ROLE OF THE COMPACT PART OF THE ELECTRICAL DOUBLE-LAYER IN THE SIMULTANEOUS SORPTION OF DIFFERENT IONS OF THE SAME CHARGE ON A REVERSED-PHASE BONDED-PHASE LIQUID-CHROMATOGRAPHY PACKING

Sorption of (4-nitrobenzyl)trimethylammonium ion (NBTA+) on Partisil-10 ODS-3 is described by the Stern-Gouy-Chapman (SGC) theory of the electrical double layer. The capacitance of the compact part of the double layer has a constant value, independent of NBTA+ activity, which corresponds to a compact layer thickness of 0.6 angstrom. Sorption of tetra-n-butylammonium ion (TBA+) on Partisil-10 ODS-3 has previously been described by SGC theory and was found to have a compact layer thickness of 12 angstrom. What is of interest in the present study is the simultaneous sorption of NBTA+ and TBA+ on Partisil-10 ODS-3. By use of solutions that have a constant NBTA+ concentration and a range of TBA+ concentrations, the sorption of both ions is measured by the column equilibration technique as a function of ionic strength and is quantified in terms of SGC theory Unlike the case of a single potential-determining ion, the capacitance of the compact layer must be treated as a differential capacitance, the value of which varies with the amounts of each ion adsorbed. The evidence is consistent with the view that NBTA+ and TBA+ adsorb in separate charge planes within the compact layer. The compact layer is divided into an inner region having an integral capacitance K(inner) and an outer region having an integral capacitance K(outer). By treating the compact layer as two capacitors in series, the potentials at the TBA+ and NBTA+ charge surfaces are evaluated.