Formation of a nanobubble and its effect on the structural ordering of water in a CH4–N2–CO2–H2O mixture

Abstract

The replacement of methane (CH₄) from its hydrate by a mixture of nitrogen (N₂) and carbon dioxide (CO₂) involves the dissociation of methane hydrate leading to the formation of a CH₄–N₂–CO₂–H₂O mixture that can significantly influence the subsequent steps of the replacement process. In the present work, we study the evolution of dissolved gas molecules in this mixture by applying classical molecular dynamics simulations. Our study shows that a higher CO₂ : N₂ ratio in the mixture enhances the formation of nanobubbles composed of N₂, CH₄ and CO₂ molecules. To understand how the CO₂ : N₂ ratio affects nanobubble nucleation, the distribution of molecules in the bubble formed is examined. It is observed that unlike N₂ and CH₄, the density of CO₂ in the bubble reaches a maximum at the surface of the bubble. The accumulation of CO₂ molecules at the surface makes the bubble more stable by decreasing the excess pressure inside the bubble as well as surface tension at its interface with water. It is found that a frequent exchange of gas molecules takes place between the bubble and the surrounding liquid and an increase in concentration of CO₂ in the mixture leads to a decrease in the number of such exchanges. The effect of nanobubbles on the structural ordering of water molecules is examined by determining the number of water rings formed per unit volume in the mixture. The role of nanobubbles in water structuring is correlated to the dynamic nature of the bubble arising from the exchange of gas molecules between the bubble and the liquid.