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Surface enhanced Raman scattering of surfactants adsorbed on silver mirror surfaces
Specmchimico Acm. Vol. 49A. No. 7. pp. 1009-1013. Printed in Great Britain
MM-8539193 96.00 + 0.00 Pergamon Press Lid
1993
Surface enhanced Raman scattering of surfactants adsorbed on silver mirror surfaces* MINGMINGFANG, TIANJIANHUANG and TIREN Gu Laboratoryof Surface and Colloid Chemistry, Department of Chemistry and Institute of Physical Chemistry, Peking University, Beijing 100871, China
and YUJUN Mo,t
ZHAOXIANG WANG and XIUYING LI
Raman and Fluorescence Spectroscopy Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing ltlfKJg0,China (Received 20 March 1992; in final form 5 August 1992; accepted 6 August 1992) Abstract-The surface enhanced Raman scattering (SERS) spectra of two surfactants: Triton X-100 and n-hexadecylpyridinium bromide adsorbed on the interfaces between chemically reduced silver mirrors and water are obtained. By comparing and analyzing their bulk and SERS spectra, we have studied the’adsorption configurations of the two surfactants on the silver/water interfaces.
SINCEits discovery in 1974, the technique of surface enhanced Raman scattering (SERS) has become a forceful and helpful tool in studying molecules on surfaces and interfaces. But SERS investigation of surface active agents (surfactants) seems to be-much less common. The conventional methods of investigating the adsorption of surfactants on solid/liquid interfaces are macroscopic ones, that is, the determination of adsorption isotherm, heat of immersion, etc. Because of its in situ detection characteristics, it is reasonable to expect that SERS can become a microscopic probe to investigate the microscopic status of surfactants adsorbed on the interfaces of solid and liquid. KNOLL [l] and BUNDINGef al. [2] have studied n-alkylpyridinium halides adsorbed on silver electrodes, but their subjects were n-methylpyridinium ions. To our knowledge, SUN et al. [3] and DENDRAMIS et al. [4] are some of the few researchers who studied MBE 84
t
m0.m Raman shift
c
n3m.m
(cm-1l
Fig. 1. The normal Raman spectrum of pure TX100 in liquid state. * A project supported by National Sciences Foundation of China. t Author to whom correspondence should be addressed. 1009
MINGMINGFANG et al.
1010
,.uaEmt.t 2m.m
1wE.m
lEEB.1
Raman
shift
[cm-'1
Fig. 2. A typical SERS spectrum of TX100 on silver surface.
surfactants (both on cetyltrimethyl ammonium bromide) by SERS, although there are many others studying surfactants using the traditional methods. In our present work, another two surfactants, Triton X-100 (TXlOO) and n-hexadecylpyridinium bromide (CPyB), which are quite different in structure from cetyltrimethyl ammonium bromide and which are nonionic and cationic surfactants, respectively, are studied. By comparing Table 1. The assignment of SERS peaks of TX100 (cm-‘) Normal Raman
SERS
Assignment
224 w
Ag-S or Ag-0 vibration [7] EO ring vibration EO skeletal deformation CCC deformation Benzene ring deformation Sym. skeletal stretch EO ring vibration Benzene ring vibration EO ring vibration C-O-C sym. stretch C-O-C sym. stretch C-C stretch C-C stretch C-C stretch C-C stretch -CHz twist and rock -CHr twist and rock -CH2 twist and rock -CH2 twist and rock -CH2 twist and rock In-plane CH deformation Benzene ring stretch Benzene ring stretch Benzene ring stretch -CHr deformation -0CHr deformation Benzene ring stretch Benzene ring stretch Benzene ring stretch Benzene ring stretch
240 m 312 w 520 w 638s
684m 732 w 750 m 804m 836 m 870 m 924m 974 w 11OOm 1138 m 1164 w 1188 m 1212 w 1248 s 1282 s 1290 s 1364 vw 1386 VW 1418 VW 1452 s 1470 s 1510 vw 1582 w 1598 VW 1612 s N.B.: relative vw = very weak.
