Frans_M_Everaerts_Isotachophoresis_378342.pdf

38 CONCEPT OF MOBILITY
where flfo is a correction factor for non-spherical particles. For water (21 C), this means
that
mf= 1.14. 10-3~f/[TCflf~)] (3.43)
From this equation, it can be concluded that the ionic mobility is a function of the shape,
charge and radius of the ion and the viscosity of the solvent. Edward [7] and Bondi [9]
calculated the contribution of different groups in a molecule to the volume of the
molecule (and hence to the radius) from the covalent radius according to Pauling and
the Van der Wads’ radii and angles [ 101 .
Perrin [ 111 derived equations for friction factors from the ratio of the axes of prolate
and oblate ellipsoids. Edward and Waldron-Edward [12] showed the possibility of
calculating friction factors from diffusion constants.
In the papers mentioned, reasonable results were obtained for the calculated values
in comparison with the experimental values, deviations being found for small ions and
strongly polar groups. Values for non-spherical and nonellipsoid ions, such as the
“knobby shape” ions, can also be calculated. Very irregular ions cannot be treated
Fig.3.6. Relationship between entropy (S) and ionic mobility (m) for some cations (a) and anions (b).
The values correspond to those given in Table 3.2.