Caprylyl Glycol: A Versatile Material to Boost Preservatives

In recent years, there has been a decline in the palette of options available for product preservation. Consequently, there is growing interest in preservative-boosting systems that use other combinations of antimicrobial choices, such as organic acids and aromatic alcohols.¹ In many formulas, the efficacy of the core preservative has been supplemented by the addition of cosmetic ingredients with one or more specified functions and an added boosting effect on the preservative system.

Caprylyl glycol, for example, is a versatile material used within personal care as a humectant and conditioning agent, as well as to modify product viscosity.¹ Other chemical names for caprylyl glycol include: 1,2-di-hydroxyoctane; 1,2-octanediol; and 1,2-octylene glycol. Its product applications include eye, skin and hair care, and to a limited extent, nail care; but its predominant focus is in leave-on dermal products.¹ In addition, caprylyl glycol has the functional benefit of increasing the efficacy of preservative systems.

Process and Manufacture

Caprylyl glycol is most often produced using catalytic oxidation of the 1-octene oxide or reduction of 2-hydroxyoctanoic acid (see Author Correction).² Industrial synthesis is commonly conducted using a two-step process, consisting of catalytic oxidation of the 1-octene oxide followed by hydrolysis with distillation and recovery of caprylyl glycol.⁵ The feedstock consists of alpha olefin and one or more oxidizing agents.

A decline in the palette of preservatives available for formulations has inspired a growing interest in alternative preservative-boosting systems.

Caprylyl glycol is manufactured synthetically using an unsaturated linear 1-octene alpha olefin.^{6, 7} General synthesis begins with the reaction of a 1-octene with hydrogen peroxide in the presence of formic acid. The hydrogen peroxide oxidizes the unsaturated α-bond on the 1-octene structure for conversion into the alkane oxide.^{2, 8, 9} When in combination with formic acid, hydrogen peroxide fosters an in situ production of performic acid to aid in the epoxidation of the 1-octene precursor.^8, 9

Formic acid has an added benefit as a solvent for the reaction and hydrolyzing agent for the epoxide ring on the alkane oxide, creating caprylyl glycol through saponification of resulting 1,2 alkane diol monoformate.⁹ Additionally, the use of hydrogen peroxide and formic acid has been found to reduce the residual hydrogen peroxide at the end of the reaction, which aids in suppressing unintended byproducts.⁸ Using this combination further helps to prevent the generation of odor-causing byproducts in the final caprylyl glycol, such as liberated sulfur, when using a sulfur catalyst and maintaining control of side reactions between the epoxy and diol that lower purity.^{8, 9}

The reaction mixture is continuously distilled during the process to increase purity and caprylyl glycol yield.² Monohydric alcohols are utilized as well in the process, to maintain the purity of the final glycol. Methanol is one type of alcohol that can be used to remove additional byproducts of formic esters formed from the reaction through transesterification and evaporation of methyl formate from the non-azeotropic mixture.^{8, 10} The final extraction of the caprylyl glycol is conducted using an organic solvent that is non-miscible with water, such as toluene, for the final recovery.⁹ Depending on the synthetic pathway and conditions, there may be variations in the purity of the caprylyl glycol.

Alternative routes of synthesis are emerging to create caprylyl glycol. Dihydroxylation catalyzed by osmium is one method being explored, although this is currently not feasible for commercial use. Osmium catalysts are more expensive and face issues relating to recycling the metal and waste, depending on the amount used for the catalyst.⁵

Chemical Structure and Properties

Caprylyl glycols are low molecular weight aliphatic organic compounds with two hydroxyl groups per molecule.^{11, 12} Commercially, they are supplied at approximately 98–100% active wt, with common use levels between approximately 0.5–5.0% wt.^2, 13 Under normal storage conditions the material is stable; however, oxidation may occur when exposed to air and heat. Stabilizers may be added to prevent the oxidation and degradation into carbonyl compounds and acid byproducts.¹¹ Depending on the storage temperature, heating may be required prior to use. Below 30°C, caprylyl glycol is present as a waxy white solid; at higher temperatures, it is a clear and colorless liquid with low odor.^13–15

