Dissolution kinetics of cellulose in ionic solvents by polarized light microscopy

Abstract

Aiming to increase the scarce information available on the kinetics of cellulose dissolution, we have applied an image-assisted technique based on the luminance evolution as cellulose dissolves to study this process under different experimental conditions. This protocol was validated via direct determination of the cellulose dissolved. In all cases datapoints were linearly fitted assuming a pseudo-zero order reaction which facilitates the comparison between datasets. To study the solvent effect we have used three ionic liquids with large basicity and found significantly different rates of dissolution, from faster to slower: [C$$_2$$ 2 C$$_1$$ 1 Im][OAc]>[C$$_1$$ 1 C$$_1$$ 1 Im][DMP]>[C$$_2$$ 2 C$$_1$$ 1 Im][DEP]. Solvatochromic parameters and viscosity of the solvent media were determined and the latter was identified as a key factor slowing the process as it reduces ionic mobility. The weight of viscosity was estimated by removing the viscosity effect with a tuned solvent media doped with DMSO. Activation energy was 70% lower at constant viscosity. On the substrate side, particle size is the main factor affecting the dissolution rate. All samples were milled to homogenise the starting material and minimise the distortions caused by the size distribution. After, dissolution of three cellulose substrates under the same experimental conditions was analysed. Dissolution of natural fibers took twice as long as dissolving pulp which dissolves slower than lyocell fibers. This trend correlates with the roughness of the samples obtained by optical perfilometry which is consistent with the dissolution mechanism proposed in the literature. Graphical abstract