Disagreement between experimental potential changes and the Gouy-Chapman theory predictions for dilute 1 : 1 electrolyte solutions: 2. Possible explanations. Version of pK variation

Three possible reasons for a disagreement between the Gouy-Chapman theory p redictions and the results of potential measurements in dilute 1 : 1 electr olyte solutions are discussed, namely, (i) shift of the shear plane, (ii) t he adsorption of anions, and (iii) changes in intrinsic surface charge dens ity sigma(i). As follows from the analysis, the observed disagreement with the theory can be due to the variation in the intrinsic surface charge dens ity at a low ionic strength. Changes in sigma(i) may result either from an increase in the density of dissociated ionizable groups with ionic strength , a small dissociation constant of these groups, or its increase with ionic strength. Such an explanation agrees with the properties of the surface di ssociation constant pK(s) resulting from the true meaning of this value, as well as from the experimental values of these constants for some surfaces (fatty acids, alkylamines, lipids, and proteins). For lipids, the hypothesi s of the variation in pK(s) at low ionic strengths is confirmed by comparin g pK(s) values obtained from the fitted dependences of electrophoretic mobi lity on ionic strength with the results of independent measurements of pote ntials on liposomes and planar lipid membranes.