immiscibility

Immiscibility is the property of two substances, typically liquids, that do not form a single homogeneous phase when mixed. Mutually immiscible liquids separate into two distinct layers rather than mixing in all proportions. Some liquid pairs are completely immiscible, while others are partially immiscible, showing limited mutual solubility at certain compositions or temperatures.

The primary cause of immiscibility is the mismatch of intermolecular forces between the liquids. Highly polar, hydrogen-bonding liquids such as water interact poorly with nonpolar or weakly polar solvents, leading to unfavorable enthalpic interactions that outweigh the entropy gain from mixing. As a result, the Gibbs free energy of mixing remains positive, and the liquids separate. Temperature, pressure, and additives can modify immiscibility; for example, heating may increase solubility for some pairs, and salts or surfactants can influence phase behavior or stabilize emulsions.

Common examples include water and many organic liquids such as oils and hydrocarbons (for example hexane). These pairs form two phases and are used in practical applications such as liquid-liquid extraction, where a solute partitions between an aqueous phase and an immiscible organic phase. In such systems, interfacial tension exists between the layers, and dispersion of droplets or emulsions may be achieved with surfactants to stabilize mixtures temporarily.

In contrast, miscibility describes the ability of two substances to mix in all proportions to form a single phase. Phase diagrams for immiscible systems often show a two-phase region bounded by a binodal curve, indicating compositions at which phase separation occurs.