From simple superficial tension modifiers to molecules able to maintain the health of skin and hair, from lather boosters to activators of skin equilibrium after washing, today's surfactants are functionalized for particular uses. Within the limits of a repetitive structure - i.e. the hydrophilic polar head and the apolar alkyl chain - there are many variants allowed. Using a series of mild surfactants able to restore the cutaneous properties depends on specific structure variables. Here we discuss a new generation of 'amido-surfactants' with the structure of 'interrupted soap', with examples of practical applications. We demonstrate that the cutaneous and sensory properties of the new cleansing formulations can be met by the adequate selection of multifunctional surfactants.
Introduction
In the 1970s, the skin aggression levels of the few available surfactants, together with the social discovery of frequent body washing, gave rise to increasing skin problems. A group of English dermatologists suggested a less frequent and abundant use of soap, bath foam and shampoo to fight the growing number of reports of mycosis and irritant dermatitis. Soaps made the skin's pH alkaline, but synthetic surfactants did not solve the skin aggression issues. The concept of cutaneous respect in cleansing was born, together with the reduction of all possible irritations both evident and hidden.
The lower layers of epidermis are all involved in the keratinisation process which leads to the formation of an equilibrated horny layer. This acts as a shield against environmental factors such as wind, dust, air, light and heat, and prevents the excessive water exchange between vital tissues and the surrounding atmosphere. The complex structure of the horny layer, defined as 'bricks' (corneocytes) and 'mortar' (hydrolipidic film) and successively, of 'lipid domains', prevents excessive water loss by having a multi-layer system soaked with lipophylic and hydro coordinating substances. The most external layer, environmentally exposed, becomes saturated with solid and volatile pollutants such as desquamation cells, oxidised lipids, residues of bacterial proliferation and secretion of sweat and sebaceous glands. So how can it be cleansed without seriously modifying its structure? Modulating the action of surfactants becomes an essential issue in performing non-aggressive hygiene. But is it enough to use substances we define as delicate? Another issue is in what amounts should these substances be used? Finally, softening actions for the skin, hygiene habits and sensory pleasantness must be compatible?
Modular structures
Among the most recent structural answers to these needs is a category of multifunctional surfactants called acylated amino acids. Their chemical structure is characterised by an alkyl-amide group, with the hydrophobic part derived from a linear fatty acid. The hydrophilic moiety is formed by an amino acid or a polypeptide. These surfactants are structurally compatible with keratin and have carboxylic groups in place of sulfate groups. They are all detergents derived from weak acids, with an acid-base equilibrium and pH buffering effects. They do not form strong ionic bonds with superficial skin layers and have a low solvent effect toward structural lipids. Moreover, they are bulky and reduce fast diffusion into skin layers. They make up a category of 'interrupted soaps' that get progressively more substantial. This work will briefly describe the main properties of three members of this category - i.e. acyl sarcosinates, acyl glutamates and acyl taurates.
Their skin compatibility, related to the space arrangement of carboxyl and amide groups, is particularly notable. A formulation puzzle then arises. If acylated amino acids have similar properties, why not select just one of them, so making the life of formulators and storekeepers easier? If we now look at an example. Just as football teams take the field with the collective aim of winning the match, the players have various tasks - e.g. defenders, midfielders, forwards; surfactants have different specific properties or particular effects. Each molecular structure in a formula may be aimed at preserving epidermal defences that are attacked by degreasing agents or to reduce the solvent impact, to avoid denaturation of superficial proteins or to improve the rinse-off action. Ingredients in a formula may then be used alone or associated, according to the quality and amount required for the product.
Acyl sarcosinates
Acyl sarcosinates are considered to be true 'interrupted soaps' due to the special conformation of their molecules. The N-methyl amide group is between the aliphatic chain and the carboxyl group of a soap structure (Figure 1).
