A Solution
to the Silicones Quandary

The Cosmetic and Personal Care Industry has enjoyed a lengthy relationship with silicone-based ingredients.  In the Cosmetic Industry, these ingredients appear under the broad term ‘siloxanes’¹. 

The versatility of the silicone atom to be incorporated into numerous polymeric structures from low molecular volatile siloxanes to high molecular weight silicone polymers and elastomers like dimethicones, provides a plethora of opportunities for formulators. In 2011, the Cosmetic Ingredient Review (CIR) determined that siloxanes and, in particular, cyclotetrasiloxane, cyclopentasiloxane, cyclohexasiloxane, and cycloheptasiloxane, are “safe in the present practices of use and concentrations” in cosmetic products². 

However, siloxanes are not entirely benign in use and recent focus has begun to examine their human safety in use and their residence in the environment, especially in waste-water treatment facilities where the bulk of siloxanes end up^3-8. 

Due to these emerging challenges, there has been a trend to begin removing siloxanes, in particular the low molecular weight cyclic siloxanes, from cosmetic and personal care formulations^9,10. Regardless of these emerging concerns, one fact is clear, siloxanes are not natural. They are derived principally through synthetic reactions employing petroleum-based starting materials. The argument has been made that even petroleum could be considered “natural” as it is the byproduct of extinct plant and animal biomass¹¹.  This gets to the core of arguments around what constitutes a truly natural ingredient and how can an ingredient be truly classified as 100 percent natural.

The Natural versus Synthetic Arguments

Dorea Reeser, a frequent contributor to Scientific American, suggested that the natural versus synthetic argument is an area often filled with misunderstandings. 

She suggested that there are three critical misconceptions of the natural versus synthetic argument including: 1) Synthetic chemicals are more toxic than natural chemicals, 
2) Organically grown food is better for you because it’s all natural, and 3) Synthetic copies of natural chemicals are not as good for you¹². 

Sometimes, substituting a synthetic material with a natural one is not an appropriate direction to go in. 

However, in the case of natural alkanes, these arguments are somewhat moot as the natural alkanes are, for all intents and purposes, equivalent to their petroleum-based counterparts. What is different is their sourcing. 

Synthesis of Natural Alkanes from 
Vegetable Biomass

Historically, low molecular weight linear alkanes have been derived from petroleum-based starting materials and, as such, are not recognised as being natural. But, even as synthetic variants, these ingredients have been commonly used in cosmetic and personal care products. Typically, they function to provide various benefits including spreading, solubilisation and textural benefits to formulations.  For many companies, the broad functionality of the alkanes can significantly outweigh the fact that they are not natural.  

Most regulatory bodies that determine naturalness allows for certain mechanical processes such as extraction (with natural solvents like water or non-petroleum-derived ethanol), grinding, crushing, distillation, and the like.  Likewise, certain kinds of chemical reactions remain available to manufacturers that can be used while still maintaining natural claims.  These include, for example, esterification, hydrolysis, hydrogenation and saponification.  

The manufacture of linear, low molecular weight alkanes proceeds from starting ingredients derived from vegetable-based resources such as fatty acids and fatty alcohols.  Common sources of these kinds of starting materials can include oils, such as palm, coconut or safflower.  Usually, the oils are isolated as triglyceride fats.  Subsequent hydrolysis of the triglycerides affords the free fatty acids and glycerol. 

The resulting natural fatty acids isolated as described above form the starting materials for manufacture of natural alkanes, Figure 1.

The manufacturing operation includes two physical processes: distillation and filtration and two chemical manipulations: hydrogenation and dehydration.  Through these processes, the starting fatty acids are reduced to fatty alcohols, purified by distillation, and then, through the process of dehydration (i.e., removal of a water molecule), the fatty alcohols are converted to linear alkenes.  

Alkenes still have carbon-carbon double bonds that need to be further reduced via hydrogenation to form the final linear alkanes. 

The various carbon chain lengths produced are C9-C12 alkanes, C13-C14 alkanes, C15-C19 alkanes, and C18-C21 alkanes.  

