A Classical Thermodynamic Model for Dispersed Nanophase Stability and Its Application for Investigating the Stability of Air Nanobubbles in Water

There is a growing need to understand the theoretical basis of nanophase stability to enable the control and manipulation of their properties. Bulk nanobubbles are examples of dispersed nanophases that show promising properties for clean, green, sustainable, and yield enhancement applications. In this work, a chemical equilibrium thermodynamic framework is presented to study the nanophase. This framework is used to prove the spontaneity of nanobubble formation and thereby its stability through the estimation of the corresponding Gibbs energy. Estimation of bubble pressure, temperature, and internal gas composition is also carried out for a typical air–water nanobubble system by employing this framework. The work shows that the curved nanointerface is critical for nanobubble stability and also exhibits unusual characteristics as it traps gas molecules at extremely high pressures, ∼1100 atm, when the bulk pressure is just 1 atm, while maintaining high surface area and reactivity. We were able to thermodynamically and mathematically show the existence of very strong interactions at the nanobubble interface, the nature of which is assumed to be primarily Coulombic, but how the Coulombic forces form extremely strong molecular traps is still unknown. The existence of these interfacial interactions, i.e., their origin and nature, have to be explored experimentally. However, there are no probes that can measure the charge present on the Stern layer around a nanobubble as well as pressure, temperature, and composition inside a nanobubble dispersed in a bulk fluid. Thus, the development of new characterization techniques or an investigation using molecular simulation in the future could validate the estimates of nanobubble pressure, temperature, and composition, as well as the interfacial interactions estimated in this work.