Measurement of the association constants through micro-Raman spectra of supersaturated lithium perchlorate droplets

High quality micro-Raman spectra of the LiClO₄ droplet with mass of nanogram scale were obtained at various concentrations from dilute to supersaturated state. From component band analysis of the v ₁-ClO₄ ^? band, four peaks at 933.3, 936.8, 942.1 and 950.7 cm^?1 were identified and assigned to free solvated perchlorate anion, solvent-shared ion pair, contact ion pair and complex ion aggregates, respectively. As expected, the signature of free solvated ClO₄ ^? ion was observed to decrease in intensity with the increase in concentration. The intensity of the signature from solvent-shared ion pair was observed to rise with increase in concentration from 1.8 mol/kg to 5.0 mol/kg before decreasing as the concentration was further increased to 5.6 mol/kg. Signatures of contact ion pair and of complex ion aggregates were shown to increase as the concentration was enhanced. Based upon the Eigen mechanism, we show that three association equilibria can be used to describe the transformations between free solvated perchlorate anion, solvent-shared ion pair, contact ion pair and complex ion aggregates. The overall association constant, K, and the stepwise association constants K _i (i = 1 to 3) in the Eigen mechanism were determined separately with values of 0.025 ± 0.003, 0.023 ± 0.002, 0.068 ± 0.033 and 0.686 ± 0.174. Based on these constants, the electronic performance can be reasonably predicted by the optimum choice of electrolyte concentrations.