Effects of emulsifiers on skin. (achieving maximum emulsifying effect with minimum irritation)

The prime concern in selecting surfactants for use in cosmetics skin products is to achieve maximum emulsifying, solubilizing, and dispersing effects while causing minimal untoward reactions with epidermal cell structures. Available information on the interaction of surfactants with epidermal tissues has been most ably reviewed by Singer and Pittz [1] and Cooper and Berner. [2] The particular effects of surfactants on skin cleansers has been reviewed by Thau. [3]

Damaging effects of surfactants on skin manifest themselves as dryness, roughness, and scaling. In addition, symptoms of inflammation (redness, swelling) can develop. In severe cases, complete destruction of the tissue occurs (necrosis). The skin is defatted by the more or less pronounced property of surfactants to emulsify lipids and thus partially or completely remove the surface film of lipids. This results in a disturbance of the skin's barrier function, leading to increased permeability for chemical substances and to loss of water. There is, in addition, loss of low molecular weight hydrophilic components, which have a role in maintaining skin flexibility and extensibility.

Anionic surfactants, especially soaps, cause swelling of the skin. As a result, they facilitate the transport of substances to lower layers where inflammation reactions can be induced. [4] The reaction of surfactants with proteins dissolves proteins out of the skin and leads to their denaturation. These changes in the matrix material have an effect on the resistance of the skin [5] and, along with degreasing and drying, are an additional cause of an increase in skin roughness. [6] Finally, the reaction of surfactants with enzymes in the tissue affects the enzyme activities and therefore metabolism in the skin. [7]

Irritancy accounts for the majority of adverse cutaneous reactions. For the general population, the normal use of surfactants in consumer products does not elicit serious skin problems. However, exaggerated use of surfactants coupled with preexisting sensitivity, extremes of temperature or pH, low humidity, co-use of other potential irritants such as abrasives, bleaches, or lipid solvents (turpentine, gasoline, etc.) can bring about reactions such as skin irritation, inflammation, chapping and roughness.

Caution must be exerted in evaluating the irritancy potential of surfactants from literature accounts of studies. Often these compounds were tested without specific determination of their exact compositions. Also, studies have been conducted in several animal species and in man; and by a variety of procedures (patch testing, immersion tests, in-use washing tests, etc.), each of which is capable of giving results widely variant from the others. To allow rational interpretation of data on skin irritant effects, only those studies that compare surfactants under identical conditions should be considered. [1]

Surfactants may be anionic, cationic, amphoteric or nonionic, ranging from highly water-soluble to highly lipid-soluble, and from those having compact hydrophilic head groups to those with large hydrophilic portions. The possession of charge confers an added dimension to the skin toxicity of ionic surfactants. However, the potential for damage cannot be deduced easily from the structural characteristics, and surface activity, per se, is difficult to relate to biological activity. Nevertheless, the affinity of surfactants for membranes and macromolecules must confer on all surface active agents a special toxicological dimension.

Anionic Surfactants

Alkyl Carboxylates ("Soaps"): Excised skin is fairly impermeable to low concentrations of soaps and charged surfactants if the pH is in a range where the surfactant is still charged. A chain length of about [C.sub.12] seemed optimal for skin penetration, with some decrease in permeation on the lower side, and little or no permeation on the higher side. When tested from solutions whose pH was not adjusted, sodium laurate penetrated excised human skin during an 18-24 hour period, while sodium lauryl sulfate (SLS) did not. [8-10] Since both penetrated at a pH greater than 11, alkali damage to the barrier was considered responsible for the difference. In other studies of the effect of pH on laurate penetration, there was no permeation in the pH range of 8.5-10.5, but significant permeation did take place at 7.0-8.5 (presumably because of lipid solubility) and at pH 10.5 (probably due to damage to the barrier). [1] Barrier damage was reversible by ringsing away the soap, indicating that the barrier components were not removed. After several days exposure of excised human abdominal skin, penetration of the epidermis was 24.4% for potassium laurate ([C.sub.12]), 5.3% for potassium caprylate ([C.sub.8]), and only 0.15% for potassium palmitate ([C.sub.16]). [11]

When trasepidermal water loss (TEWL) is used as a measure of barrier damage, comparable results are obtained; i.e., soaps were also …