Creative Developments (Cosmetics) Limited

Surfactants 1999

Surfactants are almost as ubiquitous as preservatives in cosmetic products. They are the emulsifiers, solubilisers, wetting and cleaning agents, foam producers and conditioning aids and are found in every product category from make-up to skin care as well as being the main constituents of shower gels, bath foam and shampoos. The word surfactant is short for "surface active agent". Each surfactant molecule has two chemical groups; a polar group that is water-soluble, the hydrophile, and a hydrocarbon chain that is oil-soluble, the hydrophobe. If both oil and water are present, the surfactant provides a bridge with its hydrocarbon chain dissolved in the oil and the ionic, hydrophilic, end dissolved in water.

Anionic surfactants are generally used as the prime surfactant in toiletries that foam; that is, shampoo, bath and shower products. They have a negatively charged carboxylic, sulfate or sulfonate group at the end of an alkyl, aryl or alkyl aryl chain and are commonly the sodium, potassium, magnesium or triethanolamine salts of these. Non-ionic surfactants have structures in which the hydrophilic region contains many oxygen atoms, as hydroxyl or ether groups, which can hydrogen bond to water. They are mostly used as emulsifiers and solubilisers in cosmetic applications.

Cationic surfactants have quaternary ammonium ions that are sufficiently polar to attract water. They are most often used as conditioning aids because of their substantivity to the anionic hair shaft and they may also have antimicrobial properties because they can disrupt the cell walls of some pathogenic bacteria. The fourth group of surfactants are known as amphoteric or ampholytic and can carry both a positive and negative charge. They are used to enrich the texture of foams and to improve the mildness on personal care products based on anionic systems.

With the increasingly frequent use of personal care products mildness is a much sought after property. In efforts to avoid animal testing various alternative methods have been proposed including the Hens Egg Test, Red Blood Cell (RBC), Neutral Red Release (NRR) and Neutral Red Uptake (NRU) tests. An in-vitro assessment of cytotoxicity of surfactants was described at the 1998 IFSCC Congress. Human immortalised keratinocytes (NCTC-2544) and human conjunctiva (Chang conjunctiva) were cultured in Petri dishes.

They were then inoculated with different concentrations of the test compounds and the cytotoxic effect was evaluated after 24 hours. Cells either continued to grow, had their growth inhibited or were destroyed. Results were clearly dose dependent; in the case of sodium laureth sulfate there was some cell growth at 83mg/ml, some destruction at 250mg/ml and complete destruction at 500mg/ml. Polysorbate 60, sodium cocoamphoacetate and decyl glucoside each showed growth inhibition at 83mg/ml and significant destruction at 250mg/ml. Of the substances tested there was little difference between sodium laureth sulfate and sodium cocoamphoacetate which were the most cytotoxic; the least cytotoxic were two different polyoxyethylene/polyoxypropylene copolymers, neither of which caused any cell destruction at 1,000mg/ml. [REF. 1]

A method for optimising shampoo formulation through a study of mixed micelle formation in systems containing anionic, amphoteric and non-ionic surfactants with the aim of correlating critical micelle concentration with results obtained for eye irritation tests was described by Balaguer, F et al [Ref.2]. In previous experimental work using the same strategy, a basic shampoo formulation comprising 5% sodium laureth sulfate with 1.67% each of disodium cocoamphodiacetate, cocamidopropyl betaine and decyl polyglucose was optimised. In the present study, the effect caused by the addition of diverse agents that supply the required properties of shampoo formulations were evaluated with their effects on both the critical micelle concentration and irritation potential of the resulting system being studied. It was found that materials such as laureth-2, PEG-7 glyceryl cocoate and hydrolysed wheat protein decreased the c.m.c. while polymers like polyquaternium 10 and 39 and hydroxypropyl guar hydroxypropyl trimonium chloride increased the values. Generally a decrease in c.m.c. results in a lower irritation potential. When more than one additive was incorporated in the basic formulation results tended to decrease the irritation potential; either as the result of a synergistic decrease in c.m.c. or because a decrease in c.m.c. was accompanied by a material with an affinity for the stratum corneum keratinic proteins that imparted a protective mechanism.

