Comparison of the Piezoelectric Response of Zwitterionic Liquids and Their Ionic Liquid Counterparts

We have evaluated the direct piezoelectric response of four zwitterionic liquids (ZwILs): 4-(1-(2-(2-methoxyethoxy)ethyl)-1H-imidazol-3-ium-3-yl)butane-1-sulfonate ((eo)2Im+(CH2)4SO3–), 3-(1-(2-(2-methoxyethoxy)ethyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate ((eo)2Im+(CH2)3SO3–), 3-(1-(2-(2-methoxyethoxy)ethyl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate ((eo)3Im+(CH2)3SO3–), and 3-(1-(2,5,8,11,14,17,20-heptaoxadocosan-22-yl)-1H-imidazol-3-ium-3-yl)propane-1-sulfonate ((eo)7Im+(CH2)3SO3–) in both zwitterionic and ionic forms; the ionic forms were generated by protonating the ZwILs with trifluoromethanesulfonic acid (triflic acid, TA). The data show that the direct piezoelectric responses of the zwitterionic forms are small relative to the response of the reference ionic liquid, 1-butylpyridin-1-ium bis((trifluoromethyl)sulfonyl)imide (C4Py TFSI), and that the ionic (protonated) forms of the ZwILs exhibit a piezoelectric response that depends on the extent of protonation. We relate these results to the structure-dependent dipolar properties of the species and to how these properties influence the pressure-induced crystallization process required to manifest the direct piezoelectric effect in liquids.