18-MEA AND HAIR APPEARANCE 149 such as the sum of the total ion yield, the ion peak from hydrocarbons, or the ion peak from the matrix constituent. In the case of using the sum of the total ion yield or the ion peak from hydrocarbons as a standard ion, however, these yields change due to surface contaminants such as anionic surfactants and silicone oil. Here, the ion peak from the matrix constituent was chosen as a standard ion to avoid these effects. In this study, the CN ion was used for normalization since the matrix of the hair surface is keratinous protein and the CN ion was strongly detected in the TOF-SIMS measurement of hair samples. Measurement of surface properties of hair. The wetting forces of hair were measured by the Wilhelmy method using a K100MK2 tensiometer (Kruss). Single hair fi bers were scanned over 3 mm at a velocity of 2 mm/min for the advancing mode. Dynamic contact angles were calculated from F cosθ = Sdγ where F is the wetting force, d is the diameter of the hair, γ is the surface tension of water, and θ is the contact angle of the liquid versus the fi ber surface. The hair fi ber diameter was measured on the transverse section of each fi ber with a rotating fi ber diameter measurement system equipped with a laser (Kato Tech Co.) at 20°C and at a relative humidity (RH) of 65%. The wetting force measurements were also performed at 20°C, 65% RH. Frictional properties of the outermost surface of the hair in the wet environment were measured using a Nanoscope III Dimension 3000 (Veeco Instruments, Santa Barbra, CA). Friction force microscopy (FFM) data were acquired using unmodifi ed silicon nitride (Si-N) cantilevers (spring constant of 0.12 Nm-1). After engagement of the tip with the cuticle surface, the tip was scanned parallel to the longitudinal axis of the fi ber. In order to minimize scanning artifacts, a scan rate of 1 Hz was used for all measurements. To characterize frictional properties, 2- × 2-μm scans of the cuticle faces (without edges) were performed. Trace and retrace FFM datasets were obtained, and the friction force was calculated by subtracting the retrace FFM signal from the trace FFM signal. In the FFM measurement of the cuticle surfaces before and after removing 18-MEA, trace and retrace FFM datasets were obtained at different areas of the cuticle surfaces samples were ran- domly picked up from untreated hair and 18-MEA-removed hair, and trace and retrace FFM datasets were obtained. RESULTS AND DISCUSSION INFLUENCE OF SCALE DIRECTION OF HAIR ON DYNAMIC CONTACT ANGLE A sensitive method to provide information on the outermost surface of hair is contact angle measurement. Its interpretation, however, is not always straightforward because surfaces usually give two stable contact angles, advancing and receding. The difference between the advancing and receding contact angles is referred to as contact angle hyster- esis. It is generally accepted that the contact angle hysteresis can be due to a multitude of effects, such as surface roughness, chemical heterogeneity, surface deformation, and sur- face confi guration changes (15–19).

JOURNAL OF COSMETIC SCIENCE 150 In the case of the dynamic contact angle measurements, it is necessary to take the scale direction of the hair into account because cuticle cells are attached at the root end and point forward toward the tip end of the hair fi ber. In order to examine the scale direction, a single hair fi ber was cut vertically into two pieces along the line a–b in Figure 1, and the dynamic contact angles were measured by immersing the resulting cut areas in water. Figure 2 shows the typical force curves for untreated hair in the “against scale” (AS) and “with scale” (WS) directions. Figure 3 shows the advancing and receding contact angles for untreated hair in the AS and WS directions. The error bar of each column represents the standard deviation within each group. The asterisk symbol in Figure 3 indicates a signifi cant p-value obtained from Student’s t-test. There was a signifi cant difference in the receding contact angles between the AS and WS directions while there was no signifi cant difference in the advancing contact angles between the AS and WS directions. The reced- ing contact angle for untreated hair was higher in the WS direction than in the AS direction. The results agreed with those of Molina et al. (13) and Kamath et al. (20). Kamath et al. explained their reasons for the receding contact angles for untreated hair being higher in the WS direction than in the AS direction as follows: in the receding mode the frontal edges of the scales provide a lower contact angle (possibly due to scale edge abrasion) in the AS direction, while the dorsal sides of the scales, which are hydrophobic due to the presence of 18-MEA, make a greater contribution in the WS direction, which is Figure 2. Typical contact angle force curves for untreated hair in the “against scale” (AS) and “with scale” (WS) directions. Figure 1. Schematic description of the method for studying the effect of the scale direction of hair on dynamic contact angle measurements.