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Well-ordered structure of N-octadecanoyl-l -alanine Langmuir–Blodgett film studied by FTIR spectroscopy
12 November 1999
Chemical Physics Letters 313 Ž1999. 565–568 www.elsevier.nlrlocatercplett
Well-ordered structure of N-octadecanoyl-L-alanine Langmuir–Blodgett film studied by FTIR spectroscopy Xuezhong Du ) , Yingqiu Liang Department of Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing UniÕersity, Nanjing 210093, China Received 22 June 1999; in final form 23 September 1999
Abstract N-Octadecanoyl-L-alanine Langmuir–Blodgett films have been studied by FTIR grazing angle reflection and transmission measurements. The N–H and C5O Žin amide and carboxylic acid. groups are oriented almost parallel to the substrate surface. The hydrocarbon chains are tilted regularly with a preferred orientation of C–C–C planes parallel to the substrate surface. The anisotropic arrangement of enantiomeric molecules that twist from neighbor to neighbor gives the aggregate chirality in the two-dimensional condensed state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The studies of chirality-dependent intermolecular forces in two-dimensional self-assemblies are of tremendous importance in many biological processes. Emphasis has been centered upon the investigations of monolayers consisting of N-acyl amino acid derivative w1–4x. Fluorescence microscopy provides an efficient method to visualize chiral discrimination effects in the morphologies of amphiphilic monolayers if chiral symmetry breaking is manifested in the shape of micrometer-sized domains of the ordered phase by curving in either direction or showing dendritic growth w5x. However, these observations are confined to macroscopic scales and can determine neither molecular characteristics like the orientation order of hydrocarbon chains nor the structure of the headgroups of the film-forming com)
Corresponding author. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China. Fax: q86-0411-4684839; e-mail: [email protected]
pounds w2,3x, which are assumed to be important for the macroscopic chirality-dependent domain structure Že.g., direction of curvatures.. FTIR spectroscopy appears to be suitable for the investigation of orientation order and molecular structure of welldefined organization formed by the LB technique w6x. In this Letter, we focus our interest on the study of the molecular packing and orientation order of N-octadecanoyl-L-alanine LB film and attempt to reveal the relationship between macroscopic chirality and microscopic orientation. 2. Experimental The preparation of the film-forming compound, N-octadecanoyl-L-alanine has been reported previously w7x. Y-type LB films with a transfer ratio close to unity were deposited on CaF2 substrates and silver-evaporated quartz plates for infrared transmission and grazing angle reflection ŽGAR. measurements, respectively. All FTIR spectra were recorded on a Bruker IFS 66V spectrometer equipped with a
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 4 4 - 6
566
X. Du, Y. Liangr Chemical Physics Letters 313 (1999) 565–568
DTGS detector. A KRS-5 polarizer was used for the polarization measurements of the LB film. Typically 1000 interferograms were collected to gain a satisfactory signal-to-noise ratio at a resolution of 4 cmy1 .
3. Results and discussion Shown in Fig. 1 are the transmission and grazing angle reflection ŽGAR. spectra of 11-monolayer Noctadecanoyl-L-alanine LB films. They are quite different from each other. The NH stretching Ž3324 cmy1 ., amide I w nŽC5O. at 1646 cmy1 x, CH 2 scissoring Ž1472 cmy1 ., and C5O stretching Ž1705 cmy1 in carboxylic acid. bands w7,8x are not observed in GAR spectrum in which the electric field vector is perpendicular to the metal surface, whereas those absorption bands are very strong in the transmission spectrum in which the electric field vector is parallel to the substrate surface. These spectral features indicate that the transition moments of these groups are parallel to the surface of the solid supports, moreover, the presence of the amide II band w dŽNH. at 1539 cmy1 x in the GAR spectrum also suggests a parallel orientation of amide groups. This kind of parallel orientation of the groups is attributed to the hydrogen bonds formed between the amide groups of neighboring molecules, for which evidence can be found in the frequencies of the NH stretching and amide II band, which are shifted to lower and
Fig. 1. FTIR spectra of an 11-monolayer N-octadecanoyl-L-alanine LB film: upper, transmission; lower, grazing angle reflectance.