The acid-base equilibrium of sarcosinates provides its maximum dermatological compatibility. Sarcosinates, when used in skin-cleansers for their excellent foam enhancing characteristics, decrease the eye-irritation potential. Moreover, they do not interfere with cationics. Their special property is to bind to protein substrates at pHs between 4 and 7, where the N-acyl sarcosine ion is blocked by bonds that form easily with the keratin structures1. The formation of a stable bond between skin proteins and other denaturant surfactants is prevented. Because the protein-sarcosinate bond is weak in diluted solution, the successive depletion of the surfactant during rinse-off is easier2. In the literature, simple sarcosinate-based formulations are described, sometimes in blends with soft anionics. The claim is that they have superior properties - i.e. absence of irritation, mildness, satisfactory cleansing and lack of skin-drying effect. The degreasing power toward epidermal lipids is virtually absent. N-lauroyl sarcosinate is widely used in mouthwash and toothpastes for its anti-plaque and anti-caries properties and also because of its compatibility with quaternary salts such as cetrimonium and benzethonium chloride.
Acyl sarcosinates are mainly used for their excellent foam enhancing characteristics. They are also able to influence the so-called Index of Foam, which is a combination of foam volume and water uptake of the foam. The water uptake of the foam is an indication of its creaminess. Furthermore, when the foam can take up more water, it does not slip down wet skin too fast while showering. The higher the Index of Foam, the better the quantity and features of the foam itself 3.
Acyl sarcosinates are able to decrease the freezing point of anionic surfactants, increase the cloud point of non-ionic surfactants and inhibit corrosion. They are also effective wetting agents and stable in bleach systems. All the properties of acyl sarcosinates (thickening, foaming, and wetting) depend on the pH of the system and on the fatty chain4, and can be chosen according to different needs.
While lauroyl/cocoyl sarcosinates are widely used and well- known, oleoyl sarcosine has found limited applications in cosmetics so far. Indeed, the oleoyl chain in sarcosinates decreases the CMC strongly, thus providing better detergent properties and increased potential for mild cleansing. Oleoyl sarcosine has proved to be an effective thickening agent. Thickening of surfactant solutions is a common formulation task. Nevertheless, very few molecules with a high safety profile and an ability to thicken multiple surfactants systems at low concentration are available. Frequently, thickeners require high concentrations and might negatively influence the foam characteristics and hair properties.
The highly effective thickening properties of oleyl sarcosine in blends containing surfactants were observed. Oleoyl sarcosine (in acid form and therefore insoluble in water) was neutralized first with aminomethyl propanol (AMP) and the behaviour of the AMP oleoyl sarcosinate in thickening experiments with several categories of surfactants was studied. The foaming behaviour of surfactant blends containing AMP oleoyl sarcosinate was also investigated, both at low and high energy impact. In general, AMP oleoyl sarcosinate was shown to be a powerful thickener for anionic, amphoteric and non-ionic surfactants and their blends. Its efficient concentration was about 20% to 25% lower than the usual alkanolamides level (Cocamide DEA). Its foam boosting properties at very low concentrations were surprisingly higher than any known foam booster - in terms of foam amount and how long the foam lasted, even when the thickening effect was fairly low. Foams reached 25-30% more volume and were 20% longer lasting than standard solutions (Sodium laureth sulfate + Cocamide DEA), on average, when tested under high energy mixing5.
AMP oleoyl sarcosinate allowed the coupling of oils with surfactants and the preparation of dilution-deposition systems. Oil deposition onto the skin after cleansing provides a tool for better barrier equilibrium after washing. To evaluate possible oil deposition on skin from surfactant systems obtained with AMP-neutralized oleoyl sarcosine, a test was carried out on nine volunteers in a bioclimatic room at standard temperature and humidity (24°C; 50% rh). Volunteers were required not to cleanse or moisturize their forearm for three hours before starting the test. The assessment was performed on the volar area of each forearm, where two areas measuring 18x3 cm2 were marked. Each area was treated with one detergent, according to the following randomized study design.
A = Isopropylmiristate (25%) - Neutralized oleoyl sarcosine (75%)
B = Dicaprylyl ether (25%) - Neutralized oleoyl sarcosine (75%)
C = Refined olive oil (75%) - Neutralized oleoyl sarcosine (25%)
Three subjects used the couple A - B, three subjects the couple B - C and three subjects the couple C - A. Each product was so tested on six volunteers.
