Micellar growth and network formation in acidic solutions of a sulfobetaine zwitterionic surfactant triggered by an inorganic salt

Abstract

Micellization (formation of wormlike structures) of 3-(N-erucamidopropyl-N,N-dimethyl ammonium)propane sulfonate (EDAS) in the presence of an inorganic salt, iron chloride (FeCl₃), in acidic conditions is studied using static and dynamic rheological measurements and small-angle X-ray scattering (SAXS). Infrared spectroscopy, single crystal X-ray, and UV-visible spectroscopy are used to further investigate the mechanisms of viscosity and elasticity and the enhancement and formation of an elastic gel-like solution induced by Fe³⁺ in HCl. The nature of the interaction is characterized to be hydrogen bonding between the amide groups of EDAS and coordinated water in trans-[FeCl₂(H₂O)₄]Cl structure. Such an interaction masks the repulsion forces between surfactant headgroups. This screening effect results in the formation of longer wormlike micelles in the solution. The chemical structure of FeCl₃ in concentrated HCl was predicted in the literature through theoretical and experimental techniques; however, its crystal structure, including the exact geometry (e.g., cis or trans) and the non-coordinated chloride position is reported for the first time in the present study. The findings of this study show the sensitivity of a sulfobetaine zwitterionic surfactant to pH alteration and inorganic salt.