Thermal Properties of the H₂O–CO₂–Na₂CO₃/CH₃OH/NH₃ Systems at Low Temperatures and Pressures up to 50 MPa

Victoria Muñoz-Iglesias and Olga Prieto-Ballesteros. Thermal Properties of the H₂O–CO₂–Na₂CO₃/CH₃OH/NH₃ Systems at Low Temperatures and Pressures up to 50 MPa. ACS ACS Earth Space Chem. 2021, 5, 10, 2626–2637. DOI: 10.1021/acsearthspacechem.1c00066

Experimental data on the thermal properties of aqueous solutions at low temperatures and high pressures are necessary to perform an accurate geochemical modeling of ocean worlds, that is, those planetary bodies that can sustain significant reservoirs of the interior liquid for long time periods. We used differential scanning calorimetry (DSC) to determine the values of specific heat capacity and enthalpy of dissociation of CO₂-clathrates in the presence of sodium carbonate, ammonia, and methanol at temperatures ≥233 K and pressures ≤50 MPa. We monitored the physicochemical evolution of the systems along the pressure–temperature paths through Raman spectroscopy. The protocol used to form the CO₂-clathrates influenced the thermal behavior of the system. Partial recrystallization events from ice to CO₂-clathrate during slow heating resulted in out-of-equilibrium heat capacities (C_p) of < 1 J g^–1 K^–1. After complete melting of the solids, the final aqueous solutions had C_p 2–3 J g^–1 K^–1. Final fluids richer in dissolved CO₂ had a lower C_p. The addition of carbonates and methanol led to a decrease in the melting temperature of both ice and clathrates. Ammonia reacted with CO₂ to rapidly form ammonium bicarbonate; however, ice and CO₂-clathrates could also stabilize in parallel. In all the studied systems, the formation of clathrates from aqueous solutions without the application of clathrate stabilizers, mechanical agitation, or synthesis from crushed ice led to a low guest occupancy, forming less-stable clathrates with low enthalpies of dissociation (130–275 J g^–1). These results have important planetary implications related to the thermal behavior of bodies rich in the compounds of the systems studied, as can be the case of Enceladus. In this research, we show that the level of cage occupancy in clathrates plays a key role in their thermal behavior. Highly occupied clathrates will be robust and will contribute to the retention of heat inside the planetary bodies due to their characteristic low thermal conductivity. However, poorly occupied clathrates will dissociate more easily than water ice, allowing greater heat fluxes through the icy crust and favoring the dissipation of heat to the exterior.