Gelation8,9


Up to this point, we have concentrated on lowering the surface tension of oil. While lowering of surface tension helps with oil spreading and changing the aesthetics of the formulation, there are several other interesting benefits that alkyl silicones can provide. When the alkyl content is high enough in the silicone backbone and a long alkyl chain is chosen (> C12) it can form a physical gel in oils. Physical gels are noncrosslinked solids. This means there is no covalent bonds structuring the gel and the gel can turn back into liquid form when a stimulus is applied to it. In the case of long alkyl chain silicones, the stimuli is heat. If the gel is heated above its melt point, a clear solution is obtained. Upon cooling, the gel will set back up into a solid. This is very different from silicone elastomers. Silicone elastomers are highly crosslinked polymers. The elastomer is made by a chemical reaction and once that reaction occurs, the gel can only be incorporated into another solvent by physical force (shearing, homogenising). 


When adding C26 dimethicone to olive oil a gel results. As the % of silicone in the molecule is increased, the air permeability as well as the clarity of the gel changes. The greater the % of alkyl in the product, the more occlusive the film when applied to the skin. When the amount of silicone goes up, the more air permeable the film when applied to the skin. Figure 9 shows this. 


Olive oil shown in the upper left hand corner is a yellow liquid. When 5% of a behenyl dimethicone is added to it a translucent, almost clear gel results. When behenyl dimethicone is synthesised with 35% alkyl, a gel is made using a different a:b ratio and the resulting gel is a little more opaque. As the percentage of alkyl is increased in these polymers, the gels become more and more opaque. 


US Patent 7,875,263 discloses a gel composition that comprises a specific solid alkyl silicone polymer combined with specific low viscosity liquid including natural oils, glyceryl esters, trimethylolpropane esters, and pentaerythritol esters.9 


The ability to evaluate the gelation ability of a specific alkyl dimethicone in gelation effectiveness, a term called CGC (Critical Gel Concentration) has been developed. The alkyl dimethicone is added in 10% weight concentrations ranging from 0 to 40. The gel is left for two hours and then turned upside down. The result is seen in Figure 10. 


As seen in Figure 10, the CGC for behenyl dimethicone in 50 cst fluid is in between 10% and 20%. Furthermore, gel concentration, feel and oxygen permeability can be altered by changing the structure of the alkyl dimethicone. It is also very important to note that this is not a fatty oil being thickened, but a dimethicone. 


Over the past few years, there has been an increased concern in application of trans-fats to the skin when using partially hydrogenated oils as butters, the ability to provide trans-fat free butters to the skin has become more and more important. The theoretical implications of making selfassembling gels that will melt and re-form upon cooling and alter rheology of oils is a major technical development.