Carbon Dots Combined with Zwitterionic/Nonionic Surfactants for Stabilizing Carbon Dioxide Micronanobubbles under High-Salinity Conditions

Carbon dioxide micronanobubbles (CO₂ MNBs) hold potential for enhancing gas utilization efficiency and mass transfer performance in fluids related to carbon capture, utilization, and storage (CCUS). However, their stability is significantly constrained in high-salinity environments due to pronounced coalescence and aging. To address this issue, this study proposes an interfacial synergistic regulation strategy based on a composite system of carbon dots (CDs) and zwitterionic/nonionic surfactants to stabilize CO₂ micronanobubbles. Research indicates that carbon dots themselves lack surface activity but can form composite structures with surfactants through hydrogen bonding and electrostatic interactions. This leads to synergistic adsorption at the gas-liquid interface, significantly reducing the minimum surface tension from 21.54 to 10.71 mN·m⁻¹. The established semiquantitative adsorption model characterizes the nonlinear response of interfacial tension, revealing a competitive relationship between synergistic adsorption and bulk phase dissipation. The formation of the composite interfacial layer effectively suppresses gas diffusion and Ostwald ripening, significantly enhancing the size stability of micronanobubbles. Furthermore, in high-salinity systems enriched with Ca²⁺, carbon dots exhibit pronounced interfacial charge buffering, effectively preventing charge reversal and maintaining bubbles at a stable diameter below 600 nm for 24 h. This study elucidates the mechanism by which carbon dots stabilize CO₂ micronanobubbles through interfacial regulation, providing scientific rationale for designing low-cost, salt-tolerant CO₂ enhanced oil recovery microbubble fluid systems.