What is the upper critical solution temperature?

The upper critical solution temperature (UCST) refers to the highest temperature at which two components of a mixture can coexist in a single phase. Above this temperature, the components become fully miscible, while below it, phase separation occurs. This phenomenon is particularly observed in mixtures that exhibit specific behaviors, such as the dissolution of two liquids that are only partially miscible at lower temperatures.

When a mixture surpasses the UCST, typically the two components will take on distinct phases (e.g., liquid-liquid separation), although they may be completely miscible when heated above this critical point. Hence, the defining characteristic of the upper critical solution temperature is its role in demarcating the boundary between homogeneous and heterogeneous regions in mixtures, making it a vital concept in thermodynamics and material science.