Liquid structure and the thermal pressure coefficients of cyclohexane + normal- and branched-alkane mixtures

Abstract

Velocities of sound, thermal expansion coefficients and heat capacities have been measured at 25 °C for cyclohexane mixed with n-hexadecane and with a series of highly branched alkanes (br-C_n, where n= 6, 8, 12, 16 and 19), leading to isentropic and isothermal compressibilities and thermal pressure coefficients, γ. With these data and corresponding literature values for the normal series, the mixing function Δ(γVT) has been obtained for c-C₆+ n-C_n and c-C₆+ br-C_n. For the branched series, Δ(γVT)≈–H^E, as predicted by the Flory and other theories, which assume van der Waals behaviour for the component liquids. For the n-C_n series, as n increases, Δ(γVT) deviates from –H^E towards much more negative values, indicating that the longer n-C_n are not accurately van der Waals liquids. Their behaviour is consistent with the presence of orientational order, which affects the equation of state of a liquid. The Flory theory successfully predicts dV^E/dP for both the linear and branched series, the effect of order on the compressibility apparently being small. The quantities ΔC_v and Δ(αγVT) are both strongly negative for c-C₆+ n-C_n, corresponding to a decrease of n-alkane order with increasing T, because of thermal motion and expansion of the liquid, respectively, the latter effect predominating. For the br-C_n, Δ(αγVT) is small and negative but ΔC_v is unexpectedly positive, perhaps indicating a disturbance of cyclohexane motion on the addition of the branched alkanes.