The thermodynamics of solution of non-mesomorphic solutes in the liquid crystal 6CB revisited

Results from gas liquid chromatography were analyzed to express the infinite dilution solute activity coefficient γ₂ ^∞ as a function of the Flory-Huggins size effect correction (γ_FH^∞) and an interaction parameter (κ); the sum of the thermal (γ_T^∞) and athermal (γ_I-S^∞) contributions to γ ₂^∞ From regular solution theory, solute (δ₂) and solvent (δ₁) solubility parameters were obtained at different temperatures for all the solutes in the nematic and isotropic solution phases. Plots of ΔH̄ versus ΔS̄, the solute partial molar enthalpies and entropies of solution respectively, were reasonably linear. Such ΔH̄ - ΔS̄ compensation reflects similar solubility for the solutes in the nematic and isotropic phases. Also, an average of 1.11 ± 0.02 for γ_2,N^∞/ γ_2,I^∞, where N and I refer, respectively, to the nematic and isotropic phases, indicates slightly lesser solute compatibilities with the nematic phase. For all the solutes studied plots of δ₁ versus temperature (t) showed a drastic and sharp change in δ₁ at the nematic-to-isotropic transition (29.0°C) which is reminiscent of enthalpy and density changes in first order transitions. The extent of the discontinuity in δ₁ may be a measure of the strength of the transition while δ₁ which varies continuously across the phase transition or shows a kink at the phase transition would be associated with a second order transition. This finding can help in identifying second order transitions in liquid crystals or polymers.