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,P-66.14]CI) with aluminum chloride (AICW is an exception. The viscosity of Al2CI7--dominated [P-6,P-6.6.14]CI-AlCI3 ILs (i.e., AlCI3 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 AlCI4- 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-1.6,P-6,P-6,P-14](+) coupled with Al2CI7-, while neither Cr nor AlCI4- caused bending. In general, [P-6,P-6,P-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 Al2CI7--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.