Liquid Crystals: The Fourth State Of Matter In Pharmacy 

1. Complex liquids or liquid complexes are materials intermediate between conventional liquids and solids, displaying fluid-like as well as solid-like behaviour.

Examples are polymeric melts or solutions, glasses, gels, foams and granular matter. Many of these systems are inherently disordered and strongly heterogeneous with large fluctuations on a wide range of length and time-scales. Furthermore many complex fluids, such as glasses or gels, never relax to equilibrium, which makes a theoretical analysis difficult.

2. Thus, these are binary mixtures that have a coexistence between two phases: solid-liquid (suspensions or solutions of macromolecules such as polymers), solid-gas (granular), liquid-gas (foams) or liquid-liquid (emulsions). They exhibit unusual mechanical responses that are applied stress or strain due to the geometrical constraints that the phase coexistence imposes. They mechanical response includes transitions between solid-like and fluid-like. behaviour as well as fluctuations.Their mechanical properties can be attributed to characteristics such as high disorder, caging, and clustering on multiple length scales.

3. Shaving cream is an example of a complex fluid. Without stress, the foam appears to be a solid: it does not flow and can support (very) light loads.

However, when adequate stress is applied, shaving cream flows easily like.

a fluid.

Liquid Crystals:

1. Liquid crystals (LCs) or mesomorphs are a state of matter which has properties between those of conventional liquids and those of solid crystals. For instance, a liquid crystal may flow like a liquid, but its molecules may be oriented in a crystal-like way.

There are many different types of liquid-crystal phases, which can be distinguished by their different optical properties (such as textures). The contrasting areas in the textures correspond to domains where the liquid-crystal molecules are oriented in different directions. Within a domain, however, , the molecules are well ordered. LC materials may not always be in a liquid-crystal phase (just as water may turn into ice or steam).

2. Liquid crystals can be divided into thermotropic, lyotropic and metallotropic phases. Thermotropic and lyotropic liquid crystals consists mostly of organic molecules, although a few minerals are also known.

Thermotropic LCs exhibit a phase transition into the liquid-crystal phase as temperature is changed. Lyotropic LCs exhibit phase transitions as a function of both temperature and concentration of the liquid-crystal molecules in a solvent (typically water).

Metallotropic LCs are composed of both organic and inorganic molecules, their liquid-crystal transition depends not only on temperature and concen-tration, but also on the inorganic organic composition ratio.

Glassy States:

1. Glass is a state of matter. Glasses combine some properties of crystals and some of liquids but are distinctly different from both. Glasses have the mechanical rigidity of crystals, but the random disordered arrangement of molecules that characterises liquids.

2. Glasses are usually formed by melting crystalline materials at very high temperatures. When the melt coils, the atoms are locked into a random (disordered) state before they can form into a perfect crystal arrangement.

3. As a liquid (at the melting temperature, Tm) is cooled from a high temperature, it may either crystallise or become super cooled. The particles (atoms, molecules or ions) forming crystalline materials are arranged in orderly repeating patterns, with elementary building blocks (unit cells extending to all three spatial

dimensions.

4.The structures of crystalline solids depends (predictably) on the chemistry of the material and the conditions of solidification (starting temperature and cooling rate, ambient pressures etc.) and can be described easily in detail. Super cooled liquids, on the other hand, demonstrate a rather intringing behaviour.

5. Upon further cooling below the T their particles progressively lose translational mobility, so that around the so called glass transition temperature (Ty) rearrangement to "regular" lattice sites is practically unfeasible; this behaviour is distinctive for the amorphous structures described as glasses or vitreous solids.

Types of Crystals: Types of crystals are described as belows:

1. Lyotropic Liquid Crystals

2. Thermotropic Liquid Crystals

3. Metallotropic Liquid Crystals

1. Lyotropic Liquid Crystals :

(i) A lyotropic liquid crystals consists of two or more components that exhibit-crystalline properties in certain concentration ranges.

(ii) In the lyotropic phases, solvent molecules fill the space around the compounds to provide fluidity to the system. In contrast to thermotropic liquid crystal, these lyotropic have another degree of freedom of concentration that enables them to induce a variety of different phases.

(iii) A compound that has two immiscible hydrophilic and hydrophobic parts within the same molecule is called an amphiphilic molecule.

(iv) Many amphiphilic molecules show lyotropic liquid-crystalline phase sequences depending on the volume balances between the hydrophilic part and hydrophobic part.

2. Thermotropic Liquid Crystals :

(i) Thermotropic phases are those that occur in a certain temperature range. If the temperature rise is too high, thermal motion will destroy the delicate cooperative ordering of the LC phase, pushing the material into a conventional isotropic liquid phase.

(ii) At too low temperature, most LC materials will form a conventional crystal. Many thermotropic LCs exhibit a variety of phases as temperature is changed.

(iii) Depending on the shape of liquid crystals they are of three types:

• Smectic phase-thread like
• Nematic phase-soap like
• Cholesteric phase-layered

3. Metallotropic Liquid Crystals:

1. Liquid crystal phases can also be based on low-melting inorganic phases like ZnCl, that have a structure formed of linked tetrahedra and easily form glasses. The addition of long chain slap-like molecules leads to a series of new phases that show a variety of liquid crystalline behaviour both as a function of the inorganic-organic composition ratio and of temperature. This class of materials has been named metallo-tropic.
2. Thus a metallotropic liquid crystal consists of metal ions incorporated into the liquid crystals.

Types of Glassy States: Following are the different types of glassy states:

• Formation of glass from crystalline materials: Actual formation of a glass formed by melting crystalline materials at very high temperatures.
• Glassy states of polymers cooled below (Ts) : It consists of polymers which have been cooled below the glass transition temperature (Tg). Thus these structures consists of fibrous structures of carbon atoms with small lateral branches in the form of hydrogen atoms or more complex radicals.
• Glassy states of inorganic compounds: It consists of refractory inorganic compounds of multivalent elements. These glasses in the stabilized state have the most thermostable chemical structure e.g., Silicon dioxide SiO, is as important constituent of glass.