Kinetics, nucleation definition

A subcritical aggregate having fewer subunit components than a nucleus. When this term is applied in the kinetics of precipitation, n refers to the number of subunits in a particle and n defines the number of subunits in a particle of critical size. This definition avoids confusion by distinguishing between subcritical (n < n subunits), critical (n = n subunits), and supercritical (n > n subunits) particle sizes. If a nucleus is defined as containing n n subunits, then an embryo contains n n subunits. Note that in this treatment, we are not using a phase-transition description to describe nucleation, and we are focusing on the smallest step in the process that leads to further aggregation. 

Note 4 If a mixture is thermodynamically metastable, it will demix if suitably nucleated (see Definition 2.5). If a mixture is thermodynamically unstable, it will demix by spinodal decomposition or by nucleation and growth if suitably nucleated, provided there is minimal kinetic hindrance. 

The definitive hydrate kinetic inhibition mechanism is not yet available. Some work suggests that the mechanism is to prevent hydrate nucleation (Kelland, 2006). However, a significant amount of evidence suggests that hydrate kinetic inhibitors inhibit the growth (Larsen et al., 1996). However, this apparent conflict is due to the definition of the size at which crystal nucleation stops and growth begins. To resolve this confusion, one may consider growth to occur after the critical nucleus size is achieved. 

The rate equation specifies the mathematical fimction (g(ur) = ktox AodAt = k f(ur)) that represents (with greatest statistical accuracy. Chapter 3) the isothermal yield a) - time data for the reaction. For reactions of solids these equations are derived from geometric kinetic models (Chapter 3) involving processes such as nucleation and growth, advance of an interface and/or diflEusion. f( ir) and g(ar) are known as conversion functions and some of these may resemble the concentration functions in homogeneous kinetics which give rise to the definition of order of reaction. 

As with all kinetic phenomena, (he first concept to understand is the driving force for the rate processes. While supersaiuration was represented above as the driving force for nucleation and growth, no clear definition of this quantity was given, hi fact, there are various ways in which the driving force or super-saturation can be defined ... 

The unit operation of erystallization is governed by some very complex interacting variables. It is a simultaneous heat and mass transfer process with a strong dependence on fluid and particle mechanics. It takes place in a multiphase, multicomponent system. It is concerned with particulate solids whose size and size distribution, both incapable of unique definition, vary with time. The solids are suspended in a solution which can fluctuate between a so-called metastable equilibrium and a labile state, and the solution composition can also vary with time. The nucleation and growth kinetics, the governing processes in this operation, can often be profoundly influenced by mere traces of impurity in the system a few parts per million may alter the crystalline product beyond all recognition. 

It has also been shown quite definitively that, within the precision of the kinetic data that are available, no decision can be made regarding whether a two-or three-dimensional nucleation process is operative [138]. This conclusion is true for virtually all polymers that have been studied. This is admittedly a rather frustrating situation, since nucleation plays such an important role in crystallization of polymers. In analyzing the kinetic data we shall for convenience utilize the Gibbs two-dimensional-nucleation model. The same general conclusions are reached if instead three-dimensional nucleation is assumed. In what follows, no assvunptions are made with respect to the chain structure within the nucleus. 

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