Fig. 2. FTIR transmission spectrum of an 11-monolayer silver octadecanoyl-L-alaninate LB film.
higher wavenumbers as compared with those in an unassociated state w9x. There are two possibilities for the absence of the C5O stretching band of carboxylic acid groups: one is due to the formation of silver octadecanoyl-Lalaninate on the substrate and the other is a parallel orientation of the groups w10–12x. Fig. 2 shows the FTIR transmission spectrum of an 11-monolayer LB film deposited from an aqueous subphase containing 1 mM AgNO 3 . The strong naŽCOO. stretching and weak naŽCOO. stretching bands appear at 1510 and 1401 cmy1 , respectively. It is reported that the nhexadecanoic acid self-assembled monolayer by chemisorption on a silver substrate surface exhibited a strong nsŽCOO. stretching band at approximately 1400 cmy1 in the GAR spectrum w13x. If the carboxylic acid groups of enatiomeric molecules in the 1-monolayer LB film react with metal substrate, a strong nsŽCOO. stretching band near 1400 cmy1 should be observed in the GAR spectrum. As seen in Fig. 1, the absence of an absorption band near 1510 cmy1 or around 1400 cmy1 implies that the C5O groups of carboxylic acid dimers are oriented parallel to the metal substrate. This result is supported by the fact that the 1705-cmy1 nŽC5O. stretching band is clearly observed in the transmission spectrum of the 1-monolayer LB film w7x. The GAR spectral feature of the 11-monolayer LB film in Fig. 1 indicates that in each monolayer the C5O groups of carboxylic acid dimers are oriented parallel to the substrate surface. Miyashita et al. w14x have reported
X. Du, Y. Liangr Chemical Physics Letters 313 (1999) 565–568
Fig. 3. Polarized FTIR transmission spectra of an 11-monolayer N-octadecanoyl-L-alanine LB film at the normal incidence.
a highly ordered structure in N-octadecylacrylamide LB films in each layer the N–H, C5O, and C5C groups are oriented parallel to the surface of substrate. The orientations of enantiomeric headgroups reported here are very similar to their results, with the N–H and C5O groups in amide and carboxylic acid being oriented nearly parallel to the substrate surface. Compared with the transmission spectrum, the methylene stretching bands w naŽCH 2 . at 2918 cmy1 and nsŽCH 2 . at 2850 cmy1 x in the GAR spectrum shown a marked decrease in intensity, which indicates that the long alkyl chains are standing up from the surface in the LB films. If the long alkyl chains were completely perpendicular to the surface, the absorption of the CH 2 antisymmetric stretching Ž2918 cmy1 . band would not be observed in GAR spectrum, however, the CH 2 antisymmetric stretching absorption is still detected in the GAR spectrum. Considering this fact and the relative intensity ratio of I naŽCH 2 . to I naŽCH 3 ., the long alkyl chains are inferred to be regularly tilted. In the low-frequency range, a series of CH 2 progression bands appear in both transmission and GAR spectra, which also suggests that the alkyl chains in the film are tilted regularly since the transition moment of the vibrational mode is parallel to the long axis of the chain w15x. In addition, the intensity ratio I naŽCH 2 .r I nsŽCH 2 . is also different between the two infrared modes. Nuzzo et al. w16x explained similar results in terms of close packing of CH 2 chains in a triclinic
567
packing instead of an orthorhombic packing. Since the CH 2 scissoring mode is very sensitive to the interchain interaction w17x, a singlet peak at 1472 cmy1 in the transmission spectrum is characteristic of a triclinic subcell packing where the hydrocarbon chains are packed in a parallel arrangement w18x. The different ratio of intensities between the naŽCH 2 . and nsŽCH 2 . bands is not usually observed when the C–C–C plane of the all-trans alkyl chain can take on any orientation about the chain axis, i.e., when this chain axis is oriented with a tilt angle Ž a . with respect to the surface normal w19x. So, the C–C–C plane in this case must have a preferred orientation, in which this plane is oriented with respect to the substrate surface and does not take on all possible orientations about the chain axis. This way the nsŽCH 2 . vibration has a large component parallel to the substrate and will therefore absorb strongly in the transmission mode and weakly in the GAR mode. That may be the reason why no dŽCH 2 . band can be observed in the GAR mode, since the direction of transition moment of dŽCH 2 . vibration is parallel with that of nsŽCH 2 . vibration. Fig. 3 shows the polarized FTIR transmission spectra of an 11-monolayer N-octadecanoyl-L-alanine
Fig. 4. Pair of N-octadecanoyl-L-alanine molecules in a tightly aggregated state.
