(193o) Implications of Compressed Liquid Density for Intermolecular Potential Models

The compressed liquid density of nAlkanes were studied using square well models to improve understanding of what is necessary for a potential model to achieve an accurate description. The thermodynamic perturbation theory is used in addressing this problem. In global optimization, all normal alkanes from ethane to decane and dodecane are considered. Hard diameter for CH3 and CH2 sites were determined to be 0.363 and 0.357 nm by fixing the isochors intercept to experimental data from NIST data base. Then the width of the well for each site was fit globally with average absolute deviation of 2.37% in density. Width of the well was found to be 1.8 for CH3 and 2 for CH2. When applying the same approach to different sets of hard diameters of 0.354, 0.364 and 0.374, 0.360 for CH3, CH2 the width of the well turns out to be consistently 1.8 and 2 for CH3 and CH2 with average absolute deviations of 2.03% and 1.96%. This is an indication that the compressed liquid density comprises sensitive information about the molecular interactions that is not necessarily related to the softness of the potential.