Glycerin with Humectant (Moisturizing) Properties

Humectant (Moisturizing) Properties of Glycerin, Propylene Glycol and Sorbitol: Effects and Differences on Skin

Abstract

Moisturizers are highly important products in the cosmetics industry. Their purpose is to prevent water loss from the skin and prevent skin dryness. Humectants, a group of moisturizers, have a wide range of applications. Glycerin (glycerol), sorbitol and propylene glycol are important organic humectants. These three humectants have different usage levels and application areas. In this study, the application areas, effects on skin and water-retention capacity of three different humectants were examined. Glycerin has a wider range of applications due to its high hygroscopicity and positive effect on skin development.

Introduction

Early research on moisturizers dates back to the 1950s. Moisturizers are substances that help make the skin surface and hair more moist and bring them to desired conditions, and they function by moisturizing the outermost layer of skin, the stratum corneum. However, the mechanism of action may differ depending on the chemical structure and the surface to which it is applied. Based on their behavior mechanisms, they are examined in four different groups. These different groups are given in Figure 1 [1]. Although moisturizers are commonly used alone, glycerin (glycerol), propylene glycol, sorbitol and their combinations are used in formulas at 2-10% [2].

Figure 1. Classification of moisturizers

Humectants are used in formulations for different purposes besides their effects on skin and hair. They affect the freezing point of products and the solubility of some compounds. They increase the viscosity of products, delay water evaporation, and act as preservatives and emulsion stabilizers. Additionally, they are used to dissolve oils or essences in products due to their ability to plasticize films of certain polymers. However, although many materials exhibit moisturizing properties, most are not suitable for use as moisturizing agents. Many properties are desired from an ideal humectant, and these properties are summarized in Table 1 [3].

Table 1. Properties of an ideal humectant [4]

Humectants are classified as inorganic, organometallic and organic as shown in Figure 2. Apart from inorganic materials, other materials are extensively found for use in personal care formulations.

Figure 2. Classification of humectants

Organic Humectants Commonly Used in Cosmetics: Glycerin, Propylene Glycol and Sorbitol

The physical properties of glycerin, propylene glycol and sorbitol are summarized in Table 2.

Table 2. Physical properties of glycerin, propylene glycol and sorbitol

Glycerin is a humectant widely used in cosmetic applications with a trihydric alcohol structure. It is a colorless, odorless, viscous liquid and stable under most conditions. It is used in food and cosmetic applications due to its pleasant taste and odor. It exhibits no toxic properties, is digestible and environmentally friendly. Being a versatile chemical, glycerin is a good humectant due to the degree of its hydroxyl groups and its high hygroscopic properties. However, the main problem with glycerin use is that it causes stickiness in creams and lotions to which it is added. To solve this problem, the amount of glycerin should be kept at an optimum level or it should be used in combination with sorbitol and similar other humectants [5]. In anhydrous makeup products, glycerin has a function in maintaining product integrity in addition to its humectant properties. It also has antimicrobial effects. Therefore, its areas of application are quite extensive. Propylene glycol is the second most commonly used moisturizer in personal care formulations. It is a transparent, odorless, viscous and water-soluble liquid like glycerin. Propylene glycol has a bitter taste, which is why its use in lip glosses and oral care products is limited. Sorbitol is a hexahydric alcohol, non-toxic and classified as GRAS (Generally Recognized as Safe) by the FDA as a food additive. It does not cause irritation to the face and eyes. The chemical structures of glycerin, propylene glycol and sorbitol are as shown in Figure 3. Sorbitol can be used partially or completely in place of glycerin in creams and lotions. However, due to sorbitol's poor solubility in alcohol and insolubility in organic solvents, its use in cosmetic products is limited.

