A simple molecular model for the interpretation of some properties of smectic C and A phases

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Abstract
We present a simple geometrical model in which the molecular shape is approximated by a small number of parameters for the dumbbell-like middle group and cylinder-like alkyl end chains. The pair potentials of nearest neighbours are approximated by the sum of anisotropic repulsive terms due to the contact of the different parts of the molecules and attraction due to dispersion forces between different parts of the molecules and attraction due to dispersion forces between different parts of the molecules. Since the number of nearest neighbours at the smectic C/A phase transition is unchanged, the resulting pair potentials are able to describe well the cooperative behaviour of the molecules in the non-ordered layers of the smectic C and A phases. The dependence of the tilt angle on the alkyl chain length and on the temperature and other thermodynamic and structural properties can be interpreted qualitatively very well.