Thermophysical Properties of Binary Mixtures of n-Dodecane with n-Alkylcyclohexanes: Experimental Measurements and Molecular Dynamics Simulations

DJL Prak and BH Morrow and JS Cowart and PC Trulove and JA Harrison, JOURNAL OF CHEMICAL AND ENGINEERING DATA, 64, 1550-1568 (2019).

DOI: 10.1021/acs.jced.8b01135

Thermophysical properties (densities, speeds of sound, bulk moduli, and viscosities) of binary mixtures of n-dodecane with n-alkylcyclohexanes (propyl- to dodecylcy-clohexane) were examined at various compositions and temperatures (293.15-333.15 K). Viscosities were analyzed using the McAllister three-body equation, and excess molar Gibbs energies of activation for viscous flow (Delta G*(E)) at 293.15 K were calculated. Because the Delta G*(E) values did not differ significantly from zero, the mixtures appear to behave ideally. In contrast, nonzero excess molar volume values obtained both experimentally and using molecular dynamics (MD) simulations suggest nonideal behavior. Excess molar volumes were the most negative for n-dodecylcyclohexane mixtures and increased with decreasing alkyl side-chain length eventually becoming slightly positive for mixtures containing n-propylcyclohexane. MD simulations were able to predict density, isentropic bulk modulus, and dynamic viscosity values, but the accuracy of the calculated densities decreased slightly with increasing temperature. Voronoi tessellation was used to calculate histograms of molecular volumes in the mixtures. The most probable volume of n-dodecane increases or decreases when mixed with n-propylcyclohexane or n-dodecylcyclohexane, respectively. These shifts in molar volume are responsible for the expansion and contraction upon mixing observed in the excess molar volume data. Volume contraction (negative excess molar volume) produces mixture speeds of sound that are faster than ideal (positive excess speed of sound) unless confounded by opposing compressibility differences. Excess speeds of sound were positive for n-dodecylcyclohexane mixtures, decreased as the alkyl side- chain length increased, and were negative for n-propylcyclohexane mixtures.

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