The Critical Solution Temperature (CST), also called the consolute temperature, is the critical temperature at which the mutual solubility of two partially miscible liquids changes from two-phase behavior to complete miscibility, or vice versa.
Depending on how miscibility changes with temperature, the critical solution temperature may be an Upper Consolute Temperature (UCST) or a Lower Consolute Temperature (LCST).
- Upper Consolute Temperature (UCST): The highest temperature below which the two liquids are partially miscible and above which they become completely miscible in all proportions.
- Lower Consolute Temperature (LCST): The lowest temperature above which the two liquids become partially miscible; below the LCST, they are completely miscible in all proportions.
Types of Systems
- Systems with UCST: The liquids are partially miscible below the critical temperature and become completely miscible above it. Phenol–water is a well-known example, with a UCST of approximately 66 °C at atmospheric pressure.
- Systems with LCST: The liquids are completely miscible below the critical temperature and become partially miscible above it. LCST behavior is particularly common in some polymer–solvent systems.
- Systems with both LCST and UCST: Some systems can exhibit both a lower and an upper critical solution temperature. Such systems may be completely miscible below the LCST and above the UCST, while showing limited miscibility between the two temperatures.
Comparison of LCST and UCST
| Term | Meaning | Miscibility Behavior |
|---|---|---|
| LCST | The lowest temperature associated with the onset of phase separation on heating. | Generally miscible below the LCST and partially miscible above it. |
| UCST | The highest temperature associated with the onset of complete miscibility on heating. | Generally partially miscible below the UCST and miscible above it. |
| Both LCST and UCST | The system has two critical solution temperatures. | It may be miscible below the LCST and above the UCST, with a partially miscible region between them. |
Factors Affecting Consolute Temperature
- Pressure: Changes in pressure can affect the phase equilibrium and consequently shift the consolute temperature.
- Composition: The observed phase behavior depends strongly on the composition of the liquid mixture.
- Impurities: Dissolved impurities or additional components can alter the mutual solubility and shift the critical solution temperature.
- Molecular interactions: Hydrogen bonding, polarity, and other intermolecular interactions play an important role in determining the miscibility of the liquids.
Effect of Impurities on Consolute Temperature
The presence of an impurity can change the mutual solubility of two partially miscible liquids and, consequently, alter their consolute (critical solution) temperature. The direction and magnitude of the change depend on the nature of the impurity and its interactions with the two components of the liquid mixture.
Effect on UCST Systems
In a system having an Upper Consolute Temperature (UCST), an impurity may either increase or decrease the UCST. If the impurity decreases the mutual solubility of the two liquids, a higher temperature may be required to achieve complete miscibility, resulting in an increase in the UCST.
On the other hand, if the impurity interacts favorably with both liquids and increases their mutual solubility, the temperature required for complete mixing may decrease, causing a decrease in the UCST.
Effect on LCST Systems
In a system showing a Lower Consolute Temperature (LCST), impurities can also shift the LCST by changing the intermolecular interactions and the balance between the mixed and separated phases. Depending on the nature of the impurity, the LCST may shift to a higher or lower temperature.
Why Do Impurities Affect Consolute Temperature?
Impurities affect consolute temperature mainly by changing the intermolecular forces between the components. Factors such as hydrogen bonding, polarity, preferential solvation, and differences in molecular interactions can change the mutual solubility of the liquids.
In short: An impurity can either increase or decrease the consolute temperature. The effect depends on whether the impurity increases or decreases the mutual solubility of the two partially miscible liquids.
Effect of Impurity on Consolute Temperature – Phenol-Water System
The phenol-water system is a well-known example of a partially miscible liquid system that exhibits an Upper Consolute Temperature (UCST). At atmospheric pressure, the UCST of the phenol-water system is approximately 66 °C.
The addition of an impurity can change the mutual solubility of phenol and water and, therefore, alter the consolute temperature. The effect depends on the nature and concentration of the added substance.
Effect of NaCl as an Impurity
When sodium chloride (NaCl) is added to the phenol-water system, it dissolves preferentially in the aqueous phase. The presence of dissolved salt reduces the solubility of phenol in water and produces a salting-out effect.
As a result, the mutual solubility of phenol and water decreases. A higher temperature is then required to make the two components completely miscible. Therefore, the UCST increases when suitable amounts of NaCl are added.
Effect on the Phase Diagram
In the phenol-water phase diagram, the addition of NaCl generally shifts the consolute temperature upward. The one-phase region is consequently reached at a higher temperature than in the pure phenol-water system.
Conclusion
Phenol + Water + NaCl: NaCl decreases the mutual solubility of phenol and water through the salting-out effect and generally raises the Upper Consolute Temperature (UCST).
In short: Addition of NaCl to the phenol-water system causes a decrease in mutual solubility and an increase in the consolute temperature.
Key Notes
- UCST: Miscibility increases as temperature increases.
- LCST: Miscibility decreases as temperature increases.
- Phenol–water is a classic example of a system showing UCST behavior.
- LCST behavior is commonly observed in certain polymer–solvent systems.
- Some mixtures may exhibit both LCST and UCST, producing a miscibility gap between the two critical temperatures.