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Analysis of surface coverage of benzotriazole and 6-tolyltriazole mixtures on copper electrodes from surface-enhanced Raman spectra
1 May 1998
Chemical Physics Letters 287 Ž1998. 449–454
Analysis of surface coverage of benzotriazole and 6-tolyltriazole mixtures on copper electrodes from surface-enhanced Raman spectra B.H. Loo ) , A. Ibrahim 1, M.T. Emerson Department of Chemistry, UniÕersity of Alabama, HuntsÕille, AL 35899, USA Received 18 July 1997; in final form 6 January 1998
Abstract The least-squares method has been used to analyze surface-enhanced Raman spectra of benzotriazole ŽBTA. and 6-tolyltriazole Ž6-TTA. mixtures on Cu electrodes. The fractional coverage of these molecules on Cu surfaces is dependent on their absolute solution concentration, and is consistent with an assumed Langmuir adsorption isotherm model. The adsorption equilibrium constant of 6-TTA is found to be about three times that of BTA. This indicates that 6-TTA is more strongly adsorbed on Cu than BTA. The free energy of adsorption for 6-TTA on Cu is 610 calrmol lower than that for BTA. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Surface-enhanced Raman spectroscopy ŽSERS. has been established to be a useful technique in the characterization of adsorbates at interfaces w1–5x. Specifically, molecular information such as bonding, identity and orientation of adsorbed species may be elucidated from surface-enhanced Raman ŽSER. spectra. However, SERS is still deficient in providing quantitative information on surface adsorbates, although the use of SERS as a quantitative analytical tool in determining the concentration of the SERSactive species in solution has been reported w6–11x. )
Corresponding author. Present Address: Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA. 1
In this work we demonstrate the use of the leastsquares method in elucidating the relative concentrations of coadsorbed species, namely, benzotriazole ŽBTA. and 6-tolyltriazole Ž6-TTA., on Cu electrodes. There have been two recent studies on co-adsorbates competing for sites using SERS w12,13x. However, no quantitative results were reported. BTA has been used satisfactorily over 40 years to prevent the atmospheric tarnishing and staining of Cu and its alloys w14–17x. Tolyltriazoles ŽTTAs., having three isomers with the methyl group substituted at the 4-, 5-, or 6-position on the benzene ring, are all found to be as good as or better corrosion inhibition than benzotriazole. Tornkvist et al. w18x, ¨ using FTIR reflection absorption spectroscopy and polarization resistance measurements on Cu surfaces, reported that TTAs gave better and higher inhibition
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 5 8 - X
450
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
efficiency values than BTA. O’Neal et al. w19x found that TTA isomer mixtures exhibited excellent inhibition efficiencies and retarded anodic as well as cathodic reactions w20x. Corrosion inhibition by BTA and 6-TTA is attributed to the formation of a surface protective polymeric film of inhibitors w21x. The polymeric film formation is preceded by adsorption. Hence, quantitative information on the adsorption process is important in the understanding of the initial stage of corrosion inhibition. 2. Experimental The surface-enhanced Raman ŽSER. experiments were performed on a JY MOLErU1000 Raman microprobe ŽInstruments S.A.. equipped with a thermoelectrically cooled RCA C31034 photomultiplier tube. The 647.1 nm line of a krypton ion laser ŽSpectra Physics 164-01., was used to produce the Raman excitation. The laser power used was 60 mW. An Olympus metallurgical microscope with an ultra-long 20 = objective was used as a microprobe to focus the excitation radiation on the Cu electrode surface. The spectral resolution was 4 cmy1 . All SER experiments were performed in an electrochemical cell consisting of a Cu working electrode, a Pt counter electrode and a saturated calomel reference electrode ŽSCE.. The Cu electrode was constructed from a 99.9% pure polycrystalline, 1.0 mm diameter Cu wire ŽJohnson Matthey.. A solution volume of 20 mL was used in all cases. The potentials were controlled by a Pine Instruments RDE4 potentiostat. Reagent grade chemicals BTA ŽAldrich Gold Label. and 6-TTA ŽChemalog. were used. All solutions contained 0.1 M KCl electrolyte. Reproducible SER spectra Žin spectral feature and intensity. for an absorbed inhibitor are obtained if conditions such as electrode surface preparation, laser power and beam size, etc., are carefully controlled. Electrodeposition of the inhibitor molecules on the polished Cu electrode was carried out by an in situ oxidation-reduction cycle ŽORC. using a sweep rate of 1.5 Vrmin. The ORC surface pretreatment was done with simultaneous laser illumination. The in situ method of electrode treatment was chosen over the ex situ method w22x because it produced more intense and better quality spectra.
