Considerations for Phospholipid Emulsifiers

Traditional emulsifiers allow formulators to create products with good performance and stability. However, newer innovations have allowed the formulator to incorporate natural emulsifiers for novel product textures. Such natural emulsifiers are particularly useful when they can serve a multitude of functions, such as stabilizing and developing the emulsion while also performing on a biological level. Phospholipids are a great example; they are natural and multifunctional, and can be used in the creation of both o/w and w/o emulsions. These biocompatible materials are valuable in cosmetics, but they also are commonly used in food and oral care. The present article explores how, when employed properly, these natural surfactants can provide benefits equal to those of synthetic emulsifiers.

Phospholipids in Skin

Phospholipids are composed basically of fatty acids, a glycerol unit, a phosphate group and a polar molecule. They have a hydrophilic head and two lipophilic tails, enabling them to maintain water while having a naturally high affinity to skin. These heads and tails can organize themselves into bilayers—structures similar to skin. Due to this capability, phospholipids are a major component of all cell membranes. In fact, one of their main functions is to build bilayers around cells. Since phospholipids perform active roles in skin, cosmetic formulators should be aware of their activity as well as know their formulation capabilities.

As a natural component of skin, phospholipid emulsifiers are well-tolerated; more so than ethoxylated surfactants. Due to their physical characteristics, they support the physiology of skin and are generally mild, with low irritation potential. Their mild nature was shown in vivo in a study evaluating the irritation potential of emulsifiers (see Figure 1). Hydrogenated lecithin (HPC) resulted in an irritation score of 0, while other known emulsifiers, such as cetearyl alcohol and cetearyl polyglucose (CACP), were rated at 10. In this study, sodium laureth sulfate (SLES) had the highest potential for irritation, with a score of 75. Other emulsifiers assessed included: polyglyceryl-3 methylglucose distearate (PMD), sorbitan stearate and sucrose cocoate (SSSC), polyglycerinlaurate (PGL), saponins-xanthan gum (SXG), glyceryl stearate citrate (GSC), and macromolecule and stearic acid and sodium chloride (MMSASC).¹

Phospholipids are the primary lipid found in the epidermis, accounting for nearly 50% of the total lipid content. However, the level of phospholipids decreases to less than 5% in the stratum corneum (SC). This is a result of differentiation, when the cells in the epidermis gradually move up into the SC. During differentiation, the phospholipids break down into free fatty acids to help build the “mortar” that holds the cells (corneocytes) in the SC together.

Primarily, of the phospholipids, phosphatidylcholine is found in the epidermis while phosphatidylethanolamine is found in the SC. The latter has a higher melting and as a result, the cellular membrane surrounding corneocytes is much more solid. These cells are more exposed than the keratinocytes found in the epidermis and therefore, they require a more solid membrane.

Phospholipids in Skin Care

As noted, phospholipids serve multiple functions in skin care. In addition to being emulsifiers, they can increase the delivery and efficacy of active ingredients on their own or as part of phospholipid liposomes.² Phospholipid emulsifiers themselves are associated with skin care benefits such as increased moisturization, and decreased inflammation and pigmentation.

Several studies³ have shown that phospholipid emulsifiers can increase the deposition and efficacy of active ingredients.⁴ Their ability to protect and carry actives to their proper destination allows formulators to efficiently use actives at lower levels. Phospholipids with higher phosphatidylethanolamine content deliver active ingredients further into the SC, as depicted in Figure 2; those with higher phosphatidylcholine content deliver deeper into the epidermis and dermis. Thus, phospholipids play a decisive role in the structure of any cellular membrane and contribute to essential transport and control mechanisms of skin functions.

As noted, phospholipids are able to generate cell-like structures with membranes built up in bilayers, similar to natural cell structures. These hollow vehicles, called liposomes, are used in the cosmetics and pharmaceutical industries to encapsulate active agents for penetration into the skin. They are able to penetrate the outer layers of the epidermis and reach the fibers of the skin below the SC. For this reason, liposomes can convey active components beyond the barrier layer.

Within these liposomes, the components of phospholipids show their benefits. The choline part takes over skin protection functions, and the linoleic acid improves the natural barrier function of the skin on a long-term basis, as it conforms into the ceramide.

Lecithin and its Derivatives

The most common phospholipid emulsifiers available in the cosmetic and personal care industry are lecithin and its derivatives. It is usually extracted and available from sources such as soybeans, but also has been extracted from eggs, milk, rapeseed, cottonseed and sunflower.^{5, 6} Some of the main properties of lecithin and its derivatives include strong emulsification, solubilization, moisturization and barrier repair benefits.

The use of lecithin in cosmetics is limited due to its instability against heat and oxidation. The pure form could cause discoloration in formulations. Therefore, more stable forms of lecithin are available, such as hydrogenated and hydroxylated lecithin. These materials provide the benefits of lecithin without the instability. Hydroxylated lecithin is made by reacting lecithin with hydrogen peroxide and acetic or lactic acid, or other organic acids. Hydroxylated lecithin is significantly more hydrophilic and stable than lecithin, and besides its superior stability and emulsification properties, it is a proven moisturizer.

