Effects of molecular flexibility and shape on the excess enthalpies and heat capacities of alkane systems
- 1 January 1979
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Faraday Transactions 1: Physical Chemistry in Condensed Phases
- Vol. 75, 1700-1707
- https://doi.org/10.1039/f19797501700
Abstract
Using the Picker flow microcalorimeter, molar excess enthalpies (hE) and molar excess heat capacities (cE P) have been obtained at 25°C for nonane isomers of different degrees of chain branching mixed with n-octane, and for 2,2,5-trimethylhexane and 3,3-diethylpentane (3,3-DEP) mixed with other normal alkanes and with cycloalkanes. As the degree of steric hindrance within the nonane isomer is increased, hE with n-octane changes from positive to negative and cE P from negative to positive. Values of hE are also anomalously negative for 3,3-DEP + n-Cn with n⩽ 14 and for 3,3-DEP + cyclopentane. Correspondingly, cE P is anomalously positive for 3,3-DEP + n-C6 and for 3,3-DEP + cyclopentane. The results are interpreted in terms of two contributions: (1) destruction of orientational order in the pure n-alkane giving hE > 0, cE P < 0, and (2) a “condensation” of the more freely moving molecule, e.g., n-Cn or cyclopentane onto the sterically-hindered 3,3-DEP resulting in a restriction of movement giving hE < 0, cE P > 0, i.e., a creation of order in the solution.Keywords
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