Enthalpies of solute transfer into nematic MBBA. Long-range disturbance of order

Abstract

Enthalpies, ΔH_s(T → N), have been obtained at 25°C for the transfer of solute molecules (S) from a reference solvent toluene (T) into nematic (N)p-methoxybenzylidine-p-n-butyl-aniline (MBBA). Solutes include the normal alkanes n-C_n, n= 6, 8, 12, 16 and 20, and a corresponding series of the highly-branched isomers (br-C_n) from 2,2-dimethylbutane to 2,2,4,4,6,8,8-heptamethylnonane.

Values of ΔH_s(T → N) expressed per gram of solute are positive and large (ca. 50 J g^–1) for the globular branched alkane molecules, whereas for the normal alkanes ΔH_s(T → N) decreases rapidly with n becoming negative for n-C₂₀. Using the Prigogine–Flory theory as a framework, the branched alkane values suggest a long-range breakdown of nematic order associated with the creation of the solute cavity in the MBBA which is not restored when the solute enters the cavity. For the normal alkanes of increasing chain-length the long-range disturbance of nematic order caused by the cavity seems to be increasingly balanced by the presence in the cavity of the solute, which being of anisotropic shape similar to the MBBA molecule can correlate its orientations with those of the MBBA. Enthalpies of transfer have also been obtained for other C₁₆ isomers, 2-, 4- and 6-methylpentadecane and 6-pentylundecane. They depend sensitively on the molecular structure. Other solutes include the series of alkylbenzenes and alkylcyclohexanes, cyclic alkanes and cholesterol, polyaromatic hydrocarbons, tetra-alkyltins and dimethylsiloxane isomers, all showing strong effects of solute molecular shape. In particular, trans-decalin gives a much smaller heat of transfer, 41.3 J g^–1, than the cis-isomer, 81.3 J g^–1; cholesterol gives the most negative of all the enthalpies of transfer, –20.7 J g^–1. With nineteen of the solutes for which data were available, a correlation was found between ΔH_s(T → N) and the Gibbs free energy of solute transfer from the isotropic to the nematic phase of MBBA as obtained from the depression of the nematic–isotropic transition temperature.