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

摘要

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 mu m) with WSRs of 9.5-1.0 on a quartz substrate. Upon reduction of the WSR, a blue shift of the symmetric stretching band (v(1)(NO3-)) from similar to 1048 to similar to 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 v(1)(NO3-) band increased from similar to 8.4 cm(-1) for the dilute solution (WSR = 54.0) to similar to 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 similar to 1.2 at WSR = 54.0 to similar to 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 similar to 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.