Water sorption in the interface of salt mixture Shaoheng Wang 1 , Sebastiaan Godts 2 , Michael Steiger 1 1 Department of Chemistry, University of Hamburg, Germany, 2 Royal Institute for Cultural Heritage (KIK-IRPA), Belgium Deliquescence is a first-order phase transition from solid to solution at a certain relative humidity (DRH), and it widely influences the process of aerosol formation, heritage conservation, the Martian water cycle, etc [1-3] . In blends, the solid-solution transition humidity is lower than each component, which impacts the physical stability of salt mixture system. The dynamic interaction of water sorption on the salt surface has attracted a lot of attention. Many studies focused on the predeliquescence of single salt have been reported [4-6] , and raise the mechanism that there is nanoscale thickness water film forming before deliquescence, which leads to the salt's full dissolution at DRH. However, little understanding is gained about how the deliquescence in salt mixture starts at a humidity that lower than the DRH of each component. In this work, we compare the water sorption curves of single salt NaCl, KCl, and the mixture NaCl + KCl. It is indicated that the deliquescence humidity of mixture is lower than single salt. Subsequently, in-situ observation of the water sorption was performed in an optical microscope cooperated with the environment chamber, seen in Fig.1a, 1b. We found water condensation starts at the interface of salt particles, then the dissolution spreads to bulk salts at deliquescence humidity. We propose that the deliquescence initially takes place in the nanoscale hole generated by the rough surface of particles, where the water condensation pressure is much lower than in the bulk system 7 . Then the solution film from each side, i.e. NaCl and KCl, converge in the interfacial gap, shown in Fig.1c. This confluence is a crucial step for mixture deliquescence since it produces the saturated solution containing both salts, where the water activity or vapor pressure theoretically is lower than the single salt. When the environment humidity reaches the vapor pressure of the saturated solution, this interfacial reaction would extend to the whole system, until one of the components is depleted.
Fig. 1 Water sorption of single salt and their mixture References 1. B. J. Finlayson-Pitts. Chem. Rev. 2003, 103, 4801-4822.
2. Li, D., Zhao, YY.S., Meslin, PY. et al. Commun. Earth Environ. 2022 3, 15. 3. Michael Steigr, Kirsten Linnow, et.al. Environ. Geol . 2007 , 52, 317–327. 4. George E. Ewing. Chem. Rev. 2006, 106, 1511-1526. 5. Derek A. Bruzewicz, et.al. J. Chem. Phys. 2011 , 134, 044702. 6. Jack J, Lin, et.al. Atmos. Chem. Phys., 2021 , 21, 4709–4727. 7. Michael Steiger, Tanya Talreja-Muthreja, et.al. Langmuir 2022 , 38, 36, 10963–10974
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