When formulating, it is important to know the primary phospholipid found in the source of lecithin being used. For example, one can have a hydrogenated lecithin with a high content of phosphatidylcholine or a hydrogenated lecithin with a high content of phosphatidylethanolamine. The difference, from a formulator’s perspective, is that phosphatidylcholine content will have better deposition in the epidermis, as noted above, whereas an emulsion with a phospholipid high in phosphatidylethanolamine will have better deposition in the SC. If formulating an anti-aging or skin lightening product, a high-phosphatidylcholine emulsifier might be recommended. If formulating a moisturizer, a high-phosphatidylethanolamine emulsifier might be recommended.

There are other types and derivatives of phospholipids as well. As stated, phosphatidylcholine is the most abundant among the phospholipids, representing 35% of the phospholipids found in soybeans and almost 70% of those in egg yolks. Lysolecithin is another, produced by the partial hydrolysis of phosphatidylcholine, which removes one of the fatty acid groups.⁷ Lysolecithin has been shown to have good emulsifying, moisturizing and solubilizing properties while also increasing active skin penetration. Yet another is lysophosphatidic acid, a phosphated glyceryl ester produced by removing the choline group from pure lecithin. It is known to build barrier repair properties and increase the penetration of actives.

Formulating with Phospholipids

Phospholipids such as phosphatidylcholine are amphiphilic surfactants, meaning they have a hydrophilic head and lipophilic tail in a 1:1 ratio. This amphiphilic characteristic is responsible for their emulsifying properties. They can be added in either the water or oil phase, as their amphiphilic nature will not be realized until the emulsion takes place, and the phospholipid can develop droplets to orient its tails. In many instances, they are added toward the end of the oil phase, to prevent any prolonged heating.

When used as co-emulsifiers, in combination with other emulsifiers or molecules with a high HLB, the emulsifying properties of phospholipids are enhanced. To think of it simply, imagine phospholipids as being rectangle-shaped (see Figure 3a). A membrane around a cell that is built with rectangles will be weakened by “gaps.” Therefore, the addition of other surfactants, particularly hydrophilic ones, is recommended to form the strongest membrane. Now picture hydrophilic surfactants as triangles pointing down due to their larger hydrophilic head and smaller lipophilic tail (see Figure 3b). This builds a strong membrane, surrounding oil droplets and filling the “gaps,” achieving maximum stability.

In many emulsions, formulators include ethoxylated and other lipophilic emulsifiers, where lecithin will act as a support emulsifier. However, an emulsifier’s ability is developed when it is paired with the proper companions; therefore, lipophilic surfactants are not recommended due to their small hydrophilic head and a large lipophilic tail (see Figure 3c). Here, the structure of a phospholipid with a lipophilic surfactant makes a weak membrane. Microscopic images are also shown in Figure 3, where the smallest particle size was obtained by combining phospholipid emulsifiers with hydrophilic surfactants.

In the case of liposomes, formulators also have found that applying high pressure during the process of combining liposomes with oil-based substances can enable a low emulsifier formula. The droplet size in such formulations is very uniform and stable.⁸ High pressure liposomes enable formulators to introduce actives in light textures, without the need for unnecessary stabilizers and emulsifiers.

Additional formulating considerations one must be aware of when using phospholipids include the following. Phospholipids can change color if overheated, causing the emulsion to turn brown. Although lecithin derivatives are less prone to oxidation and discoloration, formulators still must be careful in the sequence and procedure of incorporating them. It is recommended to maintain the emulsion temperature at no higher than 85°C. In many cases, formulators add the phospholipids at the end of the phase, after all solids have been melted and the phase is uniform. Typically, phospholipid emulsions also require higher shear, as they do not have the same chemical energy as ethoxylated emulsifiers. As with all emulsions, whether a high or low emulsifier load of phospholipids is used, formulators must always manage their stability. Proper evaluation in high and low temperature settings is essential. Microscopic pictures of the emulsion should be evaluated throughout varying time frames, to ensure droplet size is uniform and to prevent coalescence.

Conclusion

Formulators have many choices when selecting vehicles that deliver biological benefits as well as provide an unforgettable sensory experience. Phospholipids are another tool with benefits across many product categories. They can be used to create stable products and ensure effective actives performance, making them valuable and highly researched.

References

1. PJ Frosch and AM Kligman, The Duhring chamber. An improved technique for epicutaneous testing of irritant and allergic reactions, Contact Derm 5 73–81 (1979)
2. L Budai et al, Liposomes for topical use: A physico-chemical comparison of vesicles prepared from egg or soy lecithin, Sci Pharm 81(4) 1151 (Oct/Dec 2013)
3. Puglia et al, Formulation strategies to modulate the topical delivery of anti-inflammatory compounds, J Cosmetic Sci 64(5), 341–353 (Sep/Oct 2013)
4. H Lautenschläger, Strong effect, Phospholipids in cosmetics, Kosmetik International (2) 38-40 (2003)
5. Lecithins and phospholipids: A simple guide to use and selection, American Lecithin Company, www.americanlecithin.com/lecithin_2009.pdf (Accessed Feb 4, 2015)
6. F LaBell, Dry lecithin benefits cakes, mixes, Prepared Foods 167(5)139 (May 1998)
7. F LaBell, Lecithin: A source of vital choline, Prepared Foods 166(10) 79 (Sep 1997)
8. A De and DN Venkatesh, Design and evaluation of liposomal delivery system for L-asparaginese, J Appl Pharm Sci 8(2) 112 (Aug 2012)