Abstract
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 (CxEO6; x = 8, 12, or 16) were elucidated in quaternary-ammonium-salt-type amphiphilic gemini ionic liquids with oxygen or nitrogen-containing spacers [2Cn(Spacer) NTf2; (Spacer) = (2-O-2), (2-O-2-O-2), (2-N-2), (2/2-N-2), (3), (5), or (6); n = 10, 12, or 14 for (2-O-2) and n = 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 C12EO6 in 2Cn(Spacer) NTf2 with oxygen-containing spacers increased with increasing concentration of C12EO6, becoming close to that of C12EO6 alone, indicating that the amphiphilic ionic liquid adsorbed at the interface was replaced with CxEO6. In contrast, both 2Cn(Spacer) NTf2 with nitrogen-containing spacers and nonionic surfactants remained adsorbed at the interface at high concentrations. In the bulk, it was found that 2Cn(Spacer) NTf2 formed layer structures, in which the spacing depended on the alkyl chain length of CxEO6. These insights are expected to advance the practical applications of amphiphilic ionic liquids such as ion permeation, drug solubilization, and energy delivery systems.
Highlights
Ionic liquids are salts consisting of only cations and anions that exhibit melting points below 100 ◦ C.They have been the subject of extensive research because of their advantageous properties, such as low vapor pressure, high thermal stability, high conductivity, and unique solubility, and have attracted interest for use as novel and environment-friendly solvents [1,2,3,4,5]
We found that introducing different spacers containing oxygen or nitrogen lowered the melting point of quaternary-ammonium-salt-type amphiphilic gemini ionic liquids to below room temperature, and that the ionic liquids with nitrogen-containing spacers adsorbed efficiently at the air/water interface [19]
The addition of the nonionic surfactants to the ionic liquids increased the surface tension, and the observed behavior differed depending on the spacer structure
Summary
Ionic liquids are salts consisting of only cations and anions that exhibit melting points below 100 ◦ C. They have been the subject of extensive research because of their advantageous properties, such as low vapor pressure, high thermal stability, high conductivity, and unique solubility, and have attracted interest for use as novel and environment-friendly solvents [1,2,3,4,5]. The ionic liquids containing a long alkyl chain in molecules show surface activities such as adsorption and aggregation, which are similar to classical surfactants. These ionic liquids are called amphiphilic ionic liquids [6].
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