Self-Diffusivity Estimation by Molecular Dynamics

Abstract

Reference fluid simulations are combined with a mean field correction for attractive effects to achieve a correlation for diffusivity of unentangled species over a wide range of density, temperature, and molecular weight. Reference fluids are composed of hard united atom descriptions including branching, rings, bond angles, and chain length. The molecular model is consistent with the one studied previously by thermodynamic perturbation theory. Molecular simulations of methane through triacontane provide the basis for a diffusivity correlation of reference n-alkanes from zero density to the glass transition. The low density limit is shown to exhibit Rouse scaling, contradicting typical hard sphere models. Experimental data for n-alkanes ranging from methane to hexadecane are used to correlate the correction for attractive forces and for the softness of the potential function. The average deviation is 8.6% for 657 data points. Reference simulations of nonalkanes show that the reference simulations can be interpolated smoothly with the n-alkane correlation by treating the effective chain length as an adjustable parameter. Applying the corresponding alkane mean field correction for attractive forces results in poor predictions for nonalkanes. The attractive energy must be treated as a single adjustable parameter to achieve reasonable accuracy. Existing correlations require two parameters to achieve comparable accuracy, and the physical meaning of the two parameters is difficult to interpret. A correlation between diffusivity and entropy suggested by Rosenfeld is shown to be problematic for chain molecules. Overall, diffusivity data are treated for 35 compounds with 1801 data points with 14% deviation by the correlation developed here.