Aqueous calcium thiocyanate solution as a cellulose solvent. Structure and interactions with cellulose

Dissolution of cellulose into aqueous (aq) calcium thiocyanate solution was investigated with emphasis on solvent structure and interactions with cellulose. The aq calcium thiocyanate (Ca(SCN)(2)) solution with concentration (C-CaT) more than 48.5 wt% could dissolve any celluloses. Solvation measurement revealed that in C(CaT)greater than or equal to 48.5 wt%, the solvent system is formulated by Ca(NCS)(2) . xH(2)O (x less than or equal to 4)+yH(2)O (y less than or equal to 6), which exhibits mainly non-dissociated thiocyanate with high specific viscosity, as proved by electric conductivity, specific viscosity and IR or C-13 NMR measurements on solvent as a function of concentration of calcium thiocyanate. In this regard, the most stable 4 hydrate calcium thiocyanate is concluded from energy calculation as trans-trans configuration of Ca(-N=C=S). 4H(2)O. DSC and IR analyses suggested that dissolution of cellulose lakes place by first attack of Ca(NCS)(2) . xH(2)O (x less than or equal to 4) towards ring oxygen in cellulose. Ca(NCS)(2) . xH(2)O (x less than or equal to 4) becomes 4-hydrates-like structure (sixcoordinate complex of Ca) and cellulose forms a 5-membered ring by coordinating two oxygen atoms (O(5) and O(6)) in the glucopyranose unit, and then cellulose dissolves on heating. In this process, some ligand exchange might occur if the hydration number is more than 2. This coordination of cellulose is quite characteristic, compared with other solvents, such as cuprammonium hydroxide, which coordinate with O(2) and O(3) hydroxyl groups.