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Resonance raman spectra and tautomeric structures of a surface-active azo dye, CI Acid Red 138, adsorbed at the aqueous solution–air interface
Vibrational Spectroscopy 38 (2005) 29–32 www.elsevier.com/locate/vibspec
Resonance raman spectra and tautomeric structures of a surface-active azo dye, CI Acid Red 138, adsorbed at the aqueous solution–air interface Junzo Umemura a,*, Sang Rae Park b a
Laboratory of Solution and Interface Chemistry, Division of Environmental Chemistry, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto-fu 611-0011, Japan b Chochiwon Girls’ High School, 167 Suchang-ri, Chochiwon-eup, Yeonki-gun, Chungnam, Korea Accepted 14 February 2005 Available online 18 March 2005
Abstract Raman spectra of Gibbs monolayers adsorbed from the aqueous solution to the solution–air interface were measured for the first time using resonance Raman technique against a surface-active azo dye, CI Acid Red 138 (AR138). The tautomeric structure change of AR138 from the azo form to the protonated hydrazone form occurred upon adsorption from a basic solution. This structural change was principally the same as those observed previously for the adsorption at the aqueous solution-CCl4 interface and for micellization. # 2005 Elsevier B.V. All rights reserved. Keywords: Resonance Raman spectra; Tautomeric structure change; CI Acid Red 138; Adsorbed monolayer; Water–air interface; Azo dye
1. Introduction Adsorption of surface active substances at the aqueous solution–air interface has been one of the greatest interests for long time in surface chemistry, since it is strongly related with many interfacial processes such as forming, detergency or emulsification [1]. The Gibbs adsorption equation and surface tension measurements were primarily used to evaluate the surface excess quantity of adsorbed monolayers [1]. Radiotracer method could be used successfully to evaluate the same quantity [2]. Spectroscopic studies of adsorbed monolayers at the aqueous solution–air interface have been made by FT-IR external reflection spectroscopy for sodium dodecylsulfonate [3], water soluble polymers of poly(ethylene oxide) samples end-capped with fluorinated carbon chains [4], and sodium dodecyl sulfate [5]. To our best knowledge, however, there has been no report on the Raman spectroscopic study of Gibbs adsorbed monolayers at the aqueous solution–air interface, although there have been reports on the non-resonance [6] and
* Corresponding author. Tel.: +81 774 38 3071; fax: +81 774 38 3076. E-mail address: [email protected] (J. Umemura). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.02.003
resonance [7–8] Raman spectra of insoluble monolayers spread on the water–air interface. On the other hand, resonance Raman spectroscopy has successfully been used to monitor the structure and orientation of adsorbed monolayers of a surface-active azo dye, CI Acid Red 138 (AR138) or its commercial name Suminol brilliant red BS (BRBS), at the aqueous solution– CCl4 interface [9–12]. This is because the total reflection condition at the water–CCl4 interface could be easily realized and hence the adsorbed monolayers could be selectively monitored. It has been shown that the structural change from azo IIa to protonated hydrazone Ib (Fig. 1) forms takes place upon adsorption from basic aqueous solution below the critical micelle concentration (cmc) [11]. In this paper, we have tried and succeeded to monitor the resonance Raman signal from Gibbs monolayers of AR138 adsorbed from its aqueous solution to the solution–air interface. 2. Experimental The sample of AR138 (BRBS) was the same as that reported previously [9–11]. Aqueous solutions were pre-
30
J. Umemura, S.R. Park / Vibrational Spectroscopy 38 (2005) 29–32
Fig. 1. Tautomeric structures of the neutral and basic forms of AR138. In this figure, the less-contributed tautomers of Ia and IIb forms [8,10] are omitted. Na+ may be dissociated in the molecularly dispersed state at low concentrations.
