Experimental Observation of Mesoscopic Dynamic Fluctuations in Water-Alcohol Mixtures

Abstract

We applied a dynamic fluctuation measurement method to liquid water-methanol mixtures over a wide range of compositions and temperatures. This method, which we developed in recent years, combines ultrasonic and inelastic X-ray scattering techniques to detect mesoscopic-level fluctuations-that is, the degree of heterogeneity-in liquids using sound waves. The results showed that the strength of dynamic fluctuations increased as the alcohol composition decreased and as the temperature decreased. This trend was also observed in water-ethanol and water-glycerol mixtures. From these results, we concluded that the origin of these fluctuations lies in the low-temperature (supercooled) region of water alone, and that mixing with alcohol eliminates the dynamic fluctuations. This conclusion-that mixing substances reduces fluctuations or heterogeneity-may seem strange at first glance, but it is actually consistent with the framework of the liquid-liquid phase transition hypothesis, which explains the thermodynamic anomalies of water. It also aligns with the results of our recent measurements of the strength of dynamic fluctuations in water across a wide range of temperature and pressure conditions. It has long been known that various thermodynamic anomalies exist in water-alcohol mixtures at low compositions, and while there have been various discussions on this topic in the past, our current findings offer a new perspective on these discussions.