130 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table II Salt Effect on LAS-Induced Stratum Corneum Swelling Stratum Corneum Swelling (% Increase from Hydrated Length) Treatment 1 hr 3 hr 88 mM LAS 14.7 q- 1.5 23.0 --- 1.3 174 mM LAS 13.1 --- 1.2 20.7 q- 1.1 88 mM LAS/88 mM NaC1 12.3 q- 1.3 18.3 q- 1.6 174 mM LAS/88 mM NaC1 12.4 q- 1.6 18.3 + 1.6 Values are means q- standard deviation (S.D.). The 88 mM NaCI alone caused negligible amounts of swelling above normal hydration. Times shown are length of time incubated in the treatment solution after which readings were taken. Means differing by greater than 2% are significantly different, P • 0.05. their original lengths when in the water rinse. We also tested higher soap concentra- tions (2%), and at the longer incubation time (24 hours) the membranes became irreversibly disrupted and frequently tore (data not shown). We conclude from this and other experiments that the swelling response caused by soap is generally reversible only under conditions of shorter exposure times (less than 24 hours) and/or concentrations of 1% or less. SURFACTANT STRUCTURE EFFECTS ON STRATUM CORNEUM SWELLING A/hyl ethoxy sMfates. Systematic structural variations of several surfactants were tested for their effects on the swelling response. We examined alkyl ethoxy sulfates containing different numbers of moles of ethylene oxide (Figure 3). Increasing the degree of ethox- ylation while holding the alkyl chain constant at C•2 to C•4 average carbon atoms resulted in significantly lower levels of swelling. However, when the degree of ethoxy- lation exceeded 6 moles of ethylene oxide in the alkyl ethoxy sulfate, no additional Table III Reversibility of Soap-Induced Stratum Corneum Swelling Treatment Regimen* Stratum Corneum Swelling (% Increase from Original Hydrated Length) 0.26% Soap, pH 10 1 hour, 37øC 10.5 q- 1.8 Distilled Water Overnight, 25øC 1.4 q- 0.4 0.26% Soap, pH 10 1 Hour, 37øC 11.6 --- 1.6 Distilled Water Overnight, 25øC 1.8 --- 0.4 * Six stratum corneum samples were treated in the regimen shown above and in the order shown. Values are means q- S.D. The soap treatments both produced significant increases in swelling above the original hydrated lengths, P • 0.05.

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