Micro-Raman analysis of association equilibria in supersaturated NaClO4 droplets

We report an in situ strategy for the quantitative analysis of association equilibria in a single NaClO(4) droplet of nanogram mass deposited on a quartz substrate. In the new approach, the single droplet was forced to enter into a supersaturated state by decreasing the relative humidity (RH) of the environment, allowing accurate control over the concentration of the solute within the droplet. An analysis of the solvated structure of the ClO(4)(-) anion with change in molar water-to-solute ratio (WSR) was performed by micro-Raman spectroscopy within the confines of a single droplet. The symmetric stretching v(1)-ClO(4)(-) band was observed to shift from a Stokes frequency of 935 to 944 cm(-1) and was accompanied by a change in the full width at half-maximum (FWHM) from 11.4 to 16.6 cm(-1) as the WSR decreased from 16.1 to 2.3. From component band analysis of the spectral range of 900-970 cm(-1), four peaks at 933.3, 938.6, 944.1 and 946.0 cm(-1) were identified and assigned to the free solvated perchlorate anion, the solvent-shared ion pair, the contact ion pair and complex ion aggregates, respectively. As expected, the signature of the free solvated ClO(4)(-) ion was observed to decrease in intensity with a decrease in RH over the full range from 94 to 27%. The intensity of the signature from solvent-shared ion pairs was observed to rise with decrease in RH from 94% to 75% before decreasing as the RH was further reduced to 27%. Signatures of the contact ion pair and of complex ion aggregates were shown to increase over the full range of RH as the RH was reduced. Based upon the Eigen mechanism, three association equilibria were used to describe the transformations between the free solvated perchlorate anion, the solvent-shared ion pair, the contact ion pair and complex ion aggregates. The overall association constant K and the stepwise association constants K(i) (i = 1 to 3) were determined separately (0.27 +/- 0.01, 0.03 +/- 0.01, 5.49 +/- 0.95, 0.76 +/- 0.06).