Miscibilitet

Miscibilitet, or miscibility, is the property of two liquids to mix in all proportions to form a homogeneous phase. If two liquids are completely miscible, any combination yields a single liquid phase. In contrast, immiscible liquids do not mix to form a single phase and separate into distinct layers, while partially miscible liquids dissolve in limited amounts and may form two phases over some composition range.

Thermodynamically, miscibility is governed by the Gibbs free energy of mixing. If the free energy change for mixing is negative for all compositions at a given temperature and pressure, the liquids are completely miscible. If there is a composition range where mixing raises the free energy, phase separation occurs, described by a miscibility gap. Temperature can influence miscibility, producing phenomena such as upper and lower critical solution temperatures (UCST and LCST) where two liquids become miscible or immiscible as temperature changes. Binary phase diagrams show these relationships, with phase boundaries and tie lines indicating how compositions partition between phases at equilibrium.

Common examples illustrate the concept. Water and ethanol are completely miscible in all proportions. Water and many hydrocarbons (such as hexane or oil) are immiscible, forming two layers. Ethanol and water mix readily, while benzene and water exhibit limited miscibility at room temperature.

Applications include solvent selection, liquid–liquid extraction, distillation planning, and the design of polymer blends. Understanding miscibility helps predict phase behavior, separation efficiency, and material properties in chemical processes.