Of the anionic surfactants available the sarcosinates, isothionates and sulfosuccinates have proved of much interest for those looking for milder actives. The sulfosuccinates are claimed to be high foaming mild surfactants with a conditioning action that give optimum performance under slightly acid conditions. McIntyre produces a guide to formulating with sulfosuccinates that describes the many different derivatives available including a silicone sulfosuccinate said to be exceptionally mild for use in facial cleansers and baby wipes. Disodium laureth sulfosuccinate is recommended for use in conjunction with sodium laureth sulfate (SLES) for bath products. When used at 25% SLES:75% disodium sulfosuccinate, the combination shows a much-lowered irritation potential without significant loss of foam. McIntyre produces twenty different sulfosuccinate monoesters and eight diesters but the latter are rarely used in personal care products.

An independent testing house was commissioned by Croda Oleochemicals to undertake in-vitro testing in order to compare the mildness of sodium lauroyl sarcosinate with SLES. Using the Neutral Red Uptake bioassay method whereby the potential cytotoxicity of the test products to normal human keratinocytes is measured results show that the sarcosinate is significantly less cytotoxic than SLES. A different method, the Living Dermal Model, was commissioned by Croda to test the irritation potential of potassium wheat amino acids (Aminofoam W). This method utilises a three dimensional construct of living human fibroblasts in a collagen matrix. A bioassay quantitatively measures the irritation potential of a test solution against the known irritant, sodium lauryl sulfate (SLS), and virtually non-irritating glycerin. Results show that at concentrations of less than 5% potassium wheat amino acids are significantly less irritating than SLS.

Amphoteric compounds are usually thought to be milder than anionic surfactants and are often used in combination to reduce the irritation potential of the latter by reducing the c.m.c. Alkylpolyamphopolycarboxyglycinates (APACs) are a relatively new group of true amphoteric compounds. Unlike the betaines which are cationic at acid pH, zwitteronic (ionic-neutral) at their isoelectric point but which are never anionic, the APACs are anionic at alkaline pH. A useful side effect of adding an amphoteric surfactant to a cosmetic product is the bacteriastatic effect shown by many of them. Adding 3% lauryl betaine to a 27% active solution of sodium laureth sulfate will preserve the mixture against repeated challenge tests and less than 75ppm is said to be effective against Aspergillus niger, Staphylococcus aureus and Listeria monocytes. It is less effective against Pseudomonas aeroginosa, requiring 75,000ppm but coco APAC is effective at 18,750ppm against this organism. Using a mixture of lauryl betaine and coco APAC (Ampholak MSX-3 ex Akzo Nobel) is a means of reducing preservative levels in surfactant products while improving foam volume and adding a conditioning effect. A recently introduced amphoteric from Th. Goldschmidt is Tego Betain 810, this is a preservative-free, 35% active solution of capryl/capramidopropyl betain (CCAPB) which is shown to significantly reduce skin irritation caused by SLES and to improve foam volume and foam stability.

Cationic surfactants are commonly used as conditioning aids but as a class of compounds they tend to be more irritating than the other surfactants to be found in cosmetic products. Test results published by Croda show the following decreasing order of irritancy of a selection of cationic emulsifiers used at 1% in a simple lotion formula.

Cetrimonium chloride

Dicetyldimonium chloride

Lapyrium chloride

Behentrimonium methosulfate

Distearyldimonium chloride

Behentrimonium methosulfate

Stearalkonium chloride

Behentrimonium chloride

Behenamidopropyltrimonium methosulfate

Emulsifying Wax NF *

Dihydrogenated tallow amidoethylhydroxyethylmonium chloride

The Emulsifying Wax NF * is a commonly used non-ionic emulsifier included to give a point of reference to the results. The methods used were the in-vitro MatTek Epiderm bioassay for cytotoxicity and trans-epidermal water loss (TEWL) measurements on human volunteers. Results show that behentrimonium methosulfate is substantially non-irritating when used at 1% in the test lotion and that the emulsifier has a long-term moisturising effect.

