Thermodiffusion of CO2 mixtures in the extended critical region

Thermodiffusion, also known as Soret effect, stands for the mass flux driven by a temperature gradient, leading to partial component separation in fluid mixtures. While typically small in magnitude and often neglected, thermodiffusion can become significant under specific conditions, such as in the extended critical region. In this work, the thermodiffusion behavior of diluted supercritical mixtures of carbon dioxide with methane, ethane, or isobutane is investigated with molecular simulation techniques. Thermodiffusion is studied along the 9 MPa isobar over a temperature range from T = 290 to 345 K, where singular thermodynamic and transport properties are observed. Attention is given to the crossover region near the critical point, where asymptotic power laws no longer apply, but long-range fluctuations still influence fluid behavior. Within this region, characterized by the Widom line, extreme values for the thermodiffusion and Soret coefficients are predicted. The underlying mechanisms responsible for this behavior are explored, and it is shown to be primarily driven by the extrema of the partial molar enthalpy of the solvent in the extended critical region. • Thermodiffusion and Soret coefficients are determined by equilibrium MD simulation. • The thermodiffusion coefficient exhibits an extremum near the Widom line. • In scCO 2 , CH 4 and C 2 H 6 migrate to the warm, while C 4 H 10 goes to the cold side. • Partial molar enthalpy differences drive thermodiffusion near the critical region. • Simulations match predictive equations, depending on the chosen equation of state.