Abstract
The remarkable efficiency of chemical reactions is the result of biological evolution, often involving confined water. Meanwhile, developments of bio-inspired systems, which exploit the potential of such water, have been so far rather complex and cumbersome. Here we show that surface-confined water, inherently present in widely abundant and renewable cellulosic fibres can be utilised as nanomedium to endow a singular chemical reactivity. Compared to surface acetylation in the dry state, confined water increases the reaction rate and efficiency by 8 times and 30%, respectively. Moreover, confined water enables control over chemical accessibility of selected hydroxyl groups through the extent of hydration, allowing regioselective reactions, a major challenge in cellulose modification. The reactions mediated by surface-confined water are sustainable and largely outperform those occurring in organic solvents in terms of efficiency and environmental compatibility. Our results demonstrate the unexploited potential of water bound to cellulosic nanostructures in surface esterifications, which can be extended to a wide range of other nanoporous polymeric structures and reactions.
Highlights
The remarkable efficiency of chemical reactions is the result of biological evolution, often involving confined water
Cellulose[24], from the molecular viewpoint, has a rather simple chemical structure based on β-O1,4-linked glucopyranose repeating units that are assembled into rigid nanofibres, i.e. elementary fibrils, as the smallest subunits (Fig. 1)
Non-freezing water is directly situated at the nanofibre surface, whereas freezing water accumulates in interfibrillar pores[32]
Summary
The remarkable efficiency of chemical reactions is the result of biological evolution, often involving confined water. Current limitations are the restrictions to small molecules as reactants, the use of organic solvents and the rather complex design of these reaction systems Expanding this concept to naturally occurring and sustainable materials, which confine water due to their intrinsic native structure, would be a major contribution to the field. Nature’s best polymeric fibres are essential structural components in biological tissues: keratin[18], collagen[19] and fibroin[20,21] as examples of protein fibres, along with cellulose[22] and chitin[23], as representatives of structured polysaccharide All these materials are based on nano-scaled assemblies, covered by surface-confined water under ambient conditions. We hope to induce a paradigm shift in materials chemistry by exposing the potential of confined water to promote surface reactions of nanoporous polymeric structures This principle is demonstrated here using as example the acetylation of cellulose fibres, as representative nanoporous systems[28]
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