Dependence of the Formation of Lyotropic Uniaxial and Biaxial Nematic Phases on the Existence of Hofmeister Ions in the Intermicellar Region.

The physicochemical properties of some lyotropic mixtures, composed of surfactant, electrolyte, decanol, and water, were studied using different Hofmeister series anions and cations of electrolytes. The effects of ions present in the intermicellar region were analyzed by comparing them with those at the micelle surfaces. Three experimental techniques were employed to investigate the lyotropic nematic phase properties. The textures of the lyotropic uniaxial (discotic nematic─ND and calamitic nematic─NC) and biaxial nematic (NB) phases were characterized by polarizing optical microscopy. The transitions from the uniaxial to the biaxial nematic phases (ND to NB and NB to NC) were determined from the temperature dependences of the birefringences of the nematic phases via laser conoscopy. The ND to NB (32.75 to 38.45 °C) and NB to NC (28.00 to 34.55 °C) phase transition temperatures shifted towards higher temperatures by an increase in chaotropicity of cations. An opposite behavior was observed for anions, where the transition temperatures shifted towards smaller values (33.65 to 30.05 °C and 28.20 to 24.75 °C, respectively). While the former case was attributed to the weakening of the attractive interactions between kosmotropic surfactant head groups and oppositely charged electrolyte ions (cations) due to the formation of loosely bound ion pairs, the latter one was explained by the different influence of anions on free K+ ions. Some structural parameters, such as the micelle's core average dimensions, micelle shape anisotropy, and average area per surfactant head group at the micelle surface, were obtained from the analysis of small-angle X-ray scattering data. According to the data, the ions, which remain in the intermicellar region (or the Gouy-Chapman layer), similar to the effect of the ions located at the micelle surfaces in the Stern layer, affect micellization structural parameters. The results show that the interactions between ionic species, arising from their kosmotropic and chaotropic characters, in the Gouy-Chapman layer and/or in the intermicellar region play a crucial role in the formation of the lyotropic nematic phases, especially the biaxial one.