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-1 0 ODS-3 is described by the Stern-Gouy-Chapman (SGC) theory of the ele ctrical double layer. The capacitance of the compact part of the doubl e layer has a constant value, independent of NBTA+ activity, which cor responds to a compact layer thickness of 0.6 angstrom. Sorption of tet ra-n-butylammonium ion (TBA+) on Partisil-10 ODS-3 has previously been described by SGC theory and was found to have a compact layer thickne ss of 12 angstrom. What is of interest in the present study is the sim ultaneous sorption of NBTA+ and TBA+ on Partisil-10 ODS-3. By use of s olutions that have a constant NBTA+ concentration and a range of TBAconcentrations, the sorption of both ions is measured by the column eq uilibration technique as a function of ionic strength and is quantifie d in terms of SGC theory Unlike the case of a single potential-determi ning ion, the capacitance of the compact layer must be treated as a di fferential 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. T he compact layer is divided into an inner region having an integral ca pacitance K(inner) and an outer region having an integral capacitance K(outer). By treating the compact layer as two capacitors in series, t he potentials at the TBA+ and NBTA+ charge surfaces are evaluated.