Lyotropic System Potassium Laurate/1-Decanol/Water as a Carrier Medium for a Ferronematic Liquid Crystal: Phase Diagram Study V. Berejnov and V. Cabuil Equipe Colloı ¨des Magne ´ tiques, Laboratoire I2C, ² UniVersite ´ Pierre et Marie Curie, Ba ˆ timent F, case 74, 4 place Jussieu, 75252 Paris Cedex 05, France R. Perzynski* Laboratoire MDH, ² UniVersite ´ Pierre et Marie Curie, Tour 13, case 78, 4 place Jussieu, 75252 Paris Cedex 05, France Yu. Raikher Laboratory of Complex Fluids, Institute of Continuous Media Mechanics, Perm 614013, Russia ReceiVed: April 17, 1998; In Final Form: June 16, 1998 Synthesis of lyotropic mixtures of potassium laurate, 1-decanol, and water is presented, and the phase diagram of this ternary system is investigated in the vicinity of the nematic region. Several phases are identified and the corresponding concentration and temperature-induced phase transitions are studied. To prepare ferronematic liquid-crystalline samples, positively charged magnetic nanoparticles are introduced into the lyotropic matrix. Modifications of the phase behavior of the system induced by the particle incorporation are described. 1. Introduction Hybrid colloids resulting from association of lyotropic systems with polymers, 1,2 fine particles, 3-7 or proteins 8 are nowadays becoming a subject of extensive research. The purpose for synthesizing such complex structures is to obtain lyotropic liquid crystals (nematics, smectics, hexagonal phases) with genuine smart properties. For example, incorporation of magnetic nanoparticles into lamellar structures enables their orientation in magnetic fields of small intensity. 7 Actually, lyotropic nematic systems as themselves can respond only to rather strong (∼several kilo-oersteds) magnetic fields. A well-known example of a field-induced orientational effect is the magnetic Frederiks transition. 9 Introduction of magnetic nanoparticles into lyotropic nematics in order to enhance the magneto-orientational response has been considered in some theoretical 10,11 and experimental works. 12,13 However, any attempt to interpret the observed behavior needs a detailed description concerning the morphology of the system, its phase diagram, orientational properties, etc. Are the particles ag- glomerated? Where particularly are they located in the mixture? The answers to those and alike questions are of crucial importance. In refs 12 and 13 interesting results on introduction of magnetic nanoparticles into a lyotropic system were presented. However, the prepared mixture was not stable: the particles agglomerated and the emerging aggregates separated from the nematic matrix. In our paper 14 we proposed to synthesize ferronematic samples by admixing to the same lyotropic system hydrophilic magnetic nanoparticles of γ-Fe 2 O 3 . The essential advantage is that, due to their individual surface charges, such particles do not agglomerate in water. With the ferronematic liquid crystals, thus obtained, we observe an orientational (Frederiks-like) transition at low fields, as described in ref 15. For the synthesis of the systems in question, the problem of prime importance is to obtain a compatible phase behavior of both colloidal components when mixed. The question is a tricky one, since the size of magnetic particles in ferrofluids is of the same order of magnitude as the size of the micelles, which are the elementary structure units of the lyotropic nematic. As a departure point, one may look at the components separately. The phase behavior of ionic colloidal dispersions of magnetic nanoparticles has been clarified considerably in the recent works. 4-7 Also, the phase properties of the system potassium laurate/1-decanol/water as itself have been described more than once. 16-18 However, despite dealing with nominally one and the same system, its reported phase diagrams sometimes deviate significantly from one another. This circumstance should not be that surprising: as emphasized by the authors of refs 17 and 18, the synthesis of mesogenic lyotropic samples is a delicate point. The main difficulty is concerned with the fact that potassium laurate is not a commercially available product and is always synthesized by the team using it. The objective of the present paper is to give a precise description of the developed synthesis scheme and to character- ize (i) the components of the ternary system before mixing, (ii) the way of mixing, and (iii) the lyotropic compositions themselves. For the latter, the phase diagram is investigated in great detail that enables us to localize the domain of the nematic orientational ordering, to determine its borders and identify the types of structure organization in the adjacent regions. As the nematic state is very sensitive to any composition or temperature changes, introduction of a new componentsthe magnetic particlessconsiderably affects the phase behavior of the system. In the final part we summarize how association of nanoparticles with the surfactant micelles influences both the existence of the nematic ordering and the stability of the magnetic colloid. 2. Experimental Section 2.1. Synthesis of the Samples. 2.1.1. Materials. The basic components of the system are commercially available ingredi- * To whom correspondence should be addressed. Fax: +33 01 44 27 45 35. ² Associated with the Centre National de la Recherche Scientifique. 7132 J. Phys. Chem. B 1998, 102, 7132-7138 S1089-5647(98)01904-X CCC: $15.00 © 1998 American Chemical Society Published on Web 08/19/1998