Effect of Hofmeister anions on micellization and micellar growth of the surfactant cetylpyridinium chloride.

Controlling the morphological characteristics of micellar solutions is important for surfactant performance and for achieving desired properties. In this work we study how monovalent anions of the lyotropic series affect micellization, micellar transitions, and micellar growth of the cationic surfactant N-cetyl pyridinium chloride (CPyCl), with the aim of achieving a tool to methodically tune these self-assembly characteristics. For the first time, a set of ions of the Hofmeister series were studied by combining indirect (surface tension, conductivity, optical absorption, viscosity, dynamic light scattering) and direct-imaging cryogenic-transmission electron microscopy (cryo-TEM). Following recent literature on anionic surfactants, we considered the pyridinium headgroup as a chaotropic cation, interacting with cosmotrope and chaotrope anions (Cl(-), Br(-), NO(3)(-), ClO(3)(-)). We show that the micelles' structure is strongly influenced by both the nature and concentration of added anions and their location in the lyotropic series, but the lyotropic number by itself cannot explain all the effects measured. Especially interesting was the relatively small effect of the chlorate ion on the CMC, but its large effect on micellar transition and growth. We further test the influence of a hydrotrope on the first and second CMC and micellar growth, and compare it with the data obtained with the inorganic salts.