2.00E-09 1.S0E-09 1.60E-09 1.40E-09 � 1.20E-09 E 1.00E-09 8.00E-10 6.00E-10 4.00E-10 2.00E-10 0.00E+00 0 OIL FILMS AND MOISTURE ABSORPTION ON HAIR 10 20 30 40 50 RH(%) 60 70 80 143 90 100 Figure 7. Plot of calculated water vapor sorption diffusion rates for hair samples treated with various amounts of coconut oil as a function of RH. film from the surface of the fibers was removed with acetone. The sorption isotherms for acetone-wiped oil-treated hair along with that of the untreated hair are displayed in Figure 8. The original coconut oil-treated sample prior to wiping with acetone is also included in Figure 8. The sorption isotherm for the acetone-wiped sample is nearly 30 �------------------------------------. 25 20 10 5 0 ---+- coconut oil wiped-off - El-· untreated __.,_ coconut 10 20 30 40 50 60 Target RH (%) 70 80 90 100 Figure 8. Water vapor sorption isotherms for hair sample after the removal of the surface coconut oil.

144 JOURNAL OF COSMETIC SCIENCE identical to that of the untreated sample, suggesting that the oil has been removed from the fiber surface. Also, the diffusion coefficients were similar to those of the untreated hair. This indicates that the coconut oil that had penetrated into the fiber cortex did not affect the moisture sorption behavior of the hair sample, probably because the amount is too small. Of course, this assumes that wiping the fiber with an acetone-soaked Kimwipe® does not remove the cortex-penetrated oil. The reductions in moisture uptake and diffusion rates found in our study seem to be caused mostly by the surface oil films. NATURE OF THE OIL FILM Oil applied to hair forms a thin film on the fiber surface where the amount applied is small (0.1 ml/g). Most of the oil ends up near the cuticle edge, as shown in Figure 9. From there it will penetrate into the cuticle through the endocuticle and the cell membrane complex. A small amount (as in the case of coconut oil) may also penetrate the cortex through cortical CMCs. Since these are the same pathways used by the water vapor to diffuse into the fiber, clogging of these pathways by oil molecules can retard the rate of diffusion. Since the sorption equilibrium depends on the set dM/dt (0.0015 %/min for 20 minutes), and because of the decrease in the rate of diffusion, it is possible that the dM/dt condition is satisfied before a true equilibrium is reached. However, when we performed an additional measurement with a coconut oil-treated hair sample, allowing the hair to remain at each RH for an extended time past the previous equi­ librium condition (i.e., at least five more hours), the additional moisture uptake was negligible. This observation shows that the lowering of sorption levels is not related to the attainment of equilibrium. Thus, the more likely explanation is that the cuticle, which forms ~20% or more of the fiber volume, is partially penetrated by the oil molecules (endocuticle and CMCs), and therefore, that volume (these sites) is not avail­ able for the sorption of the water vapor. In other words, absorption of oil changes the sorption behavior of the substrate. This can lead to a lowering of the diffusion rate and the equilibrium uptake. The acetone-soaked Kimwipe® seems to remove the oil, which is penetrated into the cuticular zone. Cuticle Cells Oil Location Major Moisture Diffusion Path Figure 9. Schematic of location of oil on hair fiber surface.