Key properties of supercooled water

In order to achieve some understanding of the nucleation of hydrate crystals from supercooled water + gas systems, it is useful to briefly review the key properties of supercooled water (Section 3.1.1.1), hydrocarbon solubility in water (Section 3.1.1.2), and basic nucleation theory of ice, which can be applied to hydrates (since hydrate nucleation kinetics may be considered analogous, to some extent, to that of ice Section 3.1.1.3). The three subsections of 3.1.1 (i.e., supercooled water, solubility of gas in water, and nucleation) are integral parts of conceptual pictures of nucleation detailed in Section 3.1.2. 

Water is considered to be supercooled when it exists as a liquid at lower temperatures than its melting point, for example, at less than 0°C at atmospheric pressure. In this state, the supercooled water is metastable. The properties of supercooled water have been examined in detail in excellent reviews by Angell (1982, 1983) and Debenedetti (1996, 2003). A brief review of the properties of supercooled pure liquid water and the different liquid water models are discussed in this section. These structures comprise hydrogen-bonded water networks and/or water clusters ( cages ) that are the starting points to hydrate formation. 

The anomalies of liquid water become more pronounced when it is supercooled. For example, the volume and entropy fluctuations of liquid water become more pronounced as the temperature decreases. This is in contrast to most other liquids, in which the volume and entropy fluctuations become smaller as the temperature is lowered. Furthermore, the volume and entropy fluctuations in water at less than 4°C are anticorrelated, that is, the increase in volume which occurs when water is cooled results in a decrease in entropy (Debenedetti, 2003). 

The anticorrelation between entropy and volume for liquid water has been attributed to the formation of an open hydrogen-bonded network, in which a decrease in orientational entropy is accompanied by a volume increase (Debenedetti, 2003). This network is transient and short-ranged in liquid water (rather than being permanent and long-ranged in ice), and is the microscopic basis for water s negative thermal expansion. This open hydrogen-bonded network has a profound influence on the thermodynamics of liquid water (Debenedetti, 2003). 

In order to understand the behavior of supercooled water it is useful to briefly review the different liquid water models  

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