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Issue 3, 1989
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Acid–base equilibria in aqueous micellar solutions. Part 2.—Sulphonephthalein indicators

Abstract

The acid–base equilibria of three sulphonephthalein indictors, viz. bromocresol green, bromothymol blue and phenol red, in 1,4-dioxane–water mixtures and aqueous solutions of both non-ionic and cationic micelles have been studied. The difference between the acid–base equilibrium constant of a sulphonephthalein indicator in pure water, pKwa, and the apparent acid–base equilibrium constant of a sulphonephthalein located entirely within the interfacial microenvironment of a non-ionic micelle has been explained quantitatively in terms of the different intrinsic solvent properties of the two systems. A number of factors are considered to be primarily responsible for the difference between the apparent acid–base equilibrium constant of a sulphonephthalein residing with in the interfacial micro-environment of a cationic alkyl quaternary ammonium micelle and its pKwa value. These factors are the electrostatic micellar surface potential, the low effective interfacial dielectric constant and either an interfacial ‘salt-effect’ or the formation of an ion pair between the negatively charged sulphonate group of a sulphonephthalein and a cationic quaternary ammonium surfactant headgroup.

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Article information


J. Chem. Soc., Faraday Trans. 1, 1989,85, 537-550
Article type
Paper

Acid–base equilibria in aqueous micellar solutions. Part 2.—Sulphonephthalein indicators

C. J. Drummond, F. Grieser and T. W. Healy, J. Chem. Soc., Faraday Trans. 1, 1989, 85, 537
DOI: 10.1039/F19898500537

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