Question

With the aid of a suitable example and labelled diagrams write a detailed note on the physical behaviour of aqueous solutions of ionic surfactants. Choosing at least three physical properties to explain clearly the variance in each physical property as a function of surfactant concentration.

Answer 1

A surfactant is a compound that lowers the surface tension between two liquids or between a gas an a liquid (air and water, for example). They contain both hydrophilic ("water-loving") heads and hydrophobic ("water-hating") tails. The hydrophilic heads are logically, soluble in water, while the hydrophobic tails are soluble in non-polar solvents, such as oil. One strucutre of a surfactant can be seen in the following diagram:

In an aqueous solution, surfactants can form various structures. The one pictured is called a micelle. In a micelle, the hydrophobic tails come together in the center of a circle and the hydrophilic heads interact with the solvent outside of the circle. Another possible structure is a lipid bilayer, such as the ones present in cell membranes. The structure will depend on the chemical structure and composition of the surfactant (hydrophilic and hydrophobic parts and their size).

An ionic surfactant is one that has a hydrophilic head with a net charge (positive or negative). Positively charged surfactants are called cationic surfactants while negatively charged surfactants are called anionic surfactants. An example of an ionic surfactant is sodium stearate, which is the most common component in soap:

Depending on the concentration of the ionic surfactants, the physical properties of the solution will change.

The most important physical property that changes with the addition of ionic surfactants is the surface tension. Surface tension is an intensive property that gets dramatically reduced as the concentration of the surfactant is increased. Surface tension decreases strongly until the critical micelle concentration (CMC) is reached. At that point, adding more surfactant won't reduce the surface tension of the solution further. The graph is normally done in a logarithmic scale for concentration, because like pH, the concentration of surfactant can vary greatly. A qualitative graph of the change of surface tension vs. concentration of surfactant is shown:

Another property that changes due to to surfactant concentration is the solubility of insoluble compounds in aqueous solutions. Surfactants help dissolve more insoluble compounds in aqueous solutions, especially after the CMC is reached (where the surfactants start forming micelles and are not just individual molecules). After that point and as can be seen in the following graph, solubility increases exponentially.

Another important property that is affected by surfactant concentration is the conductivity. As the ionic surfactant is added, the conductivity of the aqueous solution logically increases with a linear relation. But once again, when the CMC is reached, another change occurs. The surfactants form micelles and the conductivity keeps increasing, but at a smaller slope than before the CMC. This can be seen in the following graph:

Surfactants are part of everyday life. Some examples of ionic surfactants are soaps and detergents. They have the characteristic structure of surfactants described earlier. When a determined concentration of soap is added to a glass of water, bubbles form and don't break down easily. This occurs because the surface tension has been drastically reduced by adding the soap. This stabilization is very important because both soap and detergent have hypochlorite or calcium carbonate that act as cleaning agents and can act much more efficiently due to the reduction of surface tension.