What is the Krafft Point?
The solubility of ionic surfactants in water generally increases slowly with rising temperature. However, when a specific temperature is reached, their solubility suddenly increases sharply. When the temperature falls below this point, the solubility of the surfactant decreases accordingly. This temperature is called the critical solubility temperature of the ionic surfactant, also known as the Krafft point. It has been demonstrated that at the Krafft point or above, the surfactant dissolves in water in the form of micelles. Below the Krafft point, similar to common water-soluble ionic compounds, the surfactant dissolves primarily in the monomeric form.
What is the Cloud Point?
For nonionic surfactants of the fatty alcohol polyoxyethylene ether type (AEOn), the EO (ethylene oxide) segments form intermolecular hydrogen bonds with water molecules, allowing AEOn to dissolve in water. As the solution temperature rises, these hydrogen bonds break, reducing the solubility of AEOn. When the temperature reaches a specific point, a 1% (w/w) aqueous solution of AEOn suddenly becomes turbid. If the temperature falls below this specific point, the solution returns to being clear and transparent. This temperature is called the cloud point of the nonionic surfactant.
What is the Significance of Measuring the Krafft Point and Cloud Point?
In practice, the Krafft point and cloud point are considered characteristic constants of ionic and nonionic surfactants, respectively. They provide an indication of the surfactant's solubility in water. Generally, a lower Krafft temperature or a higher cloud point indicates better water solubility for ionic and nonionic surfactants, respectively. The Krafft point or cloud point can be influenced by the surfactant's molecular structure, as well as by the presence of added inorganic salts or organic compounds. For nonionic surfactants, the addition of inorganic salts or polar organic substances can significantly reduce the cloud point.
The main methods for determining the Krafft point and cloud point of surfactants are spectrophotometry and visual observation of precipitation (turbidity).
Experimental Instruments and Reagents
Instruments: Analytical balance, electric heating stove, thermometer (0.1 °C scale), large test tubes, beakers, etc.
Reagents: Sodium dodecyl sulfate (SDS/AS), OP emulsifier, linear alkylbenzene sulfonate (LAS), sodium chloride, secondary distilled water (conductivity 7.8x10-7S.cm-1, prepared using a Millipore Synergy UV secondary distillation system, USA).
👉 For other anionic surfactants suitable for research and industrial applications, see [Alfa Chemistry Anionic Surfactants].
👉 For cationic surfactants, please visit [Alfa Chemistry Cationic Surfactants].
Experimental Procedures
1. Determination of Krafft Point for Ionic Surfactants
Weigh a certain amount of the ionic surfactant (AS) into a 100 mL beaker and prepare a 1 wt% aqueous solution. Transfer 20 mL of this solution into a large test tube, heat in a water bath while stirring until the solution becomes clear and transparent, and record the temperature as T1. Then, continue stirring while cooling in an ice-water bath until crystals begin to precipitate, and record the temperature as T2. Repeat the above steps three times, record the temperatures, and calculate the average values.
2. Determination of Cloud Point for Nonionic Surfactants
Weigh a certain amount of OP emulsifier into a 100 mL beaker and prepare a 1 wt% aqueous solution. Transfer 20 mL of this solution into a large test tube, heat in a water bath while stirring, and carefully observe the change in solution transparency. When the solution shows the first sign of turbidity, record the temperature as T3. Continue heating until the solution becomes completely turbid, then stop heating. Remove the test tube and stir while cooling, and record the temperature when the solution turns clear again as T4. Repeat the above steps three times, record the temperatures, and calculate the average values.
3. Effect of Additives on the Cloud Point of Nonionic Surfactants
(1) Transfer 20 mL of the OP emulsifier solution into a large test tube, add 0.1 g of sodium chloride, dissolve completely, and determine the cloud point following the procedure in Step 2.
(2) Transfer 20 mL of the OP emulsifier solution into a large test tube, add 1 wt% LAS aqueous solution (1–3 drops), and determine the cloud point following Step 2. Then, add 2.5 mL of 1 wt% LAS solution and determine the cloud point again using the same procedure.
Data Recording and Processing
1. Krafft Point of Ionic Surfactants
| Serial number | T1/°C | T2/°C | Average value |
|---|
| 1 | | | |
| 2 | | | |
| 3 | | | |
| Average value | | | |
2. Cloud Point of Nonionic Surfactants
| Serial number | OP emulsifier aqueous solution | Add 0.1 g sodium chloride | Add 1-3 drops of LAS aqueous solution | Add 2.5 mL LAS aqueous solution |
|---|
| T3/°C | T4/°C | T3/°C | T4/°C | T3/°C | T4/°C | T3/°C | T4/°C |
| 1 | | | | | | | | |
| 2 | | | | | | | | |
| 3 | | | | | | | | |
| Average value | | | | | | | | |
Recommended Nonionic Surfactants
In addition to the surfactants used in this protocol, Alfa Chemistry offers a wide range of high-quality nonionic surfactants suitable for research and industrial applications.
Frequently Asked Questions (FAQs)
Is there a difference between the cloud point measured by the heating method and the cooling method for nonionic surfactants?
Principle difference: The cloud point is the temperature at which a nonionic surfactant solution turns from clear to turbid upon heating. The heating method involves gradually raising the temperature from a low point and observing when the solution becomes turbid. The cooling method involves first heating the solution to complete turbidity and then slowly cooling it to observe when it becomes clear again.
Practical difference:
- In general, the cloud point measured by the heating method is slightly lower than that measured by the cooling method because turbidity appears as the molecules start to aggregate into micelles or clusters during heating.
- The cooling method sometimes shows a hysteresis effect, meaning the temperature at which the solution returns to clear is slightly higher than the cloud point measured by heating.
Conclusion: The cloud points measured by both methods are usually similar, but the heating method is more commonly used and provides more stable results.
How to measure the cloud point of nonionic surfactants above 100°C?
A cloud point above 100 °C is limited by the boiling point of water, making conventional measurement difficult. Solutions include:
- Using a high-pressure sealed container: Increases the boiling point of the solution and prevents water boiling from interfering with the measurement.
- Changing the solvent or adding a cosolvent: For example, adding a certain proportion of alcohols (ethanol, isopropanol) to water can lower the cloud point to a measurable temperature range.
- Diluting the sample: Lowering the surfactant concentration generally decreases the cloud point, allowing measurement below 100 °C.
Note: High-temperature measurements require safety precautions. It is recommended to use professional instruments with temperature control and pressure protection.
How to increase the cloud point of nonionic surfactants?
Increasing the cloud point can be achieved by adjusting molecular structure or solution conditions:
- Increase hydrophilic groups: For nonionic surfactants like polyoxyethylene ethers (EO), increasing the polyoxyethylene chain length enhances hydrophilicity, thereby raising the cloud point.
- Reduce hydrophobic chain length: Shortening the hydrophobic chain (e.g., fatty alcohol or alkyl chain) reduces hydrophobic interactions, making the molecules more soluble in water and increasing the cloud point.
- Solution adjustment: Adding salts (e.g., NaCl) lowers the cloud point ("salting-out effect"), while reducing salt concentration or using deionized water can slightly raise it.
- Use of additives: Adding cosolvents or surfactant mixtures (e.g., nonionic/nonionic blends) can also increase the cloud point.
