Unusual phenomena in perfluorosurfactants

Abstract

Perfluoro-surfactants form the same type of micelles and micellar phrases as hydrocarbon surfactants. It is shown that a considerable share of the different micellar phases that potentially exist in surfactant solutions can be formed with a single perfluoro-surfactant by adding different additives to the same starting solution. Spherical micelles exist in solutions of perfluoro-sulfonic acid. This compound is a strong acid both in the monomer and in the micellar state and the micelles have a large dissociation degree. Solutions of this surfactant have a low viscosity up to high concentrations. When a solution of N(C 2H 5) 4OH is added to a solution of perfluoro sulfonic acid entangled threadlike micelles are formed. For a 100 mM solution one obtains instantaneously upon mixing a highly viscoelastic solution that shows the recoil-effect. In the experiment the acid is neutralised and the N +(C 2H 5) 4-counterions bind strongly to the micellar interface. As a consequence, the cmc of the surfactant is shifted to lower values and the spheres grow into overlapping rods. The viscoelastic perfluoro-surfactant solutions show the same scaling laws as hydrocarbon surfactants. N +(C 2H 5) 4-counterions bind strongly to both the perfluorosulfonic and the perfluorocarboxylic acid but not to hydrocarbon surfactants. Viscoelastic perfluoro-surfactant solutions can also be prepared by neutralising perfluorocarboxylic acids with partially substituted alkylammonium salts. Perfluorocarboxylic acids are strong acids in the monomer state but weak acids in the micellar state, i.e. the micelles are actually completely associated and the surfactant forms bilayer structures like viscles above the cmc. The vesicles entrap surfactant- and the dissociated counter-ions from the bulk and - as a consequence - the conductivity of the perfluorocarboxylic acid passes over a sharp maximum at the cmc of the surfactant with increasing concentration. The viscoelastic properties of solutions with entangled threadlike micelles are lost by solubilisation of perfluorinated oils. This can be demonstrated by adding C 7F 16 to the viscoelastic solution which was obtained in the first experiment. The perfluoro-additive is solubilised by the micelles, enforces a rod-sphere transition and - as result - a low viscosity solution is obtained. Alternatively, the network of overlapping rodlike micelles can be transformed into another type of viscoelastic solution with densely packed multimellar vesicles. This transition is demonstrated by adding hexanol to the solution with the rodlike micelles. The transformation occurs instantaneously upon mixing and stirring. Contrary to the viscoelastic solutions from entangled threadlike micelles which have a finite zero-shear viscosity, the solutions with the vesicles have a yield stress value. The fluid can be used as a suspending fluid. A cosurfactant like hexanol is needed to transform the micelles into structures with average zero curvature. The vesicle phase is actually a L α-phase. For the experimental condition of 150 mM hexanol, the system is in the single phase region. Interestingly, these systems have a relatively high conductivity compared to multilamellar vesicles made up of hydrocarbon surfactants only which is probably an indication of a larger permeability of the vesicle bilayers. In the described experiment it was further demonstrated that densely packed vesicles can be transformed into the classic L α-phase by adding excess electrolyte which shields the charge and thus lowers the charge density of the bilayers. The yield stress value is lost by this manipulation and the shear modulus decreases.