Enthalpy-entropy compensation and solubility parameters for solutions of nonmesomorphic solutes in the smectic-A, nematic and isotropic phases of the liquid crystal p-n-octyl-p′-cyanobiphenyl

Ratios of infinite dilution solute activity coefficients (γ₂^∞) for nonmesomorphic solutes at the smectic-A-to-nematic (γ_2,SA^∞/γ _2,N^∞) and the nematic-to-isotropic (γ_2,N′^∞/γ_2,I ^∞) phase transition temperatures, T_S-N and T _N′-I respectively, of p-n-octyl-p′-cyanobiphenyl (8CB) show that the solutes are more soluble in the isotropic phase than in the smectic-A and nematic phases where they exhibit a nearly equal solubility. The Flory-Huggins size effect correction (γ_F-H^∞), and the thermal (γ_T^∞) and athermal (γ_I-S^∞) contributions to γ ₂^∞, are evaluated. Solute partial molar enthalpies (Δ_solvH) and entropies (Δ_solvS) of solvation show a compensation effect that reflects a delicate interplay between solute-solvent interactions and size and structure effects. Both the solute and solvent solubility parameters, δ₂ and δ₁ respectively, decrease as temperature increases. Additionally the δ₁ values undergo a discontinuous increase at T_S-N attributed to the disappearance of the layered structure of the Smectic-A phase and a discontinuous decrease at T_N′-I attributed to an increase in the flexibility of the alkyl tails of the liquid crystal molecules in the nematic phase. Evidence that hints at the effect of the structure of the liquid crystal solvent on the solution process is presented.