Phase Transitions of Branched Fatty-Acid Calcium Salt/Water Systems

Calcium (Ca) salts of fatty acids are powder materials, which exhibit lubricant, hydrophobic, and bactericidal properties. These physical and biological properties depend on the chemical structure of the alkyl chain of the fatty acids. In this study, the phase behavior of iso-stearic acid Ca salt/water binary systems was studied, and the effect of the branched structure in the alkyl chain was analyzed. Herein, calcium chloride is added to the iso-stearic acid aqueous solution. The state of the binary systems changed with the molar ratio (R) of [Ca2+]/[iso-stearic acid]. Precipitation occurred when R was more than 0.05, whereas the mixture changed from a micelle phase (W-m) to an oil phase (O) through a liquid crystalline phase (LC) when R was less than 0.01. The polarized microscopy images and small-angle X-ray scattering profiles showed that the LC is in a lamellar liquid crystalline state. In addition, we evaluated the crystal structure of the precipitated material. These results showed that the iso-stearic acid Ca salt was in the amorphous state. In contrast, the n-stearic acid Ca salt formed only the W-m and gel phases. These findings suggest that the branched structure of fatty acids induces loose intermolecular packing of liquid crystalline structures.