Abstract

The phase diagram of a pure substance exhibiting gas, liquid and solid phases is reasonably well understood. As the pioneering work of van der Waals showed, the fluid phase behaviour of ‘simple’ fluids can be understood in terms of the balance of spherical repulsive and attractive forces, and even the fluid-solid transition of such systems can be explained in terms of the freezing of a hard-sphere system. A more intricate picture arises, however, when the fluid phase equilibria in a mixture is considered; even if its components are relatively simple. If, in addition, molecules are not spherical, striking new transitions can be observed which lead to intermediate phases such as liquid crystals. In mixtures of liquid crystals the competition between ordering and demixing can determine the stability of many phases. Molecular shape and flexibility can also be tuned to favour the formation of the solid phases to such an extent that the fluid equilibria may become entirely metastable. The importance of taking a molecular perspective towards complex systems is highlighted as the design of novel materials is guided by a theoretical understanding.