Observations of water monomers in supersaturated NaClO4, LiClO4, and Mg(ClO4)2 droplets using Raman spectroscopy

Salts of the same anions of Mg2+, Na+, and Li + have been found to exhibit different hygroscopic properties. These differences are attributed to the molecular structural properties of the hydrogen bonding network of the water molecules in the second and first hydrated layers of Mg2+, Na+, and Li+. To study the structures of water molecules, in particular, the presence of water monomers, Raman spectra of single levitated droplets of aqueous NaClO 4, LiClO4, and Mg(ClO4)2 solutions from diluted concentrations to high supersaturations were measured. Because heterogeneous nucleation was suppressed in these levitated droplets, supersaturated droplets with a water-to-solute ratio (WSR) as low as 2 was achieved. Taking advantage of the structure breaking effect of ClO 4- on the hydrogen bonding network of water molecules, Raman spectra of water monomers in these highly supersaturated droplets were observed. At a low WSR, two peaks at 3549 and 3588 cm-1 for water monomers with one and two weak hydrogen bonds with ClO4- were observed for NaClO4 droplets. Compared with those of the NaClO4 droplets, the peaks of the water monomers in supersaturated LiClO4 and Mg(ClO4)2 droplets red-shifted to 3553 and 3546 cm-1, respectively. This observation is consistent with the order of the increase of the polarization effect of the cations, i.e., Na+ < Li+ < Mg2+. The intensity ratios of the strongly hydrogen-bonded components to the water monomers, i.e., I 3440/I3549 and I3289/I3549 and I3289/I3549 for NaClO4I3455/ I3553 and I3275/I3553 for LiClO4, and I3411/I3546 and I3440/I3549 for Mg(ClO4)2, were used to study the effects of the presence of ClO4- on the water structures of the hydration layers of Na+, Li+, and Mg2+. The results were explained in terms of the stability of water molecules in the inner spheres of these hydrated cations.