Double Freezing of (NH4)2SO4/H2O Droplets below the Eutectic Point and the Crystallization of (NH4)2SO4 to the Ferroelectric Phase

This paper presents the differential scanning calorimetry (DSC) results obtained from measurements of single droplets of different subeutectic concentrations (<40 wt % (NH4)(2)SO4) and diameters of similar to 1-1.5 mm. The measurements show that despite the fact that the freezing of the droplets takes place below the eutectic temperature of T-e approximate to 254.5 K, a phase separation into ice and a residual freeze-concentrated solution occurs. The residual solution is formed by the expulsion of NH4+ and SO42- ions from the ice lattice during the nucleation and growth of ice and may possess the eutectic concentration of similar to 40 wt % (NH4)(2)SO4. On further cooling, the residual solution freezes to the eutectic solid mixture of ice/(NH4)(2)SO4 at a temperature that is either above or below the ferroelectric "Curie" temperature of T-e approximate to 223 K. If the freezing of the residual solution takes place below the T-e, then (NH4)(2)SO4 crystallizes directly into the ferroelectric phase.