Sol-Gel Transition of Regenerated Silk Fibroins in Ionic Liquid/Water Mixtures

An acetate ionic liquid, 1-ethyl-3-methylimidazolium acetate (EMImAc), was employed to dissolve degummed silks of Bombyx mori and Antheraea pernyi, as it displays excellent solubility to both silkworm silks. Rheological measurements of those regenerated silk fibroin (RSF) solutions indicate that the solvation degree of RSF decreases with the increase of temperature and the addition of water. Generally, the sol-gel transition of both RSF/EMImAc/water systems takes place above a critical temperature at a range of water content. It is found that the gelation is irreversible when temperature decreases, which may be even lower than its "critical temperature". However, after removing significant water by heating, the gel converts into a fluid, which is able to gelatinize again with the addition of water. Modulated differential scanning calorimetry (MDSC) investigation illustrates that the sol-gel transition is most probably driven by the solvophobic interactions between the hydrophobic segments of RSF chains and then the formation of β-sheets. Such a secondary structure change of RSF is also supported by Raman and FTIR spectroscopic observations. The present study provides an insight into the conformational transition of RSF in ionic liquid from a view of thermodynamics, and also offers an environmentally friendly route for the fabrication of silk-based materials from RSF/ionic liquids solutions.