140 JOURNAL OF COSMETIC SCIENCE noticed that hair-set durability was also improved by the same treatment, as well as hair luster. Drying a wet hair, held in a given shape, with a hot dryer or a curling iron makes the hair set. This process is usually called water set. It is well known, however, that the created hair style is easily broken down with time, especially quickly in high-humidity conditions. This is due to the exchange of hydrogen bonds in the hair fiber, according to the movement of water molecules (5 ). Through treatment with an aqueous solution comprising malic acid (MA) and benzyl- oxyethanol (BOE), the setting durability of the hair was improved. This improvement was observed in the test using single hair fibers (6), suggesting that the effect is not on the basis of a surface-adhesion increase but a change in the internal property of hair. Consequently, the treatment makes it possible for hair to be set with a natural feel. This setting method is completely different from fixing by setting polymers or adhesive oils, which often are the causes of hair roughness or stickiness. Since water set is closely related to the interaction of proteins with water, near-infrared (NIR) spectroscopy is one of the best methods (7) to analyze hydrogen bonds and has been used by other researchers. The NIR region corresponds to overtone and combina- tion bands of infrared bands. The absorbance intensity of these bands is relatively weaker than that of normal modes in infrared spectroscopy, but the low absorbance leads to a high rate of transmission and gives internal information on the hair. The NIR spectrum ' is complex in general, but many bands for various materials, including proteins, have been assigned (8). For wool and human hair fibers, spectra in the NIR region have been measured and the main bands assigned (9-11). By measurements with NIR spectroscopy (6), it has also been suggested that the mecha- nism of improvement of set durability by organic acid is due to the strengthening of hydrogen bond interaction around hair proteins. For further analysis, generalized two- dimensional (2D) correlation spectroscopy was applied in this study. This method, proposed by Noda (12), has been applied extensively to analyze the IR and NIR spectra of proteins (13-16). In this method, a perturbation is applied to a system and various spectra are measured under sequential conditions. The measured fluctuations of spectral signals are then transformed into 2D spectra by use of a correlation method. The 2D correlation spectra can deconvolute overlapped bands, enabling us to know the correlations between bands due to different structures, and it provides information about the specific order of structural changes under perturbation. Any perturbation- mechanical, electrical, chemical, thermal, etc.-may be used, and temperature, concen- tration, or pH are well selected as variables. In this study, we selected the treatment time on hairs as the variable. EXPERIMENT AL HAIR SAMPLES Chemically untreated Chinese hair washed with a plain shampoo was used. The average diameter, measured by a micrometer, was 62 µm. For the acid treatment, hair fibers were immersed in an aqueous solution comprising MA (4%)/BOE (10%)/ethanol (15%) at 40 ° C for 20, 40, or 60 minutes, washed with water

INTERACTION BETWEEN HAIR PROTEIN AND ORGANIC ACID 141 for 30 sec, then naturally dried. For control samples, the hair was also treated with deionized water or an aqueous solution of 0.001 N HCl/BOE (10%)/ethanol (15%) in the same manner. CURL RETENTION MEASUREMENT IN HIGH HUMIDITY Single fibers (ca. 10-cm long) were wound around glass rods (10 mm p) under a constant tension by pending a small weight at the tip end of the fibers. The samples were then wetted with a small amount of water and blow-dried. After storage under ambient conditions (ca. 25°C, 30% RH) overnight, the hair fibers were removed from the rods. All of the hair fibers were water set in the shape of the glass rods, taking the appearance of "springs." By being cut at one side of the spring, some single fibers, having an almost round shape, were obtained. About ten curls were obtained from five hair fibers. Their shape was recorded by a digital camera, and they were kept in a glove box controlled at 26°C, 90% RH. The fibers' shape was again recorded at time t: the time left in the box. After repeating series of the process above, the radii of the curvature of the hair fibers at t, R n were measured and the hair set index, 5/Rl' where 5 is the radius of the glass rod in millimeters, was calculated. Since most of the hairs were not completely straight, the corrected hair set index was calculated according to the following equation (1) Here, R' is the radius of curvature of the hair fiber after being immersed into water, followed by natural drying. NIR SPECTROSCOPY NIR spectra were measured with a Y okogawa Infra Spec NR800 spectrometer. A total of 1024 scans were accumulated at an 8 cm- 1 resolution for each measurement. The cell of the NIR was set under dry nitrogen atmosphere at 25°C. Hair samples, untreated and treated by an aqueous solution of MA/BOE/ethanol for 20, 40, and 60 min, were dried under vacuum overnight before the NIR measurement. The NIR spectra obtained were subjected to smoothing, baseline correction, and inten- sity correction before further analysis. For the data processing, Grams software (Thermo Electron Corp.) was used. GENERALIZED 2D CORRELATION SPECTROSCOPY The mathematical background for 2D correlation spectroscopy has been described in detail by Noda (12). For the generalized 2D correlation analysis, we used a program named 2D POCHA composed by D. Adachi (Kwansei-Gakuin University). RES UL TS AND DISCUSSION HAIR-SET DURABILITY Figure 1 shows the results of the curl retention measurement for untreated hairs and for those treated with aqueous solutions of MA/BOE/ethanol or HCl/BOE/ethanol. For the