Free drainage of aqueous foams stabilized by mixtures of a non-ionic (C12DMPO) and an ionic (C12TAB) surfactant

The study at hand investigates the influence of the composition of a surfactant mixture on the free drainage of the respective foams. The Foam Conductivity Apparatus (FCA) was used to study free drainage of foams of homogeneous initial liquid fraction εinit. The foams were stabilized with mixtures of the non-ionic surfactant dodecyldimethyl phosphineoxide (C12DMPO) and the cationic surfactant dodecyl trimethylammonium bromide (C12TAB) at mixing ratios of C12DMPO:C12TAB = 1:0, 50:1, 1:1, 1:50 and 0:1. In all cases the liquid fraction ε follows a power law with respect to time t, i.e. ε ∼ tη with −1.0 < η < −0.8. The η-values are all very similar which indicates a Poiseuille-like flow (−1 < η < −2/3) for all samples irrespective of changes in composition and bubble size. For all studied mixtures drainage led to fairly dry foams (ε ∼ 10−3), which then either coalesce (C12DMPO and 50:1 mixture) or drain further (1:1 mixture). An evaluation of the critical liquid fraction εcrit (liquid fraction at which foam coalescence sets in abruptly) revealed that εcrit is independent of the foam height and the bubble size which is in line with literature. However, it depends on the composition of the surfactant mixture and decreases with increasing content of C12TAB.