文摘
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 ETN and Kamlet鈥揟aft parameters (dipolarity/polarizability (蟺*), hydrogen bond donor acidity (伪), and hydrogen bond basicity (尾)). Significant variations of polarity were observed on changing the anion and cation combination. The resulting ETN and 伪 values were strongly anion dependent; on going from bistriflimide to dicyanamide, a significant decrease in ETN and 伪 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 ETN and 伪 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 伪 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 ETN and 伪, which is able to discern between the cation core structure, functionalization, and cation鈥揳nion association, and PC2, very close to 尾, which is related principally to the anion nature.