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

Area of Science:

• Materials Science
• Chemical Engineering
• Thermodynamics

Background:

• Mixed gas hydrates, particularly those involving HFC-125a and CO2, are investigated as next-generation fire suppressants.
• These hydrates offer combined cooling and suppression capabilities, necessitating a deeper understanding of their formation and properties.

Purpose of the Study:

• To systematically investigate the formation behavior, yield, thermodynamics, and guest occupancy of HFC-125a + CO2 mixed hydrates.
• To elucidate the influence of guest-specific kinetics and cage affinity on hydrate structure and formation pathways.

Main Methods:

• Isochoric conditions were employed for hydrate formation studies.
• Pressure-temperature measurements were conducted to analyze thermodynamic characteristics.
• Raman spectroscopy was utilized to determine molecular-level guest occupancy and confirm hydrate structures.

Main Results:

• Mixed HFC-125a + CO2 hydrates exhibit distinct two-stage nucleation: rapid CO2-driven primary nucleation and lower-temperature secondary nucleation.
• The mixed system shows improved structural stability and controlled gas incorporation compared to pure HFC-125a, despite lower overall water conversion.
• Raman spectroscopy confirmed CO2 encapsulation in structure I (sI) and HFC-125a in structure II (sII) hydrates, indicating phase coexistence.

Conclusions:

• Guest-specific kinetics and cage affinity are key factors governing the formation pathways and resulting structures of mixed gas hydrates.
• The findings provide critical insights for tailoring mixed hydrate systems for efficient and controllable fire suppression applications.