614 m
774 m
930 w 980 w 109ow 1130 m 1182 m
1310 m 1364 s 1388 w 1420 w 1468W 1510 s 1574 s 1596 w 1648 s intensity
( j: s=strong;
m=medium;
Ref. [6]
Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap.
14 14 1 13 1 14 13 14 14 14 1 1 1 1 1 1 1 1 1 1 13 13 13 1 14 13 13 13 13
w= weak; and
SERS of
a.mE
m 8m.m
surfactants absorbed on silver mirror surfaces
m3m.m Raman shift
1011
c
m0.m (cm-‘)
Fig. 3. A typical SERS spectrum of CPyB on silver surface (inset: the normal Raman spectrum of CPyB).
the normal Raman spectra of pure TX100 and its SERS and analyzing the SERS spectrum of CPyB, we propose their adsorption configurations on silver/water interfaces.
EXPERIMENTAL The method for preparing the SERS substrates, silver mirrors,is the same as that described in Ref. [S]. These substrates have great enhancement factors and are quite homogeneous. The silver mirrors used are about 6x 10mm in size. TX100 (an octyl phenol ethoxylate surfactant, whose average length of polyethylene oxide chain is about 10, see Fig. 4) was a product of B.D.H. Ltd. The critical mice11 concentration (cmc) of it is 2.9 x lo-’ M (W’C). CPyB was produced by Beijing Chemistry Co. and was recrystallized with an acetone-ethanol mixture three times. The cmc of CPyB is 5.81 x lo-’ M (25°C). The chemically deposited silver mirror was placed in a cylindrical cell [S] with a 0.5 ml aqueous solution of the surfactant. The Raman measurement was begun 20 min after the substrate had been put in the solution. All the experiments were done at 25°C. The measurement system was similar to that used in Ref. [5], except it included a Spex Raman-log 1403 spectrometer and DATAMATE-1. The power of the excitation light, 5145 A, created with a Ar+ laser, was 40 mW on the sample surface. Backscattering geometry of 45” and 4OOpm width of the monochromoter slit were used.
Fig. 4. A proposed configuration of TX100 molecule on silver mirror/water interface.
MINGMING FANG et al.
1012
Table 2. The assignment of SERS peaks of CPyB (cm-‘) SERS of CPYB
SERWRaman of n-MePy
816 m 860m 1006w 1030 s 1056 w 1132 w 1174 s 1214 s 1236 w 1276 m 1396 s 1444 m 1502 w 1584 m 1634 s
800 857 1001 1026 1058 1190 1212 1266 1449 1497 1580 1632
Assignment N-R stretch N-R out-of-plane bend Sym. ring stretch Triangle ring breathe C-H in-plane bend C-H in-plane deformation of the ring N-R stretch C-H in-plane bend C-H in-plane bend N-R in-plane bend Ring stretch -CH2 bend Ring stretch Ring stretch Ring stretch
References
PI PI PI Chap. 18 in [6] 121
PI
Chap. 18 in [6]
PI Chap. 18 in [6] Chap. 1 in [6] 121 [21
PI
RESULTSAND DISCUSSION Figure 1 shows the normal Raman spectrum of pure TX100 in the liquid state. Figure 2 is a typical SERS spectrum of TX100 on a silver surface in an aqueous medium at a concentration of 1.43 x 10s4 M (0.49 cmc). The experimental results indicate that within the range of concentration from 0.08 cmc to 2.0 cmc, the Raman shifts and shapes of the SERS spectra of TX100 are the same as that shown in Fig. 2. Table 1 lists the assignments of the Raman bands of TX100 on the basis of Ref. [6]. The band at 224 cm-’ might be the vibration of Ag-S or Ag-0 [7]. It is obvious in Table 1 that all the bands related to the benzene ring appear in the SERS spectrum of TXlOO, whereas the strongest band 1470 cm-’ and another two strong bands, 836 and 870 cm-’ in the normal Raman spectrum of Fig. 1 almost disappear in the SERS spectrum of Fig. 2. The latter bands are vibrations of the polyethylene oxide EO chain, and in fact, there are no clear peaks related to EO vibration in the SERS spectrum of TXlOO. Note that SERS is very sensitive to the distance between the adsorbed molecules and the active SERS substrate, i.e. only part of the molecule that contacts directly or is very near to the substrate will show clear vibrational modes [8]. Based on the above consideration and the fact that the EO chain is strongly hydrophilic, it is reasonable to suppose that TX100 has the configuration as shown in Fig. 4: the phenol ring lies flat on the surface, and the EO chain stretches into the aqueous phase. Figure 3 is a typical SERS spectrum of CPyB adsorbed on a silver surface in aqueous solution at a concentration of 2.9 x 10T4M (0.5 cmc). Experiments show that in the range of 0.5 to 2.0 cmc, the shifts and shapes of the SERS bands of CPyB are the same as those shown in Fig. 3. There are no clear SERS bands in our experiment at wavenumbers lower than 800 cm-‘.
