Spatially Resolved Micro-Raman Observation on the Phase Separation of Effloresced Sea Salt Droplets

We report on the investigation of the phase separation of individual seawater droplets in the efflorescence processes with the spatially resolved Raman system. Upon decreasing the relative humidity (RH), CaSO 4 ·0.5H 2 O separated out foremost from the droplet at an unexpectedly high RH of∼90%. Occasionally, CaSO 4 ·2H 2 O substituted for CaSO 4 ·0.5H 2 O crystallizing first at ∼78% RH. Relatively large NaCI solids followed to crystallize at ∼55% RH and led to the great loss of the solution. Then, the KMgCl 3 ·6H 2 O crystallites separated out from the residual solutions, adjacentto NaCI at∼44% RH. Moreover, a shell structure of dried sea salt particle was found to form at low RHs, with the NaCl crystals in the core and minor supersaturated solutions covered with MgSO 4 gel coating on the surface. Ultimately, the shielded solution partly effloresced into MgSO 4 hydrates at very dry state (<5% RH).