Effect of the precipitation of neutral-soap, acid-soap, and alkanoic acid crystallites on the bulk pH and surface tension of soap solutions.

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.