The Origin of the LCST on the Liquid-Liquid Equilibrium of Thiophene with Ionic Liquids

Mixtures of thiophene with two ionic liquids, namely, [C(4)C(1)im][SCN] and [C(4)C(1)im][NTf2], were chosen as prototypes of systems presenting lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behavior, respectively. This distinct behavior is due to different interactions between the constituting species which are investigated here by means of experimental and computational studies.. Experimentally, density measurements were conducted to assess the excess molar volumes H-1 and C-13 NMR spectroscopies were used to obtain the corresponding nuclear chemical shifts with respect to those measured for the pure ionic liquids. Computationally, molecular dynamics simulations were performed to analyze the radial distribution neighborhoods of each species. Negative values of excess molar volumes and strong positive chemical shift deviations for [C(4)C(1)im][SCN] systems, along with results obtained from MD simulations, allowed the identification of specific interactions between [SCN](-) anion and the molecular solvent (thiophene), which are not observed for [NTf2](-). It is suggested that these specific [SCN](-)-thiophene interactions are responsible for the LCST behavior observed for mixtures of thiophene with ionic liquids.