Experimental observation of mesoscopic dynamic fluctuations in water–alcohol mixtures

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. As a result, it was found that dynamic fluctuations exist across a wide range of compositions and temperatures, including pure substances (water and methanol). Furthermore, in the low-methanol composition range (∼20% mol or less), the strength of dynamic fluctuation increased toward lower compositions and lower temperatures, suggesting that the origin of the fluctuation lies in the low-temperature (supercooled) region of water alone. Conversely, this indicates that in the range below 20% methanol, fluctuations in water alone are dominant rather than the mixed effects of water and methanol. We performed similar dynamic fluctuation measurements on water-ethanol and water-glycerol mixtures at room temperature. We found that dynamic fluctuations exist universally across the entire composition range and confirmed an increase in the strength of dynamic fluctuations toward lower compositions in the regions approximately below 15% and below 20%, respectively. This revealed that the phenomenon is not unique to methanol systems but is a common behavior shared by water-alcohol mixtures. It has long been known that water-alcohol mixtures exhibit various thermodynamic anomalies, but the mechanisms behind them have not yet been fully elucidated. The present results are expected to provide a new direction for discussion on this issue.