Most substances exist in three phases: solid (often crystalline), liquid and vapour. The difference between these states of matter is the degree of order in the material, which is directly related to the surrounding temperature and pressure. At low temperatures, when the material is in its solid state, the constituents (atoms, ions or molecules) cannot move about freely. Their only movements are thermal vibrations about an equilibrium position (see figure 1a).
If the temperature is raised, more energy is put into the system, leading to stronger and stronger vibrations. Finally, at the transition temperature between the solid and liquid states, the long range positional order is broken and the constituents may move about in a random fashion (figure 1b), constantly bumping into one another and abruptly changing direction of motion. The thermal energy is still not, however, high enough to completely overcome the attractive forces between the constituents, so there is still some positional order at short range. Because of the remaining cohesion, the density of the liquid is constant even though, as opposed to the solid, the liquid takes the shape of its container. The liquid and solid phases are called condensed phases. If we keep on raising the temperature until the next phase change, the substance enters its gas (or vapour) state and its constituents are no longer bound to each other (figure 1c).
|a: Solid (crystalline) phase. Periodic positional order & orientational order. Bound molecules.||b: Liquid phase. Random order. Bound molecules.||c: Gas phase. Random order. Unbound molecules.|
Certain organic substances possess more condensed phases than the basic two. They are referred to as liquid crystals and their constituent molecules are often called mesogens. Their different extra phases, found between the solid and liquid states, are called liquid crystalline phases or sometimes mesophases. The explanation to these intermediate phases lies in the fact that liquid crystal molecules are always shape anisotropic, normally having a more or less rodlike shape. They may also be disc-, or bowlshaped. The latter two groups are referred to as discotic and bowlic liquid crystals, respectively. On these pages we will concentrate on substances with rodlike molecules (sometimes called calamitic), which constitute the major class.
In the above description of the solid, liquid and vapour phases, we only took the degree of positional order into account. Because of the anisotropic shape of the liquid crystal molecules we must now also consider the orientational order in the material. The liquid crystalline phases all exhibit high orientational order while the order in molecule positions is limited. An example is shown in figure 2. The positional arrangement in the liquid crystalline phases varies from complete disorder to order in one dimension (the molecules tend to form layers) or slightly more. A substance in a liquid crystalline phase is therefore a fluid in that it assumes the shape of its container. At the same time the high orientational order gives this fluid higly anisotropic properties, which distinguishes it from normal liquids which are always isotropic (the liquid phase of a mesogenic compound is therefore normally denoted the isotropic phase).
The orientational order in the liquid crystal phases is of course
not perfect. In fact, if we were to take a snapshot of a liquid
crystalline sample we would find quite large differences in
orientation of different molecules (see figure 2). There is, however,
locally a distinct preferred direction around which the
molecules fluctuate. This average molecule orientation is described
by a unit vector
called the director,
denoted by n. It is a rather special vector as its sign is
generally of no importance; n = -n. This reflects the
fact that turning the director 180° is a symmetry operation,
which conserves all physical properties of the liquid crystal.