Formation and Structure of Acid Soap Crystals through in-situ Neutralisation in the Environments of Surfactant

Non-ionic surfactants are important co-surfactant in detergent products to enhance the detergency in hard water. And the alcohol ethoxylates are the most common non-ionic surfactants used in consumer products. However, the liquid-like surfactant can cause unwanted aggregation of washing powder. In order to accelerate the solidification of non-ionic surfactant, the acid-soaps were chosen as gelator to form crystal networks in the mixtures to encapsulate the liquid-like non-ionic surfactant.
Acid-soaps are the co-crystals formed by fatty acids and their metallic salts stoichiometrically. Although the acid-soaps were extensively studied over decades, the stoichiometric ratios are still unclear. In this project, the crystal structure of stearate acid-soap with series neutralisation ratio were determined by powder X-ray diffraction. The thermodynamic properties were determined by differential scanning calorimetry and the binary phase diagram of the stearic acid and sodium stearate system was generated. The phase behaviour during heating process was observed by small angle X-ray scattering (SAXS) and wide angle X-ray scattering. The phase transitions associate with the endo/exo-thermal peaks in DSC were assigned. The results indicated that the stoichiometric ratios of stearate acid-soaps are 2:1 and 1:1, which is contradict with the previous publications that the acid: soap ratios are 3:2, 1:1, 2:3, 2:5 and 1:5. The SAXS/WAXS with temperature stage results indicated that the acid-soaps with high acid ratio can transform to the form with low acid ratio, and the polymorphs were also observed. The morphologies of acid-soaps were observed by scanning electron microscopy.
The studies on the influences of the surfactants on the acid-soaps formation were carried out with non-ionic surfactants (alcohol ethoxylates) and cationic surfactants (alcohol ethoxylate sulfonates). The characteristic peaks associate with acid-soaps were observed in the mixtures. The d-spacings of the crystals formed in the mixtures of alcohol ethoxylate sulfonates and stearic acid were complex, which is because the AES has tendency to form acid-soap. The influences of perfume on the liquid crystal of AES and the crystals of stearic acid/acid-soaps were also studied.