Solution phase behavior and solid phase structure of long-chain sodium soap mixtures

Long-chain soaps are generally applied in industrial products as mixtures. For example, photothermographic materials often use a mixture of silver soaps consisting of silver stearate, arachidate, and behenate. Little phase information is available on long-chain soaps and none on soap mixtures, although the phase behavior and microstructure often have a direct effect on product properties. In the present study the Krafft solubility boundaries of sodium stearate, arachidate, and behenate in water were measured for low soap weight fractions. Data for the cmc showed that the observed Krafft boundary lies above the cmc in its entirety for each of the soaps. Therefore, the knee in the Krafft boundary cannot be identified with the formation of micelles. The Krafft temperature of mixtures of these three soaps was observed to have a minimum value at a high content of the shortest-chain soap. The nonlinear relationship between the soap solubility and the mixture composition can be fitted to a mixing rule based on the solid-liquid equilibrium thermodynamics. To determine if multiple solid soap phases were present, the structures of the solid phases were characterized by wide-angle X-ray scattering, FTIR, and DSC. It was found that a single mixed crystalline solid phase is formed over most of the composition range. The bilayer spacing of the soap crystals is close to that of the majority component, except when the weight fractions are roughly equal, in which case the solid phase is largely disordered. The water content of the soap crystals was found to increase continuously with increasing environmental humidity, indicating that soap hydrates are not stoichiometric.