(9j) Interactions of Cellulosic Oligomers with Different Crystallographic Surfaces of Cellulose Nanocrystals through Molecular Simulation

With added cellulosic polymers, cellulose nanocrystal (CNC) systems exhibit a plethora of enhanced properties due to non-trivial polymer-CNC surface interactions. Some of the properties include enhanced gelation, foam formation, etc. - In the presented work, we studied the interaction of two cellulosic oligomers namely, methylcellulose (MC) and cellulose or pentase (PE), with three crystallographic surfaces of CNC namely, (100), (110), and (110s) (where “s” indicates the surface is sulphated) in the presence of explicit water. Free energy of adsorption was calculated with umbrella sampling (US) combined with the COWBOE algorithm which we previously developed for optimizing the US parameter settings. The COWBOE algorithm was previously shown to give accurate potential of mean force (PMF) results with substantially lower computational time compared with the conventional approach. Extending this new method to wider polymer-interface combinations, as reported in this study, demonstrates its general applicability. We identified that the affinity of oligomers towards different surfaces ranks in the order of (100) > (110) > (110s). PE exhibited higher affinity towards (100) and (110s) surfaces compared with MC while both MC and PE displayed similar affinity towards the (110) surface. Detailed analysis of the oligomer-surface interactions, oligomer-solvent interactions, oligomer configuration and dynamics near the surface, and solvent distribution, etc., was performed to unveil the origin of the observed PMF differences in different oligomer-surface pairs. This investigation sheds light on the nature of the polymer-CNC interactions that are critical to the unusual property enhancement in those systems.