Browsing by Author "Patterson, Donald"

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    Enthalpies of Solute Transfer into Nematic MBBA Long-range Disturbance of Order
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1981, 77 (06), 1981) Croucher, Melvin D; Patterson, Donald
    Enthalpies, ΔHs(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-Cn, n= 6, 8, 12, 16 and 20, and a corresponding series of the highly-branched isomers (br-Cn) from 2,2-dimethylbutane to 2,2,4,4,6,8,8-heptamethylnonane. Values of ΔHs(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 ΔHs(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 ΔHs(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.
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    Molecular Structure and Orientational Order Effects in Enthalpies and Heat Capacities of Solute Transfer into n-C16 Part 1.—Normal and Branched Alkane Solutes
    (Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1981, 77 (06), 1981) Kronberg, Bengt; Patterson, Donald
    Enthalpies ΔH(b → n) and heat capacities ΔCp(b → n) have been obtained at 25°C for the transfer of solute molecules into normal-C16(n) from the highly-branched-C16, 2,2,4,4,6,8,8,-heptamethylnonane (b). Solutes include normal alkanes from C5 to C20 and the series of highly branched alkanes from 2,2-dimethylbutane to heptamethylnonane and polyisobutylene, as well as siloxanes and nonanes of different degrees of internal steric hindrance. With the Prigogine–Flory theory as background, results are interpreted through enthalpy and heat capacity changes associated with the elimination in b and formation in n of a cavity and of solute–cavity interactions. Interpretation is simplified by the virtual identity of the b and n liquids excepts for the presence in n of short-range orientational order. For a solute incapable of correlating its molecular orientations with those of n, e.g. a branched alkane or a lower n-alkane, ΔH(b → n) and ΔCp(b → n) are large and, respectively, positive and negative. The values are attributed to the destruction of order in forming the cavity in n. For solutes capable of correlating their molecular orientations with those of n, e.g. a longer n-alkane, interaction between the solute and the cavity in n becomes important and both ΔH(b → n) and ΔCp(b → n) change sign to become negative and positive, respectively. It is demonstrated that the orientational order can be described as a pair-wise interaction of short-range character. The order in the pure n-C16 increases intermolecular cohesion, but only slightly compared with the usual dispersion force interaction, contributing ca. 3% of the energy of vaporization at 25°C. However, due to the sensitivity of the order to temperature, the order contribution to the configurational heat capacity of pure n-C16 is ca. 20% of the total.