| Abstract
| - The formation of contact ion pairs in Mg(NO3)2 solutions and their effects on the hygroscopic properties ofthe solutions were studied using Raman spectroscopy of Mg(NO3)2 droplets levitated in an electrodynamicbalance. Upon reduction in the ambient relative humidity (RH), Mg(NO3)2 droplets lose water but do noteffloresce. The molar water-to-solute ratio (WSR) decreases exponentially with decreasing RH, but it decreaseslinearly with RH when it is less than 6. This transition of hygroscopicity at WSR = 6 coincides with anabrupt blue shift of the ν1 band of NO3- from 1049 to 1055 cm-1 in the Raman spectra, which is due tonitrate ions entering into the primary solvation shell of Mg2+ ions to form direct contact ion pairs. As theWSR further decreases, a shoulder on the higher wavenumber side of the ν1 band appears near 1060 cm-1and the separation of ν3 bands increases because of the formation of more complex contact ion pairs. Ramanimages of supersaturated droplets at WSR = 2.8 show structural heterogeneity not found in diluted droplets.Overall, the NO3- to Mg2+ ratio increases and the H2O to Mg2+ ratio decreases as the surface of the dropletis approached. However, droplets at low concentration (WSR = 9.6) show a uniform distribution ofmonodentates.
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