A study on a clathrate-like transition for carbon dioxide + propane + water via molecular dynamics simulations

The nucleation process of mixed clathrate hydrates, specifically carbon dioxide (CO₂) and propane (C₃H₈) hydrates, has not been the subject of great focus in the literature, even though this system provides potential for applications such as CO₂ capture and water desalinization. In this study, we present extensive molecular dynamics simulations of mixed CO₂/C₃H₈ at different guest ratios (1 : 2, 1 : 1, and 2 : 1) in the water phase to understand the influence of guest composition in the cage types, occupation, connectivity, and transitions in the pre-nucleation, nucleation and growth, and annealing stages to a clathrate-like phase. A total of 137 μs of trajectories is evaluated. Even though only amorphous solids are obtained, insights are proposed for the dominant cages (5¹², 4¹5¹⁰6², 5¹²6³, and 4¹5¹⁰6⁴) and for the probabilities of cage transitions. 4¹5¹⁰6⁴ and especially 5¹²6³ cages are found to be crucial for the nucleation of higher C₃H₈ composition systems, while 5¹² and 5¹²6⁴ cages are essential for higher CO₂ composition systems. A few sI and sII domains are detected, and we report the direct connection between some of them without 5¹²6³ cages as a link. A map of cage transition possibilities and probabilities is presented to demonstrate the preference of CO₂ molecules for 4¹5¹⁰6² cages instead of 5¹²6² cages.