Comparison of two modeling approaches for describing intra-diffusion in liquid Lennard–Jones mixtures containing a self-associating component

Two models for calculation of intra-diffusion coefficients of homogeneous spherical liquid Lennard–Jones (LJ) molecules including mixtures with one self-associating component are proposed. We have chosen the model proposed in [T. Merzliak, A. Pfennig, Mol. Simul. 30(7) (2004) 459–468] suggested for calculation of intra-diffusion coefficients in liquid LJ mixtures, as starting point for deriving the models. One approach to account for self-association was to explicitly describe the associate as an individual component, where the amount of dimers formed is described with a chemical equilibrium constant for the association. The second approach was to average the molecular interaction used in the model with respect to the molecular surface.Both models had three adjustable parameters, which were determined based on molecular-dynamics (MD) simulations. In order to minimize the experimental effort, the conditions at which the simulations were performed were chosen with the help of optimal experimental design.It turned out that the two proposed models cannot be discriminated based on our MD data. Since the two models have significantly different molecular basis, the results indicate that very different molecular concepts can be used to equally describe diffusion in associating molecules.