Statistical Mechanics of the Anomalous Behavior of Tetrahedral Liquids

TL;DR

This paper develops a statistical mechanical model for tetrahedral liquids like water and silica, explaining their anomalous thermodynamic behaviors without requiring a phase transition, and compares it with molecular dynamics simulations.

Contribution

The paper introduces a volume-constrained statistical mechanical theory for tetrahedral liquids that accounts for their anomalies without invoking a liquid-liquid phase transition.

Findings

The theory semi-quantitatively matches molecular dynamics simulations.

Anomalous density and heat capacity behaviors arise naturally from the model.

The model's phase transition depends on specific volume and temperature parameters.

Abstract

Tetrahedral liquids such as water and silica-melt show unusual thermodynamic behavior such as a density maximum and an increase in specific-heat when cooled to low temperatures. There is a debate in the literature whether these phenomena stem from a phase transition into a low-density and high-density liquid phases, which occur in the supercooled regime. Here we consider a model of tetrahedral liquids for which we construct a volume-constrained statistical mechanical theory which quantifies the local structure of the liquid. We compare the theory to molecular dynamics simulations and show that the theory can rationalize the simulations semi-quantitatively. We show that the anomalous density and specific heat behavior arise naturally from this theory without exhibiting a liquid-liquid phase-transition. We explain that this theory may or may not have a phase transition, depending on the volume and temperature dependence of the energy and entropy which are sensitive to small changes in the parameters of the model.