Solvation and Rotation Dynamics in the Trihexyl(tetradecyl)phosphonium Chloride Ionic Liquid/Methanol Cosolvent System B

The interactions and solvent structure in trihexyl(tetradecyl)phosphonium chloride ionic liquid ([P₁₄,₆,₆,₆⁺][Cl–], “PIL-Cl”)/methanol (MeOH) solutions across the entire range of mole fraction PIL-Cl (xIL = 0–1) are discussed. Viscosity and conductivity measurements are used to characterize the bulk solvent properties. At xIL 0.1 where the data vary linearly with mole fraction. Conductivity data show a maximum at xIL = 0.03 in good agreement with conductivity measurements in imidazolium ILs. Steady-state and time-resolved fluorescence spectroscopies were used to measure the equilibrium, lifetime, and rotational response of coumarin 153 (C153) in neat and MeOH cosolvent modified PIL-Cl. The collective set of data depicts the formation of an increasingly aggregated solvent structure that changes in proportion to the amount of PIL-Cl present in MeOH. Average solvation and rotation times are found to scale with solution viscosity. At xIL values of 0.02–0.2, the rotation times are at or near the hydrodynamic stick limit, whereas for xIL > 0.2 rotation times drop to between 40 and 70% of the stick limit, consistent with the IL literature. In this cosolvent system, the most dramatic changes in solution behavior occur between 0 and 10% PIL-Cl.