SURFACE AND INTERFACE ANALYSIS, VOL. 24, 522-528 (1996)

Determination of Negative Binding Sites on Hair Surfaces Using XPS and Ba2+ Labeling

Michael A. Stranick Colgate-Palmolive Technical Center, 909 River Road, Piscataway, NJ 08854, USA

The affinity for adsorption of cationic conditioning polymers and organics to hair fiber surfaces is largely deter- mined by the concentration of negative sites on the hair surface. In this study, XPS and Ba2+ labeling were used to determine the negative site concentration on brown and bleached hair samples. Differences in negative site concen- tration were determined for each hair type as well as along the length of the hair fibers from root to tip end. The middle and root sections of the bleached hair exhibited a higher negative site concentration than the brown hair. The negative site concentration increased from root to tip for the brown hair, but decreased from root to tip for the bleached hair. Differences in negative site concentration between hair samples were correlated with variations in the RS0,- concentrations on the fiber surfaces. Barium ion uptake in addition to that associated with RS0, - was attributed to carboxyl site adsorption or to Ba(OH), entrapment within the pores of the hair fiber. Angle-resolved XPS measurements suggested that BaZ+ ions at the outer surface of the fibers were bonded to two negative surface sites, in agreement with theory. Barium ions in the subsurface region of the fibers may be bonded to only one negative site or may be present as entrapped Ba(OH),.

INTRODUCTION

X-ray photoelectron spectroscopy (XPS or ESCA) has been shown to be useful for the characterization of hair and other keratin-based fiber surfaces. The unique capability of XPS to distinguish between the different valence states of elements has been employed to study disulfide bond oxidation in the amino acid cystine, a primary component of the proteins that comprise keratin fibers.'-4 Cystine oxidation is an important indicator of chemical degradation of keratin fiber pro- teins. X-ray photoelectron spectroscopy analysis has shown that the cystine disulfide bond is oxidized to sul- fonate due to the weathering of hair surfaces.' Weather- ing involves chemical degradation of hair fiber proteins as a result of exposure to ultraviolet radiation in the environment.' In addition, XPS has shown that chemi- cal bleaching with hydrogen peroxide at high pH oxi- dizes the cystine disulfide bond to sulfonate on hair and wool surface^.',^ Intermediate sulfur oxidation products have also been discerned using XPS for wool fibers oxi- dized by other chemical method^.^

While XPS has been used to study the chemical changes in hair and wool proteins, comparatively little XPS work has been reported regarding adsorption onto hair surfaces. X-ray photoelectron spectroscopy has been used to study differences in the substantivity of several cellulosic and vinyl-based cationic polymers to hair substrate^.^ The surface sensitivity of XPS was shown to be an advantage in this work, since traditional bulk analytical methods cannot distinguish between surface deposit and that which has penetrated the fiber.

The affinity for adsorption of substances to hair is largely determined by the chemical nature of the

CCC 0142--2421/96,/080522-07 0 1996 by John Wiley & Sons, Ltd.

surface. In aqueous solutions having a pH > 3.7, the hair surface is negatively charged.6 Thus the adsorption of cationic species, such as the cationic polymers and organic compounds often used to condition hair, is favored. The negative charge on the hair surface arises from the sulfonate and carboxyl groups present in the proteins and lipids that constitute the hair fiber. The concentrations of these negative sites on hair surfaces may vary as the hair surface is changed by chemical treatment or natural weathering. This variation in nega- tive site concentration is a factor that can affect adsorp- tion of cationic compounds onto hair surfaces. Thus, quantifying the negatively charged sites on hair surfaces would provide information useful in the study of cationic adsorption onto hair.

Chemical derivatization or labeling in combination with XPS has been used to quantify specific chemical groups on s~r faces .~ Chemical derivatization is used to identify surface chemical groups that cannot otherwise be detected by XPS due to overlap from interfering peaks or to low surface concentration. For example, XPS combined with Ba2+ labeling has been used to determine the concentration of negatively charged acid groups on carbon fiber surfaces.'-'' In the present work, the negatively charged sites on brown and bleached hair surfaces have been labeled with Ba2 + and quantified using XPS. Since Ba2 + forms low-solubility salts with sulfonate and carboxyl compounds, it is a suitable choice for labeling the negative sites on the hair surface. Differences in the negative site concentrations between brown and bleached hair, as well as that along the length of the hair fibers from the root to tip end, are reported. The results of XPS analyses for unlabeled control hair samples are also presented for reference purposes, and to illustrate the rationale behind the use of Ba2 + labeling.

