Which characteristic is MOST important while choosing the organic solvent for a liquid-liquid extraction?

The most important characteristic to consider when choosing an organic solvent for a liquid-liquid extraction is polarity. The principle behind liquid-liquid extraction relies on the differential solubility of the compounds to be separated in two immiscible liquids—typically an organic solvent and water.

When selecting an organic solvent, it's crucial to choose one that has a polarity that is appropriate for effectively dissolving the target compounds. The target solute should preferably be more soluble in the chosen organic solvent than in water (which is often the solvent used in the other layer). This solubility difference is what allows for the separation during extraction.

For example, if the compound of interest is polar, a polar organic solvent such as ethanol would be more suitable because it can solvate the compound effectively, enhancing the extraction efficiency. Conversely, if the compound is non-polar, using a non-polar solvent such as hexane would yield better results.

While factors like cost, stability, and density are also important considerations in practical applications, they do not directly influence the interaction between the solvent and solute as fundamentally as polarity does. This makes polarity the primary criterion for successful liquid-liquid extraction.