3. Results and discussion The intensity of the signals in the SER spectra is proportional to the number of scatterers on surface. The composite mixture SER spectrum, M Ž n ., is assumed to be a linear function of the SER spectra of the N pure components on surface. Thus, one can write w23x M Ž n . s Ý Ii8 Ž n . .Ci , where Ii8Ž n . is the intensity observed for the ith pure component absorbed on the electrode surface at the frequency n and Ci is the concentration of the ith component that gives rise to Ii8Ž n .. The Ci s of the components that produce M Ž n . are obtained from the solution of a set of N linear equations, written in a matrix form as ÝI 0j1 I 0j1
ÝI 0j1 I 0j2
ÝI 0j2 I 0j1
ÝI 0j2 I 0j2
...
...
ÝI 0jN I 0j1
ÝI 0jN I 0j2
...
ÝI 0j1 I 0jN
... .. . ...
ÝI 0j2 I 0jN ...
C1 C2 ...
ÝI 0jN I 0jN
CN
ÝI 0j1 M j ÝI 0j2 M j s , ... ÝI 0jN M j where I ji0 s Ii0 Ž n j . and M j s M Ž n j . for j s 1,2, . . . , K, are sets of K data points measured at equal intervals over a frequency range of n . The summation in the matrices is from j s 1 to K. The least-squares method requires that one have a reference spectrum for each of the pure components. It was assumed that only a single surface component was produced by each corrosion inhibitor on the Cu surface. The required reference spectrum was taken as the spectrum obtained when the Cu surface had undergone an ORC treatment with a single corrosion inhibitor in the electrolyte solution. Use of this approach produced reasonable results. A background spectrum for each individual reference or a mixture was collected under the same conditions as would be used for the reference or mixture spectrum collection except that no potential
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
was applied. The potential was then applied and the reference or mixture spectrum was collected. The baseline fitting routine was used to remove the background of each spectrum Žreference and mixture. before each was used in the least-squares calculation. Fig. 1 shows the representative SER spectra of BTA and 6-TTA adsorbed on Cu electrodes at y1.0 V w24x. Except for some frequency differences, the spectral features of the BTA and 6-TTA are very similar to each other, indicating both molecules are bonded to the Cu surface in a similar fashion. This conclusion can easily be rationalized when the molecular structures of BTA and 6-TTA are considered. The BTA molecule consists of a triazole ring and a benzene ring, as shown in Structure I.
There have been several studies on the bonding of BTA to Cu surfaces w21,25–30x. In all cases, it was concluded that the BTA bonded to Cu surfaces via its N atoms. The methyl group substitution at the 6-position of the benzene ring gives 6-TTA. The methyl group is not expected to interfere with the bonding characteristics of the triazole ring, as it is far
Fig. 1. Representative pure component SER spectrum of ŽA. CurBTA at y1.0 V; ŽB. Cur6-TTA at y1.0 V. The solution concentration was 20 ppm in each case.
451
Fig. 2. ŽA. The mixture SER spectrum on Cu electrode at y1.0 V for the solution concentration of the BTA and 6-TTA mixture at 20:40 ppm. ŽB. Contribution of Cur6-TTA Žafter subtracting the contribution of CurBTA from the mixture SER spectrum.. ŽC. Residual spectrum R j vs n j .
