How does liquid-liquid extraction separate substances between two immiscible liquids?

Liquid-liquid extraction separates substances by utilizing their differing relative solubilities in two immiscible or partially immiscible liquids.12

When these two liquids—typically an aqueous solution and an organic solvent—are mixed, the solutes distribute themselves between the two phases until equilibrium is reached. The extent to which a substance partitions into a specific phase depends on its relative affinity for that solvent. Once equilibrium is established, the two layers are physically separated. The phase containing the desired solute is known as the extract, while the remaining phase is called the raffinate.12

The efficiency of this separation is determined by the separation factor. If the factor is large, a single-stage extraction may suffice; however, if it is small, the process must be repeated in a multistage or countercurrent procedure. Repeated equilibrations with fresh solvent are generally more effective at removing material than a single extraction using a larger volume of solvent.32