SYNTHESIS OF STABLE LYOTROPIC FERRONEMATICS

WITH HIGH MAGNETIC CONTENT

V. Berejnov ^1,2 , Yu. Raikher ², V. Cabuil ¹, J.-C. Bacri ³, R. Perzynski ³

1. Laboratoire de Physicochimie Inorganique, Université Pierre et Marie Curie, Bat. F, case 63, 4 place Jussieu, 75252 Paris Cedex 05, France.

2. Laboratory of Kinetics of Complex Fluids, Institute of Continuous Media Mechanics, Urals Branch of the Russian Academy of Sciences, Perm, 614013, Russia.

3. Laboratoire des Milieux Désordonnés et Hétérogènes, Université Pierre et Marie Curie, Tour 13, case 78, 4 place Jussieu, 75252 Paris Cedex 05, France

Corresponding author: Dr. Valerie Cabuil, Laboratoire de Physicochimie Inorganique, Université Pierre et Marie Curie, Bat. F, case 63,4 place Jussieu, 75252 Paris Cedex 05, FRANCE

E-mail: cabuil@ccr.jussieu.fr

Telephone: 33 – 1 – 44 – 27 – 31 – 74

Telefax: 33 – 1 – 44 – 27 – 36 – 75

Abstract.

We report synthesis of colloidal solutions of ferrite nanoparticles in the surfactant micellar system potassium laurate / 1-decanol / water within the region of its nematic order. The obtained ferronematic specimens are reversibly oriented by low (£ 100 Oe) magnetic fields and retain their transparence and stability during several months.

Key words: liquid crystals, lyotropic nematics, ferrofluids, orientational transitions

Running title: Synthesis of lyotropic ferronematics

Introduction.

Introduction of magnetic nanoparticles into liquid-crystalline matrices is a problem as challenging (1) as it is tricky (2). About 20 years ago L. Liebert and A. Martinet (3) were the first to successfully approach it by assigning the role of a matrix to self-assembling surfactant solutions. They prepared and studied lyotropic ferronematic liquid crystals resulting from admixing ferrite (magnetite) grains to the ternary system: sodium dodecylsulphate / 1-decanol / water. The obtained samples proved to be sufficiently homogeneous and readily orientable by low ( £ 100 Oe) magnetic fields. Despite the tiny amount of the ferrite grains used (less than 10¹⁴/cm³), the lyotropic ferronematics (LFN) of Ref. (3) were inclined to agglomeration, and the net colloidal stability of the system was rather low. Nevertheless, due to their remarkable properties, LFN became the objects of interesting studies (4–7).

In the recent years, the stability of aqueous dispersions of magnetic nanoparticles has been improved (8). It allowed to synthesize several other types of magnetic colloids in self-assembling surfactants, isotropic (magnetic emulsions (9) and vesicles (10,11) or anisotropic (steric and electrostatic ferrosmectics (12,13) were created and examined. But the work on the synthesis of LFN did not gain any qualitative progress since the time of the initial breakthrough (3).

In this letter we report a preparation of LFN by introducing maghemite nanoparticles into a conventional lyotropic system. The essential novelties of our systems are: (i) the particles are present in relatively large amounts (up to 10¹⁷/cm³, that is 1 vol.%) and (ii) the samples are stable during several months at the least.

Experimental

Materials and methods

The lyotropic system under study is a mixture of potassium laurate (LK), 1-decanol (dOH) and water. Of those, 1-decanol is commercially available from FLUKA (purum 98 %). Potassium laurate is synthesized by ourselves by alcalinization of lauric acid (FLUKA, purum 98%) with potassium hydroxide (PROLABO, purum 86 %). Lauric acid is dissolved in 0.5 l of water-free ethyl alcohol (PROLABO, purum 99.8%) at 24° C up to the acid concentration ~ 0.25 mol/l. Then water is added to make the total volume 1 l. The aqueous solution of KOH » 9 mol/l. is added under vigorous stirring until pH of the mixture reaches 13–14. The resulting transparent solution is cooled down and mixed with aceton to precipitate the surfactant (LK). After separation, it is dissolved in water anew, heated up to 100° C and kept boiling for 3 to 6 hours until water evaporates. The emerging well-soluble white powder is once again dissolved in water at 3–5° C; the pH of the saturated solution is 12.0. The final purification is carried out by cooling this solution down to 1°C. The precipitated LK is separated and dried at room temperature. The resulting powder is soluble in water at temperatures higher than 5°C, the pH level at saturation ranges from 10.0 to 10.4.

Magnetic particles are maghemite (g-Fe₂O₃) grains ~ 6 nm in size, synthesized by alcalinization of aqueous mixtures of ferric and ferrous salts according to Ref. (8). They are dispersed in water at pH ~ 3 and have positive surface charges. Electrostatic repulsions ensure the stability of the obtained colloids which are widely known as ionic magnetic fluids or ionic ferrofluids. Being of nanoscopic dimensions, the particles are subdomain. The magnetization curve of a ferrofluid is unhysteretic and its analysis allows to determine the particle size-distribution (14). The volume fraction of the ferrite grains in ferrofluids as well as in LFN is determined from light absorption measurements with the accuracy ~ 10 ^{– 3} vol.%.

