Annealing, solvation, and mirror-plating effects in phosphonium chloroaluminate ionic liquids

The physicochemical properties of a specific ionic liquid (IL) are naturally considered to be constant at a given temperature. However, a series of eutectic melts of trihexyl(tetradecyl)phosphonium chloride ([P_6,6,6,14]Cl) with aluminum chloride (AlCl₃) is an exception. The viscosity of Al₂Cl₇⁻-dominated [P_6,6,6,14]Cl–AlCl₃ ILs (i.e., AlCl₃ mole fraction x = 0.60–0.67) gradually increased as a function of annealing. Annealing also influenced the ultraviolet–visible spectra, nuclear magnetic resonance spectra, and the glass transition temperature of the ILs. Such annealing-induced variations in physicochemical properties were not observed in the AlCl₄⁻- and Cl⁻-dominated ILs. In particular, the ionic conductivities of the ILs (x = 0.60–0.67) were strongly decoupled from their viscosities during annealing. Ab initio calculations revealed the bending of the long tetradecyl chains in [P_6,6,6,14]⁺ coupled with Al₂Cl₇⁻, while neither Cl⁻ nor AlCl₄⁻ caused bending. In general, [P_6,6,6,14]⁺-based ILs are recognized as sponge-like structures that consist of nano-scale polar and non-polar domains. We propose a gradual structurization process in Al₂Cl₇⁻-dominated ILs to account for their unusual physicochemical properties. Moreover, the addition of tetradecane, solvated in the non-polar domains of the ILs, substantially reduced the viscosity of the structured IL with x = 0.67. Mirror Al electroplating is possible without a brightener, using the x = 0.67 ILs, regardless of annealing or the admixture of tetradecane.