away from the triazole N atoms. This is substantiated by the finding that the chemisorption of 4- and 5-tolyltriazoles on Cu Žwith the methyl group substitutions at 4- and 5-positions. was the same as that of BTA w18x. In the SER spectra of pure components CurBTA and Cur6-TTA, the bands in the 1000–1600 cmy1 region shows only small differences in the frequency shifts and are overlapped in the mixture spectra. Several bands in the region below 1000 cmy1 , however, are markedly different from each other, e.g., the 558-, 637- and 789 cmy1 bands of CurBTA and the 499, 622, 764 and 832 cmy1 bands of 6-TTA. The most reliable quantitative results are obtained when component spectral bands are well separated from each other. Hence, only the bands in the region from 400 to 900 cmy1 were used in the least-squares calculations. The results of one of the solution mixtures studied are shown in Fig. 2; Fig. 2A shows the SER spectrum from a Cu electrode at y1.0 V in the BTA and 6-TTA solution mixture at 20:40 ppm, whereas Fig. 2B shows the contribution of Cur6-TTA after subtracting the contribution of CurBTA from the spectrum in Fig. 2A. A simple test of the validity of the least-squares results can be obtained by calculating the residual value R j that remains when the predicted spectral points are subtracted from the experi-
452
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
mentally observed spectral points. Thus, for a 2component system, R j s M j y I1 j C1 y I2 j C2 y Z. If the fit is perfect, R j would be everywhere zero. For the least-squares results to be reasonable, C1 and C2 must be greater than 0. The term Z accounts for any artifact constant baseline shift that was introduced by the baseline removal process. Fig. 2C shows the residual value R j from the SER spectrum shown in Fig. 2A, indicating a reasonably good fit is obtained. Table 1 shows the least-squares estimates of fractional coverages of BTA and 6-TTA on Cu electrodes in mixture solutions at several different concentrations. The fractional coverage values, u i s CirŽ C1 q C2 . for i s 1,2, give the relative amount of each component adsorbed on the unit of surface from which the spectrum was originating. For the 20:20 ppm solution mixture, u6 - TTA :u BTA is 2.1:1.0. This result shows that 6-TTA exhibits a higher surface coverage than BTA. When the solution concentration of 6-TTA is more than that of BTA, the Cu electrode surface is predominantly covered by the 6-TTA molecules. On the other hand, when the solution concentration of BTA is more than that of 6-TTA, the contribution of BTA to the mixture spectra becomes predominant. The results suggest that the absolute concentrations of the solution species affect the thermodynamics of adsorption. To better understand the adsorption on an electrode surface, we now consider the adsorption isotherm. The equilibrium adsorption at electrodes has been shown to be characterizable by the Langmuir isotherm w1x, if there are no interactions be-
Table 1 Fractional coverages of BTA and 6-TTA on Cu electrodes Solution concentration Žppm.
Fractional coverage
BTA:6-TTA
u BT A
u6 - TTA
20:20 20:40 20:100 40:20 100:20 160:20
0.32 0.26 0.10 0.53 0.64 0.77
0.68 0.74 0.90 0.47 0.36 0.23
Fig. 3. The plot of u BT A ru6 - TTA vs. wBTAxrw6-TTAx. The dots are experimental data points whereas the dashed line is the least-squares fit of the experimental data.
tween the adsorbed molecules. Chen et al. w31x have shown that the SER signals for pyridine on Ag can be represented by a simple Langmuir isotherm with the free energy of adsorption D Gads s y5.7 kcalrmol. Hence, for a dilute solution consisting of two species BTA and 6-TTA that are in equilibrium and competing for adsorption sites on the electrode surface, the fractional coverage of BTA, u BTA , can be written as w32x, assuming Langmuir adsorption isotherm,
u BTA s
K BTA w BTA x 1 q K BTA w BTA x q K 6 - TTA w 6-TTAx
.
Likewise, the fractional coverage of 6-TTA, u6 - TTA , is
u6 - TTA s
K 6 - TTA w 6-TTAx 1 q K BTA w BTA x q K 6 - TTA w 6-TTAx
,
where wBTAx and w6-TTAx are molar concentrations, and K BTA and K 6 - TTA are adsorption equilibrium constants of BTA and 6-TTA on Cu, respectively. Hence,
u BTA u6 - TTA
s
ž
K BTA K 6 - TTA
/ž
w BTAx w 6-TTAx
/
.
A plot of u BTA ru6 - TTA vs. wBTAxrw6-TTAx should yield a straight line of slope K BTA rK 6 - TTA . The slope provides a quantitative estimate of how effective a component will bond to the Cu surface relative to another adsorbate.
