Tri-n-hexylphosphine oxide and tri-n-octylphosphine oxide are primarily used as extractants in solvent extraction processes, particularly in the separation and purification of rare earth metals and other valuable metals. Their ability to selectively bind with specific metal ions makes them highly effective in hydrometallurgical applications, such as recovering metals from ores, industrial waste, or electronic scrap. These compounds are also employed in the nuclear industry for the extraction and separation of actinides and lanthanides from spent nuclear fuel. Additionally, they serve as stabilizers or ligands in catalytic reactions, enhancing the efficiency of certain chemical processes. They are further utilized as capping agents in the synthesis of semiconductor nanocrystals, such as quantum dots, which are essential for advanced optoelectronic devices. Their use in liquid-liquid extraction systems is notable for their high selectivity and stability under various conditions.