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

Abstract

Ratios of infinite dilution solute activity coefficients ($\gamma_{2}^{\infty} )$ for nonmesomorphic solutes at the smectic-A-to-nematic ($\gamma_{2,\mathrm{SA}}^{\infty}/\gamma_{2,\mathrm{N}}^{\infty} )$ and the nematic-to-isotropic ($\gamma_{2,\mathrm{N} '}^{\infty} /\gamma_{2,\mathrm{I}}^{\infty} )$ phase transition temperatures, $T_{\mathrm{S}\mbox{\scriptsize--}\mathrm{N}}$ and $T_{\mathrm{N} \mathrm{'}\mbox{\scriptsize--}\mathrm{I}}$ 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 ($\gamma_{\mathrm{F}\mbox{\scriptsize--}\mathrm{H}}^{\infty} )$, and the thermal ($\gamma_{\mathrm{T}}^{\infty} )$ and athermal ($\gamma_{\mathrm{I}\mbox{\scriptsize--}\mathrm{S}}^{\infty} )$ contributions to $\gamma_{2}^{\infty}$, are evaluated. Solute partial molar enthalpies (Δ_solv H) and entropies (Δ_solv S) 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, δ ₂ and δ ₁ respectively, decrease as temperature increases. Additionally the δ ₁ values undergo a discontinuous increase at $T_{\mathrm{S}\mbox{\scriptsize--}\mathrm{N}}$ attributed to the disappearance of the layered structure of the Smectic-A phase and a discontinuous decrease at $T_{\mathrm{N}'\mbox{\scriptsize--}\mathrm{I}}$ 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.