Study of Self-Diffusion Coefficient in Nonassociating and Associating Fluids by a New Hard-Sphere Chain Equation

The self-diffusion coefficient of non-associating fluids at moderate density and high density were calculated by the hard-sphere chain (HSC) equation obtained by the Chapman–Enskog method of solution which is corrected by molecular dynamics simulation data. Compared with the data of experiments or molecular simulations, the result shows that most of the average absolute deviation of the self-diffusion coefficient calculated by this method is about 5% when the pressure is lower than 300 MPa and the temperature is higher than 100 K. An attempt is made in this work to combine the hard-sphere chain model of the self-diffusion coefficient with the statistical associating fluid theory (SAFT). The real non-spherical associating molecules are modeled as chains of tangent hard-sphere segments with an associating site. An equation for the self-diffusion coefficient in polyatomic associating fluids is presented as a product of a non-hydrogen-bond contribution and a hydrogen-bond contribution. The SAFT equation provides the density and temperature dependence of an average number of hydrogen bonds in a molecule, and the HSC equation is used to calculate the self-diffusion coefficient for a non-associating fluid. The equation reproduces the experimental self-diffusion coefficient with an average absolute deviation of about 7.5% for water, alcohols and hydrogen fluoride over wide ranges of temperature and pressure, including supercritical water.