A very important property of alkyl silicone polymers is their solubility in oils. But even more importantly, is the realisation that these materials act as surfactants.  Meaning that they are soluble (that is clear when added), the molecules go to the air / oil interface, lower surface tension, have a CMC and alter the spreadability of the liquid.  The term CMC here is used to describe the lowering of surface tension. That is, they first lower surface tension by increasing their concentration at the interface, then when the interface is fully saturated a minimum value in surface tension is reached. Upon addition of more alkyl silicone, the surface tension remains the same, and the silicone must form a macrostructure in the bulk solution. Silicones take up more free volume at the interface due to the larger Si-O bond angle, this allows them to function at low concentrations. The very rules developed by Dr Rosen for oil and water based surfactants, work in oil / silicone systems! 


Cetyl dimethicone is the first silicone shown in Table 2. The silicone has a C16 (cetyl) group (oleophilic) and dimethicone group (siliphilic). It is a surfactant when added to oils.


Additionally, in the plot of surface tension against concentration, one sees what looks to approximate a critical micelle graph for standard oil/water based surfactants. Table 3 shows this. Clearly, one can ask what is the ‘CMC’ of cetyl dimethicone in soybean oil!


However, not all plots of this type give a clear CMC break point. A point at which adding more surfactant does not continue to drop the surface tension. We have therefore invented another term, that term is RF50. This new term represents the concentration of silicone surfactant added to reduce the surface tension of oil from around 30 to 25 Dynes/cm. The lower the RF50, the better able the silicone surfactant is to compete with the fatty surfactant for surface and the more efficient the silicone surfactant will be. This technique allows the design of molecules that will be optimised for a formulation. Not only can surfactant systems be evaluated but complex formulations can be evaluated, by simply defining the fatty surfactant’s surface tension as the formulation’s initial surface tension. Not only surface tension, but also foam and the like can be tested and optimised by evaluating foam as the property rather than surface tension.  Figure 4 shows the concept and calculation.


The lack of a clear break point when adding cetyl dimethicone to mineral oil makes the question of a CMC value somewhat difficult to answer, but there is a clear RF50. Figure 5 shows the calculation and graph. 


It is important to note that there are different cetyl dimethicones that vary in alkyl content, molecular weight and polymer structure (comb, linear) that all  fall under the same INCI name. This RF50 technique allows a formulating chemist to rapidly determine how cetyl dimethicones from different suppliers affect their formulation. It is also useful when trying to establish and optimise a structure function relationship between several alkyl silicones having a different a:b ratio in different oils. Also, blends of oils can be easily evaluated using this method and optimised.  


RF50 is 2%. RF50 as low as 0.2% by weight have been observed for soybean oil. This low surface tension makes the oil spread on the skin and feel like cyclomethicone (D5). 


When C26 dimethicone is added to soybean oil a graph like that shown in Figure 8 can be prepared. It clearly resembles the graph of sodium lauryl sulfate added to water, except the surface tension numbers on the y axis are much lower that observed in the SLS / Water analogous system. 


The ability to lower surface tension is dependent upon the type of oil chosen and how it interacts with the alkyl silicone added. Ethyl methicone, a silicone with a very short alkyl chain is much more effective in lowering surface tension than cetyl dimethicone in soybean oil. Ethyl methicone has an RF50 of 2.0% in soybean oil, while cetyl dimethicone has an RF50 of 20.0% in soybean oil.  Figure 7 shows a RF50 chart for ethyl methicone in soybean oil.