On the molecular mechanism of thermally reversible sol–gel transition with pronounced hysteresis of agarose in water

A comprehensive understanding of the thermally reversible sol-gel transition of agarose in water is explored mainly through IR spectroscopy in combination with rheology, low field (LF)-¹H NMR, and molecular dynamics simulations. Both rheological properties, in view of storage and loss modulus, and LF-¹H NMR curves, from the aspect of mobility of water molecules, demonstrate a sharp sol-gel transition with a pronounced hysteresis larger than 30 °C in an agarose/water binary system. Temperature-dependent IR spectra clearly illustrate that -C3-O-C6-CH₂- groups in agarose chains exhibit abrupt dehydration and inter-/intra-chain hydrogen bond formation along with the sol-to-gel transition upon cooling, indicating the significance of C3-O-C6 ether groups in the gelation of agarose in water, which is further confirmed by molecular dynamics simulations. Moreover, it is demonstrated that the evident hysteresis behavior is associated with molecular interactions within agarose chains, as observed from infrared spectral insights and perturbation-correlation moving window analysis. Two-dimensional correlation spectroscopy results reveal the predominant role of hydrogen bond breaking during the gel-to-sol transition, implying the contribution of inter-/intra-chain hydrogen bonds to the pronounced hysteresis.