Molecular Dynamic Simulations of Carbon and Chlorine Isotopologue Fractionation of Chlorohydrocarbons during Diffusion in Liquid Water

Until now, the magnitude of isotopologue
fractionation of organic compounds due to aqueous-phase
diffusion has been quantified only experimentally. This study
aims to determine the extent of aqueous-phase diffusion-induced
isotopologue fractionation of organic compounds for the first
time on a computational basis using molecular dynamic
simulations (MDS). The MDS were conducted for different
organic compounds including chlorinated ethenes (trichloroethene (TCE)) and ethanes (1,2-dichloroethane (1,2-DCA)) and
for different isotopologues (carbon and chlorine). The MDS
revealed a weak power law mass (m) dependency of the diffusion
coefficient (D ∝ m−β with β ≤ 0.049) for carbon and chlorine
isotopologues of TCE and 1,2-DCA, consistent with experimental results. The MDS showed that the mass of the diffusing
species is the key controlling factor for diffusion-induced isotopologue fractionation and not the molecular volume as suggested
by previous studies. Furthermore, the MDS revealed that the weak power law mass dependency of the diffusive transport rate
originates from an interplay between strongly mass-dependent short-term and mass-independent long-term solute−solvent
interactions. Hence, the presented MDS results provide for the first a time a theoretical rationale for the experimentally
observed magnitude of isotopologue fractionation of organic compounds caused by aqueous-phase diffusion.