Effect of intermolecular hydrogen-bonding and terminal substituents on the mesophase behavior of binary mixtures of dissimilarly-substituted benzoic acids

Binary mixtures of two types of 4-substituted benzoic acids, covering the whole composition range, were prepared by melting both components together, stirring to give an intimate blend, and then cooling to room temperature. The mixtures prepared were characterized for their mesophase behavior by differential scanning calorimetry (DSC) and polarized-optical microscopy (POM). The first type of the acid is substituted with a small compact polar group that cover a wide range of polarity. The other type of the acid is 4-alkoxy benzoic acid bearing an alkoxy (OCnH2n+1) group with varying chain length (n = 6–16). Binary phase diagrams were constructed whereby all phases observed were identified by POM. The smectic C (SmC) mesophase was observed in all mixtures investigated, while the nematic phase (N) was observed, together with the SmC phase, only in mixtures possessing the acid homologues with n ≤ 12 carbons.