Catanionic surfactant systems—thermodynamic and structural conditions revisited

In this work, we review shortly the current state of knowledge about catanionic surfactant systems with a focus on the detailed understanding based on the molecular buildup of such systems and of the electrostatic interaction that controls their amphiphilic monolayer and bilayer. Particularly relevant here is the extent of hydrophobicity of the oppositely charged partners, which can range from just having a more or less hydrophobic counterion until a real surfactant of opposite charge. Based on this discussion, we then investigate different systems based on cetyltrimethylammonium (CTA) combined with either laurate (L) as an oppositely charged surfactant or naphthalenesulfonate (NS) as a strongly hydrophobic counterion. Both systems were studied for the case of having salt present by combining the two amphiphilic salts but also the salt-free situation which arises from combining the hydroxide with the acid. The phase behavior was determined as well as the mesoscopic structures present, as obtained by small-angle neutron scattering (SANS) and light scattering, which allow to discern formation of wormlike micelles and vesicles. Their presence was then further confirmed by rheological measurements, where in particular normal forces allow to distinguish the two types of aggregates, and control of rheology is a key property in such systems. In addition, the thermodynamic conditions in these systems were determined by means of differential scanning calorimetry (DSC). Based on these results, a consistent understanding of the formed structures and their macroscopic properties that arise from the molecular conditions in these systems is presented.