568
X. Du, Y. Liangr Chemical Physics Letters 313 (1999) 565–568
LB film. At normal incidence, all vibrational bands exhibit infrared anisotropy with a dichroism Ž D s A H rA 5 . of 1.1 ; 1.2, which indicates that both headgroup components and hydrocarbon chains in the film take a biaxial orientation. Under the same conditions of film fabrication, the hydrocarbon chains in octadecanoic acid LB film are uniaxially oriented and inclined at 25–348 from the normal at different surface pressures, which is consistent with the tilt angles of 27–358 reported previously w20x. It seems impossible that the infrared dichroism of N-octadecanoyl-L-alanine LB film results from the flow of water on the surface during the course of monolayer transfer as suggested in Ref. w14x. The orientation anisotropy is inferred to originate from the chiral centers of N-octadecanoyl-L-alanine molecules. On the basis of Nandi’s effective pair potential ŽEPP. approach w21,22x, which may predict the senses of helical amphiphilic assemblies including chiral monolayers and bilayers, the pairwise intermolecular interaction between the groups attached to the chiral carbons of the neighboring molecules depends on the size of attached groups, their separation distance, and their relative orientation. The sizes of the four groups attached to the chiral carbons of the molecules decrease as follows: CH 3 ŽCH 2 .16 CONH) COOH) CH 3 ) H. In the plane of the CH 3 and H groups, the adjacent CH 3 groups are farther away from each other compared to the two H groups. Similarly, in the plane of the CH 3 ŽCH 2 .16 CONH and COOH groups, the adjacent CH 3 ŽCH 2 .16 CONH groups are farther away and the neighboring COOH groups are closer to each other, which would force the two COOH groups to rotate around their respective C–C a bonds to deviate from the C–C–C planes of alkyl chains and to form an out-of-plane dimer between the neighboring molecules, shown schematically in Fig. 4. The minimum-energy configuration of a pair of the same enantiomers favors a twisted angle between them, and this twist from neighbor to neighbor gives rise to the chirality of the aggregate. This could be correlated with the curved domains and dendritic growth observed by fluorescence spectroscopy w5x. These macroscopic anisotropic curvatures probably
originate from the microscopic twisted orientation of groups attached to the chiral carbon in the condensed state.
Acknowledgements This work is financially supported by a major research project grant from the State Science and Technology Commission of China.
