Institute of Physical Chemistry, University of Innsbruck, A-6020 Innsbruck, Austria.
Institute of Materials Chemistry, Vienna University of Technology, A-1060 Vienna, Austria.
J Chem Phys. 2016 Dec 7;145(21):211923. doi: 10.1063/1.4965434.
Here we investigate the freezing and thawing properties of aqueous solutions in oil emulsions, with a particular focus on investigating the influence of the oil and surfactant and the stirring time of the emulsion. Specifically, we employ optical cryomicroscopy in combination with differential scanning calorimetry to study the phase behavior of emulsified 25 wt. % ammonium sulfate droplets in the temperature range down to 93 K. We conclude that the nucleation temperature does not vary with oil-surfactant combination, that is, homogeneous nucleation is probed. However, incomplete emulsification and non-unimodal size distribution of dispersed droplets very often result in heterogeneous nucleation. This in turn affects the distribution of freeze-concentrated solution and the concentration of the solid ice/ammonium sulfate mixture and, thus, the phase behavior at sub-freezing temperatures. For instance, the formation of letovicite at 183 K critically depends on whether the droplets have frozen heterogeneously or homogeneously. Hence, the emulsification technique can be a very strong technique, but it must be ensured that emulsification is complete, i.e., a unimodal size distribution of droplets near 15 μm has been reached. Furthermore, phase separation within the matrix itself or uptake of water from the air may impede the experiments.
在这里，我们研究了油乳液中水溶液的冻结和融化特性，特别关注研究油和表面活性剂的影响以及乳液的搅拌时间。具体来说，我们采用光学低温显微镜结合差示扫描量热法研究了温度范围降至 93 K 下乳化的 25wt%硫酸铵液滴的相行为。我们得出结论，成核温度与油-表面活性剂组合无关，即探测到均相成核。然而，不完全乳化和分散液滴的非单峰尺寸分布常常导致异相成核。这反过来又会影响冷冻浓缩溶液的分布以及固体冰/硫酸铵混合物的浓度，从而影响亚冷冻温度下的相行为。例如，在 183 K 下形成莱托维采特取决于液滴是异相还是均相冻结。因此，乳化技术可以是一种非常强大的技术，但必须确保乳化是完全的，即达到了接近 15μm 的液滴的单峰尺寸分布。此外，基质本身的相分离或从空气中吸收水分可能会阻碍实验。
