Calorimetric investigations of hydrogen-bonded liquid crystal binary mixtures

Abstract

Double hydrogen-bonded liquid crystals formed between methyl malonic acid (MM) and p-n-alkyloxy benzoic acids (nBAO) are characterized. Variation in the molar proportion of MM + nBAO (n = 8, 11 and 12) exhibiting good phase polymorphism yields two different sets of binary mixtures labeled as X and Y (where X = MM + 8BAO and Y = MM + 11BAO, MM + 12BAO) which yields 18 hydrogen-bonded binary mixtures as a result. The molar proportions of X and Y are varied in steps of 0.1–0.9 to obtain 18 binary mixtures. Chemical, optical and thermal analysis are carried out for all the 18 binary mixtures formed. Hydrogen bond existence and the chemical environment of the precursor are confirmed by Fourier transform infrared spectroscopy and nuclear magnetic resonance spectroscopy studies, respectively. Phase variance and the corresponding transition temperatures of the binary mixtures are studied by the observation of textures through polarizing optical microscope (POM), and the same is verified by recording the differential scanning calorimetry (DSC). Phase diagram of the two binary mixtures is built from the POM and DSC data. Thermal equilibrium, odd–even effect, thermal stability factor, order of phase transition, stability of phase through enthalpy values and specific heat value possessed by the mixtures are derived from the DSC datum obtained. These investigations are performed for the prepared binary mixtures with a curiosity to understand the optical and calorimetric properties exhibited by them.