References w1x J.G. Heath, E.M. Arnett, J. Am. Chem. Soc. 114 Ž1992. 4500. w2x K.J. Stine, J.K.-J. Uang, S.D. Dingnam, Langmuir 9 Ž1993. 2112. w3x D.P. Parazak, J.K.-J. Uang, B. Turner, K.J. Stine, Langmuir 10 Ž1994. 3787. w4x A. Gericke, H. Huhnerfuss, Langmuir 10 Ž1994. 3782. ¨ w5x H.M. MoConnel, Annu. Rev. Phys. Chem. 42 Ž1991. 171. w6x B. Schrader, in: J.R. Ferraro, K. Krishnan ŽEds.., Practical Fourier Transform Infrared Spectroscopy, Academic Press, San Diego, CA, 1990, p. 168. w7x X. Du, B. Shi, Y. Liang, Langmuir 14 Ž1998. 3631. w8x X. Du, B. Shi, Y. Liang, Spectrosc. Lett. 32 Ž1999. 1. w9x R.S. Clegg, J.E. Hutchison, Langmuir 12 Ž1996. 5239. w10x F. Kimura, J. Umemura, T. Takenaka, Langmuir 2 Ž1986. 96. w11x Y. Liang, Y. Jiang, Y. Tian, Acta Phys. Chim. Sin. 7 Ž1991. 72. w12x J. Umemura, S. Takeda, T. Hasegawa, T. Takenaka, J. Mol. Struct. 297 Ž1993. 57. w13x Y.-T. Tao, W.-L. Lin, G.D. Hietpas, D.L. Allara, J. Phys. Chem. B 101 Ž1997. 9732. w14x T. Miyashita, T. Suwa, Langmuir 10 Ž1994. 3387. w15x J.F. Rabolt, F.C. Burns, N.E. Schlotter, J. Chem. Phys. 78 Ž1983. 946. w16x R.G. Nuzzo, A. Fusco, D.L. Allara, J. Am. Chem. Soc. 109 Ž1987. 2358. w17x R.G. Snyder, J. Mol. Spectrosc. 7 Ž1961. 116. w18x R.F. Holland, J.R. Nielsen, J. Mol. Spectrosc. 9 Ž1962. 436. w19x P.J. Werkman, R.H. Wieringa, A.J. Schouten, Langmuir 13 Ž1997. 6755. w20x T. Takenaka, K. Nogami, H. Gotoh, R. Gotoh, J. Colloid Interface Sci. 35 Ž1971. 395. w21x N. Nandi, B. Bagchi, J. Am. Chem. Soc. 118 Ž1996. 11208. w22x N. Nandi, B. Bagchi, J. Phys. Chem. A 101 Ž1997. 1343.
Chemical Physics Letters 313 Ž1999. 565–568 www.elsevier.nlrlocatercplett
Well-ordered structure of N-octadecanoyl-L-alanine Langmuir–Blodgett film studied by FTIR spectroscopy Xuezhong Du ) , Yingqiu Liang Department of Chemistry and State Key Laboratory of Coordination Chemistry, Nanjing UniÕersity, Nanjing 210093, China Received 22 June 1999; in final form 23 September 1999
Abstract N-Octadecanoyl-L-alanine Langmuir–Blodgett films have been studied by FTIR grazing angle reflection and transmission measurements. The N–H and C5O Žin amide and carboxylic acid. groups are oriented almost parallel to the substrate surface. The hydrocarbon chains are tilted regularly with a preferred orientation of C–C–C planes parallel to the substrate surface. The anisotropic arrangement of enantiomeric molecules that twist from neighbor to neighbor gives the aggregate chirality in the two-dimensional condensed state. q 1999 Elsevier Science B.V. All rights reserved.
1. Introduction The studies of chirality-dependent intermolecular forces in two-dimensional self-assemblies are of tremendous importance in many biological processes. Emphasis has been centered upon the investigations of monolayers consisting of N-acyl amino acid derivative w1–4x. Fluorescence microscopy provides an efficient method to visualize chiral discrimination effects in the morphologies of amphiphilic monolayers if chiral symmetry breaking is manifested in the shape of micrometer-sized domains of the ordered phase by curving in either direction or showing dendritic growth w5x. However, these observations are confined to macroscopic scales and can determine neither molecular characteristics like the orientation order of hydrocarbon chains nor the structure of the headgroups of the film-forming com)
Corresponding author. Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China. Fax: q86-0411-4684839; e-mail: [email protected]
pounds w2,3x, which are assumed to be important for the macroscopic chirality-dependent domain structure Že.g., direction of curvatures.. FTIR spectroscopy appears to be suitable for the investigation of orientation order and molecular structure of welldefined organization formed by the LB technique w6x. In this Letter, we focus our interest on the study of the molecular packing and orientation order of N-octadecanoyl-L-alanine LB film and attempt to reveal the relationship between macroscopic chirality and microscopic orientation. 2. Experimental The preparation of the film-forming compound, N-octadecanoyl-L-alanine has been reported previously w7x. Y-type LB films with a transfer ratio close to unity were deposited on CaF2 substrates and silver-evaporated quartz plates for infrared transmission and grazing angle reflection ŽGAR. measurements, respectively. All FTIR spectra were recorded on a Bruker IFS 66V spectrometer equipped with a
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 Ž 9 9 . 0 1 1 4 4 - 6
566
X. Du, Y. Liangr Chemical Physics Letters 313 (1999) 565–568
DTGS detector. A KRS-5 polarizer was used for the polarization measurements of the LB film. Typically 1000 interferograms were collected to gain a satisfactory signal-to-noise ratio at a resolution of 4 cmy1 .
