Influence of Amphiphilic Block Copolymers on Lyotropic Liquid Crystals in Water-Oil-Surfactant Systems Christian Frank, † Thomas Sottmann, ‡ Cosima Stubenrauch,* ,§ Ju ¨ rgen Allgaier, † and Reinhard Strey ‡ Forschungszentrum Ju ¨ lich GmbH, Institut fu ¨ r Festko ¨ rperforschung, D-52425 Ju ¨ lich, Germany, Institut fu ¨ r Physikalische Chemie, Universita ¨ t zu Ko ¨ ln, Luxemburger Strasse 116, D-50939 Ko ¨ ln, Germany, and Department of Chemical and Biochemical Engineering, University College Dublin, Belfield, Dublin 4, Ireland Received June 3, 2005. In Final Form: July 20, 2005 In ternary water-oil-nonionic alkyl polyglycol ether (CiEj) microemulsions, an increase in efficiency is always accompanied by the formation of a lamellar (LR) phase. The addition of an amphiphilic block copolymer to the ternary base system increases the efficiency of the microemulsion drastically while suppressingsat least partlysthe formation of the L R phase. However, amphiphilic block copolymers can be used not only to suppress the formation of lyotropic liquid crystals but also for the opposite effect, namely, to induce their formation. To understand to what extent the increase in efficiency is accompanied by the formation of lyotropic liquid crystals, we studied phase diagrams of water-n-alkane-n-alkyl polyglycol ethers (C iEj)-PEPX-PEOY at a constant volume fraction of oil in the water/oil mixture. Using polymers of the poly(ethylene propylene)-copoly(ethylene oxide) type, with MPEP ) X kg mol -1 and MPEO ) Y kg mol -1 , we determined phase diagrams as a function of the polymer concentration, size, and symmetry. Moreover, the influence of a particular polymer mixture was studied, which turned out to be the best system if both a high efficiency and a low tendency to form an L R phase are needed. 1. Introduction Lamellar (L R ) phases and other lyotropic liquid crystals exist in many chemical and biological systems. They are well-known in binary water-surfactant and ternary water-oil- surfactant systems. In the former systems, the L R phase consists of surfactant bilayers surrounded by water, whereas in the latter systems, stacked mono- layers separate oil and water domains. Liquid crystalline phases play a key role in many technical processes in which they are used as nanoreactors or templates for the synthesis of nanoparticles and mesoporous solid material. 1-4 Moreover, L R phases form the membranes of biological cells. 5 On the other hand, there are certain applications and processes in which the formation of lyotropic liquid crystals is undesirable because they are often highly viscous. Thus, the challenge is to control the stability of lyotropic liquid crystals in general and that of the L R phase in particular, that is, to induce or suppress them depending on the respective needs. Looking at the phase diagrams of binary water- surfactant systems, one sees that the L R phase can expand over a broad concentration range, namely, from surfactant concentrations of more than 80 wt % down to surfactant concentrations of less than 1%. Such expanded L R regions can be found using, for instance, the nonionic alkyl polyglycol ethers C 12 E 5 (L R down to 1% 6 ) or C 10 E 4 (L R down to 8% 7 ) as surfactants. Although the general phase behavior of binary water-surfactant systems is reviewed in detail by Laughlin, 8 the control of the stability of the L R phase in these kinds of systems was discussed recently. 7 The L R phase can also be observed over a broad concen- tration range in ternary water-oil-surfactant systems. 9 When one considers the Gibbs phase triangle at the phase inversion temperature, it appears that the L R phase extends deep into the water and oil corners. 10 These diluted L R phases consist of oil- and water-swollen surfactant bilayers, respectively. At equal volumes of water and oil, however, surfactant monolayers separate the two sub- domains. Note that L R phases formed by monolayers are not as stable as those stabilized by bilayers, which is most likely due to the smaller bending rigidity of the mono- layers. For example, at equal volume fractions of water and n-octane, at least 7 wt % of C 12 E 5 molecules are needed to form a (monolayer) L R phase, 11 whereas a (bilayer) L R phase is already observed at 1 wt % in the corresponding water-C 12 E 5 system. 6 Upon dilution, both types of L R phases become unstable, which leads to the formation of isotropic phases. In the binary system, an L 3 phase is formed in which a randomly oriented bilayer is found to divide the space into two equivalent water-continuous subvolumes. 12 In the ternary system, however, a bicontinuous microemulsion is formed in which it is a randomly oriented monolayer that separates the continuous water- and oil-rich subvol- * Corresponding author. E-mail: cosima.stubenrauch@ucd.ie; phone: +353-1-716-1923; fax: +353-1-716-1177. † Forschungszentrum Ju ¨ lich GmbH. ‡ Universita ¨t zu Ko ¨ln. § University College Dublin. (1) Attard, G. S.; Glyde, J. C.; Goltner, C. G. Nature 1995, 378, 366. (2) Go ¨ ltner, C. G.; Berton, B.; Kramer, E.; Antonietti, M. Adv. Mater. 1999, 11, 395. (3) Kao, C.-P.; Lin, H. P.; Mou, C. Y. J. Phys. Chem. Solids 2001, 62, 1555. (4) Han, B.-H.; Antonietti, M. J. Mater. Chem. 2003, 13, 1793. (5) Structure and Dynamics of Membranes; Lipowsky, R., Sackmann, E., Eds.; Elsevier: New York, 1995. (6) Strey, R. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 182. (7) Stubenrauch, C.; Burauer, S.; Strey, R.; Schmidt, C. Liq. Cryst. 2004, 31, 39. (8) Laughlin, R. G. Aqueous Phase Behavior of Surfactants; Academic Press: New York, 1994. (9) Kahlweit, M.; Strey, R.; Firman, P. J. Phys. Chem. 1986, 90, 671. (10) Olsson, U.; Wu ¨ rz, U.; Strey, R. J. Phys. Chem. 1993, 97, 4535. (11) Strey, R. Colloid Polym. Sci. 1994, 272, 1005. (12) Porte, G.; Marignan, J.; Bassereau, P.; May, R. J. Phys. (Paris) 1988, 49, 511. 9058 Langmuir 2005, 21, 9058-9067 10.1021/la051463r CCC: $30.25 © 2005 American Chemical Society Published on Web 08/19/2005