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.