Confined phase behavior of subcritical carbon dioxide in nanoporous media: the effects of pore size and temperature

This study investigates the effect of confinement on the phase behavior of carbon dioxide (CO₂) and its implications for storage in nanometer-scale pores. A patented gravimetric apparatus was employed to experimentally measure the adsorption and desorption isotherms at varying pore sizes and temperatures. The isotherms were generated at temperatures below the critical point of CO₂ (from -23.1 to 20 °C) using mesoporous material MCM-41 with pore sizes of 6, 8, 10, and 12 nm. The capillary condensation and bulk saturation pressures were measured from the adsorption isotherms for each pore size and temperature. Meanwhile, the evaporation pressures under confinement were determined from the desorption branches. The experimentally measured bulk saturation and dew point pressures were successfully compared against the NIST data, confirming the accuracy of measurements. All isotherms showed reversible behavior, exhibiting no adsorption-desorption hysteresis, indicating that all temperatures studied here were above the hysteresis critical temperature. For all pore sizes, the amount of CO₂ adsorbed in confined spaces decreased with ascending temperatures. Furthermore, the CO₂ uptake during capillary condensation showed an inverse correlation with the pore size; smaller pores adsorbed more CO₂ due to the higher interaction strength with pore walls than larger counterparts. Furthermore, the results provide an in-depth understanding of the effect of pore size and temperature on the equilibrium behavior of CO₂ molecules in confined and bulk spaces of porous media. The results from the present study can significantly aid the storage applications of CO₂ in a wide range of natural and synthetic porous materials.