Do anions influence the polarity of protic ionic liquids?

Article abstract of DOI:10.1039/c2cp23256a

Polarity studies in two classes of imidazolium-based protic ionic liquids (PILs) possessing [HSO₄]^-, [HCOO]^-, [CH ₃COO]^- 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 [CH ₃CH₂COO]^- anions. The E_T(30) value decreases in both the classes upon varying the anions ([HSO₄] ^-, [HCOO]^-, [CH₃COO]^- and [CH ₃CH₂COO]^-). 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 Bronsted acid to Bronsted base. the Owner Societies 2012.