A disjoining pressure study of n-dodecyl-beta-D-maltoside foam films

The disjoining pressure Pi as function of film thickness h of aqueous n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) solutions has been measured using a newly assembled thin film pressure balance. The theoretical analysis of the experimental Pi(h) curves in terms of the DLVO theory confirms the existence of electrical charges and provides numerical values for their density at the water/air interface of this nonionic sugar surfactant system. The origin of the charges is discussed on the basis of a recently proposed adsorption model for OH- ions. The surface charge density is found to decrease with increasing surfactant concentration, to increase with increasing electrolyte concentration and stay constant in the range 4 < pH < 8. Furthermore, it is shown that the molecular structure of sugar surfactants has no influence on the magnitude of the long-range repulsive forces stabilizing the common black film. However, the structure influences the stability of the film as well as its ability to form a Newton black film. The good agreement between the results presented and those for nonionic alkyl polyglycol ethers (CiEj) suggests a common origin of the properties of thin liquid films stabilized by nonionic surfactants.