Phase behavior of nematic-nonnematic binary systems: III. The effect of hydrogen bonding interaction

For the binary mixtures of nematic 4-pentyl-4'-cyanobiphenyl (5CB) with some kinds of nonnematic solutes, which showed 'unusual' increase in nematic-isotropic phase transition temperature compared with that of pure 5CB, we have measured the temperature dependence of the IR dichroic ratio for several bands of 5CB and of the solutes. The dichroic ratios and their temperature dependence distinctly showed the presence of rather firmly hydrogen-bonded complexes in these binary systems. We found some interesting results, e.g., the dichroic ratios for the -OH stretching mode of phenol, which is hydrogen bonded to the -CN group of 5CB, and for p-ethyl benzoic acid were larger than the dichroic ratio of the -CN band. The temperature dependence of the second-order parameters derived from IR dichroic ratios was analyzed based on a Maier-Saupe-type mean field theory.