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

• Published in: Journal of physical organic chemistry Vol. 18; no. 8; pp. 850 - 855
• Main Authors: Blagoeva, Iva B., Ouarti, Nadia, El Seoud, Omar A., Ruasse, Marie-Françoise
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.08.2005
• Subjects: buffers; cationic micelles; interfacial exchanges; micellar solutions; mono- and divalent ions; pH metry
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.953

The pH changes of an aqueous carbonate buffer solution (9 × 10−3 M Na2CO3, 10−3 M NaHCO3, pHo 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 exCI/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. The pH change of an aqueous carbonate solution induced by addition of cetyltrimethylammonium chloride (CTACl) micelles is analyzed (solid line) with equations appropriate to mono‐divalent ion exchanges, derived from an extended pseudophase ion‐exchange model including a β dependence on [CTA+]. The exchange constant, K exCI/CO 3, is interpreted in terms of salt effects on the micellization parameters.