Surface tensions of multicomponent aqueous electrolyte solutions: Predictive models based on binary limits

Most simple inorganic acids and organic bases (for example amines) decrease surface tension in aqueous solutions, but common inorganic salts have the opposite effect. Therefore, the surface tension of an aqueous acid-base solution does not change linearly between acid and base binary limits. Surface tensions of two acids (hydrochloric and sulfuric acids), two bases (ammonia and dimethylamine), and their four salts were explained by surface activities of single ions and molecular bases. Estimates of surface activities are based on solute properties (charge, size, and structure), Gibbs adsorption equation and surface tensions, and published molecular dynamic studies. In addition to published surface tensions, new data were measured for aqueous dimethylammonium chloride and ammonia/dimethylamine-hydrochloric acid solutions. Surface tensions of the ternary acid-base solutions change quite linear between acid, base, and salt (neutral solution) binary limits. On the basis of this linearity, several simple models, which need only binary solution surface tensions, were tested by calculating model deviations from the experimental data. As a result, two simple surface tension models were suggested for aqueous acid-base mixtures.