Thermophysical Properties of Two-Component Mixtures of n‑Nonylbenzene or 1,3,5-Triisopropylbenzene with n‑Hexadecane or n‑Dodecane at 0.1 MPa: Experimentally Measured Densities, Viscosities, and Speeds of Sound and Molecular Packing Modeled Using Molecular Dynamics Simulations

This work presents experimentally measured densities, viscosities, and speeds of sound and calculated bulk moduli of binary mixtures of a C15H25 aromatic compound (n-nonylbenzene or 1,3,5-triisoproylbenzene) with either n-dodecane or n-hexadecane. Most properties increased monotonically as the amount of the compound with the higher property value increased. In contrast, minima in the speeds of sound for the 1,3,5-triisopropylbenzene mixtures were found at x 1 = 0.1 or 0.2 in n-dodecane mixtures and x 1 = 0.8 or 0.9 for n-hexadecane systems at 293.15 K. Also, the 1,3,5-triisopropylbenzene and n-hexadecane mixtures had viscosities below those of the individual compounds at 293.15 K. Excess molar volumes were all positive or close to zero; excess speeds of sound were all positive, while excess isentropic compressibilities and viscosity deviations were negative. Excess molar Gibbs energies of activation for viscous flow did not differ statistically from zero. Molecular dynamics simulations qualitatively reproduced the trends in excess molar volumes with 1,3,5-triisopropylbenzene mixtures having more positive excess molar volumes compared to the n-nonylbenzene mixtures. Examination of fluid structure using radial distribution functions and angular radial distribution functions showed that the relative positions and orientations of the aromatic rings in pure 1,3,5-triisopropylbenzene and n-nonylbenzene were very different. In addition, the arrangement of the n-nonylbenzene aromatic rings did not change as linear alkanes were added. In contrast, the addition of linear alkanes to 1,3,5-triisopropylbenzene altered the packing of the aromatic rings. This disruption of the preferential arrangement of 1,3,5-triisopropylbenzene aromatic rings in mixtures with linear alkanes is likely the reason these systems have larger excess molar volumes than the n-nonylbenzene mixtures.