The critical micelle concentration (CMC) and reduction factor 50% (RF50) both are measures of a surfactant's ability to lower surface tension. The complication arises when there is both a silicone surfactant (with a surface tension in the 20 dynes/cm range) and a fatty surfactant (with a surface tension in the 30 dynes/cm) in the same solution. The RF50 graph differs both in the range of surface tension achieved and generally the shape of the graph. The measurement of the surface tension in mixed systems is important to the formulator, as it allows for a direct measure of silicone surfactant effectiveness.
When added into a solvent, surfactant molecules, while clear in the solution, orientate themselves at the surface at the solvent/air interface. As these molecules arrive on the surface, they reduce surface tension because the concentration of the surfactant increases, resulting in two distinct regions. The first is a linear decrease from the original surface tension to a level plateau region. In the linear region, more surfactant molecules are arriving at the interface; the more molecules at the interface, the lower the surface tension.
As the solvent/air interface becomes saturated, the surface tension plateau and little change occurs, forming micelles (Figure 1).1 Micelles are macromolecular structures of surfactant molecules. In Figure 1, The first box shows pure water with a surface tension is around 72 dynes/ cm. As surfactant is added, demonstrated by the second box, surface tension is falling as dilute surfactant organizing at the surface. As the surface reaches saturation, a significant situation develops. The surface tension no longer drops, even with additional surfactant. It is at this concentration, called critical micelle concentration (CMC) that micelles become the dominant form of surfactant. The third box shows this situation.
CMC charts are shown for SLES-2 and PEG 8 dimethicone are shown in Figure 2 and Figure 3, respectively. The key factor here is that the water is about 72 dynes/cm before addition of fatty surfactant and about 32 dynes/cm after the addition. The addition of the silicone surfactant lowers the surface tension to about 20 dynes/cm.
When one considers the effect of adding silicone surfactant (PEG-8 dimethicone for example) to a solution of a fatty surfactant (SLES for example), the surface tension will drop from 32 dynes/cm. The ability of a silicone surfactant to lower the surface tension will be dependent upon its ability to compete with the fatty surfactant at the air/solvent interface. The reduction in surface tension beyond the CMC surface tension for the fatty surfactant is a direct result of competition between the fatty surfactant and the silicone surfactant. The significant lowering of the surface tension with addition of low concentration silicone surfactant indicates the silicone surfactant competes well with the fatty surfactant for surface at the interface. Figure 4 illustrates what happens if one plots the surface tension against the concentration of added silicone surfactant.
Reduction Factor 50% (RF50) is defined as the concentration of silicone surfactant added to reduce the surface tension by half of the difference between the fatty surfactant’s surface tension and the silicone surfactant’s surface tension.
A lower RF50 allows the silicone surfactant to better compete with the fatty surfactant for surface and results in a more efficient silicone surfactant. This technique allows one to design molecules that will be optimized for a particular formulation. Surfactant systems and complex formulations can both be evaluated by simply defining the fatty surfactant’s surface tension as the formulation’s initial surface tension. Surface tension and foam can be tested and optimized by evaluating foam as the property rather than surface tension.
When comparing two silicones (INCI: PEG-8 Dimethicone) with different molecular weights, it can be concluded that more higher molecular weight silicone is needed to obtain the same reduction in surface tension as the lower molecular weight silicone. Adding the silicone surfactant PEG-8 dimethicone at different molecular weights to the fatty surfactant SLES-2, will result in a lower concentration of the lower molecular weight silicone surfactant (1.2 RF50) needed to reduce the surface tension by half compared to a higher concentration of the higher molecular weight silicone surfactant (3.5 RF50) needed to do the same. The implication to the formulator is that the higher molecular weight silicone will be less efficient and consequently more costly.
1. Technical Brochure Measuring principles of KRÜSS Tensiometers, KRÜSS, www.kruss.info (Accessed Mar 2, 2011)