Physicochemical and solution properties of quaternary-ammonium-salt-type amphiphilic gemini ionic liquids with spacers containing oxygen or nitrogen

Quaternary-ammonium-salt-type amphiphilic gemini compounds 2C(n)(Spacer) NTf2 were synthesized by the quaternization of a tertiary diamine and an n-alkyl bromide, followed by an ion-exchange reaction with potassium bis(trifluoromethanesulfonyl)imide [(Spacer) = (2-O-2), (2-O-2-O-2), (2-N-2), (2/2-N-2), (2/2)-OH, (5), (6), and n represents the alkyl chain lengths; n = 10, 12, 14 for the 2-O-2 spacer, and n = 12 for all others]. The compounds with melting points below 100 degrees C were treated as ionic liquids during the subsequent measurements. The melting points of the amphiphilic gemini compounds could be arranged in the following order, based on the spacer present: (2/2)-OH > 2/2-N-2 > (2-N-2) > (2-O-2) > (5) > (6) > (2-O-2-O-2). The melting points for 2C(n)(2-O-2) NTf2 (n = 10, 12), 2C(12)(2-O-2-O-2) NTf2, and 2C(12)(2-N-2) NTf2 could be reduced to < 0 degrees C. The amphiphilic gemini ionic liquids exhibited significantly lower conductivities and higher viscosities than the corresponding monomeric ionic liquids. This was attributed to a decrease in the mobility of the cationic molecules caused by the gemini structure, in which the two monomeric compounds are connected by a spacer. Furthermore, the amphiphilic gemini ionic liquids, particularly those containing oxygen in the spacer, exhibited efficient adsorption at the air/water interface.