Received 14 March 1996 Accepted 13 May 1996

XPS OF Ba2+-LABELED HAIR 523

EXPERIMENTAL

X-ray photoelectron spectroscopy analysis

A 1.2 cm long, 20-30 fiber hair sample was attached to a slotted, stainless-steel sample mount for XPS analysis. The fibers were placed over the slot, to eliminate any possible interfering signal from the sample mount itself. The ends of the fibers were loosely attached to the sample mount using double-sided adhesive tape. The fibers were further secured by placing an Mo mask over the fibers and affixing it to the mount with two machine screws. A 5 mm diameter hole in the mask exposed the hair fiber surfaces for XPS analysis. Samples from the tip, middle and root sections of brown and bleached hair tresses were analyzed.

The XPS analyses were conducted with a Physical Electronics (PHI) model 5600 ESCA spectrometer employing monochromatic A1 Ka x-rays (hv = 1486.7 eV). The x-ray source was operated at 400 W in the diffuse spot mode. The XPS spectra were obtained with the sample at an angle of 65" with respect to the analyzer axis, and with the analysis area set to a 2 x 0.8 mm spot. Selected samples were also analyzed at angles of 15" and 40", utilizing an 800 pm spot. To minimize the effects of the irregular sample topography on the angular distribution of the detected photoelectrons, the hair fibers were oriented so that the axes of the fibers were parallel to that of the analyzer. The spherical sector analyzer was operated in the fixed analyzer trans- mission mode using pass energies of 187.85 and 29.35 eV (corresponding to Ag 3d,,, FWHM of 2.42 and 0.82 eV) for survey and high-resolution scans, respectively. The analyzer was calibrated to the Cu 2p3,, line at 932.67 eV and the Au 4f,,, line at 84.00 eV. Com- pensation for sample charging was accomplished by use of an electron flood gun. The voltage and emission current of the flood gun were adjusted for each sample so as to place the C 1s line at -284 1 eV, while achieving a minimum peak width. Data acquisition and analysis were performed using PHI software and an HP Apollo 425s computer. Peak positions were corrected to C 1s at 284.6 eV. Quantification was accomplished using the PHI data analysis software and sensitivity factors. Curve fitting of spectral data was performed using a least-squares fitting routine contained in the PHI software. The curve-fitting routine utilized an inte- gral background subtraction and Gaussian-Lorentzian peak shape.

Barium ion labeling of hair

Hair samples comprising approximately 30-50 fibers of 5 cm in length were taken from the tip, middle and root sections of 20 cm long brown and bleached hair tresses. The virgin European brown and bleached hair tresses were obtained from DeMeo Brothers, New York, NY. The tresses had previously been washed in a 20% solu- tion of sodium lauryl sulfate. To facilitate handling, the samples were bound at one end using laboratory tape. Barium labeling was accomplished by suspending the samples to within 3 mm of the tape binding in a beaker containing 80 ml of an aqueous 0.1 M BaC1, (Baker

Analyzed reagent) solution. The samples were immersed for a period of 15 min with constant stirring. Three hair samples were treated at a time. In a similar fashion, the samples were rinsed for two 5 min periods in separate 80 ml volumes of absolute methanol (Baker Analyzed reagent) to remove any excess BaC1, solution from the hair fiber surfaces. The samples were air dried prior to mounting for XPS analysis. Chlorine was not detected by XPS for the labeled hair samples, indicating that complete rinsing of residual BaCI, was achieved. Addi- tional methanol rinsing did not produce a reduction in the surface barium content. Also, no change in surface composition was detected by XPS for a control hair sample, measured before and after methanol rinsing. This indicated that the protein and lipid composition of the hair surface was not significantly altered by the rinsing procedure.

RESULTS AND DISCUSSION

X-ray photoelectron spectroscopy analysis of control hair

The XPS survey spectrum of a control sample of brown hair is shown in Fig. l(a). The survey spectrum is typical of those measured for keratin surfaces, as photoelectron peaks due to carbon, oxygen, nitrogen and sulfur are evident. The presence of these elements is consistent with the protein and lipid nature of the hair surface.6 Low-intensity peaks due to calcium and silicon are also evident in the spectrum, and represent Ca2+ and sili- cone contaminants often found on hair surfaces.

