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

Abstract

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-C_n 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: V^E, dV^E/dT, dV^E/dP, Δ(γVT), C_p^E, Δ(αγVT) and ΔC_v. In all cases the variation of the excess volume V^E with n is well predicted by the Flory theory, which is also successful with dV^E/dP for all systems. Experimental dV^E/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)=–H^E 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 C^E_p and its components Δ(αγVT) and ΔC_v are negative for all systems except p-xylene–n-C₆, where ΔC_v 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 dV^E/dT, Δ(γVT) and C_p^E, 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. C^E_p results for benzene–n-C_n are also consistent with weak order in pure liquid benzene.