Comparatively Speaking: CMC vs. RF₅₀

By: Anthony J. O'Lenick Jr., Siltech LLC; and Thomas O'Lenick, University of Tennessee

Posted: March 2, 2011

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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.

References
1. Technical Brochure Measuring principles of KRÜSS Tensiometers, KRÜSS, www.kruss.info (Accessed Mar 2, 2011)