Mesomorphic state disordered crystals

Liquid crystals are intermediate states of matter or mesophases, halfway between an isotropic liquid and a solid crystal. In nature, some substances, or even mixtures of substances, present these mesomorphic states. These liquid crystalline phases exhibit a local disorder ( liquid-like behavior) and are dynamic at a molecular level, but a long-range order exists, which endows it with unique rheological, mass transport, and optical properties. Liquid crystals offer a number of useful properties for the drug delivery. Solubilization of drug in the liquid crystalline is similar to the solubilization of drug in micelles. Simultaneously, increase in viscosity of the system helps to provide more... 

The term mesomorphic was proposed by Friedel in 1922 for materials of middle (Greek mesos) form (Greek morphe) to address materials in a condensed phase having intermediate characteristics between liquids and crystals [1]. More generally, the term mesomorphic maybe used to address all the states of matter, which may be considered as intermediate between the crystalline and the liquid (or amorphous) state, as for instance positionally disordered crystals or orientationally ordered liquids and/or glasses [2]. 

Cormational disorder exist in the a2 nd p2 polymorphs of PDES. Additional disorder is introduced in the am polymorph, leading to a mesomorphic state that resembles the isotropic melt. The residual order of this state however promotes the crystallization of PDES. Upon shearing changes occur within the am polymorph indicating the possibility of a phase transition. 

Fig. 1.2 Solid form system illustrating long-range ordered (translational, orientational, and conformational) crystals on one end and completely disordered amorphous material on the other end. Solid forms can assume various length scale of order (long range, medium range, short range) and/or mesomorphic states (smectic, nematic)...

The terms liquid crystal, mesophase, or mesomorphic state are used synonymously to describe a number of different states of matter in which the molecular order lies between the almost perfect long-range positional or orientational order of solid crystals and the long-range disorder found in ordinary isotropic liquids. Two main classes of liquid crystals are usually distinguished lyotropic and thermotropic. In lyotropic mesophases, the combination of order and mobility can be achieved by using a solvent thermotropic mesophases are based on the temperature-induced mobility of form-anisotropic molecules in the melt. Surfactants can often form both thermotropic and lyotropic liquid crystals i.e., they possess amphitropic properties [2]. 

Liquid crystal state - recommended symbol LC - a mesomorphic state having long-range orientational order and either partial positional order or complete positional disorder. 

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... 

States exist in which the molecules have long range translational order in some respects but disorder in others. These are called liquid crystals. Liquid crystals exhibit fluid flow but are anisotropic. Transitions in which at least one phase is a liquid crystal are mesomorphic transitions. Compotmds of relatively long rod-shaped molecules with polar groups form tluee main types of liquid ciystals. They are nematic, cholesteric, and smectic. Disc-shaped molecules may also form several types of liquid crystals having the general designation of discotic. 

The detailed descriptions of the structures of calamitic phases allow us to classify the mesomorphic liquid crystal state, and to place this state in context with the crystalline and amorphous liquid states [3]. Table 1 describes the relationship between ordered crystals, disordered or soft crystals, liquid crystals and the isotropic liquid. 

Physical studies of biomembranes have concentrated mainly on the organization of the lipid matrix. Perhaps the best studied physical property of membranes is their thermotropic mesomorphism. This change of state induced by temperature is reflected in an order-disorder phase transition of the lipid matrix and is commonly referred to as the gel to liquid crystal phase transition. 

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