Nucleation Kinetics in Mixed NaNO₃/Glycerol Droplets Investigated with the FTIR-ATR Technique

The in situ infrared spectra of sodium nitrate (NaNO₃) and mixed NaNO₃/glycerol droplets with organic to inorganic molar ratio (OIR) of 1:8, 1:4, 1:2, 1:1, and 2:1 on the ZnSe substrate were collected using the Fourier transform infrared attenuated total reflection (FTIR-ATR) technique in the RH linearly decreasing process. When the efflorescence process occurred in the RH decreasing process, the stochastric transformation from NaNO₃ droplets to NaNO₃ solid particles resulted in gradually increasing of a new band at 836 cm^-1 and contineously decreasing of an initial band at 829 cm^-1, which were assigned to the v₂-NO₃^- mode in crystal phase state and in liquid state, respectively. There were excellent isobesic points between the two bands in the transformation processes, indicating the synchronization between the disappearence of NO₃^- in solutions and the production of NaNO₃ crystal. The nucleation ratio, i.e., the amount of the droplets crystallized at a given RH upon the total amount droplets, was obtained by using the absorbance of ν₂-NO₃^- band at 836 cm^-1, which was used to calculate the nucleation rates of NaNO₃ either for heterogeneous or for homogeneous nucleation process. While the glycerol molecules delayed the efflorescence RHs (ERH) of NaNO₃ in the mixed NaNO₃/glycerol droplets (OIR = 2:1) to 15%, greatly lower than the ERH for pure NaNO₃ droplets at 62.5%, they also greatly suppressed the heterogeneous nucleation rate with increase of the OIR ratio. Two different kinetic mechanisms were suggested in the mixed droplets with OIR = 1:8, 1:4, 1:2, and 1:1, i.e., homogeneous nucleation at higher supersaturation and heterogeneous nucleation at lower supersaturation. For the mixed droplets with 2:1 OIR, they fell into the homogeneous nucleation region completely.