Reply to Comment on “Hydrophobic Forces in the Foam Films Stabilized by Sodium Dodecyl Sulfate: Effect of Electrolyte” and Subsequent Criticism

Stubenrauch et al. 1 also cited our study, 3 presenting its contents in an incorrect way. Here, we restrict ourselves to a brief response to the comments regarding our article. The authors of ref 1 write “The long-range hydrophobic force concept was also used to describe film thinning, that is, drainage. In refs 3 -5, it was found that the Reynolds equation (in which the driving force for the drainage is given by the disjoining pressure) does not describe the drainage: the foam films drain faster than theoretically predicted. Attributing the faster drainage to an attractive longrange hydrophobic force and extending the DLVO expression with the respective term for the long-range hydrophobic force indeed leads to a good fit of the experimental data. However, the Reynolds equation was derived for film thinning between flat solid surfaces. Thus, the assumption of tangentially immobile surfaces is incorrect in the case of many foam films as was shown in ref 6. The lower the surfactant concentration and the thicker the film, the more pronounced is this effect. 6,7 As the foam films studied by the authors are indeed stabilized by low surfactant concentrations and as they are relatively thick, the accelerated film thinning could be partly due to the convective surface motion of the surfactant (Marangoni effect).” First, in our article3 we report and interpret data for the dependence of the critical thickness of film rupture, hcr on the film radius,R(see Table 1 and Figure 5 therein) rather than data for the dependence of the film thickness, h, on time,t (i.e., for the process of film drainage). Physically, these are two quite different dependencies. Second, in our article the dependence hcr(R) is interpreted by means of stability analysis of liquid-film breakup based on the theory of the growth of fluctuation capillary waves 8 (see eqs 8-10 and 13-15 in ref 3) rather than the Reynolds equation, which describes the drainage of plane-parallel films. We applied the latest and the most complete version of the theory of film breakup by capillary waves, 8 coauthored by one of us (K.D.D.), which takes into account all effects related to the mobility of the film surfaces, including the surface dilatational elasticity (the Marangoni effect), the surface convection and diffusion, the effects of bulk and surface viscosities, and the exchange of surfactant molecules between the bulk and the film surfaces. This theory has two versions: with linear and nonlinear stability analysis. 8 The theory was tested against available experimental data, and very good agreement was found without using any adjustable parameters. 8,9 However, our quantitative analysis of the data for the rupture of films at low SDS concentrations 3 showed that the measured greater values of hcr cannot be attributed to the effects of surface mobility. In the considered case, it turned out that data processing by means of the full computer program taking into account all mobility effects 8