Anion-Exchange Extraction of Doubly Charged Anions by Higher Quaternary Ammonium Salts with Different Steric Accessibility of the Exchange Center

An increase in the steric accessibility of a quaternary ammonium salt (QAS) exchange center leads to a significant increase in the exchange constant for most of the studied doubly charged anions (sulfate, sulfite, molybdate, tungstate, hydrogen phosphate, and selenate), which reaches for “pure” anion-exchange systems of more than five orders of magnitude. The effect of minimizing of an anion-exchange affinity was detected when a solvating additive, heptyl p -trifluoroacetylbenzoate, was introduced into the organic phase. The effect of steric accessibility is not entirely lost due to the conservation of charge of the solvate. The introduction of a solvating additive improves the affinity of anions for the QAS phase significantly (up to 6.5 orders of magnitude). The effect of anion size on the effect of steric accessibility of the exchange center was found for sulfate, sulfite, and thiosulfate ions. An inverse relationship was found for the tetrathionate ion; that is, with a decrease in steric accessibility of the QAS exchange center, anion-exchange extraction is weakened due to the large size of the ion.