(272e) Molecular Dynamics Simulations of Diffusion and Clustering Along Critical Isotherms of Medium-Chain n-Alkanes

Understanding the transport properties of n-alkanes and other molecular fluids in the critical region is important for a number of industrial and natural systems. It is well-established that the Fickian diffusion coefficient vanishes at the critical point in binary fluids. However, there are strongly conflicting results for the behavior of the self diffusion coefficient D_s near the critical point in single-component systems. Further, there is no molecular-scale understanding of the behavior of D_s in single-component molecular fluids in the critical regime. Thus, we perform extensive molecular dynamics simulations of single-component fluids composed of medium-chain n-alkanes (n-pentane, n-decane and n-dodecane) using anisotropic united-atom potentials. For each system, we quantify the self-diffusion coefficient, cluster sizes and lifetimes along critical isotherms. We show that D_s decreases and the average size of molecular clusters increases as a function of density along critical isotherms, but D_s remains finite at the critical point. This work confirms that the self-diffusion coefficient is finite at the critical point in single-component molecular fluids.