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Search for isotope effects in heavy-ion diffusion in liquids


THERE is some controversy as to the existence of isotope effects in liquid diffusion in which isotopically labelled molecules diffuse in the same liquid medium. Friedman1 has predicted from the standpoint of the correlation-function theory of diffusion that there should be little or no effect and certainly nothing approaching the inverse square mass relationship and this has been shown2,3 to be true for organic liquids. Miller4 claims, however, that uranyl ions diffusing in aqueous solution do show an isotope effect and also that heavier ions diffuse faster than lighter ions. He attributed the phenomenon to kinetic effects in the molecular diffusion process; in particular, to persistence of velocities and backscattering. The effects should apply generally to heavy ions diffusing in light solvents, and in view of the reported separation factors the difference in diffusion coefficients may be expected to be measurable. I report here the results of experiments designed to test this supposition.

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  1. 1

    Friedman, H. L., Molecular Motions in Liquids (edit. by Lascombe, J.),(Reidel, D., Dordrecht, 1974).

  2. 2

    Allen, G. G., and Dunlop, P. J., Phys. Rev. Lett., 30, 316 (1973).

  3. 3

    Mills, R., J. phys. Chem., 79, 852 (1975).

  4. 4

    Miller, L., Nature, 243, 32 (1973).

  5. 5

    Mills, R., and Woolf, L. A., The Diaphragm Cell (Australian National University Press, Canberra, 1969).

  6. 6

    Miller, L., J. phys. Chem. (in the press).

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