The disjoining pressure P as function of film thickness h of aqueous n-dodecyl-b-D-maltoside (b-C 12 G 2 ) solutions has been measured using a newly assembled thin film pressure balance. The theoretical analysis of the experimental P(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 À 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 (C i E j ) suggests a common origin of the properties of thin liquid films stabilized by nonionic surfactants.

The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces were chosen. The interactions between the air/water interfaces were measured with a thin film pressure balance in terms of disjoining pressure as a function of film thickness. The interactions between the solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the nature of the surface on the interaction forces was investigated at surfactant concentrations below and above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged silanated glass and the charged air/water interface results in a lowering of the surface charge density. The reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at the air/water interface as well as its decrease with increasing surfactant concentration is discussed and a new mechanism for generation of OH- ions at this particular interface is proposed.

No abstract