|It has long been known that the addition of salts to water favors vitrification. The identification of two distinct glass transitions in calorimetric experiments of LiCl-water glasses prompted the hypothesis of a liquid-liquid phase separation in supercooled water. (J. Phys. Chem. 91, 1967-1971, 1987). These experiments, however, could not confirm the existence of two phases, nor their composition or dimensions of phase segregation. |
We present evidence by molecular simulations of the liquid-liquid immiscibility that develops in supercooled aqueous solutions. Two water nanophases form on hyperquenching of some water solutions: a pure low-density liquid phase that vitrifies to low density amorphous ice, and a higher density water-solute mixture. The immiscibility of supercooled aqueous solutions is driven by the transformation of water from a high to a low density liquid (LDL): the hydrophilic solutes cannot be incorporated into the tetrahedral LDL network and are expelled to form an ion-rich water nanophase. We conclude that the low-density liquid is solutophobic (as its closely related phase, ice, is). The simulation results agree with the interpretation of raman spectra by Suzuki and Mishima (Phys. Rev. Lett. 85, 1322-1325, 2000) and quantitatively reproduce the relative amounts of low and high density water as a function of solute content of their experiment. Our simulations predict dimensions of phase segregation of 4-5 nm that should be observed by SAXS of the glass. Our results yield further insight into the mechanism of growth of the low-density liquid domain and the relationship between the size of these domains and the nucleation of ice.
PHYS Poster Session - Water Mediated Interactions
7:30 PM-10:00 PM, Wednesday, August 20, 2008 Pennsylvania Convention Center -- Hall C, Poster