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
Fig. 3 shows the plot of u BTA ru6 - TTA vs. wBTAxrw6-TTAx, using the data in Table 1. A good linear fit for the experimental data is obtained. The slope of the plot gives K 6 - TTA s 2.8 K BTA . The results indicate that the adsorption coefficient for 6TTA on Cu is larger than that for BTA on Cu, or 6-TTA is more strongly adsorbed on Cu than BTA. These results are consistent with the known corrosion inhibition efficiency of these compounds, i.e., 6-TTAG BTA w18–20,33x. They are in accordance with a model in which inhibitor molecules are in direct competition with each other for the surface active sites – the more efficient one is adsorbed preferentially on the surface which is related to the strength of the bonds formed with the surface. The solubility for BTA is determined to be 1.76 gr100 ml H 2 O at 208C and that for 6-TTA is 0.44 gr100 ml H 2 O at 208C. Thus, the less soluble 6-TTA is more strongly adsorbed on the Cu surface. This result is in agreement with the general observation that the less soluble the material, the more strongly it will tend to be adsorbed w34x. From the relative adsorption equilibrium constants, the free energy of adsorption DGads for 6-TTA on Cu is calculated to be 610 calrmol more negative than that for BTA on Cu. Since BTA and 6-TTA differ only by CH 2 in mass, we note that the above value is comparable to the values obtained on homologous series of surfactants and linear aliphatic compounds such as alcohols and acids for each increment in the -CH 2 group. For the surfactants, the DGads per -CH 2 group increment is 590 calrmol w35x and for the linear aliphatic compounds, the corresponding value is 640 calrmol w34x. 4. Conclusions We have demonstrated that least-squares analysis of the SER spectra can be used to elucidate relative surface concentration of coadsorbed species on electrode surfaces, using Langmuir isotherm. Reproducible surface coverage values can be obtained when the reference and mixture spectra are collected under identical experimental conditions. The relative surface coverages provide information on the thermodynamics such as adsorption equilibrium constants and hence the adsorptivity of coadsorbed species on surfaces.
453
Acknowledgements The authors thank Dr K.G. Baikerikar of Iowa State University for discussions, and the reviewer for suggestions.
References w1x R.P. van Duyne, in: C.B. Moore ŽEd.., Chemical and Biochemical Applications of Lasers, vol. 4, Academic Press, New York, 1979, p. 479. w2x J.A. Creighton, in: R.F. Willis ŽEd.., Vibrational Spectroscopy of Adsorbates, Springer, Berlin, 1980, p. 145. w3x R.L. Birke, J.R. Birke, J.R. Lombardi, L.A. Sanchez, in: K.M. Kadish ŽEd.., Electrochemical and Spectrochemical studies of Biological Redox Components, Am. Chem. Soc., Washington, DC, 1982, p. 69. w4x R.K. Chang, T.E. Furtak ŽEds.., Surface-Enhanced Raman Scattering, Plenum Press, New York, 1982. w5x . M. Fleischmann, I.R. Hill, in: R.E. White, J.O’M. Bockris, B.E. Conway, E. Yeager ŽEds.., Comprehensive Treatise of Electrochemistry, vol. 8, Plenum Press, New York, 1984, p. 373. w6x P.D. Enlow, M. Buncick, R.J. Warmack, T. Vo-Dinh, Anal. Chem. 58 Ž1986. 1119. w7x F. Ni, T.M. Cotton, Anal. Chem. 58 Ž1986. 3159. w8x M.M. Carrabba, R.B. Edmonds, R.D. Rauh, Anal. Chem. 59 Ž1987. 2559. w9x J.C. Rubin, J. Electroanal. Chem. 220 Ž1987. 339. w10x R.D. Freeman, R.M. Hammaker, C.E. Meloan, W.G. Fateley, Appl. Spectrosc. 42 Ž1988. 456. w11x J.J. Laserna, A.D. Campiglia, J.D. Winefordner, Anal. Chem. 61 Ž1989. 1697. w12x G. Zhou, S. Cai, L. Song, H. Yang, A. Fujishima, A. Ibrahim, Y.G. Lee, B.H. Loo, Appl. Surf. Sci. 52 Ž1991. 227. w13x D. Sockalingum, M. Fleischmann, M.M. Musiani, Spectrochim. Acta 47A Ž1991. 1475. w14x Proctor and Gamble Ltd., British Patent 652, 1947, p. 339. w15x S. Dugsdale, J.B. Cotton, Corros. Sci. 3 Ž1963. 69. w16x Miles Laboratories, Ger. Offen. 2,014,883, 1970. w17x Ciba-Geigy A.G., Ger. Pat. 1,521,758, 1971. w18x C. Tornkvist, D. Thierry, J. Bergman, B. Liedberg, C. Ley¨ graf, J. Electrochem. Soc. 136 Ž1989. 58. w19x C. O’Neal Jr., R.N. Borger, Mater. Perform. 15 Ž1976. 9. w20x T. Notoya, G.W. Poling, Boshuku Gijutsu 30 Ž1981. 381. w21x J.B. Cotton, I.R. Scholes, Br. Corros. J. 2 Ž1967. 1. w22x K.D. Beer, W. Tanner, R. L Garrell, J. Electroanal. Chem. 258 Ž1989. 313. w23x L.L. Tyson, Y.-C. Ling, C.K. Mann, Appl. Spectrosc. 36 Ž1984. 663. w24x A. Aruchamy, A. Fujishima, A. Ibrahim, B.H. Loo, J. Electroanal. Chem. 281 Ž1990. 299. w25x G.W. Poling, Corr. Sci. 10 Ž1970. 359.
