Samples of aqueous solutions are often contaminated with dust or other particles, which may lead to heterogeneous ice nucleation at higher temperature instead of homogeneous ice nucleation. The large volume of a bulk sample enhances the probability of such a contamination, and one good ice nucleus is sufficient that the sample freezes heterogeneously. To overcome this problem, the volume of the sample can be reduced to the volume of a micrometer sized droplet. The crystallization heat of such a droplet, on the other hand, is smaller than the detection limit of a regular differential scanning calorimeter (DSC). But the sum of the crystallization heat of a large number of micrometer sized droplets can be detected by a regular DSC. Such a situation is realized in inverse (water-in-oil) emulsions, which contain two immiscible substances, usually a hydrophobic and a hydrophilic substance, combined with an emulsifier. Since an emulsion consists of a large number of droplets, one can be ensured that at most a small fraction of the droplets will contain unintentionally an ice nucleus. Therefore, emulsion calorimetry allows investigating homogeneous ice nucleation. However, the droplets can also be seeded by a specific ice nucleus, and hence emulsions are also a potent tool to observe heterogeneous ice nucleation.
Here, a commercial Differential scanning calorimeter (DSC, TA Q10) with a LNCS cooling system is used. This setup allows measuring the temperature in the range of 130 to 600 K, with a precision of ±0.01 K. The cooling and heating rates can be adjusted between 0.01 and 50 Kmin−1.
Figure 2 shows the raw data for a cooling and heating curve of a typical experiment with an emulsion made of a 9.9 wt% NaCl solution. The sample is cooled with a constant cooling rate (10 Kmin−1) and a large exothermic peak appears at about 220 K, which is assigned to freezing of the water in the emulsion. The ice freezing point of the sample is determined by the onset of the peak (marked by the black lines). In the heating cycle (rate = 1 Kmin−1) two endothermic peaks appear, a larger one at about 250 K and a smaller one close to 270 K. The first one is due to the eutectic melting of the NaCl·2H2O
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