On the Vitality of the Classical Theory of Crystal Nucleation; Crystal Nucleation in Pure Own Melt; Atmospheric Ice and Snow; Ice in Frozen Foods

Abstract

The main reason for the longevity of the Classical Nucleation Theory (CNT) is its firm thermodynamic basis; reviewing the discussion about the molecular-scale mechanism of crystal nucleation from solutions, and especially the mechanism of protein crystal nucleation, we note that the diverse nucleation pathways across the metastable phase cannot contradict the thermodynamic conclusions of the CNT. In this review paper, revisiting the basic postulates of CNT, we argue that not only the energy barrier for crystal nucleation but the entire dependence of Gibbs’ thermodynamic potential on the crystal size is worth interpreting. In doing so, two supplementations to CNT have been elaborated. The first one concerns the theoretical method employing Equilibration between the Bond energy (i.e., the intra-crystalline cohesive energy which maintains the integrity of a crystalline cluster), and the surface Destructive Energy (tending to tear-up the crystal) - abbreviated EBDE. Second, we show that the dependence of the Gibbs’ thermodynamic potential on the crystal size determines not only the birth, but also the initial growth (or dissolution during Ostwald ripening) of the just born nuclei of the new phase; this is predicted in the negative branch of the said dependence. Initially, EBDE was used for explaining crystal nucleation from solutions, but most recently, this method was redefined for considering crystal nucleation in melts. The purposively redefined EBDE was applied for considering ice nucleation, which is an important case of spontaneous melt crystallization in nature - the quantitative consideration of the ice crystal nucleation is needed for better understanding of atmospheric processes, such as snowfall, white frost, sleet, hail, and ice fog. By focusing on the action of ice nucleating particles (INPs), which engender heterogeneous nucleation of ice, the snowfall is elucidated in a new way - ice nucleation in the atmosphere is considered as a two-step process, the first one being vapor condensation in liquid droplets, and the second one - water freezing. Also, ice nucleation in frozen foods is re-considered applying EBDE. (It is known that freezing ensures a high-quality product and long shelf life of a wide range of food products, such as fish, meat, vegetables, tropical fruits, coffee, flavor essence, etc.) And because numbers and sizes of ice crystals are decisive for the degree of deterioration of food quality due to freezing, the mean sizes of the ice crystals (which depend on their number) are considered in a quantitative manner. Also, another consideration concerns ice crystal nucleation and growth occurring by freeze concentration of liquid foods. Although aimed at reviewing fundamental aspects of crystal nucleation, it is to be hoped that some results of the considerations in this paper may also be beneficial for practical applications; suggestions in this respect are mentioned throughout the paper. For instance, the direct comparison between ice crystal nucleation in pure water and in frozen foods suggests how the dynamic food freezing step may be optimized, etc. The review ends with a short paragraph presenting the advantages and disadvantages of CNT.