Multiscale Modeling Reveals Physical Interactions that Influence the Mechanical Behaviour of Polysaccharide-Surfactant Hydrogels

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Chitosan is a gel-forming polysaccharide biopolymer. The amine groups on chitosan monomers ionize with a pKa ∼6.5 which facilitates pH-responsive transition from an insoluble physically crosslinked hydrogel at basic pH, to a soluble polycationic state at acidic pH. However, addition of anionic surfactants such as sodium dodecyl sulfate (SDS) to cationic chitosan allows the formation of electrostatically linked hydrogels. In contrast to the elastic nature of chitosan gels at basic pH, these acid-stable SDS:chitosan gels have been shown to display viscoelasticity and self-healing properties. This tunable nature of chitosan makes it an attractive biomaterial for bioelectronics fabrication, and other biomedical applications. Here, we use a multiscale molecular modeling approach to identify the molecular interactions responsible for the orthogonal dependence of mechanical properties on pH and SDS concentration. We use coarse-grained molecular dynamics to self-assemble large, space-spanning SDS:chitosan networks. We then back-map these networks to atomistic resolution to further investigate the finer differences between their interaction profiles. Our findings show that water mediated contacts are the predominant crosslinking interaction at basic pH, but not at acidic pH. Instead, SDS-chitosan salt bridges and hydrogen bonds are the dominating crosslinks at acidic pH. Furthermore, direct chitosan-chitosan hydrogen-bonds appear to have a much smaller role in hydrogel structure than previously thought. These findings offer valuable insights into the fine molecular interactions that stabilize these polysaccharide-surfactant hydrogels. Taken together with the known mechanical behaviours of these hydrogels, our results offer directions for the rational design of chitosan materials from the bottom up.