Effect of Protonation on the Solution and Phase Behavior of Aqueous Sodium Myristate

Abstract

Aqueous sodium myristate solutions have been shown to have unusually low dynamic tensions (1–10 mN/m) under pulsating area conditions. These solutions have no sharp solubility limit, evidently because they are protonated (or “hydrolyzed”) to form the much less soluble myristic acid and acid soaps. With no added electrolytes, the protonation fraction is 1% or less. The apparent protonation equilibrium “constant” increases with increasing concentration, indicating strong solution nonidealities, in addition to micellization. This protonation seems to affect the solution and phase behavior of aqueous sodium myristate strongly, as evidenced by the effect of added NaOH. Ion-selective electrodes (for Na+ and H+) and conductimetry indicate that at 25°C dissolved surfactant concentrations keep increasing well after dispersed particles are observed (2 mM). A cmc of about 4.5 mM, micelles of aggregation number n=70 and counterion binding parameter β=0.7 are inferred from these techniques. The cmc of sodium myristate increases slightly with temperature from 25 to 45°C. FTIR analysis of the filtered particles indicates that the dispersed particles are mainly acid soaps for concentrations less than 6 mM. With 10 mM NaOH, the particles observed above 2 mM consist mostly of sodium myristate. From both conductivity and IR data, the solubility of sodium myristate in water at 25°C is estimated to be about 6 mM, and as expected, it increases with increasing temperature and decreases with increasing sodium ion concentration.