Fluorocarbon vapors slow down coalescence in foams: influence of surfactant concentration

Even though it has been known for a long time that traces of perfluorinated vapors suppress gas exchange (coarsening) between the bubbles of aqueous foams, its stabilizing impact on foam coalescence has been evidenced only recently. While previous work has demonstrated this effect for different surfactant types, we investigate here the influence of the surfactant concentration. We compared the foam properties of aqueous solutions of the non-ionic surfactant dodecyldimethylphospine oxide (C₁₂DMPO) in the absence and presence of perfluorohexane (PFH) in the gas phase. In order to decouple foam coarsening and coalescence, we accompany the foam stability experiments with (a) lifetime statistics of individual, vertical foam films pulled from the same solutions and (b) surface tension measurements in controlled gas environments. All measurements show a clear increase of foam and film stability in the presence of PFH, its effect being most pronounced above the cmc of the surfactant. The surface tension measurements show a clear co-adsorption of the PFH up to the formation of macroscopic films at saturation. We complete the analysis by showing that alkane vapors also have a stabilizing effect for the same surfactant solutions, yet much less pronounced than that of PFH. The precise origin of the stabilizing action of these hydrophobic vapors remains to be elucidated.