Determination of complex stabilities with nearly insoluble host molecules. Part II. Complexation of alkali and alkaline earth metal cations with dibenzo crown ethers in aqueous solution

Abstract

An experimental method was developed to determine the stability constants with nearly insoluble ligands in homogeneous solution. This method was tested using dibenzo crown ethers, which have a very low solubility in aqueous solution. The stability constants for the complexation of alkali and alkaline earth metal cations with different dibenzo crown ethers were determined in aqueous solution. Owing to the complex formation the total concentration of the ligand in solution increases. Dibenzo crown ethers absorb in the ultraviolet spectral range, hence the increase in the ligand concentration can be easily detected. Without the knowledge of the molar absorptivities of the ligands and of the corresponding complexes and of the solubilities, the stabilities of the complexes formed can be calculated under certain assumptions. To verify these assumptions, the solubilities of the dibenzo crown ethers and the molar absorptivities were determined. The kinetics of the solubilization process of the ligands was followed by spectrophotometric measurements.

Theoretical Studies on Selectivity of Dibenzo-18-Crown-6-Ether for Alkaline Earth Divalent Cations

“…7 The 18-crown-6-ether (18C6) and dicyclohexo-18-crown-6-ether (DCH18C6) also displayed the same order of selectivity in aqueous solution. 8 The study of complex formation of DCH18C6 with Mg…”

Crown Ethers Assisted Cesium Extraction from Aqueous Solutions into a Hydrophobic Room Temperature Ionic Liquid 1-Butyl-3-methylimidazolium Bis[(trifluoromethyl)sulfonyl]imide

“…[9] In our case within the set of crowns 12C4, 15C5, 18C6, DB18C6, DB21C7 and DB24C8 the D Cs L are changing in a non monotonous way as the cavity size [22,23] is increasing, indicating a maximal value for DB21C7 (Table 1, Figure 2). Crown cavity size R L , nm [22,23] 12C4 Extraction is not detected 1.42 [19] No data 0.06 15C5 17 2.0 0.30 1.75 [19] 0.80 [17] 0.086-0.092 18C6 50 19 1.3 3.40 [19] 0.96 [17] 0.134-0.143 DB18C6 26 14 1.3 3.47 [20] 0.74 [21] 0.134-0.143 DB21C7 82 47 1.7 3.52 [20] No data 0.17 DB24C8 46 8.6 0.9 2.91 [20] 1.41 [21] 0.20 Indeed, for 12С4, 15С5, 18С6 and DB18C6 the cavity size is smaller than cesium ion diameter (0.17 nm [22] ), while for DB24C8 it is remarkably bigger. According to the crystallographic data and the structural data for the aqueous phase [24,25] the cesium cation in its complexes with 18C6 is placed slightly above the plane, constituted by the oxygen atoms of the ligand, being a bit bigger than the cave size.…”

Thermodynamics of complexation of cesium with dibenzo-21-crown-7 and dibenzo-24-crown-8 in 1-butyl-3-methylimidazolium bis((trifluoromethyl)sulfonyl)imide

“…They are very efficient in the removal of cesium and strontium radioisotopes as Cs + and Sr 2+ cations from radioactive liquid waste [2,3]. 18 Crown 6 (18C6) substantially increases the recovery factor in cesium extraction from aqueous solutions into hydrophobic ionic liquids [7,8]. The recovery factor in this process correlates well with the stability constants of the cesium-18C6 complexes.…”

“…The ΔS 1 values were then calculated. These results and the same thermodynamic data for molecular solvents [13][14][15][16][17][18][19] are presented in Table 3 and in the figure.…”