Isotope fractionation in phase-transfer processes under thermodynamic and kinetic control – Implications for diffusive fractionation in aqueous solution

Highlights: • Hydrogen isotope fractionation was measured for partitioning of benzene, toluene, cyclohexane between water and n-octane • Relative aqueous-phase diffusion coefficients of isotopologues were derived under kinetic control. • Light and heavy isotopologues of all investigated solutes show very similar aqueous-phase diffusion coefficients. • Thermodynamic control of partitioning produces much stronger isotope fractionation than kinetic control. Diffusive isotope fractionation of organic compounds in aqueous solution was investigated by means of liquid-liquid and liquid-gas partitioning experiments under kinetic control. The two-film model was used to describe phase-transfer kinetics. It assumes the diffusion of solutes across a stagnant water boundary layer as the rate-controlling step. For all investigated solutes (benzene-D₀ and -D₆, toluene-D₀, -D₅, and -D₈, cyclohexane-D₀ and -D₁₂), there was no significant observable fractionation effect between nondeuterated and perdeuterated isotopologues, resulting in a ratio of diffusion coefficients D_light: D_heavy = 1.00 ± 0.01. In addition, isotope fractionation due to equilibrium partitioning of solutes between water and n-octane or gas phase was measured. The deuterated compounds are more hydrophilic than their light isotopologues in all cases, giving rise to fractionation coefficients αH_part = K_{octane/water,H}: K_{octane/water,D} = 1.085 to 1.15. Thus, thermodynamic fractionation effects are much larger than diffusion fractionation effects. Methodical and environmental implications of these findings are discussed.