The enthalpies of dissolution of fibroin in imidazole-based ionic liquids were determined, and the contribution of polymer crystal structure decomposition to the enthalpy of dissolution was estimated. The heat of mixing of imidazole ionic liquid with water was described.Biopolymers (cellulose, chitosan, fibroin, keratin) are widely used in various fields of human activities. Processing of biopolymers gives rise to a lot of waste materials which may be reused, e.g., in the fiber spinning and film casting from concentrate solutions for domestic needs, technics, biotechnology, and medicine. A general property of the above biopolymers is their very poor solubility due to the presence of a large number of intermolecular bonds in the supramolecular structure. Appropriate solvents for the preparation of concentrated solutions may be aqueous and organic solutions of salts, N-methylmorpholine Noxide, and ionic liquids on the basis of imidazole [1,2]. The latter have been used as solvents for biopolymers relatively recently; therefore, some aspects of dissolution of natural polymers therein still remain unstudied; this is especially relevant to film-forming polypeptides such as fibroin. Nevertheless, the use of articles made of such polymers seems to be quite promising taking into account their compatibility with living matter, high strength, relatively high hydrophobicity, and resistance to bacteria. Fibroin and other polypeptides may be useful as materials for biochemistry, medicine, and biotechnology, for example, as fibers and films for wound repair and immobilization of enzymes and as many other articles.There are very limited published data on the solubility of fibroin in 1-butyl-3-methylimidazolium chloride and acetate and on some properties of the solutions [2][3][4]. These data are insufficient to understand specificity of intermolecular interactions in the system fibroin-ionic liquid, effect of water on the dissolution and regeneration of the polymer from solution. In the present work we used thermochemical method to study solvent-solute interactions. Calorimetric parameters of dissolution make it possible to unambiguously describe processes occurring in a complex system, quantitatively estimate their energies, and compare the data for different solvents. Figure 1 shows the plots of the enthalpies of fibroin (as washed silk fibers) dissolution in 1-butyl-3-methylimidazolium chloride (curve 1) and acetate (curve 2) versus concentration of the polymer in solution. It is seen that the enthalpy of fibroin dissolution does not depend on its concentration in the examined range. The dissolution of fibroin in 1-butyl-3-methylimidazolium acetate is accompanied by a higher exothermic effect as compared to 1-butyl-3-methylimidazolium chloride. Taking into account that the overall enthalpy of dissolution is the sum of some components: ΔH dis = ΔH s-s + ΔH p-p + ΔH p-s + ΔH dil , where the first two terms characterize endothermic effect of bonds breaken between solvent and polymer molecules, respectively, ΔH p-s describes t...
