Indifferent symmetric electrolyte

To get the main idea of the charge effect on adsorption kinetics, it is sufficient to consider an aqueous solution of a symmetric (z z) ionic surfactant in the presence of an additional indifferent symmetric (z z) electrolyte. When a new interface is created or the equilibrium state of an interfacial layer disturbed a diffusion transport of surface active ions, counterions and coions sets in. This transport is affected by the electric field in the DEL. According to Borwankar and Wasan [102], the Gouy plane as the dividing surface marks the boundary between the diffuse and Stem layers (see Fig. 4.10). When we denote the surfactant ion, the counterion and the coion, respectively, with the indices / = 1, 2 and 3, the transport of the ionic species with valency Z/ and diffusion coefficient A, under the influence of electrical potential i, is described by the equation [2, 33] ... 

The space charge density p(z) neutralizing the surface charge is related to the local electrolyte concentrations. For a symmetrical indifferent electrolyte p(z) can be written as ... 

The concentrations of small ions inside the Donnan phase can be obtained from the condition of electroneutrality in this phase. In the presence of a symmetrical indifferent electrolyte one obtains ... 

The inert electrolytes are, indeed indifferent at low concentrations and near the PZC, For example the values of C potentials and cro are often symmetrical with respect to the PZC and insensitive to the nature of these salts. However certain results, e.g. Hoffmeister s series, studies of uptake of ions from solution, and even shifts in the isoelectric point lEP and PZC suggest non-electrostatic interactions of these ions and surfaces of materials, especially at high concentrations and far from the PZC. These results are shown and briefly discussed next. 

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