3. Results and discussion Shown in Fig. 1 are the transmission and grazing angle reflection ŽGAR. spectra of 11-monolayer Noctadecanoyl-L-alanine LB films. They are quite different from each other. The NH stretching Ž3324 cmy1 ., amide I w nŽC5O. at 1646 cmy1 x, CH 2 scissoring Ž1472 cmy1 ., and C5O stretching Ž1705 cmy1 in carboxylic acid. bands w7,8x are not observed in GAR spectrum in which the electric field vector is perpendicular to the metal surface, whereas those absorption bands are very strong in the transmission spectrum in which the electric field vector is parallel to the substrate surface. These spectral features indicate that the transition moments of these groups are parallel to the surface of the solid supports, moreover, the presence of the amide II band w dŽNH. at 1539 cmy1 x in the GAR spectrum also suggests a parallel orientation of amide groups. This kind of parallel orientation of the groups is attributed to the hydrogen bonds formed between the amide groups of neighboring molecules, for which evidence can be found in the frequencies of the NH stretching and amide II band, which are shifted to lower and
Fig. 1. FTIR spectra of an 11-monolayer N-octadecanoyl-L-alanine LB film: upper, transmission; lower, grazing angle reflectance.
Fig. 2. FTIR transmission spectrum of an 11-monolayer silver octadecanoyl-L-alaninate LB film.
higher wavenumbers as compared with those in an unassociated state w9x. There are two possibilities for the absence of the C5O stretching band of carboxylic acid groups: one is due to the formation of silver octadecanoyl-Lalaninate on the substrate and the other is a parallel orientation of the groups w10–12x. Fig. 2 shows the FTIR transmission spectrum of an 11-monolayer LB film deposited from an aqueous subphase containing 1 mM AgNO 3 . The strong naŽCOO. stretching and weak naŽCOO. stretching bands appear at 1510 and 1401 cmy1 , respectively. It is reported that the nhexadecanoic acid self-assembled monolayer by chemisorption on a silver substrate surface exhibited a strong nsŽCOO. stretching band at approximately 1400 cmy1 in the GAR spectrum w13x. If the carboxylic acid groups of enatiomeric molecules in the 1-monolayer LB film react with metal substrate, a strong nsŽCOO. stretching band near 1400 cmy1 should be observed in the GAR spectrum. As seen in Fig. 1, the absence of an absorption band near 1510 cmy1 or around 1400 cmy1 implies that the C5O groups of carboxylic acid dimers are oriented parallel to the metal substrate. This result is supported by the fact that the 1705-cmy1 nŽC5O. stretching band is clearly observed in the transmission spectrum of the 1-monolayer LB film w7x. The GAR spectral feature of the 11-monolayer LB film in Fig. 1 indicates that in each monolayer the C5O groups of carboxylic acid dimers are oriented parallel to the substrate surface. Miyashita et al. w14x have reported
X. Du, Y. Liangr Chemical Physics Letters 313 (1999) 565–568
Fig. 3. Polarized FTIR transmission spectra of an 11-monolayer N-octadecanoyl-L-alanine LB film at the normal incidence.
