CHARACTERIZATION OF HUMAN SKIN CHEMISTRY 289 natural, cosmetically modified, old and freshly generated human skin in situ. Referring again to Figure 3, it should be noted that the internal reflection prism shown there is labelled as being constructed ofgermanium, an important requirement not only for the application of ellipsometry and contact potential measurements to the films in question but also assuring biological safety for the volunteer subjects especially when deliberate skin wounding was employed. Thallium bromide salts, such as employed in the more commonly used KRS-5 internal reflection prisms, can cause contact dermatitis and potential toxicity in such cases, while the biological acceptability of germanium seems much better (14). RESULTS WETTING AND SPREADINO ON CLEAN HUMAN SKIN Table 1 lists the variety of pure diagnostic liquids utilized in determination of the potential wettability and spreadability of cosmetic ingredients on clean human skin. These same liquids are useful in assessing the new surface condition of skin after treat- ment with specific ingredients in proofs of efficacy for cosmetic or medicinal prepara- tions. In Figure 4, the average contact angle values obtained for these liquids on the surface of the skin of a male volunteer, after that skin had been cleansed with a liquid hand soap, well rinsed, towel-dried and equilibrated in clean room conditions, are plotted in the standard Zisman format (8, 9) to yield a critical surface-tension intercept near 38 dynes/cm. This is a typical value for uncontaminated, fibrous protein prepara- tions and compares well with values already published for human skin, keratin, collagen and gelatin (15, 16). It might therefore be taken as a general case, for human skin treated by simple cleaning procedures as described here, that cosmetic or me- dicinal preparations having operational liquid/vapor surface tensions lower than 35 dynes/cm will wet and spread well upon those skin surfaces. Fhfids with lower surface tensions will give excellent coverage and appearance, while cosmetic preparations with operational surface tensions greater than 35 to 38 dynes/cm will tend to bead up, retract, or leave interstitial voids on the same skin surfaces. Table I Wettability of Clean Human Skin, In Situ Wetting Liquid and Surface Average Tension (•/LV) Contact Angle (dynes/cm, 20øC) (0 in degrees) Water 72.8 65 Glycerol 63.4 66 Formamide 58.2 51 Thiodiglycol 54.0 54 Methylene Iodide 50.8 53 Sym-Tetrabromoethane 47.5 34 !- Bro toonaphthalene 44.6 25 O-DiBromobenzene 42.0 18 1-Methylnaphthalene 38.7 24 Dic yclohexyl 33.0 0 n- Hexadecane 27.7 0

290 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS HUMRN SKIN, IN SITU SURFACE TENSION DTNE$/CM 2o so 4o so 6o ' (]) • (D • • • 37.8 DYNES/CM + • •LOPE = -0.022 CM/DYNE P•ZHT• •[TH • •Y•L• •BT IND. LI!•J [# •'TNRI{•T L[I,E 7o 8o I :-, i. --. /- NRSHED NITH HF•ND SORP RINSED DRIED Ealspa. • Figure 4. Contact angle data plot typifying the wettability and surface energy of human skin free of cos- metic preparations or greasy residues Figure 5. Infrared spectra illustrating the utility of internal reflection spectroscopy for the noninvasive evaluation of cosmetic preparations on human skin in situ. Compare with spectra of the cosmetics, alone, in Figure 6. O Residue of liquid hand soap used to establish "clean" skin conditions. (Continued on Page 291.)