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

• Published in: The journal of physical chemistry. B Vol. 118; no. 45; pp. 12979 - 12992
• Main Authors: Barra, Kathleen M, Sabatini, Randy P, McAtee, Zachery P, Heitz, Mark P
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13.11.2014
• Subjects: Chlorides; Dynamical systems; Dynamics; Ionic liquids; Methyl alcohol; Moles; Solvation; Solvents
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp5092784

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 (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.