Controllable preparation and formation mechanism of monodispersed bulk nanobubbles in dilute ethanol-water solutions

Unraveling the Surface Activity of Ethanol-Water Mixtures through Experiments and Molecular Dynamics Simulations

Ethanol's complete miscibility in water makes it a widely used solvent in various applications, such as organic compound synthesis, paint manufacture, chromatography, and cosmetics preservation. Studies suggest that ethanol's concentration at interfaces can be higher than in the bulk due to its amphiphilic nature, especially at lower concentrations, making it a surface-active agent. Accordingly, ethanol plays a crucial role in controlling the emulsion stability, foam formation, heat transfer, and coating adhesion. However, the precise concentration ranges up to which ethanol's surface activity dominates its interfacial properties, and the underlying molecular mechanism is not fully understood in the literature. In this context, our foamability experiments, coupled with film stability experiments conducted via ethanol drop impact on varying concentration ethanol-water mixture pools, indicate that the surface-active nature of ethanol is observed up to a maximum of 10% molar ethanol concentration in water. We next employ all-atom molecular dynamics simulations to reveal that the surface tension and other interfacial properties are most significantly affected only up to the molar concentration in the range of 0-10% of ethanol in water. This observation is further supported by free energy analyses, indicating that the stabilization free energy of an ethanol molecule at the interface becomes comparable to that in the bulk region beyond this concentration range. The transition from surface-active to a behavior resembling a homogeneous solution occurs when the molar concentration of ethanol in water exceeds 10%. This transition is attributed to distinctive alterations in the number and strength of ethanol-water hydrogen bonds. These findings provide valuable insights into the interfacial molecular structure, which can be suitably exploited to modulate interfacial properties and dynamic behavior in a wide array of industrial and scientific applications.

Polarizing Perspectives: Ion- and Dipole-Induced Dipole Interactions Dictate Bulk Nanobubble Stability

The origin of the stability of bulk Nanobubbles (NBs) has been the object of scrutiny in recent years. The interplay between the surface charge on the NBs and the Laplace pressure resulting from the surface tension at the solvent-NB interface has often been evoked to explain the stability of the dispersed NBs. While the Laplace pressure is well understood in the community, the nature of the surface charge on the NBs has remained obscure. In this work, we aim to show that the solvent and the present ions can effectively polarize the NB surface by inducing a dipole moment, which in turn controls the NB stability. We show that the polarizability of the dispersed gas and the polarity of the dispersing solvent control the dipole-induced dipole interactions between the solvent and the NBs, and that, in turn, determines their stability in solution.

Synergism of Surfactant Mixture in Lowering Vapor-Liquid Interfacial Tension

Through employing molecular dynamics, in this work, we study how a two-component surfactant mixture cooperatively reduces the interfacial tension of a flat vapor-liquid interface. Our simulation results show that in the presence of a given insoluble surfactant, adding a secondary surfactant would either further reduce interfacial tension, indicating a positive synergistic effect, or increase the interfacial tension instead, indicating a negative synergistic effect. The synergism of the surfactant mixture in lowering surface tension is found to depend strongly on the structure complementary effect between different surfactant components. The synergistic mechanisms are then interpreted with minimization of the bending free energy of the composite surfactant monolayer via cooperatively changing the monolayer spontaneous curvature. By roughly describing the monolayer spontaneous curvature with the balanced distribution of surfactant heads and tails, we confirm that the positive synergistic effect in lowering surface tension is featured with the increasingly symmetric head-tail distributions, while the negative synergistic effect is featured with the increasingly asymmetric head-tail distributions. Furthermore, our simulation results indicate that minimal interfacial tension can only be observed when the spontaneous curvature is nearly zero.

Fundamental Investigation on a Foam-Generating Microorganism and Its Potential for Mobility Reduction in High-Permeability Flow Channels

This study proposed a novel foam EOR technique using Pseudomonas aeruginosa to generate the foam and investigated the potential of the microbial foam EOR to modify the permeability of a high-permeability porous system. We investigated oxygen nanobubble, carbon dioxide nanobubble and ferrous sulfate concentrations to discover the optimal levels for activating the foam generation of the microorganism through cultivation experiments. We also clarified the behavior of the microbial foam generation and the bioproducts that contribute to the foam generation. The potential of the foam to decrease the permeability of high-permeability porous systems was evaluated through flooding experiments using sand pack cores. The foam generation became more active with the increase in the number of nanobubbles, while there was an optimal concentration of ferrous sulfate for foam generation. The foam was identified as being induced by the proteins produced by the microorganism, which can be expected to bring about several advantages over surfactant-induced foam. The foam successfully decreased the permeability of high-permeability sand pack cores to half of their initial levels. These results demonstrate that the microbial foam EOR has the potential to decrease the permeability of high-permeability porous systems and improve the permeability heterogeneity in oil reservoirs.

Stability and Free Radical Production for CO2 and H2 in Air Nanobubbles in Ethanol Aqueous Solution

In this study, 8% hydrogen (H₂) in argon (Ar) and carbon dioxide (CO₂) gas nanobubbles was produced at 10, 30, and 50 vol.% of ethanol aqueous solution by the high-speed agitation method with gas. They became stable for a long period (for instance, 20 days), having a high negative zeta potential (−40 to −50 mV) at alkaline near pH 9, especially for 10 vol.% of ethanol aqueous solution. The extended Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory was used to evaluate the nanobubble stability. When the nanobubble in ethanol alkaline aqueous solution changed to an acidic pH of around 5, the zeta potential of nanobubbles was almost zero and the decrease in the number of nanobubbles was identified by the particle trajectory method (Nano site). The collapsed nanobubbles at zero charge were detected thanks to the presence of few free radicals using G-CYPMPO spin trap reagent in electron spin resonance (ESR) spectroscopy. The free radicals produced were superoxide anions at collapsed 8%H₂ in Ar nanobubbles and hydroxyl radicals at collapsed CO₂ nanobubbles. On the other hand, the collapse of mixed CO₂ and H₂ in Ar nanobubble showed no free radicals. The possible presence of long-term stable nanobubbles and the absence of free radicals for mixed H₂ and CO₂ nanobubble would be useful to understand the beverage quality.