Counterion effects on aggregate size and shape in dilute binary solutions of fluorinated ammonium carboxylate surfactants

The dilute liquid phase of a range of substituted ammonium perfluorodecanoate surfactants in water have been investigated using cryo-transmission electron microscopy, nuclear magnetic resonance, and optical techniques. As the counterion became more "hydrophobic", at a fixed surfactant volume fraction, the aggregates formed in the liquid phase possessed decreasing interfacial curvature. This manifests itself as a structural transition in the aggregates from micelles to vesicles then on to flat bilayers. The aggregation form in the dilute region was found to influence the structure of the lyotropic liquid crystalline phases formed at higher concentrations. The experimental observations made in this contribution are discussed within a simple framework of enhanced counterion binding.