Biphasic liquid systems are important in many areas of chemistry and biology. For example, in living cells, some organelles do not have membranes and appear to be unbound regions of liquid that sustain different composition and properties to their surroundings. In the chemical industry, biphasic liquids have been used to perform separations for decades. Recently, switchable biphasic mixtures have come into focus as an emerging separation method, where the application and removal of a stimulus (e.g., temperature, pH, pressure, light, or CO2) triggers a drastic change in material properties, such as mutual solubility of two liquids. For example, amine-based switchable solvents have shown potential for desalination, water-softening, anti-solvent precipitation, and oil extraction. 2 2A). Understanding the molecular-level interactions governing the temperature-switchable mixing behavior will enable the informed advancement of these new switchable solvent-based separations, allowing enhanced energy efficiency and improved sustainability. LCST behavior implies a particular combination of weakly negative enthalpy of mixing, and therefore a thermodynamic tendency to mix at low temperature, combined with negative entropy of mixing, so that a de-mixed, phase separated state becomes favored at higher temperatures. Systems of ideal particles gain entropy when mixed: ideal particles have a positive entropy of mixing due to the increase in configurational disorder upon mixing. This would favor mixing over phase-separation at high temperature. Consequently, many biphasic, liquid-liquid systems fully mix only at high temperature, the well-known upper critical solution temperature (UCST) behavior, which is used in chemistry reference texts to explain the role of entropy in phase behavior. Paradoxically, the behavior of LCST systems is the opposite: LCST mixtures become less miscible at high temperatures.