Molecular level study of carbon isotope fractionation in Knudsen number flows induced by thermo-osmosis

Gas molecules/isotopes under thermal gradient (e.g., thermo-osmosis) can exhibit different paths: some move toward hot patches while others move toward cold spots. However, the mechanism has not been thoroughly investigated. In this study, carbon isotope fractionation (i.e., variation in the concentration ratio of 13CH4 to 12CH4) induced by thermo-osmotic flows with different Kn (Knudsen number) in 2 nm and 5 nm organic pores is conducted, respectively, by employing molecular dynamic simulations. 13CH4 molecules have larger mass (i.e., inertia) and are prone to accumulate in low-temperature zone (ZL) under low Kn flows (Kn 0.1) dominated by intermolecular collisions both in 2 nm and 5 nm pores. Because of frequent collisions with 13CH4 molecules, the velocity of 12CH4 molecules under low Kn flows oscillates violently. This trend becomes more intensive as temperature gradients increase between cold and hot sides under flows with Kn = 0.09 due to the large thermal force, especially in 2 nm pores. For high Kn flows (Kn 10), more 12CH4 molecules in 5 nm pores gather in ZL because of high diffusion capacity and very low probability of collision with 13CH4 molecules under high Kn flows (Kn = 17.87). However, this trend is not found in 2 nm pores at Kn = 22.34. In addition, 13CH4 molecules with high mass are subjected to large thermo-osmotic forces both in 2 nm and 5 nm because of the exchange of energy and momentum between molecules and walls, which further promotes isotope fractionation.