The XPS elemental analyses of samples taken from the tip, middle and root sections of brown and bleached hair are presented in Table 1. A small decrease in the carbon concentration and an increase in oxygen and nitrogen concentrations are observed from the root to the tip of the brown hair. This indicates that the amount of surface lipid decreases from root to tip along the length of hair. The reduction in carbon for the tip of the hair is indicative of a lower surface lipid content, while the increased nitrogen and oxygen levels reflect

c 1s (A) Control 0 i s

I R r n m e

u 1 *

(6) Ba Labeled

0 l S ,

1000 800 600 400 200 0 Binding Energy, eV

Figure 1. X-ray photoelectron spectroscopy survey spectra of control brown hair (a) and Ba2+-labeled brown hair (b).

524 M. A. STRANICK

Table 1. Surface composition of control hair determined by XPS Atomic per cent

Sample C 0 N S,,,,, RSO,-' RS-SRb

Brown hair Root 70.1 18.6 8.4 2.9 0.9 2.0 Middle 68.0 19.5 9.6 3.0 1 .o 2.0 Tip 67.0 20.3 9.8 3.0 1.2 1.8

Root 62.9 22.3 11.6 3.2 2.1 1.1 Middle 63.1 22.5 11.6 2.9 2.1 0.8 Tip 63.8 22.0 11.8 2.3 1.7 0.6

Bleached hair

a Percentage of sulfur as sulfonate. Percentage of sulfur as disulfide.

Ratio

RS0,-/RS-SR

0.45 0.50 0.67

1.91 2.63 2.83

that a greater proportion of the surface is comprised of protein. A decrease in lipid from root to tip may reflect the greater length of time that hair towards the tip end is exposed to the environment compared to hair at the root end. For the bleached hair, the carbon, oxygen and nitrogen levels are invariant from the root to tip sec- tions of the hair. This indicates that the relative amounts of protein and lipid are similar along the length of the bleached hair. In addition, Table 1 reveals that the nitrogen and oxygen concentrations for the bleached hair surface are greater than those for the brown hair. Also the atomic percentages of carbon are less for the bleached hair compared to the brown hair. These differences in surface composition between the brown and bleached hair samples are consistent with the removal of lipid from the hair surface during the bleaching process.'

Figure 2 presents typical S 2p spectra for samples of brown and bleached hair. Two S 2p doublets are evident in the spectra, having 2p,,, binding energies of 163.3 and 167.7 eV. The binding energy of the former peak is the same as that measured for sulfur in the amino acid L-cystine at 163.1 eV, and is characteristic of disulfide (RS-SR). Cystine is the amino acid that occurs at the highest concentration in the proteins that com- prise hair fiber surfaces.6 The latter peak has the same binding energy as that determined for sulfur in sodium

Brown Hair

176 172 168 164 160 156 Binding Energy, eV

Figure 2. X-ray photoelectron spectroscopy S 2p spectra for brown and bleached hair.

paraffin sulfonate (RSO, -) at 167.8 eV. These binding energies are also in agreement with those reported for other disulfide and sulfonate compound^."^ The sul- fonate species detected on the hair surface forms from oxidation of the disulfide bond in the proteins of the hair that contain cystine.' Oxidation of cystine occurs as a result of weathering or chemical treatment of the hair.' It is evident from the S 2p spectra in Fig. 2 that chemical bleaching oxidizes a significant number of the disulfide bonds at the hair surface, resulting in an increased sulfonate concentration.

The RSO,- and RS-SR concentrations, as well as the RS0,-/RS-SR ratios, provide an indication of the extent that hair surfaces have been exposed to oxidizing conditions. The sulfonate and disulfide concentrations for tip, middle and root samples of brown and bleached hair are presented in Table 1, along with the RS03-/RS-SR ratios. For brown hair, the RS0,- con- centrations increase, while the RS-SR concentrations decrease from root to tip. This indicates that oxidation of the proteins in the hair fiber increases along the length of the hair from the root to the tip, due to the increased exposure to weathering conditions. For the bleached hair, Table 1 shows that the RSO,- and RS-SR concentrations decrease, while the RSO,-/RS- SR ratio increases from the root to the tip. The decrease in the sulfur species indicates that the concentration of sulfur-containing proteins decreases from root to tip along the hair fiber. This effect may be due to greater solubilization of sulfur-containing proteins toward the tip end of the hair, as a result of bleaching.6 The RSO, -/RS-SR ratios indicate that oxidation of the pro- teins in the hair increases along the fiber from root to tip. Thus, bleaching does not result in a uniform level of oxidation for the proteins along the length of the hair. Also, the RSO, -/RS-SR ratios for the bleached hair are greater than those for the brown hair, indicating that bleached hair has a greater fraction of sulfur species present in an oxidized form.

