Mixtures of anionic and cationic surfactants with single and twin head groups: solubilization and adsolubilization of styrene and ethylcyclohexane

ABSTRACT: This research reports on the adsorption and precipitation of mixtures of anionic and cationic surfactants having single and twin head groups. The surfactant mixtures investigated were: (i) a single-head anionic surfactant, sodium dodecyl sulfate (SDS), in a mixture with the twin-head cationic surfactant pentamethyl-octadecyl-1,3-propane diammonium dichloride (PODD)-adsorption was studied on negatively charged silica; and (ii) a twin-head anionic surfactant, sodium hexadecyldiphenyloxide disulfonate (SHDPDS), and the single-head cationic surfactant dodecylpyridinium chloride (DPCl)-adsorption was studied on positively charged alumina. Whereas the mixed surfactant system of SHDPDS/DPCl showed adsorption on alumina that was comparable to that of SHDPDS alone, the mixed surfactant system of SDS/PODD showed increased adsorption on silica as compared with PODD alone. The adsorption of the SDS/PODD mixture increased as the anionic and cationic system approached an equimolar ratio. Precipitation diagrams for mixtures of single-and twin-head surfactant systems showed smaller precipitation areas than for single-head-only surfactant mixtures. Thus, the combination of single-and doublehead surfactants helps reduce the precipitation region and can increase the adsorption levels, although the magnitude of the effect is a function of the specific surfactants used. Paper no. S1508 in JSD 9, 21-28 (Qtr. 1, 2006). KEY WORDS: Alumina, anionic surfactant, cationic surfactant, mixed surfactant, precipitation, silica. Mixtures of anionic and cationic surfactants act synergistically, as evidenced by ultralow critical micelle concentrations (CMC), increased surface activity (1,2) and improved detergency performance (3). The main disadvantage of mixed anionic and cationic surfactant systems is their tendency to form precipitates or liquid crystal phases (4). Precipitation negatively affects surfactant use in many applications, such as detergency performance and subsurface remediation of oil contamination (4,5). The main goals of this work are to evaluate synergism of surfactant adsorption onto solid surfaces by using anionic and cationic surfactant mixtures, and to determine how properties of these adsorbed mixtures affect the co-adsorption or adsolubilization of different types of solutes. While of secondary interest, we also evaluate the precipitation of anionic/cationic surfactant mixtures to define isotropic concentration regimes in which to conduct the adsorption studies. We hypothesize that by using mixtures of anionic and cationic surfactants we will observe a synergistic adsorptive behavior as evidenced by having higher surfactant adsorption at sub-CMC surfactant concentrations and by reaching the adsorption plateau (Region IV) at lower surfactant concentrations compared with single surfactant systems; this hypothesis is based on the lower CMC observed for mixtures of anionic and cationic surfactants compared with mixtures of similarly structured surfactants. We also hypothesize that an increased level of plateau surfactant adsorption will result because of the tighter packing density in adsorbed aggregates of these mixed surfactants owing to a reduction in charge repulsion between adjacent adsorbed surfactants compared with single surfactant systems