Phase transitions of confined water

Various phase transitions of confined water and related phenomena may play an important role in technological and biological processes. First, we consider the effect of confinement on liquid-vapor phase transition of water. Then, freezing and melting transitions of confined water are analyzed. Finally, we discuss how confinement may affect the liquid-liquid phase transitions of supercooled water. 

Similar to other fluids, a liquid-vapor phase transition of confined water appears, for example, as a rapid change in the mass adsorbed when the pressure of external bulk water is varied. Numerous examples of the adsorption isotherms of water in various pores, obtained in experiments or in simulations, can be found in literature (some of them we consider below in Section 4.3). However, there are only a few studies 

The most detailed studies of the liquid-vapor coexistence curve of water [10, 28, 30, 32, 205, 249, 250] were performed by simulations 

The evolution of the pore critical temperature of water in slit-like pores with the pore width is shown in Fig. 55. Note that a thickness of water layer in pore is notably smaller than pore width Hp in narrow pores because the space of about 1.25 A width near each pore wall is not accessible for water molecules. Therefore, a real thickness of water phases is equal to Hp — 2.5 A. A critical temperature of quasi-2D water, which may be considered as being confined in pore of width Hp = 5 k, and the respective critical temperatures of water in various pores are shown in Fig. 55. To compare the water critical temperature in pores with theoretical equations (15) and (16), ATI is analyzed as a function of (Hp — 2.5 A) in double-logarithmic scale (Fig. 56). When all data points in Fig. 56 being fitted to equation (16), the value of 0 0.82 was obtained [250]. However, the most of the data points may be well fitted by 

The observed evolution of the pore critical temperature with the size of the slit-like pore is in general agreement with theoretical predictions. To make a comparison with simulations for the 3D Ising model in pores, one should express the results obtained for fluids in terms of layers. In particular, the shift of the critical temperature of water in the pores. 

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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. 

Brovchenko 1, Oleinikova A. Effect of confinement on the liquid-liquid phase transition of supercooled water. I. Chem. 58. Phys. 2007 126 214701. 

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. 

As we have mentioned in the Introduction, the location of the critical point of the lowest density liquid-liquid transition of real water is unknown and both scenarios (critical point at positive or at negative pressure) can qualitatively explain water anomalies. Recent simulation studies of confined water show the way, how to locate the liquid-liquid critical point of water. Confinement in hydrophobic pores shifts the temperature of the liquid-liquid transition to lower temperatures (at the same pressure), whereas effect of confinement in hydrophilic pores is opposite. If the liquid-liquid critical point in real water is located at positive pressure, in hydrophobic pores it may be shifted to negative pressures. Alternatively, if the liquid-liquid critical point in real water is located at negative pressure, it may be shifted to positive pressures by confinement in hydrophilic pores. Interestingly, that it may be possible in both cases to place the liquid-liquid critical point at the liquid-vapour coexistence curve by tuning the pore hydrophilicity. We expect, that the experiments with confined supercooled water should finally answer the questions, concerning existence of the liquid-liquid phase transition in supercoleed water and its location. 

Hansen, E.W., StScker, M., and Schmidt, R. 1996. Low-temperature phase transition of water confined in meso-pores probed by NMR. Influence on pore size distribution. J. Phys. Chem. 100 2195-2200. 

Confinement in pores affects all phase transitions of fluids, including the liquid-solid phase transitions (see Ref. [276, 277] for review) and liquid-vapor phase transitions (see Refs. [28, 278] for review). Below we consider the main theoretical expectations and experimental results concerning the effect of confinement on the liquid-vapor transition. Two typical situations for confined fluids may be distinguished fluids in open pores and fluids in closed pores. In an open pore, a confined fluid is in equilibrium with a bulk fluid, so it has the same temperature and chemical potential. Being in equilibrium with a bulk fluid, fluid in open pore may exist in a vapor or in a liquid one-phase state, depending on the fluid-wall interaction and pore size. For example, it may be a liquid when the bulk fluid is a vapor (capillary condensation) or it may be a vapor when the bulk fluid is a liquid (capillary evaporation). Only one particular value of the chemical potential of bulk fluid provides a two-phase state of confined fluid. We consider phase transions of water in open pores in Section 4.3. 

Ability of water molecules to form various kinds of local order in condensed state causes variety of its crystalline and amorphous phases at low temperatures. The transitions between liquid water phases with different local orders at low temperatures strongly affect the properties of water at ambient conditions. This effect is presumably responsible for various water properties, which makes water different from most other fluids and often called anomalous (liquid density maximum, heat capacity minimum, etc.). Naturally, the bulk polyamorphism appears also in water properties near surfaces. A transition of liquid water to strongly tetrahe-drally ordered water upon cooling is the most important manifestation of this phenomenon as it occurs at ambient pressures. This transition is extremely difficult to detect in bulk water due to unavoidable crystallization. However, it is observed in many systems containing a confined water owing to the drastic change in various properties. 

This methodology developed to observe water freeze-thaw in concrete materials, may be used quite generally to observe solid-liquid phase transitions in many different materials of industrial and technological interest. The method could be also applied to other problems involving freezing and thawing of water in confined pores. 

Another type of physical explosion can occur upon rapid vaporization of a liquid when contacted with a significantly hotter material (e.g., water added to vessel containing hot oil). This is also referred to as a rapid phase transition explosion. In addition to blast, physical explosions can also generate fragments when initially confined. 

Therefore, the maximum of Cp °° occurs where the correlation length associated with the tetrahedral order is maximum, i.e. along the Widom line associated with the LL phase transition." In MF we may compare Cp calculated for the LLCP scenario J(j > 0) with Cp calculated for the SF scenario J(j = 0) [Fig. 7(b)]. We see that the sharper maximum is present only in the LLCP scenario, while the less sharp maximum occurs at the same T in both scenarios. We conclude that the sharper maximum is due to the fluctuations of the tetrahedral order, critical at the LLCP, while the less sharp maximum is due to fluctuations in bond formation. The similarity of our results with the experiments in nanopores is striking. Data in ref. [ °] show two maxima in Cp. They have been interpreted as an out-of-equilibrium dynamic effect in [ °], but more recent experiments show that they are a feature of equilibrated confined water. Therefore, our interpretation of the two maxima is of considerable interest. 

The capillary condensation phenomenon is of course not exclusive to water. It can be found in any confined system, where the surfaces prefer one phase over another and there is a first order phase transition between the phases of the material between the surfaces. A nematic liquid crystal is an example of such a system exhibiting a first order phase transition between the isotropic and the nematic phase. For this system, the nematic capillary condensation has been predicted by P. Sheng in 1976 [17]. Since the isotropic-nematic phase transition is only weakly first order, the phenomenon is not easy to observe. One has to be able to control the distance between the surfaces with a nanometer precision and the temperature within 10 K, which is unachievable to methods like NMR, SEA, DSC, etc., and very difficnlt to achieve in dynamic light scattering experiments [18,19]... 

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