An instrumental experiment was then carried out to determine the amount of oils absorbed on the skin after the above treatment. The forearm of each volunteer was wiped with paper disks wet with hexane. The disks were collected and extracted three times with hexane. The extracts were added to the internal standard, treated with NaHCO3, evaporated, dissolved again in hexane and injected into a gas chromatograph. Sample C (olive oil) was trans-esterified with acetyl chloride in methanol before the analysis. Gas chromatography was carried out on the extracts to determine the residual oil on the skin5.
The quantity results were:
- 18.5 µg/cm2 for dicaprylyl ether;
- 62.8 µg/cm2 for isopropylmiristate; and
- 166.3 µg/cm2 for refined olive oil.
Thus oleoyl sarcosine was shown to have oil dilution-deposition effects with film forming and emollient properties. It is also able to thicken surfactant solutions even at a very low concentration and exhibits high foam boosting.
Acyl glutamates
Figure 2 shows the structure of acyl glutamates.
Acyl glutamates are obtained from glutamic acid and long chain fatty acids. A large number of enzymes can be found on the skin’s surface. Thanks to these naturally occurring enzymes, the application of an acylated amino acid on the skin can lead to formation of the original components. For acyl glutamates we obtain glutamic acid and free fatty acids. Amino acids and fatty acids are very important for skin and hair. The skin's Natural Moisturizing Factor contains about 40% of free amino acids. Free fatty acids represent about 25% of total lipids in the horny layer, while in the lower layers this is only 5%. They are essential for the barrier effect of the skin, keeping it supple and moisturized6. Glutamic acid constitutes about 15% of keratins. Furthermore, with its two carboxylic acid groups it helps the skin to maintain its natural acidity7. This is important because there is a proven correlation between skin pH values and some skin disorders8. Observations via scanning electron microscopy show that acyl glutamates have a protecting and repairing effect on the cuticle layer of hair9.
Using acyl glutamates gives perceptible benefits in cosmetic formulations. A product based on acyl glutamate (10% active matter in aqueous solution) used three times a day for 10 consecutive days by 10 volunteers, showed that acyl glutamates are a non-irritant for the skin (Flex Wash Test)10 and were shown to be hypoallergenic11. Acyl glutamates are also 'tear free'.
An in vitro RBC-Test (Red Blood Cell Test) to determine eye irritation was done using lauroyl glutamate to compare it with other surfactants. Figure 3 shows the results12. The numeric values on the y axis are MIOI (Mean Index of Ocular Irritation).
Sodium laureth sulfate, one of the most commonly used anionic surfactants, has a high affinity to the epidermis. Almost everyone has quantifiable amounts of this detergent on the skin. The addition of sodium cocoyl glutamate to a formulation where sodium laureth sulfate is present was shown to lead to a significant reduction of this affinity while increasing skin moisturization13. A market cleanser formulation claimed as a 24 hour-lasting moisturizer was compared with the same formulation with 5% sodium cocoyl glutamate added. Both formulations were shown to be effective, but the one with sodium cocoyl glutamate was more active in a statistically significant way14.
If we change the fatty chain we obtain other interesting properties. For example, in disodium capryloyl glutamate, the presence of the caprylic fatty chain does not only impart cleansing properties, but also skin compatibility. It can be used in products for treating skin disorders, it helps to maintain skin equilibrium, and it is useful in dandruff and sebum control and is suitable for inclusion in formulations for body odour control.
N-methyl acyl taurates
Long fatty chain derivatives of taurine - e.g. N-methyl acyl taurates15, 16, have been known for a long time and have been shown to decrease dandruff and protect hair from cuticle damages17, 18, 19. Moreover, scalp cell turnover has been shown not to increase on treatment with acyl taurates20, 21. The main problem in their use is due to their physical form, because they are available as a powder or as a solution that becomes a paste at temperature below 20 °C-25 °C. Blending sodium methyl cocoyl taurate with sodium myristoyl sarcosinate gives the best performances and cold stability22. Figure 4 shows the structures of the two blend components.
