Surfactants are used in many industries for various reasons but their functionality primarily rests in the ability to lower the surface tension of a liquid and the interfacial tension between liquids. They may act as detergents, wetting agents, emulsifiers, solubilizers, foaming agents and pigment dispersants. In personal care, surfactants are used to create lather and cleanse the skin and hair of dirt and excess sebum; in fact, soap is the oldest surfactant still used today.
Emulsifiers are an important category of surfactants for personal care. They are essential to produce creams or lotions by enabling oil and water/aqueous components to mix and remain stable over a long period of time. Choosing an optimum emulsifier system helps create evenly dispersed, small droplets, thus providing kinetic stability and an elegant texture, skin feel and appearance. Typically, emulsions have a milky white, opaque appearance due to the type and levels of emulsifiers used; however, there are microemulsions that appear clear or transparent to the human eye. These are used in specialized applications, such as enhancing skin permeation of active substances.1 Emulsifiers often impart a specific texture or sensory aspect to the end product, so their selection is important for marketing appeal as well as technical aspects.
The global market for personal care surfactants was estimated to be around US $10.5 billion in 2012.2 Green or renewably based surfactants, although initially a small market base, are predicted to grow rapidly over the next decade, as new technologies emerge to produce lower cost raw materials. Currently, many certified organic shampoos and body washes use soaps such as potassium cocoate as their primary cleansing agent, in part due to a lack of suitable, organically approved foaming alternatives. This article reviews the use of modern surfactants and emulsifiers developed based on a green and eco-conscious philosophy.
Concerns and Claims
Certain commonly used surfactants have come under fire for different reasons. Sulfate-based surfactants, particularly sodium lauryl sulfate (SLS), are known to cause stinging upon contact with the eyes, and to irritate the skin. They, therefore, are often formulated with other, milder surfactants to ameliorate irritancy potential. Regardless, sulfate-containing products have become associated with harsh shampoos that strip the hair of oils or color, and high foaming body washes that are not suitable for sensitive skin. This has led to the sulfate-free claim used on a number of shampoos to indicate the omission of alkyl sulfate or alkyl ether sulfate surfactants. Such products are often marketed as prestige and touted as being milder.
Similarly, PEG-free is a claim seen on skin creams or lotions. This relates to the use of ethoxylation based on petrochemically derived ethylene oxide to produce polyethylene glycol (PEG) derivatives. Issues fueling this controversy are reports by nongovernmental organizations (NGOs) of what they feel are unacceptably high levels of 1,4-dioxane in some cleansing products, such as baby shampoos. Although the use of 1,4-dioxane itself for cosmetics is banned under European legislation—being a petroleum-derived, suspected carcinogenic compound—it can be generated during the manufacture of ethoxylated surfactants such as sodium laureth sulfate (SLES).
There is little consensus between NGOs, manufacturers and regulatory bodies, such as the FDA and Health Canada, over what constitutes a safe level of 1,4-dioxane, and impurity levels can vary between manufacturers of the same surfactant materials. Reputable raw material suppliers closely monitor this and strip out the 1,4-dioxane so that only trace amounts remain, i.e., typically less than 10 ppm. The suspicion is that less reputable manufacturers of low priced commodity materials might not be so vigilant. Whatever the scientific validity of these concerns is, the damage has already been done in the mind of the consumer, particularly in some mothers of young children and consumers generally looking to purchase “eco-friendly” products.
In addition, some suppliers of raw materials are also promoting products as nitrosamine-free; nitrosamines have been found to act as carcinogens when tested at high levels in animals. This marketing relates to the absence of alkanolamines, particularly diethanolamine, which under certain circumstances could have the potential to release nitrosamines in a cosmetic product over time. Materials such as cocamide DEA, or coconut diethanolamide, commonly used as a foam boosters and stabilizers in some conventional products, are being phased out of many green cleansing products. There is even a new trend for betaine-free ingredients since these workhorse amphoteric surfactants, including cocoamidopropyl betaine (CAPB) or coco betaine, have been associated with skin allergies by some dermatologists. However, it would appear that the reported cases may be due to irritant reactions rather than true allergies from impurities such as amidoamine and/or dimethylaminopropylamine in less well-controlled raw materials. Certain CAPB grades, coco betaine and other similar betaines, in fact, are currently approved for use in Ecocert Greenlife-certified natural products.3
Over and above human health concerns, the world of personal care is being driven to provide more sustainable alternatives for all types of cosmetic ingredients. For surfactants, factors such as biodegradability, aquatic toxicity and the fate of these materials in the environment are just as important. This has led to a plethora of new raw material launches designed to provide greener alternatives to conventional petrochemical-derived surfactants.
