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 μm) with WSRs of 9.5-1.0 on a quartz substrate. Upon reduction of the WSR, a blue shift of the symmetric stretching band (v₁(NO₃^-)) from ∼1048 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 ∼8.4 cm ^-1 for the dilute solution (WSR = 54.0) 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/I₃₂₅₆, increased from ∼1.2 at WSR = 54.0 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 ~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.