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w26x J. Rubim, I.G.R. Gutz, O. Sala, W.J. Orville-Thomas, J. Mol. Struct. 100 Ž1983. 571. w27x D. Thierry, C. Leygraf, J. Electrochem. Soc. 132 Ž1985. 1009. w28x B.S. Fang, C.G. Olson, D.W. Lynch, Surf. Sci. 176 Ž1986. 476. w29x J.O. Nilsoon, C. Tornkvist, B. Liedberg, Appl. Surf. Sci. 37 ¨ Ž1989. 306. w30x K.T. Carron, G. Xue, M.L. Lewis, Langmuir 7 Ž1991. 2. w31x C. Chen, T. Heinz, D. Ricard, Y. Shen, Chem. Phys. Lett. 58 Ž1981. 83.
w32x R.A. Alberty, R.J. Silbey, Physical Chemistry, 2nd ed., John Wiley, New York, 1997. w33x N. Hussein, K. Nobe, paper no. 26, Corrosionr73, NACE Annual Meeting, Anaheim, CA, March 1973. w34x A.W. Adamson, Physical Chemistry of Surfaces, 5th ed., John Wiley, New York, 1990. w35x M.J. Rosen, Surfactants and Interfacial Phenomena. John Wiley, New York, 1978.
Chemical Physics Letters 287 Ž1998. 449–454
Analysis of surface coverage of benzotriazole and 6-tolyltriazole mixtures on copper electrodes from surface-enhanced Raman spectra B.H. Loo ) , A. Ibrahim 1, M.T. Emerson Department of Chemistry, UniÕersity of Alabama, HuntsÕille, AL 35899, USA Received 18 July 1997; in final form 6 January 1998
Abstract The least-squares method has been used to analyze surface-enhanced Raman spectra of benzotriazole ŽBTA. and 6-tolyltriazole Ž6-TTA. mixtures on Cu electrodes. The fractional coverage of these molecules on Cu surfaces is dependent on their absolute solution concentration, and is consistent with an assumed Langmuir adsorption isotherm model. The adsorption equilibrium constant of 6-TTA is found to be about three times that of BTA. This indicates that 6-TTA is more strongly adsorbed on Cu than BTA. The free energy of adsorption for 6-TTA on Cu is 610 calrmol lower than that for BTA. q 1998 Elsevier Science B.V. All rights reserved.
