nonmiscible

Nonmiscible, also described as immiscible, refers to a pair of liquids that do not mix to form a single homogeneous phase. When such liquids are combined, they separate into two distinct liquid layers rather than forming a uniform solution. The mutual solubility of immiscible liquids is typically negligible under ordinary conditions, though temperature or pressure can alter solubility for some pairs.

The phenomenon arises from thermodynamic and molecular factors. Immiscible liquids usually have different polarities, hydrogen-bonding capabilities, or molecular structures that make mixing energetically unfavorable. The result is a positive Gibbs free energy of mixing, so the liquids remain distinct. They exhibit interfacial tension at the boundary between the two phases, and the denser liquid often forms the lower layer.

Temperature and pressure can influence miscibility. For many pairs, immiscibility persists across standard temperature ranges, but some exhibit partial or complete miscibility at higher temperatures or under unusual conditions. Phase diagrams for immiscible systems typically show two liquid phases over a range of compositions.

Practical relevance includes liquid-liquid extraction and separation processes, where immiscible solvents are used to partition solutes based on polarity or solubility. In the laboratory, choosing immiscible solvent pairs enables efficient separation by simple decantation or centrifugation. Emulsions can temporarily suspend droplets of one liquid within another, often stabilized by surfactants, but immiscibility remains the underlying tendency of the liquids to form separate phases.