Cation effects in the separation of calix[4]pyrroles by nonaqueous capillary electrophoresis with tetraalkylammonium chloride salts as background electrolytes

The migration behavior of three calix[4]pyrroles (C4Ps) in acetonitrile (ACN) and in the mixtures of ACN/ethanol (EtOH) (95:5; v/v)with tetraalkylammonium chloride as background electrolyte (BGE) was studied. Electroosmotic flow (EOF) mobilities and absolute mobilities in ACN and mixture of ACN/EtOH (95:5; v/v) were investigated. A decrease of EOF mobilities in EtOH/ACN was observed compared with that of in pure ACN. Variation of the C4Ps effective mobilities as a function of the square root of concentration of tetraakylammonium chloride salts such as tetramethylammonium chloride (Me4NCl), tetraethylammonium chloride (Et4NCl) and tetrabutylammonium chloride (Bu4NCl) in both solvents was plotted, and straight lines were exhibited when Bu4NCl was used as BGE. Absolute mobilities of C4Ps were obtained by extrapolation from the mentioned plots. It was found that the absolute mobilities of C4Ps is independent of the nature of BGE and the values are: meso-octamethylcalix[4]pyrrole (OMCP) > meso-tetracyclopentylcalix[4]pyrrole (TPCP) > meso-tetracyclohexylcalix[4]pyrrole (THCP). The ion-pairing constants follow the order under given solvent: Me4N+ > Et4N+ > Bu4N+, and ion-pairing constant of C4P follows the order: THCP > TPCP > OMCP. As though, ion-pairing association is intensified by steric hindrance.