Pore water, supercooling

The extent of hydrate formation depends on the supercooling of the system (ATsc) with respect to the temperature of the hydrate stability limit the supercooling of pore water reached from one up to several degrees (Table 2) which is comparable to earlier measurements for CH4 hydrate formation in the same soils (excluding the sample with 7% of kaolinite where the values are different). As shown in Table 2, the supercooling for the second cycle showed a marked increase in contrast to methane saturated samples where a typical decrease of the supercooling is seen. 

The comparison of supercooling values in samples with different clay additives shows in samples with kaolinite values up by 2 degrees and more while supercooling in samples with montmorilIonite do not exceed 1.3 degree. Evidently, the presence of montmorillonite as compared to kaolinite does not complicate the formation of gas hydrate crystals in pore water and even may favour it as was observed earlier... 

It will be recalled that Sivashinsky and Tanny also favored the idea of supercooled water whose structure is influenced by being in pores as opposed to the idea of freezing water. While the NMR experiments of Sivashinsky and Tanny were performed on the Na+ form, Tc at 250 K was 1.7 x 10 s, which is in the midrange of those obtained by MacMillan et al. for different water contents. It is difficult to imagine water as forming ice in the usual bulk sense in these confined spaces having high surface/volume. 

K. Striking is the broad distribution of jump times of water in cell walls coextending from times of liquid water to ice. We can compare water in cell walls with supercooled water with a broad scale of mobilities. The reduction of the apparent T may be induced by the interaction water/mucopolysaccharid groups. Water in charcoal with mean pore radius of 13 A shows a broader distribution of t but with a... 

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. 

Figure 62 Shift of the liquid-liquid transition of supercooled water in pores. Solid line liquid branch of the Uquid-vapor coexistence curve of ST2 bulk water with a step at 270 K, indicating liquid-liquid transition at zero pressure (dotted line). Liquid branches of the coexistence curves of water in pores of various hydrophilicity are shown by open circles. Temperatures of the liquid-liquid transitions at ambient pressure are indicated by arrows [10].

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