34 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (with the possible exception of the lauryl mono- and triethoxylate) removed any more than that removed by water itself. Preliminary studies showed that only above the critical micelle concentration of the surfactants was there any increased extraction of amino acids and protein from the stratum corneum. To investigate the effect of lipophilic chain length upon extraction an homologous series of sodium alkyl sulphates and sodium soaps were studied and the results are shown in Table III. Table IIIa shows that all alkyl sulphates, especially those of chain length 11, 12 and 13, removed more amino acids and soluble proteins from the skin. Sodium lauryl sulphate was the most effective. Table IIIb, for the sodium soaps showed a similar trend, but here sodium myristrate removed far more soluble protein, and sodium laurate removed more amino acids from the skin than other members of the homologous series. The overall conclusions to be drawn from this study were that the ability to remove compounds from the skin, which probably contributes directly to the irritancy of a surfactant, is a function of both the nature of the polar head group and the length of the lipophilic chain, but the nature of the head group polarity of surfactants seems to be more important than length of the lipophilic chain in determining whether surfactants are able to extract materials from the stratum corneum. Table IIIa. Effect of lipophilic chain length on ability of alkyl sulphate sur- factants to extract material from the skin during washing Alkyl sulphate (25 mm) % increase in extraction relative to washing with water Soluble protein Total amino acids Sodium nonyl sulphate (C9) 50.8 Sodium decyl sulphate (C10) 166.1 Sodium undecyl sulphate (C11) 119.5 Sodium dodecyl sulphate (C12) 238.9 (sodium lauryl sulphate) Sodium tridecyl sulphate (C13) 198.5 Sodium tetradecyl sulphate (C14) 163.9 Sodium pentadecyl sulphate (C15) 77.9 62.7 84.2 100.4 194.8 141 '7 110.3 41.3 Experimental conditions resembled those described in Table II, except that total soluble protein in wash liquors was measured by the Folin-Ciocalteu method and total amino acids by a spectrocolorimetric assay, both of which are more sensitive than uv absorption as used in Table II.

SKIN IRRITATION POTENTIAL OF SURFACTANTS Table IIIb. Eftbct of lipophilic chain length on ability of soaps to extract material from the skin during washing 35 Soap (25 mM) •o increase in extraction relative to washing with water Soluble Total amino protein acids Sodium caprylate (C8) 125.7 0.0 Sodium caprate (C10) 196.8 43.3 Sodium laurate (C12) 186.7 234.9 Sodium myristate (C14) 792.8 147.5 Sodium palmitate (C16) 147.6' 160.5* ((insoluble at 22 ø ) Assay methods as in Table Illa. * Presence of insoluble soap make these readings doubtful. Percutaneous absorption of [x•C] labelled surfactants through guinea-pig skin in vivo As little is known of the rates of penetration of surfactants through the skin of live animals we studied a series of radioactively-labelled pure sur- factants: these were applied to the dorsal skin of guinea-pigs and the amounts penetrating the skin barrier determined. The radioactive surfac- tants were all of 12-carbon chain length, labelled with •4C at the c•(1)-carbon position of the alkyl chain, the point of linkage to the head groups. These were either purchased from the Radiochemical Centre, Amersham, or synthesized in our laboratory by Mr C. T. James. The precise methods of synthesis and details of the penetration studies will be published elsewhere (9), and closely resemble the method described by Howes in the previous paper (11). The fate of the eight cutaneously applied compounds during the 24 h following application is shown in Table IV. Attempts were made to account for all of the radioactivity applied to the animals and in most cases the percentages recovered are reasonable. In every case, by far the most radio- activity was accounted for in the skin rinsings, on the non-occlusive patches or bound to the skin of the animals at the original sites of application. With the exception of lauryl hexaethoxylate and lauryl alcohol there was much activity remaining in the skin. No attempts were made to determine if this activity was in the epidermis or dermis (i.e. whether it represented