The physiochemical nature of the metal-extractant species in organic solvent has been a matter of debate over liquid-liquid extraction of transition metals by bis(2-ethylhexyl) phosphate. Molecular modeling was employed to study the aggregation of nickel-extractant molecules for better understanding the solvent extraction of metal ions from aqueous to nonpolar organic phase. Sodium hydroxide is usually used to adjust the pH of the aqueous phase during the solvent extraction of divalent metal ions such as Ni+2, Co+2, Zn+2 and Pb+2. Since sodium bis(2-ethylhexyl)phosphate (NaDEHP) species tend to form reversed micelles in the organic phase, the aggregation behavior of Nickel (II) bis(2-ethylhexyl)phosphate (Ni(DEHP)2) was investigated using molecular modeling with the present of NaDEHP molecules. The recently proposed “open” water-channels Ni(DEHP)2 rodlike reversed micelles which is in contact with the nonaqueous solvent rather than in an inner core (or “closed” water channel) of the nickel-extractant species by Ibrahim and Neuman appears to need modifications. The present of NaDEHP molecules with the Ni(DEHP)2 species behave in a different manner. The Ni(DEHP)2 species form rodlike reversed micelles, with the formation of “closed” water channel by the NaDEHP molecules. The water molecules trapped in channel of Ni(DEHP)2 complexes and NaDEHP molecules. The formed reversed micelles are in accord with the conventional view of reversed micelles.

Key words: reversed micelles; HDEHP; solvent extraction; divalent metals; molecular modeling.