In personal care, caprylyl glycol's utility is based on its amphipathic nature. The balance of hydrophobic and hydrophilic content alters the emolliency of the material due to the contribution from hydroxyl groups and the fatty alkyl chain (see Figure 1).^{15, 16} The hydrophilic moieties on the structure enable the absorption and retention of moisture from the atmosphere. During product use, the water absorbed by the caprylyl glycol structure aids in moisture-retention in the upper layers of skin, providing a moisturizing benefit.¹¹ The polyhydroxy component of the structure promotes the absorption of water from the atmosphere and permits water solubility at all use levels.

The combined hydrophilic and hydrophobic nature of the structure creates surface active properties and promotes its use as a solubilizer for active ingredients; as such, in combination with other systems, within formulas the ingredient can reduce the need for other solubilizers.^{12, 16} Its action as a solvent is based on the polyhydroxy moiety in combination with the carbon chain length of the structure.¹² Caprylyl glycols may also enhance visual clarity for formulations by incorporating immiscible ingredients into aqueous systems via coupling.

The robust nature of caprylyl glycol and associated multifunctional benefits provide an advantage for the development of future personal care products.

The structure of caprylyl glycol is also responsible for its antimicrobial benefit, which boosts total preservation efficacy in conjunction with a variety of preservatives. The alkyl chain length provides the preservation benefit, although its potency decreases rapidly beyond the C8 carbon chain length. This octyl carbon chain length has been shown to disrupt cell membranes or outer cytoplasmic membranes, accelerating anti-microbial activities.^{13, 15, 18, 19} Caprylyl glycol itself may function as a preservative against bacteria within oil and water formulations; however, it has limited efficacy against fungi.^{20, 21} Consequently, for broad-spectrum antimicrobial protection, caprylyl glycol is often used in conjunction with other preservatives in a system.^{13, 20, 22}

The enhanced antimicrobial efficacy that caprylyl glycol lends to other preservatives is based on its ability to increase the partitioning of preservatives into the aqueous solution, wherein microbial efficacy is desired.^17, 23 Caprylyl glycol’s function as a cosolvent imparts this action by increasing the aqueous wetting and solubility of some traditional preservatives, such as parabens and phenoxyethanol.^{21, 24, 25} Reductions in use levels of traditional materials are thus possible when efficacy is increased in this manner.²⁵

The optimum use levels of caprylyl glycol must be considered due its surface-active nature. Caprylyl glycol may be incorporated into the micelles of nonionic surfactants, thereby reducing the preservation of the system due to inavailability.²⁶ The interactions with emulsifiers and cleansers in the system may also promote instability of the formulation, in addition to lowering preservation efficacy, depending on the composition.¹⁶

Technology and Applications

The antimicrobial properties of caprylyl glycol have made it one of the predominant multifunctional ingredients to help reduce the use of traditional preservatives. It also holds promise to aid in preservation for applications in systems with extreme pH conditions and reduced free water content.^{27, 28} As stated, in conjunction with other preservatives, there is also an increase in the antimicrobial efficacy due to its innate structure and function; combination blends of caprylyl glycol with chloroxylenol or chlorphenesin or both have shown broad-spectrum preservative efficacy.²⁹

Finally, synergistic preservative systems including caprylyl glycol and enzymatic compositions demonstrate enhanced antifungal, antibacterial and antimicrobial efficacy.³⁰ This flexibility also allows for a variety of formats for use when considering non-conventional preservation systems and consumer benefits such as conditioning, emolliency and humectancy in cosmetics.³¹ The robust nature of caprylyl glycol and associated multifunctional benefits provide an advantage for the development of future personal care products.

