J . Phys. Chem. 1989, 93, 3255-3258 sound absorption related to the concentration fluctuations, suggest that it is possible to study complex systems such as microemulsions as a function of concentration 4 because they also hold for temperature regions far from the critical one. Following Fenner's suggestion that for high frequencies (than the relaxation one) and in the critical region the aeX f dependence predicted by FB theory is general, we have analyzed our data in terms of the excess in absorption and a classical background. These two contributions have been respectively connected
-
3255
with the structural properties and the shear viscosity of the system. In these terms we have extracted information about the changes in the structure and in the interaction type and about the cooperative effects occurring as a function of concentration in the potassium oleate microemulsions. A comparison of the excess of absorption results with the dielectric susceptibility data, obtained at lower frequencies, reinforces the validity of the analysis. Registry No. Potassium oleate, 143-18-0.
An X-ray Photoelectron Spectroscopic Study of an Alumina-Glucose Adsorption System Jiro Nakatani, Sentaro Ozawa, and Yoshisada Ogino* Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku University, Aramaki-Aoba, Sendai, 980 Japan (Received: June 28, 1988)
X-ray photoelectron (XP) spectra of glucose adsorbed by alumina containing Na sites have been measured in order to elucidate the adsorbed state of glucose. A supplemental adsorption study has revealed that glucose molecules are almost monomolecularly adsorbed by the adsorbent used. The XPS data have shown that most of glucose molecules adsorbed survive the X-ray irradiation under ultrahigh vacuum conditions. It has been shown that the &A1 portion of the adsorbent surface provides glucose molecules with adsorption sites and no preferential adsorption of glucose onto the Na site takes place.
1. Introduction An appli~ationl-~ of XPS to the study of chemical states of molecules adsorbed by solid surfaces brings out useful information into the field of surface chemistry. Even polymeric molecules as well as protein molecules interacting with solid surfaces are reported"I0 to be characterized by this method. The present authors have thus conducted an XPS study of the adsorbed state of glucose molecules over an alumina catalyst which is active for the glucose mutarotation." This reaction in which a-D-glucosewith an optical rotation ([aI2'D) of 112O isomerizes to 0-D-glucose with [aI2'D = 18.7' is a fundamental biochemical reaction and has thoroughly been studied in homogeneous ~ y s t e m s . ' ~On * ~the ~ contrary, only a few studies' i ~ 1 4 - 1 on 6 this reaction over heterogeneous catalysts have been reported. It must be pointed out that information about biochemical reactions over solid surfaces is important in view of its wide applicability to the design of biocompatible or biomimetic ( I ) Carlson, T. A. Photoelectron and Auger Spectroscopy; Plenum: New York, 1975;p 275. (2) Barr, T.L. Practical Surface Analysis by Auger and X-ray Photoelectron SDectroscoov: Wilev: Chichester. U. K., 1983: D 283. (3) Somorjai, G: A . Chemistry in Two Dimensions; Surfaces; Cornell University Press: Ithaca, NY, 1981;p 65. (4) Rater, B. D.; Horbett, T. A.; Shuttleworth, D.; Thomas, H. R. J . Colloid Interface Sci. 1981, 83, 630. (5) Lindberg, B.; Maripuu, R.; Siegbahn,K.; Larson, R.; Golander, C. G.; Ericksson. J. C. J . Colloid Interface Sci. 1983. 95. 308. (6) Sundgren, J. E.;Bode, P.: Ivarsson, B.; Lundstrom, I. J . Colloid Interface Sci. 1986, 113, 530. (7) Zadorecki, P.; Ronnhult, T. J . Polym. Sci. A, Polym. Chem. 1986, 24, 131
I>/.
(8) Golander, C. G.; Ericksson, J. C. J . Colloid Interface Sci. 1987, 119, 38.
(9)Golander, C. G.;Kiss, E. J . Colloid Interface Sci. 1988, 121, 240. (10) Briggs, D.Practical Surface Analysis by Auger and X-ray Photoelectron Snectroscoov: Wilev: New York. 1983:D 359. ( I 1) Oiawa, S.; Nakatani, J.; Sato, M.;'Ogino,'Y. Bull. Chem. SOC.Jpn.
.
