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Journal of Solution Chemistry

, Volume 9, Issue 10, pp 753–769 | Cite as

Thermodynamics of mixtures of hexane and heptane isomers with normal and branched hexadecane

  • Michel Barbe
  • Donald Patterson
Article

Abstract

Molar excess mixing enthalpies h E , Gibbs free energies g E and hence entropies s E have been obtained using calorimetry and the vapor sorption method at 25°C for hexane isomers+2,2,4,4,6,8,8-heptamethylnonane, a highly branched C 16 . The h E and g E are negative while Ts E are positive, but small. The values are explained by the Prigogine-Flory theory through negative free volume contributions to h E and Ts E , counterbalanced in the case of Ts E by the positive combinatiorial Ts E for mixing molecules of different size. No contribution is seen from the interaction between methyl and methylene groups. The excess quantities are also obtained for hexane and heptane isomers mixed with n-hexadecane. Values of h E and Ts E are now strongly positive, while those of g E are only slightly less negative. The interpretation requires two recently advanced contributions in addition to those of the Prigogine-Flory theory: 1) a decrease of order when correlations of orientations between n-C 16 molecules in the pure liquid are replaced in the solution by weaker correlations whose strengths depend on the shapes of the lower alkane isomers. For lower alkane isomers of the same shape, but highly sterically hindered, h E and Ts E are small, manifesting, 2) a negative contribution, ascribed to a rotational ordering of n-C 16 segments on the sterically-hindered molecule. Enthalpy-entropy compensation is observed for these new contributions, arising from their rapid fall-off with increase of temperature.

Key words

Excess enthalpy excess entropy enthalpy-entropy compensation orientational order hexane isomers heptane isomers branched n-C16 compound rotational ordering 

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Copyright information

© Plenum Publishing Corporation 1980

Authors and Affiliations

  • Michel Barbe
    • 1
  • Donald Patterson
    • 2
  1. 1.Pulp and Paper Reserach Institute of CanadaPointe ClaireCanada
  2. 2.McGill UniversityMontrealCanada

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