Ion-Modulated Ostwald Ripening Dynamics of Nitrogen Nanobubble Pairs

Understanding the stability of bulk nanobubbles, especially how electrolyte ions influence their coarsening dynamics, holds significant importance for widespread practical applications, but the fundamental microscopic mechanisms remain not fully understood and warrant further investigation. In this work, using machine-learning molecular dynamics simulations, we captured the Ostwald ripening of bulk nanobubbles, which is consistent with experimental observations, and systematically investigated its behavior under neutral, acidic, alkaline, and saline conditions. Notably, the ripening rates exhibit a system-dependent order: acidic > alkaline ≈ pure water > saline. Further analyses reveal that counterions critically regulate the orientational alignment and hydrogen bonding of interfacial water molecules and ions through hydration and electrostatic effects. Furthermore, in the saline system, the coupling of Na+ and Cl- ions leads to the enrichment of "lying-flat" ion pairs near the interface, forming a dense hydration layer. Based on how these interfacial structural alterations regulate gas diffusion to impact the bubble ripening rate, we established an ion-modulated microscopic mechanism grounded in the relative dominance of enthalpy and entropy. This framework, which incorporates specific-ion effects, provides a self-consistent explanation for the divergent ripening rates observed across various environments. Ultimately, these findings bridge atomistic interfacial restructuring with thermodynamic principles, providing a comprehensive physicochemical landscape for understanding ion-modulated nanobubble coarsening dynamics.