HUMAN HAIR MOISTURIZATION WITH COSMETIC PRODUCTS 309 The sorption–des orption plots in Figure 2 show that the difference in the moisture con- tents of hair treated with conditioning actives from that of untreated hair is very small ( 1%). A closer examination of the S-D isotherms suggests that comparing hysteresis of treated samples with those of the untreated sample at various humidities can provide use- ful information regarding the moisture retention effect of conditioning actives. Figure 3 shows the hysteresis of hair treated with conditioning actives along with that of untreated hair as a function of relative humidity. In Figure 3, the hysteresis plot of the untreated hair goes through a maximum at ~70% RH and drops off beyond that value. The hysteresis plots of all conditioner- treated hair fall below that of the untreated in this region of humidity ( 70% RH). The plot of polymer JR-400 is higher than all others below 40% RH, indicating it has the best moisture holding capacity at low humidities. Next is Jaguar C-17 below 40% RH. Here, we did an additional experiment with hair treated with Jaguar C-17 only once, and the result is that the plots for both 10X and 1X treatment are virtually coincident. This suggests that with polymeric conditioners which deposit on the surface without penetration, multiple treatments may not have an added ef- fect. All top layers rinse off except the one which is strongly bound to the charged surface. This has been established by repeated wetting force measurements on treated fi bers (6). These ionic polymers have the capability of reducing the thermodynamic activity [fugacity (7) and escaping tendency, G. N. Lewis] of water sorbed on/in the hair. Figure 3. Hyste r esis plots of conditioned hair as a function of humidity.
JOURNAL OF COSMETIC SCIENCE 310 The low molecul a r weight conditioner CETAB behaves differently. Treatment over long periods of time repeatedly allows for considerable penetration of CETAB into the bulk of hair (8). This increases the hydrophobicity of the cortex, thus limiting the penetration of water molecules into the hair. Because of low interaction with water molecules, they do not retard the diffusion of water out of the hair. Therefore, low molecular weight conditioners have limited moisturizing capability. Hydro- philic polymers, on the other hand, because of their hydrophilicity, even with thin fi lms bound to the surface of hair, and limited penetration into the cuticular zone reduce the evaporation of water. There may be an additional effect of the very low molecular weight component of polymeric actives penetrating into the hair. HYSTERESIS RATIO S AND MOISTURIZATION Hysteresis ratio s for the plots in Figure 3 for conditioning actives were evaluated accord- ing to equation (5) using i = 1–5 or 1–9 to see how the value of i affects the overall HR. The values are summarized in Table II. The data in Tabl e II show that if the isotherms are well defi ned, then even a small value of i yields satisfactory ratios. Values of HR for 5 and 9 values of i are very close. The values of HR also show that CETAB is not a good moisturizer, although it gives a very smooth surface by forming a monolayer on the surface of the cuticle cells. The polymers which deposit on the surface (low molecular weight components may pen- etrate) have a weak moisturizing effect (HR 1). This is because the moisture associ- ated with the polymer has a lower activity and, therefore, diffuses very slowly from the polymer–fi ber composite. JR-400 seems to do better than Jaguar-C-17. CETAB shows a split behavior. Below 30% RH, the value of HR = 1.1, whereas in the range of 30–60% RH, the value of HR = 0.92. Because CETAB penetrates into the fi ber (8), the lipid chains packed into the matrix reduce its hydrophilicity and facilitate the release of moisture better than in the untreated fi ber. Overall, the data show that the moisturizing effect of ordinary rinse-off conditioners is weak, in spite of multiple Table II Hysteresis Ratios of Hair Treated with Conditioning Actives from Figure 3 RH (%) Hysteresis (%) (Hi) Untreated CETAB JR-400 Jaguar C-17 20 1.98 2.32 3.03 2.47 25 2.08 2.29 2.84 2.42 30 2.19 2.26 2.68 2.37 35 2.34 2.29 2.68 2.47 40 2.45 2.32 2.68 2.47 45 2.63 2.37 2.73 2.58 50 2.74 2.42 2.79 2.68 55 2.84 2.45 2.82 2.76 60 2.92 2.50 2.87 2.84 HR (1–5) 1.00 0.96 1.14 1.04 HR (1–9) 1.00 0.96 1.13 1.04 HR (1–5) is for RH 20, 30, 40, 50, and 60. HR (1–9) is for 20, 25, 30, …, 60 .
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