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 ₄·0.5H₂O separated out foremost from the droplet at an unexpectedly high RH of ∼90%. Occasionally, CaSO₄· 2H₂O substituted for CaSO₄·0.5H₂O crystallizing first at ∼78% RH. Relatively large NaCl solids followed to crystallize at ∼55% RH and led to the great loss of the solution. Then, the KMgCl₃·6H₂O crystallites separated out from the residual solutions, adjacent to NaCl 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 MgSO₄ gel coating on the surface. Ultimately, the shielded solution partly effloresced into MgSO₄ hydrates at very dry state (<5% RH).