Abstract
The pressure dependence of the excess enthalpy HE, dHE/dP, has been calculated from experimental excess volumes VE and dVE/dT using dHE/dP=VE−TdVE/dT. dHE/dP at zero pressure are reported at 25°C and equimolar concentration for the mixtures: cyclohexane with the series of normal alkanes (n-C n , where n=6,8,10,12,14 and 16) and with the series of highly branched alkanes (br-C n , where n=6,8,12 and 16), benzene, toluene and p-xylene +n-C n and 1-chloronaphthalene +n-C n and br-C n . Experimental and Flory theory dHE/dP values are in good agreement for the whole cyclohexane +br-C n series. For the n-C n series, dHE/dP becomes increasingly positive deviating from the Flory predictions. This discrepancy is due to the presence of short-range orientational order in the higher n-C n pure liquids which makes dH/dP more negative and which, upon mixing, is destroyed producing a positive contribution to dHE/dP not accounted for by the theory. The discrepancy between theoretical and experimental dHE/dP is large for benzene, but progressively smaller for toluene, p-xylene and 1-chloronaphthalene. These results are consistent with creation of order between the aromatic plate-like molecule and the long n-C n in solution. For 1-chloronaphthalene +n-C n , this order creation process produces a negative contribution to dHE/dP which balances the positive order-destruction contribution originated by the rupture, upon mixing, of short-range orientational order in pure n-C n .
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Aicart, E., Tardajos, G. & Costas, M. The effect of pressure on order destruction and order creation in linear or branched alkane mixtures. J Solution Chem 18, 369–377 (1989). https://doi.org/10.1007/BF00656774
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DOI: https://doi.org/10.1007/BF00656774