Solvents, thermotropic liquid crystals

A compound which displays liquid crystal properties is referred to as a mesogen and said to exhibit mesomorphism. Liquid crystals may be considered either as disordered solids or ordered liquids, and their properties are very dependent on temperature and the presence or absence of solvent. In thermotropic liquid crystals the phases of the liquid crystals may be observed to change as the temperature is increased. In lyotropic liquid crystals the ordered crystalline state is disrupted by the addition of a solvent, which is very commonly water. For these systems temperature changes may also be... 

The liquid crystal state represents the fourth state of matter and exists between the solid and liquid states, which form its boundaries. The liquid crystal state is reached from the solid state either by the action of temperature (thermotropic liquid crystals) or of solvent (lyotropic liquid crystals) and it is the former that will be the subject of this chapter. 

Starting with the crystalline state, the mesophase is reached by increasing the temperature or by adding a solvent. Accordingly, a differentiation can be made between thermotropic and lyotropic liquid crystals, respectively. As with thermotropic liquid crystals, a variation of the temperature can also cause a phase transformation between different mesophases with lyotropic liquid crystals. 

Lyotropic liquid crystals differ from thermotropic liquid crystals. They are formed by mesogens which are not the moleeules themselves but their hydrates or solvates as well as associates of hydrated or solvated molecules. In the presenee of water or a mixture of water and an organie solvent as the most important solvents for drug molecules, the degree of hydration or solvation, respectively, depends on the amphiphilic properties of a drug moleeule. Hydration of the mostly rod-shaped molecule— and the same holds for solvation—results in different geometries, eone or cylinder [5] (Fig. 3). 

As compared to the cholesteric LC, the lyotropic LC consists of two or more components that exhibit liquid-crystalline properties (dependent on concentration, temperature, and pressure). In the lyotropic phases, solvent molecules fill the space around the compounds (such as soaps) to provide fluidity to the system. In contrast to thermotropic liquid crystals, these lyotropics have another degree of freedom of concentration that enables them to induce a variety of different phases. A typical lyotropic liquid crystal is surfactant-water-long-chain alcohol. 

These structures are extensively described in the current literature (Fanum, 2008 Friberg, 1976 Birdi, 2002 Holmberg, 2004 Somasundaran, 2006). Even within the same phases, their self-assembled structures are tunable by the concentration for example, in lamellar phases, the layer distances increase with the solvent volume. Lamellar structures are found in systems such as the common hand soap, which consists of ca. 0% soap + 20% water. The layers of soap molecules are separated by a region of water (including, salts etc.) as a kind of sandwich. The x-ray diffraction analysis shows this structure very clearly. Since lyotropic liquid crystals rely on a subtle balance of intermolecular interactions, it is more difficult to analyze their structures and properties than those of thermotropic liquid crystals. Similar phases and characteristics can be observed in immiscible diblock copolymers. 

The mesogenic structures of glycolipids are due to the occurrence, on the same molecule, of a hydrophilic and a hydrophobic moiety often referred to as head and tail respectively. As a result, glycolipids are able to self-organize into a large variety of mesophases also called liquid crystals (Fig. 2) [ 10]. Supramolecular assemblies of mesogenic compounds can be caused by a rise in temperature (thermotropic liquid crystals) or by the addition of water (lyotropic liquid crystals) they result from different responses of the carbohydrate and the alkyl chain to temperature or solvent (water), respectively. 

A liquid crystal compound in more cases Ilian not takes on more than one type of mesomorphic structure as the conditions of temperature or solvent are changed. In thermotropic liquid crystals, transitions between various... 

In this review we are mainly concerned with thermotropic materials, i.e. with liquid crystals and LC-glasses which do not contain a solvent. The transitions of the macro-molecular, thermotropic liquid crystals are governed then by temperature, pressure and deformation. In lyotropic liquid crystals and LC-glasses a solvent or dispersing agent is present in addition. The transitions then also become concentration dependent. 

