Hypothesis: The incorporation of carbon dioxide into a sodium hydroxide solution containing cellulose may induce the formation of a transient cellulose carbonate intermediate, which readily hydrolyzes to carbonate ion, and this process is responsible for the instantaneous formation of loose cellulose aggregates. Simulations: We employed molecular dynamics simulations to gain insight into the role of carbon dioxide and related ions in the cellulose aggregation process. By performing free energy calculations using OPLS/AA force fields between cellulose chains at different ionic environments, we were able to gain additional information regarding the behavior and thermodynamics of the involved species and propose a potential mechanism for the aggregation of cellulose in these systems. Findings: Our hypothesis of the formation of an intermediate cellulose carbonate in the solution, which strongly favors carbon dioxide absorption and carbonate ion formation, is supported by the simulation results. These results suggest that the aggregation process is driven by entropy upon the introduction of carbonate ions into the system.