Synthesis of LFN specimens

Potassium laurate is mixed with water and then with 1-decanol under stirring at room temperature. The obtained lyotropic is foamed, and needs centrifugation at 2000–3000 rad/sec to remove the air bubbles. The reference quantitative composition of the solution on this stage of synthesis is (wt.%): LK 29.8, dOH 7.2, H₂O 63.0. The water content of this preparation (“empty” matrix) is chosen in such a way that some addition of water (which will be brought in with the particles) would not cause any important modifications of the nematic structure, i.e., no borders on the phase diagram would be crossed. This implies a detailed knowledge of the phase diagram of the ternary system. Our investigations of this issue are described elsewhere (Berejnov, V., Cabuil, V., et al. – in preparation), but its main features are shown in Fig. 1. The closed area (N) corresponds to the nematic phase abutting the isotropic phase (I) from one side and smectic phase (Sm) from the side of enhanced surfactant concentration. On further growth of the surfactant contents, the system looses its fluidity and turns into a polycrystal.

An acidic magnetic fluid (pH 2–3) constituted by cationic (positively charged) maghemite particles synthesized as indicated above, is then added drop by drop under vigorous stirring. The mixture is centrifugated once more, yielding the final product. The content of the latter, corresponding to the above-given example of the “empty” matrix composition, is (wt.%): LK 28.1, dOH 6.9, H₂O 65.0. The specimens are kept for several days at rest on a magnet in order to verify that the particles are indeed incorporated to the nematic supramolecular structure. We consider as LFN only the specimens which do not segregate under a field gradient ~ 10 T/m.

Results and discussion

Macroscopically, monophase ferronematics appear as transparent viscous birefringent fluids of red-brown color (Fig. 2). If they are put into a flat glass microslide of thickness 0.1 mm (VITRO DYNAMIC Inc., used as purchased) and observed under an optical microscope between crossed polaroids, LFN samples are rather uniformly oriented (Fig. 3a). The samples are uniaxial with a homeotropic texture with regard to the capillary plane. This texture is confirmed by the observed defect patterns, if existing.

Application of a magnetic field along the director, i.e., normal to the capillary plane, leads to an orientational transition with a threshold H 100 Oe (Berejnov, V., Bacri, J.-C., et al. -- submitted to Europhys. Lett.) – see Fig. 3b. The effect is well reversible: upon turning off the field, the homeotropic texture restores in a few tens of seconds.

It appears that a given initial composition of the nematic (especially, the content of 1-decanol) determines the maximal amount of particles that it can hold. When exceeding a certain volume fraction, macroscopic exclusion occurs associated with irreversible precipitation. A typical composition of a monophase LFN is given in Fig. 3.

By our technique, we obtain homogeneous LFN with the concentrations up to 1 vol.% (10¹⁷ particles per cm³), that is about 1000 times greater than those reported in Ref. (3). If properly sealed and kept at constant temperature, they remain unchanged on the time scale of six months.

We remark that the very fact of existence of the described ferronematics raises up interesting questions. The main one concerns the sign of the particle surface charge in our LFN. In the first attempts we tried to introduce in nematic systems anionic (negatively charged) particles, since they may be stabilized against agglomeration at the pH level of the matrix (alcaline medium). However, it appears that only cationic particles, stabilized usually in the acidic media (pH < 5) yield stable and concentrated LFN. This result is rather surprising and has to be understood in order to find out the actual location of the grains relative to the surfactant micelles.

Conclusions

We describe the method and present the evidence of successful preparation of lyotropic ferronematic specimens with an enhanced magnetic content, that was never achieved before. The obtained systems display high magneto-orientational susceptibility and are rather stable. At the same time, the problem of determination of their internal structure and field-induced behavior calls on further investigations.

Acknowledgements

This work was supported by “Le Réseau Formation – Recherche” № 90R0933 of MENESRIP, by the grant № 96–1149 of the French Direction of Armament(DGA) and from the Russian side by the grant № 95–02–03953 of RFBR.

References

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Figure Captions

Figure 1 Phase diagram of the potassium laurate (LK) / 1-decanol (dOH) / water system for T=20–22° C and pH = 10.0–10.4. The regions are marked: isotropic solution (i), nematic phase (N), smectic phase (Sm), polycrystal (PC); all the concentrations are given in wt.%
Figure 2 LFN specimens in tubes (diameter 1 mm) observed between crossed polaroids with their axes at ± 45° to the vertical direction. The magnetic content in vol.% (from top to bottom) is: 4.4´ 10^–2, 2.2´ 10^–1, 1.1. The equivalent (extrapolated to zero particle content) lyotropic matrix composition (in wt.%) is: LK 26.5, dOH 7.2, H2O 66.3
Figure 3 Micrographs of LFN in a flat microslide (with 1 mm) between crossed polaroids at ± 45° to its axis; a – no applied field, homeotropic texture; b – field ~ 100 Oe normal to the microslide, planar texture. The lyotropic matrix composition is the same as in Fig. 2, the magnetic content is 0.022 vol.%