Balance of pH and Ionic Strength Influences on Chain Melting Transition in Catanionic Vesicles Claire Vautrin,* ,† Thomas Zemb, † Matthias Schneider, ‡ and Motomu Tanaka ‡ SerVice de Chimie Mole ´ culaire, CEA Saclay, 91191 Gif-sur-YVette, France, and Lehrstuhl fu ¨r Biophysik E22, Technische UniVersita ¨t Mu ¨nchen, D-85748 Garching, Germany ReceiVed: December 10, 2003; In Final Form: March 23, 2004 We study the thermotropic phase transition of “salt-free” mixtures of single-tailed cationic and anionic surfactants, called “catanionic systems” in this paper, for concentrated (10 wt %) and diluted (0.5 wt %) samples. Small-angle neutron scattering (SANS) and differential scanning calorimetry (DSC) demonstrate that the chain melting phase transition is identical for diluted and concentrated samples at the same mole fraction (r) of anionic surfactants. The addition of anionic surfactants (for r ) 0.5-0.7) induces an increase of the global surface charge, followed by an increase of the main transition temperature. This evolution may be interpreted as the hydrogen bonding formation between carboxylic acid headgroups. At r ∼ 0.6, corresponding to the region where facetted vesicles are formed, the “melting” of the alkyl chains is a progressive process extending over 15 °C. Here, molten and crystallized bilayers coexist in swollen lamellar phases of catanionic bilayers. This coexistence explains the increase of the viscosity of the solution, via structural reorganization of the bilayers or friction between the partially fused vesicles. Introduction Mixtures of single-tailed cationic and anionic surfactants (catanionics) are known to produce a very rich range of aggregate microstructures and, in particular, present simulta- neously low spontaneous curvature due to ion-pair forming. 1,2 In the absence of additional salt, catanionic bilayers are expected theoretically to show high bending rigidity. 3 This rigidity of salt-free catanionics has been shown by observation of a very limited binding by cryoelectron microscopy. 4 Since such aggregates are formed by electrostatic interactions between the two oppositely charged headgroups, it is necessary to study so-called “salt-free” (also called “true”) catanionics. Here, influence of counterions still dominates, and therefore it is possible to compensate the free energy associated with local segregation of the components. 5 Salt-free catanionics were studied first by Bengt Jo ¨nsson and P. Jokela, 6-9 reporting a limited swelling of lamellar phases without charge at equimolar ratio. However, they observed an increase in the swelling at nonequimolar ratio, which can only be explained by coexistence of small, crystallized colloids in lamellar phases. 4 Highly charged aggregates can be prepared by mixing the corresponding acid and hydroxide surfactants without counter- ions, so no excess salt is formed. 10 In such ternary systems, two variables control the molecular interactions: the weight fraction Φ of surfactants and the molar ratio r of anionic surfactant (r ) n acid /n acid + n base ). For such “rigid” systems with a two-dimensional network of opposite charges, it is therefore interesting to study the influence of surface net charges on the thermotropic phase transition (e.g., chain melting) as a function of pH of the solution, since changing pH after formation of the aggregate will change the surface effective charge. The role of charge screening has been described in the literature for classical catanionic systems, 11 as well as pH and ionic strength effects on phospholipid systems, 12-14 and more recently on catanionic vesicles. 15 The first step in this direction is to determine if chain melting transitions are identical for the concentrated and diluted phases present in the phase diagram. Following our recent calorimetry experiments on phase behavior of concentrated, swollen lamellae (Φ ) 10 wt %), 16,17 we study here the thermotropic phase transition of salt-free catanionics of myristic acid and cetyltrimethylammonium hydroxide in diluted phases (Φ ) 0.5 wt %), which form polyhedral-shaped vesicles. 18 Materials and Methods The cationic surfactant cetyltrimethylammonium hydroxide (CTAOH) was prepared by ion exchange (resin Bio-Rad 100- 200 Mesh) with a basic ion exchanger from the bromide salt, purchased from Fluka (Germany) and used without further purification. The quality of the ion exchange was checked by electrophoresis with a Waters capillary ion analyzer. The quantity of residual bromide counterions was evaluated to be 2.2 mol %. Tetradecanoic acid (myristic acid, abbreviated as C13COOH) was purchased from Fluka and used after recrystal- lization in acetonitrile. The composition of the samples is given by the molar ratio of C13COOH to total surfactant concentration (r), and the total surfactant concentration (Φ) in weight percent. Heat capacity profiles were recorded on a Microcal MC2 calorimeter (Northampton, MA) by using a scan rate of 15 °C/ h. The resolution of this instrument was proven to be around 0.15 µW with a standard deviation of 0.025 µW. 19 The cell volume was fixed to 0.5 mL. To confirm the equilibration, two successive heating/cooling cycles repeated. The obtained heat capacity scans were analyzed using the routine of the Origin software. Viscosity measurements have been performed with a Couette geometry rheometer (Rheometrix RFS2) with a gap of 0.25 mm (6.28 rad s -1 , 50% deformation). -Potentials of the vesicle suspensions were determined by measuring the electro- phoretic mobility using a Zetameter Coulter Delsa 440SX. The * Corresponding author. E-mail: vautrin@scm.saclay.cea.fr. † CEA Saclay. ‡ Technische Universita ¨t Mu ¨nchen. 7986 J. Phys. Chem. B 2004, 108, 7986-7991 10.1021/jp037787a CCC: $27.50 © 2004 American Chemical Society Published on Web 04/29/2004