Micelles form in other solvents

Surfactants & Critical Micellar Concentration (CMC)

Surfactants are surface-active substances. They consist of a polar head group and a long non-polar hydrocarbon chain (see Figure 1). The polar part of the molecule can strongly interact with a polar solvent such as water, which is why it is also called hydrophilic (water-loving). The non-polar part of the molecule can interact strongly with a non-polar solvent such as oil, which is why it is called lipophilic (fat-loving) or hydrophobic (water-avoiding).

For example, surfactants can be classified based on their head group charge:

  • anionic surfactants have a negatively charged head group
  • Cationic surfactants have a positively charged head group
  • amphoteric surfactants have a zwitterionic head group (positive and negative charge)
  • nonionic surfactants have an uncharged polar head group.

Figure 1: Schematic structure of a surfactant

Due to their two-part structure, surfactants preferentially accumulate at interfaces where they find the most favorable energetic conditions for them. The polar part is oriented towards the more polar side of the interface. On a water surface, for example, the surfactants orient themselves in such a way that the head group points into the water and the hydrocarbon chain into the gas phase (see Figure 2). The interfacial tension is reduced by the accumulation of surfactants at the interface. The surfactants can interact well with one phase as well as with the other and thus “mediate” between the atoms / molecules of the two phases. The addition of surfactants therefore facilitates the mixing of non-polar and polar phases, which is used, for example, in the detergent and cleaning agent industry.

Figure 2: Surfactants at an interface

The lowering of the interfacial tension by surfactants is greater, the more surfactants are attached to the interface. However, if the interface (as well as the adjacent volume phases) is saturated with surfactant molecules, the addition of surfactant does not lead to a further lowering of the interfacial tension (see Figure 4). Instead, the surfactant molecules self-organize in a volume phase. Micelles are formed, for example, when several surfactant molecules assemble their non-polar chains and shield them from a surrounding aqueous phase with their polar head groups (see Figure 3). The minimization of the unfavorable contact between the non-polar surfactant chains and the polar phase compensates for the loss of entropy due to micelle formation.

In addition to the micelles shown in Figure 3, there are also so-called inverse micelles in which the head groups are clustered and the non-polar surfactant chains are aligned with a surrounding non-polar phase. Micelles can be spherical, but if parameters such as temperature and system composition vary, they can also assume elongated or worm-like structures. For example, layer structures of surfactant molecules also occur in liquid crystals.

Figure 3: Spherical micelle

The critical micelle concentration (CMC) is the surfactant concentration at which micelle formation begins. It can be determined for a surfactant solution by measuring the surface tension at different concentrations. This decreases with the surfactant concentration below the CMC and is constant above the CMC, since the number of surfactant molecules in the interface does not change any further. A logarithmic plot of the surface tension versus the surfactant concentration shows two linear areas, below and above the CMC (see Figure 4). The CMC can therefore be read off at the point of intersection by extrapolating corresponding best-fit straight lines.

The CMC can be determined automatically with the aid of a dynamic contact angle measuring device and tensiometer of the DCAT series if a liquid dosing system LDU 25 is connected.

Figure 4: Surface tension as a function of the surfactant concentration

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