| Summary: | The amphiphilic ionic liquids containing an alkyl chain in molecules form nano-structure in the bulk, although they also show surface activity and form aggregates in aqueous solutions. Although insights into the layer structures of ionic liquids were obtained using X-ray and neutron scattering techniques, the nanostructures of ionic liquids remain unclear. Herein, the surface adsorption and bulk properties of homogeneous polyoxyethylene (EO)-type nonionic surfactants (C<i><sub>x</sub></i>EO<sub>6</sub>; <i>x</i> = 8, 12, or 16) were elucidated in quaternary-ammonium-salt-type amphiphilic gemini ionic liquids with oxygen or nitrogen-containing spacers [<i>2</i>C<i><sub>n</sub></i>(Spacer) NTf<sub>2</sub>; (Spacer) = (2-O-2), (2-O-2-O-2), (2-N-2), (2/2-N-2), (3), (5), or (6); <i>n</i> = 10, 12, or 14 for (2-O-2) and <i>n</i> = 12 for all other spacers] by surface tension, small- and wide-angle X-ray scattering, cryogenic transmission electron microscopy, and viscosity measurements. The surface tension of C<sub>12</sub>EO<sub>6</sub> in 2C<i><sub>n</sub></i>(Spacer) NTf<sub>2</sub> with oxygen-containing spacers increased with increasing concentration of C<sub>12</sub>EO<sub>6</sub>, becoming close to that of C<sub>12</sub>EO<sub>6</sub> alone, indicating that the amphiphilic ionic liquid adsorbed at the interface was replaced with C<i><sub>x</sub></i>EO<sub>6</sub>. In contrast, both 2C<i><sub>n</sub></i>(Spacer) NTf<sub>2</sub> with nitrogen-containing spacers and nonionic surfactants remained adsorbed at the interface at high concentrations. In the bulk, it was found that 2C<i><sub>n</sub></i>(Spacer) NTf<sub>2</sub> formed layer structures, in which the spacing depended on the alkyl chain length of C<i><sub>x</sub></i>EO<sub>6</sub>. These insights are expected to advance the practical applications of amphiphilic ionic liquids such as ion permeation, drug solubilization, and energy delivery systems.
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