Study on solute–solvent and solute–solute interactions for the dibenzo-24-crown-8-alkali metal picrate extraction system

Abstract

The constants of overall extraction equilibrium ( K ex) and distribution ( K D,MLA) for various diluents and aqueous ion-pair formation ( K MLA) of dibenzo-24-crown-8 (DB24C8)–alkali metal (Na–Cs) picrate (MA) 1:1:1 complexes (M(DB24C8)A) were determined at 25 °C; the partition constants of DB24C8 were also measured at 25 °C. The distribution behavior of DB24C8 and M(DB24C8)A complexes can be explained by regular solution theory; the molar volumes and the solubility parameters of DB24C8 and M(DB24C8)A were determined. The log K D,MLA value for the same diluent decreases from Na to K and increases from K to Cs; however, the reverse is true of the log K MLA values. The extraction selectivity of DB24C8 for the alkali metal picrates decreases in the order Cs>Rb>K>Na or Cs>K≥or >Rb>Na. This is governed largely by the aqueous stability order for the M(DB24C8) + complex ions. For every diluent, the plot of log K ex values against the reciprocal effective ionic radii of the alkali metals gives a straight line, which is related to Born's formula of solvation free energy. The reason for this was elucidated.