Nanophase Segregation in Supercooled Aqueous Solutions and Their Glasses Driven by the Polyamorphism of Water

We use large-scale molecular dynamics simulations to investigate the phase transformation of aqueous solutions of electrolytes cooled at the critical rate to avoid the crystallization of ice. Homogeneous liquid solutions with up to 20% moles of ions demix on cooling producing nanophase segregated glasses with characteristic dimensions of phase segregation of about 5 nm. The immiscibility is driven by the transformation of water to form a four-coordinated low-density liquid (LDL) as it crosses the liquid-liquid transformation temperature T-LL of the solution. The ions cannot be incorporated into the tetrahedral LDL network and are expelled to form a solute-rich water nanophase. The simulations quantitatively reproduce the relative amounts of low and high-density liquid water as a function of solute content in LiCl glasses [Suzuki and Mishima, Phys. Rev. Lett. 2000, 85, 1322-1325] and provide direct evidence of segregation in aqueous glasses and their dimensions of phase segregation.