Observations on the unique phase transitions of inorganics relevant due to gluconic acid in particles

The phase state of atmospheric aerosols has significant impacts on global climate and atmospheric environment chemistry but remains indistinct. Previous study believed that deliquescence on hydration and efflorescence on dehydration for inorganics independent of circumstances. However, the present studies showed an unexpected phase transition of NaCl and (NH₄)₂SO₄ when they mixed with ultraviscous gluconic aid. When gluconic acid and NaCl were mixed in equimolar ratio, no efflorescence was observed on dehydration, in contrast, a unique phenomenon of “efflorescence upon hydration” was observed clearly by optical microscope technology. When NaCl was replaced by (NH₄)₂SO₄, the similar phenomenon is also observed by optical images, which was further demonstrated by feature bands of solid (NH₄)₂SO₄ in FTIR spectra. And Raman spectra intuitively identify that ammonium sulfate moves to the centre and become crystal in a single particle on hydration. In view of ultraviscous property of gluconic acid, the “efflorescence upon hydration” should be caused by slower water diffusion in the mixed particles. Meanwhile, we also explore gluconic acid/NaCl mixed system in different mole ratios. But no “efflorescence upon hydration” appeared. For 2:1 mixed system, the particles have no obvious morphological changes, exhibiting the behaviour similar to that of pure gluconic acid. Whereas for the system rich in NaCl, deliquescence and efflorescence both appear as usual, and the corresponding critical RHs are semblable with pure NaCl particle. Our data fill the database of phase transitions and hygroscopicity in the atmosphere and provide insights for future studies in ultraviscous polyhydroxy organic acid.