INTERPRETATION OF ION-EXCHANGE CHROMATOGRAPHIC RETENTION BASED ON AN ELECTRICAL DOUBLE-LAYER MODEL

An ion-exchange chromatographic model based on the Stern-Gouy-Chapman electrical double-layer theory is presented. We assume several equilib ria occurring at the surface of an ion-exchange resin, such as the ion -pair formation of counterions with an ion-exchange site, the adsorpti on of ions, and the ion-pair formation of an adsorbed ions with their counterions, These equilibria are affected by the potential at the pla nes (the surface and the Stern layer potential) where the reactions oc cur. in addition, the nonselective accumulation of ions in the diffuse layer is also taken into account. Based on the developed model and de rived equations, the nature of log k'-logX (X is the concentration of an eluent ion) plots is investigated for various sets of parameters an d is compared with that of possible experimental results. Three extrem e cases can be distinguished: (1) ion-pair-controlled, (2) adsorption- controlled, and (3) accumulation in the diffuse layer-controlled reten tion, Though log k'-log X plots, when they are studied over wide eluen t concentration ranges (more than 2 orders), do not necessarily show p recise linearity especially in the presence of eluent adsorption and e xtremely high eluent ion-pair formation, the linear regression analyse s give negative unit slopes (within 10% deviations) for monovalent sol ute-eluent combinations. Since the deviation from linearity relation i s mostly found in very low k' ranges (e.g., k' < 1), it is possible on ly in very limited cases to confirm nonlinearity experimentally. This may have mistakenly led to the idea that selectivity coefficients can be regarded as constants and thus the log k'-log X plots should be lin ear with the slope equal to (charge ratios) x (-1) in ion-exchange chr omatographic experiments. The efficiency of the developed model is ver ified by its ability to predict experimental results, e.g., nonlinear log k'-logX plots, changes in separation factors with varying ion-exch ange affinity of an eluent ion, adsorption isotherms at different salt concentration, etc.