A new approach to study interaction parameters in cyanobiphenyl liquid crystal binary systems

•The phase transition of 7CB and 5CB liquid crystals studied using the DSC.•This work includes the determination of the eutectic in the 7CB/5CB mixture.•The excess functions and interaction parameters calculated in the 7CB/5CB mixtures.•The P∗ randomicity parameter used to describe the phase transitions of C–N and N–I.•A small amount of P∗ showed a non-random identity of the C–N phase transition.

The phase transition of heptylcyanobiphenyl 7CB and pentylcyanobiphenyl 5CB liquid crystals was investigated using the differential scanning calorimetry DSC technique. Then, the phase transition of different compositions of 7CB/5CB binary mixture was studied to determine the eutectic point. The phase diagram of mentioned binary system in 7CB mole fraction of 0.45 at T = 273.45 K is in good agreement with that of predicted from Schroder–van Laar equation. The thermodynamic excess functions and interaction parameters were calculated to describe the phase transition physically using the non-random mixing for the first time. The P∗P∗ randomicity parameter was used to describe the phase transitions of C–N and N–I in which a small amount of P∗P∗ shows a non-random identity of C–N phase transition. Contrarily, the P∗P∗ is greater in N–I phase transition showing a random mixing process.

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