Relating Hygroscopic Properties of Magnesium Nitrate to the Formation of Contact Ion Pairs

The formation of contact ion pairs in Mg(NO₃)₂ solutions and their effects on the hygroscopic properties of the solutions were studied using Raman spectroscopy of Mg(NO₃)₂ droplets levitated in an electrodynamic balance. Upon reduction in the ambient relative humidity (RH), Mg(NO₃)₂ droplets lose water but do not effloresce. The molar water-to-solute ratio (WSR) decreases exponentially with decreasing RH, but it decreases linearly with RH when it is less than 6. This transition of hygroscopicity at WSR = 6 coincides with an abrupt blue shift of the v₁ band of NO₃^- from 1049 to 1055 cm ^-1 in the Raman spectra, which is due to nitrate ions entering into the primary solvation shell of Mg²⁺ ions to form direct contact ion pairs. As the WSR further decreases, a shoulder on the higher wavenumber side of the v₁ band appears near 1060 cm^-1 and the separation of v₁ bands increases because of the formation of more complex contact ion pairs. Raman images 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 droplet is approached. However, droplets at low concentration (WSR = 9.6) show a uniform distribution of monodentates.