A surface rheological study of non-ionic surfactants at the water–air interface and the stability of the corresponding thin foam films

The dilational elasticities ɛ and viscosities η of the two non-ionic surfactants n-dodecyl-β-d-maltoside (β-C12G2) and tetraethylene-glycol-monodecylether (C10E4) were studied as a function of the frequency ν and the surfactant concentration c. To have access to a large frequency range two different oscillating drop tensiometers were used. The first is based on evaluating the drop profile and allows to study dilational rheology in a frequency range from 0.005 to 0.2 Hz, while the second is based on measuring the capillary pressure, thus extending the frequency range up to 100 Hz. The concentrations investigated have been chosen similar, or even equal, to those for which the disjoining pressure Π versus thickness h curves of the respective foam films were known. The results are discussed in the light of the stability of the respective foam films focussing on the observation that the stability of foam films cannot be explained solely by the magnitude of the surface forces. Indeed, the correlation between the surface visco-elasticity and the stability of the respective foam films, which was suggested on the basis of low frequency data in a previous study, is here confirmed and extended to higher frequency.