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Liquid-liquid refers to systems in which two liquids are mutually immiscible, forming distinct phases separated by an interface. In such a system, molecules distribute between the phases according to their solubility, described by the partition coefficient. The interface exhibits interfacial tension and can influence mass transfer, mixing, and reaction rates.

Liquid-liquid extraction is a common application, using a solvent pair such as water and an immiscible organic solvent. A solute partitions between phases; recovery and purification rely on adjusting pH, temperature, or ionic strength. The equilibrium distribution is quantified by the distribution coefficient (D) or partition coefficient (K).

Phase separation behavior can be influenced by emulsions, surfactants, and processing conditions; methods to enhance separation include adjusting density, adding salts (salting out), using coagulants, or employing centrifugation. Emulsions are metastable mixtures where droplets are dispersed; breaking them can require demulsification.

Beyond extraction, liquid-liquid phase separation also describes a phenomenon in polymer and biomolecule solutions, where a homogeneous liquid separates into two liquid phases with different compositions, important in intracellular organization and materials science.

Applications span purification and separations in chemical, pharmaceutical, and environmental contexts, including metal ion recovery, drug formulation, and contaminant removal. Considerations include solvent choice, safety, environmental impact, and process economics, as well as methods to control interfacial phenomena and improve phase disengagement.