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

Significant retardation of the evaporation rate of levitated aqueous MgSO4 droplets has been found at high concentrations using an electrodynamic balance. Raman spectroscopy was used to study the structural changes, in particular, the formation of contact ion pairs, in supersaturated aqueous MgSO4 droplets at ambient temperatures. As the relative humidity (RH) decreases, single levitated droplets lose water and become supersaturated. A molar water-to-solute ratio as low as 1.54 was obtained, facilitating the study of contaction pairs of unhydrated Mg2+ and SO4 2- ions in MgSO4 solutions. The characteristics of the

The hygroscopic properties of sulfate-containing particles are important to understanding the behavior of atmospheric aerosols. At high concentrations, chemical interactions between sulfate ions with the countercations are significant and lead to the formation of contact pairs. In this paper, Raman spectroscopy was used to study the structural changes of single aqueous droplets of equal molar Na2SO4/MgSO4 mixture, Na2SO4, (NH4)2SO4, MgSO4, ZnSO4, and CdSO4 in relation to their hygroscopic properties in an electrodynamic balance. The molar water-to-solute ratio (WSR) and the Raman spectra of droplets equilibrated at different ambient relative humidities were measured. When RH is reduced, the WSR of the Na2SO4/MgSO4 droplet decreases from 25.3 to 4.2 without crystallization but with two distinct transition points. The first one appears at WSR) 18.9, where the WSR is more sensitive to RH and the shoulder at 995 cm-1 of the Ó1-SO4 2-band of the Raman spectrum shows an abrupt change in the ratio of the intensity at 995cm-1 to that at 984cm-1. This WSR is close to the minimum ratio (18) required to support the hexaaquo structures for both Mg2+ and Na+ ions. A mixture of the contact ion pairs of Mg2+O6-x(SO4 2-)Ox(H2O)‚Na+O6-y(SO42-) Oy (H2O) (y < x < 6), which change the spectral characteristic of the shoulder at 995cm-1, is formed. These contact ion pair mixtures share sulfate ions and water molecules, which invalidate empirical mixing rules of water activity of atmospheric aerosols such as the ZSR model. As RH is further reduced, the second transition point appears at WSR) 7.7, where the WSR becomes almost insensitive to RH and another shoulder at 1002cm-1 appears in the spectrum. The mixture of the contact ion pairs finally evolves into the double salt of MgSO4, Na2SO4, 4H2O, giving two new Raman peaks at 1007 and 1043 cm-1. Transitions of the hygroscopic properties coinciding with the formation of monodentates were also observed for MgSO4 and ZnSO4 droplets.

The ATR-FTIR spectra of aqueous LiClO4, NaClO4, and Mg(ClO4)2 solutions with ClO4 concentrations ranging from 0 to 3.00 mol dm 3 were obtained. After subtracting the spectra of pure water, positive peaks on the high wavenumber side and negative peaks on the low wavenumber side of the O-H stretching bands are observed in the difference spectra. The positive peaks appear constantly at about 3580cm 1 independent of cation, which are assigned to the water molecules weakly hydrogen-bonded with ClO4. However, the negative peaks appear at 3203, 3196, and 3254cm 1 for LiClO4, NaClO4, and Mg(ClO4)2 solutions, respectively, and the peak areas show significant difference with increasing the concentration of perchlorate anions and are dependent on cations. The negative peaks are attributed to the "structure breaking" effect of perchlorate ions on the hydrogen bond network of water, which is in agreement with Raman spectroscopic studies. Besides the "structure breaking" effect of ClO4 on destroying the strong hydrogen bonds of the water molecules with fully hydrogen-bonded five-molecule tetrahedral nearest neighbor structure, the difference of the negative peaks are the results of the different "structure making" effect of the three cations, which is consistent with the ability of the polarization and hydration, in the order of Naþ<Liþ Mg2þ. The overall shifting of the v3 band of perchlorate ions towards low wavenumber with increasing the concentration of perchlorates is attributed to the presence of solvent separated ion pairs, i.e., M…(H2O)n…ClO4. The symmetric stretching vibration (v1) of perchlorate ions, which is an infrared inactive mode for free perchlorate ions, shows a weak band at 930cm 1 in a wide concentration range of the three systems. The appearance of the weak band is considered as the perturbation of the ZnSe/water interface on perchlorateions.

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 NaClO4, 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 ClO4-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., I3440/I3549 and I3289/I3549 for NaClO4, I3455/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. Hygroscopicity is one of the most important properties of atmospheric aerosols. Aerosols change their size and concentrations through absorbing water with increasing relative humidity (RH) or evaporating water with decreasing RH. Such processes affect the size distributions, radiative properties, deposition characteristics, and chemical reactivity of the aerosols.1 Na+ and Mg2+ are the major cations in sea-salt aerosols. However, their salts of the same anions have very different hygroscopic properties.2,3 On a molecular level, the water content of an aerosol is controlled by the interactions between the water molecules and the solutes, including the hydration of ions, the formation of ion pairs and contact ion pairs, as well as the hydrogen bonding between water molecules. Previously, when we coupled a single particle levitation system with a Raman spectroscopic system, we showed that the Raman spectra of aqueous sulfate droplets can be used to correlate the changes of molecular structures, the formation of contact ion pairs in particular, with their hygroscopic properties. 4,5 This technique facilitates the study of extremely supersaturated solutions, in which the interactions between solute and water molecules are much more extensive than in a typical bulk study. This study examines the interactions between water molecules in the inner spheres of these hydrated cations.