Viscosity of Freeze-Concentrated Solution Confined in Micro/Nanospace Surrounded by Ice

An aqueous solution separates into ice and a freeze-concentrated solution (FCS) when frozen at temperatures above the eutectic point. The FCS acts as important reaction media in natural environment and industrial processes. The viscosities of the FCS in frozen glycerol/water solutions are evaluated by two spectrometric methods with different principles: (1) the reaction rate of the diffusion controlled emission quenching and (2) fluorescence correlation spectroscopy. Thermodynamics indicates that the concentration of glycerol in the FCS is constant at a constant temperature regardless of the glycerol concentration in the original solution before freezing (SI). However, the viscosity of the FCS measured at a given temperature increases with decreasing and this trend becomes more pronounced with decreasing measurement temperature. Further, the viscosity of the FCS in a rapidly frozen solution is higher than that in a slowly frozen solution. These results suggest, that the viscosity of the FCS depends on the size of the space in which the FCS is confined and is enhanced in smaller spaces. This result agrees well with several reports of anomalous phenomena in a microspace confined in ice. These phenomena should originate from the fluctuation of the ice/FCS interface, which is macroscopically stable but microscopically dynamic and undergoes continuous freezing and thawing. Thus, the FCS near the interface has ice-like physicochemical properties and structures, giving higher viscosity than the corresponding bulk solution.