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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.

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