Surface adsorption and bulk properties of polyoxyethylene–polyoxypropylene random copolymer-type double-chained surfactants in quaternary-ammonium-salt-type amphiphilic gemini ionic liquids

Abstract

Recently, a few studies have been reported on the layer structure of ionic liquids using X-ray and neutron scattering. However, the nanostructural features of ionic liquids remain unclear. Herein, the surface adsorption and bulk properties of polyoxyethylene (EO)–polyoxypropylene (PO) decyl tetradecyl ether-type nonionic surfactants (C10-C12POxranEO24PO13−x, where x represents the length of the PO chain linked to alkyl chains; x = 0, 5, 8, and 13) in quaternary-ammonium-salt-type amphiphilic gemini ionic liquids containing oxygen or nitrogen in the spacer (2C12(Spacer) NTf2, where (Spacer) = (2-O-2), (2-O-2-O-2), (2-N-2), (2/2-N-2), and n represents the alkyl chain length; n = 10, 12, and 14 for the 2-O-2 spacer, and n = 12 for all others) are elucidated by surface tension, small- and wide-angle X-ray scattering, and viscosity measurements. The surface tension of the C10-C12POxranEO24PO13−x surfactants in the gemini ionic liquids containing oxygen in the spacer increased as the surfactant concentration increased, and it became comparable to that of C10-C12POxranEO24PO13−x alone, indicating that the gemini ionic liquid adsorbed at the air–liquid interface was replaced with C10-C12POxranEO24PO13−x following its addition. In the bulk, 2Cn(Spacer) NTf2 and C10-C12POxranEO24PO13−x formed a layered structure, and the layer spacing depended on the alkyl chain length and spacer structure of the gemini ionic liquid and the length of the PO chain linked to the alkyl chains of the surfactants.