Impact of Substrate, Aging, and Size on the Two Freezing Events of (NH4)2SO4/H2O Droplets

Below the eutectic point of T-e approximate to 254.5 K, millimeter-scaled (NH4)(2)SO4/H2O droplets placed on an Al substrate produce two heterogeneous freezing events: the freezing out of pure ice at T-ice and the subsequent freezing of a residual freeze-concentrated solution at T-res. Whether the appearance of T-ice and T-res is governed by the substrate, aging, and the size of droplets was unclear and is investigated here. We present differential scanning calorimetry results of the study of freezing aqueous 25 wt % (NH4)(2)SO4 droplets placed on 11 substrates of different surface properties, including four oil-surfactant mixtures. The analysis shows the following: (i) Independent of substrates, the two freezing events always appear below T-e, and T-ice is always higher than T-res. (ii) The freezing out of ice consists of a fast stage, during which the majority of ice is formed, and a sluggish stage, during which the ice crystallization continues until the residual solution starts freezing. (iii) T-res is more sensitive to surface properties than T-ice; that is, the second freezing event related to the residual solution takes place heterogeneously. (iv) The heterogeneous freezing of the residual solution most likely begins from the nucleation of (NH4)(2)SO4 crystals. (v) The lanolin surfactant impacts the freezing behavior of 25 wt % (NH4)(2)SO4 droplets; that is, in contrast to the past belief, lanolin can also impact the freezing behavior of emulsified droplets. (vi) The measurements of 2.5 month-old 25 wt % (NH4)(2)SO4 droplets show that there is no reaction between lanolin and NH4+ or SO42-. (vii) A 20-fold increase of the mass of 25 wt % (NH4)(2)SO4 droplets does not impact the appearance of T-ice and T-res.