Surface Adsorption and Bulk Properties of Surfactants in Quaternary-Ammonium-Salt-Type Amphiphilic Monomeric and Gemini Ionic Liquids

The physicochemical properties of ionic liquids can be readily controlled. Currently, it is necessary to investigate the properties of different surfactants to elucidate the mixtures used in quaternary-ammonium-salt-type ionic liquids. Herein, the surface adsorption and bulk properties of homogeneous polyoxyethylene (EO)-type nonionic surfactant, quaternary-ammonium-salt-type cationic surfactant, and sulfobetaine-type zwitterionic surfactant are elucidated in quaternary-ammonium-salt-type amphiphilic monomeric ionic liquids and gemini ionic liquids with bis(fluorosulfonyl)imide or bis-(trifluoromethanesulfonyl)imide as counterions. The monomeric amphiphilic ionic liquids that adsorbed at the interface were replaced with CxEOy (where x and y represent alkyl and EO chain lengths, respectively) as the concentration of CxEOy increased. On the other hand, in the gemini amphiphilic ionic liquids, the surface tensions of CxEOy, were lower than those of the monomeric ionic liquids. Consequently, both gemini amphiphilic ionic liquids and CxEOy adsorbed efficiently at the interface and oriented themselves because of a synergistic effect. Furthermore, for ionic liquids with short alkyl chains, an orderly bulk nanostructure was not observed at low concentrations in CxEOy, while a layer structure formed at higher concentrations; in contrast, ionic liquids with long alkyl chains formed a layer structure. The alkyl chains, which were interlocked in the bilayer structure, resulted in a densely packed layer structure.