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

The $NaNO3$ droplets with sizes of $1–5μm$ 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 $NaNO3$ 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 $∼3539cm−1$⁠, together with three relatively weak peaks at $∼3400$⁠, $∼3272$⁠, and $∼3167cm−1$⁠, 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 $∼3400cm−1$ 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 $ν3-NO3−$ band at 1363 and $1390cm−1$ at the RH of $∼72%$⁠, instead of the single $ν3-NO3−$ band at $1357cm−1$ for the initial solid $NaNO3$⁠, 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 $ν3-NO3−$ band showed two overlapping peaks at 1348 and $1405cm−1$ similar to those of bulk $NaNO3$ solutions. In the efflorescence process of the $NaNO3$ droplets, the $ν1-NO3−$ band presented a continuous blueshift from $1049cm−1$ at $∼77%$ RH to $1055cm−1$ at $∼36%$ RH, indicating the formation of contact ion pairs between $Na+$ and $NO3−$⁠. Moreover, in the RH range from $∼53%$ down to $∼26%$⁠, two peaks at 836 and $829cm−1$ were observed in the $ν2-NO3−$ band region, demonstrating the coexistence of $NaNO3$ solid particles and droplets.