Molecular dynamics simulations of diffusion and clustering along critical isotherms of medium-chain n-alkanes

摘要

Understanding the transport properties of molecular fluids in the critical region is important for a number of industrial and natural systems. In the literature, there are conflicting reports on the behavior of the self diffusion coefficient D_s in the critical region of single-component molecular systems. For example, D_s could decrease to zero, reach a maximum, or remain unchanged and finite at the critical point. Moreover, there is no molecular-scale understanding of the behavior of diffusion coefficients in molecular fluids in the critical regime. We perform extensive molecular dynamics simulations in the critical region of single-component fluids composed of medium-chain n-alkanes - n-pentane, n-decane, and n-dodecane - that interact via anisotropic united-atom potentials. For each system, we calculate D s, and average molecular cluster sizes κ_(cl) and numbers N_(cl) at various cluster lifetimes τ, as a function of density ρ in the range 0.2ρ_c ≤ ρ ≤ 2.0ρ_c at the critical temperature T_c. We find that D_s decreases with increasing ρ but remains finite at the critical point. Moreover, for any given τ < 1.2 × 10~(-12) s, κ_(cl) increases with increasing ρ but is also finite at the critical point.