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Liquid structure and second-order mixing functions for benzene, toluene and p-xylene with n-alkanes

1986; Royal Society of Chemistry; Volume: 82; Issue: 9 Linguagem: Inglês

10.1039/f19868202977

2050-5655

G. Tardajos, Emilio Aicart, Miguel Costas, Donald Patterson,

Chemical Thermodynamics and Molecular Structure

Expansion coefficients (α), thermal pressure coefficients (γ), isothermal compressibilities, densities and heat capacities have been measured at 25 °C for pure components and the following mixtures at equimolar composition: benzene and toluene with the series of normal alkanes n-Cn where n= 6, 8, 10, 12, 14 and 16 and for p-xylene with n-C6 and n-C16. Some exceptions ocur when data are in the literature. With these data the following first-and second-order mixing quantities are reported: VE, dVE/dT, dVE/dP, Δ(γVT), CpE, Δ(αγVT) and ΔCv. In all cases the variation of the excess volume VE with n is well predicted by the Flory theory, which is also successful with dVE/dP for all systems. Experimental dVE/dT values become increasingly negative as n increases, deviating from the Flory predictions. This discrepancy is large for benzene, but progressively smaller for toluene and p-xylene. As n increases, the mixing function Δ(γVT) deviates from the equality Δ(γVT)=–HE which is predicted by the Flory theory and other theories which assume van der Waals behaviour. Here again, the deviation is strong for benzene, decreasing for toluene and p-xylene. The excess heat capacity CEp and its components Δ(αγVT) and ΔCv are negative for all systems except p-xylene–n-C6, where ΔCv is positive and small. Contrary to theory, the three quantities become increasingly large and negative as n increases, the effect decreasing from benzene to p-xylene. The results are explained by temperature-sensitive ordering of (2–2) and (1–2) contacts. The (2–2) order between long-chain alkanes is partially destroyed on mixing, giving negative contributions to dVE/dT, Δ(γVT) and CpE, while the (1–2) order formed in solution has the opposite effect. The (1–2) order is small for benzene–alkane contacts, increasing for toluene and p-xylene. CEp results for benzene–n-Cn are also consistent with weak order in pure liquid benzene.