1. Introduction Surface-enhanced Raman spectroscopy ŽSERS. has been established to be a useful technique in the characterization of adsorbates at interfaces w1–5x. Specifically, molecular information such as bonding, identity and orientation of adsorbed species may be elucidated from surface-enhanced Raman ŽSER. spectra. However, SERS is still deficient in providing quantitative information on surface adsorbates, although the use of SERS as a quantitative analytical tool in determining the concentration of the SERSactive species in solution has been reported w6–11x. )
Corresponding author. Present Address: Department of Chemistry, Mississippi State University, Mississippi State, MS 39762, USA. 1
In this work we demonstrate the use of the leastsquares method in elucidating the relative concentrations of coadsorbed species, namely, benzotriazole ŽBTA. and 6-tolyltriazole Ž6-TTA., on Cu electrodes. There have been two recent studies on co-adsorbates competing for sites using SERS w12,13x. However, no quantitative results were reported. BTA has been used satisfactorily over 40 years to prevent the atmospheric tarnishing and staining of Cu and its alloys w14–17x. Tolyltriazoles ŽTTAs., having three isomers with the methyl group substituted at the 4-, 5-, or 6-position on the benzene ring, are all found to be as good as or better corrosion inhibition than benzotriazole. Tornkvist et al. w18x, ¨ using FTIR reflection absorption spectroscopy and polarization resistance measurements on Cu surfaces, reported that TTAs gave better and higher inhibition
0009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 8 . 0 0 0 5 8 - X
450
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
efficiency values than BTA. O’Neal et al. w19x found that TTA isomer mixtures exhibited excellent inhibition efficiencies and retarded anodic as well as cathodic reactions w20x. Corrosion inhibition by BTA and 6-TTA is attributed to the formation of a surface protective polymeric film of inhibitors w21x. The polymeric film formation is preceded by adsorption. Hence, quantitative information on the adsorption process is important in the understanding of the initial stage of corrosion inhibition. 2. Experimental The surface-enhanced Raman ŽSER. experiments were performed on a JY MOLErU1000 Raman microprobe ŽInstruments S.A.. equipped with a thermoelectrically cooled RCA C31034 photomultiplier tube. The 647.1 nm line of a krypton ion laser ŽSpectra Physics 164-01., was used to produce the Raman excitation. The laser power used was 60 mW. An Olympus metallurgical microscope with an ultra-long 20 = objective was used as a microprobe to focus the excitation radiation on the Cu electrode surface. The spectral resolution was 4 cmy1 . All SER experiments were performed in an electrochemical cell consisting of a Cu working electrode, a Pt counter electrode and a saturated calomel reference electrode ŽSCE.. The Cu electrode was constructed from a 99.9% pure polycrystalline, 1.0 mm diameter Cu wire ŽJohnson Matthey.. A solution volume of 20 mL was used in all cases. The potentials were controlled by a Pine Instruments RDE4 potentiostat. Reagent grade chemicals BTA ŽAldrich Gold Label. and 6-TTA ŽChemalog. were used. All solutions contained 0.1 M KCl electrolyte. Reproducible SER spectra Žin spectral feature and intensity. for an absorbed inhibitor are obtained if conditions such as electrode surface preparation, laser power and beam size, etc., are carefully controlled. Electrodeposition of the inhibitor molecules on the polished Cu electrode was carried out by an in situ oxidation-reduction cycle ŽORC. using a sweep rate of 1.5 Vrmin. The ORC surface pretreatment was done with simultaneous laser illumination. The in situ method of electrode treatment was chosen over the ex situ method w22x because it produced more intense and better quality spectra.
3. Results and discussion The intensity of the signals in the SER spectra is proportional to the number of scatterers on surface. The composite mixture SER spectrum, M Ž n ., is assumed to be a linear function of the SER spectra of the N pure components on surface. Thus, one can write w23x M Ž n . s Ý Ii8 Ž n . .Ci , where Ii8Ž n . is the intensity observed for the ith pure component absorbed on the electrode surface at the frequency n and Ci is the concentration of the ith component that gives rise to Ii8Ž n .. The Ci s of the components that produce M Ž n . are obtained from the solution of a set of N linear equations, written in a matrix form as ÝI 0j1 I 0j1
ÝI 0j1 I 0j2
ÝI 0j2 I 0j1
ÝI 0j2 I 0j2
...
...
ÝI 0jN I 0j1
ÝI 0jN I 0j2
...
ÝI 0j1 I 0jN
... .. . ...
ÝI 0j2 I 0jN ...
C1 C2 ...
