HUMAN HAIR MOISTURIZATION WITH COSMETIC PRODUCTS 315 Based on the areas of hysteresis loops, Figures 7–9 clearly show that product 1 retains more moisture than product 2 with reference to the control hair. This is clearly shown in the hysteresis plots of Figure 10. Figure 10 shows that the data useful for product comparison lies in the 10–60% RH range. This is desirable because this is also the range of humidities over which cosmetic products perform the best. The area of the hysteresis loops from Figures 7–9 are shown in Table IV. The data in Table IV show that product 1 is more like a conditioner capable of moistur- izing hair. HR value of 1.42 suggests that it is 42% better in retaining moisture than the untreated hair on the other hand, product 2 (HR = 0.58) is 42% worse in retaining water than the untreated hair and, therefore, may be useful as a styling (and style holding) and anti-frizz product. To check the reliability of hysteresis measurement by this method, fi ve measurements of hysteresis were made on an untreated sample of hair. Hysteresis data are shown in Table V. The data in Table V show that both hysteresis values as a function of RH, as well as the areas of the hysteresis loops, have good reproducibility. Of course, it depends on the sample uniformity. Because of this, we could quantify the effi cacy of cosmetic prod- ucts and grooming processes in moisturizing hair with a limited number of experiments, using carefully prepared hair samples. THE NATURE OF WATER SORPTION IN HAIR Sorption of water in hair from vapor occurs by molecular diffusion. Molecules from the vapor condense on the surface to form an assemblage of water molecules, establishing a Figure 8. Sorption–de so rption isotherms of hair treated with product 2.

JOURNAL OF COSMETIC SCIENCE 316 concentration gradient necessary for diffusion into the hair. Because the cuticle cells are highly cross-linked with cystine, the diffusion does not occur through the cuticle cells. Instead, they diffuse through the CMC between the cuticle cells, then on to the CMCs between the cuticle and the cortex and then fi nally through the CMCs of the cortex into the cortical cells. Adsorption occurs on the inner surfaces of the pores in the entire fi ber, rather than the geometrical surface of the fi ber only (which is generally much smaller than the total surface area of the pores). The sorption isotherm refl ects the nature of this adsorption. For example, the sorption of water into hair from the vapor is known to fol- low the BET isotherm (11). Molecules diffusing into the hair close to 0% RH form a monolayer which is tightly bound to the keratin by strong hydrogen bonds (5–8 kcal/ mole). Subsequent adsorption occurs in multiple layers until about 60–70% RH. Follow- ing this, adsorption occurs by capillary condensation, leading to signifi cant swelling of the fi ber. The hydrogen bond strength decreases in subsequent layers following the fi rst monolayer, until it approaches the strength of the hydrogen bond in water at that temperature. The amount of water adsorbed per unit mass of hair depends on two characteristics of the substrate, e.g., the surface area of the pores and the total volume of the pores. Thielmann et al. (12) have determined the BET surface area of hair fi bers by inverse gas chromatog- raphy (IGC) using hexane and water as the dispersive and polar probes, respectively. Their value is 0.3 m2/g. Surface area of hair has also been determined by Hessefort et al. (13) by nitrogen adsorption and their value of 0.4 m2/g (total pore volume = 0.000689 cm3/g) agrees reasonably well with that determined by the IGC method. The geometric cylindri- cal (assumed) surface area of hair based on a density of 1.38 g/cm3 and 100 μm in diam- eter is only 0.055 m2/g. The surface area of the pores (BET surface area) is nearly seven times the geometric surface area of the assumed cylindrical hair fi bers. Figure 9. Sorption–desor p tion isotherms of untreated control hair.