It was, however, observed that such systems under appropriate conditions of concentration, solvent, molecular weight, temperature, etc. form a liquid crystalline solution. Perhaps a little digression is in order here to say a few words about liquid crystals. A liquid crystal has a structure intermediate between a three-dimensionally ordered crystal and a disordered isotropic liquid. There are two main classes of liquid crystals lyotropic and thermotropic. Lyotropic liquid crystals are obtained from low viscosity polymer solutions in a critical concentration range while thermotropic liquid crystals are obtained from polymer melts where a low viscosity phase forms over a certain temperature range. Aromatic polyamides and aramid type fibers are lyotropic liquid crystal polymers. These polymers have a melting point that is high and close to their decomposition temperature. One must therefore spin these from a solution in an appropriate solvent such as sulfuric acid. Aromatic polyesters, on the other hand, are thermotropic liquid crystal polymers. These can be injection molded, extruded or melt spun. 

For molecules dissolved in a nematic thermotropic liquid-crystal, the direct coupling constants can be determined and from these the molecular geometry can be calculated. If the satellites are determined, not only the proton structure but the carbon skeleton of the molecule can be established. Oxirane has been measured in two laboratories. It proved possible to determine the orientation, the sign of the indirect coupling constant, and the geometry. Enantiomers can readily be determined by recording measurements in optically active liquid-crystals as solvents. ... 

Phase Equilibrium in a Rigid-Chain Polymer-Solvent System 2.1 Lyotropic and Thermotropic Liquid Crystals... 

In this connection let us consider a fragment of a schematic phase diagram in the region of high concentrations of a polymer capable of forming the liquid crystalUne phase (Fig. 2). In a crystalUne polymer containing no solvent (100% polymer, vf), the transition from the crystalline state (c) to the Uquid crystalline state (Ic) must take place at the temperature T -.ic and further transition into isotropic state (i), at the temperature Such transitions are called thermotropic, and the system formed at I, is called the thermotropic liquid crystal. The transition to the Uquid crystalline state can also occur by adding to a polymer a solvent at a temperature below T -. c-... 

It should be mentioned that crystal structure data are often obtained from co-solvent systems (Desai and Klibanov, 1995). For example, crystals of ubiqui-tin, papain and a heptapeptide were grown from 30% polyethylene glycol (PEG) 4000, 62% MeOH and a DMSO/isopropanol mixture, respectively (Kamphius et al., 1984 Karle et al., 1993 Love et al., 1997). The orthogonal and tetratogonal crystal forms of cyclosporine were prepared from 25% PEG 300 and acetone, respectively (Petcher et al., 1976 Kessler et al., 1985 Loosli et al., 1985 Verheyden et al., 1994a). Furthermore, cyclosporine and leuprolide form thermotropic liquid crystals when dried from EtOH and lyotropic liquid crystals when solubilized in PG, respectively (Tan et al., 1998 Lechuga-Ballesteros et al., 2003 Stevenson et al., 2003). 

Liquid crystal phases, or mesophases, are characterized by a partial order, intermediate between the full orientational and translational disorder of the isotropic liquid phase and the full orientational and translational order of the crystalline phase. Thermotropic liquid-crystal phases are obtained for a given compound (or possibly a mixture) as a function of temperature, while the so-called lyotropic liquid-crystal phases are obtained as a function of the concentration of a given solute in a solvent Typical examples of the latter systems are the various types of aggregates formed by amphiphilic molecules either in water or in organic solvents. In this chapter we will be interested only in thermotropic systems. An interesting review on lyotropic ionic liquid crystals can be found in Ref. [2],... 

Techniques for phase identification are similar to those employed for thermotropic liquid crystals, although there are differences. Thus, optical microscopy is the most common technique used, but rather than prepare multiple samples at a range of concentrations it is more common to perform Lawrence Penetration Experiments [189]. In this experiment, some solid surfactant (maybe up to 50 mg) is placed on a microscope slide and a cover slip placed on the top. Water (or which ever solvent is to be used) is placed at the end of the cover slip and proceeds from one end to the other by capillary action, thus, setting up a concentration gradient across the sample. Now, at any given temperature, it is possible to have a snapshot of the whole phase diagram, and clear phase boundaries can often be seen. This experiment can be very... 

At 80°C, the polymers with more than 10 carbon atoms are in the liquid crystalline state. Low mobility of the side chain prevents the polymers of n 9 to be in the thermotropic liquid crystal. The existence of the discontinuity is evidence for the solvent like nature of the side chains in the thermotropic liquid crystalline phase. 

Thermotropic Liquid Crystals as Photochemical Reaction Solvents... 

