Abstract
Experimental data for the disjoining pressure of foam films stabilized by anionic surfactant in the presence of 1:1, 1:2, 1:3, and 2:2 electrolytes: NaCl, Na2SO4, Na3Citrate, and MgSO4 are reported. The disjoining pressure predicted by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory coincides with the experimental data in the case of a 1:1 electrolyte, but it is considerably greater than the measured pressure in all other cases. The theory is extended to account for the effects of ionic correlations and finite ionic radii. Original analytical expressions are derived for the local activity coefficient, electrostatic disjoining pressure, and asymptotic screening parameter. With the same parameter of counterion binding as for a 1:1 electrolyte, the curves predicted by the extended theory are in perfect agreement with the experimental data for 1:2 and 1:3 electrolytes. In comparison with the DLVO theory, the effect of ionic correlations leads to more effective screening of electrostatic interactions, and lower electric potential and counterion concentrations in the film’s midplane, resulting in lower disjoining pressure, as experimentally observed. The developed theory is applicable to both multivalent coions and multivalent counterions. Its application could remove some discrepancies between theory and experiment observed in studies with liquid films from electrolyte solutions.
Highlights
The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1,2] of surface forces in thin liquid films provides a quantitative description of the behavior of colloidal dispersions
In to the valence of the coion. (ii) The dependencies of both surface charge and potential on h are rather the case of DLVO theory, Figure 5a,b, the results indicate the following: (i) ρs and φs are weakly weak: ρs varies with about 5%, whereas φs varies with only 1.4%
We report experimental data for the disjoining pressure of foam films stabilized by anionic surfactant in the presence of 1:1, 1:2, and 1:3 electrolytes: NaCl, Na2 SO4, and Na3 Citrate
Summary
The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory [1,2] of surface forces in thin liquid films provides a quantitative description of the behavior of colloidal dispersions. The results about the importance of the effect of ionic correlations for the electrostatic double-layer interactions in thin liquid films appear to be controversial In part, this is due to the fact that in many cases the correlation effect could be incorporated in an effective surface charge (or potential) [9], which can be determined as an adjustable parameter, obtaining an apparent agreement with the DLVO theory. It is very important that the binding energy of the Na+ ion to the sulfate headgroup of the SDS molecule, characterized by the Stern constant KSt, , is known from surface-tension studies [27,28,29,30] and has not been used as an adjustable parameter in the present study In such a case, the effect of the ionic correlations on the surface force (if any) should be detected as a deviation from the predictions of the DLVO theory. 5, an analytical expression is derived for the asymptotic decay length of and a considerable effect of ionic correlations was detected in the cases of 1:2, 1:3 and 2:2 electrolytes, double‐layer interactions affected by the ionic correlations
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