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

摘要

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