Relaxation of the ionic cloud on the basis of a dressed-ion theory

The dynamic response of a bulk electrolyte or colloid solution to an extern al field is investigated on the basis of a dressed-ion theory (DIT) in a hy drodynamic point of view. The radial part of the perturbed electric field a cting on a given ion is explicitly calculated in terms of the DIT quantitie s derived from the linear response function calculated from the modified me an-spherical approximation (MMSA) and its static and frequency-dependent li mits are analyzed. In the static case, the asymptotic behavior is analyzed and Onsager's result is reformulated in terms of the effective charges and effective screening length and in the limit of vanishing concentration Deby e-Falkenhagen- Onsager results are recovered. In the frequency-dependent DI T transport theory a relation between the field frequency and the time of r elaxation of the ionic atmosphere is shown to be needed in order to get rea l renormalized charges and screening lengths. A decay of the perturbed elec trostatic field as the inverse square root of the field frequency is obtain ed at high frequency and vanishing concentration along with a model-indepen dent phase factor between the external and internal fields. The radial depe ndence of the perturbed average potential in the neighborhood of a quasipar ticle is also calculated in the static case and several behaviors ranging f rom classical Derjaguin-Landau-Verwey-Overbeek colloidal stability theory i nteraction to pure attraction and repulsion are obtained. The results are a nalyzed in terms of a splitting of the ionic cloud into three different par ts each one contributing to the radial dependence of the perturbed potentia l. (C) 1999 American Institute of Physics. [S0021-9606(99)51007-6].