How do anions affect self-assembly and solubility of cetylpyridinium surfactants in water

We report the specific effects of a series of anions (chloride, nitrate, and oxalate) on the solubility and self-assembly of cationic cetylpyridinium surfactants in water. The anion influence on micellization was evidenced by tensiometry and determination of Krafft temperatures. Anions strongly affect these parameters, depending on their position in the lyotropic series as well as on their "bridging" character. Scattering techniques (light, X-rays, and neutrons) were used to characterize the structures of micelles, and by solving a lateral equation of state approach, we show that chaotropic anions can be considered as adsorbed on the pyridinium head groups, inducing a decrease of the surface polarity and a Krafft temperature shift. Mixing different counteranions in various ratios led to a competition with a preferential adsorption at the micellar surface.