MECHANISM OF CHROMATOGRAPHIC RETENTION OF ORGANIC IONS ON A NON-IONIC ADSORBENT

Adsorption of organic ions on the styrene-divinylbenzene copolymer Amberlite XAD-2 is studied by means of adsorption isotherms and microelectrophoresis measurements. The results obtained with the sample ions diphenylguanidinium and benzylammonium are quantitatively explained in terms of the Stern-Gouy-Chapman (SGC) theory of the electrical double layer in which the sample ion is adsorbed onto the surface and small inorganic counterions, such as chloride, occupy the diffuse part of the electrical double layer. The addition of quaternary ammonium ions to the mobile phase produces a constant surface potential. Under chromatographic conditions where the surface potential is constant, the reciprocal of the adjusted retention volume is linearly dependent on the reciprocal of the mobile phase ionic strength. For a constant lonic strength, the log of the adjusted retention volume varies directly as the negative log of the concentration of potential-determining ion. The general effects of electrolyte type in the mobile phase on retention behavior of ionic samples are also discussed. © 1979, American Chemical Society. All rights reserved.