DISSOCIATION OF TETRAHEXYLAMMONIUM PICRATE ION-PAIRS ADSORBED AT THE CHLOROFORM WATER INTERFACE

Two membrane phase separators, made of porous Teflon and of paper, are used to measure isotherms for adsorption of tetrahexylammonium picrate (QP) at the chloroform-water interface In a rapidly stirred liquid-liquid dispersion. The ion-pair species QP and the cation Q+ are both surface-active but the picrate anion P− is not. In the presence of a large excess of P− dissociation of the adsorbed species QP is suppressed. Its adsorption follows a Langmuir isotherm. Without a large excess of P present, the adsorbed QP dissociates appreciably on the interface. The Ion-pair dissociation constant is KDis,QP = (1.2 ± 0.1) × 10−5 mol/L. Aspects that are peculiar to ion-pair dissociation at an interface, as opposed to ion-pair dissociation In bulk solution, are discussed and quantified. These include competitive adsorption between Q+ and QP as well as the presence of an electric charge on the interface as a result of adsorbed Q+. © 1990, American Chemical Society. All rights reserved.