The Anomalously High Rate of Crystallization, Controlled by Crystal Forms under the Conditions of a Limited Liquid Volume

Non-isothermal evaporation and crystallization in a thin layer of an aqueous salt solution have been studied experimentally. The results of these studies are important for a wide range of modern technologies, associated with obtaining thin coatings and crystallization in thin layers and films. The crystallization patterns are shown to differ for different salts and change both over the crystalline hydrate surface and over time. The growth kinetics of crystal hydrates has been examined. A qualitative behavior of crystallization curves may both coincide and differ from the behavior of the curves, obtained using a statistical approach. It is shown that this difference is determined by a changing ratio of rates of evaporation to crystallization. By changing the evaporation velocity, it is possible to change the crystal forms and crystallization velocity. It is well-known that the crystallization rate increases as a square root of time. However, according to our results, the crystallization rate may both rise and fall over time and have an extremum. For all studied crystalline hydrates of salts, it was possible to find dendrites. For the NaCl crystals, the dendrites were not found. A neglect of the crystal form (crystal habit) may result in both overestimating and underestimating the crystallization rate by 3 orders of magnitude. The morphology of the crystal forms is shown to control the kinetics and leads to anomalously high crystallization rates. The kinetic constant derived in the experiment is three orders higher than the constant, calculated by the known kinetic expressions, which do not take into account the crystal habit. Ahead of the crystallization front, there is a moving thermal front, arising in a metastable solution. Consideration of heat transfer and crystal forms allows quantitatively and qualitatively describing the difference in the behavior of crystallization.