Experimental estimation of the location of liquid-liquid critical point for polyol aqueous solutions.

Abstract

To solve a mystery of low-temperature liquid water, a liquid-liquid critical point (LLCP) hypothesis that the two kinds of waters, low-density and high-density liquids (LDL and HDL), and a critical point relating to the two waters exist is thought to be the most realistic idea. However, there is no conclusive evidence showing the existence of LLCP. I measured the polyamorphic volume changes of the glassy dilute polyol (ethylene glycol, glycerol, meso-erythritol, xylitol, and D-sorbitol) aqueous solutions during the compression and decompression processes and estimated the location of LLCP for the polyol aqueous solution by a new analysis of the concentration dependence of polyamorphic transition. The LLCP of glycerol aqueous solution around 150 K is estimated to be around 0.045 GPa and around 0.135 molar fraction. This indicates that the solvent water in the glycerol aqueous solution at 1 atm changes continuously from the LDL-like state to the HDL-like state with the increase of solute concentration. The concentration region in which the crossover between LDL-like solvent water and HDL-like solvent water occurs is located near the region that the liquid-liquid transition line is extended to the concentration axis at 1 atm. Moreover, the formation of LDL-like solvent water relates deeply to the homogeneous nucleation of ice Ih in the polyol aqueous solution. This conclusion shows that the LLCP hypothesis of water has an important implication for understanding the dynamics of aqueous solution such as solubility, hydration, segregation, aggregation of solute, nucleation of ice Ih, glass formation, glass transition, and so on.