Structure-Dependent Formation of HFC-125a/CO₂ Mixed Hydrates: Raman Evidence and Implications for Fire Suppression

Mixed gas hydrates containing 1,1,1,2,2-pentafluoroethane (HFC-125a) and carbon dioxide (CO₂) have emerged as promising candidates for next-generation fire suppression due to their combined cooling and capabilities. In this study, the formation behavior, hydrate yield, thermodynamic characteristics, and molecular-level guest occupancy of HFC-125a + CO₂ (20-80 mol %) mixed hydrates were systematically investigated under isochoric conditions. Pressure-temperature measurements revealed distinct differences between pure and mixed systems, with the mixed hydrate exhibiting a two-stage nucleation behavior. Primary nucleation was associated with rapid CO₂-driven hydrate formation, subsequent structural reorganization, and gas incorporation, while secondary nucleation occurred at lower temperatures and pressures. Comparative hydrate yield analysis showed that although the mixed system exhibits a lower overall water-to-hydrate conversion than that of pure HFC-125a, it provides improved structural stability and controlled gas incorporation. Raman spectroscopic analysis confirmed the encapsulation of CO₂ predominantly in structure I (sI) hydrates and HFC-125a in structure II (sII) hydrates, supporting the coexistence of sI and sII phases in the mixed system. These findings demonstrate how guest-specific kinetics and cage affinity govern hydrate structure and formation pathways, providing critical insight into tailoring mixed hydrate systems toward efficient and controllable fire-suppression applications.