Naturally Preserving pH-Neutral Formulations

Smart Combinations Natural-sourced 1,2-alkanediols are ideal ingredients for modern natural cosmetics due to their multifunctional properties, gentleness on skin and environmental friendliness. Formulations containing 1,2-alkanediols are self-preserving even at neutral pH. Smart combinations of 1,2-alkanediols with other natural-sourced additives increase formulation mildness while reducing costs. However, achieving successful results requires the correct selection among different 1,2-alkanediols and various additives. This article examines existing options by providing supporting evidence for safely and cost-effectively protecting natural cosmetic products.

Modern Cosmetics

In today's world focused on "healthy living," consumer demand for natural and responsibly produced cosmetic products continues to grow. Natural beauty products, with clean labels such as "vegan" and "GMO-free," are becoming the new ideal according to the principle of return to roots. Cosmetics launched to market with claims of being "natural" and "sustainable" have recorded an average annual growth of 35% over the past 10 years.[1] Of course, modern cosmetic products today must be gentle on skin and contain only those components strictly necessary for efficacy. In this context, scrutiny of preservatives has increased. Most traditional preservatives are derived from fossil carbons, which are harmful to the environment. Various components such as formaldehyde donors, methylisothiazolinone or parabens have faced media criticism due to controversial side effects. For this reason, safe and effective natural alternatives are in high demand. Currently, many natural cosmetics are preserved using carboxylic acids. However, these are only effective in acidic environments, which can irritate skin. Another common natural preservative is ethanol, which is known to be volatile and flammable. Ethanol is not accepted in any country due to these hazards. As a result, there are only a few solutions for preserving natural cosmetics with a pH value between 5.5 and 7. Selection should be made considering that the ideal pH range for light and skin-friendly cosmetic products is between 5.5 and 7.

Natural-Sourced and Sustainable

Using natural-sourced 1,2-alkanediols to preserve natural cosmetics is a relatively new option. Two options offering significant advantages over ethanol were recently launched; natural-sourced Pentylene Glycol and natural-sourced Caprylyl Glycol provide serious advantages with their moisturizing, odorless, non-volatile and non-flammable properties. 1,2-alkanediols offer more reliable solutions compared to carboxylic acids. They provide formula protection even in skin-friendly neutral pH ranges. 1,2-alkanediols are found in nature only as metabolites of certain microorganisms. For this reason, commercially available types are man-made products produced from fossil carbons or plant-based raw materials. Natural-sourced versions are produced in accordance with sustainable chemistry principles.[2] Natural-sourced Pentylene Glycol is derived from hemicellulose, while natural-sourced Caprylyl Glycol is obtained from natural oils. (Figure 1) Both components comply with COSMOS and NATRUE eco-certification standards and have a natural origin index of "1" according to ISO 16128. These components are multifunctional as humectants, humectants and emollients. In doing so, they comply with both current clean-ingredient beauty product concepts and can replace numerous traditional ingredients. This reduces the number of materials to be sourced, providing an advantage. [caption id="attachment_131457" align="aligncenter"] Figure 1. Ways to obtain multifunctional ingredients with sustainable chemistry principles.[2][/caption]

Supporting 1,2-Alkanediols

It is possible to provide preservation in cosmetic formulas by using only Pentylene Glycol in a formulation. However, in this case, to pass ISO 11930 tests or comply with European Pharmacopoeia standards, it is necessary to add Pentylene Glycol at 4% - 5% to the formula. This would be very costly. By using Pentylene Glycol in smaller quantities and combining it with other multifunctional additives, significant antimicrobial efficacy can be achieved. Such combinations reduce the required amount of Pentylene Glycol to approximately 1.5% - 2.5%. This effect is referred to as "boosting," referring to Pentylene Glycol's effect of disrupting microbial cell membranes. According to this, different antimicrobial agents assist one another in penetrating microorganisms. Thus, lower concentrations than would be used alone are sufficient for complete protection. Pentylene Glycol, by dissolving in water, also stabilizes microbial components in the aqueous phase where most microorganisms are found. In addition to all this, among Pentylene Glycol's other functions, which are known to be multifunctional, are moisturizing skin, aiding pigment dispersion, assisting in the hydration of gelling agents, and creating a soft and oil-free skin feel.[3] Compared to water-soluble Pentylene Glycol, the longer-chain Caprylyl Glycol is more lipophilic. Caprylyl Glycol's water solubility is limited to 0.75%, but its use at only 0.5% is sufficient to completely protect a cosmetic product from microbial invasion. Its extraordinary antimicrobial efficacy makes natural-sourced Caprylyl Glycol one of the safest and most cost-effective solutions for creating self-preserving natural formulations.[4] However, Caprylyl Glycol is a waxy solid that melts at 30-35°C. The melting process can take a long time and the melted product tends to form supercooled melts, making the freezing point unpredictable. As material freezes, its volume increases, which can cause barrels and pipes to burst. All of this makes Caprylyl Glycol a rather difficult component to handle. Combining natural-sourced Caprylyl Glycol with other components in a stable liquid mixture can help improve its use while preserving its benefits. Thanks to some boosting effects, 1,2-alkandiol concentration can be reduced to approximately 0.2% - 0.4%, which helps reduce costs and increase the softness of the final formulation. The simplest way to stabilize natural-sourced Caprylyl Glycol in liquid form is to combine it with inert additives such as water or glycerol. Adding water to a mixture of Caprylyl Glycol and glycerol does not lead to phase separation. Instead, stable liquids are formed with a melting point below 20°C, consisting of 75% - 80% Caprylyl Glycol. In this form, Caprylyl Glycol is easy to process and suitable for cold processing. Simple storage at room temperature is sufficient for it to remain in liquid form. This saves time and energy. It also provides safe use without heating or localized air extraction. Considering positive aspects such as low odor, low usage cost and lack of pH restrictions, Caprylyl Glycol is quite a good natural-sourced option.

