Ion-pair solid-phase extraction with membranes: group contributions and steric effects of ions on the extraction behaviour

Abstract

The group contribution of the components of organic ions and the effect of the position of the charge in the ions on ion-pair solid-phase extraction with a membrane filter (MF) were studied in terms of extraction constant. The extraction constants for many combinations of MFs made of several materials, anionic surfactants and cationic dyes having various substituents were determined. The results were compared with those obtained in solvent extraction. The extraction constant K_ex is defined as K_ex=[C⁺·A^–]_s/([C⁺][A^–]), where the subscript s denotes solid or solvent phase and no subscript denotes aqueous phase. Cellulose nitrate (CN), cellulose acetate (CA), poly ether sulfone (PS) and polytetrafluoroethylene (PTFE) were the materials of the MFs. Crystal Violet ion (CV⁺) and bis[2-(5-X-2-pyridylazo)-5-diethylaminophenolato]-cobalt(III) ion {[Co(5-X-PADAP)₂]⁺, ×= H, Cl, CF₃} were the cations. Alkylsulfonates, alkylcarbonates, alkylsulfates and alkylbenzenesulfonates with different alkyl groups were the anions. The magnitude of the contribution of several components of organic ions for solid-phase extraction was found to be fixed and smaller than that for solvent extraction for any combinations of MFs, cations and anions. Although the extractabilities of CV⁺ and [Co(5-Cl-PADAP)₂]⁺ are almost same in solvent extraction, in solid-phase extraction with CN, CA and PS MFs CV⁺ is much more extractable than [Co(5-Cl-PADAP)₂]⁺. In solid-phase extraction with PTFE, the situation is the converse, that is, [Co(5-Cl-PADAP)₂]⁺ is more extractable than CV⁺. Probably CV⁺, a planer cation where the positive charge distributes on the surface on the ion, interacts strongly with the negatively polarized site of the membrane material through electrostatic forces. On the other hand, [Co(5-Cl-PADAP)₂]⁺, a steric cation where the positive charge is at the centre of the ion, interacts with PTFE essentially through hydrophobic forces.