First-order phase transition liquid silica

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. 

First-Principles Simulations In the cases of carbon and silica, computer simulations using classical empirical potentials have shown a liquid-liquid transition [17,92], but first-principle MD (FPMD) simulations [93,94] show results that are not consistent with classical simulations. In silicon, Jakse and Pasturel [22] and independently Ganesh and Widom [23] have reported first-principle simulation results, both of which support the proposed liquid-liquid transition in silicon. In the work of Ganesh and Widom, the authors report the emergence of a van der Waals-like loop (shown in Fig. 13), as signature of a first-order phase transition at temperatures below 1182K. The maximum time span of these simulations is around 40 ps [22], which seems to be very small compared to the relaxation times of LDL (tens to hundreds of nanoseconds see below) obtained from simulations of SW silicon [21]. But the FPMD calculations are computationally very expensive compared to classical MD simulations. Hence, it would be of interest to compare the equilibration times of the system simulated in FPMD and classical MD and also do a systematic study of relaxation processes in these two different methods of simulation. A comparison of properties obtained in different simulations are discussed in a later section. 

Lipid bilayer membrane systems, having gel (solvated crystalline state)-to-liquid crystalline phase transitions are attractive as specific organic media for separation chemistry. The first approach in HPLC was direct immobilization of a phosphatidylcholine lipid onto silica. This modified silica shows interesting selectivity against amino acids, but the separation mode is too complicated, due to the zwitter-ionic property of the immobilized molecule. In addition, no lipid membrane function is realized on the silica because of the direct immobilization with covalent bonding, which prohibits lateral diffusion of lipids from forming highly-ordered structures that lead to supramolecular functions of lipid membrane systems. 

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