Fibrillar Structure of Methylcellulose Hydrogels

Abstract

It is well established that aqueous solutions of methylcellulose (MC) can form hydrogels on heating, with the rheological gel point closely correlated to the appearance of optical turbidity. However, the detailed gelation mechanism and the resulting gel structure remain poorly understood. Herein the fibrillar structure of aqueous MC gels was precisely quantified with a powerful combination of (real space) cryogenic transmission electron microscopy (cryo-TEM) and (reciprocal space) small-angle neutron scattering (SANS) techniques. The cryo-TEM images reveal that MC chains with a molecular weight of 300 000 g/mol associate into fibrils upon heating, with a remarkably uniform diameter of 15 ± 2 nm over a range of concentrations. Vitrified gels also exhibit heterogeneity in the fibril density on the length scale of hundreds of nanometers, consistent with the observed optical turbidity of MC hydrogels. The SANS curves of gels exhibit no characteristic peaks or plateaus over a broad range of wavevector, q, from 0.001-0.2 Å(-1). The major feature is a change in slope from I ∼ q(-1.7) in the intermediate q range (0.001 - 0.01 Å(-1)) to I ∼ q(-4) above q ≈ 0.015 Å(-1). The fibrillar nature of the gel structure was confirmed by fitting the SANS data consistently with a model based on the form factor for flexible cylinders with a polydisperse radius. This model was found to capture the scattering features quantitatively for MC gels varying in concentration from 0.09-1.3 wt %. In agreement with the microscopy results, the flexible cylinder model indicated fibril diameters of 14 ± 1 nm for samples at elevated temperatures. This combination of complementary experimental techniques provides a comprehensive nanoscale depiction of fibrillar morphology for MC gels, which correlates very well with macro-scale rheological behavior and optical turbidity previously observed for such systems.

Influence of pH on mechanical relaxations in high solids LM-pectin preparations

“…29 The mechanism of gel formation is based on the extent of hydrophobic interaction among MC chains that associate to form a fibrilar gel. 30 As temperature increases, hydrophobic interactions strengthen and chains are able to assemble and form the gel.…”

An ultra melt-resistant hydrogel from food grade carbohydrates

“…[19][20][21] The gelation process is attributed to an initial association of the hydrophobic methylated regions, followed by formation of a turbid gel and phase separation. 20,[22][23][24][25] The heat setting nature of MC, along with its thermoreversibility, 26 makes it appealing for use in cooking as a thickener or a binder. 18 Previous studies have shown that MC-agar solutions have much quicker gelation times compared to MC alone when held at a constant temperature.…”

Cellulose-Derivatives-Based Hydrogels as Vehicles for Dermal and Transdermal Drug Delivery

“…The same group of researchers has also demonstrated by cryogenic transmission electron microscopy and small-angle neutron scattering that MC hydrogels have a heterogeneous fibrillar structure that is responsible for their turbidity [66,67].…”