Effect of Pressure, Temperature, and NaCl on Formation and Coarsening of Nanobubbles in Aqueous Methane System

Studying the effect of various factors on the growth of nanobubbles (NBs) is crucial for understanding their role in applications such as water treatment and mineral separation and in processes such as natural gas extraction, given that increasing size reduces the residence time of NBs in liquids. Tracking the evolution of NBs is challenged by their high diffusivity and exchange of gas molecules between bubbles, which makes it difficult to assign a persistent identity to any single bubble within a cluster. To overcome this, we present a new approach that involves molecular dynamics simulations and identification and ranking of NBs present based on their size. This approach is applied to study NB formation and coarsening in CH₄ + H₂O system, which emulates the liquid phase formed during methane hydrate dissociation. NBs formed in this system are identified by their ranks, and the growth of the largest bubble is examined, as mass transfer during coarsening primarily occurs toward this bubble. This provides estimates of the contributions of dissolved gas absorption and that of coarsening toward NB growth. The effect of temperature (300-330 K), pressure (40-300 bar), and mole fraction (<i>X</i>_NaCl = 0-0.10) of NaCl, an important hydrate inhibitor, on methane NB evolution is examined. The study reveals that coarsening becomes more prominent with an increase in salinity or temperature. In contrast, the contribution of coarsening decreases progressively as pressure increases. Examining the contribution of coalescence and Ostwald ripening toward NB coarsening showed that an increase in temperature or <i>X</i>_NaCl result in an initial increase in Ostwald ripening, which decreases with further increase in these parameters. The observations are explained based on the number of NB nuclei formed and dynamic exchange of gas between the bubble and liquid. This work provides insight into methane NB growth in hydrate melts and presents a new general approach to study coarsening in NB clusters.