CHJ /
Fig. 5. A proposed configuration of CPyB molecule on silver mirror/water interface.
SERS of surfactants absorbed on silver mirror surfaces
1013
The inset of Fig. 3 shows the normal Raman spectrum of pure CPyB in the polycrystallized state. Because of strong fluorescence, no Raman peaks can be seen in the range of 200 to 2000 cm-‘. However, KNOLL [l] and BUNDINGet al. [2] have studied the normal Raman spectrum of pure n-methylpyridinium bromide and its SERS spectrum on a silver surface. Considering that there are some similarities in the chemical structure between CPyB and n-methylpyridinium bromide, and that CPyB has an alkane chain longer than n-methylpyridinium bromide, we list the assignments of the SERS bands of CPyB in Table 2. It can be seen from Table 2 that the peaks related to the pyridinium ring are more numerous and stronger than other peaks in the same spectrum, and only the peaks near 1444 cm-’ (medium intensity) are related to the alkane chain. The above information means that the pyridinium cycle lies close to the surface. Also considering the chemical structure of the CPyB molecule, we propose that the configuration of the CPyB molecule adsorbed on the silver surface is as shown in Fig. 5: the pyridinium cycle lies flat on the surface, the alkane chain stretches tortuously on the surface with some parts of the chain close to the silver surface.
CONCLUSION
Summarizing the above discussion, it can be concluded that the technique of SERS is a helpful approach in studying the adsorption state of surfactants. Our present work has analyzed and clearly given the adsorption configurations of TX100 and CPyB on the silver surfaces. By studying the concentration dependence of SERS intensitities of some bands of the molecule, some information about the process of the surface micellization can be obtained. These results will be published in a later paper. Acknowledgemenr-The authors of the present paper are thankful to the National Foundation of Natural Sciences of China for its financial support.
REFERENCES (I] W. Knoll, J. Chem. Phys. 77, 219 (1982). [2] K. A. Bunding, MI. I. Bell and R. A. Durst, Chem. Phys. Lett. 89, 54 (1982). [3] S. Sun, R. L. Birke and J. R. Lombari, J. Phys. Chem. 94,2005 (1990). [4] A. Dendramis, E. W. Schwinn and R. P. Sperline, Surf. Sci. 134,675 (1983). (51 Y. MO, I. Miirke and P. Wachter, Surf. Sci. 133, LA52 (1983). [6] F. R. Doltish, W. G. Fateley and F. F. Bentley, Characteristic Raman Frequencies of Organic Compounds. John Wiley (1974). [7] Y. J. MO, H. von Kaemel, W. Barsa, P. Wachter, U. Martin and F. K. Reinhart, Proceedings ofrhe 12th International Conference on Raman Spectroscopy, p. 310. Columbia, South Carolina, August 1990. [8] P. N. Sanda, J. E. Demuth, J. C. Tsang and J. M. Warlaumont, in Surface Enhanced Raman Scattering (Edited by R. K. Chang and T. E. Furtuk), p. 189. Plenum, New York (1982).
