DLVO and non-DLVO surface forces in foam films from amphiphilic block copolymers

Abstract

Systematic research on foam films from PEO–PPO–PEO triblock copolymers is carried out. These commercial (Synperonic, ICI, Witton, UK) non-ionic polymeric surfactants are rather pure though polydispersed. The microscopic foam film is used as a tool for investigating the interaction between two aqueous solution/air interfaces. The influence of electrolyte concentration, copolymer concentration, pH, and capillary pressure are monitored with the microinterferometric technique. Stable foam films are obtained at concentrations below CMC. Film stability is either electrostatic or steric in origin. Electrostatic repulsion arises from the charge at the water/air interface due to the preferential adsorption of OH − ions. It can be screened (by increasing the ionic strength), obliterated (by decreasing pH), or outweighed (by increasing the capillary pressure). Electrostatic interaction can be described within the framework of the DLVO theory (weak van der Waals attraction is also detected). In this respect PEO–PPO–PEO polymeric surfactants are analogous to low molecular non-ionic surfactants. In the absence of electrostatic repulsion non-DLVO forces are responsible for film stability. At concentrations between C Γ∞ and CMC the copolymers form a PEO-brush at the water/air interface. However, brush-to-brush contact is established only at higher capillary pressure and then the disjoining pressure isotherm follows de Gennes' scaling prediction. At lower pressure a softer steric repulsion occurs. It is governed by the bulk surfactant concentration and hence is fundamentally different from the brush-to-brush repellency. Chain length is of utmost importance for steric interaction. The foam film is an effective model for studying de facto separately and quantitatively DLVO and non-DLVO surface forces.