Molecular events in deliquescence and efflorescence phase transitions of sodium nitrate particles studied by Fourier transform infrared attenuated total reflection spectroscopy Author(s): Lu PD ()1, Wang F (Wang, Feng)1, Zhao LJ ()1, Li WX ()1, Li XH (

The NaNO3 droplets with sizes of 1-5 mu m 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 N-2 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 similar to 3539 cm(-1), together with three relatively weak peaks at similar to 3400, similar to 3272, and similar to 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 similar to 72%, slightly lower than the deliquescence RH (74.5% ), the band component at similar to 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 nu(3)-NO3-band at 1363 and 1390 cm(-1) at the RH of similar to 72%, instead of the single nu(3)-NO3-band at 1357 cm(-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 similar to 87% to similar to 96% RH according to the fact that the nu(3)-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 nu(1)-NO3- band presented a continuous blueshift from 1049 cm(-1) at similar to 77% RH to 1055 cm(-1) at similar to 36% RH, indicating the formation of contact ion pairs between Na+ and NO3-. Moreover, in the RH range from similar to 53% down to similar to 26%, two peaks at 836 and 829 cm(-1) were observed in the nu(2)-NO3- band region, demonstrating the coexistence of NaNO3 solid particles and droplets.