Mesoscopic Inhomogeneities in Ethanol-Water Mixtures: Are They Nanobubbles, Impurity Aggregates, or Nanoscale Gas-Water Composite Structures?

The study of bulk nanobubbles is a rapidly expanding field, and mixing water with alcohol has been proposed as a simple method for generating such structures. However, previous light-scattering investigations of alcohol-water mixtures have produced inconsistent and often contradictory results. In this work, we employed static light scattering (SLS), dynamic light scattering (DLS), nanoparticle tracking analysis (NTA), and transmission electron microscopy (TEM) to examine ethanol-water mixtures prepared from two distinct ethanol sources. Our findings indicate that the strongly light-scattering objects observed in these mixtures originate from impurities in the ethanol. These impurities could not be removed by degassing alone but were effectively eliminated through ethanol purification methods such as distillation. NTA measurements revealed high concentrations of dim, colloidal-like particles in mixtures prepared with high-purity ethanol. These particles could be removed through vacuum degassing, suggesting that their presence is linked to gas supersaturation. Based on our observations, we propose that the impurities likely originate from additive molecules leached from plastic containers used to store ethanol. Upon mixing with water, these molecules may aggregate into nanoparticles, contributing to the strong scattering signals. In contrast, the dim colloidal-like particles appear to be gas-containing nano-objects, most likely mesoscopic clathrate hydrate structures, as confirmed by TEM imaging. Our results provide a coherent explanation that reconciles the seemingly contradictory findings in the literature.