One might define a naturally derived surfactant as having a molecular structure containing at least 50%—preferably much more—of sustainably sourced, renewable raw materials.4 Further, necessary chemical modifications should ideally be carried out using only mild reaction processes, such as those accepted by the major certification standards bodies, especially if the material is to be certified for natural cosmetics. However, there is little consensus between many of these bodies regarding allowable green chemistry processes, and their selection may seem somewhat arbitrary (see Table 1 and Table 2).
For example, Ecocert8 and the Cosmetics Organic Standard (COSMOS)9 currently allow the use of alkyl sulfates such as SLS; sulfosuccinates including disodium lauryl sulfosuccinate; amphoacetates, e.g., disodium cocoamphodiacetate; betaines such as CAPB; and certain grades of sodium lauryl sulfoacetate. Of this list, only SLS and similar alkyl sulfates would be acceptable by the International Natural and Organic Cosmetics Association (NATRUE) standards.10 Amphoacetates and betaines, however, are due to be phased out of the newly harmonized COSMOS standard when it is fully implemented in 2015. While raw materials are being launched to fill the void this will create, it is still challenging to check off all the desirable safety, green and environmental requirement boxes.
In addition to those already mentioned, other factors include low environmental toxicity, and reduced energy consumption and carbon dioxide emission during processing and distribution of the materials. Further, natural surfactants and emulsifiers should be sourced in an ethical or socially responsible manner. Also, water is a precious commodity in many parts of the world, so reduced water footprint is another factor to consider. It almost goes without saying these days that the material should not use animal-derived ingredients, and for Europe in particular, should not be subject to animal testing. Finally, many companies prefer products derived from organic agriculture, whether wholly or in part.
What constitutes the “perfect” green surfactant? First, it should perform as well as equivalent synthetics when used alone or in combination with other green surfactants. Important performance characteristics include foaming capacity, i.e., lather volume, texture and longevity; sensory properties such as the residual skin and hair feel; and cleansing ability. It should respond to salt and/or other commonly available rheology modifiers to thicken or add body to the end product, and it should be non-toxic and non-irritating, preferably with an established safe history of consumer use. Specifications might include an acceptably low odor and color, consistency in quality from batch to batch, and no tendency to discolor the formulation. For emulsifiers, the stability of the finished cream or lotion is paramount, along with the sensory aspects of the product, which as noted, can be heavily influenced by the emulsifier type and level.
Considering all these constraints, what materials are available to the green product formulator, and are any of them close to “perfect”?
Saponins: A number of physically extracted natural surfactants contain saponins as their primary active foaming agents. More than 1,000 different saponin structures are found in plants, which can be divided into the three main categories of triterpene glycosides, steroid glycosides and steroid alkaloid glycosides. An example of a typical chemical structure is shown in Figure 1.
One saponin-containing plant useful to the cosmetics industry is the Indian soapnut tree, or Sapindus mukorossi. Interestingly, dried soapnut shells can be used in small muslin laundry wash bags to replace synthetic laundry liquids and powders. Soapnut extracts sold for cosmetics possess anti-irritant, antimicrobial and anti-fungal properties. The lather produced is quite rich, and the commercial materials are promoted as natural replacements for SLS, particularly for sensitive applications such as oral care.
Two other major commercial sources of saponins are the yucca tree, Yucca schidigera, which grows primarily in Mexico and the Mohave Desert in the United States, and the soapbark tree, Quillaja saponaria, from Chile. For specialized applications, these materials can be useful. For example, Rigano describes the use of Quillaja triterpenic saponins for acne prevention.12 He showed that in acne-prone skin, saponins significantly decrease the number and intensity of inflammatory lesions and contribute to skin normalization.
