Different freezing behavior of millimeter- and micrometer-scaled (NH₄)₂SO₄/H₂O droplets

Although the freezing of aqueous solutions is important for nature and different branches of science and freeze-applications, our understanding of the freezing process is not complete. For example, numerous measurements of micrometer-scaled (NH(4))(2)SO(4)/H(2)O droplets report one freezing event below the eutectic point. However, measurements of larger millimeter-scaled droplets reveal two freezing events: the freezing out of ice and subsequent freezing of a residual freeze-concentrated solution. To resolve this apparent contradiction we performed numerous calorimetric measurements which indicate that the freezing of a residual solution of millimeter-scaled 5-38 wt% (NH(4))(2)SO(4) droplets occurs mainly between ∼ 210 and 225 K. We also find that micrometer-scaled droplets produce one freezing event which is within or in the vicinity of the ∼ 210-225 K region. This fact and the analysis of thermograms suggest that the residual solution of micrometer-scaled droplets may partly crystallize simultaneously with ice and partly transform to glass at T(g)≈172 K. Our results suggest for the first time that the size of (NH(4))(2)SO(4)/H(2)O droplets may affect the number of freezing events below the eutectic point.