Interfacial Structure and Reactions of Nanobubbles in Pure Ethanol

Bulk nanobubbles (NBs) exist as stable microbubbles with high surface charge density and long lifetimes in pure water. However, the behavior of NBs in solvents other than water remains largely unexplored. While NBs in electrolytes have been studied, their properties in organic solvents have received little attention. In this study, we investigated the behavior of NBs in ethanol using in situ infrared absorption spectroscopy (IRAS) and nanoparticle tracking analysis (NTA) to examine their stability and interfacial chemical states. The NTA results revealed that NBs encapsulating oxygen or nitrogen remain relatively stable in ethanol, whereas NBs encapsulating CO₂ exhibit a slightly fragile interface and expel the gas from the NBs. The IRAS data suggested the formation of ethanol molecular clusters at the NB interface. The formation of ethyl acetate was observed in suspensions of NBs containing oxygen and nitrogen. Based on these findings, we propose an interfacial structure model in which the NB interface forms a two-dimensional layer of alcohol molecular clusters, and the stability of the interface is maintained by the dipole moments of these clusters pointing inward. This interfacial layer may generate a local potential and strong electric field, which could facilitate the formation of ethyl acetate at the NB interface under the observed conditions.