1987..60. _ ..,
4273. ~
~
(12) Pigman, W.; Isbell, H. S . Ado. Carbohyd. Chem. 1968, 23, 11. (13) Isbell, H. S.;Pigman, W. Ado. Carbohyd. Chem. 1969, 24, 13. (14) Sato, M.; Nakatani, J.; Ozawa, S.;Ogino, Y. J . Chem. SOC.Jpn., in press. (15) Dunstan, T. D. J.; Pincock, R. E. J . Phys. Chem. 1984, 88, 5684. (16) Dunstan, T. D.J.; Pincock, R. E. J . Org. Chem. 1985, 50, 863.
0022-3654/89/2093-3255$01.50/0
substances. Thus the present authors have intended this work, in order to accumulate information about the biochemical surface catalysis.
2. Experimental Section 2.1. Materials. Neobead-P (Mizusawa Kagaku; X-alumina; specific surface area = 243 m2/g; pore volume = 0.67 cm3/g) was used as an adsorbent. This alumina contained a few percent of N a which acted as a basic site" in the mutarotation of glucose. a-D-Glucose was obtained commercially (Nakarai Kagaku) and used as an adsorbate. Dimethyl sulfoxide (DMSO) was used as a solvent after a thorough dehydration under controlled conditions.' 2.2. Adsorption Measurement and Sample Preparation. The adsorbent alumina was pulverized and dried at 120 O C for 24 h and a given amount of the powder was contacted with a glucose/DMSO solution of a known concentration. This was continued for 24 h at 25 OC under magnetic stirring. The amount of glucose adsorption was calculated from the concentration change during this period (the adsorption equilibrium was reached within 0.5 h). The glucose concentration was determined by the anthrone colorimetric method;"J* an absorption intensity at 620 nm was measured with a Hitachi 330 spectrophotometer. The adsorbent was then filtered, rinsed, and dried at 72 O C for 6 h under a vacuum of 2 X lo2 Pa to remove the solvent DMSO. Finally the adsorbent alumina containing glucose was pressed into a disk with a thickness of ca. 100 km. 2.3. XPS Measurements. X P spectra of the sample disk were recorded on a Shimazu ESCA 750 spectrometer by averaging 10 scans (Mg Ka X-ray source (1253.6 eV); vacuum level attainable was 3.0 X Pa). Data processings including corrections for static charging, background drifts, and cross sections (sensitivities) of elements were carried out using the computer of the spectrometer. The binding energy of carbon (1s) at 285.0 eV was adopted as a reference value in determining line positions of other elements. In addition, every peak position including that of C(1s)
'
~
~~
(17) Morris, D. L. Science 1948, 107, 254. (18) Koehler, L. H.Anal. Chem. 1952, 24, 2004.
0 1989 American Chemical Society
3256 The Journal of Physical Chemistry, Vol. 93, No. 8, 1989
Nakatani et al.
6
10
4
'0 10
"
" " " " "
'
2 4 6 8 10 12 IL Equilibrium Concentration C l g llOOml DMSO b
Nals I
L
2 f-$-g-$-O4-+---
O b '
"
" '
10 ' ' ' C-' l lOOml DMSOlg
" 05
"
OO
" 15
10
5
15
C I gllOOml DMSO
Figure 1. The adsorption isotherm of glucose at 25 OC: (a) ( 0 )observed, (-) calculated with eq 1; (b) a Langmuir plot showing a linearity between y-' and C'.
Figure 2. The relative intensities of XP spectra as a function of the adsorption equilibrium concentration of glucose (C): (a) C(ls), (b) O(ls), (c) AWp), (d) Na(ls).
was occasionally checked by using the Au 4f7/* (84 eV) peak as a standard. In this case, the standard material (Au) was evaporated onto the sample disk by using a JEOL JEE4B equipment at 20 A under a vacuum of 1 X Pa.
