Physicochemical characterization of green sodium oleate-based formulations. Part 2. Effect of anions

In Part 1 of this work we reported the behavior of a moderately concentrated dispersion of sodium oleate (NaOL) in water that produces elongated wormlike micelles (WLMs). Prompted by the striking effect induced by adding potassium chloride to the original NaOL dispersion, here we investigate the effect of different anions (with fixed cation) on NaOL or KOL-based hydrogels upon addition of different strong electrolytes. The interest in these investigations relies on the fact that they are among the best candidates for the production of eco-friendly stimulus-responsive materials.

The thermal and rheological properties of a 0.43 M dispersion of NaOL or KOL in water were investigated by steady-state and oscillatory rheology, and DSC experiments in the presence of different potassium or sodium salts at the same concentration (0.54 m), respectively.

This paper highlights the occurrence of a Hofmeister phenomenon in the case of oleate-based WLMs and illustrates the remarkable effect induced by kosmotropic and chaotropic anions in terms of rheology and hydration of the rod-like nanoassemblies, that reflect the different ion adsorption at the WLM interface. We also discuss the different ion condensation of sodium and potassium ions at the interface that can lead to a significant change in the curvature of the elongated rods.