pared using distilled water from a modified Mitamura Riken model PLS-DFR automatic lab still consisting of a reverseosmosis module, an ion-exchange column, and a double distiller. The pH of the water was 6.2 and adjusted to pH 14 with NaOH if necessary. The concentrations of AR138 were 1.0 10 5 and 1.0 10 3 M. The concentration of 1.0 10 5 M is below the cmc of AR138 at pH 6.2 (1.1 10 3 M) and at pH 14 (cmc is expected to be close to 2.5 10 4 M at pH 13.2) [13]. On the other hand, the concentration of 1.0 10 3 M is above the cmc at pH 14, and just near the cmc at pH 6.2. Aqueous solutions of AR138 were sealed into Pyrex capillary tubes with inner diameter of 1.5 mm and they are mounted vertically. Resonance Raman spectra of bulk solutions and adsorbed monolayers at the solution and air were measured by focusing the horizontal 488.0 nm laser beam (100 mW) from a Spectra-Physics model 2016 Ar+ laser into the center of the solution part and into the concaveshaped solution–air interface from the solution side, respectively. The scattered Raman signals were collected at the right angle to the incident beam and introduced, after reflections by two mirrors, vertically to the 100 mm slit of a Spex Triplemate with a Photometrics model PM512 CCD detector. The CCD detector was operated at 125 8C by a Photometrics Series 200 cryogenic camera system. The optical resolution was about 10 cm 1. The exposure time of 30–200 s was spent to record resonance Raman spectra. It has been known that at all concentrations at pH 6.2 and at concentrations above cmc in basic conditions like pH 13.2, the electronic absorption spectra of aqueous AR138 solution show a doublet at 514 and 542 nm (552 nm above cmc) typical of hydrazone Ib form [13,14]. At concentrations below cmc in basic conditions, the absorption maxima shift to a singlet peak of 458 nm typical of azo IIa form. An isosbestic point was observed at ca. 470 nm in the visible absorption spectra when the pH was varied, and hence the 488.0 nm excitation close to this isosbestic point is best suited to monitor both the azo and hydrazone forms of AR138 [13]. We also measured spectra using Ar+ 514.5 nm line which is more sensitive to the hydrazone Ib form, but they were essentially the same as those excited with 488.0 nm line and will not be presented in this paper.
3. Results and discussion Fig. 2 shows the typical resonance Raman spectra of AR138 observed from a 1.0 10 3 M bulk aqueous solution at pH 6.2 and from adsorbed monolayer at the interface between the solution and air. In the interface spectrum, signals come mainly from the interface and partly from the solution phase. Since the surface excess quantity is much more than the solute quantity in the solution [1], however, the information from the interface can be obtained easily. This condition is identical throughout all of the following figures. In principle, there is no difference in spectra between bulk and interface. These spectral features represented by the most intense peak at 1280 cm 1 are characteristic of the hydrazone Ib form depicted in Fig. 1 [11,13]. Marks of H as appendices to frequency readings represent those of the hydrazone form. Assignments of the observed peaks are mainly made after Machida’s work on azobenzene derivatives [15–18] and are given in Table 1. It has been known that the more micelle-rich bulk solutions at higher concentrations exhibit only spectra typical of the hydrazone Ib form [11]. At this neutral pH, therefore, the protonated hydrazone form prevails both in solutions and in Gibbs monolayers. At this stage, the reader might wonder if
Fig. 4. Resonance Raman spectra of AR138 from a 1.0 10 3 M bulk aqueous solution at pH 14 and from the interface between the solution and air.
J. Umemura, S.R. Park / Vibrational Spectroscopy 38 (2005) 29–32 Table 1 Observed peak frequencies in cm and azo forms of AR138a Hydrazone form Assignments
1606 1590 1575 1509 1500 1431 1409
Ring vibration Ring vibration Ring vibration Ring vibration – Ring vibration C–C stretching
1363 m
Frequency
Assignments
1433 m
Ring vibration
1389 m
–
1320 s 1302 w
N N stretching –
1229 m
Ring vibration
1146 m
C–N stretching
–
1280 vs 1225 m 1178 m
C–N stretching Ring vibration N–N stretching
1112 m
Ring vibration
a
and their assignments for hydrazone Azo form
Frequency m m m m sh sh s
1
31
m: medium; sh: shoulder; s: strong; vs: very strong; w: weak.
Fig. 2. Resonance Raman spectra of AR138 from a 1.0 10 3 M bulk aqueous solution at pH 6.2 and from the interface between the solution and air.
