Influence of Structural Variations in Cationic and Anionic Moieties on the Polarity of Ionic Liquids

The polarity of a series of ionic liquids (ILs) arising from the quaternarization of N-methylmorpholine, N-methylpyrrolidine, N-methylpiperidine, N-methylazepane, 4-hydroxy-1-methylpiperidine, 1,2-dimethylimidazole, and 1-methylimidazole with simple alkyl chains and/or hydroxyl (mono- or dihydroxyl) functionalized alkyl chains and having bistriflimide, dicyanamide, or nitrate as counteranions has been investigated using solvatochromic dyes and expressed in terms of E-T(N) and Kamlet-Taft parameters (dipolarity/polarizability (pi*), hydrogen bond donor acidity (alpha), and hydrogen bond basicity (beta)). Significant variations of polarity were observed on changing the anion and cation combination. The resulting E-T(N) and alpha values were strongly anion dependent; on going from bistriflimide to dicyanamide, a significant decrease in E-T(N) and a values was observed. On the other hand, the alkyl chain length has only a moderate effect on these parameters; either an increase or decrease in E-T(N) and a values was observed on increasing the alkyl chain length, depending on the cation core. In the case of cyclic onium salts, the size of the cation ring affected the a parameter; the ILs based on the seven-membered ring system N-methyl-N-butylazepanium (also named N-methyl-N-butylhexamethyleneiminium, [HME1,4](+)) have high polarity values, comparatively to the ILs based on analogous five- and six-membered cyclic cations (pyrrolidinium and piperidinium). The introduction of the OH groups on the cation alkyl chain increases the polarity; the effect is substantial for the first OH group and more moderate for the second. Also, the thermosolvatochromism (changes in solvatochromic properties with the change in temperature) was studied for four dihydroxyl functionalized ILs. Finally, the principal component analysis (PCA) carried out on 67 ILs has shown that there are only two statistically relevant parameters: PC1, a weighted sum of E-T(N) and alpha, which is able to discern between the cation core structure, functionalization, and cation-anion association, and PC2, very close to beta, which is related principally to the anion nature.