Cellulose solutions and emulsions: From polymer physics to micro-bioreactors

Cellulose is the world’s oldest, most abundant and renewable polymer. It’s unique macromolecular and supramolecular structure was perfected by evolution, since the first cyanobacteria produced it for protective function to the structural function of higher plants and trees. Humankind has benefited from cellulose since early years for energy, construction and clothing. However, only a minute fraction of its annual natural production is utilized as raw material for fabrication of synthetic products, or as a source for biofuels or higher value chemicals, as cellulose processing requires harsh solvents or procedures considered to be detrimental to the environment and are increasingly regulated. In the last two decades there has been significant advance in finding new solvent systems for cellulose processing. Notable among these, ionic liquids (IL) gain attention as "green" solvents with unique properties such as stability, low toxicity and non-volatility. In this lecture I review some of the recent studies with my students and colleagues.

Using a combination of scattering methods, molecular dissolution of isolated cellulose chains as expanded-coils of highly persistent units is revealed, even when the IL is a minor component mixed with a polar organic liquid, itself a non-solvent. Dissolution is thus due to strong polymer-solvent interactions, leading to “tight-adsorption” of IL ions. Moreover, the dissolved cellulose chains exhibit an amphipilic nature, readily forming an encapsulation coating in oil-in-water emulsions. This leads to several potential applications such as phase-change materials or controlled release of essential oils.

The novel emulsions have a unique structure: the hydrophobic core is encapsulated by an inner hydrogel shell covered by an external coating of continuous amorphous cellulose. These unique emulsion microstructures can be utilized for efficient biochemical reactions. Thus we show simultaneous cellulose saccharification and fermentation by emulsion droplets integrated with celullases and yeast, that produce ethanol or butanol. Lipase assembled at the inner oil-hydrogel interface effectively catalyze transesterification reactions of heavy oil in the core. These results are promising steps towards “one-pot” processes for cellulose valorization by a cascade of biochemical reactions utilizing integrated cellulose-coated emulsion particles.