a highly ordered structure in N-octadecylacrylamide LB films in each layer the N–H, C5O, and C5C groups are oriented parallel to the surface of substrate. The orientations of enantiomeric headgroups reported here are very similar to their results, with the N–H and C5O groups in amide and carboxylic acid being oriented nearly parallel to the substrate surface. Compared with the transmission spectrum, the methylene stretching bands w naŽCH 2 . at 2918 cmy1 and nsŽCH 2 . at 2850 cmy1 x in the GAR spectrum shown a marked decrease in intensity, which indicates that the long alkyl chains are standing up from the surface in the LB films. If the long alkyl chains were completely perpendicular to the surface, the absorption of the CH 2 antisymmetric stretching Ž2918 cmy1 . band would not be observed in GAR spectrum, however, the CH 2 antisymmetric stretching absorption is still detected in the GAR spectrum. Considering this fact and the relative intensity ratio of I naŽCH 2 . to I naŽCH 3 ., the long alkyl chains are inferred to be regularly tilted. In the low-frequency range, a series of CH 2 progression bands appear in both transmission and GAR spectra, which also suggests that the alkyl chains in the film are tilted regularly since the transition moment of the vibrational mode is parallel to the long axis of the chain w15x. In addition, the intensity ratio I naŽCH 2 .r I nsŽCH 2 . is also different between the two infrared modes. Nuzzo et al. w16x explained similar results in terms of close packing of CH 2 chains in a triclinic
567
packing instead of an orthorhombic packing. Since the CH 2 scissoring mode is very sensitive to the interchain interaction w17x, a singlet peak at 1472 cmy1 in the transmission spectrum is characteristic of a triclinic subcell packing where the hydrocarbon chains are packed in a parallel arrangement w18x. The different ratio of intensities between the naŽCH 2 . and nsŽCH 2 . bands is not usually observed when the C–C–C plane of the all-trans alkyl chain can take on any orientation about the chain axis, i.e., when this chain axis is oriented with a tilt angle Ž a . with respect to the surface normal w19x. So, the C–C–C plane in this case must have a preferred orientation, in which this plane is oriented with respect to the substrate surface and does not take on all possible orientations about the chain axis. This way the nsŽCH 2 . vibration has a large component parallel to the substrate and will therefore absorb strongly in the transmission mode and weakly in the GAR mode. That may be the reason why no dŽCH 2 . band can be observed in the GAR mode, since the direction of transition moment of dŽCH 2 . vibration is parallel with that of nsŽCH 2 . vibration. Fig. 3 shows the polarized FTIR transmission spectra of an 11-monolayer N-octadecanoyl-L-alanine
Fig. 4. Pair of N-octadecanoyl-L-alanine molecules in a tightly aggregated state.
