Drainage of foam films stabilized with mixtures of non-ionic surfactants

The drainage of single foam films stabilized with a mixture of the non-ionic surfactants n-dodecyl-beta-D-maltoside (beta-C(12)G(2)) and hexaethyleneglycol monododecyl ether (C12E6) was studied at two different mixing ratios (beta-C(12)G(2):C12E6 = 1:1 and 50:1) as a function of the electrolyte and the total surfactant concentration, respectively. The electrolyte concentration was varied from 10(-3) to 10(-)1 M NaCl and the total surfactant concentration ranged from 0.01 to 10 CMC. Moreover, the influence which the film radius has on the drainage of the foam films was investigated. From the It (film thickness) versus t (time) dependence the values of the drainage coefficients (a) were determined for all films according to the equation derived by Manev et al. [E. Manev, R. Tsekov, B. Radoev, J. Disper. Sci. Tech. 18 (1997) 769]. It was found that the drainage of these foam films is generally in line with the theory. First, at concentrations below the CMC the value of a decreases with an increase in the total surfactant concentration and levels off at C-surf similar to CMC in all cases except one. Second, increasing the ionic strength increases the rate of drainage at C-surf < CMC, while it does not significantly affect alpha at C-surf = CMC of the respective solution. Last but not least, films of smaller radius drain faster regardless of their composition. However, the results obtained for the 1: 1 and the 50:1 mixture differ quantitatively. These differences are discussed and some of the results are compared with those obtained for the pure surfactants. (c) 2006 Elsevier B.V. All rights reserved.