References

(All websites accessed June 12, 2018)

1. happi.com/contents/view_features/2011-05-03/a-matter-of-self-preservation
2. online.personalcarecouncil.org/ctfa-static/online/lists/cir-pdfs/PR576.PDF
3. (see Author Correction)
4. (see Author Correction)
5. U Sundermeier et al, Recent Developments in the Osmium-catalyzed Dihydroxylation of Olefins, in Modern Oxidation Methods, JE Bäckvall (ed), John Wiley & Sons (2000)
6. EF Lutz, Shell higher olefin process, J Chem Educ 63(3) 202 (1986)
7. Chemistry and Technology of Surfactants, R Farn (ed), Blackwell Publishing, Ames, IA USA (2006) pp 119–121
8. JP Pat 2,008,007,463 A, Method for producing 1,2-alkanediol, T Yamamoto et al, assigned to Osaka Organic Chem Ind Ltd (Jan 17, 2008)
9. UK Pat 2,145,076 A, A process for producing 1,2-alkanediols, Z Mazour and P Radimerski, assigned to Ciba-Geigy AG (March 20, 1985)
10. US Pat 7,385,092 B2, Process for preparing alkanediols and alkenetriols having a vicinal diol group, C Osterholt et al, assigned to Evonik Degussa GmbH (June 10, 2008)
11. msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_091b/0901b8038091b508.pdf
12. Interfacial Phenomena in Apolar Media, HF Eike and GD Parfitt (eds), Marcel Dekker, Inc., New York (1987)
13. inolexcosmetics.ulprospector.com/en/na/PersonalCare/Detail/353/10336/Lexgard-O
14. minasolve.com/wp-content/uploads/2018/03/MINSO_BOOK2017_A4_REV2_SCREEN.pdf
15. H Iwata and K Shimada, Formulas, Ingredients and Production of Cosmetics: Technology of Skin- and Hair-Care Products in Japan, Springer Science & Business Media (2012)
16. eservices.personalcarecouncil.org/Science/15SS/paperlesssites/micro/schnittgerpresentation.pdf
17. academia.edu/23891939/Effect_of_ethanol_propylene_glycol_and_glycerol_on_the_interaction_of_methyl_and_propyl_p-hydroxybenzoate_with_Staphylococcus_aureus_and_Pseudomonas_aeruginosa
18. CosmeticsandToiletries.com/formulating/function/preservatives/premium-12-alkanediols-for-cosmetic-preservation-228198361.html
19. P Ziosi et al, Caprylyl Glycol/Phenethyl Alcohol Blend for Alternative Preservation of Cosmetics, Cosmetics & Toiletries 128(8) 538–549 (2013)
20. products.symrise.com/cosmetic-ingredients/actives/hydroliter-5/
21. A Thiemann and J Jänichen, The Formulator’s Guide to Safe Cosmetic Preservation, Personal Care Europe Nov 39–43 (2014)
22. cen.acs.org/articles/95/i12/new-tack-discovery-funding.html
23. A Varvaresou, Self-preserving cosmetics, International Journal of Cosmetic Science 31 163–173 (2009)
24. in-cosmetics.com/__novadocuments/5121
25. akema.it/pdf/KEM_DIOL.pdf
26. cbinet.com/sites/default/files/files/English_Don_Pres%20day%202-updated.pdf
27. midwestscc.org/blog2/wp-content/uploads/presentations/Jan2012CurrentTrendsinCosmeticPreservation.pdf
28. A Kerdudo et al, Optimization of cosmetic preservation: Water activity reduction, International Journal of Cosmetic Science 37(1) (2014)
29. US Pat 7,854,940, Broad spectrum preservation blends, D Ciccognani et al, assigned to Arch Chemicals Inc (Dec 21, 2010)
30. US Pat 20,080,226,568, Synergistic preservative systems and their use in topical compositions, E Rozsa et al, assigned to Playtex Products, Inc. (Sept 18, 2008)
31. DS Orth, Preservative-free (Self-preserving) Products, in Insights into Cosmetic Microbiology, Allured Publishing Corp, Carol Stream, IL USA (2010) pp 154