the information from the interface is really obtained or not. However, it will become obvious at the last stage of this presentation. Fig. 3 shows the resonance Raman spectra of AR138 observed from a 1.0 10 5 M bulk aqueous solution at pH 6.2 and from an adsorbed monolayer at the interface between the solution and air. In the bulk solution, AR138 molecules are molecularly dispersed as monomers since the concentration is appreciably lower than its cmc of 1.1 10 3 M. These spectral features are also the same as in Fig. 1, and they are ascribed to those of the hydrazone Ib form. In short, at neutral pH of 6.2, the AR138 takes the protonated hydrazone form, irrespective of the molecular states as monomer, micelle, or Gibbs adsorbed monolayer at the interface. Fig. 4 depicts the resonance Raman spectra of AR138 observed from 1.0 10 3 M bulk aqueous solution at pH
14 and from adsorbed monolayer at the interface between the solution and air. In the bulk solution, peaks ascribable to the hydrazone Ib form (marked by H as appendices to frequency readings) appear as well as those ascribable to the azo IIb form (marked by A as appendices). This situation is slightly different from that of the bulk solution at pH 13.2 where the hydrazone peaks are predominant [11], but is reasonable if we consider the fact that intensity ratio of the hydrazone 1280 cm 1 band to the azo 1320 cm 1 band decreases with increasing pH in the bulk phase [13]. Therefore, the azo and hydrazone forms coexist in the bulk solution at this concentration and pH. Assignments of the azo peaks are also given in Table 1. In the spectrum at the solution–air interface in Fig. 4, the relative intensities of azo peaks decreases from those in the bulk solution. The selectivity of the interface region is not so good as the total reflection method at the CCl4–aqueous solution interface [12], but the data in Fig. 4 clearly
Fig. 3. Resonance Raman spectra of AR138 from a 1.0 10 5 M bulk aqueous solution at pH 6.2 and from the interface between the solution and air.
Fig. 5. Resonance Raman spectra of AR138 from a 1.0 10 5 M bulk aqueous solution at pH 14 and from the interface between the solution and air.
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J. Umemura, S.R. Park / Vibrational Spectroscopy 38 (2005) 29–32
demonstrate that the information from the Gibbs monolayer can be obtained by this experimental setup and that the protonated hydrazone form is preferred in the Gibbs monolayer. The resonance Raman data at pH 14 and 1.0 10 5 M are given in Fig. 5. In the bulk solution, barely discernible peaks from the noisy background are only those of the azo form. The other several data that were not shown here support this conclusion. This is in conformity with the fact that in this condition AR138 molecules are molecularly dispersed as monomers in the azo form [13]. At the interface, however, hydrazone peaks are observed clearly and more prominently than azo peaks. The data at basic pH of 14 in Figs. 4 and 5 indicate that the protonated hydrazone form is more stable in the micelle and in the Gibbs monolayer at the solution–air interface. This result is consistent with the previous data in the bulk solution [13] and at the soution–CCl4 interface [11]. The promotive force of this azo-hydrazone tautomerism is in reducing the Coulombic repulsion forces, which are prevailed between charged azo IIb species dispersed molecularly in the solution, by protonation in the aggregated states as micelle or Gibbs monolayer.
4. Conclusions The resonance Raman signal from Gibbs monolayer adsorbed from aqueous solution of AR138 to solution–air interface could be obtained by illuminating the interface encapsulated into a Pyrex capillary tube. Gibbs monolayers adsorbed from basic solutions of pH 14 convert their tautomeric forms from the azo form into the protonated
hydrazone form, by reducing the Coulombic repulsion between charged azo forms.
References [1] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1978. [2] K. Tajima, M. Muramatsu, T. Sasaki, Bull. Chem. Soc. Jpn. 43 (1970) 1991. [3] Y.S. Tung, T. Gao, M.J. Rosen, J.E. Valentini, L.J. Fina, Appl. Spectrosc. 47 (1993) 1643. [4] Y. Ren, M.S. Shoichet, T.J. McCarthy, H.D. Stidham, S.L. Hsu, Macromolecules 28 (1995) 358. [5] T. Kawai, H. Kamino, K. Kon-No, Langmuir 14 (1998) 4964. [6] T. Kawai, J. Umemura, T. Takenaka, Chem. Phys. Lett. 162 (1989) 243. [7] D.N. Batchelder, C. Cheng, W. Mu¨ ller, B.J.E. Smith, Makromol. Chem., Makromol. Symp. 46 (1991) 171. [8] G.A. Schick, M.R. O’Grady, Thin Solid Films 215 (1992) 218. [9] T. Nakanaga, T. Takenaka, J. Phys. Chem. 81 (1977) 645. [10] T. Takenaka, Chem. Phys. Lett. 55 (1978) 515. [11] H. Takahashi, J. Umemura, T. Takenaka, J. Phys. Chem. 87 (1983) 739. [12] T. Takenaka, J. Umemura, in: J.R. Durig (Ed.), Applications of Infrared and Raman Spectroscopies to the Study of Surface Chemistry, Vibrational Spectra and Structure, 19, Elsevier, Amsterdam, 1991 (Chapter 5). [13] H. Takahashi, J. Umemura, T. Takenaka, J. Phys. Chem. 86 (1982) 4660. [14] St. Stoyanov, T. Iijima, T. Stoyanov, L. Antonov, Dyes Pigments 27 (1995) 237. [15] K. Machida, B.-K. Kim, Y. Saito, K. Igarashi, T. Uno, Bull. Chem. Soc. Jpn. 47 (1974) 78. [16] H. Terada, B.-K. Kim, Y. Saito, K. Machida, Spectrochim. Acta A31 (1975) 945. [17] K. Machida, H. Lee, A. Kuwae, J. Raman Spectrosc. 9 (1980) 198. [18] Y. Saito, B.-K. Kim, K. Machida, T. Uno, Bull. Chem. Soc. Jpn. 47 (1974) 2111.