3. Results and Discussion 3.1. Adsorption Isotherm. The adsorption isotherm of glucose at 25 O C is shown in Figure la. The ordinate and abscissa represent respectively the amount of adsorption (y) and the equilibrium concentration (C) of glucose. It is interesting that this adsorption isotherm obeys a Langmuir type adsorption equation y = y"KC/(l
+ KC)
(1)
where y" is the amount of adsorption at saturation (the monolayer adsorption capacity) and K is the adsorption equilibrium constant. As can be seen in Figure 1b, plots of y-l against C1are linear, indicating the validity of the application of eq 1 to the adsorption isotherm observed. The intercept of the straight line with the mol/g. The Langmuir adordinate gives that y" = 6.7 X sorption isotherm withy" = 6.7 X lo-" mol/g and K = 0.219 100 mL of DMSO/g agrees well with the observed value, as shown in Figure la. It appears worth mentioning that they" value obtained above is close to the theoretical value obtained by assuming that glucose molecules (each has an molecular area of 50 A2)are monomolecularly covering the surface with an area of 243 m2/g at saturation. This is consistent with the use of the Langmuir type adsorption equation (eq 1) and justifies the use of a monomolecular adsorption model. 3.2. XP Spectral Intensity. The relative intensities of X P spectra ([intensity of an element] X 100/[sum of intensities of C( Is), O( ls), A1(2p), and Na( Is)]) obtained in this study are shown in Figure 2a-d as a function of the equilibrium glucose concentration C which qualitatively represents the amount of adsorption; the use of C as abscissa is only for convenience in displaying the experimental data. As can be seen in this figure, the relative intensity of C( 1s) increased with the amount of glucose adsorption, while the relative intensities of O( 1s) and AI(2p) decreased. On the other hand, the relative intensity of Na( 1s) remained almost constant. Although these results suggest that glucase molecules mostly cover the A 1 4 portion of the adsorbent, scattering of the data points hinders us from extracting further information. Although no figure has been shown here, it must be noted that the use of an intensity normalized by the AI(2p) peak intensity
25Y o b ,
'
'
'
I
5
'
'
'
'
IO
'
'
'
' I
C i g i l O O m l DMSO
Figure 3. XPS peak widths as a function of the adsorption equilibrium concentration of glucose (C): (a) O(ls), Na(ls), and AI(2p); (b) C(1s).
brought out little information about the behavior of surface sites such as 0 and Na. The normalized intensity of the valence band also remained constant and gave no sign of change due to the glucose adsorption (the valence band spectra were obtained by averaging 100 scans). 3.3. XPS Peak Width. The half-width values at the highest peak positions for A1(2p), O( Is), Na( Is), and C( 1s) are shown in Figure 3a,b as a function of the glucose adsorption. The half-width values of A1(2p), O(ls), and Na(1s) remained almost constant. On the other hand, the value of C( 1s) increased greatly with the increase in the glucose adsorption. The scatterings of the data points are not serious and hence we can convince that the half-width increase observed would be caused by the glucose adsorption. A comparison of Figure 3 with Figure l a also supports this view. The C(1s) X P spectra shown in Figure 4 reveal interesting aspects of the peak width broadening mentioned above. It can be pointed out in this figure that the half-width increase of the C(1s) peak is almost solely ascribed to the increase in the peak
XPS Study of an Alumina-Glucose Adsorption System I
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 3257 TABLE I: Characteristics of Glucose XP Spectra peak amount of positions3' sample glucose adsorbed, lo4 mol/n I I1 no. ,I
1 2 3 4 5 6 I Binding
Energy /eV
1.2 1.9 2.7 3.8 4.3 4.4 5.6
288.1 288.3 288.3 288.4 288.3 288.3 288.3
286.6 286.7 286.7 286.7 286.1 286.6 286.6
peak area ratio. 1/11 4.60 4.47 4.24 4.23 4.24 5.01 5.03
uLiterature values are 286.6 eV (I) and 288.1 eV (11); ref 5.
Figure 4. Changes in the C(ls) peak shape upon changing the amount of glucose adsorption (y): y = 0, 1.9, 2.7,4.3,4.4, and 5.6 in lo4 mol/g, respectively, for a, b, c, d, e, and f. a
b
,\
NO1
l
*/ y I
d--
No2 A
-\
, '
NOT " '
e-
NO&
+v
NO 5
77NO 6
R e l a t i v e Intensity of /
7NO 7
Cls
I
Figure 6. The relative XPS intensities of O(ls), A1(2p), and Na(1s) as a function of the relative intensity of C(1s); values for AI(2p) are shown
in parentheses on the left-hand side ordinate.
,
peak positions with that reported in literature5 enabled us to assign the peak I to C
Figure 5. Difference XP spectra in the C(1s) binding energy region: (a) difference spectra of samples with different glucose loadings (the amount of glucose loaded is shown in Table I); (b) an example of the deconvolution of the difference spectra (sample no. 6).
