Observation of mesoscopic clathrate structures in ethanol-water mixtures

Ing-Shouh Hwang^1*, Wei-Hao Hsu¹, Tzu-Chieh Yen¹, Chien-Chun Chen¹, Chih-Wen Yang¹, Chung-Kai Fang¹

¹Institute of Physics, Academia Sinica, Taipei, Taiwan

* Presenter:Ing-Shouh Hwang, email:ishwang@phys.sinica.edu.tw

Alcohol in water is one of the fundamental liquid–liquid solutions in chemical and biological processes. Both water and alcohol (monohydric alcohols such as methanol and ethanol) are liquid and highly miscible with each other under ambient conditions. Interestingly, many abnormal physicochemical properties of the mixtures (relative to ideal solutions) have puzzled researchers for decades [1]; anomalies are prominent in the low-alcohol concentration regime in many cases. If two liquids are miscible and form a homogeneous mixture, then in principle the total entropy of the mixture should be higher than that of the pure components prior to mixing; in reality, water-alcohol mixtures exhibit large negative excess entropy [1]. It is well known that pure liquid water forms a fluctuating three-dimensional network of hydrogen bonds, suggesting that mixing a small percentage of alcohol molecules into water would lead to endothermic destruction of water-water hydrogen bonds. Surprisingly, mixing water with alcohol leads to excessive heat evolution [1]. In addition, the presence of a high concentration of mesoscale objects (detected with light scattering methods) in such mixtures has caused debate. Many researchers attribute these objects to bulk nanobubbles, nanoscale gas bubbles in bulk water, because their concentration is strongly affected by the amount of dissolved air gases. However, other experimental findings challenge this interpretation, suggesting that the observed structures may not be gas bubbles at all, and many researchers attribute the light-scattering nano-objects in water-alcohol mixtures to nanoscale segregation of hydrophobic contaminants. To resolve these issues, we use transmission electron microscopy (TEM), nanoparticle tracking analysis (NTA), and atomic force microscopy (AFM) to study ethanol–water (EW) mixtures. TEM reveals a special type of mesoscopic clathrate hydrate structures with water molecules forming a crystalline matrix hosting a high density of tiny cells [2]. The presence of these mesoscopic clathrate structures is further supported by a refractive index of 1.27 ± 0.02 at 405 nm measured via NTA and the hydrophilic nature of the mesoscopic structures implied by AFM observations. Formation of the low-energy, low-entropy mesoscopic clathrate hydrate structures in EW mixtures explains the previously reported negative enthalpy, appreciable entropy loss, conflicting nanobubble data, strengthening of hydrogen bonds, and many other experimental observations related to water-alcohol mixtures.

[1] F. Franks, D.J.G. Ives, The structural properties of alcohol–water mixtures, Q. Rev. Chem. Soc. 20 (1), 1–44 (1966).
[2] W.-H. Hsu, T.-C. Yen, C.-C. Chen, C.-W. Yang, C.-K. Fang, and I.-S. Hwang*, “Observation of mesoscopic clathrate structures in ethanol-water mixtures”, Journal of Molecular Liquids 366, 120299 (2022).

Keywords: alcohol-water mixtures, nanostructures, transmission electron microscopy, clathrate hydrate structures