Nanophase segregation in supercooled aqueous solutions driven by water's polyamorphism.

PHYS 331

Valeria Molinero, and Ly Le. Department of Chemistry, University of Utah, 315 South 1400 East, Rm 2020, Salt Lake City, UT 84112
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

Division of Physical Chemistry

The 236th ACS National Meeting, Philadelphia, PA, August 17-21, 2008