719 PHYSICOCHEMICAL PROPERTIES OF TEXTURED HAIR the general property of the rotation of the elliptical fiber is the same as that found in extremely tightly curled African hair. HAIR LIPID DISTRIBUTION IN HAIR We investigated the lipid distribution in African and Caucasian hair using FTIR imaging. Figure 7 presents FTIR images of the two hair types. In addition to the differences in shape of the cross-sections—African hair is much more elliptical than Caucasian hair—the lipid levels are greater in the African hair samples. These data were generated by normalizing the integrated intensity of the symmetric and asymmetric C–H stretch vibrations (2,800– 2,980 cm−1) to the peak area of the amide II frequency at 1,548 cm−1. As found in previous studies, the lipid levels are relatively higher in the medulla region, followed by the cortex and then the cuticle (33,34). In Caucasian hair, the medulla region is discontinuous (as compared to Asian hair) therefore, we do not always observe a relatively distinct lipid distribution. Interestingly, almost all of the African fiber cross-sections in Figure 7 have some element of a lipid reservoir in the medulla region. Overall, our results are in agreement with previous studies that demonstrate higher levels of external lipids in African hair as compared to Asian and Caucasian hair (8–10). However, a study published by Ji et al. reported higher levels of integral lipids in Asian hair as compared to African and Caucasian hair (29). WATER MANAGEMENT PROPERTIES OF AFRICAN VERSUS CAUCASIAN HAIR The amount of moisture absorption and desorption by hair is often monitored to elucidate the internal structural properties of the fiber. This is typically achieved using dynamic vapor sorption (DVS), which facilitates the generation of sorption isotherms and the determination of diffusion coefficients. The amount of water in hair has a profound impact on its mechanical and material properties. Hair samples from the top, middle, and bottom of the tress were cut Figure 7. Spatial distribution of lipids in (A) Caucasian and (B) tightly curled African hair. Spectral images were obtained by taking the ratio of the integrated C–H symmetric and asymmetric stretching vibrations at 2,800 to 2,980 cm−1 to the integrated amide II band in the spectra at 1,548 cm−1.

720 JOURNAL OF COSMETIC SCIENCE into 1–2 mm snippets to probe the absorption/desorption behavior of the hair cortex. The snippets were thoroughly blended and then analyzed at 25°C. Essentially, the hair sample was placed into the DVS chamber, which contains a sensitive microbalance in a humidity- and temperature-controlled environment. Weight changes experienced by the sample are reported in terms of moisture content, which is the amount of water absorbed/desorbed per mass of hair. The samples were dried for 120 min at 60°C. The humidity was then ramped up from 0% to 90% RH at 10% RH steps (720 min at each step) to measure absorption and then ramped back down to 0% RH to monitor desorption. There has been some controversy in the literature as to whether chemical treatments (bleaching, perming, etc.) or cosmetic treatments (e.g., a hydrophobic cationic surfactant) affect the water absorption/desorption properties of hair (35–38). Regardless, we sought to investigate if any differences could be identified between the two African hair types and Caucasian hair. Figure 8 contains isotherms for the three hair types investigated, which follow the typical sigmoidal pattern for an isotherm of hair. The absorption and desorption isotherms for tightly curled and extremely tightly curled African hair demonstrate the lower capacity of the two African hair samples to uptake moisture as compared to Caucasian hair. At lower humidity levels, all three samples behave similarly however, at high humidity, differences between them are more discernible. These data could suggest that the pathway for water to get into the fiber structure is more tortuous in African hair due to the greater presence of lipids. On the other hand, these differences may also be attributed to differences in protein structure. To determine if the absorption-desorption properties are influenced by protein composition, experiments on 0 5 10 15 20 25 0 10 20 30 40 50 60 70 80 90 100 % RH Caucasian Tightly curled African Extremely tightly curled African Figure 8. Hair-water absorption-desorption isotherms for (A) Caucasian, (B) tightly curled African, and (C) extremely tightly curled African hair. MoistureContent (