Crystal embryos formation

Interface reaction is another necessary step for crystal growth and dissolution. After formation of crystal embryos, their growth requires attachment of molecules to the interface. The attachment and detachment of molecules and ions to and from the interface are referred to as interface reaction. (During nucleation, the attachment and detachment of molecules to and from clusters are similar to interface reaction.) For an existing crystal to dissolve in an existing melt. 

There are two approaches in developing the conditions that govern favorable nucleation of a crystal embryo out of the liquid phase of a fluid without dissolved substances. From a structural point of view, nucleation implies that under certain favorable conditions the bond formation rendering a crystalline lattice may be competitive with the thermal motion trying to randomize and destroy bonds. A detailed molecular theory of freezing is necessarily a many-bodied problem. There exists, however, a classic thermodynamic theory of freezing that provides much insight into the process of ice nucleation (Hobbs, 1974). 

It was proposed to use the thermodynamics of small systems and Avrami equations to describe the formation processes of carbon nanostmctures during recrystallization (graphitization) [6, 7]. These equatiorrs are successfully applied [8] to forecast permolecular stmctures and prognosticate the conditions on the level of parameters resulting in the obtaining of nanostmctures of definite size and shape. The equation was also used to forecast the formation of fibers [9]. The application of Avrami equations in the processes of nanostmcture formation a) embryo formation and crystal growth in polymers [8... 

Figure 3. Intracellular freezing of 8-cell mouse embryos cooled at 20 °C/min in 2 M DMSO. The black "flashing" occurring in cells at -31 °C to -46 °C is characteristic of intracellular ice formation, and is caused by the scattering of light by many small highly branched ice crystals. (Modified from Rail et al., 1983.)...

The problems of phase transition always deeply interested Ya.B. The first work carried out by him consisted in experimentally determining the nature of memory in nitroglycerin crystallization [8]. In the course of this work, questions of the sharpness of phase transition, the possibility of existence of monocrystals in a fluid at temperatures above the melting point, and the kinetics of phase transition were discussed. It is no accident, therefore, that 10 years later a fundamental theoretical study was published by Ya.B. (10) which played an enormous role in the development of physical and chemical kinetics. The paper is devoted to calculation of the rate of formation of embryos—vapor bubbles—in a fluid which is in a metastable (superheated or even stretched, p < 0) state. Ya.B. assumed the fluid to be far from the boundary of absolute instability, so that only embryos of sufficiently large (macroscopic) size were thermodynamically efficient, and calculated the probability of their formation. The paper generated extensive literature even though the problem to this day cannot be considered solved with accuracy satisfying the needs of experimentalists. Particular difficulties arise when one attempts to calculate the preexponential coefficient. 

Crystal nuclei may form from various kinds of particles molecules, atoms, or ions. In aqueous solutions these may be hydrated. Because of their random motion, in any small volume several of these particles may associate to form what is called a cluster—a rather loose aggregation which usually disappears quickly. Occasionally, however, enough particles associate into what is known as an embryo, in which there are the beginnings of a lattice arrangement and the formation of a new and separate phase. For the most part, embryos have short lives and revert to clusters or individual particles, but if the supersaturation is large enough, an embryo may grow to such a size that it is in thermodynamic equilibrium with the solution. It is then called a nucleus, which is the smallest... 

Figure 10. a) Energy balance of the crystal for the formation of a precipitate (after /7/ arbitrary units) b) Density of embryos as a function of embryo size (after 715,18/). 

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