Mixtures of anionic and cationic surfactants with single and twin head groups: Adsorption and precipitation studies

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 hexadecyl‐diphenyloxide disulfonate (SHDPDS), and the single‐head cationic surfactant dodecylpyridinium chloride (DPCI)—adsorption was studied on positively charged alumina. Whereas the mixed surfactant system of SHDPDS/DPCI showed adsorption on alumina that was comparable to the 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 double‐head 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.