On the origin of electrostatic and steric repulsion in oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers

Oil-in-water emulsion films were used to investigate the origin of electrostatic and steric surface forces between two oil droplets in aqueous solution of nonionic polymeric surfactants. The Thin Liquid Film–Pressure Balance Technique was used. Equivalent thickness of oil-in-water emulsion films stabilized by PEO-PPO-PEO triblock copolymers (Pluronic® F108 and P104) vs. pH was measured at constant capillary pressure and ionic strength. The isoelectric point at the negatively charged film interfaces, corresponding to zero diffuse double layer potential was found. This demonstrates that the origin of the electrostatic repulsion in the emulsion films studied is associated to preferential adsorption of OH− ions. Disjoining pressure isotherms for the emulsion films obtained at different pH were fitted to de Gennes’ scaling theory for steric interaction between polymer ‘brush’ layers with the ‘brush’ thickness as a fitting parameter. The fit obtained was satisfying and yielded PEO-‘brush’ thickness of 9.6 nm. These results allowed the conclusion that in absence of diffuse double layer forces the stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers was due to ‘brush-to-brush’ steric repulsion.