Applications and Importance of Lyotropic Liquid Crystals

One material that demonstrates lyotropic liquid crystalline behavior is simple household soap. Soaps work better than pure water at removing dirt and grease because the nonpolar insides of the micelles are capable of dissolving nonpolar substances that will not dissolve in water. (This also works in reverse if the solvent is nonpolar and some of the substance to be removed is polar.) Soaps also help water dissolve more because the molecules tend to remain at the surface, hydrocarbon tail away from the water, thus lowering the surface tension of the water and allowing more material to enter it and be dissolved.

Other diverse applications exist for amphiphilic molecules. Because of their ability to dissolve both polar and nonpolar substances, a mixture of water and an amphiphilic compound can be pumped into a depleted oil well in order to remove much of the residual oil. In the medical professions, a lyotropic liquid crystal can coat a drug to keep it from being destroyed in the digestive tract. The drug can then be taken orally, and after it reaches the proper location in the body, the liquid crystal breaks down and the drug is released.

Lyotropic liquid crystals have been used to make a stable hydrocarbon foam. Hydrocarbon foams have been difficult to produce in the past because the surface tension of an the hydrocarbon is low enough that adsorption to an oil-soluble surfactant would have no significant effect. Without the adsorption, the hydrocarbon simply behaves as a liquid. When lyotropic liquid crystal molecules change from inverse micelles to lamellar sheets, they lower the surface tension enough for a foam to form. The hydrocarbon and the surfactant can dissolve in each other, and the surfactant cannot dissolve in water, although water can dissolve in the surfactant and mix into the liquid crystal.

Many other substances are also more soluble in lyotropic liquid crystals. One example is the drug hydrocortizone. It is often taken in topical applications, but its uses have been limited because the highest concentration possible has been only 1%. When the drug was blended into a liquid crystal of lecithin and water, the concentration went up to 4%. In time, liquid crystals may become a primary solvent for topical medications.

The outermost layer of skin is primarily a lyotropic liquid crystal made of fatty acids. At least some of the fatty acids must be unsaturated because saturated chains will simply crystallize rather than forming a liquid crystal structure. Experiments with crystals made only from saturated fatty acids produced layers that did not prevent water transport across them. This is a symptom that sometimes occurs in people whose diets are deficient in essential fatty acids of the sort found in unsaturated fats.

Biological Membranes

Lyotropic liquid crystals are also extremely important because of their role in biological membranes. Membranes are composed of amphiphilic lipids - mostly phospholipids and cholesterol, with a small percentage of glycolipids. The phospholipids vary in the polar head compositions and the hydrocarbon chain lengths, but almost all have two hydrocarbon tails - one saturated and one unsaturated. These tails are flexible, with the most freedom of movement found at the greatest distance away from the polar head. (See the picture to the left for a representative phospholipid structure and the one to the right for a cholesterol structure.) Although there is still much to be learned about biological membranes, it is now generally accepted that the lipids form lyotropic bilayers. In the first experiment to provide evidence for a bilayer, all the lipids were removed from red blood cell membranes, spread out on the surface of water, and forced into a monolayer. The surface area was found to be twice the surface area of the original red blood cells, suggesting that the molecules were ordinarily in a double row. Further investigations have supported the concept of an amphiphilic bilayer as the basic structure of a cell membrane. Also evidence in its favor is the fact that the component lipids form bilayers when dissolved in water even if they are not part of a living cell. If the structure is caused by purely physical hydrophobic interactions, then we do not need to ask how the cell creates or maintains it. Those same hydrophobic interactions also cause any holes appearing in the bilayer to immediately close, a very important characteristic for a cell membrane.

The membrane also contains proteins, the placement of which may be determined by the hydrophobic interaction. This would leave polar parts of proteins on the outside exposed to water either inside or outside the cell, while nonpolar parts would stay within the lipid bilayer, isolated from the water on either side. It also seems likely that the bilayer serves as a solvent for the various proteins and that it provides contact with specific polar heads that may be needed for a protein to function properly. The image of a cell membrane (lower right) shows lipids in green and proteins interspersed through the membrane in blue.

Phase transitions

The phase transitions exhibited by biological membranes are an important aspect of their properties and are directly related to their structure. In a typical case, calorimetric measurements (such as those discussed in connection with Differential Scanning Calorimetry) show a rather broad specific heat anomaly near or just below the temperature at which the membranes formed when the cell was grown. Studies of a variety of membranes indicate that this transition is related to a disordering of the hydrocarbon chains. As the temperature is raised through the transition region, the chains which were primarily in an all-trans configuration (See Polymer Structures) of the carbon-carbon bonds are freed to undergo greatly increased thermal motion. This includes the formation of gauche conformations with a corresponding excluded volume increase and a necessity for cooperative effects to accommodate the increased range of motion.

Existing at or above the phase transition temperature provides some possible advantages to the cell. Since a cell is three-dimensional with no edge, membrane growth involves forcing new lipid molecules into the existing membrane. The higher compressibility at or above the phase transition temperature makes it easier for the membrane to make room for additional molecules. Furthermore, the more fluid structure makes it easier for lipids and proteins to move around the cell membrane. Studies of living cells have shown that movement and rotation within a layer are indeed very common, although movement between layers is rare.

It has also been observed that the structure of the membrane molecules affects the phase transition temperature. An introductory Physics Today article (Nagle and Scott, 1978) explains that a membrane with two additional CH₂ groups on each chain would have a phase transition temperature about fifteen degrees higher than one that did not. Unsaturating the lipids and forming a double bond on each chain lowers the phase transition temperature approximately forty-five degrees, and removing a CH₃ group from each of the polar head areas raises it by about twenty degrees. Changing the length and bonding of the hydrocarbon tails also affects the degree to which the lipid molecules can be compressed together, and thus the membrane fluidity. In fact, single-celled organisms whose temperatures change with their surroundings maintain a consistent fluidity by altering the varieties of molecules synthesized as the temperature changes. It has been observed that cold-blooded animals have a higher percentage of unsaturated chains in their cell membranes than do warm-blooded animals. Presumably, the unsaturated chains give the liquid crystal phase greater stability despite the temperature changes that the animal endures. Animals that can regulate their own body temperature do not need such a safeguard. Animals that live deep in the ocean use molecules that can remain in the liquid crystal state at enormous pressures. When the animals are brought to the surface, their cell membranes are unable to adapt to the much lower pressure. The cells may rupture, killing the animal. The presence of cholesterol molecules in animal cells also affects the membrane consistency. At temperatures below the liquid crystal phase transition, cholesterol makes hydrocarbon chains more fluid, and at temperatures above the transition, it makes the chains more rigid. Thus cholesterol helps to maintain the fluidity of the membrane despite changes in temperature. Cholesterol also appears to play a role in the addition of proteins to the lipid bilayer. The specific molecules in the cell membrane, then, help to determine its properties.

Other interesting behaviors are found when bilayers with saturated and unsaturated lipids are studied. The bilayer then has two phase transition points, one for each kind of lipid. In these synthetic bilayers, lipids at their freezing point group together, separating the bilayer into regions composed of different lipids. Biological membranes, however, incorporate lipids with both a saturated and an unsaturated tail on each molecule, which makes the separations far less frequent. In the true biological membranes that have been studied, the lipids on the inside and outside layers have been different. In red blood cells, the molecules on the inner layer are less saturated, making the inner layer more fluid. Furthermore, one of the molecules found on the inside carries a net negative charge, causing a charge difference between the inside and the outside of the cell. The structure and function of biological membranes is currently an area of active research.

Thermodynamic Considerations