Influence of Surface Aging on the Drainage of Foam Films Stabilized by Aqueous Solutions of Ethyl Hydroxyethyl Cellulose

Abstract

The drainage times of microscopic horizontal foam films stabilized by dilute aqueous solutions of ethyl hydroxyethyl cellulose (EHEC) was shown to be dependent on the aging effects (configuration changes of the adsorbed macromolecules) occurring in the freshly created air/solution interface. At low polymer concentration (5 ppm), the films drained fairly rapidly from thicknesses of about 400 to 300 nm with drainage times about 5 to 6 times greater than theoretical values calculated using the Reynolds equation. However, at higher polymer concentrations (100 ppm) at extended surface aging (15−180 min) the film drainage times were shown to increase drastically giving values 50 times greater than theoretical values. Although these aging effects could not be directly related to surface tension data, diffusion coefficients were calculated from interfacial tension profiles using classical diffusion theory. As the concentration of polymer increased, the diffusion coefficients were shown to decrease and were considerably smaller than previously reported experimentally values determined in bulk solution by NMR. This difference between experimental and theoretical results endorsed a kinetic rather than a diffusion or mass transport model for the transfer of EHEC molecules to the interface. The increase in drainage times with extended aging times could be explained by the gradual formation of a steric energy barrier caused by configuration changes of the adsorbed polymer. This probably involved the progressive extension of the EHEC tails into the aqueous phase increasing the disjoining pressure, decreasing the drainage rate, and producing thick stable films.