H 'C/
c/
and
\o
H'
Similarly, the peak I1 was assigned to 0
H
area of the high-energy side of the reference peak (C(1s) of -CHz-CHz-; 285.0 eV).'O It can be seen in the figure that the larger the amount of glucose adsorption the larger the area of the high-energy side of the C ( 1s) peak. On the other hand, the small peak appearing on the right-hand side of the reference peak exhibited no systematic dependence upon the glucose adsorption. The peak was seen even for the sample without glucose adsorption. Thus we assigned the small peak to graphitic carbon19 originally contained in the adsorbent as an impurity (there remains a possibility that part of the graphitic carbon was produced by the decomposition of glucose under the irradiation of X-ray under ultrahigh vacuum conditions). The experimental results shown in Figure 4, in particular the X P spectral behavior at the high-energy side of the C(1s) peak, appear worthy of further discussion. For this purpose, difference spectra were obtained by subtracting the C(1s) peak for the bare alumina (but containing impurities such as -CHz-CH2-) from those of the samples containing glucose. The difference spectra thus obtained are shown in Figure 5a. It can be seen that the intensity of the difference spectra increases with the amount of glucose adsorption. Thus we can expect that information about glucose adsorbed would be extracted by analyzing the difference spectra. Illustrated in Figure 5b is a result of deconvolution of the difference spectra. It can be seen that the difference spectra consists of two peaks (I and 11). Positions of these peaks for different samples are given in Table I. A comparison of these (19) Hamrin, K.; Johansson, G.; Gelivs, U.; Nordling, C.; Siegbahn, K. Phys. Scr. 1970, 1, 277.
C'
\C'
c/
\o
All of these groups constitute a glucose molecule. The ratio of the contents of the type I carbons and that of the type I1 carbon is theoretically 5.0. The experimental value for this ratio is 4-5, as can be seen in Table I. It is thus possible to consider that most of glucose molecules adsorbed have survived the X-ray irradiation over the alumina surface. 3.4. Information about Adsorption Site. Plots shown in Figure 6 represent the percentages of XPS intensities of O(ls), A1(2p), and Na(1s) as a function of the percentage of the C( 1s) intensity. Qualitatively speaking, this figure represents changes in relative concentrations of 0 site, A1 site, and Na site upon the glucose adsorption. It is interesting that the concentration of the 0 site and that of the AI site decrease with the glucose adsorption. The rate of the decrease is more evident for the 0 site than the A1 site. The smaller contribution of the A1 site as the adsorption site would be ascribed to the smaller surface stoichiometric concentration of this site than the 0 site. Thus, it appears probable that the adsorption of glucose takes place over the 0-A1 portion of the adsorbent surface. On the contrary, the Na sites appear to play little role in the glucose adsorption. As can be seen in Figure 6, the concentration of the Na site remains constant. This shows that the glucose molecules adsorbed do not cover the Na site. It must be noted here that the escaping depth X of XP electrons will be much larger than the thickness d of the glucose monolayer, but nevertheless the glucose layer would appreciably reduce the XPS intensities of elements covered by glucose. The sample A1203 surface is not flat and hence an emission angle of XP electrons will be less than 7r/2 and larger than 0 on average. In such a case,
3258
J . Phys. Chem. 1989, 93, 3258-3260
the surface layer can reduce the intensity of X P electrons from an underlying substance20 even when d/X is 0.1 or less. The invariance of the XPS intensity of Na( 1s) thus strongly suggests that the Na site is not covered by glucose; neither a dissolution of Na into the solvent nor an adsorption of an external N a contaminant explained the experimental result that the Na content measured by XPS was almost identical with the original Na content of the sample. (20) Hohmann, S.Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy; Wiley: New York, 1983; p 141.
The glucose adsorption onto the 0-A1 sites is probable considering the result obtained by the adsorption measurement described in section 3.1. It has been pointed out in section 3.1 that the adsorption capacity obtained by applying a Langmuir adsorption isotherm to the experimental result is equivalent to the amount of glucose necessary to cover the surface area of the adsorbent alumina. This clearly means that major components (0,Al) constituting the adsorbent surface provide glucose molecules with adsorption sites. Registry No. A1,03, 1344-28-1; N a , 7440-23-5; glucose, 50-99-7.
