Interfacial and temperature behaviors of water and solutions bound to water-soluble lignin powder

Production of valuable industrial materials from lignins as byproducts of different origin is of importance to solve the utilization problem for large amounts of these byproducts. One of the corresponding pathways is the production of water-soluble lignins, for which, interactions with water and aqueous solutions play an important role in practical application efficiency of these materials. One could assume that interactions of lignins with bound water and solutions depend strongly on the dispersion media and the presence of various (e.g., polar and ion-generating) solutes in water. The interfacial and temperature behaviors of water (solutions) bound to lignin particles in wetted powders could be effectively studied using low-temperature 1H NMR spectroscopy applied to static samples that allows one to separate mobile and immobile phases vs. temperature below freezing point (Tf,b) of bulk liquid. At T < Tf,b several effects could be observed in wetted lignin powders such as confined space effects (CSE) in pores or voids between or inside crosslinked macromolecules, cryoscopic effects (CE) in bound solutions, partial freezing of liquids with cryoconcentration and differentiation of solutions, clusterization of bound liquids, etc. To elucidate some of these effects, wetted water-soluble lignin powders at a constant degree of hydration (h = 0.3 g/g) were studied in such dispersion media as air and chloroform-d alone or in a mixture with deuterated trifluoroacetic acid using the low-temperature 1H NMR spectroscopy method. Additionally, hydrated lignin molecules of different sizes were studied using quantum chemistry methods. Obtained results show that both CSE and CE affect the temperature behavior of bound water and related solutions. However, there is no their strong synergetic impact because the stronger bound the water (solution) the lower the activity of water as a solvent; i.e., CSE could partially inhibit CE. The obtained results are of interest in order to better understand different aspects of applications of water-soluble lignins under various conditions.