Interfacial ion exchange between monovalent and divalent anions in cationic micelles, revised in the light of correlation analysis

The pH changes of an aqueous carbonate buffer solution (9 × 10 −3 M Na 2 CO 3 , 10 −3 M NaHCO 3 , p H o 10.79), induced by addition of cetyltrimethylammonium chloride (CTA + Cl − ) in concentrations varying from 10 −3 to 10 −1 M , are employed to determine the concentration of dianionic base in the water pseudophase. The pH decreases markedly up to −0.6 pH units at 0.1 M CTA + . The pH profile is analysed in terms of the pseudophase ion‐exchange model from which the equations for mono‐/divalent anion exchanges are derived. The fit of the pH data to these equations is satisfactory only when a [CTA + ] dependence of β (the degree of counterion binding) is considered: β = 0.5 + 2[CTA + ]. The resulting exchange constant, K   ex CI/CO   3 = (6.75 ± 0.22) × 10 −2 , is markedly larger than that measured previously at only one surfactant concentration without taking into account the β variation. This surfactant concentration dependence of β is discussed in terms of recent results on salt effects on the composition of micellar interfaces, which also show that β depends on the surfactant concentration in the presence or absence of any added salt. Copyright © 2005 John Wiley & Sons, Ltd.