Ion Exchange in Catanionic Mixtures: From Ion Pair Amphiphiles to Surfactant Mixtures Eva Maurer, Luc Belloni, Thomas Zemb, and David Carrie `re* LIONS (Laboratoire Interdisciplinaire sur l’Organisation Nanome ´ trique et Supramole ´ culaire), CEA/Saclay, F-91191 Gif-sur-YVette Cedex, France ReceiVed January 22, 2007. In Final Form: March 29, 2007 We have studied concentrated equimolar mixtures of tetradecanoic acid (myristic acid, C 13 COOH) and hexadecyltrimethylammonium hydroxide (CTAOH) in which the OH - counterions are gradually exchanged by other anions (Cl - , Br - , CH 3 COO - , CH 3 -(C 6 H 4 )-SO 3 - ). We demonstrate that the stability of a L  phase can be achieved at equimolarity between both surfactants, provided that the phase contains also a sufficient number of anions exchanged with OH - . In the absence of exchange (equimolar mixture of C 13 COOH and CTAOH), a three-dimensional crystalline L c phase is produced. As the OH - ions are replaced by other ions, a swollen L  lamellar phase appears, first in coexistence with the L c (D* ) 400 Å) and then in coexistence with a dilute phase only (D* ) 215 Å). In the latter regime, the repeating distance depends very little on the exchange ratio, but rather on the nature of the counterion. If too many OH - ions are exchanged, the L  phase becomes unstable again. A Poisson-Boltzmann model with charge regulation computed for a closed system predicts qualitatively the existence of this narrow domain of stability for the L  phase. Introduction Mixtures of surfactants of opposite charge, also called “catanionic” mixtures, have motivated a broad interest from the scientific community for different reasons. 1,2 For instance, dilute mixtures of surfactants are known to form thermodynamically stable vesicles, 3 the stability of which arises from the highly nonideal mixing between the surfactants. 4 These stable catanionic vesicles show potential applications in drug delivery devices and microreactors. 5,6 The highly synergetic effects between surfactants of opposite charge have also been used for the supramolecular synthesis of biologically active compounds 7 and for the enhancement of bioavailability by solubilization of fatty acids. 8 The study of catanionic aggregates is also helpful for addressing fundamental questions about self-assembly. The phase diagrams have been discussed in terms of force balance, 9-11 and the morphology of the aggregates themselves require inspection of the curvature, rigidity, and crystalline state, i.e., molten or solid, of the bilayer itself. 12,13 In most of these systems, each ionic surfactant is accompanied by its own counterion in solution. One particular catanionic system consists of a mixture of surfactants in which the organic or inorganic counterions of the surfactants are absent, preventing electrostatic screening upon surfactant association. 2,14,15 In the myristic acid/cetyltrimethylammonium hydroxide/water system, a rich polymorphism has been reported, with lamellar phases, 16 discs, 17 or facetted vesicles 18 depending on the composition. The peculiar morphologies in this system originate from the lateral segregation of the surfactants, leading to a cocrystallization in the membrane and rejection of the excess surfactants into defects such as pores or rims. 12 It has been argued that the use of H + and OH - as the initial surfactant counterions allows recombination into water and thus the absence of salt in solution. The resulting large Debye length would allow electrostatics to be the driving force for this in-plane segregation since typical electrostatic pair potentials would become of the order of kT. As already underlined by Tondre and Caillet, 2 this illustrates a fundamental difference between “surfactant mixtures”, where salt is released from the combination of surfactants, as opposed to “ion pair amphiphiles” where the salt is eliminated or intrinsically absent. In this paper, we study the transition from one situation to the other. A myristate/cetyltrimethylammonium ion pair amphiphile is progressively converted into a surfactant mixture by exchanging the OH - anions with other X - anions, all other parameters remaining unchanged. We first demonstrate experimentally that the introduction of X - anions promotes the stability of lamellar phases in a narrow domain of composition. We then compare the experimental results with a Poisson- Boltzmann model with charge regulation in a closed system. The qualitative agreement between the experiments and the model allows general conditions for the stabilization of catanionic phases by ion exchange to be proposed. Materials and Methods Myristic acid (Fluka) was recrystallized twice from hot acetonitrile. A 10% weight solution of CTAOH in water (Sigma) was freeze- * To whom correspondence should be addressed. E-mail: david. carriere@cea.fr. Tel.: +33 1 69 08 54 89. Fax: +33 1 69 08 66 40. (1) Khan, A.; Marques, E. COCIS 1999, 4, 402. (2) Tondre, C.; Caillet, C. AdV. 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(15) Zemb, T.; Dubois, M. Aust. J. Chem. 2003, 56, 971. (16) Dubois, M.; Gulik-.Krzywicki, T.; Deme ´, B.; Zemb, T. C. R. Acad. Sci. Paris II C 1998, 1, 567. (17) Zemb, T.; Dubois, M.; Deme ´, B.; Gulik-Krzywicki, T. Science 1999, 283, 816. (18) Dubois, M.; Deme ´, B.; Gulik-Krzywicki, T.; Dedieu, J. C.; Vautrin, C.; De ´sert, S.; Perez, E.; Zemb, T. Nature 2001, 411, 672. 6554 Langmuir 2007, 23, 6554-6560 10.1021/la070184w CCC: $37.00 © 2007 American Chemical Society Published on Web 05/11/2007