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

The NaNO(3) droplets with sizes of 1-5 microm 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 NaNO(3) 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 approximately 3539 cm(-1), together with three relatively weak peaks at approximately 3400, approximately 3272, and approximately 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 NaNO(3) particles, respectively. When the RH approached approximately 72%, slightly lower than the deliquescence RH (74.5%), the band component at approximately 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 NaNO(3) particles. A splitting of the nu(3)-NO(3)(-) band at 1363 and 1390 cm(-1) at the RH of approximately 72%, instead of the single nu(3)-NO(3)(-) 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 NaNO(3). The metastable solid phase deliquesced completely in the region from approximately 87% to approximately 96% RH according to the fact that the nu(3)-NO(3)(-) band showed two overlapping peaks at 1348 and 1405 cm(-1) similar to those of bulk NaNO(3) solutions. In the efflorescence process of the NaNO(3) droplets, the nu(1)-NO(3)(-) band presented a continuous blueshift from 1049 cm(-1) at approximately 77% RH to 1055 cm(-1) at approximately 36% RH, indicating the formation of contact ion pairs between Na(+) and NO(3)(-). Moreover, in the RH range from approximately 53% down to approximately 26%, two peaks at 836 and 829 cm(-1) were observed in the nu(2)-NO(3)(-) band region, demonstrating the coexistence of NaNO(3) solid particles and droplets.

Confocal Raman observation of the efflorescence/deliquescence processes of individual NaNO3 particles on quartz.

Confocal Raman spectroscopy was used to study the structural changes of bulk NaNO3 solutions with molar water-to-solute ratios (WSRs) of 54.0-12.3 and NaNO3 droplets (10-100 microm) with WSRs of 9.5-1.0 on a quartz substrate. Upon reduction of the WSR, a blue shift of the symmetric stretching band (nu(1)(NO3-)) from approximately 1048 to approximately 1058 cm(-1) was observed in the confocal Raman spectra with high signal-to-noise ratios. Accordingly, the full width at half-height of the nu(1)(NO3-) band increased from approximately 8.4 cm-1 for the dilute solution (WSR = 54.0) to approximately 15.6 cm-1 for the extremely supersaturated droplet (WSR = 1.0), suggesting the formation of contact ion pairs with different structures. For the O-H stretching band, the ratio of weak hydrogen-bonding components to strong ones, i.e., I(3488)/I(3256), increased from approximately 1.2 at WSR = 54.0 to approximately 7.3 at WSR = 1.0, indicating that the strong hydrogen bonds were heavily destroyed between water molecules especially in the supersaturated droplets. In the humidifying process, two hygroscopic behaviors were observed depending on the morphology of solid NaNO3 particles. No surface water was detected for a solid NaNO3 particle with rhombohedral shape at relative humidities (RHs) below 86%. When the RH increased from 86% to 93%, it suddenly absorbed water and turned into a solution droplet. For a maple-leaf-shaped NaNO3 particle with a rough surface, however, a trace of residual water originally remained on the rough surface even at very low RH according to its Raman spectrum. Its initial water uptake from the ambient occurred at approximately 70% RH. The small amount of initially adsorbed water induced surface rearrangement of the maple-leaf-shaped particle. A further increase of RH made the particle gradually turn into a regular solid core swathed in a solution layer. Eventually, it completely deliquesced in the RH region of 86-93%, similar to the case of the NaNO3 particle with rhombohedral shape.