Phase diagram of bulk water

Although the liquid-vapor phase transition of bulk water is well studied experimentally, this is not the case for the phase transitions of interfacial and confined water, which we consider in the next sections. Therefore, studies of the phase transitions of confined water by computer simulation gain a special importance. For meaningful computer simulations, it is necessary to have water model, which is able to describe satisfactorily the liquid-vapor and other phase transitions of bulk water. The coexistence curves of some empirical water models, which represent a water molecule as a set of three to five interacting sites, are shown in Fig. 1. Some model adequately reproduces the location of the liquid-vapor critical point and. 

Upon heating, the densities of the coexisting vapor and liquid phases approach each other, and, asymptotically close to the critical temperature, their difference follows the universal power law  

The diameter p of the coexistence curve is the average value of the densities of the coexisting liquid and vapor phases. It is equal to the critical density at T = Tc and changes mainly regulary with r. In the close vicinity of the critical point, diameter of fluids shows a critical anomaly, which may behave as [14] or [15], or as superposition of 

Temperature dependences of the fraction of water molecules with tetrahedral arrangement of the nearest four neighbors in liquid water calculated at the liquid-vapor coexistence curve for two water models are shown in Fig. 5. There are less than 10% of such water molecules at the liquid-vapor critical point. Upon cooling, more water molecules gain tetrahedral ordering and their fraction achieves 50% at ambient conditions. At some temperatures below the freezing temperature, fraction of tetrahedrally ordered water molecules in supercooled liquid water shows a rapid or even a stepwise increase. 

Another important structural characteristic of the local order is a coordination number NN, the number of the neighbors in the first coordination shell. There is a clear correlation between NN and the degree of the tetrahedral ordering. The fraction of water molecules with NN = 4, showing tetrahedral ordering, exceeds 90% at low temperatures. This fraction is essentially lower for water molecules having more neighbors. 

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In Section 1 of this book, we give a brief description of the phase diagram of bulk water. This includes analysis of the liquid-vapor coexistence curve of water, a possibility to describe it in a universal way, effect of the liquid-vapor critical point on the properties of supercritical... 

Fig. 7-9 P-T phase diagram for bulk water. (Based on data of the Smithsonian Meteorological Tables.)...

For objects above the Thomas et al. volume threshold, bulk density should be closely related to the densities and therefore the compositions of the major constituents— rock and ice in most cases. The complications arising from the high-pressure phases of minerals under conditions found in the interiors of the larger terrestrial planets are much less severe for these objects. Interior pressure in even the largest outer planet satellites reaches only —3.5 MPa, which will not affect the densities of minerals in the rock portion significantly (Schubert, 1986). The major pressure-related effect that must be taken into account is the phase diagram of the water-ice system, where temperatures and pressures in the larger icy... 

MLO-water bulk phase with excess water and an MLO-based dispersion. (B) Temperature-dependent phase diagram of MLO-water system. 

The corresponding relations between temperatures and monolayer film pressure of forms I and II in the case of monoelaidin is shown in Fig. 5.11. Monoelaidin, having a trans double bond, exhibit monolayers with liquid crystalline chains (form II) up to about 30 °C.The relation between the monolayer transitions and the corresponding bulk phase transition in the binary phase diagram of monoelaidin/water (Fig. 5.2 (b)) is thus very close. Monoolein shows only monolayers with liquid chains (form I) at all temperatures between 0 and 100 °C, which is in agreement with the phase diagram of monoolein/water shown in Fig. 5.2 (c). 

Despite the intensive studies of interfacial and confined water, many aspects of its behavior remain not well studied or even unclear. There are only a few studies of the phase diagram of confined water and of water adsorbed on the surface. Most of these studies are the simulations with very simple smooth surfaces. Clearly, experimental studies and simulations with more realistic surfaces are necessary. Repulsion between hydrophilic surfaces in liquid water gained much less attention than attraction between hydrophobic surfaces. However, this effect may be responsible, for example, for the destruction of some solids in environment with varying humidity. The liquid-liquid transitions of water, confined in various pores, should be studied because of their importance not only in understanding the properties of interfacial water but also aiming to locate these transitions in bulk water. 

Figure 4.19 Phase diagrams of the mixture in the T-wa projection for the model slit-pore for different water substrate interaction strengths es- The bulk coexistence curve is represented by the thin solid line. Dots ( ) indicate critical points.

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