First the foaming properties of the blend were evaluated. The sodium methyl cocoyl taurate and sodium myristoyl sarcosinate blend shows a superior foam height under strong energy than sodium laureth sulfate. Association with sodium laureth sulfate in the ratios 1:1 and 1:3 gives comparable high results in terms of height and very high foam quality. High foam stabilisation is obtained with cocamide DEA, mainly in terms of foam height at low energy impact and foam quality. Disodium laureth sulfosuccinate, disodium cocoamphodiacetate and cocamidopropyl betaine plus their combinations, as associated surfactants, provide high foam enhancements23.
We then studied the possible applications of the blend in hair care. Its effects on hair colouring, colour retention and conditioning of dyed hair after treatments were examined23. Hair colouring was done using a permanent hair dye formula and with a semi-permanent hair dye formula. Three variants containing respectively the new blend or reference surfactants (sodium laureth sulfate (SLES) or cocamidopropyl betaine - (CAPB) were tested. Washing fastness, expressed as a decrease of initial colour intensity and increase of luminosity, were evaluated instrumentally after 10 (permanent dyes) or five (semi-permanent dyes) wash cycles with SLES solution. Sensory parameters such as colour homogeneity, brightness, colour intensity and conditioning/softness were also evaluated.
Sodium methyl cocoyl taurate and sodium myristoyl sarcosinate blend based oxidation hair dye showed the same colour uptake as the benchmarks and the same or better colour stability after washing. For the semi-permanent dye formula, initial covering and colour intensity stability were better for formulae with cocamidopropyl betaine or with sodium methyl cocoyl taurate and sodium myristoyl sarcosinate blend than with sodium laureth sulfate, while hue changes were significantly lower for the formula with the studied blend.
In a comparison between a standard sodium laureth sulfate - cocamide DEA shampoo formula (Formula A'') and a sodium methyl cocoyl taurate - sodium myristoyl sarcosinate shampoo based formula (Formula B''), sensory parameters like hair conditioning, colour maintenance and homogeneity, showed improved results for the latter. The two formulations were used for the evaluation of washing fastness on coloured tresses by a repeated series of the permanent hair dye and five (temporary hair dye) standard wash and rinse cycles. In all cases, colour intensity was evaluated instrumentally. Sensory evaluation was carried out on parameters of colour homogeneity, brightness, colour intensity and sensory properties. The results are shown in Figures 5 and 6. These evaluations confirm that cleansing with taurate - sarcosinate based shampoo provides statistically better hair conditioning and long-lasting colour permanence on dyed hair.
The studied N-methyl cocoyl taurate and myristoyl sarcosinate new blend shows significant improvements in the diffusion of colorants into the hair and in its capability to maintain the acquired shade. Additionally, sensory elements support the possibility of using this surfactant blend in hair colouring and colour fading proof formulations.
Formulations
In a set of developed formulations, the special combinations of the new surfactants used in balanced equilibrium with usual foaming agents are shown. Similarly with emulsions, where the fatty phase performs an equilibrated blend of properties, the detergent phase is balanced to obtain optimum foam, softness, respect of congested or irritated skin, bacteriostatic effects, easy rinse-off, iso-epidermic pH, controlled soil elimination, rheology characteristics and sensory performances. This formulation strategy is allowed by the use of the acylated amino acids.
Conclusions
The use of acylaminoacid type surfactants allows the formulation of selective cleansers. It is possible in this way to obtain cosmetics not only able to clean skin and hair but also to respect their structure, their constitutive elements, their resident micro-organisms and pH. Acylated aminoacids are mild, natural, biodegradable and non-mutagenic. Their use can help the formulator to obtain a balance of properties and functionality: optimum foam, softness, care and protection for normal and damaged skin or hair, mildness to eyes and mucous membranes, conditioning, deodorant effect, and so on. The surface activity of these molecules, combined with their functional properties, provides numerous opportunities for the development of effective and innovative products for skin and hair care applications.