Performance Evaluation Under Realistic Conditions

Each of two natural-sourced 1,2-alkandiol components, Pentylene Glycol and Caprylyl Glycol, was combined separately with two natural-sourced additives, Phenylpropanol and Glyceryl Caprylate/Caprate. The mixtures were formulated as storage-stable liquid mixtures. (Table 1). For comparison, the efficacy of two natural-sourced 1,2-alkanediols was also tested separately without additives. Natural-sourced Caprylyl Glycol was also applied in a physically stabilized liquid form ("E-Leen 8"). Table 1 summarizes the different combinations tested. The components listed in Table 1 were evaluated for antimicrobial performance in three different personal care products. Thus, common mass-market applications were also considered. • Natural Oil/Water Emulsion (Table 2 A), • Sulfate-Free Shampoo (Table 2 B), • Environmentally-Friendly, Cellulose-Based Wet Wipes (Figure 2). The mixtures and individual 1,2-alkanediols were tested in microbial tests according to ISO 11930 standard at different usage levels and pH values. Each product was separately inoculated with five different microorganisms (one yeast, one mold, three different bacteria). Microbial counts were determined after 7, 14 and 28 days. Based on the reduction in microbial counts, formulations were found to meet either A criteria (completely protected) or B criteria (partially protected), or failed neither criterion (unprotected). Table 3 shows a summary of the results obtained for the oil/water emulsion and sulfate-free shampoo. [caption id="attachment_131458" align="aligncenter"] Table 1. Combinations of natural-sourced 1,2-alkanediols with natural-sourced additives.[/caption]

Results and Discussion

Test results show that Pentylene Glycol and Caprylyl Glycol can be regarded as complementary components. Each of the two components is particularly suitable for specific applications. For Pentylene Glycol-based E-Leen Green A and E-Leen Green C mixtures, usage levels of 2-3% are relatively high (Table 3, entries 2 and 3). Therefore, these mixtures are particularly suitable for formulations that benefit from the functions of Pentylene Glycol, such as skin moisturizing, dispersing, wetting, emollient or solubilizing properties. Since Pentylene Glycol is a water-soluble 1,2-alkandiol, it has a low tendency to migrate into oil phases or become trapped within micelles. On the other hand, it has the ability to stabilize lipophilic components within aqueous phases. For all these reasons, Pentylene Glycol is particularly suitable for formulas that are rich in oils or contain high concentrations of surfactants. [caption id="attachment_131459" align="aligncenter"] Table 2. Natural oil/water emulsions and sulfate-free shampoo subjected to microbial testing.[/caption] [caption id="attachment_131460" align="aligncenter"] Table 3. Microbial test results according to ISO 11930 standard.[/caption] As a result of testing, it was proven that the two mixtures containing Pentylene Glycol provided more versatile solutions compared to other mixtures. In addition to the oil/water emulsion, it also provided protection for sulfate-free shampoo at acceptable usage levels. (Table 3, entries 2 and 3) The tested sulfate-free shampoo is based solely on mild surfactants. Protection of such formulations can be very difficult. In contrast, many common cleaning products contain sulfate or sulfonate surfactants. Products with these properties provide natural antimicrobial effects. As a result, sulfate or sulfonate-containing products are generally self-preserving. Therefore, they can be protected with smaller amounts of antimicrobial additives. During testing, Caprylyl Glycol was proven to be an economical preservative. Caprylyl Glycol offers less versatile solutions compared to Pentylene Glycol. The main reason for this is the lower amount of Caprylyl Glycol used. The tested oil/water emulsion was protected with small amounts of Caprylyl Glycol. The physically stabilized liquid form "E Leen 8" (Table 3, entry 5) is as effective as the pure form "A-Leen 8" (Table 3, entry 4). Additionally, the application of the liquid form is much easier. E-Leen P8, a boosted mixture containing Phenylpropanol, was proven to be even more effective than pure 1,2-alkandiol. E-Leen P8 provides enhanced safety for challenging formulations (Table 3, entry 6). On the other hand, the skin-friendly mixture E-Leen GC 8 (Table 3, entry 7) allowed complete protection of the oil/water emulsion to be achieved at lower Caprylyl Glycol levels. Interestingly, despite its tremendous antimicrobial activity, relatively high concentrations of Caprylyl Glycol and related mixtures were found to be required to protect sulfate-free shampoo. (Table 3, entries 4-7) This is because Caprylyl Glycol is lipophilic. Because Caprylyl Glycol is lipophilic, it is more easily retained within surfactant micelles compared to the hydrophilic Pentylene Glycol. The addition of Phenylpropanol provided limited improvement, as aromatic alcohols also tend to be partially inactivated within nonionic surfactants. In contrast, solutions containing Caprylyl Glycol can be a high-performance and economical choice for cleaning products containing sulfate or sulfonate surfactants.

