Lyotropic liquid crystal molecules belong to a class of substances called amphiphilic compounds. These compounds are characterized by a sort of split personality - one end of the molecule is polar and attracted to water while the other end is nonpolar and attracted to hydrocarbons, or lipophilic. The diagram to the left shows sodium laurate, a common amphiphilic molecule. In solution, the molecules situate themselves such that either the polar ends are dissolved in a polar solvent or the nonpolar ends are dissolved in a nonpolar solvent. The opposite end is kept isolated from the unlike solvent. As the concentration of the molecules in solution increases, they take on different arrangements or phases.
For the purpose of this discussion, it will be assumed that the amphiphilic molecules are dissolved in water, so the molecules will be arranging themselves with the polar heads in contact with the water.
At low concentrations, the solution looks like any other - particles of solute distributed randomly throughout the water. When the concentration gets high enough, however, the molecules begin to arrange themselves in hollow spheres, rods, and disks called micelles. In some reactions, the type of micelle affects the reaction rate, most likely because the parts of the molecule involved in the reaction are more likely to be exposed in some formations than in others. The surface of a micelle is a layer of polar heads dissolved in the water, while the inner portion consists of hydrophobic tails screened from the water by the hydrophilic heads. Micelles come in varied sizes, but the smallest ones have a diameter about twice as long as the length of a hydrocarbon chain with all trans- bonds. As the weight concentration of amphiphile increases, the micelles become increasingly able to dissolve nonpolar substances. When this occurs, the micelles become large and swollen. If they reach a large enough size, the solution becomes cloudy and is called an emulsion. At lower concentrations, the swollen micelles are not large enough to interfere with light, but they are still extremely stable and exist in equilibrium. This phase is referred to as a microemulsion. A later section in the tutorial will show a movie of micelles forming in an amphiphilic solution.
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| Spherical micelle | Cross-section | ||
As the concentration increases, the micelles begin to arrange themselves into loose patterns. These patterns are the actual liquid crystal aspects of the molecular behavior. One of the first liquid crystal phases has micelles forming a structure similar to a face-centered or body-centered cubic crystal lattice. The illustration below shows a body-centered cubic crystal structure. Micelles take the place of individual atoms, ions, or molecules. It should also be noted that the pattern is not as stable or as rigid as that of a solid crystal like graphite or table salt, hence the term liquid crystal. Rod-shaped micelles often form into hexagonal arrays made out of six rods grouped around a central one for a total of seven, as illustrated in the picture below. In the enlargement of a single rod, notice that the micelle surface is composed of hydrophilic heads. The hydrophobic tails are isolated inside the micelle. Hexagonal liquid crystals generally exist in solutions that are forty to seventy percent amphiphile. The liquid crystals may come apart if too much water or salt is added to the solution, but many varieties can absorb oil by expanding the diameter of the rod-shaped micelle.
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| Cubic liquid crystal | Hexagonal liquid crystal | Rod micelle close-up | |||
At even higher concentrations the molecules move into another liquid crystalline phase - the lyotropic liquid crystal bilayer. This structure has a double layer of molecules arranged a bit like a sandwich with polar heads taking the place of the bread and nonpolar tails as the filling. This pattern is similar to that of smectic liquid crystals in the thermotropic category. Because the sheet-like layers can slide easily past each other, this phase is less viscous than the hexagonal phase, at least in the direction of the sliding, despite its lower water content. The bilayer, or lamellar, phase has a focal conic texture. (See the section on liquid crystal phases for more information.) Another structure, called the ribbon phase, may be the precursor to the bilayer. Ribbon phases involve finite bilayers that end in cylindrical half-micelles. Bilayers may form when these ribbons fuse together. Lyotropic liquid crystals rarely exist in solutions that are less than half amphiphile by weight. If the amphiphile concentration is lower, the mixture reverts to a hexagonal phase or a solution of micelles.
Amphiphilic monolayer
Other behaviors occur when the situation is something other than a simple water solution. If the molecules are placed on the surface of water without actually being dissolved in it, they form a monolayer in which the polar heads are in contact with the water and the hydrophobic tails point into the air. These monolayers are often referred to as Langmuir films and are a subject under investigation by ALCOM.
The following generic sort of phase diagram shows the changes in structure as concentration of amphiphilic molecules increases. The concentration at which micelles form in solution, called the critical micelle concentration, is shown as a dotted line. Also notice the dark line below which few liquid crystals form. This line represents a boundary temperature, referred to as the Krafft temperature. Below the Krafft temperature, a few liquid crystals may be suspended in the solution, but for the most part the amphiphilic molecules stay widely distributed. The reasons for this phenomenon will be explored in the next section. For further information, the Physics Today article (Pershan, 1982) covering the topic is recommended.
If the concentration by weight of amphiphilic molecules is higher than that of water, the molecules form a sort of matrix with water droplets scattered inside, in contact with the polar heads. If the molecules are dissolved in a nonpolar solvent, their behavior is similar to that when dissolved in water, except that now the nonpolar tails are in contact with the solvent and the polar heads are isolated in the centers of the micelles and bilayers. If the solution contains both water and a higher concentration of nonpolar solvent, similar inverse micelles form with water droplets quarantined inside the micelle and nonpolar solvent on the outside. See the illustration to the left for a cross-section of one of these reverse micelles. Finally, if weaker amphiphilic molecules and simple salts are dissolved together in water, they form "lyotropic nematic phases." In these crystals, as in thermotropic nematics, the director orientation can be changed by applying a magnetic field.
If water, a hydrocarbon, and a surfactant are mixed together, it is possible to get a microemulsion known as a ringing gel. This phase forms when micelles shift from rod to sphere shapes in the presence of a hydrocarbon. If the gel is placed in a container and the container is tapped, the gel will vibrate with an audible resonance frequency.
Other interesting behaviors can arise if a polymer is in solution with the amphiphilic molecules. Sometimes polymers will adsorb to micelles, creating a group of micelles all in a row like a necklace. Anionic amphiphiles form micelles at lower concentrations when a polymer is present in the solution. Other liquid crystals will break down into small micelles in the presence of a polymer. If a polymer is caught between the two sides of a bilayer it can poke holes right through. It is also possible for micelles adsorbed to a polymer to coexist in solution with free micelles.
NOTE: This page was completed in its current form July 21, 2003.
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Introduction to Lyotropic Liquid Crystals |  |
Intermolecular Chemistry and Lyotropic Structures |  |