Isotopic fractionation of helium during solution: A probe for the liquid state

Abstract

The isotopic fractionation of ³ He/ ⁴ He has been determined in pure H ₂ O, D ₂ O, seawater and two alcohols. The measurements have revealed the temperature dependence of the isotopic fractionation in each liquid. Substitution of D ₂ O for H ₂ O decreases the fractionation by a small but measurable amount. The fractionation varies with salinity and an equation is given for equilibrium values in seawater. The effect in ethanol is approximately 25% less than in water. Interpretation of the experimental results in terms of a zero-point-energy argument suggests that gas molecules dissolved in the different liquids occupy cavities with different sizes. In pure water the cavity size seems to be independent of the solute and is approximately 6 Å.