The interactions and solvent structure in trihexyl(tetradecyl)phosphonium chloride ionic liquid ([P(14,6,6,6)(+)][Cl(-)], "PIL-Cl")/methanol (MeOH) solutions across the entire range of mole fraction PIL-Cl (x(IL) = 0-1) are discussed. Viscosity and conductivity measurements are used to characterize the bulk solvent properties. At x(IL) < 0.1, the log(η) data show a nonlinear dependence on mole fraction in contrast to the data for x(IL) > 0.1 where the data vary linearly with mole fraction. Conductivity data show a maximum at x(IL) = 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 x(IL) values of 0.02-0.2, the rotation times are at or near the hydrodynamic stick limit, whereas for x(IL) > 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.

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