76 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table I Correlation Between "C-Values" and Soap Chamber Scores (Toxicol) for Anionic Surfactants (see Figure 13) Surfactant "C-value" Soap chamber score Run 1 Proprietary formula 1 0.34 0.30 Proprietary formula 2 0.60 0.65 SLEC (3.8EO) 21.00 0.70 SLES-coco 32.00 0.89 SLES-oxo 47.83 1.36 MgLS 2.18 1.62 SLS 103.00 4.75 Run 2 ALES 11.80 2.95 Proprietary formula 3 14.70 4.14 ALS 17.74 4.75 to small changes in the chemical structure of the surfactant molecules of homologous series from the same manufacturer. In vivo data are seldom as specific. Thus we were able to describe the effects of the shape of the alkyl chain, the degree of ethoxylation, and the effect of counterions. The liposome assay was also used to test surfactant mixtures. It is important to realize, however, that we were dealing entirely with commercial grade surfactants with a dis- tribution of alkyl chain lengths and the possible presence of unreacted components and impurities. The results, therefore, apply only to the actual samples tested, and a number of unidentifiable factors may contribute to the observed effects. The same is true, of course, for samples sent for in vivo testing. For practical reasons all concentrations had to be expressed as weight percent and not as millimoles. Effects originating from changes in alkyl chain length or configuration, or the degree of ethoxylation, are common to amphipathic substances in general. These effects will, therefore, be discussed here as they pertain to all surfactant classes investigated. Counterion effects are described as they were observed for anionic surfactants. ALKYL CHAIN AND ETHOXYLATION EFFECTS Each batch of test surfactants was run against reagent grade sodium dodecyl sulfate, which was used as reference compound throughout. Commercial grade sodium lauryl sulfate with branched (oxo) alkyl chains was confirmed as more aggressive than the corresponding coco-derived product (Figure 4). Polyoxyethylene groups are frequently inserted between the alkyl chains and the charged head groups to modify surfactant irritancy. Ethoxylation of surfactants increases the hydrophilicity. In anionics this counterbalances the effect of the alkyl portion in non- ionics it is the basis for the detergent effect. Depending on the charge distribution or the hydrophilic/lipophilic balance, ethoxylation may reduce or enhance the aggressive- ness of a surfactant toward liposomes. Sodium lauryl ether sulfate (2% ethoxylation) (SLES) and lauryl ether carboxylate (3.8% ethoxylation) (SLEC) interacted less strongly with the liposomes than the lauryl sulfates (Figure 5). Figure 6 illustrates the increasing

SURFACTANT-SKIN INTERACTIONS 77 ß - T • ---• o-SLES-•oco • .... • .... SLES-oxo k. '• ................... • .... • .................................. ..•..: ...--:v.:::...-:=.:.•.. I I I I I I I I I I I 0.• 0.• 0,• 0.•0 0.• 0.•4 0.• 0.• 0.• 0.• s eurfactant Figure 4. Sur•ctant/liposome interactions--Response to changes in alkyl chains. 0,0 0,00 I I I I I I I I I ! I I O.Ol• 0.04 0.0• 0.0• O.l.O s eurfactant Figure 5. Surfactant/liposome interactions--Response to different anionic surfactant classes. = SLS .--O--SLES $ SLEC aggressiveness of a homologous series of straight-chain ethoxylated alcohols with iden- tical cetyl-stearyl alkyl chains bearing 10, 15, and 20 moles of ethoxylation respec- tively, i.e., with increasing hydrophile-lipophile balance. All three interacted extremely aggressively with liposomat membranes, and more so with increasing ethoxylation. In contrast with the above, an increase in the length of the polyoxyethylene chains has been