Calorimetric study of the nematic to smectic-A phase transition in octylcyanobiphenyl-hexane binary mixtures

TL;DR

This study investigates the nematic to smectic-A phase transition in octylcyanobiphenyl-hexane mixtures using calorimetry, revealing a solvent concentration-driven crossover to tricritical behavior and changes in transition characteristics.

Contribution

It provides new insights into how solvent concentration influences the N-SmA transition, demonstrating a crossover towards tricriticality in liquid crystal mixtures.

Findings

Heat capacity peak shifts non-monotonically with solvent concentration.

Transition exhibits hysteresis and a crossover to tricritical behavior.

Effective critical exponent increases with solvent concentration.

Abstract

The continuous nematic to smectic-A (N-SmA) phase transition has been studied by high-resolution ac-calorimetry in binary mixtures of the liquid crystal octylcyanobiphenyl(8CB) and a non-mesogenic, low-molecular weight, solvent n-hexane(hex) as a function of temperature and solvent concentration. Heating and cooling scans about the N-SmA transition temperature were repeatedly performed on pure and six 8CB+hex samples having hexane molar concentration ranging from x_{hex}= 0.02 to 0.12. All 8CB+hex samples in this range of x_{hex} remain macroscopically miscible and exhibit an N-SmA heat capacity peak that shifts non-monotonically to lower temperature and evolves in shape, with a reproducible hysteresis, as x_{hex} increases. The imaginary part of heat capacity remains zero up to x^{TCP}_{hex}\simeq 0.07$ above which the distinct peak is observed, corresponding to a jump in both the real and imaginary enthalpy. A simple power-law analysis reveals an effective exponent that increases smoothly from 0.30 to 0.50 with an amplitude ratio A^{-}/A^{+}\rightarrow 1 as x_{hex}\rightarrow x^{TCP}_{hex}. This observed crossover towards the N-SmA tricritical point driven by solvent concentration is consistent with previous results and can be understood as weakening of the liquid crystal intermolecular potential promoting increased nematic fluctuations.