RHEOLOGY OF STRATUM CORNEUM--II Table I Assignments of molecular modes in stratum corneum 17 Frequency (cm -•) Assignment 3 289 (s) 3 063 (w) 2 955 (w) 2 919 (s-m) 2 872 (w) 2 854 (w-m) 1 740 (w-sh) 1 653 (vs) 1 543 (s-vs) 1 518 (sh) 1 464 (sh) 1 454 (m-w) 1 395 (m) 1 340 (w-sh)* 1 295 (w) 1 270 (vm)* 1 245 (m) 1171 (m) 1 126-1 100 (sh) 1 078 (w) 970 (vw)* 934 (vw) 897 (vw) 826 (vw) 775 (vw)* 745 (sh) 720 (shp-w)* 700 (w) 665 (w) 620 (w) 580 (vw)* 425 (sh) Amide A N-H str from peptide groups Amide B overtone of Amide II at 1 550 cm -• Antisym C-H str from CHa groups Antisym C-H str from CH2 groups Sym C-H str from CHa groups Sym C--H str from CH2 groups C = O str probably from lipid esters Amide I, largely C = O str + a small contribution from NH bend Amide II, complex mode C-N str + N-Hb from peptide groups CH3 CHz CH• (Wag) CH3 ? ? Amide III complex mode similar to Amide II but includes energy contributions from str C = O, and O --- C-N. ? ? C-O or C-C str C-C str ? 9 ? Amide band possibly? r CH2 from lipids Amide IV (probably)O -- C-N + other Amide VI out of plane C = O str Amide V out of plane NH bend from lipids from peptide groups vs, very strong s, strong m, medium w, weak vw, very weak sh, shoulder shp, sharp str, stretch b, bend r, rock. * Believed to be associated with lipid material. affected by the extractions or by mechanical stress (9). However, bands arising from lipid material were modified by treatments. The adsorption intensities in the C-H stretching region of the spectrum contain com- ponents arising from the lipid content of untreated stratum corneum (14). After chloroform extraction, bands at 2 920 -• and 2 851 cm -x were absent these assignments are well established as the symmetric and antisymmetric

18 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS C-H stretching vibrations coming from CH• groups of the lipids. Con- firmation of the removal of lipids from the corneum was observed by the reduction, and in some cases disappearance, of the C-H rocking mode at 720 cm 4 after treatment with chloroform. The ir spectrum of stratum corneum after treatment with SDS showed no change in those absorptions assigned to the main peptide vibrations. It was found that extensive washing was necessary to remove the anionic detergent completely and that those samples treated at pH 3 required a significantly greater wash time than the ones treated at pH 7 (90 and 70 h, respectively). An interesting feature of these spectra was that the SDS (at both pH's) reduced the lipid content and water-retaining ability of the corneum, the effect being similar to that observed with CHCla/H20 extraction. The conclusions from the TGA results confirmed the effects of these solvents in reducing the water-holding power of the stratum corneum and the restfits are given in Table IL Chloroform extraction alone had no effect Table II Percentage water loss in the temperature range 100ø-140 ø for stratum corneum before and after extraction (average of three samples) Extraction procedure Weight loss (50 None 2.43 CHCIa 2.42 CHC1,/H20 0.19 SDS(pH 7/H20) 0.33 SDS(pH 3/HaO) 0.56 on water retention and the slightly higher values of percentage weight loss recorded for the samples extracted by SDS/water (compared with those extracted by chloroform/water) are probably due to incomplete wash out of the sulphate. Information could not be gained from TGA runs on SDS- treated corneum before wash out since a large weight loss occurred in the temperature region of interest (100-140øC) due to breakdown of the sulphate (SOa driven off). DISCUSSION If stratum corneum is treated with chloroform or detergent solutions followed by aqueous extraction, the water-retaining ability of the substrate is reduced and this is reflected in a change in its elastic properties, i.e. after