Cellulose is an excellent raw material for the development of renewable, biodegradable materials that can replace or complement, for example, single-use plastic articles and fossil-based textiles. Many of these applications require processing in order to shape the cellulose into films, membranes, textile fibers, etc. This requires dissolution-based processing, however, since cellulose degrades before it melts. In order to dissolve cellulose, cellulose-solvent interactions are required to overcome the attractive stabilising forces between the cellulose chains: cellulose chains are, namely, stabilised through strong intra- and intermolecular hydrogen bonding as well as hydrophobic interactions and are, as such, organized in semi-crystalline fibrils that are further assembled in complex layered hierarchical morphology. Despite this, numerous solvents have been developed and most of them are complex systems, such as ionic liquids (Wang et al.
2012), specific salt-solvent combinations [e.g. DMAc/LiCl (McCormick et al.
1985), DMSO/TBAF (Liebert and Heinze
2001)], aqueous solutions of bases or acids [e.g. NaOH(aq) (Sobue et al.
1939; Davidson
1934), quaternary ammonium hydroxides(aq) (Powers and Bock
1935), phosphoric acid(aq) (Boerstoel et al.
2001)], hydrated metal amine salts [e.g. Schweizer’s reagent (Schweizer
1857)] as well as those relying on the derivatization of cellulose [e.g. industrially important CS
2/NaOH(aq)]. Aqueous solution of NaOH is of particular interest since it is inexpensive, non-toxic, readily available and already in use in the pulp and paper industry. Dissolution of cellulose in NaOH(aq), however, only occurs below +1 °C and in solutions with a NaOH concentration between 7 and 10 wt.% (Budtova and Navard
2016). The use of this solvent system has also been held back, partly due to its inability to dissolve cellulose with a DP over ca 200 and partly due to problems with the instability of the solutions as they gel with increasing time, temperature and/or concentration of cellulose (Roy et al.
2003). Considerable efforts have therefore been made to improve dissolution in the cold NaOH(aq) system and different additives have been identified, including urea (Zhou and Zhang
2000), thiourea (Zhang et al.
2002), ZnO (Yang et al.
2011) and polyethylene glycol (Yan and Gao
2008). Whilst a general stabilisation mechanism for these additives has not been established, emphasis in current research is now being placed on the importance of hydrophobic interactions in solvent systems, since the amphiphilic nature of cellulose has been investigated widely. It has been shown, for example, that increasing concentrations and molecular weights of cellulose can be dissolved by increasing the hydrophobicity of the cation in quaternary ammonium hydroxide bases (Wang et al.
2018).
The aim of this work was to increase understanding of the dissolution of cellulose in aqueous solvents and, more specifically, the role of the cation, by combining different hydroxide bases and investigating whether or not cellulose displays an affinity for different cations. The resulting solutions were investigated using differential scanning calorimetry to identify the hydrate structures of the bases in solution, and how these are affected by each other and by cellulose. Moreover, NMR spectra of selected solvents were analysed to shed additional light on molecular interactions. In order to investigate if these solutions displayed properties different to those of single-base solutions, intrinsic viscosity analysis was used to compare the solvent quality, while dynamic rheology measurements were performed to investigate effects on solution stability.