Vantage offers four unique products. Jeechem NDA-LC (INCI: C9-12 Alkane), Jeechem NDA-CC (INCI: C13-14 Alkane), Jeechem NDA-HC (INCI: C18-21 Alkane), and Jeechem NDA-5 (INCI: C15-19 Alkane) that encompass each of the carbon chain length ranges noted above, respectively.  Interestingly, three of the products indicated above, the Jeechem NDA-LC, Jeechem NDA-CC and Jeechem 
NDA-5, are liquids at room temperature while the fourth, Jeechem NDA-HC (the C18-C21 fraction), is a low melting solid at room temperature. The crystals of purified Jeechem NDA-HC will quickly melt when applied to the skin and the melting process of the alkane, requiring absorption of heat from the body, provides an interesting cooling effect on the skin. This cooling effect is, however, lost if the alkane is dissolved. 

The physical properties of each product can be seen in Table 2.

The corresponding parameters for cyclopentasiloxane (D5) are: Flash Point (73 oC), Molecular Weight (370.77 g/mol), Viscosity @ 30 oC (3.74 cP) [16].  From these comparisons, the Jeechem NDA-LC has the closest flash point to D5, but the Jeechem NDA-5 has the closest kinematic viscosity.  Where the linear alkanes and the cyclic siloxanes differ more profoundly is in their surface tension defined as “o/(mN/m)” which dictates their ability to spread upon contact with a surface.  For D5, the surface tension is 18.29 mN/m.  For the Jeechem NDA-LC, the surface tension is 22.92 mN/m.  The linear alkanes will behave slightly differently than cyclic siloxanes with how they spread upon contact with a surface like hair or skin. The surface tension may also impact the tactile properties of each.  Vantage anticipates that formulators using the linear alkanes will likely experiment with combinations of each to attain desirable properties that meet their formulating needs.  

The linear alkanes do have one advantage over their siloxane counterparts.  They will tend to have a superior ability to dissolve other cosmetically important ingredients.  
Table 3 shows various solubility parameters for a variety of cosmetic ingredients in 30% linear alkane.  

Likewise, the linear alkanes may function very well to help solubilise volatile fragrances or possibly organic sunscreen actives.  This can be particularly important for formulators who are trying to maintain a high level of natural ingredients in their formulations.  

Vantage has also explored the use of the natural alkanes as pigment dispersing solvents. The mineral pigments commonly used in dispersions are Titanium Dioxide (TiO2), Zink Oxide (ZnO), and the various iron oxide pigments like yellow, red, and black iron oxide. By themselves, the alkanes will not be able to maintain a pigment suspension for extended periods of time. This can be overcome by co-formulating the dispersions with various rheology controlling ingredients that can improve the suspending capabilities of the solvents.  If settling does occur, it is desirable that the dispersion can be re-mixed with simple operations such as use of a mixer or rolling of the dispersion container prior to use.

Formulating with the Jeechem NDA 
Linear Alkanes

Formulating with the Jeechem linear alkanes is straightforward and would occur as would be typical for use of silicones oils.  Generally speaking, the alkanes will either form, or be added to, the oil phase of an emulsion. They may function in both O/W and W/O emulsions or they may even have some interesting opportunities in three phase emulsions (W/O/Silicone, for example). Each alkane has a flash point that formulators need to be cognizant of to make sure that headspace vapors do not accumulate, resulting in a possible vapor flash.  These concerns are common for organic ingredients and should not be a surprise to skilled formulators and operators. Operators handling large scale quantities of the linear alkanes should wear respirators or vapor facial masks to minimise inhaling the vapors. 

Biodegradability of Low Molecular Weight Linear Alkanes  

It is often common for people to speak of the biodegradability of various ingredients. However, it is important when discussing such things to keep in mind how ingredients are used. For materials that find their way into waterways and, ultimately to water treatment plants, one of the more accurate measures of molecules’ ability to decompose harmlessly into the environment are Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD)¹⁷.  These values are dependent, of course, on the individual waste water treatment facility and how effectively the water is treated at the site. Materials that do not decompose typically end up in the final “sludge” taken from the treatment process which is typically disposed of by landfilling or incineration and then landfilling. 

