The orthobaric coexisting densities of {xSi(CH3)4(1−x)C(CH3)4} fromT≈ 360 K up to the gas–liquid critical line: a possible experimental route for the verification of the Lorentz or arithmetic-mean combining rule for mixtures of quasi-spherical molecules

The orthobaric densities of the liquid and vapour phases of (tetramethylsilane+2,2-dimethylpropane) have been measured with a hydrostatic density balance from a temperature of about 360 K up to near the (gas+liquid) critical locus. The results were used to test the applicability of the extended-scaling equation of Wegner to fit the densities of these mixtures along a single limb of the coexistence curve; the data were well represented using only two scaling correction terms. The observed critical temperatures show a linear composition dependence between the values for the pure substances, in good agreement with the Lorentz–Berthlot rules for mixtures of molecules of near-equal size and energy. A verification of these results is finally accompanied with a set of values for Σii, Σjj, ϵii, and ϵjjas obtained from the numerical technique of Rummens and Rajan.