Salt-Dependent Structure in Methylcellulose Fibrillar Gels

Methylcellulose (MC) is a commercially important, water-soluble polysaccharide. Many food applications exploit the thermoreversible gelation behavior of MC in aqueous media. The mechanism of MC gelation upon heating has been debated for decades; however, recent work has demonstrated that gelation is concurrent with the formation of ca. 15 nm diameter fibrils, which percolate into a network. The fibrillar network dictates the properties and mechanical behavior of the resulting hydrogel. The addition of salt to MC gels has also been an area of academic and commercial interest. It has been reported that MC solutions containing salts exhibit an increase or decrease in the gelation temperature, which generally follows the Hofmeister series. To build upon these investigations, we study the effect of salt on the MC fibril structure. We demonstrate the effect of salt (NaCl, NaI, NaBr, NaNO3, KCl, NH4Cl, LiCl, and CaCl2) on the gelation and dissolution temperatures using rheology and cloud point measurements. From small-angle X-ray scattering (SAXS) and high contrast cryogenic transmission electron microscopy (cryo-TEM) we show that salty MC gels are also composed of fibrils. Fitting the SAXS curves to a semiflexible cylinder model, we demonstrate that the fibril diameter decreases monotonically with increasing salt molarity, largely independent of the salt anion or cation type.