MM-8539193 96.00 + 0.00 Pergamon Press Lid
1993
Surface enhanced Raman scattering of surfactants adsorbed on silver mirror surfaces* MINGMINGFANG, TIANJIANHUANG and TIREN Gu Laboratoryof Surface and Colloid Chemistry, Department of Chemistry and Institute of Physical Chemistry, Peking University, Beijing 100871, China
and YUJUN Mo,t
ZHAOXIANG WANG and XIUYING LI
Raman and Fluorescence Spectroscopy Laboratory, Institute of Physics, Chinese Academy of Sciences, Beijing ltlfKJg0,China (Received 20 March 1992; in final form 5 August 1992; accepted 6 August 1992) Abstract-The surface enhanced Raman scattering (SERS) spectra of two surfactants: Triton X-100 and n-hexadecylpyridinium bromide adsorbed on the interfaces between chemically reduced silver mirrors and water are obtained. By comparing and analyzing their bulk and SERS spectra, we have studied the’adsorption configurations of the two surfactants on the silver/water interfaces.
SINCEits discovery in 1974, the technique of surface enhanced Raman scattering (SERS) has become a forceful and helpful tool in studying molecules on surfaces and interfaces. But SERS investigation of surface active agents (surfactants) seems to be-much less common. The conventional methods of investigating the adsorption of surfactants on solid/liquid interfaces are macroscopic ones, that is, the determination of adsorption isotherm, heat of immersion, etc. Because of its in situ detection characteristics, it is reasonable to expect that SERS can become a microscopic probe to investigate the microscopic status of surfactants adsorbed on the interfaces of solid and liquid. KNOLL [l] and BUNDINGef al. [2] have studied n-alkylpyridinium halides adsorbed on silver electrodes, but their subjects were n-methylpyridinium ions. To our knowledge, SUN et al. [3] and DENDRAMIS et al. [4] are some of the few researchers who studied MBE 84
t
m0.m Raman shift
c
n3m.m
(cm-1l
Fig. 1. The normal Raman spectrum of pure TX100 in liquid state. * A project supported by National Sciences Foundation of China. t Author to whom correspondence should be addressed. 1009
MINGMINGFANG et al.
1010
,.uaEmt.t 2m.m
1wE.m
lEEB.1
Raman
shift
[cm-'1
Fig. 2. A typical SERS spectrum of TX100 on silver surface.
surfactants (both on cetyltrimethyl ammonium bromide) by SERS, although there are many others studying surfactants using the traditional methods. In our present work, another two surfactants, Triton X-100 (TXlOO) and n-hexadecylpyridinium bromide (CPyB), which are quite different in structure from cetyltrimethyl ammonium bromide and which are nonionic and cationic surfactants, respectively, are studied. By comparing Table 1. The assignment of SERS peaks of TX100 (cm-‘) Normal Raman
SERS
Assignment
224 w
Ag-S or Ag-0 vibration [7] EO ring vibration EO skeletal deformation CCC deformation Benzene ring deformation Sym. skeletal stretch EO ring vibration Benzene ring vibration EO ring vibration C-O-C sym. stretch C-O-C sym. stretch C-C stretch C-C stretch C-C stretch C-C stretch -CHz twist and rock -CHr twist and rock -CH2 twist and rock -CH2 twist and rock -CH2 twist and rock In-plane CH deformation Benzene ring stretch Benzene ring stretch Benzene ring stretch -CHr deformation -0CHr deformation Benzene ring stretch Benzene ring stretch Benzene ring stretch Benzene ring stretch
240 m 312 w 520 w 638s
684m 732 w 750 m 804m 836 m 870 m 924m 974 w 11OOm 1138 m 1164 w 1188 m 1212 w 1248 s 1282 s 1290 s 1364 vw 1386 VW 1418 VW 1452 s 1470 s 1510 vw 1582 w 1598 VW 1612 s N.B.: relative vw = very weak.