Zajic et al. examined the BOD and COD for various petroleum-based hydrocarbons, similar in most respects to the natural alkanes described here¹⁸.  Depending on the chain length, they reported BOD and COD values that were between 67–98% of theoretical values indicating that in properly functioning waste water treatment facilities, the low molecular weight linear alkanes should be quite degradable.  Any alkanes that don’t degrade that might end up in sludge would be easily incinerated if the waste water treatment facility employed incineration to treat sludge.

Summary

Vantage continues to push the envelop to introduce new and highly functional ingredients to the Cosmetic and Personal Care Industry.  With the current global focus on natural ingredients in the market and with the high concern around use of low molecular weight cyclic siloxanes in finished products, the need for innovation could not be greater.  Through careful development and sourcing, Vantage can provide customers with a variety of unique, naturally-derived, linear, low molecular weight alkanes that will allow formulators to develop new textures and new natural products to excite their demanding customers.

References:

1. Berthiaume MD. Silicones in Cosmetics. In: Principals of Polymer Science and Technology in Cosmetics and Personal Care.  Ed., Goddard ED, Gruber JV. Francis Taylor, NY, 1999, Pages 275-324.
2. Johnson W et al., Safety assessment of cyclomethicone, cyclotetrasiloxane, cyclopentasiloxane, cyclohexasiloxane and cycloheptasiloxane. Int J Toxicol. 2011;30(Supp):149S-227S.
3. Mojsiewicz-Pieńkowska K, et al., Evolution of consciousness of exposure to siloxanes-review of publications.  Chemosphere. 2018;191:204-217.
4. Dekant W, et al., Toxicology of decamethylcyclopentasiloxane (D5). Regul Toxicol Pharmacol. 2016;74 Suppl:S67-76.
5. Klaunig JE, et al., Biological relevance of decamethylcyclopentasiloxane (D5) induced rat uterine endometrial adenocarcinoma tumorigenesis: Mode of action and relevance to humans. Regul Toxicol Pharmacol. 2016;74 Suppl:S44-56.
6. Jean PA, et al., Chronic toxicity and oncogenicity of decamethylcyclopentasiloxane in the Fischer 344 Rat. Regul Toxicol Pharmacol. 2016;74 Suppl:S57-66.
7. Li B, et al., The occurrence and fate of siloxanes in wastewater treatment plant in Harbin, China. Environ Sci Pollut Res Int. 2016;23:13200-9.
8. Bletsou AA, et al., Mass loading and fate of linear and cyclic siloxanes in a wastewater treatment plant in Greece.  Environ Sci Technol. 2013;47(4):1824-32.
9. Lee S, et al., A nationwide survey and emission estimates of cyclic and linear siloxanes through sludge from wastewater treatment plants in Korea. Sci Total Environ. 2014;497-498:106-112.
10. Gutierrez D. New Toxic Chemical in Cosmetics: D4 and D5 Siloxanes. Nat News, April 2009.
11. Marta. Silicones - Should we avoid them? Truth in Aging, October2008.
12. Johnson C. Greener Formulation Options. NYSCC SYMPOSIUM 2018 - Sustainable Cosmetic Science. October 2018.
13. Reeser D. Natural versus Synthetic Chemicals Is a Gray Matter. Sci Am. April 2013.
14. Garside J et al., Verifying natural ingredients via carbon-14 testing.  Personal Care Mag. 2017; Nov: 63-65.
15. Schultz H. Carbon-14 verification sets curcumin ingredient apart, supplier says.  https://www.nutraingredients-usa.com/Article/2017/07/14/Carbon-14-verification-sets-curcumin-ingredient-apart-supplier-says , Last Accessed 060623.
16. ASTM D6866-18, Standard test methods for determining the biobased content of solid, liquid and gaseous samples using radiocarbon analysis, ASTM International, West Conshohocken, PA, 2018.
17. Wikipedia Search: Decamethylcyclopentasiloxane.
18. Zajic J. et al., BOD and COD Analyses on Paraffinic Hydrocarbons. J Am Water Works Assoc. 1970;62:784-786.