HUMAN HAIR MOISTURIZATION WITH COSMETIC PRODUCTS 311 treatments. Single treatment with regular rinse-off products is unlikely to have a signifi cant moisturizing effect (there may be other benefi cial effects from emollients, similar to the plasticizing effect of water). EFFECT OF HEAT T REATMENT ON S-D HYSTERESIS OF HAIR Heat treatment ( blow-drying or fl at ironing) is a common grooming process used mainly to dry wet hair, and sometimes straighten hair from its curly or frizzy state. The temperatures and the devices used vary, but the overall effect is one of reducing the post-drying water uptake (the amount depends on the temperature and time) which helps prevent curl reversal and reduce frizziness. Covalent cross-linking of hair is possible at relatively high tempera- tures ( ~100°C) by dehydration reactions involving reactive side groups of amino acids, such as –COOH, –OH, and –NH2 groups. Because all the three groups are centers for hydrogen bonding, their elimination by cross-linking reduces the amount of water sorbed by hair. Since sorption and desorption occur by diffusion, cross-linking hair reduces the rate of diffusion by increasing tortuosity. In this work, we selected a heat treatment tem- perature of 120°C for 1 h. This was performed to intensify the effect. The S-D isotherms of untreated and heat-treated (1 h at 120°C) hair are shown in Figure 4. From the isotherms, we can see that the heat-treated hair absorbs signifi cantly less water compared with the untreated. The ratio of the areas of the hysteresis loops of heat-treated hair to that of untreated hair is 1.45. The hysteresis plots are shown in Figure 5. From Figures 4 and 5, it is easy to see that heating hair at elevated temperatures reduces the amount of moisture sorbed by the hair however, it also helps retain that water in hair Figure 4. S–D is o therms of untreated and heat-treated hair.

JOURNAL OF COSMETIC SCIENCE 312 during desorption. Lower diffusion rate should have no effect on the amount of water retained because desorption is carried to equilibrium. Therefore, higher water contents in the desorption phase suggest that cross-linking generates additional areas in the matrix where water is “locked-in” by cross-links. WATER DIFFUSION IN HEAT-TREATED HAIR A unique advantage of a DVS experiment is that it provides a mass change plot as a function of time at a chosen RH. Such a plot can be changed to a plot of Mt/Meq vs —t. From the initial rate of this plot, we can calculate the diffusion coeffi cient D (9) assum- ing an average fi ber diameter. A plot of diffusion coeffi cient as a function of RH is shown in Figure 6 for the untreated and heat-treated hair. The diffusion coeffi cients show a maximum in the mid-humidity range with minima at the extremes, suggesting a transition in the diffusion behavior of water in hair. A possible mechanism for this behavior has been discussed by Keis et al. (9). The lower diffusion coeffi cients of heat- treated hair compared with untreated hair during both sorption and desorption clearly suggests heat-activated cross-linking of proteins via residual side chain functionalities of amino acids. This is also the cause of higher hysteresis compared with that of the untreated hair. The fact that in the mid-humidity range water retention is higher in spite of higher diffusion coeffi cients suggests that the diffusion rate alone is not related to hysteresis. Kinetics of water va por diffusion in hair is controlled by thermodynamic activity and the tortuosity of the diffusion path. The diffusion of water molecules follows random walk kinetics (10). Based on this concept, we can calculate the distance penetrated by a single water molecule by the equation: x2 = 2 Dt, where x is the distance penetrated, D is the Figure 5. Hysteres i s plots of heat-treated and untreated hair.