614 m
774 m
930 w 980 w 109ow 1130 m 1182 m
1310 m 1364 s 1388 w 1420 w 1468W 1510 s 1574 s 1596 w 1648 s intensity
( j: s=strong;
m=medium;
Ref. [6]
Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap. Chap.
14 14 1 13 1 14 13 14 14 14 1 1 1 1 1 1 1 1 1 1 13 13 13 1 14 13 13 13 13
w= weak; and
SERS of
a.mE
m 8m.m
surfactants absorbed on silver mirror surfaces
m3m.m Raman shift
1011
c
m0.m (cm-‘)
Fig. 3. A typical SERS spectrum of CPyB on silver surface (inset: the normal Raman spectrum of CPyB).
the normal Raman spectra of pure TX100 and its SERS and analyzing the SERS spectrum of CPyB, we propose their adsorption configurations on silver/water interfaces.
EXPERIMENTAL The method for preparing the SERS substrates, silver mirrors,is the same as that described in Ref. [S]. These substrates have great enhancement factors and are quite homogeneous. The silver mirrors used are about 6x 10mm in size. TX100 (an octyl phenol ethoxylate surfactant, whose average length of polyethylene oxide chain is about 10, see Fig. 4) was a product of B.D.H. Ltd. The critical mice11 concentration (cmc) of it is 2.9 x lo-’ M (W’C). CPyB was produced by Beijing Chemistry Co. and was recrystallized with an acetone-ethanol mixture three times. The cmc of CPyB is 5.81 x lo-’ M (25°C). The chemically deposited silver mirror was placed in a cylindrical cell [S] with a 0.5 ml aqueous solution of the surfactant. The Raman measurement was begun 20 min after the substrate had been put in the solution. All the experiments were done at 25°C. The measurement system was similar to that used in Ref. [5], except it included a Spex Raman-log 1403 spectrometer and DATAMATE-1. The power of the excitation light, 5145 A, created with a Ar+ laser, was 40 mW on the sample surface. Backscattering geometry of 45” and 4OOpm width of the monochromoter slit were used.
Fig. 4. A proposed configuration of TX100 molecule on silver mirror/water interface.
MINGMING FANG et al.
1012
Table 2. The assignment of SERS peaks of CPyB (cm-‘) SERS of CPYB
SERWRaman of n-MePy
816 m 860m 1006w 1030 s 1056 w 1132 w 1174 s 1214 s 1236 w 1276 m 1396 s 1444 m 1502 w 1584 m 1634 s
800 857 1001 1026 1058 1190 1212 1266 1449 1497 1580 1632
Assignment N-R stretch N-R out-of-plane bend Sym. ring stretch Triangle ring breathe C-H in-plane bend C-H in-plane deformation of the ring N-R stretch C-H in-plane bend C-H in-plane bend N-R in-plane bend Ring stretch -CH2 bend Ring stretch Ring stretch Ring stretch
References
PI PI PI Chap. 18 in [6] 121
PI
Chap. 18 in [6]
PI Chap. 18 in [6] Chap. 1 in [6] 121 [21
PI
RESULTSAND DISCUSSION Figure 1 shows the normal Raman spectrum of pure TX100 in the liquid state. Figure 2 is a typical SERS spectrum of TX100 on a silver surface in an aqueous medium at a concentration of 1.43 x 10s4 M (0.49 cmc). The experimental results indicate that within the range of concentration from 0.08 cmc to 2.0 cmc, the Raman shifts and shapes of the SERS spectra of TX100 are the same as that shown in Fig. 2. Table 1 lists the assignments of the Raman bands of TX100 on the basis of Ref. [6]. The band at 224 cm-’ might be the vibration of Ag-S or Ag-0 [7]. It is obvious in Table 1 that all the bands related to the benzene ring appear in the SERS spectrum of TXlOO, whereas the strongest band 1470 cm-’ and another two strong bands, 836 and 870 cm-’ in the normal Raman spectrum of Fig. 1 almost disappear in the SERS spectrum of Fig. 2. The latter bands are vibrations of the polyethylene oxide EO chain, and in fact, there are no clear peaks related to EO vibration in the SERS spectrum of TXlOO. Note that SERS is very sensitive to the distance between the adsorbed molecules and the active SERS substrate, i.e. only part of the molecule that contacts directly or is very near to the substrate will show clear vibrational modes [8]. Based on the above consideration and the fact that the EO chain is strongly hydrophilic, it is reasonable to suppose that TX100 has the configuration as shown in Fig. 4: the phenol ring lies flat on the surface, and the EO chain stretches into the aqueous phase. Figure 3 is a typical SERS spectrum of CPyB adsorbed on a silver surface in aqueous solution at a concentration of 2.9 x 10T4M (0.5 cmc). Experiments show that in the range of 0.5 to 2.0 cmc, the shifts and shapes of the SERS bands of CPyB are the same as those shown in Fig. 3. There are no clear SERS bands in our experiment at wavenumbers lower than 800 cm-‘.