Thermotropic liquid crystals and also lyotropic liquid crystals generate functional molecular assemblies. lyotropic liquid crystalline phases are exhibited by amphiphilic molecules in appropriate solvents. They form nano-segregated structures because the molecular structures consist of hydrophilic and hydrophobic components. In Chapter 6, Gin and co-workers describe how lyotropic liquid crystals may be used to form functional materials. Lyotropic liquid crystals can act as templates for inorganic materials, ion conductors, catalysts, drug delivery systems, and nanofilters. 

The results reported in this paper demonstrate that colloidal dispersions in thermotropic liquid crystals are challenging systems for discovering and studying new physical effects and structures. The anisotropy of the continuous phase leads to the observation of phenomena that are markedly different from that known in isotropic solvents. We have shown that a thermally induced phase sep-... 

Matter is often considered as existing in one of three forms—solid, liquid, and gas. Yet, there exists a state of matter between the solid and liquid states that possesses properties reminiscent of each, so that like a solid it has order, while like a liquid it is fluid. This true state of matter is the liquid-crystal state and is generated in one of two ways. When a compound passes between the solid, liquid, and liquid-crystal state as a primary function of temperature and in the absence of solvent, the liquid-crystal behavior is termed thermotropic. When the phase transitions ate driven by the concentration of the liquid crystal in a solvent, the behavior is termed lyotropic. ... 

Liquid crystals are broadly classified as nematic, cholesteric and smectic (I)- There are at least nine distinct smectic polytypes bearing the rather mundane labels smectic A, B, C,... I, by the chronological order of their discovery. Some of the smectics are actually three-dimensional solids and not distinct liquid-crystal phases at all. There are three t s of liquid crystals. Thermotropic liquid-crystal phases are those observed in pure compounds or homogeneous mixtures as the temperature is changed they are conventionally classified into nematic, cholesteric, and smectic phases in Fig.2. Lyotropic liquid-crystal phases are observed when amphiphilic molecules, such as soaps, are dissolved in a suitable solvent, usually water. Solutions of polymers also exhibit liquid-crystalline order, the polymeric phases. Most of our knowledge about liquid crystals is based on the thermotropic phases and much of this understanding can be transferred to elucidate polymeric and lyotropic phases. 

Lyotropic liquid crystals are principally systems that are made up of amphiphiles and suitable solvents or liquids. In essence an amphiphilic molecule has a dichotomous structure which has two halves that have vastly different physical properties, in particular their ability to mix with various liquids. For example, a dichotomous material may be made up of a fluorinated part and a hydrocarbon part. In a fluorinated solvent environment the fluorinated part of the material will mix with the solvent whereas the hydrocarbon part will be rejected. This leads to microphase separation of the two systems, i.e., the hydrocarbon parts of the amphiphile stick together and the fluorinated parts and the fluorinated liquid stick together. The reverse is the case when mixing with a hydrocarbon solvent. When such systems have no bend or splay curvature, i.e., they have zero curvature, lamellar sheets can be formed. In the case of hydrocarbon/fluorocarbon systems, a mesophase is formed where there are sheets of fluorocarbon species separated from other such sheets by sheets of hydrocarbon. This phase is called the La phase. In the La phase the molecules are orientationally ordered but positionally disordered, and as a consequence the amphiphiles are arranged perpendicular to the lamellae. The La phase of lyotropics is therefore equivalent to the smectic A phase of thermotropic liquid crystals. 

There are two principal categories of mesophases, thermotropic and lyotropic. Thermotropic liquid crystals are formed within a particular range of temperature in a molten material, with no solvent present, whereas lyotropic liquid crystals are formed by some substances when they are dissolved in a solvent. Within each of these categories there are three distinct classes of mesophases, which were first identified by Friedel in 1922. The simplest of these to describe are the nematic and smectic classes, illustrated schematically in fig. 12.16. These phases are formed by long thin rigid molecules which tend to line up parallel to each other. 

Thermotropic liquid crystals show ordering of the crystals based on the temperature of the system, whereas, the lyotropic liquid crystals show the ordering of the crystals based on the addition of the solvents.Lyotropic liquid crystals are explored as potential drug delivery vehicles due to their biocompatible, biodegradable, bioadhesive, and nontoxic properties. 

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