AGGREGATION, REVERSED MICELLES, AND MICROEMULSIONS IN LIQUID-LIQUID-EXTRACTION - THE TRI-NORMAL-BUTYL PHOSPHATE-DILUENT-WATER-ELECTROLYTE SYSTEM

The experimental evidence (e.g. distribution, viscosity, conductivity, and molecular weight measurements) suggesting the presence of aggregates in TBP extracts is assembled and critically reviewed. A variety of TBP-based systems are considered, such as TBP-Diluent, TBP-H2O, TBP-Diluent-H2O, TBP-H2O-Acid, TBP-Diluent-H2O-Acid, TBP-H2O-Acid-Metal Salt, TBP-Diluent-H2O-Acid-Metal Salt, TBP-H2O-Metal Salt, and TBP-Diluent-H2O-Metal Salt. It is demonstrated that aggregation is favored by high acidities and high metal concentrations. Organic extracts derived from highly acidic aqueous solutions and/or highly concentrated metal salt solutions tend to contain ion-pairs or ion-multiplets predominantly. These species possess significant amphiphilic character since they contain both hydrophobic (i.e., trialkyl radicals) and hydrophilic (i.e., P = O(H2O)xMz+ and/or P = O(H2O)xH+) groups. On the basis of current notions of aggregation and solubilization in surfactant solutions, arguments are presented for possible formation of reversed micellar structures and microemulsions in TBP solvent systems.