CHJ /
Fig. 5. A proposed configuration of CPyB molecule on silver mirror/water interface.
SERS of surfactants absorbed on silver mirror surfaces
1013
The inset of Fig. 3 shows the normal Raman spectrum of pure CPyB in the polycrystallized state. Because of strong fluorescence, no Raman peaks can be seen in the range of 200 to 2000 cm-‘. However, KNOLL [l] and BUNDINGet al. [2] have studied the normal Raman spectrum of pure n-methylpyridinium bromide and its SERS spectrum on a silver surface. Considering that there are some similarities in the chemical structure between CPyB and n-methylpyridinium bromide, and that CPyB has an alkane chain longer than n-methylpyridinium bromide, we list the assignments of the SERS bands of CPyB in Table 2. It can be seen from Table 2 that the peaks related to the pyridinium ring are more numerous and stronger than other peaks in the same spectrum, and only the peaks near 1444 cm-’ (medium intensity) are related to the alkane chain. The above information means that the pyridinium cycle lies close to the surface. Also considering the chemical structure of the CPyB molecule, we propose that the configuration of the CPyB molecule adsorbed on the silver surface is as shown in Fig. 5: the pyridinium cycle lies flat on the surface, the alkane chain stretches tortuously on the surface with some parts of the chain close to the silver surface.
CONCLUSION
Summarizing the above discussion, it can be concluded that the technique of SERS is a helpful approach in studying the adsorption state of surfactants. Our present work has analyzed and clearly given the adsorption configurations of TX100 and CPyB on the silver surfaces. By studying the concentration dependence of SERS intensitities of some bands of the molecule, some information about the process of the surface micellization can be obtained. These results will be published in a later paper. Acknowledgemenr-The authors of the present paper are thankful to the National Foundation of Natural Sciences of China for its financial support.
REFERENCES (I] W. Knoll, J. Chem. Phys. 77, 219 (1982). [2] K. A. Bunding, MI. I. Bell and R. A. Durst, Chem. Phys. Lett. 89, 54 (1982). [3] S. Sun, R. L. Birke and J. R. Lombari, J. Phys. Chem. 94,2005 (1990). [4] A. Dendramis, E. W. Schwinn and R. P. Sperline, Surf. Sci. 134,675 (1983). (51 Y. MO, I. Miirke and P. Wachter, Surf. Sci. 133, LA52 (1983). [6] F. R. Doltish, W. G. Fateley and F. F. Bentley, Characteristic Raman Frequencies of Organic Compounds. John Wiley (1974). [7] Y. J. MO, H. von Kaemel, W. Barsa, P. Wachter, U. Martin and F. K. Reinhart, Proceedings ofrhe 12th International Conference on Raman Spectroscopy, p. 310. Columbia, South Carolina, August 1990. [8] P. N. Sanda, J. E. Demuth, J. C. Tsang and J. M. Warlaumont, in Surface Enhanced Raman Scattering (Edited by R. K. Chang and T. E. Furtuk), p. 189. Plenum, New York (1982).