Polyelectrolyte-induced structural changes in the isotropic phase of the sulfobetaine/ pentanol/toluene/water system

Abstract

The paper describes the behavior of the sulfobetaine/pentanol/toluene/water system. An isotropic inverse micellar region (L₂ phase) can be observed in the oil corner. The area of the L₂ phase is decreased by adding an anionic polyelectrolyte, i.e. sodium polyacrylate. The incorporation of the cationic polyelectrolyte poly(diallyl-dimethylammonium chloride) (PDADMAC) induces the extension of the isotropic phase in the direction of the water corner under the formation of an isotropic phase channel. The aim of the work presented here was to study this phase channel at constant surfactant concentration (i.e. 20%) by means of electrical conductivity, ¹H NMR self-diffusion experiments, rheology, polarizing microscopy, small-angle X-ray spectroscopy, and micro-differential scanning calorimetry (DSC). Macroscopically, no indication of a phase transition is observed when going from the oil-rich side to the water-rich side. However, above a critical content of the aqueous PDADMAC solution, the diffusion coefficients are changed significantly, freezable water becomes detectable in DSC measurements, and lamellar liquid-crystalline textures become visible after shearing. This behavior can be explained by a shear-induced transition from an L₂ phase to a lamellar liquid-crystalline phase, and is combined by only small viscometric and conductometric effects. The transition from the L₂ to the L_α phase can be explained by a significant change of the curvature of the surfactant film due to Coulombic interactions with the polycation.