ELECTRICAL DOUBLE-LAYER MODEL FOR SORPTION OF IONS ON OCTADECYLSILYL BONDED PHASES INCLUDING THE ROLE OF RESIDUAL SILANOL GROUPS

Sorption isotherms on a highly end-capped octadecylsilyl (ODS) silica bonded phase packing have been measured at various ionic strengths, adjusted with NaCl, both for tetra-n-butylammonium cation (TBA+), from a solution of its chloride salt, and for p-nitrobenzenesulfonate anion (NBS-), from a solution of its sodium salt, using the column equilibration technique. At constant ionic strengths the NBS- isotherms are strictly Langmuirian, while the TBA+ isotherms are Langmuirian only after subtracting the moles of TBA+ that are strongly adsorbed on residual silanol groups. The completeness of elution of TBA+ in the experiment has been verified by neutron activation analysis. Sorption of each of the organic ions, NBS- and TBA+, is quantitatively described by the Stern-Guoy-Chapman (SGC) model of the electrical double layer. The surface potential psi-o is Nernstian for the TBA+ system but is markedly sub-Nernstian for the NBS- system. The latter is due to the presence of a small number of anionic silanolate sites on the ODS packing.