Comparative Adsorption of Phenol and p-Chlorophenol on a Chitosan–Cellobiose Dimer in an Aqueous Medium: A DFT Study of Hydrogen Bonding and Noncovalent Interactions

A comparative study was carried out using density functional theory of the adsorption of phenol and p-chlorophenol on two molecular models of biopolymers in aqueous medium: a chitosan dimer and cellobiose. Twelve adsorbent–adsorbate complexes with three initial orientations per system were optimized, and their structural, electronic, and non-covalent properties were analyzed using boundary orbitals, molecular electrostatic potential, NCI/RDG, and QTAIM. In all four systems, the most stable geometry corresponded to the anchoring of the contaminant hydroxyl group to an adsorbent hydroxyl group, identifying O–H···O as the guiding motif of molecular recognition. However, conformational selectivity was strongly dependent on the adsorbent and the aromatic substituent. For phenol, the alternative orientations were 2.7 and 21.2 kcal mol−1 in chitosan and 6.6 and 48.9 kcal mol−1 in cellobiose. For p-chlorophenol, chitosan showed a much more severe discrimination, with penalties of 43.6 and 46.44 kcal mol−1. In contrast, in cellobiose, the alternative orientations remained close to the minimum, with differences of 5.1 and 3.5 kcal mol−1. The effect of Cl was also reflected in the electron topology: PC increased from 3.2 × 10−2 to 6.34 × 10−2 a.u. in chitosan and from 3.2 × 10−2 to 4.2 × 10−2 a.u. in cellobiose, while |V|/G went from 3.6 to 7.5 in chitosan and from 3.00 to 3.1 in cellobiose. Overall, the results show that p-chlorophenol interacts more intensely and selectively with chitosan, whereas cellobiose favors a more flexible, less topologically differentiated adsorption. These results clarify how a para-chloro substituent reorganizes hydrogen-bond-driven adsorption on two biopolymer microenvironments with different functional heterogeneity.