Salt Dependent Structure in Methylcellulose Fibrillar Gels

Methylcellulose (MC) is a commercially important water-soluble polysaccharide, which thermoreversibly gels upon heating. Although MC has been studied and exploited in applications for many decades, it has only recently been discovered that the gelation occurs via self-assembly of the polymer chains into ca. 15 nm diameter fibrils, which percolate into a network. The network structure dictates the properties and mechanical behavior of the resulting hydrogel. The addition of salt to MC gels has been an area of academic and commercial interest. MC solutions containing salts exhibit an increase or decrease in the gelation temperature, generally following the Hofmeister series. We build upon those investigations and explore the effect of salt on MC fibril structure. We demonstrate the effect of salt on the gelation and dissolution temperatures using rheology and cloud point measurements. From SAXS and cryo-TEM, we show that salty MC gels are also comprised 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. We propose two different mechanisms of fibril diameter reduction with the addition of salt.