Figure 3. Chemical structures of glycerin, propylene glycol and sorbitol

When examining the effects of humectants on skin, parameters such as relative humidity and temperature should be considered. The water-retention capacity of the same humectant can vary at different temperatures and relative humidity values. Table 3 provides water-retention capacities for glycerin, propylene glycol and sorbitol. When the three humectants are compared at 76% relative humidity, glycerin is found to have the highest water-retention capacity [6]. Since glycerin has approximately 1.35 times more water-binding capacity than propylene glycol and approximately 4 times more than sorbitol, it has greater use in creams and lotions [7]. Nevertheless, the effect of glycerin on skin is as important as its effect on the product to which it is added. When undiluted glycerin is used in solid soap, it significantly reduces water content loss. In a study conducted by Griffin and colleagues, it was reported that in an o/w emulsion containing 20% glycerin, water loss occurred at less than 95% ambient relative humidity. When water-based products come into contact with air, what causes humectants to retain water is not balancing the water level but slowing down or delaying water loss. In another cream formulation based on soap, although sorbitol is a weak humectant, it was stated to reduce water loss more than glycerin and propylene glycol. With 2% sorbitol and even at 30% ambient relative humidity, water loss decreased between 10-50% in periods ranging from 30 minutes to 48 hours. For this reason, different results can sometimes be obtained in anionic and nonionic formulations [8].

Table 3. Water-retention capacities of humectants at different relative humidity levels

In a study conducted by Sloan and Labuza [9], the water activity of humectants mixed with water at different concentrations is as shown in Table 4. Low water activity is a result of the humectant undergoing chemical interaction with water. It is observed that glycerin is used at lower concentrations at a certain water activity. However, in addition to water-binding capacity, the capacity of humectants to absorb water into the skin should also be an important performance evaluation criterion. In this respect, although propylene glycol values appear close to glycerin, it should be borne in mind that glycerin is present in high amounts in skin structure, while propylene glycol has a synthetic and volatile nature. Glycerin synthesis is regulated in channels called Aquaporin-3 in the pilosebaceous unit. Glycerin prevents the lipids in the stratum corneum from transitioning from liquid to solid state and thus improves skin barrier properties.

Table 4. Water activity (aw) of different humectants at 23°C

In a study conducted by Guillot and colleagues [10], the effects of 26 cosmetic products, including 15 glycols, sorbitol and glycerin, on ocular and skin irritation were evaluated. Glycerin caused very low irritation at both 100% and 20% concentrations. In a study conducted by Bisset and McBride [11], the effects of different glycerin concentrations on pig skin were examined in vivo. As a result of the study, maximum improvement in the epidermis and stratum corneum occurred at 20-40% glycerin levels. In another study, the effect of different glycerol concentrations on human skin was examined. The study was conducted in winter and glycerin was applied to dry skin on the lower leg of women and men aged 25-55. As a result of the study, a maximum increase occurred up to the 20% glycerin level, after which there was no significant increase. Considering the similarity of pig and human skin structures, a similar trend was observed in the studies in terms of moisturization degree as indicated in Figure 4 [11].

*Rate of skin improvement after two-week application (water was used as reference).

Figure 4. Effects of glycerin on human and pig skin [11]

Studies were conducted on pig skin to examine the moisturizing effect of many polyols on skin. Ethylene glycol, propylene glycol and 1,3-propanediol did not provide long-lasting beneficial effects on skin. This is because these liquids are volatile and evaporate easily from the skin. Polyols such as sorbitol, xylitol and erythritol provided only marginal benefits. These solids tend to crystallize on the skin surface when water evaporates [8]. In studies conducted on healthy and allergic skin, glycerin at 20% concentration does not cause any irritation or sensitivity. However, propylene glycol can cause irritation and sensitivity at 10% in healthy skin and at 2% in allergic skin [6].

Conclusion

Polyols such as sorbitol and propylene glycol are not effective in the long term because they are crystalline and volatile liquids. Sorbitol is not used alone on skin but is used together with glycerin. Sorbitol has lower hygroscopicity compared to glycerin and propylene glycol, and propylene glycol can cause irritation and sensitivity in healthy skin at low concentrations such as 10%, making glycerin much more effective and widely used than these two humectants. In conclusion, glycerin is a much more effective humectant on skin because it is hygroscopic and non-volatile. Merve Yılmazer / R&D Engineer / Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş. Bahar Kafadar / Senior R&D Engineer / Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş. Burak Saka / R&D Category Manager / Evyap Sabun Yağ Gliserin Sanayi ve Ticaret A.Ş.

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