Critical micelle formation concentration explained

Figure 1: Measuring the critical micelle formation concentration helps in the formulation of surfactants to achieve the best effect while minimising the use of resources.

If an interface between two phases is saturated with surfactants, excess surfactant molecules form micelles in the liquid phase. The concentration at which the first micelles are formed is known as the critical micelle formation concentration (CMC for short). It can be determined using a series of measurements with different surfactant concentrations. In practice, the critical micelle formation concentration is an important indicator of how many surfactant molecules need to be added to a particular mixture in order to achieve the greatest effect while conserving resources.

What are surfactants?

Surfactants are surface-active substances. Surfactant molecules consist of a hydrophobic, non-polar chain and a hydrophilic, polar head. They prefer to accumulate at the interface between two phases and reduce the interfacial energy, so that less energy is required to enlarge the interface. Thus, they enable better mixing of two phases (see: How surfactants influence the interface).

What are micelles and how do they form?

Surfactant molecules like to accumulate at the interface or surface between two phases. However, if surfactant molecules no longer find a place at the interface because it is saturated with surfactant molecules, the surfactants start to form groups in the liquid phase. These groups of surfactants are called micelles. If the surrounding liquid is polar, the molecules will assemble their non-polar chains so that their polar head groups protrude into the liquid phase (see Figure 2). If the surrounding liquid is non-polar, so-called inverse micelles are formed, in which the headgroups are stored together and the non-polar hydrocarbon chains are orientated towards the surrounding phase.

Micelles can take various forms. One common form are spherical micelles, in which the surfactants are arranged in a spherical shape (see Figure 2). However, when parameters such as temperature and system composition are varied, they can also form elongated or even worm-like structures. Layered structures of surfactant molecules occur in liquid crystals.

Figure 2: Spherical micelles are one of the most common forms of micelles. Here, the non-polar hydrocarbon chains are clustered together and the polar molecular heads protrude outwards into the surrounding polar liquid.

What is the critical micelle formation concentration (CMC)?The critical micelle formation concentration (CMC) is the surfactant concentration at which the formation of micelles begins. It can be determined for a surfactant solution by measuring the interfacial or surface tension at different surfactant concentrations.
When surfactants are added, the interfacial or surface tension decreases until the critical micelle formation concentration is reached and the interface is saturated with surfactants. Once the critical micelle formation concentration is reached, the interfacial or surface tension remains constant, as the number of surfactant molecules in the interface no longer changes.
The critical micelle formation concentration can be determined graphically with the aid of a logarithmic diagram: the value of the interfacial or surface tension is entered on the y-axis and the surfactant concentration is entered on a logarithmic x-axis.
There are two areas in the logarithmic plot: on the one hand, the decreasing interfacial or surface tension before the critical micelle formation concentration is reached, and on the other hand, the constant interfacial or surface tension after the critical micelle formation concentration has been reached (see Figure 3). The critical micelle formation concentration (CMC) can therefore be read off at the point of intersection of corresponding regression lines.

									
										
											
											
										
									
								
Figure 3: The surface tension depends on the concentration of the surfactant. Once the critical micelle formation concentration (CMC) is reached, the surface tension remains constant with further addition of surfactants.
The critical micelle formation concentration in practiceKnowing the critical micelle formation concentration is crucial in many industries as it is a measure of the efficiency, stability, and effectiveness of surfactants. 
Quantitative measurements of the critical micelle formation concentration can help manufacturers of detergents and cleaning solutions to achieve increased cleaning efficiency of their products. Additionally, determining the critical micelle formation concentration makes it possible to use surfactants in the lowest possible concentration. This ensures high cleaning power with low surfactant consumption.
Dispersibility is often a challenge when formulating medicines. This is where the critical micelle formation concentration comes into play to promote the formation of micelles, which can enclose active ingredients. This enables the production of drugs with better bioavailability.
In food processing, emulsifiers are needed to disperse fats in water. The critical micelle formation concentration is crucial to determine the optimal concentration of these emulsifiers. Stable micelles ensure a long-lasting emulsion, which is important for products such as margarine or ice cream.
In environmental protection applications, micelle formation is used to remove harmful substances from soil and water. Micelles can coat pollutants and convert them into water-soluble forms.
In the petroleum industry surfactants are characterised using the critical micelle formation concentration. These surfactants form micelles that help to mobilise crude oil from porous rock layers. This makes the extraction of crude oil more efficient and economical.
Surfactants and dyes are often used together to colour textiles. Precise knowledge of the critical micelle formation concentration helps to find the optimum conditions for dyeing. This contributes to the uniform colouring of the textiles.

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