(384b) Molecular Interactions in Nonpolar+Polar Binary Mixtures: Measurements of N-Hexadecane + Butyl Benzoate

Minima of the viscosity-composition dependence occur in binary mixtures of a nonpolar and a polar compound but the physical origins of the phenomenon are not well understood and current mixture viscosity models lack the flexibility to represent, much less predict the feature.

Continuing a systematic study, the binary mixture n-hexadecane + butyl benzoate was investigated to test the anticipated existence of the phenomenon. Rapid characterization instruments are used for automated measurements at ambient pressure and temperatures from 233.15 K to 373.15 K. A rotating concentric cylinder device senses viscosity, density is determined with two vibrating-tube instruments and the speed of sound is clocked with a pulse-echo method. These instruments invalidate the conventional perception that property measurements are either time-consuming and expensive or fast and inaccurate. Still, care has to be taken when measuring milliliter samples to assure their composition and to avoid contamination traps.

The measurement results provide a range of new insights. Deviations between -30% and 15% from the DIPPR viscosity correlation for butyl benzoate indicate the potential of such rapid characterizations to improve the general data quality. The anticipated viscosity-composition minima in the mixture were confirmed. Minima with respect to composition were also found in the sound speed whereas the mixture density varies linearly with both temperature and mole fraction. Two UNIFAC-based mixture viscosity models fail to represent the measured results. A starting point to develop a quantitative prediction method may be the viscosity of the mixture at infinite dilution of the polar compound. When a polar molecule is surrounded by only nonpolar molecules, its interactions due to electrostatic forces are screened out and the polar molecule interacts as if nonpolar, only due to its size and shape. Thus, mixture viscosities at increasing dilution of the polar compound allow to discern between interactions due to molecular size and shape and those due to polarity. Considering thermodynamic and transport properties in the same context provides additional stimulus. Can the viscosity minima be connected to the commonly found azeotropy in nonpolar+polar binaries and to their liquid-liquid equilibria? In turn, can models for thermodynamic properties such as ERAS (extended real associated solution) be adapted to represent or predict transport properties?