SURFACTANT INDUCED SKIN TIGHTNESS 153 +0.4. •0.2 -0.2 -0.4 o ß oe r =0.25 ../' '••r 0.21 Oß Oß 0 ß I I i I r= 0.84 i I / 0 ,I II O •/•ß ß 0.94 o 2oo o 0:5 i I ß Amino Acids (nmol/cm =) oSqualene & (nmol/cm =) ß Cholesterol o lb 2b oUrocanic Acid ( n mol/cm 2) =- . ß Indigocarmin Staining (A E) Figure 4. Interrelationship between relative degree of skin tightness (main effect) and skin amino acid extraction, lipid extraction, and surfactant sorption of surfactants. Figure 4 summarizes the relationship of skin tightness to the relative intensities of various physicochemical effects induced by these surfactants. The order of skin tightness is parallel to both lipid removal (r = 0.94, P 0.01) and adsorptive ability (r = -0.99, P 0.01), but not to the amino acid- or the urocanic acid-releasing abilities (r = 0.21). In order to confirm the relationship of the tightness sensation to lipid removal and adsorptive abilities of these surfactants and to differentiate between these two factors, several experiments have been performed. In these experiments surface chemical actions of surfactants are simulated by a combination of treatment consisting of removal of lipid by ether or ethanol, application of water by towel, and surfactant application before or after removal of lipid. Table V shows the intensity of skin tightness observed after each of these procedures. The application of ether or ethanol to facial skin produces mild scaling with an average value of 1.0 for tightness intensity. Additional treatment with water by application of a towel containing water at 40øC produces no substantial change in tightness. A single treatment with 5 % SLS solution provides a skin tightness value of 1.3, and an additional application of water is not effective in changing the tightness level. However, an application of SLS solution subsequent to ether treatment produces an average tightness value of 2.3 and therefore is more effective in inducing tightness than a single appli- cation of SLS alone. Tightness is not effectively changed by an additional treatment with water by towel application or by removal of excess surfactant after rinsing. A single treatment with 5% aqueous MAP solution following removal of skin surface lipids with ether produced a tightness level similar to that of SLS, but subsequent rinsing with water dramatically diminished the tightness to an average value of 1.4, which is equivalent to that produced by water washing alone.

154 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Table V Model Procedure for Clarifying Mechanisms of Skin Tightness Treatment Average of Skin Tightness (n = 10) No treatment 0.3 Washed with water 1.3 Lipid removal by ethyl ether (A) 1.0 Lipid removal by ethyl alcohol (B) 1.0 (A) + water application 0.9 (B) + water application 1.2 5% SLS application (C) 1.3 (C) + water application 1.4 (A) + (C) 2.3 (A) + (C) + water application 2.0 (A) + (C) + rinsed with water 1.9 (A) + 5% MAP application (D) 2.1 (A) + (D) + rinsed with water 1.4 DISCUSSION The physicochemical factors associated with skin tightness may be classified into at least two actions: 1) removal of skin surface lipids (5,9, 16) and loss of hygroscopic materials (4,17) which may occur during washing, and 2) adsorption of surfactant onto the skin (7,10,18). Of the primary causative factors described above, our in vivo studies suggest that the lipid removal ability of surfactants is strongly associated with the induction of skin tightness. Although it had been suggested that the removal of hy- groscopic or water-soluble materials like amino acids and urocanic acid led to induction of skin tightness (19), our studies reveal that the removal of amino acids and urocanic acid from the horny layer does not correlate with skin tightness. Our efforts to test the role of skin surface lipid removal in inducing tightness has revealed that almost com- plete removal of skin surface lipid by ether does not lead to a marked induction of tightness, suggesting that lipid removal may be viewed as a prerequisite but not the actual cause of skin tightness. A certain amount of surfactant tends to adsorb onto skin, is difficult to wash away, and remains on the horny layer (7,10,18). Our in vivo study reveals a positive correlation (r = 0.99) between the amount of adsorbed surfactant and tightness. In an attempt to confirm adsorption as an essential factor for the induction of tightness, we carried out two different experimental procedures: applying surfactant before and after removal of skin surface lipid. The removal of skin surface lipid enhances skin tightness but is not essential for tightness. The importance of adsorbed surfactant was corroborated by the fact that although MAP application without rinsing produces tightness with an intensity similar to that of SLS, subsequent rinsing of the MAP with water dramatically reduces tightness to the level induced by water. This is presumably due to the weak ability of MAP to sorb to the skin as demonstrated in our previous studies (5). One possible explanation for the mechanism underlying tightness can be represented by the following scheme (Figure