Adsorption of tetrahexylammonium – bromothymol blue ion-pairs at the chloroform–water interface

Abstract

Porous membrane phase separators are used to study the adsorption of the cation tetrahexylammonium (Q⁺), of the anion bromothymol blue (HB⁻) and of the ion-pair formed between them (QHB) at the liquid–liquid interface in a rapidly stirred mixture of chloroform and aqueous buffer. Adsorption isotherms in all three cases follow the Langmuir equation. The anion HB⁻ is much more strongly adsorbed than the ion-pair QHB. The porous membrane technique readily permits measurement of simultaneous adsorption of the two species HB⁻ and QHB, and thereby allows a study of their competitive adsorption. When QHB is adsorbed in the presence of an excess of HB⁻ both the saturated (monolayer) interfacial concentration of QHB and the logarithm of the adsorption equilibrium constant for QHB decrease linearly with an increase in interfacial concentration of HB⁻. This shows quantitatively that coadsorption of QHB and HB⁻ involves a direct competition for space at the interface and also that the presence of adsorbed HB⁻ changes the adsorbent properties of the interface. Analytical implications for solvent extraction are discussed. Key words: interfacial adsorption, ion-pairs, liquid–liquid interface.