Simple model of induced smectic phases in binary mixtures of liquid crystals

Abstract

The origin of the occurrence of induced smectic phases (ISP) in binary mixtures of liquid crystals is discussed. First the experimental evidence is reviewed, which leads to a division in strong complexing (electron–donor–acceptor complex as indicated by a charge–transfer band) and weak complexing, where only indirect evidence is available. Within the mean field approximation a theoretical model is given in which the influence of complex formation on the nematic–smectic-A transition temperature is investigated for ideal orientational order. The ISP mechanism is attributed to the dispersive interactions between the complexes. However, in principle in a two-component mixture without complexing also ISP can be obtained due to interactions involving permanent dipoles. From fits to experimental phase diagrams and from estimates of parameters it follows that the first mechanism is more likely. Three nonstandard groups of phase diagrams with characteristic maxima of minima are predicted. The location of the extrema is calculated as a function of complex concentration and effective, intermolecular interactions. The model correctly describes presently existing, very rich, experimental data.