Adsorption-Controlled Redox Activity. Surface-Enhanced Raman Investigation of Cystine versus Cysteine on Silver Electrodes Tadashi Watanabe* and Hiroyuki M a e d a
Institute of Industrial Science, University of Tokyo, Roppongi. Minato- ku, Tokyo 106, Japan (Received: July 12, 1988)
A quasi-reversible disulfide thiol redox process was clearly detected by surface-enhanced Raman spectroscopy on a silver electrode in a potential range between -0.3 and +0.3 V vs Ag/AgCl when cystine (disulfide) was adsorbed from the electrolyte solution. In contrast, no electrochemistry was observed when its reduced partner, cysteine (thiol), was adsorbed. Such a conspicuous difference in redox activity is rationalized in terms of both the strong Ag-S bonding and the operation of steric hindrance by adsorption of two cysteine molecules to neighboring silver sites.
Introduction Surface-enhanced Raman spectroscopy (SERS) is a powerful tool for the in situ investigation of the nature and surface orientation of adsorbed species at an electrode/electrolyte interface, although the electrode material is limited to such metals as silver, gold, and copper.Iv2 Recently several workers studied aromatic (diphenyl and dipyridyl) disulfides on and gold6 electrodes by SERS and confirmed that all these disulfides are adsorbed as ionized thiols due to S-S bond cleavage. To date, no cases are known in which a disulfide retains the S-S bond on these electrode surfaces, and no S-S bond reformation by electrooxidation has been demonstrated in the framework of SERS investigation. In the present work we studied a pair of nonaromatic disulfide (cystine) and thiol (cysteine) on silver electrodes by SERS and found that adsorbed cystine undergoes a quasi-reversible disulfide + thiol interconversion electrochemically. Surprisingly, no such electrochemical process was observed when the reduced partner (cysteine) was first adsorbed on the silver electrode. A model is proposed to account for this unprecedented finding. Experimental Section Reagent grade L-cystine and L-cysteine (>98% purity) from Tokyo Kasei Kogyo Co., Ltd., were used without further purification. Water was used after deionization and ultrafiltration. One end of a 99.95% silver rod (6.0 mm in diameter), with the side face electrically isolated with thermally coated Teflon, was cut at an angle of 30°, and the resulting 0.57-cm2 oval area served as a working electrode. A platinum wire and an Ag/AgCl (1) Surface Enhanced Raman Scattering, Chang, R . K., Furtak, T. E., Eds.; Plenum: New York, 1982. (2) Papers collected in Surf. Sci. 1985, 158, no. 1-3. (3) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1981, 85, 248. (4) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J . Electroanal. Chem. 1984, 175, 341. (5) Takahashi, M.; Fujita, M.; Ito, M. Surf. Sci. 1985, 158, 307. (6) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J . Electroanal. Chem. 1985, 186, 299.
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electrode (KCl saturated) were employed as the counter and reference electrodes, respectively, and the potential of the working electrode was controlled with a Hokuto Denko Model NPGFZ2501 potentiostat. Raman measurements were conducted on a Jasco Model R-800 laser Raman spectrophotometer equipped with an NEC Model GLS-3200 argon ion laser. The excitation wavelength was 488.0 nm, the beam power about 50 mW, the wavenumber resolution 5 cm-I, and the wavenumber scan rate 0.5 cm-'/s throughout. For SERS measurements, the silver electrode surface was first submitted to seven oxidation-reduction cycles (reformation charge: -200 mC/cm2 in each) in a deoxygenated 0.5 M HCl aqueous solution to obtain a SERS-active roughened surface, and then an equal volume of a 3.5 M HCI aqueous solution and cystine or cysteine was added so that the final solution was 2 M in HC1 and 1-200 mM in the compound to be studied. The solution of a total volume of 2 mL in a Pyrex cell was kept under nitrogen atmosphere during measurements. The scattered light was collected at right angles to the excitation beam through a thin solution layer and a flat window.
Results and Discussion Figure 1 compares the normal Raman scattering (NRS) spectrum of powdery cysteine (A) with the SER spectrum from a silver electrode surface in contact with a 100 mM cysteine solution (B). At this concentration the N R S from the latter solution was negligible. The outstanding features in going from A to B are the disappearance of the S-H stretching peak at 2565 cm-I and significant broadening and wavenumber lowering of the 670 cm-'). This evidences, as in C-S stretching' peak (690 the case of thiourea on a silver electrode: the formation of a strong Ag-S bond (or more specifically a bond between the thiolate moiety -S- and surface Ag' ion9) on adsorption of cysteine. The
-
(7) Susi, H.; Byler, D. M.; Gerasimowicz, W. V. J . Mol. Struct. 1983, 102, 63. (8) Loo, B. H . Chem. Phys. Lett. 1982, 89, 346.
0 1989 American Chemical Society
