Surfactants & critical micelle concentration (CMC)
Surfactants are interfacially active compounds. They consist of a polar head group and a non-polar hydrocarbon chain (see figure 1). The polar part of the molecule can interact strongly with polar solvents, like water, and is therefore also called the hydrophilic part. The non-polar part, on the other hand, can form strong interactions with non-polar solvents, like oil, and is therefore also called lipophilic or hydrophobic part.
Surfactants can be classified according to the charge of their polar head group:
- anionic surfactants have a negatively charged head group
- cationic surfactants have a positively charged head group
- zwitterionic surfactants have a zwitterionic head group (positive and negative charge)
- nonionic surfactants have an uncharged polar head group
Surfactants adsorb preferably at interfaces where they find the energetically most favourable conditions due to their two-part structure. At a water surface, for example, the surfactants orient themselves in such a way that the head group resides in the water and the hydrocarbon chain points to the gaseous phase (see figure 2). Thus surfactants can
mediate between two phases as they can form strong interactions with both of them. The interfacial tension consequently decreases. The addition of surfactants hence facilitates the mixing of non-polar and polar phases, which is used in the detergent industry for example.
The decrease of the interfacial tension caused by surfactants becomes stronger the more surfactants are adsorbed at the interface. Once the interface (and the adjacent volume phases) are saturated the addition of more surfactants will not decrease the interfacial tension any further (see figure 4). Instead a self-organisation of the surfactant molecules takes place inside a volume phase. For example micelles form which consist of several clustered surfactant molecules that shield their non-polar chains from the surrounding aqueous phase with their polar head groups (see figure 3). The minimization of the unfavourable contact between non-polar surfactant chains and the polar solvent compensates the loss of entropy by micelle formation.
In addition to the micelles shown in figure 3 so-called inverse micelles exist which cluster their head groups and orient their chains towards a surrounding non-polar phase. Moreover, varying parameters like temperature or system composition micelles can also adopt forms other than spherical, such as elongated and worm-like structures. In liquid crystals also layered structures are found.
The critical micelle concentration CMC is the surfactant concentration at and above which micelles are formed. It can be determined for surfactant solutions by measuring the surface tension at different concentrations. Below the CMC the surface tension decreases with increasing surfactant concentration as the number of surfactants at the interface increases. Above the CMC, in contrast, the surface tension of the solution is constant because the interfacial surfactant concentration does not change any more. In a logarithmic representation of the surface tension versus the surfactant concentration there are two linear regimes below and above the CMC (see figure 4). An extrapolation of respective regression lines yields the CMC at the intersection.
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