Other work also describes the sebum-reducing properties of saponins. One blended botanical surfactanta is based on plants from around the globe that have a natural saponin content— “skikakai” from India, desert date from Africa and Gypsophila paniculata from the Mediterranean. This blend is useful as a foaming agent in dilute aqueous systems such as non-aerosol mousse facial cleansers or leave-in hair conditioners since it produces rich, creamy foam at fairly low levels. As with other saponin-based surfactants, it can be used to boost cleansing and improve the volume, texture and stability of product lather when suitably blended with naturally derived surfactants.13
However, the use of these natural surfactants is currently in more niche product applications or certified organic products, as they tend to be relatively expensive and have some functional drawbacks. For instance, the color and odor of these materials has often precluded their use at high enough levels to produce totally natural foaming products. So, are saponins the “perfect” natural surfactants? Unfortunately, no. Although some progress has been achieved by companies in this area, it appears unlikely that saponins will ever become major players in the surfactant world.
Alkylpolyglucosides: Materials that have made great strides toward the “perfect” naturally derived surfactant are alkypolyglucosides (APGs). These form the major component of many natural cleansing product formulations, as they are mild, biodegradable surfactants produced either from fatty alcohols, i.e., coconut or palm oil-based, and glucose from corn or potatoes. APGs are generally considered the most natural of the chemically produced surfactants; while they are chemically produced, they are derived from natural, renewable feedstocks. Figure 2 shows a typical reaction scheme for APG surfactant production.
The feedstocks for many green surfactants are derived from sugar, palm oil, coconut oil or amino acids, but more exotic or “healthy” sources of fatty acids or alcohols are also used. These include amphoacetates or betaines produced from, for example: Orbignya oleifera (babassu) seed oil, Argania spinosa (argan) oil, Vitis vinifera (grape) seed oil), Prunus amygdalus dulcis (sweet almond) oil, Theobroma cacao (cocoa) seed butter, Butyrospermum parkii (shea) or karite butter, and Olea europaea (olive) fruit oil.
The foaming of most APGs is reasonable—it depends upon the chain length of the fatty alcohol group used—but it certainly is not as rich and copious as conventional SLES/betaine blends. However, the recent introduction of APGs produced by a reportedly patented process15 has given rise to materials with better flash foam characteristics and improved rinseability. The issue with APGs in general is their residual feel on skin when used as the sole surfactants in a product. It is different to conventional products, being described as “raspy” on hair; therefore, it is suggested to blend APGs with other mild green surfactants for better results.
Other options for high foaming but mild sulfate-free cleansing products are surfactants with highly renewable carbon content. While not all the current chemical processes used for their production are accepted by standards bodies, many resulting bio-based materials are gaining interest from companies wishing to enhance the sustainable nature of their operations. In fact, the U.S. Department of Agriculture (USDA) introduced its BioPreferred certification for consumer products via the 2002 Farm Bill. Its purpose was to increase the purchase of bio-based products and display the obligatory seal/logo on the product package.16 A small number of personal care products are already certified to this standard, but new applications are currently suspended due to lack of government funding.
Two examples of surfactants with high amounts of bio-based content include the sodium salts of cocoyl/lauroyl methyl isethionate (see Figure 3) and methyl cocoyl/methyl oleyl taurate.18 These surfactants are typically made with 80–86% naturally derived materials, denoted as naturally derived carbon content. These surfactants have been blended with other mild, naturally derived surfactants to produce ready to dilute, sulfate-free, high renewable content surfactant bases for specific applications such as baby cleansing products.19
Turning now to emulsifiers, note that sources of physically extracted materials for their production are quite limited and tend to originate from the food industry. For example, lecithins or phospholipids extracted from soybean, sunflower, egg or milk are an interesting option for green product formulators, and these have long been considered natural, especially coming from the food industry. It is even possible to source organic grades of lecithin; however, lecithin can be challenging and expensive to use.
Consisting of long molecules with a hydrophilic head and two lipophilic chains, lecithin has the ability to form protective bi-layers; further, it has a high affinity to skin. Thus, this material can provide additional moisturizing benefits and active ingredient delivery functions. It is also common to combine lecithin, lysolecithin or hydrogenated lecithin with other higher HLB materials or naturally derived gelling agents to form emulsifier blends that are easier to use and have interesting sensory properties.
