A light scattering study on hydroxyethyl cellulose in binary mixtures of water‐dimethyl sulphoxide and water‐formamide

Studies have been made on hydroxyethylcellulose in binary mixtures of water/dimethylsulphoxide and water‐formamide. Light scattering and viscosity measurements were made in the mixture ranges 0–100% dimethyl sulphoxide Me2SO and 0–80% formamide (vol.‐%). For both solvent systems, somewhat anomalous results were obtained. Water and dimethyl sulphoxide are both solvents for hydroxyethyl cellulose. However, light scattering and viscosity results reveal polymer aggregation in the mixtures containing 30–75% dimethyl sulphoxide. This behaviour is a consequence of strong dipole‐dipole interactions between water and dimethyl sulphoxide, leading to a solvent structure that greatly diminishes interactions with the polymer. Formamide is a non‐solvent (at 25°C.) for hydroxyethyl cellulose, although water‐formamide mixtures ( < 80% formamide) are thermodynamically good solvents. Light scattering data show uniformily high coil extensions in this range, and also that the solvent environment of the polymer is that of the bulk solvent. Such behaviour is interpreted by the associated structure of pure formamide, which is substantially altered by the presence of water.