Molecular events in deliquescence and efflorescence phase transitions of sodium nitrate particles studied by Fourier transform infrared attenuated total reflection spectroscopy

The NaN O3 droplets with sizes of 1-5 μm generated from a nebulizer were deposited on a ZnSe substrate in a Fourier transform infrared attenuated total reflection (FTIR-ATR) chamber. After solidification of the droplets with dry N2 gas passing through the chamber, the solid NaN O3 particles were monitored by in situ FTIR-ATR spectra in cycles of deliquescence and efflorescence processes with varying relative humidities (RHs). With an increase in the RH, a dominant peak at ∼3539 cm^-1, together with three relatively weak peaks at ∼3400, ∼3272, and ∼3167 cm^-1, in the O-H stretching band of water was resolved by the high signal-to-noise ratio FTIR-ATR spectra. The dominant peak and the three relatively weak peaks were contributed by the water monomers and the aggregated water molecules adsorbed on the surfaces of solid NaNO₃ particles, respectively. When the RH approached ∼72%, slightly lower than the deliquescence RH (74.5%), the band component at ∼3400 cm^-1 became the main peak, indicating that the water monomers and the aggregated water molecules aggregated to form a thin water layer on the surfaces of solid NaNO₃ particles. A splitting of the v₃-NO₃^- band at 1363 and 1390 cm^-1 at the RH of ∼72%, instead of the single v₃-NO₃ ^- band at 1357 cm^-1 for the initial solid NaNO ₃, was observed. We suggested that this reflected a phase transition from the initial solid to a metastable solid phase of NaNO₃. The metastable solid phase deliquesced completely in the region from ∼87% to ∼96% RH according to the fact that the v₃-NO₃ ^- band showed two overlapping peaks at 1348 and 1405 cm^-1 similar to those of bulk NaNO₃ solutions. In the efflorescence process of the NaNO₃ droplets, the v₃-NO₃ ^- band presented a continuous blueshift from 1049 cm^-1 at ∼77% RH to 1055 cm^-1 at ∼36% RH, indicating the formation of contact ion pairs between Na⁺ and N O₃^-. Moreover, in the RH range from ∼53% down to ∼26%, two peaks at 836 and 829 cm^-1 were observed in the v₃-NO₃ ^- band region, demonstrating the coexistence of NaN O3 solid particles and droplets.