The Challenges of Protecting Wet Wipes

The final step for a green and sustainable antimicrobial is protecting wet wipes. With their water content and large surface areas, wipes are highly conducive to microbial growth. The more natural the wipes, the more ideal they become for microbes. To further challenge the testing, an eco-friendly, airlaid cellulose-based tissue supplied by Asutec was used.[5] The water of the tested wet wipe consists only of a mixture of water, a natural-sourced detergent (NatragemTM S140 NP, Croda) and a citrate buffer (Figure 2). This aqueous solution was preserved with E-Leen 8 (1%), E-Leen P8 (1%) or E-Leen GC 8 (1%), respectively. Two weight equivalents of wet wipe solution were applied per weight equivalent of a dry wipe. Wipes were inoculated with microbes and incubated in closed plastic bags. Microbial counts were enumerated after 7, 14 and 28-day incubation according to ISO 11930 standard. (see Figure 2). The results of the challenge testing are summarized in Figure 2. Unprotected wipes showed very high growth for all tested microbial types. (results not shown) Wet wipes protected with E-Leen 8 and E-Leen P8 passed testing and met Criterion A. The E-Leen GC 8 mixture demonstrated broad-spectrum antimicrobial activity, but required a higher usage level than 1% to meet Criterion A. These results demonstrate that solutions containing natural-sourced Caprylyl Glycol are a suitable option for producing skin-friendly wet wipes. Thus, self-preserving wipes simultaneously provide skin care benefits. Use of irritating or persistent additives can thus be avoided. These mixtures are 100% natural-sourced, COSMOS and NATRUE compliant and readily biodegradable. Low usage levels ensure economical and efficient production. [caption id="attachment_131461" align="aligncenter"] Figure 2. Formulation of wet wipe solution and results of microbial challenge tests on impregnated wipes. Red bars show target values for microbial count reductions after 28-day incubation.[/caption] [caption id="attachment_131462" align="aligncenter"] Figure 3. Natural-sourced mixtures containing Pentylene Glycol and Caprylyl Glycol offer complementary functionalities.[/caption]

Summary

Natural-sourced versions of the two 1,2-alkanediols Pentylene Glycol and Caprylyl Glycol have proven to be versatile ingredients. Combined with other natural-sourced additives, they represent an economical option providing time and cost savings for the production of self-preserving natural cosmetics. (Figure 3) Natural-sourced Pentylene Glycol is suitable for many products due to its multifunctional properties. With its high water solubility, it can be used as an additive in oil-based emulsions. It provides a lighter skin feel with faster absorption. Another positive aspect is its low tendency to be trapped in micelles, a feature particularly beneficial for antimicrobial additives. Natural-sourced Caprylyl Glycol is a high-performance alternative suitable for common oil/water emulsions and surfactant products. Due to its low usage level, it is an unrivaled cost-effective option. It is particularly suitable for eco-certified wet wipes. Natural-sourced Phenylpropanol and Glyceryl Caprylate/Caprate are suitable as complementary diols. Phenylpropanol is resistant to high and low pH values. It provides additional support to antimicrobial efficacy and provides enhanced protection for challenging applications. It is a safe and pleasant perfume component soluble in water at appropriate concentrations. Glyceryl Caprylate/Caprate provides safe and cost-effective protection for sensitive skin products and is a component that does not damage skin. Due to its lipophilic nature and low water solubility, its primary application area is oil/water emulsions. Through these recommendations, cosmetic manufacturers and consumers can benefit from products with fewer ingredients and lighter formulations. Being sustainably sourced, it perfectly meets today's demands and expectations.  

Author: Markus Nahrwold Technical Director Minasolve

Translator: Gülce Çay Sales Manager Arerko

References:

[1] MINTEL GNPD: number of cosmetics launched worldwide in 2011 – 2020 with claim "sustainable". [2] P.T. Anastas, J. C. Warner, "Green chemistry: theory and practice", Oxford University Press, 1998. [3] N. Konaté, M. Nahrwold, "Sustainably Sourced Pentylene Glycol – a Green Allrounder", SOFW Journal (English Edition) 2016, 142, 44-48. [4] J. Toliver, S. Narasimhan, "Caprylyl Glycol: A Versatile Material to Boost Preservatives", Cosmetics & Toiletries, July/August 2018, vol. 133, N° 7, 18-23. [5] http://www.ascutec.de/flushable-news-english/