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Identification of Lyotropic Liquid Crystalline Mesophases

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Identification of Lyotropic Liquid Crystalline Mesophases CHAPTER 16 Identification of Lyotropic Liquid Crystalline Mesophases Stephen T. Hyde Australian National University, Canberra, Australia 1 Introduction: Liquid Crystals versus 2.1.8 Intermediate mesophases: Crystals and Melts .................. 299 (novel bi- and polycontinuous 2 Lyotropic Mesophases: Curvature and space partitioners) ......... 316 Types 1 and 2 ..................... 301 2.2 Between order and disorder: 2.1 Ordered phases ................. 307 topological defects .............. 317 2.1.1 Smectics: lamellar (“neat”) 2.2.1 Molten mesophases: mesophases .............. 307 microemulsion (L1,L2)and 2.1.2 Gel mesophases (Lβ ) ....... 307 sponge (L3) phases ........ 318 2.1.3 Lamellar mesophases (Lα) ... 308 2.3 Probing topology: swelling laws .... 319 2.1.4 Columnar mesophases ...... 308 3 A Note on Inhomogeneous Lyotropes .... 321 2.1.5 Globular mesophases: discrete 4 Molecular Dimensions Within Liquid micellar (I1,I2) ........... 310 Crystalline Mesophases ............... 323 2.1.6 Bicontinuous mesophases .... 310 5 Acknowledgements .................. 326 2.1.7 Mesh mesophases ......... 315 6 References ........................ 327 1 INTRODUCTION: LIQUID positions, due to thermal motion of the atoms. How- CRYSTALS VERSUS CRYSTALS AND ever, this motion is small, and typically far less the MELTS average spacing between the atoms. (In some solids, the material forms a glass rather than a crystal, in which case the solid consists of a spatially disordered and Liquid crystals are a distinct phase of condensed materi- non-crystalline arrangement of atoms, although it does als, which typically form under physical conditions that exhibit short-range order, and is locally similar to a lie between those giving rise to solids and melts. crystal.) The melt is typically freely flowing, and char- In the (crystalline) solid phase, the relative locations acterized by large fluctuations in the atomic positions, of all atoms in the material are fixed, defining the crystal both in time and space, so that it is invariably optically structure that can adopt a variety of symmetries com- isotropic. Those fluctuations imply that it is difficult to mensurate with our three-dimensional (3D) Euclidean associate the atomic arrangement with a geometric struc- space. The crystal is optically isotropic if it crystal- ture, in contrast to crystals. lizes in a cubic space group, and anisotropic otherwise, Liquid crystals share features of both a crystal and a with distinct physical features along different directions melt, i.e. with partial order/disorder of atomic species. in the crystal. There is some uncertainty in atomic The ordering may be purely orientational, with no spatial Handbook of Applied Surface and Colloid Chemistry. Edited by Krister Holmberg ISBN 0471 490830 2001 John Wiley & Sons, Ltd 300 ANALYSIS AND CHARACTERIZATION IN SURFACE CHEMISTRY order (nematics and cholesterics), or spatial. Spatial Lyotropic mesophases contain at least two chemi- order in liquid crystals can occur on the atomic scale cal components: the organic molecule and its solvent. (with melting of some atoms in the molecule, while the The organic moiety must exhibit some chemical com- others remains frozen) or on longer length scales, such as plexity, or otherwise the solvent will simply dissolve the mesoscale ordering found in many solvent-induced the molecule, forming a structureless – and certainly liquid crystals. In the latter case, the material may not liquid crystalline – molecular solution of dispersed exhibit no atomic ordering, so that the material is a pure and disordered molecules. The simplest examples are melt on the atomic scale. Collective order of molecular amphiphilic molecules. The addition of a solvent such aggregates leads to a well-defined structure on a larger as water will selectively hydrate the hydrophilic moi- length scale, typically beyond 20 A.˚ Like crystals, liquid ety of each molecule, avoiding the hydrophobic regions. crystals are sometimes optically isotropic (cubic phases), This “schizophrenic” relationship between the solvent or otherwise optically anisotropic (lamellar “smectics”, and solute drives the molecules to self-assemble, thereby hexagonal and intermediate phases). Liquid crystalline minimizing the exposure of hydrophobic moieties to phases are called, to quote a pioneer of the field, Jacques the water. (Clearly, the argument holds in reverse if a Friedel, mesophases, from the Greek prefix denoting lipophilic solvent, such as an alkane, is used. Indeed, a an intermediate. In addition to the clear biological and combination of hydrophobic and hydrophilic solvents materials importance of liquid crystals, they are of can also lead