568
X. Du, Y. Liangr Chemical Physics Letters 313 (1999) 565–568
LB film. At normal incidence, all vibrational bands exhibit infrared anisotropy with a dichroism Ž D s A H rA 5 . of 1.1 ; 1.2, which indicates that both headgroup components and hydrocarbon chains in the film take a biaxial orientation. Under the same conditions of film fabrication, the hydrocarbon chains in octadecanoic acid LB film are uniaxially oriented and inclined at 25–348 from the normal at different surface pressures, which is consistent with the tilt angles of 27–358 reported previously w20x. It seems impossible that the infrared dichroism of N-octadecanoyl-L-alanine LB film results from the flow of water on the surface during the course of monolayer transfer as suggested in Ref. w14x. The orientation anisotropy is inferred to originate from the chiral centers of N-octadecanoyl-L-alanine molecules. On the basis of Nandi’s effective pair potential ŽEPP. approach w21,22x, which may predict the senses of helical amphiphilic assemblies including chiral monolayers and bilayers, the pairwise intermolecular interaction between the groups attached to the chiral carbons of the neighboring molecules depends on the size of attached groups, their separation distance, and their relative orientation. The sizes of the four groups attached to the chiral carbons of the molecules decrease as follows: CH 3 ŽCH 2 .16 CONH) COOH) CH 3 ) H. In the plane of the CH 3 and H groups, the adjacent CH 3 groups are farther away from each other compared to the two H groups. Similarly, in the plane of the CH 3 ŽCH 2 .16 CONH and COOH groups, the adjacent CH 3 ŽCH 2 .16 CONH groups are farther away and the neighboring COOH groups are closer to each other, which would force the two COOH groups to rotate around their respective C–C a bonds to deviate from the C–C–C planes of alkyl chains and to form an out-of-plane dimer between the neighboring molecules, shown schematically in Fig. 4. The minimum-energy configuration of a pair of the same enantiomers favors a twisted angle between them, and this twist from neighbor to neighbor gives rise to the chirality of the aggregate. This could be correlated with the curved domains and dendritic growth observed by fluorescence spectroscopy w5x. These macroscopic anisotropic curvatures probably
originate from the microscopic twisted orientation of groups attached to the chiral carbon in the condensed state.
Acknowledgements This work is financially supported by a major research project grant from the State Science and Technology Commission of China.
References w1x J.G. Heath, E.M. Arnett, J. Am. Chem. Soc. 114 Ž1992. 4500. w2x K.J. Stine, J.K.-J. Uang, S.D. Dingnam, Langmuir 9 Ž1993. 2112. w3x D.P. Parazak, J.K.-J. Uang, B. Turner, K.J. Stine, Langmuir 10 Ž1994. 3787. w4x A. Gericke, H. Huhnerfuss, Langmuir 10 Ž1994. 3782. ¨ w5x H.M. MoConnel, Annu. Rev. Phys. Chem. 42 Ž1991. 171. w6x B. Schrader, in: J.R. Ferraro, K. Krishnan ŽEds.., Practical Fourier Transform Infrared Spectroscopy, Academic Press, San Diego, CA, 1990, p. 168. w7x X. Du, B. Shi, Y. Liang, Langmuir 14 Ž1998. 3631. w8x X. Du, B. Shi, Y. Liang, Spectrosc. Lett. 32 Ž1999. 1. w9x R.S. Clegg, J.E. Hutchison, Langmuir 12 Ž1996. 5239. w10x F. Kimura, J. Umemura, T. Takenaka, Langmuir 2 Ž1986. 96. w11x Y. Liang, Y. Jiang, Y. Tian, Acta Phys. Chim. Sin. 7 Ž1991. 72. w12x J. Umemura, S. Takeda, T. Hasegawa, T. Takenaka, J. Mol. Struct. 297 Ž1993. 57. w13x Y.-T. Tao, W.-L. Lin, G.D. Hietpas, D.L. Allara, J. Phys. Chem. B 101 Ž1997. 9732. w14x T. Miyashita, T. Suwa, Langmuir 10 Ž1994. 3387. w15x J.F. Rabolt, F.C. Burns, N.E. Schlotter, J. Chem. Phys. 78 Ž1983. 946. w16x R.G. Nuzzo, A. Fusco, D.L. Allara, J. Am. Chem. Soc. 109 Ž1987. 2358. w17x R.G. Snyder, J. Mol. Spectrosc. 7 Ž1961. 116. w18x R.F. Holland, J.R. Nielsen, J. Mol. Spectrosc. 9 Ž1962. 436. w19x P.J. Werkman, R.H. Wieringa, A.J. Schouten, Langmuir 13 Ž1997. 6755. w20x T. Takenaka, K. Nogami, H. Gotoh, R. Gotoh, J. Colloid Interface Sci. 35 Ž1971. 395. w21x N. Nandi, B. Bagchi, J. Am. Chem. Soc. 118 Ž1996. 11208. w22x N. Nandi, B. Bagchi, J. Phys. Chem. A 101 Ž1997. 1343.