Liquid−Liquid Immiscibility in Single-Component Network-Forming Fluids: Model Calculations and Implications for Polyamorphism in Water

Abstract

Analytical calculations and Monte Carlo simulations of a lattice model of a tetravalent network-forming fluid show liquid−liquid immiscibility at low temperatures. Three types of phase behavior are found to occur: vapor−liquid equilibrium, vapor−liquid equilibrium plus liquid−liquid immiscibility with an upper critical temperature, and vapor−liquid equilibrium plus closed-loop liquid−liquid immiscibility. The coexisting liquids differ appreciably in density, structure, and molecular dynamics. The low-density liquid displays more structured, open networks, and slower translational and network-restructuring dynamics than the high-density liquid. The model provides useful insights into the molecular basis of low-temperature immiscibility in network-forming fluids, and its relation to the unusual transition between distinct amorphous phases in vitreous water.