On the use of excess entropy scaling to describe single-molecule and collective dynamic properties of hydrocarbon isomer fluids.

We use molecular simulation to study the ability of excess entropy scaling relationships to describe the kinetic properties of four hydrocarbon isomers: n-octane, 2,2-dimethylhexane, 2,5-dimethylhexane, and 3-methyl-3-ethylpentane. Four dynamic properties are considered: translational and rotational diffusivities, a characteristic relaxation time for rotational motion, and a collective relaxation time stemming from analysis of the coherent intermediate scattering function. For each of the dynamic properties considered, reduced data collapse onto a species-specific common curve when expressed as a function of the thermodynamic excess entropy. Because each isomer exhibits a quantitatively distinct excess entropy scaling relationship, straightforward corresponding states principles do not provide an effective means to predict dynamic properties.