Physical Chemistry of the Freezing Process of Atmospheric Aqueous Drops

In supercooled aqueous solutions, ice nucleation is the initial stage of the freezing process. In this paper, we present experimental results that indicate that during the freezing of aqueous solutions, freeze-induced phase separation (FIPS) into pure ice and a freeze-concentrated solution (FCS) takes place. Our observations involve the use of an optical cryo-microscope (OC-M) to record images and movies. The results visually indicate for the first time that there are two freezing processes for (NH₄)₃H­(SO₄)₂/H₂O solutions: (i) contact freezing, as is the case for pure water drops, and (ii) the Wegener–Bergeron–Findeisen process, which is the growth of frozen drops (ice) at the expense of liquid ones. We also present OC-M images of frozen micrometer-scaled H₂SO₄/H₂O drops that support our previous finding that freezing of these solutions generates mixed-phase particles, namely an ice core coated with a FCS. These results are relevant for atmospheric as well as for pharmaceutical sciences