Application of irreversible thermodynamics to isotopic diffusion

From literature sources irreversible thermodynamic parameters may be obtained for all alkali metal chloride salt solutions at 298.16 K in the concentration range 0–3.0 mol dm–3. Comparable data for isotopic diffusion coefficients of component ions and water are incomplete. These have been obtained in this study, using a diaphragm cell, for all but rubidium ion in rubidium chloride. It is shown that these two sets of data may be combined to yield Onsager frictional coefficients (rii*) or mobility coefficients (lii*) which measure the kinetic interaction between isotopes as a function of concentration for each salt solution. Interpretation of lii* for ions is made difficult because of large frame-of-reference contributions with water solvent, but these may be compared with values calculated from the Onsager limiting law. Frictional interpretations are more explicit. The main factor contributing to the experimental diffusion coefficient for ions, Dii, remains ion-water friction which amounts to some 90 % for many slats even at 3.0 mol dm–3. The primary cause is that interionic friction between ions of opposite charge is largely cancelled by isotope–isotope friction in the expression for Dii.Water diffusion, D00, unlike the ionic case, is due primarily to isotope–isotope friction (between water and water) which is the sole contribution to D00 for pure water. These two components have been isolated for all salts and the effects of the solvation tendencies for different cations is clearly shown in variations of water-to-water friction in the more concentrated solutions.