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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.
Bibliographical noteFunding Information:
This work was supported primarily by the National Science Foundation through the University of Minnesota under award numbers DMR-1420013 and DMR-2011401. The authors thank Dow Chemical Company for generously providing the MC samples. The SAXS measurements were conducted at the DuPont-North-western-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by Northwestern University, E.I. DuPont de Nemours & Co., and Dow Chemical Company. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. Cryo-TEM imaging was performed at the Technion Center for Electron Microscopy of Soft Matter, supported by Technion Russel Berrie Nanotechnology Institute (RBNI).
© 2021 American Chemical Society.
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9/1/20 → 8/31/26
Project: Research project