Speed of sound measurement and mixing-rule evaluation of (n-butanol + n-heptane) binary mixtures

The speed of sound of the (n-butanol + n-heptane) mixtures was measured by the Rayleigh-Brillouin light scattering method at temperatures ranging from 298.05 to 618.29 K and pressures up to 9.0 MPa. The relative expanded uncertainty of the speed of sound experimental system is 1.2% (coverage factor k = 2). The mixing effects were analyzed from molecular interactions: the trend that speed of sound rises first and then decreases with the n-butanol concentration mainly depends on the rupture of alkanol structures and the molecular association, respectively. Lacking other thermophysical properties of mixtures at high temperature and pressure, six classical speed-of-sound mixing rules only based on pure component properties were evaluated by experimental data, including Wood, Nomoto, Van Dael, Impedance dependence relations, and Time-average model and Volume-average model. It was found that the estimates by Wood's relation (recognized as Junjie's relation) agree best with experimental data. The applicability of mixing rules was further evaluated by 21 mixtures of (n-alkanol + n-alkane), where Wood's relation still obtains great accuracy (average of rRMSDs = 0.91% and maximum = 1.84%). For these (n-alkanol + n-alkane) mixtures of weak interaction, Wood's relation is expected to give reliable estimates for the under a wide temperature and pressure range, for just insignificant volume change being ignored.