Nucleation of Gas Hydrates

Brief Overview of Ice Nucleation

“…Creation of such an interface costs free energy which is given by the product of the interfacial area and the specific interfacial free energy between a thermodynamically stable phase and a metastable parent phase [ 31 ]. In this feature article, we use the term “specific interfacial free energy” to refer to the interfacial free energy of a unit area of an interface between two well-defined media that has a unique value under a given temperature and pressure [ 34 ]. Likewise, we use the term “specific surface energy” for the surface energy of a unit area of a well-defined surface in vacuum that has a unique value under a given temperature and pressure.…”

“…We note that all the three curves of g ( r ) bulk , g ( r ) interfacial and Δ g ( r ) activation are functions of temperature and pressure, and that Figure 1 is shown for a constant temperature and pressure for which the prevailing phase is metastable . The figure would look very different depending on the temperature/pressure in question [ 34 ].…”

“…However, they may exist temporarily. Such temporary formations of thermodynamically unstable clusters are at the heart of nucleation theory and this is possible because of the uncertainty principle of quantum mechanics [ 34 ]. Simply put, the lifetime of an unrealistically high energy state is very short, but not zero, and the lifetime progressively increases as the energy level becomes lower and more realistic.…”

“…Then, the probability that a monomer attaches to a sub-critically sized cluster, which is thermodynamically unstable and hence can only exist briefly, increases with the lifetime of the cluster. Because of this non-zero probability of the system to “get around” or “tunnel through” the activation barrier, nucleation becomes an intrinsically stochastic event under a modest metastability (i.e., before the driving force becomes so large that the height of the activation barrier falls all the way to zero) [ 34 ]. It is therefore experimentally impossible to “repeat” or “reproduce” a nucleation event, in a deterministic sense, because the nature of the phenomenon is probabilistic.…”

“…It is therefore experimentally impossible to “repeat” or “reproduce” a nucleation event, in a deterministic sense, because the nature of the phenomenon is probabilistic. Instead, a probabilistic approach based on a large number of measurements of metastable systems is generally required (it remains open as to how best to analyze such statistical data [ 34 , 36 ]). Theoretically, this intrinsically probabilistic nature of nucleation is incorporated in the form of probability distributions of the Boltzmann form of classical statistical physics [ 37 ].…”