Analytical Sciences

Abstract − Analytical Sciences, 17(12), 1413 (2001).

Effects of Electrolytes on the Configuration Change of Cobalt(II)-Halide Complexes in Chloroform/Water Reverse Micelle Systems
Hironori NAGAYAMA, Masashi HOJO,  Tadaharu UEDA, Yumehiro NISHIMORI, Mariko OKAMURA, and Chihiro DAIKE
Department of Chemistry, Faculty of Science, Kochi University, Akebono-cho, Kochi 870-8520, Japan
The effects of various salts and HClO4 on the configuration change of cobalt(II)-halide complexes in CHCl3/CTAC or CTAB/H2O reverse micelle systems were examined at 25°C by means of spectrophotometry, where CTAC and CTAB represent cetyltrimethylammonium chloride and bromide, respectively. The formation of the [CoCl4]2- or [CoBr4]2- species of the tetrahedral configuration from [Co(H2O)6]2+ of the octahedral configuration in the reverse micelles was greatly promoted not only by a decrease in the W value (W = [H2O]/[surfactant]), but also, at a constant W value (e.g., W = 2.0), by the addition of relatively low concentrations of salts or the acid (e.g., 4.0 mol dm-3 in the aqueous phase or 4.0 x 10-2 mol dm-3 in the whole reverse micelle system). The effects of perchlorate salts increased as Na+ ≤ Li+ ~ H+ < Sr2+ < Ca2+ < Mg2+. Non-metallic salts, various tetraalkylammonium (R4N+) salts at lower concentrations, gave minor effects. The enhanced effects of metal salts on the configuration change of the cobalt(II)-halide complexes were interpreted by a further distortion of the hydrogen-bonded structure of the water in a ''water pool'' in the presence of salts of even relatively low concentrations. A conformation change with increasing temperature was also attributed to a further distortion of the water structure. An almost completed formation of [CoBr4]2- as well as [CoCl4]2- was attained in the reverse micelles at a low W value of 0.69 containing LiClO4 or HClO4. A partial transfer of the [CoX4]2- species from a ''water pool'' into the CHCl3 phase by the addition of the metal salts may be suspected. An examination of cobalt(II)-bromide complexes in dichloromethane/CTAB/H2O at W = 1.3 - 5.55 justified all the arguments concerning the chloroform systems. The Raman spectra of D2O containing concentrated LiBr and LiClO4 have supplied conclusive evidence that the hydrogen-bonded structure of the bulk water is completely distorted by extremely concentrated salts.