Immiscibility

Immiscibility is the inability of two substances to form a single homogeneous phase under specified conditions. It can apply to liquids, solids, or other mixtures and depends on temperature, pressure, and composition.

Gibbs free energy of mixing: The change in Gibbs free energy when components are combined at fixed temperature and pressure. Mixing is unfavorable when its free-energy change favors separated phases.
Phase: A region of matter with uniform physical and chemical properties, separated from other regions by boundaries. Immiscibility means coexistence of distinct phases rather than one uniform phase.
Liquid–liquid extraction: A separation method that transfers a solute between two immiscible liquid phases. The distinct phases make selective transfer and subsequent separation possible.
Diffusion: The net movement of particles from regions of higher concentration to lower concentration. Diffusion can occur across phases without making immiscible substances form one phase.
Chemical potential: The change in a system's thermodynamic potential when the amount of a component changes. Phase equilibrium requires each component to have equal chemical potential across coexisting phases.
Phase diagram: A graph showing which phases are stable at different temperatures, pressures, and compositions. It maps the conditions under which components are miscible or phase-separated.
Oil–water separation: The separation of oil and water into distinct liquid layers or droplets. Oil and water are a familiar pair whose immiscibility enables simple decanting.
Kinetic stability: Resistance to change caused by slow reaction or transport rates, rather than thermodynamic favorability. A mixture may remain uniform temporarily despite being thermodynamically immiscible.
Flory–Huggins solution theory: A lattice model that estimates polymer-mixture free energy from entropy and interaction energies. Its interaction parameter predicts when polymer blends demix.
Solubility: The maximum amount of one substance that can dissolve in another under specified conditions. Limited solubility can leave two phases even when some material dissolves.

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