Effects of Water Molecules on Metal Complexes of Hydroxyoxime and Carboxylic Acid Extractants

Abstract

Water molecules around the cobalt(II) (Co) and manganese(II) (Mn) complexes of the hydroxyoxime extractant, LIX63 and/or of the carboxylic acid extractant, Versatic acid 10 (VA10), in cyclohexane solvent were investigated using molecular dynamics calculations with simulated annealing. For the complexes of VA10, water molecules were coordinated to the metal ions, and VA10 molecules were found around the complexes. For the complexes of LIX63, a few water molecules were located at ~ 0.4 nm from the metal ions, in addition to the LIX63 molecules were also coordinated to the metal ions. Our trajectory analysis indicates that the water molecules at ~ 0.4 nm are hydrogen-bonded to LIX63 or other water molecules, suggesting that the hydrogen bonding contributes to the stabilization of the metal complex.

Permeation Process of Metal Complexes with Hydroxyoxime and Carboxylic Acid Extractants through Organic-Aqueous Interface

“…These molecules consisting of the hydrogen bonds were located at ~ 0.5 nm around Co 2+ , contributing to the stabilization of the Co complexes. However, VA10 molecules were also found at the region and this is not agreement with our previous study [16]. Thus, the Co complexes in this calculation are transient and this result suggests that longer calculation time should be needed for the Co complexes in equilibrium.…”

“…-24 -interface (Figure 4(c) and (b)) and staying in the aqueous phase (Figure 4(b)) account for about 10, 60 and 30%, respectively. The structure shown in Figure 4(e) is good agreement with one of the Co-LIX63 complexes in the organic phase, but not with the most stable structure where the Co complex in the organic phase is composed of 3 LIX63 molecule coordinated to Co 2+ [16]. Thus, because there were some of Co 2+ and LIX63 molecules not permeating through the interface, this complex seems to be in one of local minimal.…”

“…Although water molecules coordinated to Co 2+ were exchanged with VA10 molecules in the organic phase by using the hybrid MC/MD method, other water molecules were immediately coordinated to Co 2+ and the Co complexes moved back to the aqueous phase. Our previous study [16] shows that water molecules were mainly coordinated to Co 2+ in the Co-VA10 system and VA10 molecules were located at ~ 0.4 nm around the Co complexes. Thus, the hybrid MC/MD method might not be suitable for application where extractant -25 -molecules surrounding metal complexes coordinated with water molecules contribute to the stabilization of the Co complexes.…”

“…MD simulations were performed with the GROMACS software package [13,14]. The force field parameters, the atomic charges and the calculation conditions used in this study were the same as the previous studies [15,16]. The initial model systems were heterogeneous biphasic ones composed with 508 CHX molecules and ~ 3,300 water molecules.…”

“…The systems were embedded in rectangular boxes in a periodic boundary condition, and H 3 O + and Clwere added to neutralize the systems. In addition, H 3 O + could stabilize the Co complexes, reported in our previous studies [16]. The box sizes were set to be 4.6 nm x 4.6 nm x 9.8 nm and described by the coordinate systems XYZ with the Z axis along the interface normal (shown in Figure 2).…”