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 perfluorodecanoa te surfactants in water have been investigated using cryo-transmission elec tron microscopy, nuclear magnetic resonance, and optical techniques. As the counterion became more "hydrophobic", at a fixed surfactant volume fractio n, the aggregates formed in the liquid phase possessed decreasing interfaci al curvature. This manifests itself as a structural transition in the aggre gates from micelles to vesicles then,on to flat bilayers. The aggregation f orm in the dilute region was found to influence the structure of the lyotro pic liquid crystalline phases formed at higher concentrations. The experime ntal observations made in this contribution are discussed within a simple f ramework of enhanced counterion binding.