Electrostatic and steric interactions in oil-in-water emulsion films from Pluronic surfactants.

Stabilization of oil-in-water emulsion films from PEO-PPO-PEO triblock copolymers is described in terms of interaction surface forces. Results on emulsion films from four Pluronic surfactants, namely F108, F68, P104 and P65 obtained with the Thin Film Pressure Balance Technique are summarized. It is found that film stabilization is due to DLVO (electrostatic) and non-DLVO (steric in origin) repulsive forces. The charging of the oil/water film interfaces is related to preferential adsorption of OH(-) ions. This is confirmed by pH-dependent measurements of the equivalent film thickness (h(w)) at both constant capillary pressure and ionic strength. With reducing pH in the acidic region, a critical value (pH(cr,st)) corresponding to an isoelectric state of the oil/water film surfaces is found where the electrostatic interaction in the films is eliminated. At pH≤pH(cr,st), the emulsion films are stabilized only by steric forces due to interaction between the polymer adsorption layers. Disjoining pressure (Π) isotherms measured for emulsion films from all the four Pluronic surfactants used at pH<pH(cr,st) show a transition to a Newton black film with increasing Π. The experimental data before the NBF-transition in the disjoining pressure isotherms are fitted to the Alexander-de Gennes' scaling theory for steric interaction between polymer brushes with the PEO-brush thickness as a free parameter. The NBF observed are stabilized most probably by short-range steric forces that may differ from the brush-to-brush interaction.