ABSTRACT
In this work, we identify and characterize a new intriguing capability of carboxylated cellulose nanofibrils that could be exploited to design smart nanomaterials with tuned response properties for specific applications. Cellulose nanofibrils undergo a multivalent counter-ion induced re-entrant behavior at a specific multivalent metal salt concentration. This effect is manifested as an abrupt increase in the strength of the hydrogel that returns upon a further increment of salt concentration. We systematically study this phenomenon using dynamic light scattering, small-angle X-ray scattering, and molecular dynamics simulations based on a reactive force field. We find that the transitions in the nanofibril microstructure are mainly because of the perturbing actions of multivalent metal ions that induce conformational changes of the nanocellulosic chains and thus new packing arrangements. These new aggregation states also cause changes in the thermal and mechanical properties as well as wettability of the resulting films, upon water evaporation. Our results provide guidelines for the fabrication of cellulose-based films with variable properties by the simple addition of multivalent ions.
ABSTRACT
Low-concentration oil-in-water emulsions stabilized by cellulose nanofibrils (CNFs) extracted from primary plant cell wall materials are used to prepare thin biobased CNF-oil composite films by solvent casting. Flexible, transparent, and biodegradable composite films are obtained, with increased thermal stability (up to 300 °C) as the oil concentration increases. Examination of the microstructure demonstrates a clear dependency on the oil content, as a multilayered structure where the oil phase trapped between two layers of CNFs is appreciated at high oil concentrations. The embedded oil significantly influences the mechanical and wetting properties of the films, confirming their potential for use in packaging systems. Encapsulation of curcumin in the composite films leads to an increased antioxidant (up to 30% radical scavenging activity) and antimicrobial activity, inhibiting the growth of foodborne bacteria such as Escherichia coli. The resulting composite films show promising results in the field of active packaging for applications in the food, pharmaceutical, and cosmetic industries.
ABSTRACT
It is long known that oil-in-water emulsions can be stable against coalescence in homogenized plant cell wall dispersions because of the presence of surface-active biopolymers. When plant cell wall material is homogenized to the extent of deagglomeration of the cellulose microfibrils (CMFs), a much more complex dispersed system is obtained. Here we show that in such complex systems both surface active soluble polymers and individual CMFs are at the origin of this stabilization against coalescence, as they form a shell around the oil droplets providing Pickering-like stabilization. Individual CMFs and bundles of them in the presence of soluble biopolymers form a hybrid network in the continuous phase linking the droplets, creating a viscoelastic network that prevents the droplets from coalescing. Depletion induced attraction caused by soluble biopolymers and dispersed CMFs induces the formation of oil droplet clusters at low CMF concentrations leading to a highly heterogeneous distribution of oil droplets. This effect diminishes at high CMF concentrations at which the strong viscoelastic network arrests the droplets. These findings are important steps toward controlling complex dispersed systems comprising CMF-polymers mixtures with a second liquid or solid dispersed phase.
