Grand canonical Monte Carlo simulations are performed in a hybrid adsorbent model in order to interpret the CO₂ solubility behavior. The hybrid adsorbent is prepared by confining a physical solvent (OMCTS) into the pores of a mimetic MCM-41 solid support. As a result, simulated adsorption isotherms of CO₂ nicely match the experimental data for three distinctive systems: bulk solvent, raw MCM-41, and hybrid MCM-41. The microscopic mechanisms underlying the apparition of enhanced solubility are then clearly identified. In fact, the presence of solvent molecules favors the layering of CO₂ molecules within the pores; therefore, the CO₂ solubility in the hybrid adsorbent markedly increases in comparison to that found in the raw adsorbent as well as in the bulk solvent. In addition, a good understanding of confined solvents’ properties and solid surface structures is essential to fully evaluate the efficiency of hybrid adsorbents in capturing CO₂. The sorbent–solid interactions along with the solvent molecular size’s impact on CO₂ solubility are therefore investigated in this study. We found that an ideal hybrid system should possess a weak solvent–solid interaction but a strong solvent–CO₂ interaction. Besides, an optimal solvent size is obtained for the enhanced CO₂ solubility in the hybrid system. According to the simulation results, the solvent layer builds pseudomicropores inside the mesoporous MCM-41, enabling more CO₂ molecules to be absorbed under the greater influence of spatial confinement and surface interaction. In addition, the molecular sieving effect is clearly observed in the case of larger solvent molecular sizes.