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

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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.

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