354 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS It is essential to realize the difference between the influence on skin lipids by a fatty oil (a liquid triglyceride such as soybean oil) and by G2. Stratum corneum treated with soybean oil (Figure 4) showed little difference in lipid interlayer spacing, while treat- ment with G2 caused a new band to appear at 30-45 fk. Hence, the comparison of soybean oil-treated stratum corneum with G2-treated stratum corneum demonstrates that the 30-45 /ix band is neither the result of G2's triglyceride structure nor trace impurities remaining in the purified G2. From the studies of G2 treatment on washed stratum corneum, we find that G2 can interact with the stratum corneum even after the bulk of the lipid has been removed by washing. This is concluded after comparison of the 30-45 A band for G2-treated stratum corneum with the 30-45 fk band of G2-treated stratum corneum after washing (Figure 1). It is also shown that most of the lipid structural change caused by G2 is removed with the lipids during washing, since the spectrum for stratum corneum washed after treatment was identical to washed untreated stratum corneum. SUMMARY From the small-angle x-ray diffraction study of human thin skin stratum corneum, it is proposed that the broad diffraction band from 50-80/ix is due to epidermal lipids. Whereas washing removes epidermal lipids, treatment with G2 changes the packing structure of epidermal lipids. It is proposed that this interaction is the primary source of the skin-softening properties of G2. This interaction can occur even after removal of epidermal lipids by washing however, when the stratum corneum is washed after treatment with G2, the characteristic x-ray reflection caused by addition of G2 was removed. REFERENCES (1) (2) (3) (4) (5) (6) (7) (8) (9) (lO) (11) (12) G. Swanbeck, Macromolecular organization of epidermal keratin, Acta Dermato-Venerelogica, 39, 1- 37 (1959). G. Swanbeck and Nils Thyresson, A study of the state of aggregation of the lipids in normal and psoriatic horny layer, Acta Dermato-Venerelogica, 42, 445-457 (1962). L. A. Goldsmith and H. P. Baden, Uniquely oriented epidermal lipid, Nature, 225, 1052-1053 (1970). G. L. Wilkes, An-Lac Nguyen, and R. Wildnaver, Structure-property relations of human and neo- natal rat stratum corneum. I. Thermal stability of the crystalline lipid structure as studied by x-ray diffraction and differential thermal analysis, Biochimica et Bio-Physica Acta, 304, 267-275 (1973). A.M. Kligman, "The Biology of the Stratum Corneum," in The Epidermis, W. Montagna and W. C. Lobitz, Jr., Eds. (Academic Press, New York, 1964), pp 387-433. D. T. Downing, J. S. Strauss, and P. E. Pochi, Variability in the chemical composition of human skin surface lipids,J. Invest. Dermatol., 53, 322 (1969). B. A. Gilchrest, Skin and Aging Processes (CRC Press, Inc., Boca Raton, Florida, 1984), p 39. M. A. Lampe, M. L. Williams, and P.M. Elias, Human epidermal lipids: Characterization and modulations during differentiation, J. Lipids Res., 24, 131-140 (1983). S. E. Friberg and D. W. Osborne, A lameliar liquid crystal as an in situ surface balance. II. The conformation of a glyceridacid, Colloids and Surfaces (in press). D. W. Osborne, Ph.D. dissertation (under preparation). E. Haahti, Major lipid constituents of human skin surface with special reference to gas-chromato- graphic methods., Stand. J. C/in. Lab. Invest., 13 (Suppl. 59), 1-108 (1961). H. J. O'Neil, L. L. Gershbein, and R. G. Scholz, Identification of pristance in human sebum and related lipid sources, Biochem. Biophysical Res. Comm., 35, 946-952 (1969).

j. Soc. Cosmet. Chem., 36, 355-362 (September/October 1985) In vitro and in vivo study of the substantivity of p-amino benzoic acid and two of its esters M. I. MORASSO, A. M. THIELEMANN, C. PINTO, M. FIGUEROA, and A. ARANCIBIA, Departamento de Ciencias Farmacologicas, Universidad de Chile, Casilla 233, Santiago, Chile. Received November 26, 1984. Synopsis The substantivity of p-amino benzoic acid (PABA) and two of its esters (Escalol 507 © and Amerscreen "P"©), in an alcoholic vehicle, has been studied both in vitro and in vivo in order to establish the possible correlation between the two methods. Eight healthy volunteers participated in the in vivo method. Solutions of the sunscreen substances were prepared in isopropanol (IPA): PABA at 5%, and Escalol 507 © and Amerscreen "P"©, both at 2%. In order to study the substantivity of each of the sunscreen substances to the skin, the subjects placed their hands in the solutions. The hands were then rinsed with water and extracted with IPA at 50øC to quantitate the amount of substance retained by the skin. A crossover design was used all the subjects participated in experiments with each of the substances at one week intervals. In the in vitro experiment, keratin powder obtained from hard, insensible tissues (human calluses of the feet) was used. The keratin was kept in an ethanol solution of the sunscreen for 72 hr, then treated with water for 48 hr, and finally extracted with ethanol at 50øC for 24 hr in order to determine the amount of sunscreen retained by the keratin. The sunscreen concentrations in the different solutions in both in vitro and in vivo methods were determined by spectrophotometry. The in vivo percent substantivities (i.e. percentage of sunscreen deposited which was not rinsed off ,by water) obtained in the study were: PABA = 0.30 _+ 0.05 Escalol 507 © = 57.96 --- 0.63 Amerscreen "P"© = 0.46 + 0.08. In the in vitro method the following values were obtained: PABA = 0.30 +-- 0.02 Escalol 507 © = 50.84 + 0.66 Amerscreen "P"© = 0.47 + 0.03. The differences are statistically significant (p 0.01) and Escalol 507 © is the substance that shows this property in the highest degree. The results obtained by the in vitro and in vivo methods were linearly correlated. INTRODUCTION The function of a sunscreen preparation is to absorb as completely as possible the erythemic ultraviolet radiation of sunlight while transmitting a maximum of the other wavelengths. During the last few years, several researchers have focused their attention on the loss of efficacy of sunscreens due to their removal from the skin by perspiration and/or swimming (1,14). The term substantivity is currently used to express the ca- 355