ÝI 0jN I 0jN
CN
ÝI 0j1 M j ÝI 0j2 M j s , ... ÝI 0jN M j where I ji0 s Ii0 Ž n j . and M j s M Ž n j . for j s 1,2, . . . , K, are sets of K data points measured at equal intervals over a frequency range of n . The summation in the matrices is from j s 1 to K. The least-squares method requires that one have a reference spectrum for each of the pure components. It was assumed that only a single surface component was produced by each corrosion inhibitor on the Cu surface. The required reference spectrum was taken as the spectrum obtained when the Cu surface had undergone an ORC treatment with a single corrosion inhibitor in the electrolyte solution. Use of this approach produced reasonable results. A background spectrum for each individual reference or a mixture was collected under the same conditions as would be used for the reference or mixture spectrum collection except that no potential
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
was applied. The potential was then applied and the reference or mixture spectrum was collected. The baseline fitting routine was used to remove the background of each spectrum Žreference and mixture. before each was used in the least-squares calculation. Fig. 1 shows the representative SER spectra of BTA and 6-TTA adsorbed on Cu electrodes at y1.0 V w24x. Except for some frequency differences, the spectral features of the BTA and 6-TTA are very similar to each other, indicating both molecules are bonded to the Cu surface in a similar fashion. This conclusion can easily be rationalized when the molecular structures of BTA and 6-TTA are considered. The BTA molecule consists of a triazole ring and a benzene ring, as shown in Structure I.
There have been several studies on the bonding of BTA to Cu surfaces w21,25–30x. In all cases, it was concluded that the BTA bonded to Cu surfaces via its N atoms. The methyl group substitution at the 6-position of the benzene ring gives 6-TTA. The methyl group is not expected to interfere with the bonding characteristics of the triazole ring, as it is far
Fig. 1. Representative pure component SER spectrum of ŽA. CurBTA at y1.0 V; ŽB. Cur6-TTA at y1.0 V. The solution concentration was 20 ppm in each case.
451
Fig. 2. ŽA. The mixture SER spectrum on Cu electrode at y1.0 V for the solution concentration of the BTA and 6-TTA mixture at 20:40 ppm. ŽB. Contribution of Cur6-TTA Žafter subtracting the contribution of CurBTA from the mixture SER spectrum.. ŽC. Residual spectrum R j vs n j .
away from the triazole N atoms. This is substantiated by the finding that the chemisorption of 4- and 5-tolyltriazoles on Cu Žwith the methyl group substitutions at 4- and 5-positions. was the same as that of BTA w18x. In the SER spectra of pure components CurBTA and Cur6-TTA, the bands in the 1000–1600 cmy1 region shows only small differences in the frequency shifts and are overlapped in the mixture spectra. Several bands in the region below 1000 cmy1 , however, are markedly different from each other, e.g., the 558-, 637- and 789 cmy1 bands of CurBTA and the 499, 622, 764 and 832 cmy1 bands of 6-TTA. The most reliable quantitative results are obtained when component spectral bands are well separated from each other. Hence, only the bands in the region from 400 to 900 cmy1 were used in the least-squares calculations. The results of one of the solution mixtures studied are shown in Fig. 2; Fig. 2A shows the SER spectrum from a Cu electrode at y1.0 V in the BTA and 6-TTA solution mixture at 20:40 ppm, whereas Fig. 2B shows the contribution of Cur6-TTA after subtracting the contribution of CurBTA from the spectrum in Fig. 2A. A simple test of the validity of the least-squares results can be obtained by calculating the residual value R j that remains when the predicted spectral points are subtracted from the experi-
452
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
mentally observed spectral points. Thus, for a 2component system, R j s M j y I1 j C1 y I2 j C2 y Z. If the fit is perfect, R j would be everywhere zero. For the least-squares results to be reasonable, C1 and C2 must be greater than 0. The term Z accounts for any artifact constant baseline shift that was introduced by the baseline removal process. Fig. 2C shows the residual value R j from the SER spectrum shown in Fig. 2A, indicating a reasonably good fit is obtained. Table 1 shows the least-squares estimates of fractional coverages of BTA and 6-TTA on Cu electrodes in mixture solutions at several different concentrations. The fractional coverage values, u i s CirŽ C1 q C2 . for i s 1,2, give the relative amount of each component adsorbed on the unit of surface from which the spectrum was originating. For the 20:20 ppm solution mixture, u6 - TTA :u BTA is 2.1:1.0. This result shows that 6-TTA exhibits a higher surface coverage than BTA. When the solution concentration of 6-TTA is more than that of BTA, the Cu electrode surface is predominantly covered by the 6-TTA molecules. On the other hand, when the solution concentration of BTA is more than that of 6-TTA, the contribution of BTA to the mixture spectra becomes predominant. The results suggest that the absolute concentrations of the solution species affect the thermodynamics of adsorption. To better understand the adsorption on an electrode surface, we now consider the adsorption isotherm. The equilibrium adsorption at electrodes has been shown to be characterizable by the Langmuir isotherm w1x, if there are no interactions be-
Table 1 Fractional coverages of BTA and 6-TTA on Cu electrodes Solution concentration Žppm.
