Co-solvency phenomenon and thermodynamic mechanism of cyanoethyl cellulose in acetone-water binary solvent

Cyanoethyl cellulose (CEC), which is insoluble in both water and acetone individually, exhibits unexpected solubility in acetone-water mixtures-a phenomenon that has not been previously reported or mechanistically explained. Herein, we investigate this anomalous co-solvency behavior by analyzing solution transmittance alongside Huggins and Kraemer parameters to evaluate solvent quality. The results reveal that with increasing substitution degree, CEC becomes more soluble in solvents with higher acetone content and often exists in aggregated forms. Molecular dynamics simulations further show that CEC induces a local stratification of solvent molecules, significantly enhancing its contact with both water and acetone molecules while markedly reducing the repulsive energy among solvent molecules. The solvation free energy of a single CEC chain in mixed solvents varies with both the acetone mole fraction and substitution pattern, with the solvation free energy difference between the optimal mixed solvent and pure water ranging from -1.60 to -10.75 kJ/mol/anhydroglucose unit (AGU), and between the mixed solvent and pure acetone ranging from -6.18 to -17.65 kJ/mol/AGU. The solvation process is predominantly driven by enthalpy. This work sheds light on the microscopic origin of CEC's unusual dissolution behavior in acetone-water systems and offers new insights into optimizing solvent environments for its broader applications.