Determination of Pore Size of Catanionic Icosahedral Aggregates Karine Glinel,* ,† Monique Dubois, ‡,§ Jean-Marc Verbavatz, | Gleb B. Sukhorukov, ‡ and Thomas Zemb* ,§ “Polyme`res, Biopolyme`res, Membranes”, UMR 6522 CNRS - Universite´ de Rouen, F-76821 Mont Saint Aignan, France, Max Planck Institute of Colloids and Interfaces, Interfaces Department, D-14476 Golm/Potsdam, Germany, Service de Chimie Mole´culaire, CEA/DSM/DRECAM, CE Saclay, F-91191 Gif-sur-Yvette, France, and Service de Biophysique des Fonctions Membranaires DSV/DBJC, CEA/Saclay, F-91191 Gif-sur-Yvette, France Received March 15, 2004. In Final Form: July 19, 2004 We show that it is possible to measure the porosity of facetted micron-sized hollow icosahedra of catanionic solutions by performing fluorescence recovery after photobleaching measurements. The size of spontaneous permanent pores in bilayers formed via molecular segregation is compatible with what is observed by freeze-fracture electron microscopy and is discussed versus theoretical expressions of bending energy. Introduction In the absence of salt, mixtures of single-tailed anionic and cationic surfactants produce so-called catanionic aggregates resulting from electrostatic interactions be- tween oppositely charged headgroups. 1 Lamellar phases, stiff nanodisks, cylindrical micelles, and facetted vesicles are present in the room-temperature triangular cut of the ternary phase prism. 2 The association of cationic and anionic surfactants produces bilayers with variable charge per unit area and hence allows a direct and unique measurement of intermolecular forces between bilayers of controlled surface charge in the domain -30 to +30 µC/cm 2 . 3 In the presence of salt formed by a com- bination of counterions, quaternary systems including at least one hydrophobic counterion such as tosylate spon- taneously form multilamellar vesicles in equilibrium with lamellar phases at maximum swelling. 4 A particular type of catanionic system is obtained by mixing the corre- sponding acid and hydroxide surfactants: The counterions react to form a water molecule so that, in salt-free conditions, a salt-free system is produced. 5,6 A common, but yet unexplained,feature of all catanionic systems studied so far is their surprisingly low aptitude to encapsulate water-soluble molecules. A review on all available data related to this surprisingly quick release of solutes once encapsulated in catanionic globular or facetted vesicles has been published. 7 The equilibrium phase diagram of the mixture of tetradecanoic acid, so-called myristic acid, and cetyltri- methylammonium hydroxide was studied in water. 6 We showed that this system spontaneously forms vesicles, micelles, lamellar phases, and disk aggregates depending on the composition of the mixture. 6,8-11 Moreover, in the very dilute range of concentrations (<1%)and in the presence of a slight excess of myristic acid (i.e., the molar fraction of myristic acid in the surfactant mixture is equal to 0.57), we observed the formation of micrometer-sized rigid vesicles of icosahedral shape. 12 This atypical geo- metrical shape was clearly evidenced by freeze-fracture electron microscopy (FFEM)(Figure 1). Various other techniques such as X-ray and neutron scattering, confocal and Normasky microscopies, cryogenic transmission electron microscopy, and differential scanning calorimetry (DSC) were applied to confirm the morphology and the microstructure of these polyhedral-shaped aggregates. 3,12,13 It was shown that the faces are composed of crystalline bilayers resulting from a very regular close packing of amphiphile chains and the vertexes and the edges are enriched in anionic surfactant. Chain melting of ag- gregates is a process extending thermodynamically over 10 °C. 14 When the bilayer is completely frozen, a noticeable feature of the facetted crystallized aggregates is the presence of pores on some of the vertexes, which are clearly seen by electron microscopy at high magnification (Figure 1a). The formation of these pores is linked to the excess of myristic acid in the mixture. 12 Indeed, a possible topology to evacuate the surfactant in excess from stoichiometric cocrystallization is to segregate them in pores. These pores are of importance since they control the permeability of the aggregates and consequently their ability to entrap and to release probes or drug. * To whom correspondence should be addressed. K. Glinel: e-mail, karine.glinel@univ-rouen.fr; fax, 00 33 2 35 14 67 04. † “Polyme`res, Biopolyme`res, Membranes”, UMR 6522 CNRS - Universite´ de Rouen. ‡ Max Planck Institute of Colloids and Interfaces,Interfaces Department. § Service de Chimie Mole´culaire,CEA/DSM/DRECAM, CE Saclay. | Service de Biophysique des Fonctions Membranaires DSV/ DBJC, CEA/Saclay. (1) Pa¨ ivi, J.; Bengt, J.; Eichmu¨ ller, B.; Fontell, K. Langmuir 1988, 4 (1), 187. (2) Zemb, Th.; Dubois, M. Aust. J. Chem. 2003, 56, 971. (3) Meister, A.; Dubois, M.; Belloni, L.; Zemb, Th. Langmuir 2003, 19, 7259. (4) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. J. Phys. Chem. 1992, 96, 6698. (5) Dubois,M.; Zemb,Th. Book of Abstracts, 215th ACS National Meeting,Dallas, Mar 29-Apr 2, 1998;American ChemicalSociety: Washington, DC, 1998. (6) Dubois, M.; Gulik-Krzywicki, Th.; Deme´, B.; Zemb, Th. C. R. Acad. Sci. 1998, IIC, 567. (7) Tondre, C.; Caillet, C. Adv. Colloid Interface Sci. 2001, 93, 115. (8) Dubois, M.; Belloni, L.; Zemb, Th.; Deme´, B.; Gulik-Krzywicki, T. Prog. Colloid Polym. Sci. 2000, 115, 238. (9) Vautrin, C.; Dubois, M.; Zemb, Th.; Schmo¨lzer, S.; Hoffmann, H.; Gradzielski, M. Colloids Surf., A 2003, 217, 165. (10) Zemb, Th.; Dubois, M.; Deme´, B.; Gulik-Krzywicki, Th. Science 1999, 283, 816. (11) Dubois, M.; Dedieu, J.-C.; Deme´, B.; Gulik-Krzywicki, Th.; Zemb, Th. ACS Symp. Ser. 1999, 736, 86. (12) Dubois,M.; Deme´, B.; Gulik-Krzywicki, Th.; Dedieu, J.-C.; Vautrin, C.; De´sert, S.; Perez,E.; Zemb,Th. Nature 2001, 411,672. (13) Vautrin, C.; Zemb, T.; Schneider, M.; Tanaka, M. Submitted. (14) Vautrin, C.; Zemb, Th.; Schneider, M.; Tanaka, M. J. Phys. Chem. B 2004, 108, 7986. 8546 Langmuir 2004, 20, 8546-8551 10.1021/la049328m CCC: $27.50© 2004 American Chemical Society Published on Web 08/28/2004