Silicone compounds, unlike aliphatic compounds, have a unique lower surface tension. The ability to reduce surface tension leads to many interesting properties of silicones in formulations. The slow addition of dohenylsimethicone to soybean oil causes the surface tension to decrease and condense to form micelles above the CMC of the soybean oil in a manner similar to the surface tension reduction shown in Figure 1. Figure 3 shows the effect. The structure of the specific alkylsiloxane added to soybean oil will determine the amount of alkylsiloxane required to reduce the surface tension to a given value. In other words, the effectiveness of alkylsiloxanes in reducing the surface tension of an oil is not the same for all alkylsiloxanes, nor is it the same for alkylsiloxanes with the same INCI name.
The INCI name does not reveal not only the type of alkyl groups present, but also the percentage of alkyl groups relative to the silicone present, nor the size of the molecule. The concept of RF504 has been proposed as a way to determine the effectiveness of alkylsiloxane compounds in reducing the surface tension of oils. When more than one type of surfactant is present, it is important for the formulator to evaluate the effect of surfactant concentration on surface tension because these so-called hybrid systems are what is present in the formulation. To quantify the effect of adding silicone surfactant to an aliphatic surfactant, follow these steps: Add varying amounts of silicone surfactant to a 1% aliphatic surfactant solution and measure and plot the surface tension . This approach allows one to study the effect of adding silicone surfactants to fatty surfactants to observe interactions and synergies. Since all cosmetic formulations contain water and silicone surfactants, the interaction of silicones with the formulation is critical to the formulation.