Effect of pH and temperature on the solubility of a surface active carboxylic acid

We have examined the effect of pH and temperature on the solubility of 3-(4-heptylbenzoyl) benzoic acid (1), a surface active carboxylic acid with a pKa of 4.83 and a critical micelle concentration (cmc) of 4 X 10(-4) M. Our results show that below a pH of 7.0 and a temperature of 50 degrees C, the solubility of 1 is less than the cmc; under these conditions, 1 behaves like a typical weak acid. Above pH 7.0, the solubility can equal or exceed the cmc, depending on the temperature, and dramatic deviations from typical weak acid solubility behavior are observed. The temperatures at which the solubility becomes equal to the cmc at various pH values were determined; these temperatures are termed apparent Krafft Points (KPapp). The KPapp varies with pH; the higher the pH, the lower the KPapp. For example, the KPapp at pH 7.0 is 49 degrees C, and at pH 8.0 is less than 5 degrees C. We report the pH-temperature-solubility relationship for 1 and use it to construct simple phase diagrams.