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

Abstract

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 (π*), hydrogen bond donor acidity (α), and hydrogen bond basicity (β)). Significant variations of polarity were observed on changing the anion and cation combination. The resulting E(T)(N) and α values were strongly anion dependent; on going from bistriflimide to dicyanamide, a significant decrease in E(T)(N) and α 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 α 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 α parameter; the ILs based on the seven-membered ring system N-methyl-N-butylazepanium (also named N-methyl-N-butylhexamethyleneiminium, [HME(1,4)](+)) 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 α, which is able to discern between the cation core structure, functionalization, and cation-anion association, and PC2, very close to β, which is related principally to the anion nature.

Pyrazolium- versus Imidazolium-Based Ionic Liquids: Structure, Dynamics and Physicochemical Properties

“…33 As previously observed for (5) is eliminated and the interaction with H(7), H(8) and H(9) is reduced, probably by steric effects.…”

Polarity of Some [NR1R2R3R4]+[Tf2N]− Ionic Liquids in Ethanol: Preferential Solvation versus Solvent–Solvent Interactions

“…Both IL cations account primarily for the differences found in the acidity parameter plots as the anion is the same in both IL; even more, it is generally accepted that the ILs acidity is predominantly attributed to cations. 32,33,40,41 It is perceptible that, in the range of greatest IL concentration, the addition of EtOH molecules to [N 4111 N] structure by EtOH molecules seems to be easier to occur. Simultaneously, there is the possibility of the occurrence of IL-EtOH interactions leading to an entity S12 with higher polarity expressed by the estimated parameter ν̃1 2 = 22.74 kK relative to those estimated for the pure solvents ν̃1 = 18.05 kK and ν̃1 = 19.64 kK (using RB as a probe molecule, Table 2).…”

Overview of the Effect of Salts on Biphasic Ionic Liquid/Water Solvent Extraction Systems: Anion Exchange, Mutual Solubility, and Thermomorphic Properties

“…69 The inherent solubility of the IL in the aqueous phase can vary a lot depending on the IL structure and the polarity of its anions and cations. 87 For the IL anions (Table 3), the hydrophobicity of the ILs follows the charge density series (Figure 1). Anions can alter the hydrophobicity of the IL very much: [P 8444 ][Cl] is watersoluble, while [P 8444 ][Br] forms a biphasic system with only around 2 wt % of IL in the aqueous phase.…”