Pallab Kumar Borah, Vlad Dinu, Amir H Irani, Benjamin Westberry, Joshua E S J Reid, Stefan Baier, and Gleb E Yakubov
University of Nottingham, UK
The design of structured and functional food materials relies on our ability to tune polysaccharide chain conformations, internal length scales, and self-association, which are intrinsically coupled with solvent properties. In this study, we utilise pectin as a flexible high-molecular weight (Mw, 3 × 106 g mol-1) model polysaccharide to study the role of solvent effects of aqueous glycerol on polysaccharide chain conformation and excluded volume-to-hydrodynamic interactions in governing macroscopic rheological behaviour. Steady-state shear and extensional rheology reveal that the viscosity of the polysaccharide scales with increasing glycerol exchange in water, and , respectively. Upon relating the viscosity with shear rate, a crossing over in the normal stress difference from -2/3 to -1/3 power-law scaling is observed, indicating a conformational change from coil to rod-like chains. Intrinsic viscosity results supported the observed spatial extension and contraction of polysaccharide chains. Complementary computational single polysaccharide chain molecular dynamics simulations demonstrated an increasing radial probability distribution of water in the first solvation shell (r, ca. 0.20 nm), whilst glycerol occupying the fourth solvation shell (r, ca. 0.60 nm), and is supported by water sorption experiments. It appears that nonspecific interactions amidst polysaccharide chain and solvent components are at play, whereby glycerol promotes polysaccharide chain self-association by steering the water towards the excluded volume of the chains, whilst itself being preferentially excluded. Based on the thermodynamic unaffordability of such exclusion, a geometric argument is postulated whereby the incompatibility in the system is reduced by decreasing the area of polysaccharide chain-solvent contact through enhancement of chain self-association. This work sets the foundation for future research in rheology and nonequilibrium dynamics of polysaccharide-solvent interactions in governing macroscopic behaviour and presents new sets of opportunities for designing structured and functional foods.