The role of cation and anion on the dissolution rate of flax yarns in ionic liquids

Abstract This study investigates the dissolution of flax yarns in two imidazolium based ionic liquids ILs (1-butyl-3-methylimidazolium acetate) ([C4mim] + [OAc] − ), and (1-ethyl-3-methylimidazolium octanoate) ([C2mim] + [Oct] − ). These results are combined with our previous published work on (1-ethyl-3-methylimidazolium acetate) ([C2mim] + [OAc] − ) (Albarakati et al.2023) using the same flax yarns, to establish an understanding of the effect of the anion and cation on dissolution behaviour. The dissolution process involved submerging the flax yarns in the ILs for a range of temperatures and times, followed by coagulation in water. The dissolved coagulated material produced an outer ring that surrounded the centre undissolved yarn. Optical microscopy was used to follow the growth of this region, which was found to follow an Arrhenius behaviour. The dissolution activation energies of the ILs [C4mim][OAc] and [C2mim] [Oct] were found to be 67 ± 1 kJ/mol and 79 ± 1 kJ/mol, respectively. In addition, the growth of the outer coagulated ring’s thickness was measured with time and temperature, enabling the IL’s diffusion to be determined. Nuclear Magnetic Resonance, viscosity and density measurements were combined through a Stokes–Einstein analysis to further understand the dissolution mechanisms. Comparing the resultant data for all three ILs shows that the dissolution rate goes from fastest to slowest in the order [C2mim][OAc] > [C4mim][OAc] > [C2mim][Oct]. Our key observation is that the dissolution of the flax yarns (in all three ILs) is controlled by the diffusion of each IL through a region of swollen cellulose/IL solution around each yarn as it dissolves.