Critical thickness of foam films stabilized by nonionic, ionic surfactants and their mixtures

The critical thickness (hcr) for foam films of n-dodecyl β-D maltoside (C₁₂G₂) and of its mixed solutions with dodecanol (C₁₂Е₀), hexaethyleneglycol dodecyl ether (C₁₂E₆) and dodecyl trimethylammonium bromide (C₁₂TAB) of different molar ratio (50:1; 1:1;1:50) at low and high ionic strength was measured interferometrically. It was found that the hcr increases with the increase of the film radius independently of solutions composition. At low ionic strength the type of surfactant affects the critical thickness and the equilibrium state of the film. hcr for the films of mixture with C₁₂E₀ increases with the increase of the total surfactant concentration, while hcr for the films of mixture with C₁₂TAB decreases. For the values of the critical thicknesses for films from individual surfactant solutions the following sequence hcr (C₁₂TAB) > hcr (C₁₂E₆) > hcr (C₁₂G₂) is found. At high ionic strength the quantity of nonionic additive does not substantially affect the value of hcr, while the quantity of ionic additive influences it by two different ways (i) in 50:1 mixture C₁₂TAB supports C₁₂G₂ in the reducing of negative charge; (ii) in 1:1 mixture C₁₂TAB recharges the film surfaces.