Liquid crystal is a fourth state of matter in which a special condition arises. The molecules in solids exhibit both positional and orientational order resulting them in certain direction and position. The molecules of liquids do not have any positional or orientational order.
Liquid crystals are a phase of matter whose order is intermediate between a liquid and that of a crystal. Molecules in liquid crystal do not exhibit any positional order, but they do possess a certain degree of orientational order.
The liquid crystalline phase is also referred as mesophase and the compounds which exhibit mesophase are referred to as liquid crystal.
Liquid crystals molecules possess orientational order. The direction of preferred orientation in a liquid crystal is called the director. As the molecules do not possess any positional order they are in constant motion and they spend more time pointing along the director than along any other direction.
Order Parameter: Liquid crystals possess some degree of orientational order but not as good as solid material. To quantify how much order is present in a material an order parameter (s) is defined.
Order of parameter for, Solid = 1; Liquid- 0 and for Liquid crystal = 0.3 to 0.9.
Order parameter depends on temperature. As the temperature increases order parameter decreases.
Liquid crystals are mainly divided in to two main classes i.e. Thermotropic, (temperature dependent) and lyotropic (concentration dependent). Within thermotropic liquid crystals there are high molar mass materials (suitable for polymer) and low molar mass materials.
Within the polymeric kind there are main chain polymers where the chains align in a direction or a chain where there is comb-like structure with side chains that hang off the polymer. The director is oriented either along the main chain or parallel to the side chains.
In the low molar mass materials there are disc like molecules (discotic) and rod like molecules (calamitic liquid crystals).
Rod-like LC are nematic (N), Chiral nemetic and smectic and can be described as follows:
Nematic LC: Molecules have no positional order but tend to point in the same direction along the director.
Chiral nematic or cholesteric LC: In this type of liquid crystals, intermolecular forces favor alignment between molecules at a slight angle to one another, directors form a continuous helical pattern.
Smectic LC: These liquid crystals show a degree of translational order and tend to align themselves in layers or planes. The director may be perpendicular or tilted to the plane.
Nematic discotic LC: when the disc like molecules of liquid crystals orient in a sheet like fashion known as discotic nematic liquid crystals. The director is perpendicular to the plane.
Columnar LC: When the disc like molecules of Liquid crystals orient in form of columns and packed together to form a two-dimensional crystalline array. The director is perpendicular to the plane.
Lyotropic liquid crystals: Lyotropic liquid crystals are formed by dissolving an amphiphilic mesogen (molecules consisting of two parts of contrasting character that are hydrophilic and hydrophobic) in a suitable solvent, under appropriate conditions of concentration,temperature, and pressure. These liquid crystals arise by the arrangement of multimolecular units instead of individual molecules.
APPLICATION OF LIQUID CRYSTALS IN PHARMACY
- Polymer liquid crystals have been extensively studied. They are found to be the important phases to form super strength polymers for novel drug delivery systems.
- Liquid crystals are also seen to be intimately involved in the functions of biological structures such as living tissue thus can be used as bio-physical model.
- Special liquid crystal devices can be attached to the skin to show a "map" of temperatures. This is useful because often physical problems, such as tumors have a different temperature than the surrounding tissue. Thus, it can be used as diagnostic purposes.
- Stability of emulsions can be improved due to smectic mesophase, which possess high viscosity.
- Lyotropic liquid crystals include DNA, proteins and cholesterol. Liquid crystal pharmaceuticals (LCP) are a unique class of lyotropic liquid crystals that represent novel drug candidates for the treatment of a wide range of diseases.
- A new Liquid crystal pharmaceuticals anti-tumor drug called Tolecine, a compound that has antiviral and antibacterial applications and another LCP, Apatone, which attacks cancer cells via multiple pathways to offer improved efficacy.
- Liquid crystal thermography is the technique to evaluate inflammatory conditions or describing viability of tissues by the differences in coloration that occur with the use of these unique substances. The technique has proved useful in studying hemophilia, tumors of the extremities, arthritis and vascular conditions, including the complications of diabetes.