0.025 0.020 :::::, 0.015 co ·en 0.010 0.005 0.000 0 0.05 0.04 ::J ro o.o3 ·oo 0.02 0.01 0 PENETRATION OF OILS INTO HAIR FIBERS C coconut oil 24 hrs. w. heat T-R ¼ R-T /"�··/ I I I I I I I j 20 40 60 Scattering angle (degrees) I\ oil removed I\ R-T I 20 I I I I I I 40 D oil removed T-R 60 Scattering angle (degrees) Figure 7. Continued. 293 80 80 T-R and R-T positions appeared after additional heat treatment. Surprisingly, for hair treated with sunflower oil, the scale edges were partly exposed after 24 hours, indicating at least partial penetration of sunflower oil into the fiber. As GP scans characterize only
294 JOURNAL OF COSMETIC SCIENCE the hair surface condition, it is not possible to determine if sunflower oil penetration has occurred only within the cuticular layer or throughout the hair fiber, involving the cortex. In general, polyunsaturated oils do not seem to diffuse into hair. Saturated and monounsaturated oils seem to diffuse into the hair structure. DIFFUSION MECHANISM It is known that diffusion of small molecules such as dyes and surfactants in wool fibers occurs through intercellular pathways known as cell membrane complexes, CMCs (8). It is also known that surfactants such as sodium lauryl sulfate diffuse into protein struc­ tures effectively because of their ability to solubilize proteins by complexation (9). Since we are considering the diffusion of oils that are uncharged triglycerides, this may be irrelevant in this instance. Based on the work referenced above, the diffusion or pen­ etration ability of small molecules would be expected to be through the CMCs, con­ trolled by the affinity of the molecules for nonkeratinous proteins in the CMCs, mo­ lecular structure, and molecular weight. For example, mineral oil does not diffuse into hair because it is nonpolar, containing long linear hydrocarbon chains with lengths above C-20. Even though the straight-chain molecular structure is favorable for diffusion by reptation, lack of affinity seems to prevent it from diffusing into hair. Vegetable oils, on the other hand, consist of triglyceride molecules in which three fatty acid molecules are naturally esterified to the three hydroxyl groups of a glycerol backbone. For example, coconut oil is rich in lauric acid (C-12 triglyceride), whereas in sunflower oil linoleic acid (C-18:2 triglyceride) is predominant. Because of its polar character, coconut oil seems to have an affinity toward protein molecules in the CMCs, and, therefore, this oil penetrates more readily into hair fibers compared to mineral oil. Since C-12 fatty acid is a straight­ chain fatty acid, the molecular structure of the triglyceride is also favorable for diffusion. Diffusion occurs by an acid-base type of interaction involving the ester groups in the oil and the carboxyl and amine groups in the protein. Oils containing triglycerides with unsaturated fatty acids seem to encounter greater resistance to diffusion. These molecules tend to be more spread out because of the presence of multiple double bonds and are therefore difficult to enter and move through the narrow channels of the CMCs. These aspects need to be explored further, based on the dynamics of these molecules in a protein environment. CONCLUSIONS The work reported in this paper shows that the formation of oil films in hair fiber assemblies can be studied by two very different methods, interfiber adhesion and re­ flectance of light. There seems to be good correspondence between the two methods, as shown by the agreement between the two methods for coconut, mineral, sunflower, and olive oils. In general, saturated and monounsaturated oils penetrate into the hair because of a compact molecular structure and the polar head group of the triglyceride molecules that constitute these oils. In a dynamic mode these molecules can reptate and squeeze through the CMCs. On the other hand, polyunsaturated oils do not penetrate into hair, most likely because of the more open and spread-out structures of their triglyceride molecules and because of the presence of multiple double bonds. The results reported in
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