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

Abstract

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 order to control the film stability. Studies at different pH values support the assumption that the negative charges are caused by an excess of hydroxide ions. Addition of different types of salt anions seems to lead to an increase in surface charges with decreasing hydration shell. Both water-soluble and water-insoluble amphiphiles replaces the charges. The air/water surface remains negatively charged in case of non-ionic surfactant (dodecyl-β maltoside, C₁₂G₂) and anionic ampihiphiles (stearic acid and SDS). The charge will be reversed in case of cationic amphiphiles. 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. Respective 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 stabilised by electrostatic repulsion. Potential adsorption of water-soluble surfactants on oppositely charged surfaces is also taken into account and is studied by contact-angle measurements.