Cloud point – definition, mechanism and measurement methods

The mechanism of surfactant solubility in water

Solubility is a physical property of chemical compounds, depending primarily on the type of substance being dissolved and the solution in which it is dissolved. It is influenced, among other things, by the temperature of the solution used and the pressure applied. The dissolution mechanism varies depending on the substance being dissolved and may be related to:

• Dissociation and ion formation in the case of ionic compounds,
• The formation of hydrogen bonds between water molecules and ether oxygen in the oxyalkylene group in the case of non-ionic compounds.

In addition, there are relationships that should be mentioned when considering the solubility of surfactants:

• The more ether groups in a surfactant molecule, the greater its solubility in water. This is due to increased hydrophilicity.
• The higher the temperature, the lower the solubility, which causes the solution to become cloudy.

The temperature above which two phases begin to separate in the solution and the solution becomes heterogeneous is the cloud point.

The presence of inorganic salts in water can lower the cloud point of surfactant solutions.

Hydrotropes, a specific group of chemical compounds, are used to prevent the cloud point from lowering.

What are hydrotropes?

They are substances that have the ability to modify the solubility of chemical compounds in water. They are characterised by amphiphilicity, i.e. they have both hydrophobic and hydrophilic fragments in their molecules. Above a certain concentration, called the hydrotropic concentration, they form micelles. Their main function is therefore to prevent the precipitation of components from formulations rich in surfactants at low temperatures. They are similar in structure to surfactants, but their hydrophobic tail is shorter.

See the hydrotropic agents available from the PCC Group.

Flocculation and coagulation

Surfactants in the form of micelles, like all colloidal solutions under certain conditions, tend to combine into larger clusters, or aggregates. Coagulation leads to a reduction in the interfacial area, resulting in the separation of the individual phases. Flocculation also leads to the formation of larger aggregates, but these can move freely in the dispersion medium. The factor influencing this process is the nature of the electric double layer around the micelles.

See flocculants in the PCC Group’s product range.

Non-ionic surfactants

The course of processes aimed at producing larger clusters is strictly dependent on the hydration capacity of the -OH or -(CH2CH2O)n- groups. As the temperature increases, the degree of hydration decreases. This results in the flocculation of micellar surfactant solutions and the subsequent mixing of the phases with each other.

During this process, a transparent aqueous phase, practically devoid of surfactants, and a clearly cloudy phase containing a highly concentrated colloidal solution of surfactant can be observed. The turbidity of the solution is therefore a consequence of the presence of large surfactant aggregates in it, which cause the scattering of visible light passing through the solution. Phase separation can be observed within a certain temperature range, near the cloud point.

Mixtures of anionic and cationic surfactants

The phenomenon of solution turbidity is also observed in mixtures of anionic and cationic surface-active substances, but it is caused by other mechanisms. Depending on the ratio of anionic and cationic surfactants present in the solution, as well as the presence of electrolytes, the system may be a clear micellar solution or a precipitate-coacervate system with various mutual equilibria.

Depending on the composition of the mixture used, the relationship between the cloud point and the molar fraction of one of the components can be expressed using a characteristic graph. Empirical studies have shown that an increasing fraction of anionic surfactant from ~0.47 to ~0.51 causes a decrease in the cloud point. From ~0.51 to ~0.57, an increase in CP was observed. In general, it can also be said that the presence of electrolytes in the solution causes a decrease in the cloud point.

Cloud point – reversibility of the transformation

Solutions of surfactants become cloudy when heated, but thanks to the reversibility of the transformation, they become clear again when cooled to a certain temperature.

What affects the cloud point?

1. 1. Contaminants: the more contaminants in the solution, the lower the cloud point. Additional particles disrupt the structure and hinder the formation of micelles.
   2. Pressure: the higher the pressure, the higher the cloud point. High pressures promote the formation of more compact structures.
   3. Solvent,
   4. Surfactant: for the most common non-ionic surfactants, the following relationships apply:
      1. Chemical structure – hydrophobic chain length,
      2. Degree of oxyethylation

The longer the hydrophobic chain and the lower the degree of oxyethylation, the lower the cloud point.

Methods for determining the cloud point

1. 1. Visual methods
      1. Visual observation during gradual heating of the solution,
      2. Use of dedicated devices that employ the technique of controlled heating,
   2. Spectrophotometric methods
      1. Measurement of light absorption or transmittance, which change with temperature,

Significance of the cloud point parameter

• Food and chemical industry, including the determination of the properties of surfactant formulations.
• Quality control of fuels and oils,
• Determination of the properties of materials, such as polymers and fats,