SURFACTANT SWELLING OF STRATUM CORNEUM 131 ii i Moles of Ethylene Oxide Figure 3. Swelling of human stratum corneum by alkyl (C12-C14 average) sulfates with different moles of ethylene oxide units. Incubation times are 1 and 6 hours as shown. Each was tested at 70 mM surfactant. Swelling values are percent increases from hydrated lengths. All treatments were significantly different (P • 0.05) except the difference between one and two moles of ethylene oxide and between six and twelve moles of ethylene oxide. reductions in the swelling response occurred. The swelling response for these higher ethoxylates is only slightly above normal hydration. Nonionic surfactants. Several ethoxylated nonionic surfactants were also tested for effects on stratum corneum swelling, i.e. alfonic 1012-60 and tergitol 25-L-7. Both produced virtually no swelling (above normal hydration) at concentrations between 1 and 30% (w/w). Homologous series of various surfactants. We tested a homologous alkyl chain series of several anionic and cationic surfactants for effects on crosswise swelling (Figure 4). For

132 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 4 ) •' )1 Alkyl Benzene • Sulfonate$ Alkyl Chain Length 4• 3• Alpha Olefln $ulfonate$ Alkyl Chain Length E i. o 2 ) Paraffin • . Sulfonates/ 2 4 hr•'7• Alkyl Trimethyl Ammonium Bromides lhr Alkyl Chain Length Alkyl Chain Length Figure 4. Dependence of swelling of human stratum corneum on alkyl chain length for various surfac- rants. Studies were all run at 1% (w/w) surfactant concentration. Time periods shown are incubation times of the treatments after which readings were taken. Swelling values are percent increases from hydrated lengths. Means differing by greater than 2% are significantly different (P • 0.05). Please note that swelling results for one homologous surfactant series in this figure cannot be compared with results for a different homologous series because they were run in separate experiments. alpha olefin sulfonates, paraffin sulfonates, linear alkyl benzene sulfonates, and alkyl sulfates (see reference 2), we found maximal swelling for the C 12 or C 14 homologue in all cases. These anionic surfactants were tested on a weight percent basis (1%, w/w),