Turbidimetric studies of the interaction of aqueous micelles of phosphatidic acid with cations

Aqueous micelles of phosphatidic acid are prepared by ultrasonic radiation. In the pH range studied, ≧ 7, these dispersions consist of highly charged polyanions and have low turbidities. The interactions of buffered systems of phosphatidic acid with cations can be studied by turbidimetric titrations. At pH 7.2, increased turbidity occurs in 0.2 N LiCl or 0.4 N NaCl, however, KCl and tetramethylammonium chloride produce no change up to concentrations of 0.75 N. In 10⁻⁵ M CaC1₂ the turbidity increases and rises steeply to 2 × 10⁻⁴ M. Somewhat higher concentrations are necessary with MgCl₂ . Additions of univalent cations to phosphatidic acid containing Ca reduces the turbidity by exchange with Ca.

In systems containing sodium citrate, the concentrations of CaCl₂ required to produce given turbidity changes are greater than in comparable systems containing NaCl. This gives a measure of the concentration of CaCit⁻ formed. From the known formation constant of CaCit⁻, the Ca⁺² concentration in equilibrium with the anions of citric and phosphatidic acids is calculated. This permits the evaluation of the formation constant of CaPA. Values obtained at pH 7.2, 8.8, and 9.6 are 1.2 × 10⁴.