We propose a new model for calculating infinite dilution diffusion coefficients for carbon dioxide and water mixtures. The model takes into account temperature dependence of the dipole moment of water and polarizability of CO 2 , and fits experimental CO 2 AH 2 O data at low and high pressures with an accuracy of 4.9%. Remarkably, the proposed model also accurately predicts infinite dilution diffusion coefficients for other binary water mixtures where solute polarizability is close to that of CO 2 , such as CH 4 , C 2 H 6 , C 3 H 8 , and H 2 S. Moreover, we present-to the best of our knowledge-the first predictions of composition-based Fickian diffusion coefficients for CO 2 AH 2 O mixtures over the temperature range 298.15-413.15 K, and pressures up to 50 MPa.

No abstract

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.