Negative charges at the air/water interface and their consequences for aqueous wetting films containing surfactants

Negative charges at the air/water interface have been proven previously by the authors via disjoining pressure measurements of wetting films. In the present study the surface charges of wetting film are modified in oreder to control the film stability. Under all of those conditions, films against likely charged solid surfaces are stable and unstable against oppositely charged solid substrates. For this purpose, the charge of silicon wafers was precoated with polyelectrolytes before. At concentrations well below the CMC cationic surfactants are not able to reverse the charges at the air/water interface. Reprective wetting films are less stable and show a partial dewetting even on likely charged surfaces. These results lead to the conclusion that the films are stablilised by electrostatic repulsion. Potential adsorption of water-soluble surfactants on oppositely charged surfaces is also taken into account and is studied by contact-angel measurements.