The Marcelja-Luckhurst theory, together with its variants, are becoming widely used both to understand and to predict the behaviour, at the molecular level, of liquid crystals composed of flexible molecules. Although the theory has been relatively successful, its development was essentially ad hoc, being built from elements taken from molecular theories of other liquid-crystalline systems and from polymer science. Here we attempt to remove this unsatisfactory feature of the Marcelja-Luckhurst theory by rederiving it using a variational approach first employed by de Gennes for nematogens composed of rigid, uniaxial particles. This new derivation makes clear the nature of the fundamental approximations on which the theory is based and allows for its further development. The additional approximations necessary to simplify the computations needed to implement the theory are also discussed.
