Salinity and Thermal Effects on CO ₂ Hydrate Formation and Distribution Probed by Raman and Optical Microscopy in Micro-capillaries

Clathrate hydrates of CO₂ were synthesized in NaCl solutions (0-15 wt %) and investigated using micro-Raman spectroscopy and optical microscopy in newly developed high-pressure microcapillaries. This combined approach provides spatially resolved insights into hydrate nucleation, growth, and phase distribution under controlled salinity and thermal conditions, with direct relevance for hydrate-based desalination and CO₂ capture. Spectral markers from the CO₂ Fermi dyad (ν^-, ν⁺) and O-H stretching region enabled clear discrimination between hydrate-rich domains, hydrate-brine slurries, and residual aqueous phases. In particular, the position and full width at half-maximum of the ν^- component were shown to be robust indicators for distinguishing dissolved CO₂, liquid CO₂, enclathrated CO₂, and mixed-phase environments. Regions where hydrates coexist with liquid CO₂ or aqueous solution exhibit intermediate spectral signatures that differ unambiguously from those of the corresponding pure phases. Two thermal sequences were examined following primary hydrate formation below 246 K: (i) heating to (near-) dissociation and (ii) cooling under moderate to strong subcooling. Dissociation temperatures decreased systematically with salinity, from ∼279 K in pure water to ∼273 K at 15 wt % NaCl, consistent with thermodynamic inhibition. Pure water exhibited relatively homogeneous hydrate coverage, whereas saline systems developed pronounced interfacial crystallization and heterogeneous hydrate-brine coexistence. During secondary formation, moderate subcooling (∼4 K) favored hydrate reconversion at the gas-liquid interfaces and on residual seed crystals, while deeper subcooling (≥12 K) increased hydrate coverage but amplified salinity-dependent heterogeneities. The interplay between seed availability, hydrate memory effects, salt-induced inhibition, and gas-bubble dynamics was found to govern nucleation pathways and spatial patterns of growth. These results establish a mechanistic link between microscale hydrate distributions and macroscopic formation kinetics, providing a foundation for optimizing hydrate-based water recovery and carbon management processes.