Oscillating bubble measurements of the compression viscoelasticity of mixed surfactant–polyelectrolyte surface layers

Mixed polyelectrolyte surfactant layers co-adsorbed at the surface of aqueous solutions have an unusual behavior. The adsorption is irreversible and the response to large compressions is highly nonlinear, as evidenced by Brewster angle microscopy. As a consequence, it is very difficult to measure equilibrium surface tension with bubble shape techniques, as soon as bubble volume is varied. These difficulties seem, however, specific to strongly viscoelastic layers. They are never found with surfactant solutions, and perhaps more surprisingly not seen either with more rigid layers such as those obtained with proteins. Long equilibration times are found, much longer than in Wilhelmy plate experiments. Uncontrolled convective motions occurring with the plate technique might explain the observed differences. It is easier to obtain reliable compression viscoelastic moduli with the oscillating bubble technique.