Models of hydrates

Equation (15) permits a straightforward analysis of dielectric continuum models of hydration that have become popular in recent decades. The dielectric model, also called the Bom approximation, for the hydration free energy of a spherical ion of radius R with a charge q at its center is... 

Leermakers, F. A. M., Rabinovich, A. L. and Balabaev, N. K. (2003). Self-consistent field modeling of hydrated unsaturated lipid bilayers in the liquid-crystal phase and comparison to molecular dynamics simulations, Phys. Rev. E, 67, 011910. 

Figure 5.8 Model of hydrated glass surface showing the development of the hydrated layer. (Reprinted from Doremus, 1975 Figure 3. Copyright 1975 with permission from Elsevier.)...

An alternative mesoscale approach for high-level molecular modeling of hydrated ionomer membranes is coarse-grained molecular dynamics (CGMD) simulations. One should notice an important difference between CGMD and DPD techniques. CGMD is essentially a multiscale technique (parameters are directly extracted from classical atomistic MD) and it... 

As discussed in Section 6.5.3, coarse-grained molecular modeling approaches offer the most viable route to the molecular modeling of hydrated ionomer membranes. The coarse-grained treatment implies simplification in interactions, which can be systematically improved with advanced forcematching procedures, but allows simulations of systems with sufficient size and sufficient statistical sampling. Structural correlations, thermodynamic properties, and transport parameters can be studied. 

There is a conceptual model of hydrated ions that includes the primary hydration shell as discussed above, secondary hydration sphere consists of water molecules that are hydrogen bonded to those in the primary shell and experience some electrostatic attraction from the central ion. This secondary shell merges with the bulk liquid water. A diagram of the model is shown in Figure 2.3. X-ray diffraction measurements and NMR spectroscopy have revealed only two different environments for water molecules in solution of ions. These are associated with the primary hydration shell and water molecules in the bulk solution. Both methods are subject to deficiencies, because of the generally very rapid exchange of water molecules between various positions around ions and in the bulk liquid. Evidence from studies of the electrical conductivities of ions shows that when ions move under the influence of an electrical gradient they tow with them as many as 40 water molecules, in dilute solutions. 

Mancera, R. L. (2007) Molecular modeling of hydration in drug design. Curr Opin Drug Discov Devel 10, 275-280. 

A somewhat intermediate view has also been adopted by Horne and Birkett (SO), who also propose a multilayer model of hydration where both the firmly bonded, first hydration layer and the disordered zone of the Frank-Wen model are accepted. However, they suggest the existence of a second layer of water molecules (separating the primary hydration shell and the disordered zone) around the ion, consisting of rarified or extended clusters of water molecules with density less than waters but definitely not of Ice-I like structure. We return to this aspect later. In this connection, compare also our discussion of the studies by Vaslow (150), Griffith and Scheraga (67), and Luz and Yagil (103). 

We have described several properties of aqueous solutions, some of which appear anomalous. It is now appropriate to discuss briefly what bearing these observations have on the degree and nature of involvement of the water structure in ion hydration. Specifically, are the observed concentration-dependent anomalies determined by the nature of the hydrated structures or are they manifestations of structural changes, induced by the ions, in the pure solvent The information which we have discussed also bears on the question of which model of hydration is most likely to be correct—the Frank-Wen (48) model or that of Samoilov (115). Some anomalies are amazingly abrupt. Vaslows occur over rather narrow concentration ranges, and those observed by Zagorets, Ermakov and Grunau are even sharper. Sharp transitions could be ex-... 

All modern pictures and models of hydrate crystal growth include mass transfer from the bulk phases to the hydrate. Unfortunately, some confusion arises due to the fact that two interfaces are usually considered, and the driving forces may not be intuitive for those not familiar with the area. In order to provide a basis for the modeling section, a brief overview of the diffusional boundary layer is given. 

FIGURE 3.32 Physical models of hydrate film growth along the water-hydrate former fluid interface. (Reproduced from Mochizuki, T., Mori, Y.H., J. Cryst. Growth, 290, 642 (2006). With permission from Elsevier.)... 

There are three current models of hydrate formation in the literature (1) in situ formation from biogenic methane, (2) formation from free (perhaps recycled) gas traveling upward, and (3) formation by upward mobile water which exsolves the gas used for hydrate formation. Each model is discussed briefly in Sections 7.2.3.2.1-7.2.3.2.3. 

FIG U RE 7.10 Proposed model of hydrate formation by upward migration and recycling of gas. (Reproduced from Pauli, C.K., Ussier, W., Borowski, W.S., inProc. First International Conference onNatural Gas Hydrates, 715,392 (1994). With permission from the New York Academy of Sciences.)... 

The above two accomplishments are milestones in the knowledge development of hydrates in nature. It is now beyond question that gas can be produced from hydrates, and that data from such production can be accurately modeled. However, because only a few days were spent proving the concept, the transient results prevented the unambiguous long-term modeling of hydrate production, as shown in the sections that follow. As one result of this work, it appears to be important to provide a longer production test, to enable the long-term projection of gas production from hydrates. 

One cannot yet rule out that other interactions contribute to the hydration, such as the disruption of the hydrogen bond networks when two surfaces approach each other. However, at least a part of this disruption is already contained in the dipole—dipole interactions included in the polarization model. In addition, the polarization model of hydration can relate the magnitude of the hydration force to the density of dipoles on the surface. This can explain the dependence of the hydration repulsion on the surface dipolar potential18 or the restabilization of some colloids at high ionic strength16 observed experimentally.10... 

Much of the new information has arisen because of novel and improved methods for the study of hydrate structures and processes Of course, modeling of hydrates, especially phase... 

Figure 5. Four-state model of hydration-mediated equilibrium between unbound and sidechain-associated counterions in ionomeric membranes (9,10, 13).

The shells do not surround the C3S grains this could suggest the effect of Si substitution by A1 in their formation. Therefore the hydrating C3S phase is not a good model of hydrating cement [9]. 

Meehanisms of hydration of anhydrite, especially the role of excitation agents in the hydration process, have been extensively investigated. It was generally accepted that excitation agents could exert great effect on hydration of anhydrite, however, the hydration process is still in large debate. Mainly two models of hydration of anhydrite were proposed to explain the hydration process, namely dissolution-crystallization and double salt models. 

Popular models of hydration of clinker minerals can be divided into the delayed nucleation model and the protective layer model. 

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