Polymorphic water model systems

In this work, we review briefly the phenomenology associated to LLPTs based on results obtained from computer simulations of different systems, such as silica, water, and atomic model systems. When possible, results from computer simulations are compared to available experiments. This work is organized as follows. In the next section, we present the phase diagram of polymorphic liquids supported by many computer simulations and experiments. We review the thermodynamics of first-order phase transitions and show how it is observed in computer Simula tions of polymorphic liquids. The relationship between liquid polymorphism and anomalous properties in liquids is also discussed. The next section also includes a description of glass polymorphism, its relation to liquid polymorphism, and a close comparison between experiments and simulations. In Section III, we describe computer simulation models of systems that present liquid polymorphism, with emphasis on the molecular interactions and common properties of these models that are thought to originate LLPTs. A summary and discussion are presented in Section IV. 

The water-sorption experiments were carried out in a dynamic flow vapor sorption apparatus (Model SGA100, VTI Corporation, Hialeah, FL). Samples of the two polymorphs were placed in the instrument s sample chamber and their moisture uptake as a function of relative humidity (RH) was measured. Water-sorption isotherms for both polymorphs were carried out imder the temperature conditions of 20, 25, 35, and 45°C. The amoimt of sample used for an analysis depends on the sample s tendency to pick up water. If the sample is highly hygroscopic, about 2-5 mg is sufficient for the test, but if the sample is nonhygroscopic, a larger mass is needed, about 25 mg or more. For this study, water-sorption isotherms for both polymorphs were carried out using the flow system and a sample size of about 50 mg. 

To investigate the kinetic explanation for the step rule, we model the reaction of three silica polymorphs — quartz, cristobalite, and amorphous silica — over time. We consider a system that initially contains 100 cm3 of amorphous silica, the least stable of the polymorphs, in contact with 1 kg of water, and assume that the fluid is initially in equilibrium with this phase. We include in the system small amounts of cristobalite and quartz, thereby avoiding the question of how best to model nucleation. In reality, nucleation, crystal growth, or both of these factors might control the nature of the reaction we will consider only the effect of crystal growth in our simple calculation. 

Multicomponent systems that present polyamorphism have also been reported in computer simulation studies. For example, in Ref. [35], it is found that silica has a LLCP at very low temperature. Silica is also a tetrahedral liquid and it shares many of the thermodynamic properties observed in water. In Ref. [35], two silica models were considered. In both models, the interactions among O and Si atoms are isotropic, due to single point charges and short-range interacting sites located on each atom. Both models considered in Ref. [35] are characterized by a LLCP at very low temperature and coexistence between two liquids is observed in out of equilibrium simulations close to one of the spinodal lines (see Fig. 2b). The location of the LLCP was estimated to be below the glass transition in real silica and hence, unaccessible in experiments. We note that polyamorphism in the glass state is indeed observed in compression experiments on amorphous silica [14], and is qualitatively reproduced in computer simulations [89]. Other examples of multicomponent systems that show LLPT in simulations are presented in Refs [65,90]. In these cases, a substance that already shows polymorphism is mixed with a second component. 

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