Abstract
The characteristic nature of ion-binding of much charged polyions has been investigated mainly on the basis of the Poisson-Boltzmann (P.B.) equation and partly through the conductance experiment. In general, the ion-binding can be classified into two types, the ψ-binding and the P-binding. The former is due to the electrostatic field around polyions, while the latter is due to the short-range interaction between ionized groups of polyions and salt-ions. Through the theoretical analysis of non-linear property of the P.B. equation, we have found, for every shape of polyions, the strong ψ-binding of counterions which greatly depresses the polyion-charge within a thin ion-condensation phase around the polyion. By exactly solving the two-dimensional P.B. equation, we have also obtained the ion-distribution around many rod-like polyions and found the two-step ion-condensation phenomenon; at the first step, counterions are condensed in the neighbourhood of the assembly of polyions, and at the second step, a similar condensation phenomenon occurs around each of the polyions. The alteraction of spatial arrangement of polyions causes a larges change of activity coefficient of small ions.
Through the conductance measurement, we have found the strong P-binding of divalent cations to polyions. Generally, the P-binding is very much strengthened by the effect of φ-binding.
