Miscible

Miscible describes the property of two liquids to mix in all proportions to form a single homogeneous liquid phase at a given temperature and pressure. When two liquids are miscible, their mixture has uniform composition throughout and does not separate into distinct layers. If two liquids are not completely miscible, they may be immiscible, forming two phases, or partially miscible, mixing only to a limited extent or only under certain conditions.

Common examples of miscible pairs include water with ethanol, water with acetone, and water with methanol. Many organic solvents are miscible with water, while immiscible pairs include oil and water. Some mixtures are temperature dependent: certain liquids are miscible at higher temperatures but separate at lower temperatures, and there can be a miscibility gap in more complex systems such as polymer blends or multicomponent solutions.

The concept is governed by thermodynamics. The Gibbs free energy of mixing, ΔG_mix, must be negative for mixing to occur across all compositions. This outcome depends on enthalpy and entropy changes upon mixing; favorable interactions between unlike molecules can drive complete miscibility, even if the enthalpy is slightly positive. In practice, solubility parameters and related models help predict miscibility between liquids. The term is also used in polymer science and materials engineering to describe the compatibility of components in blends, where incomplete miscibility can lead to phase separation and distinct domains.