Adsorption and Aggregation Behavior of Mixtures of Quaternary-Ammonium-Salt-Type Amphiphilic Compounds with Fluorinated Counterions and Surfactants

Abstract

The surface adsorption and aggregation behavior of a mixture of quaternary-ammonium-salt-type amphiphilic monomeric compounds (C₄ FSA, C₈ FSA, and C₄ NTf₂) or gemini compounds (C₁₀-2-C₄ FSA) and various surfactants (nonionic hexaoxyethylene dodecyl ether (C₁₂EO₆), anionic sodium dodecyl sulfate (SDS), cationic dodecyltrimethylammonium bromide (C₁₂TAB), and zwitterionic N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (C₁₂Sb)) was investigated. Both types of compounds contained alkyl chains of nonidentical lengths that used bis(fluorosulfonyl)imide (FSA^–) or bis(trifluoromethanesulfonyl)imide (NTf₂^–) as counterions. The mixtures were analyzed for surface tension, viscosity, electrical conductivity, and pyrene fluorescence, in addition to evaluation by cryogenic transmission electron microscopy, small-angle X-ray scattering, and dynamic light scattering. Our results showed that the surface tension depended on the surfactant structure. For the mixture of C₈ FSA and SDS, as the SDS concentration increased, the surface tension first decreased and became constant at the critical micelle concentration (CMC). In this concentration range, C₈ FSA and SDS were approximately equimolar (2.5 mmol dm^–3), the mixture adsorbed efficiently at the air–water interface, and vesicles and linear-type micelles were formed in the solution owing to the decreased electrostatic repulsion between the hydrophilic groups. As the SDS concentration further increased, the surface tension increased and reached another constant value. The C₈ FSA at the interface was replaced by SDS and the aggregates transformed into spherical micelles. The surface tension plot of the mixture of the amphiphilic compounds and C₁₂Sb showed a minimum at the CMC. The lowest CMC and surface tension were observed for C₁₀-2-C₄ FSA, indicating that the gemini compounds offer excellent adsorption and orientation at the air–water interface. It was revealed that the quaternary-ammonium-salt-type amphiphilic compounds in this study acted as ionic liquids on their own and as surfactants in aqueous solution. Further, they could improve the surface activity of conventional ionic surfactants.