Phase behaviour of systems of a non-ionic surfactant and a non-ionic polymer in aqueous solution

The phase behaviour of different non-ionic surfactant/non-ionic polymer/water systems has been studied. The surfactants investigated are tetraethylene glycol mono(n-dodecyl) ether (C₁₂E₄) and octaethylene glycol mono(n-dodecyl) ether (C₁₂E₈), and the polymers chosen are a linear random copolymer of ethylene oxide and propylene oxide (UCON) and ethyl hydroxyethyl cellulose (EHEC); all components display a lower consolute curve. The phase behaviour is strongly dependent on the distances from the phase separation limits of the two components, except that the phase behaviour of systems with a given polymer is similar. However, on changing the polymer, a marked change in phase behaviour is displayed. For systems with UCON, the miscibility between the surfactant and the polymer is of crucial importance in determining the phase behaviour. The polymer and the surfactants simply act as a preferred solvent for each other compared to water. In this case, the cloud points of the ternary mixtures depend essentially on the component with the lowest solubility, i.e. UCON in the C₁₂E₈/UCON/water mixture and C₁₂E₄ in the C₁₂E₄/UCON/water mixture. When the component with the higher solubility (C₁₂E₈ in C₁₂E₈/UCON/water and UCON in C₁₂E₄/UCON/water) is substituted for water, the lower consolute boundary shifts upwards. In systems with EHEC, however, the lower consolute boundaries pass through a minimum on adding surfactant to a binary EHEC—water solution, which could be attributed to an attractive interaction and the formation of a strong complex between the polymer and the surfactant.