RHEOLOGY OF STRATUM CORNEUM--II 15 TGA runs were carried out on a Du Pont thermogravimetric analyser. The weight loss for approximately 5 mg samples of stratum corneum was recorded at a constant heating rate over the temperature range from room temperature to 200 ø. A number of heating rates were screened from zero (temperature held at specific values) to 10 ø min -• in order to arrive at the optimum value for this substrate of 5 ø min 4. Corneum samples were pre- dried for 24 h under vacuum in an attempt to standardize initial environ- mental conditions. Chloroform and solutions of sodium dodecyl sulphate (BDH, specially pure) were used as extractants for the stratum corneum. Sodium dodecyl sulphate solutions were made up to the required concentration by weight, and adjustments of pH were made with sulphuric acid and sodium hydroxide. Pieces of corneum (• 2 x 10 -3 m square) were normally ex- tracted for 2 days with chloroform (•200 ml) or sodium dodecyl sulphate (• 200 ml) solutions, and then for a further day with water. Extraction was facilitated by stirring and the solvents were renewed several times during these periods. RESULTS The variation of elastic modulus with relative humidity, between 30 and 100•o, was recorded for samples extracted with chloroform/water or 0.1 M SDS (pH 7 and 3)/water after conditioning (9). Within the limits of experimental error, the effects of the treatments were not graphically dis- tinguishable and the normalized data (9) are combined in a single curve (a) illustrated in Fig. 1. Included in Fig. 1, for comparison, is the curve (b) previously obtained for untreated stratum corneum (9). Although these curves have the same general shape, the moduli of the extracted samples are higher than those for untreated corneum at every rh and they (i.e. the moduli of extracted samples) decrease at a much slower rate between 30 and 70•o rh. The ir spectrum of untreated stratum corneum is illustrated in Fig. 2. Frequencies, band assignments (11-13) and qualitative intensifies are listed in Table I. Deuteration of the corneum was carried out to assist in establish- ing some of the assignments. Deuteration of the material also showed that even after attempts to remove water, by desiccation, some still remained, as evidenced in the normal spectrum by the weak shoulder on the high frequency side of the amide A (at 3 400 cm -•) and by the broad band (800- 500 cm -x) in the amide IV, V and VI region. Removal of most of this

16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS 6000'- 3000 - •' 1000- •z 600- • 300 - , • Ioc - u 60- 3C- 10- 6- 30 40 60 80 100 % relotive humidity Fi•,ure I. Normalized ,variation of elastic modulus of stratum corncure with relati,ve humidity for (a) sol,vent extracted and (b) untreated material. tenaciously bound water was only accomplished by a chloroform/water extraction followed by desiccation. A striking feature of the spectra of treated and untreated stratum corneum was that the adsorption bands attributable to protein modes were not 2.5 • 4 5 6 7 8 9 I 0 I 5 20•.25 •)4050 4000 3000 2000 1800 1600 1400 1200 1000 800 600 400 cm-•200 Figure 2. [nlra-red spectrum ooe stratum corneum.