Self-preservation ice layer mechanisms in mixed CO_2-CH_4 gas hydrate systems: Implications for icy planetary environments

Gas hydrates can develop self-preserving ice layers when pre-existing hydrate phases are exposed to near-freezing conditions outside their thermodynamic stability field. We investigated the energetic feasibility and equilibrium thickness of such coatings on mixed CO_2-CH_4 hydrates under conditions approaching hydrate quadruple-point regimes and assessed their implications for the buoyancy of micron-scale clusters in icy ocean environments. We modelled multi-layer hydrate–ice–water systems using Casimir--Lifshitz theory combined with experimental and ab initio dielectric data. Casimir--Lifshitz interaction energies were calculated for hydrate cores with gas-depleted surface regions, and equilibrium ice-layer thicknesses were obtained through Gibbs free-energy minimisation. Cage occupancy was treated parametrically to explore near-surface non-equilibrium states rather than thermodynamic stability domains. The model predicts energetically favoured self-preserving ice layers for CH_4-rich compositions or reduced gas occupancy. These coatings can significantly modify effective density and enable flotation of micron-scale hydrate clusters even at relatively high CO_2 fractions, depending on cluster size, composition, and ice-layer thickness. Self-preserving ice layers sustained by dispersion forces may influence buoyancy and volatile redistribution under near-freezing metastable conditions relevant to some icy ocean environments. The results provide a physically constrained framework for equilibrium ice-layer formation. However, kinetic stability and long-term survival were beyond the scope of this study.