Surface phase separation and collapse of the stearate anion--alkaline earth cation complex.

The surface properties of fatty acid and fatty acid-alcohol mixtures were examined at 22-24 degrees C. At pH 12, sodium stearate forms a rigid surface film that generates an equilibrium spreading pressure of 16.5 dynes/cm. At pH 12, stearate-alkaline earth cation films collapse at the air-water interface and do not generate significant equilibrium spreading pressures. The rate of film collapse depends on the counterion decreasing in the sequence Ba2+ greater than Sr2+ greater than Ca2+. Stearate-stearyl alcohol mixtures form solid (condensed) films that are relatively stable and behave initially as homogeneous surfaces in their selectivities for counterions. Stearate-oleyl alcohol mixtures form fluid (expanded) films that are unstable. Lateral phase separations occur rapidly in fluid films and the stearate-alkaline earth cation phase collapses. The rate of film collapse in the fluid mixtures also depends on the counterion decreasing in the sequence Ba2+ greater than Ca2+. These surface properties suggest how a lipid anion may function as an ionophore in the translocation of alkaline earth cations.