Mixtures of cationic and anionic surfactants dispersed in water show a rich polymorphism depending on the mixing ratio. For an excess of the anionic component, hollow facetted polyhedrons are formed which have been subject to detailed structural studies in the past.1, 2
We focus here on the nano-mechanical properties of these objects. While our aim in this study is to understand the impact of electrostatic interactions on the mechanics of the catanionics, our results are also relevant for studies on viruses or facetted vesicles which are mechanically analogous.
In this purpose, Atomic Force Microscopy (AFM) is used to measure elastic properties and other surface interaction forces.
In order to measure the mechanical properties with the AFM apparatus, one needs to immobilize the polyhedrons onto a flat surface. z-potential measurements have revealed that the surface of the facetted polyhedron is positively charged. However, it has been shown that strongly negatively charged surfaces (silica) involves an important spreading of the facetted objects leading to the formation of a flat membrane onto the surface. The point was to find a negatively charged surface of suitable charge density. This has been achieved using polylelectrolytes of tailored charge density.
Combining force measurements and imaging allowed demonstrating the strong dependence between the stiffness and the shape of the facetted polyhedrons.3
The mechanical measurements have shown that compared to a supported lipid bilayer in the gel-state, the catanionic membrane is very rigid, showing an effective bending modulus of up to 450 kBT.4 In order to understand the origin of the rigidity of the catanionic membrane, the evolution of the mechanical properties of the polyhedron were investigated as function of the nature of salt and the salt concentration, which are know to change the electrostatic contribution to the bending stiffness.
(1) Dubois, M.; Demé, B.; Gulik-Krzywicki, T.; Dedieu, J.-C.; Vautrin, C.; Désert, S.; Perez, E.; Zemb, T. Nature 2001, 411, 672-675.