Explanation of Ionic Sequences in Various Phenomena. II. Reversal of Colloid Charge and Ion Binding

Abstract

Previous proposed models for the structure of hydrated ions and the calculated values of the effective dielectric constant of such hydrated ions were used to explain the reversal of colloid charge and ion binding phenomena. In contrast to the conclusions made by Bungenberg de Jong, it is shown that the more soluble the counter -cation or counter-anion of a colloid charge, the greater is the ability of the counterion to reverse the electrical charge of the colloid. The reversal of charge phenomenon is therefore associated with the counterions's solubility, not its insolubility. The solubility sequence is determined by whether or not the carboxylate, sulfate, or phosphate ion is positively (A regions) or negatively hydrated. The phosphate group of DNA or RNA must be associated with a base by means of ion-ion bonds in order to produce the observed reversal of charge sequence. Just as in the reversal of charge phenomenon, the ion-binding phenomenon involves the electrostatic attraction of a counterion with the polyelectrolyte rather than a binding or insolubilization of the counterion. The reverse ion-binding sequence can be obtained if one dialyzes extensively in the presence of sufficient salt before physical measurements are made. This is because the solubility of a counterion determines the true electrostatic charge of the polymer. In other words, different concentrations of salt arise in the dialysis bag when different counter-ions are added because the activity coefficient of the counterion is determined by the solubility of the ion-ion complex between the counterion and the colloid′s charged group.