Influence of organic acids on surfactant self-assemblies in surfactant/oil/water systems

• The structure of surfactant self-assemblies was dependent on the affinity between the cosurfactant and oil.
• Oil more unsaturated allowed high solubilization of acid with bigger carbon chain.
• Oil less unsaturated allowed high solubilization of acid with smaller carbon chain.
• Acetic acid was more susceptible to partition in comparison to propionic acid.

The cosurfactant effect of short-chain organic acids (acetic and propionic acids) and their behavior in relation to the oil prevailing unsaturation (HOSO or sunflower oil) was evaluated from SAXS and rheological measurements in systems formed by water, oil, Tween80 and the organic acid. Self-assemble of the surfactants was influenced by the structure and ratio of the components, which formed different structures (microemulsions and liquid crystal) with different rheological behaviors. The water content exerted influence on the organization of the surfactant moieties and on the dissociation of the cosurfactant, modifying its partition to the water phase. Systems with acetic acid formed more rigid structures with higher viscosity, shear-thinning behavior and bigger distances between the structures due to the tendency of this acid to migrate to water phase. On the other hand, propionic acid was less susceptible to dissociation, being in its neutral form, which makes it able to remain at the interface and solubilize higher oil content. As a result, structures were softer with lower viscosity and slightly smaller distances between them. The interaction between oil and cosurfactant exerted influence on the colloidal properties that reflected on bulky properties. In general, the more hydrophobic oil (sunflower oil) facilitated the partition of the acetic acid to the water, giving to this structure greater distance, as compared to HOSO + acetic acid. Thus, small differences in the structure of the components can influence the interaction between them, changing the partition of the components and the surfactant behavior.

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