The C 1s spectrum of a control brown hair sample is shown in Fig. 3. This spectrum illustrates the carbon peaks observed for all hair samples analyzed. Curve fitting the spectrum reveals the presence of three peaks. A hydrocarbon peak, CH, , is evident at 284.6 eV and is consistent with the protein and lipid surface of the hair. An amide carbon peak, N-C=O, from surface pro- teins can also be identified at 288.0 eV. The peak at 286.0 eV can be assigned to the carbon adjacent to the amide group, singly bonded to nitrogen (C-N). This

XPS OF Ba2+-LABELED HAIR 525

c 1s

Brown Hair RHx

I I I 292 288 284 280

Binding Energy, eV

Figure 3. Typical XPS C 1 s spectrum for brown hair.

assignment is also consistent with surface proteins. The C-N/N-C=O peak area ratio is greater than unity for hair samples, however, suggesting that additional carbon functional groups contribute to the 286.0 eV peak. For example, the XPS peak for other C-N groups present in amino acid side chains will also occur at 286.0 eV. X-ray photoelectron spectroscopy studies of the homopolymers of several amino acids have recently shown that the peak representing carbon singly bonded to oxygen (C-0) occurs at approximately the same binding energy (k 0.2 eV) as that for C-N type carbon.', Thus, the 286.0 eV peak for hair surfaces can include contributions from various C-N and C-0 species. The presence of these species is consistent with the various amino acids and lipids that constitute the hair fiber surface.6

It is well known that carboxyl groups terminate one end of protein chains; however, there is no indication of a carboxyl peak in the C 1s data. This suggests that the carboxyl concentration on the hair surface is low in comparison to the other carbon species present. The

proximity of the carboxyl peak to the amide peak also complicates the identification of carboxyl groups on hair surfaces. Thus, the concentration of surface carbox- yl groups present in the proteins that comprise hair fibers cannot be determined by curve fitting the XPS C 1s data.

The negative sites on the hair surface consist of sul- fonate and carboxyl groups. It is evident from the above data that sulfonate concentrations vary between hair types and also along the length of the hair fiber. Since the sulfonate group is responsible for some fraction of the negative charge on the hair surface, the concentra- tion of negative sites should also vary with hair type and location along the fiber. In contrast to the sulfonate groups, the surface carboxyl concentration cannot be determined from XPS data. Thus, the contribution of carboxyl groups to the total negative site concentration on the hair surface is unknown. The inability to directly determine the carboxyl concentration on hair surfaces necessitates the use of a labeling agent, such as Ba2+, to quantify differences in the total negative site concentra- tion for hair fibers.

X-ray photoelectron spectroscopy analysis of Ba2 +-labeled hair

Figure l(b) shows the XPS survey spectrum of a BaZ+- labeled brown hair sample. In addition to the carbon, oxygen, nitrogen and sulfur peaks in the spectrum, lines due to barium are also evident, indicating Ba2+ adsorp- tion onto the hair surface. The absence of a C1 2p peak at - 197 eV indicates that there is no residual BaCl, on the hair surface.

Table 2 presents the XPS elemental analysis data for the Ba2+-labeled brown and bleached hair. Since hair is a natural material that is affected by environmental conditions, some variation in surface chemical composi- tion between samples is not unusual. Thus, several samples from both hair types and from each location along the hair fiber were Baz+-labeled and analyzed by XPS. The elemental analysis results for each sample presented in Table 2 represent the mean values from a

Table 2. Surface composition of Ba2+-labeled hair determined by XPS Atomic per cent

Sample C 0 N S,,,,, KO, - RS-SRb

Brown hair Root

Middle

Tip

Bleached hair Root

Middle

Tip

70.3 18.6 7.6 2.6 1 .o 1.6 (1.8)' (0.7) (1.0) (0.1) (0.1) (0.2) 67.9 19.1 9.4 2.8 1 .o 1.8 (0.6) (0.2) (0.5) (0.1) (0.2) (0.2) 70.1 18.5 7.8 2.5 1.2 1.4 (1.2) (0.5) (0.6) (0.1) (0.1) (0.1)