A wide range of naturally derived emulsifiers are available for both o/w and w/o systems, where acceptable green chemistry has been used to produce materials based on natural raw materials such as sugars, hydrophilic acids, coconut, palm/palm kernel oil, milk protein, etc. Examples of some naturally derived emulsifier types meeting these standards are given in Table 1 and Table 2.
Thankfully, there is somewhat better agreement between the natural and organic standards bodies for acceptable emulsifier chemistries than for what constitutes a naturally derived surfactant. The COSMOS organic standard aims for 30% of the Chemically Processed Agro Ingredients (CPAI) to be certified organic by the start of 2015. NATRUE also now accepts the organic component of an approved “derived natural” ingredient to be counted in the overall organic content calculation. Some emulsifier manufacturers have the capability to produce commonly used, naturally derived emulsifiers from organic agricultural materials, although the economics may not always stack up. One example is self-emulsifying glyceryl stearate, an o/w anionic emulsifier that can be productd at > 98% certified organic.20 Based on glycerin and palm oil of organic origin, this type of emulsifier is widely used for creams and lotions, particularly those with a higher viscosity and rich skin feel. However, the organic industry must ensure that these organic ingredients are utilized, even though they are somewhat more expensive than the natural alternative, or they risk being discontinued. Guaranteed palm oil-free emulsifiers are also a trend for those looking to satisfy some major retailer requirements for suppliers to move away from the use of non-sustainable palm oil.
Advancing Green Chemistry
So, where is the industry on this quest for the “perfect” green surfactant or emulsifier? Some would say at the beginning of a long road ahead, but at least there is a road. So what does the future hold? One report21 showed that after years of double-digit growth, the annual growth rate for the global natural and organic cosmetics and personal care market slowed to 7–9% in 2011. However, many companies producing conventional or semi-natural cosmetics, which currently account for around 97% of the world market, are also increasingly seeking more sustainable, renewable raw materials, particularly in an era of high, fluctuating oil prices.
Advances in green chemistry and an increased use of renewable feedstock will drive the production of green surfactants in the near future. In fact, it is reported that the global market for bio-based chemicals in general has continued steady and solid growth in the past two years—helped by stabilized glycerin prices—reaching a market value of US $3.6 billion in 2011. Glycerin and lactic acid accounted for just over two-thirds of this value.22
The use of biomass/waste biomass for the production of both hydrophobic and hydrophilic building blocks for surfactants and emulsifiers is also part of the move to greater “upcycling;” there are already cosmetic raw materials available based on the waste by-products from other, much larger industries, such as food. One example is fusel wheat bran/straw glycoside-based emulsifiers. These materials are obtained by the glycosylation of natural alcohols derived from fusel oil from fermentation with monosaccharides derived from hydrolyzed wheat bran and hydrolyzed wheat straw.
Looking even further into the future, new green production technologies will offer massive opportunities for creating vital cosmetic ingredients in a highly sustainable way.23 These include the use of new catalysts to enable coupling reactions for the synthesis of surfactants from renewable components to be carried out at lower temperatures and with higher yields. Bio-processes are likely to be developed utilizing more efficient or new micro-organisms for bio-surfactant production. Enzyme technologies will be harnessed for more efficient coupling reactions—i.e., so-called “protein engineering.” Bio/fermentation routes for the production of important surfactant building blocks such as amino acids, organic acids, carbohydrates, ethylene and acetic acid (from bio-ethanol), are also likely to become the norm. One of the most exciting new technologies is the potential for mass-scale use of algae production systems for a range of cosmetic ingredients. Finally, the conversion of anthropogenic carbon dioxide into useful chemical building blocks could take “green” and man-made full circle. So clearly, the future can be “green” and clean—but for now, it just depends what shade of green one aspires to be!
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12. Ibid Ref 11
13. K Bird, Croda launches foamer to improve sensory qualities of natural products, available at: cosmeticsdesign.com/Formulation-Science/Croda-launches-foamer-to-improve-sensory-qualities-of-natural-products (Accessed Jun 4, 2013)
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17. Innospec Ltd., internal literature
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