to the formation of liquid crystalline fundamental importance to the issue of disorder. Despite mesophases.) its frequent use in the scientific literature, the term The essential phenomenon common to all liquid is a difficult one to define precisely. (For example, crystalline states is the presence of (at the bare mini- all melts are conventionally “disordered”, but how can mum) orientational order. In order to characterize the generic phase behaviour of liquid crystalline systems, one compare disordered materials at a structural level?) it is essential to acknowledge the existence of other It is becoming clear that the notion of disorder is a mesophases, which are not (stricto sensu) liquid crystals. “prickly” one, with the possibility of many distinct types These include the isotropic microemulsion and sponge of disorder disguised within that classification. mesophases (and, if one ignores the somewhat academic Many pure molecular substance form liquid crys- constraint of thermodynamic equilibrium, emulsions). tals during the melting process. Where the chemically We include some discussion of their role in the scheme, pure material is found to melt over a temperature range, as their physical properties also lie between those of rather than the abrupt first-order transition expected crystals and molecular melts. They retain a central fea- for the solid–melt phase transformation, the formation ture of lyotropes: self-assembly of the chemical moi- of liquid crystals within that range is certain. Such eties into multi-molecular domains, thus reducing the substances exhibit thermotropic liquid crystals, whose miscibility of one species in the other from the mono- stability is a function of temperature. (The term is molecular miscibility characteristic of a pure melt phase. used loosely to denote the temperature-dependent phase This self-assembled mesostructure is often idealized in behaviour of any liquid crystal condensate, including terms of the geometry and topology of the interface(s) chemical mixtures.) A good overview of thermotropic separating immiscible domains. From such a perspec- phases is that of Seddon (1). Materials (possibly mixed) tive, microemulsions and sponge mesophases are closely that form liquid crystals by the addition of solvents related to lyotropic liquid crystals, although the inter- are lyotropic liquid crystals. Many materials exhibit faces may themselves be devoid of any translational both thermotropic and lyotropic liquid crystalline tran- and/or orientational order. They are partial melts of the sitions, i.e. mesomorphism. Liquid crystals are typically liquid crystalline mesophases, although not dispersed at organic molecules, ranging from polyelectrolytes (e.g. the molecular scale. DNA, vegetable gums, etc.) to small molecules (mem- We have introduced two distinct perspectives of the brane lipids, detergents, etc.), in the presence of (some- liquid crystalline state. The first views them as partially times aromatic) hydrocarbon “oils” and water. Other defective crystals. The second as partially self-organized solvents, including glycerol, formamide, etc., result in melts. Such alternative understandings lead to subtly lyotropic mesophases in the absence of water. The different approaches to measuring and understanding hydrophobic chemical moieties are not limited to hydro- lyotropic mesophase behaviour. carbons: perfluorinated species – both as amphiphiles The former has led to a crystallographic analysis of and solvents – also exhibit lyotropic liquid crystalline lyotropes. This view has been championed particularly mesophases. by one of the pioneers of the field, Vittorio Luzzati. LYOTROPIC LIQUID CRYSTALLINE MESOPHASES 301 There is a strong emphasis on the (typically small-angle likely that such hybrid inorganic/organic liquid crystals X-ray or neutron) diffraction features of the mesophases, are already synthesised in vivo. and most mesophases are conventionally distinguished on that basis. Diffraction, a Fourier-transform technique, probes the geometric correlations within the material. 2 LYOTROPIC MESOPHASES: Detailed models of the idealized mesostructure can CURVATURE AND TYPES 1 AND 2 therefore be constructed. For this reason, mesophase structure is dominated by such an approach. A brief catalogue of currently known and labelled The latter is less developed, but, in this author’s lyotropic mesophases follows. For the reasons outlined opinion, equally important, certainly in an applied or above, we will focus first on those mesophases that industrial context. It implies the investigation of X-ray or are spatially ordered and lead to Bragg diffraction neutron scattering, rather than diffraction (if the reader (sometimes in an approximate sense, with rather diffuse will allow some momentary pedantry). Furthermore, an diffraction “spots”). explicit focus on the “molten”
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