Effect of temperature on critical micelle concentration and thermodynamic potentials of micellization of anionic ammonium dodecyl sulfate and cationic octadecyl trimethyl ammonium chloride

The critical micelle concentrations (CMCs) of anionic ammonium dodecyl sulfate (ADS) and cationic octadecyltrimethylammonium chloride (OTAC) were determined at various temperatures using a du Noüy ring tensiometer. The CMC decreased to a certain minimum and then increased with the temperature, displaying a U-shaped behavior. This behavior was analyzed using a power-law equation on the basis of reduced variables. The exponents of the power-law equation were found to be 1.05 for ADS and 1.12 for OTAC. These values were different from previous literature values of 1.74 for ionics, 3.54 for commercial ionics, and 5.80 for commercial amphoterics. Also, the U-shaped behavior is explained in terms of hydrophobic forces, as described by the thermodynamic potentials and parameters in the pseudo-phase separation model.