64.7 21.2 10.0 2.7 1.8 0.9 (2.1) (1.0) (1.0) (0.2) (0.1) (0.1) 65.7 20.6 10.1 2.4 1.7 0.7 (2.3) (1.0) (1.3) (0.1) (0.1) (0.1) 65.5 20.7 10.6 2.1 1.5 0.6 (1.1) (0.4) (0.7) (0.1) (0.1) (0.1)

Ratio Ba RSO,-/RS-SR

0.9 0.63

0.9 0.56

1.1 0.86

(0.04)

(0.1 )

(0.04)

1.5 2.00

1.3 2.43

1.2 2.50

(0.04)

(0.1 1

(0.1 1 * Percentage of sulfur as sulfonate.

Percentage of sulfur as disulfide. Standard errors of the means in parentheses.

526 M. A. STRANICK

minimum of three labeled samples. For the labeled brown hair, the average carbon, oxygen and nitrogen levels for the middle and root sections of the hair do not differ significantly from those of the control. Higher carbon and lower oxygen and nitrogen concentrations for the brown hair suggest that the labeled tip samples contain slightly more lipid than the corresponding control hair. The tip, middle and root of the labeled bleached hair samples exhibit similar carbon, oxygen and nitrogen concentrations as observed for the control hair. The same trends in RS0,-/RS-SR ratios from root to tip are also observed for the labeled brown and bleached hair samples as for the controls. Thus, the data in Table 2 indicate that the protein and lipid composi- tion of the hair surface is not affected by the BaZ+ label- ing procedure.

The surface BaZ+ concentrations for the labeled brown and bleached hair samples are also presented in Table 2. For the brown hair, the Ba2+ concentrations on the surface increase from root to tip. Thus, the nega- tive charge on the surface of the brown hair is greatest at the tip end. The concentration of negative sites on the tip end of the hair is larger than that of the root end by 28%. Conversely for the bleached hair, the Ba2+ con- centration decreases from root to tip. This indicates that the negative charge is greatest at the root end of the bleached hair. For the bleached hair the concentration of negative sites at the root end is greater than the tip end by 23%. In addition, the Ba2+ levels on the middle and root sections of the bleached hair were greater than those for the brown hair by 48% and 68%, respectively. Thus the negative site concentrations for the middle and root sections of the bleached hair are significantly greater than those of the brown hair. The concentration of negative sites at the tips of the bleached and brown hair are similar. For both brown and bleached hair, the trends in surface Ba2 + concentration follow the same trends as the RSO, - concentrations. Thus, differences in negative site concentration between hair types and along the length of the hair fiber correlate with the number of oxidized disulfide bonds in the proteins that comprise the hair fiber. It should also be noted that the observed negative site concentrations are specific to the hair samples used in this study. Hair that has been exposed to different environmental or chemical condi- tions may exhibit a different negative site distribution.

The nature of Ba2+ adsorption on the hair surface can be further elucidated by considering the Ba2+/RS0,- ratio. Adsorption of dibasic BaZ+ to two adjacent RSO,- sites on the hair surface should produce a Ba2+/RS0,- ratio of 0.5. However, the average Ba2+/RS03- ratio for the hair samples, calcu- lated from the data in Table 2, is 0.85. This suggests that Ba2+ is adsorbing to sites other than RS0,- or that Ba2+ can adsorb to single negative sites on the hair surface. The possibility also exists that some Ba2+ ions are non-specifically adsorbed or remain physically entrapped within the hair fibers. Entrapment of BaC1, is unlikely, since chlorine was not detected by XPS. Barium hydroxide particles have been detected on the surfaces of BaZ +-labeled carbon fibers, however." Thus, the entrapment of Ba(OH), within the pores of the hair fibers cannot be excluded.

