Towards CMC measurement using an inductive RF technique for non-ionic surfactants in complex media

Due to their unique chemical feature – associating hydrophilic and lipophilic parts in the same molecule, surfactants are strongly used in cosmetics applications. In particular, the stability of emulsions is due to their ability to reduce surface tension. When the concentration of surfactants is enough, the barrier that they form at interfaces being saturated, they begin to aggregate in spherical micelles which are also amphiphilic structures. The associated concentration is known as Critical Micellar Concentration (CMC). On their own, i.e. with surfactants in solution, the formation of micelles can help with active product delivery control or optimization of detergency properties. In order to measure the CMC value, most methods resort to surface tension, conductivity, or fluorescence measurements. However, those techniques can lead to inaccurate evaluations due to impurities, additional compounds necessary for the stability of the product, and even surfactant nature. The CMC is particularly difficult to obtain for a complex fluid containing multiple surfactants. This work proposes an alternative technique based on inductive measurement of the complex permittivity at RF frequencies. This quantity is directly linked to the polarizability of the electric charges dispersed around the micelles, as explained by Gouy-Chapman’s and Stern’s models. This inductive RF technique is tested with surfactants dispersed in water, thickened water, and oil. The results are compared with ones obtained from classic methodology using tensiometer. Furthermore, the assessment of CMC value of mixture is discussed. Results demonstrate that the technique is sensitive enough to measure the CMC of one or multiple surfactants in various media, anticipating the interest of the technique for complex fluids usually used in cosmetic field.