Raman spectroscopic studies on single supersaturated droplets of sodium and magnesium acetate

Raman spectroscopy was used to study structural changes, in particular, the formation of contact-ion pairs in supersaturated aqueous NaCH ₃COO and Mg(CH₃COO)₂ droplets at ambient temperatures. The single droplets levitated in an electrodynamic balance (EDB), lost water, and became supersaturated when the relative humidity (RH) decreased. For NaCH₃COO droplet the water-to-solute molar ratio (WSR) was 3.87 without solidification when water molecules were not enough to fill in the first hydration layer of Na⁺, in favor of the formation of contact-ion pairs. However, the symmetric stretching vibration band (v ₃ mode) of free -COO^- constantly appeared at 1416 cm ^-1, and no spectroscopic information related to monodentate, bidentate, or bridge bidentate contact-ion pairs was observed due to the weak interactions between the Na⁺ and acetate ion. On the other hand, the band of methyl deformation blue shifted from 1352 to 1370 cm^-1 (at RH = 34.2%, WSR = 2.43), corresponding to the solidification process of a novel metastable phase in the highly supersaturated solutions. With further decreasing RH, a small amount of supersaturated solution still existed and was proposed to be hermetically covered by the metastable phase of the particle. In contrast, the interaction between Mg²⁺ and acetate ion is much stronger. When WSR decreased from 21.67 to 2.58 for the Mg(CH ₃COO)₂ droplet, the band of C-C-symmetric stretching (v₄ mode) had a blue shift from 936 to 947 cm^-1. The intensity of the two new shoulders (∼1456 and ∼1443 cm^-1) of the v₃ band of free -COO^- at 1420 cm^-1 increased with the decrease of WSR. These changes were attributed to the formation of contact-ion pairs with bidentate structures. In particular, the small frequency difference between the shoulder at ∼1443 cm^-1 and the v₃ band of the free -COO^- group (∼1420 cm ^-1) was proposed to be related to the formation of a chain structure based on the contact-ion pairs of bridge bidentate. The continuous formation of various contact-ion pairs started at higher WSR value (WSR = 15.5) greatly reduced the hygroscopic properties of Mg(CH₃COO)₂ droplet, so that the WSR of Mg(CH₃COO)₂ droplets was even lower than that of NaCH₃COO in the RH range of 40-60%.