Figure 4 shows the XPS-determined BaZ+ (at.%) adsorbed on the hair surface plotted as a function of the

1.60 a,

1.20 -I

I 0.5

r 0 - 65"

0.00

0.00 0.40 0.80 1.20 1.60 2.00

XPS Atomic O h RSOj

Figure 4. X-ray photoelectron spectroscopy atomic per cent of BaZ+ vs. atomic per cent of RS0,- for Ba2+-labeled hair.

atomic percent RSO, -, for all labeled hair samples. It can be seen that the surface Ba2+ concentration increases linearly with increasing RSO, - concentration on the hair surface. Thus, differences in the negative site concentration are directly correlated to the concentra- tion of oxidized disulfide bonds on the hair surface. A linear least-squares fit of the data in Fig. 4 yields an intercept at 0.24 at.% Ba". The non-zero intercept for the plot in Fig. 4 suggests that some Ba2+ present is not associated with RSO,- sites on the hair surface. This additional contribution to Ba2 + uptake may be associ- ated with Ba2 + adsorption to surface carboxyl groups, or to Ba(OH), that is entrapped within the hair fiber. It is not possible, however, to distinguish between adsorbed Ba2+ and that which is entrapped within the fiber. The relative fractions of carboxyl adsorbed and entrapped Ba2 + are therefore unknown. The non-zero intercept, however, may be considered an estimate for the upper limit of the carboxyl group concentration on the hair surface. Thus, Fig. 4 shows that the differences in negative site concentration on hair surfaces are due primarily to variations in the degree of cystine disulfide bond oxidation to RSO, -. Additional uptake of Ba2+ can be attributed to carboxyl site adsorption and/or to Ba(OH), entrapment within the fiber.

The slope of the linear fit to the data in Fig. 4 is 0.66, slightly greater than the theoretical value of 0.5 for stoi- chiometric Ba2+ to RSO,- adsorption. Barium ion adsorption to only one negative site on the hair surface may account for this difference between the experimen- tal and theoretical slopes. Ideally, dibasic Ba2 + requires two negative sites, either RSO, - or carboxyl, in order to bond to the hair surface. Two negative surface sites may not always be in close enough proximity to allow this type of binding to occur. For example, on carbon fiber surfaces, it was estimated that the maximum separation between adjacent carboxyl groups that would allow binding of Bit2+ to occur was 0.4 nm.* In some instances, adjacent negative sites on the hair surface may be too widely separated, so that BaZ + ions have only one negative site with which to bond. In this case, charge neutrality can be maintained through the bonding of an additional singly charged anion obtained from the labeling solution. The hydroxyl ion is the most

XPS OF Ba*+-LABELED HAIR 521

likely candidate for this additional anion, since chloride was not detected on the hair surfaces. If Ba2+ adsorp- tion to a single negative site is occurring, it would account for - 30% of the total Ba2+ uptake.

The entrapment of Ba(OH)2 within the pores of the hair fibers may also be a factor contributing to the dif- ference between the experimental and theoretical data observed in Fig. 4. The porosity of hair fibers is likely to vary between hair types and along the length of the hair fibers. Thus, the quantity of Ba(OH), which may be entrapped within the pores of the hair fibers is also likely to vary from one sample to another. Variation in the entrapped Ba2+ levels could also account for the experimental results shown in Fig. 4.

The deviation between the measured and predicted slopes for the plot shown in Fig. 4 was further investi- gated using angle-resolved XPS. Samples from the middle sections of brown and bleached hair tresses were Ba2+ labeled and analyzed by XPS at angles of 15", 40" and 65". The chemical compositions of these samples as a function of analysis angle are presented in Table 3. For the brown hair, the RSO, -/RS-SR ratio increases with decreasing analysis angle, indicating that the RSO, - groups occur primarily at the outer surface of the hair fiber. The observation that RSO,- occurs at the outer surface of the fiber is consistent with weather- ing as the cause for oxidation of cystine in the hair fiber proteins. The outer surface proteins of the hair fibers will be more sensitive to the UV weathering conditions that cause oxidation.' Subsurface proteins should be less affected. For the bleached hair, the RSO,- levels decrease - 20% with decreasing analysis angle. This trend indicates that while most of the RSO,- groups occur at the outer surface of the fiber, some RSO, - also exists in the subsurface regions. The presence of RSO, - in the subsurface region of the fiber is not unreasonable because the bleaching solution will penetrate the fiber to some extent. An increase in the RSO,-/RS-SR ratio with decreasing angle for the bleached hair was also observed, further indicating that the greatest fraction of RS0,- groups is at the outer fiber surface. Thus, for both the brown and bleached hair, the negative sites attributable to RSO, - groups are concentrated at the outer surfaces of the fibers.

