDS-C12TAB mixture adsorbed to the PMMA-aqueous interface through acid-base interactions between the positively charged surfactant headgroups and the basic groups at PMMA surface and increased the hydrophobicity of the PMMA substrate at low surfactant concentrations. When the surfactant concentration exceeded the critical micelle concentration (CMC), the surfactant started to adsorb to the PMMA-aqueous interface through hydrophobic interactions between the hydrophobic surfactant tails and the nonpolar sites at PMMA surface, which further improved the wettability of PMMA. Overall, the wettability of low energy substrates can be significantly improved under the strong synergistic effect of commercial anionic and cationic surfactants without need of complex synthesis of new amphiphiles. Therefore, the catanionic surfactant mixtures may be a very promising system for regulating wettability.