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 soa ps consisting of silver stearate, arachidate, and behenate. Little phase in formation is available on long-chain soaps and none on soap mixtures, altho ugh the phase behavior and microstructure often have a direct effect on pro duct properties. In the present study the Krafft solubility boundaries of s odium stearate, arachidate, and behenate in water were measured for low soa p weight fractions. Data for the cmc showed that the observed Krafft bounda ry 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 mic elles. 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 composit ion can be fitted to a mixing rule based on the solid-liquid equilibrium th ermodynamics. To determine if multiple solid soap phases were present, the structures of the solid phases were characterized by wide-angle X-ray scatt ering, FTIR, and DSC. It was found that a single mixed crystalline solid ph ase is formed over most of the composition range. The bilayer spacing of th e 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 large ly disordered. The water content of the soap crystals was found to increase continuously with increasing environmental humidity, indicating that soap hydrates are not stoichiometric.