Many cationic surfactants have antimicrobial properties and some are listed preservatives, however most are known to be irritant and their permitted concentration is low e.g. benzalkonium chloride is only permitted at 0.1% as a preservative and to a maximum of 3% for other functions such as hair conditioning. A cationic that combines excellent conditioning action with a low level of irritation potential and good antimicrobial properties is PCA ethyl cocoyl arginate available as CAE from Ajinomoto Ltd. Its antimicrobial activity, shown in the following table, makes it suitable as a germicidal agent for shower gels and face and hand wash formulations.

Minimum concentration of growth inhibition (µg /cc)

Strain CAE Benzethonium

Chloride

Benzalkonium

Chloride

Triclosan

Escherichia coli 50 50 --- 50

Pseudomonas aeruginosa 100 100 --- 1000

Proteus vulgaris 10 10 --- ---

Staphylococcus aureus 25 10 3 10

Bacillus subtilis 10 10 --- 10

Corynebacterium diptheriae 10 10 --- ---

Candida albicans 50 10 --- ---

Enterococcus faecalis 100 --- --- ---

Lactobacillus fermenti 100 --- --- ---

Malassezia furfur 300 --- 150 ---

Propyonibacterium acnes 9 --- 7 7

The ease with which anionic surfactant systems could be thickened by the simple addition of sodium chloride resulted in the majority of retail products being quite viscous and the consumer came to equate viscosity with quality. The introduction of amphoteric surfactants without a thickening action and of different actives and the need to suspend such materials as silicone droplets brought new problems to the formulating chemist. While simply increasing the cocamide DEA and salt content may work this is not suitable for today’s claims of mildness. Carbomer ETD 2020 is an attempt to provide thickening action and yield value to a product containing electrolytes but the salt content of many surfactants is greater than even this grade of carbomer can tolerate. PEG-150 distearate is a cheap and effective additive but the final viscosity is difficult to control: it imparts very little yield value and the rheology of the finished product is not always thought desirable. Methyl glucoside is easier to work with but its dilatant rheology can be a problem, not least in the filling of the product. Veegum is suitable for some opaque systems and xanthan gum may be used in clear ones and both give a useful yield value.

Some cellulose-derived thickeners are salt-tolerant and have a Newtonian rheology. Hydroxyethyl cellulose (Elfacos CD 481 from Akzo Nobel) is a high molecular weight non-ionic cellulose ether with substantial surface and interfacial activity. By virtue of its molecular weight and amphiphilic nature it shows many of the properties and functions of traditional cellulose thickeners, but is said to provide additional benefits because of its polymer surfactant character. In aqueous systems and surfactant solutions hydroxyethyl cellulose shows strong polymer-surfactant interactions that influence phase behaviour and interfacial activity. Depending on the nature and amount of surfactant added, this may lead to enhanced absorption of cationic conditioning aids at surfaces, and stronger foaming of anionic surfactant systems.

Finally, many suppliers are now aware of the benefits of placing their information directly onto the computer of prospective users. Rewo (now Witco Surfactants) and Henkel Cospha (now Care Chemicals) were first with 3.5 in floppies but CD-ROMs are now the preferred medium. Disks have recently been issued by Dow Corning, excellent but space hungry if loaded onto a hard disk and too slow if not. By Akzo Nobel, very informative and quick enough without the need for downloading, and Th. Goldschmidt; again very informative with many applications on it but it requires a Pentium 100 or better if it is not to be down-loaded.

Ref. 1: An in vitro assay for the assessment of cytotoxicity of surfactants in two human immortalised cell lines: Nctc2544 and clone 1-5c-4; Dolfini E, Lavazza M, Meloni M., IFSCC Congress, Cannes 1998.

Ref. 2: Mildness evaluation of hair and body shampoo formulations; Balaguer, F. et al., IFSCC Congress, Cannes 1998