Effect of Aromatic Solvents Dissolved in an Aqueous Phase on Ion-pair Solid-phase Extraction

Abstract

The effect of aromatic solvents dissolved in an aqueous phase at the 10^–3 mol dm^–3 level on the distribution of an ion pair between octadecylsilane (ODS)-modified silica gel and cellulose nitrate was studied systematically. Bis[2-(5- chloro-2-pyridylazo)-5-diethylaminophenolato]cobalt(III) ion and dodecanesulfonate ion were the cation (C⁺) and anion (A^–) studied. The extent of distribution was expressed in terms of the ion-pair extraction constant, K_ex, which is defined as K_ex = [C⁺·A^–]_s/[C⁺][A^–], where the subscript s denotes the solid phase and no subscript the aqueous phase. In the ODS–silica gel system, nitrobenzene (NB), p-methylnitrobenzene (p-MNB), p-ethylnitrobenzene (p-ENB) and o-, m- and p-xylene enhanced the K_ex value by more than two orders of magnitude. Other aromatic solvents such as toluene, chlorobenzene and benzene also enhanced K_ex. Non-aromatic solvents, such as chloroform, dichloromethane and octan-2-ol had no effect on the extraction. The solvent effect cannot be described in terms of a model of ion-pair solvent extraction. The effect was interpreted through mutual interactions of key constituents of the solid phase, organic solvent and ion pair. The key structual fragments present in an effective solvent are a phenyl group and nitro and alkyl groups on the phenyl group. In the proposed model, the alkyl group on the phenyl group interacts with the octadecyl group on the ODS–silica gel through a hydrophobic force, and the negatively polarized nitro group and/or π electrons in the benzene ring of the solvent adsorbed on the ODS interact with the ion pair through electrostatic forces. In the cellulose nitrate system, the solvent effects were small even in the case of NB, p-MNB and p-ENB. The effect must be saturated by the nitro group originally present in the matrix of cellulose nitrate.