Do anions influence the polarity of protic ionic liquids?

Abstract

Polarity studies in two classes of imidazolium-based protic ionic liquids (PILs) possessing [HSO₄]⁻, [HCOO]⁻, [CH₃COO]⁻ and [CH₃CH₂COO]⁻ anions were carried out using a solvatochromic method from 298.15 to 353.15 K. For 1-methylimidazolium class of PILs, E_T(30) was found to be independent over the entire range of temperature, while E_T(30) was noted to decrease with a rise in temperature in the case of 1-butylimidazolium class of PILs containing [CH₃COO]⁻ and [CH₃CH₂COO]⁻ anions. The E_T(30) value decreases in both the classes upon varying the anions ([HSO₄]⁻, [HCOO]⁻, [CH₃COO]⁻ and [CH₃CH₂COO]⁻). The E_T(30) value is controlled by hydrogen bond acceptor basicity, β, and dipolarity/polarizability, π*. The E_T(30) value for PILs varies inversely to the strength of the coulombic interaction between ions in PILs. Strong interactions between ions lead to lower E_T(30) values. Unlike the poor thermal effect on E_T(30), the Kamlet–Taft parameters i.e. α, β and π* have pronounced thermal effect in the imidazolium-based PILs. Variation in the Kamlet–Taft parameters is controlled by the stabilization of ions and the degree of proton transfer from Brønsted acid to Brønsted base.