Abstract. Here we present experimental surface tension isotherms of mixed solutions of two surfactants, sodium dodecylsulfate (SDS) and cocoamidopropyl betaine (Betaine), measured by means of the Wilhelmy plate method. The kinetics of surface tension relaxation exhibits two characteristic timescales, which have been distinguished to determine correctly the equilibrium surface tension. The transition from the zwitterionic to the cationic form of Betaine is detected by surface tension measurements. The critical micellization concentration (cmc) increases monotonically with the rise of the mole fraction of SDS in the surfactant blend. The experimental surface tension isotherms are fitted by means of the two-component van der Waals model, and an excellent agreement between theory and experiment was achieved. Having determined the parameters of the model, we calculated different properties of the mixed surfactant adsorption layer at various concentrations of SDS, Betaine and salt. Such properties are the adsorptions of the two surfactants; the surface dilatational elasticity, the occupancy of the Stern layer by bound counterions, the surface electric potential, etc. In particular, the addition of a small amount of Betaine to SDS significantly increases the surface elasticity. The results could be further applied to predict the thickness and stability of foam films, or the size of the rodlike micelles in the mixed solutions of SDS and Betaine.

The natural pH of sodium dodecanoate (laurate), NaL, and sodium tetradecanoate (myristate), NaMy, solutions is measured as a function of the surfactant concentrations at 25 degrees C, and at several fixed NaCl concentrations. Surface tension isotherms are also obtained. Depending on the surfactant concentration, the investigated solutions contain precipitates of alkanoic acid, neutral soap, and acid soaps. The latter are complexes of alkanoic acid and neutral soap with a definite stoichiometry. A method for identification of the different precipitates from the experimental pH isotherms is developed. It is based on the analysis of precipitation diagrams, which represent plots of characteristic functions. This analysis reveals that for the NaL solutions there are three concentration regions with different precipitates, including lauric acid and 1:1 acid soap. In the case of NaMy solutions, we identified the existence of concentration regions with precipitates of myristic acid: 4:1, 3:2, and 1:1 acid soaps, and coexistence of two solid phases: 1:1 acid soap and neutral soap. The solubility products of the precipitates have been determined. Modeling the acid soaps of different stoichiometry as solid solutions of alkanoic acid and 1:1 acid soap, we derived a theoretical expression for their solubility products, which agrees well with the experiment. The kinks in the surface-tension isotherms of the investigated solutions correspond to some of the boundaries between the regions with different precipitates in the bulk. The theoretical analysis indicates that for the NaL solutions the adsorption layer is composed mostly of lauric acid, while for the NaMy solutions + 10 mM NaOH the adsorption layer is composed of nondissociated molecules of neutral soap. The developed approach could be applied to analyze the type of precipitates and the behavior of the surface tension for solutions of sodium and potassium alkanoates with different chain lengths at various temperatures and concentrations.

Equilibrium surface tension isotherms of sodium dodecylbenzene sulfonate (DDBS) are obtained at various fixed concentrations of NaCl. The contents of unsulfonated dodecylbenzene (DDB) in the used surfactant sample is determined by processing the surface-tension data. Having determined the parameters of the best fit, we computed the adsorption of surfactants (anionic DDBS and nonionic DDB), the binding of counterions in the Stern layer, the surface electric potential, the surface elasticity, etc., each of them for various surfactant and salt concentrations. The results show that for the solutions without added NaCl, the adsorption layer consists mostly of the nonionic DDB, irrespective of its small mole fraction in the surfactant blend. The admixture of DDB in the sample of DDBS leads to a significant increase of the surface elasticity. Moreover, even minimal added amounts of CrCl3 or Fe2(SO4)3 cause a considerable reduction in the surface tension, which is due to the greater binding energies of some of the counterions released by the latter electrolytes. The paper gives a quantitative analysis and description of the adsorption from aqueous solutions of a technical ionic surfactant. The followed strategy, which was to determine the contents of the admixtures and to account for their presence in the theoretical model, rather than to purify the surfactant, may find applications to other mixed surfactant systems.

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