Surface Immobilization and Mechanical Properties of Catanionic Hollow Faceted Polyhedrons Nicolas Delorme, ² Monique Dubois, ‡ Se ´ bastien Garnier, § Andre Laschewsky, §,| Richard Weinkamer, ² Thomas Zemb, ⊥ and Andreas Fery* ,² Max Planck Institute for Colloid and Interface Research, 14424 Potsdam, Germany, SerVice de Chimie Mole ´ culaire, CEA Saclay, 91191 Gif-sur-YVette, France, UniVersita ¨t Potsdam, 14415 Potsdam, Germany, Fraunhofer Institute for Applied Polymer Research FhG-IAP, D-14476 Potsdam, Germany, and Laboratoire Claude Frejacques, CNRS/CEA URA 331, DRECAM/SCM, 91191 Gif-sur-YVette, France ReceiVed: August 10, 2005; In Final Form: NoVember 24, 2005 We report here for the first time on surface immobilization of hollow faceted polyhedrons formed from catanionic surfactant mixtures. We find that electrostatic interaction with the substrate dominates their adhesion behavior. Using polyelectrolyte coated surfaces with tailored charge densities, polyhedrons can thus be immobilized without complete spreading, which allows for further study of their mechanical properties using AFM force measurements. The elastic response of individual polyhedrons can be locally resolved, showing pronounced differences in stiffness between faces and vertexes of the structure, which makes these systems interesting as models for structurally similar colloidal scale objects such as viruses, where such effects are predicted but cannot be directly observed due to the smaller dimensions. Elastic constants of the wall material are estimated using shell and plate deformation models and are found to be a factor of 5 larger than those for neutral lipidic bilayers in the gel state. We discuss the molecular origins of this high stiffness. Introduction Above the chain melting temperature, ionic surfactants in water self-assemble into a large number of stable shapes (sphere, cylinder, bilayers, ...). 1 When cationic and anionic surfactants are mixed in water, a catanionic system is formed. 2 These catanionic systems are stabilized by the hydration force only. 3 When anionic or cationic components are added in excess, one obtains colloids of controlled surface charge, spanning from nonionic to highly charged (80 µC/cm 2 ) and hence highly curved mesostructures. 4 One of the consequences of ion pairing in catanionic surfactant aggregates is an increase of 20-40 °C of the hydrophobic chain melting temperature sometimes called the “Krafft point”. Stable catanionic aggregates in the crystalline state have been first proven to exist as bilayers with the shaped of nanodisks (sometimes dubbed “bicelles” of controlled size). 5 Most textbooks consider crystalline surfactant aggregates as artifacts. These rigid dispersed colloids can only be obtained in catanionic systems, since the concentration of monomers in equilibrium with “frozen micelles” lies in the range of micro- moles, and not millimoles as for all other known single chain surfactant systems. The osmotic pressure, of electrostatic origin, is the dominant mechanism responsible for stability of this new kind of aggregates. 6 Due to crystallization of hydrophobic chains, bilayer rigidity of catanionic aggregates is higher than for single surfactant components because of the ion pair forming between the surfactant heads. 7 Furthermore, in the case of the absence of additional salt (so-called “true catanionic”), strong long-range electrostatic interactions occur and lead to the formation of giant colloidal structures, showing a rich polymorphism depending on the mixing ratio. 8 One of the most striking faceted stable shapes found is the faceted hollow polyhedron. It has been recently shown that if the edge bending energy is dominant, the free energy of formation of a polyhedron of fixed initial area is an icosahedron. 9 For this reason most of the identified objects have a local pentagonal symmetry. Since the shapes are not uniform, we design the object under investigation as a faceted poly- hedron. This shape occurs with micron-sized hollow objects of well-defined thickness set by chain length. 10 This regular shape is well suited to investigating bulk material properties of these catanionic bilayers. Indeed, by mixing mystiric acid (C 13 H 27 COO - ,H + ) and cetyltrimethylammonium hydroxide (C 16 H 33 N-(CH 3 ) 3 + ,OH - ) with an excess of the anionic com- ponent, hollow faceted polyhedrons can be formed. 6,8-11 At room temperature the chains are in the crystallized state, and freeze-fracture electron microscopic measurements 10 have shown that the faceted polyhedrons have a narrow radius distribution (from 0.7 to 2 µm) whereas combined small-angle neutron (SANS) and light scattering (SALS) experiments have demonstrated that the wall thickness of the faceted objects is 4.8 nm, corresponding to the frozen catanionic bilayer thickness. SANS has also demonstrated a surprisingly high wall rigidity since the persistence length is in the micrometer range, leading to an indirect estimation of Young’s modulus (E > 100 MPa). 10 So far, little is known on the interaction of catanionic polyhedrons with flat surfaces, and in the first part of the paper we study the interactions of the polyhedrons with different substrates and the resulting morphological changes. We find that, by using surfaces of suitable charge density, the interaction * Corresponding author. Telephone: 49-(0)-331-567-9202. Fax: 49-(0)- 331-567-9202. E-mail: andreas.fery@mpikg.mpg.de. ² Max Planck Institute for Colloid and Interface Research. ‡ CEA Saclay. § Universita ¨t Potsdam. | Fraunhofer Institute for Applied Polymer Research. ⊥ DRECAM/SCM. 1752 J. Phys. Chem. B 2006, 110, 1752-1758 10.1021/jp054473+ CCC: $33.50 © 2006 American Chemical Society Published on Web 01/11/2006