Effect of nonmesogenic impurities on the liquid crystalline phase transitions of octylcyanobiphenyl

The effect of the nonmesogenic solutes cyclohexane (CH), biphenyl (BP), and water (W) on the nematic-isotropic (N-I) and the nematic-smectic- A (N-SmA) phase transitions in the liquid crystal octylcyanobiphenyl (8CB) has been studied by means of adiabatic scanning calorimetry. For BP and CH a linear decrease of both transition temperatures was observed with increasing solute mole fraction. For water the transition temperature stayed nearly constant (after a slight decrease for mole fractions of water up to 0.1) over the entire investigated region. For all concentrations of biphenyl and water studied the N-SmA transition remains second order. For 8CB+CH, however, crossover from second order to first order is observed at a tricritical point of the mole fraction x of CH around 0.046. For water it was found that for mole fractions of water above 0.1 phase separation between an 8CB rich and a water rich phase occurred. The changes in the transition temperatures and differences between the three systems, and in particular the crossover from second order to first order in the 8CB+CH system, is explained in terms of a mean-field free energy density expression including coupling terms of the solutes mole fraction with the nematic and smectic- A order parameters.