Fractional coverage
BTA:6-TTA
u BT A
u6 - TTA
20:20 20:40 20:100 40:20 100:20 160:20
0.32 0.26 0.10 0.53 0.64 0.77
0.68 0.74 0.90 0.47 0.36 0.23
Fig. 3. The plot of u BT A ru6 - TTA vs. wBTAxrw6-TTAx. The dots are experimental data points whereas the dashed line is the least-squares fit of the experimental data.
tween the adsorbed molecules. Chen et al. w31x have shown that the SER signals for pyridine on Ag can be represented by a simple Langmuir isotherm with the free energy of adsorption D Gads s y5.7 kcalrmol. Hence, for a dilute solution consisting of two species BTA and 6-TTA that are in equilibrium and competing for adsorption sites on the electrode surface, the fractional coverage of BTA, u BTA , can be written as w32x, assuming Langmuir adsorption isotherm,
u BTA s
K BTA w BTA x 1 q K BTA w BTA x q K 6 - TTA w 6-TTAx
.
Likewise, the fractional coverage of 6-TTA, u6 - TTA , is
u6 - TTA s
K 6 - TTA w 6-TTAx 1 q K BTA w BTA x q K 6 - TTA w 6-TTAx
,
where wBTAx and w6-TTAx are molar concentrations, and K BTA and K 6 - TTA are adsorption equilibrium constants of BTA and 6-TTA on Cu, respectively. Hence,
u BTA u6 - TTA
s
ž
K BTA K 6 - TTA
/ž
w BTAx w 6-TTAx
/
.
A plot of u BTA ru6 - TTA vs. wBTAxrw6-TTAx should yield a straight line of slope K BTA rK 6 - TTA . The slope provides a quantitative estimate of how effective a component will bond to the Cu surface relative to another adsorbate.
B.H. Loo et al.r Chemical Physics Letters 287 (1998) 449–454
Fig. 3 shows the plot of u BTA ru6 - TTA vs. wBTAxrw6-TTAx, using the data in Table 1. A good linear fit for the experimental data is obtained. The slope of the plot gives K 6 - TTA s 2.8 K BTA . The results indicate that the adsorption coefficient for 6TTA on Cu is larger than that for BTA on Cu, or 6-TTA is more strongly adsorbed on Cu than BTA. These results are consistent with the known corrosion inhibition efficiency of these compounds, i.e., 6-TTAG BTA w18–20,33x. They are in accordance with a model in which inhibitor molecules are in direct competition with each other for the surface active sites – the more efficient one is adsorbed preferentially on the surface which is related to the strength of the bonds formed with the surface. The solubility for BTA is determined to be 1.76 gr100 ml H 2 O at 208C and that for 6-TTA is 0.44 gr100 ml H 2 O at 208C. Thus, the less soluble 6-TTA is more strongly adsorbed on the Cu surface. This result is in agreement with the general observation that the less soluble the material, the more strongly it will tend to be adsorbed w34x. From the relative adsorption equilibrium constants, the free energy of adsorption DGads for 6-TTA on Cu is calculated to be 610 calrmol more negative than that for BTA on Cu. Since BTA and 6-TTA differ only by CH 2 in mass, we note that the above value is comparable to the values obtained on homologous series of surfactants and linear aliphatic compounds such as alcohols and acids for each increment in the -CH 2 group. For the surfactants, the DGads per -CH 2 group increment is 590 calrmol w35x and for the linear aliphatic compounds, the corresponding value is 640 calrmol w34x. 4. Conclusions We have demonstrated that least-squares analysis of the SER spectra can be used to elucidate relative surface concentration of coadsorbed species on electrode surfaces, using Langmuir isotherm. Reproducible surface coverage values can be obtained when the reference and mixture spectra are collected under identical experimental conditions. The relative surface coverages provide information on the thermodynamics such as adsorption equilibrium constants and hence the adsorptivity of coadsorbed species on surfaces.
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Acknowledgements The authors thank Dr K.G. Baikerikar of Iowa State University for discussions, and the reviewer for suggestions.
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