The surface Ba2+ concentrations for the brown and bleached hair samples, measured as a function of analysis angle, are also presented in Table 3. It can be seen that for both the brown and bleached hair samples, the BaZ+/RSO3 - ratios decrease with decreasing

analysis angle. This indicates that, relative to RSO, -, there is a higher Ba2+ concentration in the subsurface region of the fibers than at the surface. The RS0 , - groups, which provide many of the binding sites for Ba2+, were shown to exist, however, mainly at the outer fiber surface. Thus, Ba2+ that has penetrated the fiber will be less likely to have two adjacent negative RSO, - sites with which to bond. This means that some sub- surface Ba2+ may be bonded to only one negative site. Barium ions at the outer surface of the fiber should have a higher probability of bonding to two adjacent negative RSO, - sites. The decrease in the Ba2 +/RSO, - ratios with decreasing analysis angle can thus be accounted for by the bonding of BaZ+ to single RSO, - sites in the subsurface region of the fibers. In addition, any Ba(OH), that is entrapped in the subsurface pores of the fibers would also cause a decrease in the Ba2+/RS03 - ratios with decreasing angle. Based on this reasoning, the Ba2+ and RSO, - concentrations measured at 15" should more closely reflect the theoreti- cal stoichiometry required for bonding of a dibasic cation to the singly charged negative sites on the hair surface. The XPS Ba2 + concentration (at.%) plotted us. the RS0,- concentration (at.%) for the brown and bleached hair samples, measured at an angle of 15", is shown in Fig. 4. It is evident from Fig. 4 that the data obtained at 15" are in agreement with the theoretical values for stoichiometric Ba2+ to RSO, - adsorption. Thus, comparison of the data obtained at 15" and 65" suggests that Ba2+ ions at the outer surface of the fibers are bonded to two negative sites, in agreement with theory. Barium ions that bond to only one negative site or become entrapped as Ba(OH), are located in the subsurface regions of the hair fibers. Thus, these sub- surface Ba2 + species account for the deviation between the theoretical and experimental data presented in Fig. 4.

CONCLUSIONS

(1) The combination of XPS and Ba2+ labeling can be used to determine differences in the concentrations of negative sites on hair surfaces.

(2) For brown hair, the negative site concentration is greater at the tip end of the hair than at the root

~~~~~ ~~~~

Table 3. Angle-resolved XPS analysis of Ba2+-labeled hair-middle section

Atomic per cent Analysis angle C 0 N S,,,,, RSO,-' RS-SR" Ba

Brown hair 65" 69.0 18.7 8.8 2.6 1 .I 1.5 0.9 40" 72.4 17.1 7.3 2.3 1.1 1.2 0.9 15" 76.2 15.5 5.6 1.8 1 .I 0.7 0.8

65" 70.7 18.4 7.4 2.1 1.7 0.4 1.5 40" 73.3 16.9 6.6 1.9 1.6 0.3 1.3 15" 80.7 12.6 4.3 1.5 1.3 0.2 0.9

Bleached hair

Percentage of sulfur as sulfonate Percentage of sulfur as disulfide.

Ratio

RS0,-/RS-SR Ba/RSO,-

0.73 0.82 0.92 0.82 1.61 0.73

4.25 0.88 5.33 0.81 6.50 0.69

528 M. A. STRANICK

end. Conversely for bleached hair, the concentra- tions of negative sites increase from tip to root.

(3) The concentrations of negative sites on the middle and root sections of the bleached hair are greater than those on the brown hair. The negative site con- centrations for the tips of the brown and bleached hair were similar.

(4) Differences in the negative site concentrations between brown and bleached hair samples or along the length of hair fibers correlate with variations in the RSO, - concentrations on the hair surface. The uptake of BaZ+ in excess of that associated with

RS0,- can be attributed to carboxyl site adsorp- tion or to Ba(OH), entrapment within the hair fiber.

(5 ) Angle-resolved XPS measurements indicate that RS0,- groups are concentrated at the outer sur- faces of both brown and bleached hair fibers. The angle-resolved data also suggest that Ba2+ ions at the outer surfaces of the hair fibers are bonded to two negative sites, in agreement with theory. The data further suggest that subsurface Ba2+ ions may be bonded to only one negative site or be indicative of Ba(OH), entrapment.

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