Alkylether derivatives of choline as cationic surfactants for the design of soluble catanionic systems at ambient conditions

Abstract

Mixtures of cationic and anionic surfactants are promising systems with great potential for various purposes in surface and interfacial science. However, so far their applicability has been severely limited by poor solubility at and around equimolar ratios, where too strong interactions typically cause precipitation. In the present study, we hypothesized that the insertion of ethylene oxide units between hydrophobic alkyl chains and a positively charged choline headgroup should give cationic surfactants with high intrinsic flexibility, hindering phase separation and thus allowing soluble catanionic mixtures to be formed at room temperature with conventional sulfate-based anionic surfactants. To that end, different alkylether derivatives of choline were synthesized and combined with alkyl or alkylether sulfates at varying ratios in water. The resulting mixtures of cationic and anionic surfactants were characterized in detail with respect to solubility, static and dynamic activity at surfaces and liquid/oil interfaces, adsorption on solid substrates, and cytotoxicity. In further application-oriented tests, the potential of the developed catanionic surfactant systems for wetting of hydrophobic surfaces, stabilization of liquid foams, and stain removal in lab-scale washing experiments was assessed. Our results demonstrate that the synthesized cationic surfactants can be combined with conventional anionic surfactants to give water-soluble systems across the entire range of mixing ratios, including equimolarity. This remarkable enhancement of solubility is found to be accompanied by pronounced synergistic effects in terms of interfacial activity, evidenced among others by low surface tensions, strong adsorption on solid interfaces as well as fast and complete spreading on hydrophobic substrates. These promising, and readily tunable, physicochemical properties are also reflected in improved foaming and cleaning performance, highlighting the potential of soluble catanionic surfactant formulations for a broad range of industrially relevant applications.