6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS course of dyeing, the hair samples were pretreated by immersing the hair samples in about 300 ml of buffer solution of appropriate pH overnight, prior to dyeing. In addition to maintaining a uniform pH during dyeing, pretreatment was found to improve the reproducibility of the dyeing kinetics, since the fibers have reached equi- librium swelling prior to dyeing. This pretreatment, however, was performed only in the case of the aqueous dyeing experiments. To establish the effbct of stirring the dye solution on the rate of diffusion, experiments were performed with and without stirring under otherwise identical conditions (HC Red 3, C = 1.0 g/l, 50 vol.% aqueous ethanol, pH = 10.8, and T = 42øC). It was found that stirring the dyebath did not produce any significant differences in either D or M•. Consequently all dyeing experiments were performed without stirring in order to avoid felting of the hair. Dyeing was performed by immersing about 1.0 g of the hair sample (--2-3 inches in length) in the dyebath (liquor-to-hair ratio 200:1) and removing aliquots of the hair sample at given time intervals. The dyed hair was rinsed well, blotted between layers of filter paper, and immediately dried in a vacuum desiccator. The weight of the dried dyed hair was determined, and the sample was then placed in a 50-ml groundglass stoppered Erlenmeyer flask for extraction. The dyestuff was extracted either at room temperature or at 60øC using 50 vol.% aqueous ethanol as the extracting medium. Two to four successive extractions were carried out using 5.0 ml of fresh solvent for each extraction lasting about 2 h. The solvent fractions were combined and their absorbance measured with a Beckman DU- 6 UV-visible spectrophotometer. The amount of dye uptake was calculated from the absorbance values. Linear relationships according to Beer's law were found between absorbance and concentration for HC Red 3 in this solvent (½)491 = 4950. DESORPTION EXPERIMENTS Dye desorption experiments were carried out using a flow cell system attached to the Beckmann DU-6 UV-visible spectrophotometer. This arrangement permits continuous monitoring of solution absorbance and thus has the advantage of covering the important early stages of dye desorption. The hair samples used for desorption experiments were dyed at specified conditions for 24 h to ensure that equilibrium sorption conditions were attained, and were then dried and stored as described previously. These samples were immersed in a specified volume of solvent to desorb the dye at a given temperature, and the absorbance of the solution was monitored as a function of time. Desorption equilibrium measurements were used to determine the partition coefficients of the dye between the hair fiber and the dyebath solution. MICROSPECTROPHOTOMETRY A Leitz MPV 1.1 microscope photometer equipped with an interference filter for spectroscopic measurements was used in this study (Figure 2). This instrument is basically a microscope fitted with a photomultiplier to measure the intensity of the light transmitted through microscopic specimens at selected wavelengths. A variable

SEMIPERMANENT DYE DIFFUSION IN HAIR 7 PHOTOMULTI PLIER i NTE RFE R E NCE•'""•'I FILTER (360-700n rn) I X-Y RECORDER INTERFACE COMPUTER --i--- •.• IBM-RDC • DIAPHRAGSTAGESPECIMEN OBJECTIVE • I---'1 i• CONDENSER I LIGHT SOURCE ( TUNGSTEN-HALOGEN ) Figure 2. Schematic diagram of the Leitz MPV l. 1 microscope photometer. diaphragm is introduced between the objective and the photomultiplier to restrict the size of the beam, so that selected areas of the specimen as small as --0.5 •m 2 can be viewed and analyzed. The specimen stage of the microscope is moved horizontally at a constant speed (--3 •m/s), thus permitting scanning of the specimen by the light beam. The system is designed to take four readings of intensity per second as the fiber cross section moves through the light beam, and these data are transferred to a computer via an IBM research device coupler. A plot of light intensity as a function of position in the fiber cross section, i.e., the dye concentration profile, can then be constructed. From the dye concentration profile, dye diffusion coefficients at specific concentrations can be determined using eq. 3, and the fractional dye uptake, Mt/M•, can be deter- mined from eq. 10. For this study, white hair fibers dyed as described in the previous section were em- bedded in an epoxy resin and cured. The cured blocks were microtomed on a Du Pont JB-4 microtome to 10 •m thickness, and the fiber cross sections were examined at an appropriate wavelength (491 nm). RESULTS AND DISCUSSION pH EFFECTS The effects of dyebath pH on the rate of dye uptake of HC Red 3 from aqueous solution