DFT study of metal cation-induced hydrogelation of cellulose nanofibrils

We report a density functional theory study of cation-induced bonding between carboxylated cellulose nanofibrils (CNFs). We describe a methodology of using cleaved cellulose crystal unit cells to develop simple surface and molecular models of charged CNFs. We compare bond lengths, binding energies, and displaced solvation volumes for interfibril models intercalated with alkali, alkaline earth, main group, or transition metal cations, surrounded by an implicit solvent. We characterize the type of bonding interactions that occur between metal cations, Mn+ and carboxylated CNF surfaces by calculating the electronic density of states and Mayer bond orders. We find that Mn+-O interactions for alkaline earth metal systems are predominantly electrostatic whereas transition metal cations form stronger, more covalent bonds with enhanced valence orbital overlap. Our results show that multivalent-as opposed to monovalent-ions can create CNF networks by effectively crosslinking multiple fibrils through surface carboxylate anions. Our computational results agree with empirical models of metal-carboxylate binding, while also providing a deeper understanding of the bonding mechanisms for different cations. Our findings help to explain trends in recent CNF hydrogelation experiments, and we also predict the existence of two new hydrogels-CNF-Mg2+ and CNF-Zr4+.