Resonance raman spectra and tautomeric structures of a surface-active azo dye, CI Acid Red 138, adsorbed at the aqueous solution–air interface Junzo Umemura a,*, Sang Rae Park b a
Laboratory of Solution and Interface Chemistry, Division of Environmental Chemistry, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto-fu 611-0011, Japan b Chochiwon Girls’ High School, 167 Suchang-ri, Chochiwon-eup, Yeonki-gun, Chungnam, Korea Accepted 14 February 2005 Available online 18 March 2005
Abstract Raman spectra of Gibbs monolayers adsorbed from the aqueous solution to the solution–air interface were measured for the first time using resonance Raman technique against a surface-active azo dye, CI Acid Red 138 (AR138). The tautomeric structure change of AR138 from the azo form to the protonated hydrazone form occurred upon adsorption from a basic solution. This structural change was principally the same as those observed previously for the adsorption at the aqueous solution-CCl4 interface and for micellization. # 2005 Elsevier B.V. All rights reserved. Keywords: Resonance Raman spectra; Tautomeric structure change; CI Acid Red 138; Adsorbed monolayer; Water–air interface; Azo dye
1. Introduction Adsorption of surface active substances at the aqueous solution–air interface has been one of the greatest interests for long time in surface chemistry, since it is strongly related with many interfacial processes such as forming, detergency or emulsification [1]. The Gibbs adsorption equation and surface tension measurements were primarily used to evaluate the surface excess quantity of adsorbed monolayers [1]. Radiotracer method could be used successfully to evaluate the same quantity [2]. Spectroscopic studies of adsorbed monolayers at the aqueous solution–air interface have been made by FT-IR external reflection spectroscopy for sodium dodecylsulfonate [3], water soluble polymers of poly(ethylene oxide) samples end-capped with fluorinated carbon chains [4], and sodium dodecyl sulfate [5]. To our best knowledge, however, there has been no report on the Raman spectroscopic study of Gibbs adsorbed monolayers at the aqueous solution–air interface, although there have been reports on the non-resonance [6] and
* Corresponding author. Tel.: +81 774 38 3071; fax: +81 774 38 3076. E-mail address: [email protected] (J. Umemura). 0924-2031/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2005.02.003
resonance [7–8] Raman spectra of insoluble monolayers spread on the water–air interface. On the other hand, resonance Raman spectroscopy has successfully been used to monitor the structure and orientation of adsorbed monolayers of a surface-active azo dye, CI Acid Red 138 (AR138) or its commercial name Suminol brilliant red BS (BRBS), at the aqueous solution– CCl4 interface [9–12]. This is because the total reflection condition at the water–CCl4 interface could be easily realized and hence the adsorbed monolayers could be selectively monitored. It has been shown that the structural change from azo IIa to protonated hydrazone Ib (Fig. 1) forms takes place upon adsorption from basic aqueous solution below the critical micelle concentration (cmc) [11]. In this paper, we have tried and succeeded to monitor the resonance Raman signal from Gibbs monolayers of AR138 adsorbed from its aqueous solution to the solution–air interface. 2. Experimental The sample of AR138 (BRBS) was the same as that reported previously [9–11]. Aqueous solutions were pre-
30
J. Umemura, S.R. Park / Vibrational Spectroscopy 38 (2005) 29–32
Fig. 1. Tautomeric structures of the neutral and basic forms of AR138. In this figure, the less-contributed tautomers of Ia and IIb forms [8,10] are omitted. Na+ may be dissociated in the molecularly dispersed state at low concentrations.
pared using distilled water from a modified Mitamura Riken model PLS-DFR automatic lab still consisting of a reverseosmosis module, an ion-exchange column, and a double distiller. The pH of the water was 6.2 and adjusted to pH 14 with NaOH if necessary. The concentrations of AR138 were 1.0 10 5 and 1.0 10 3 M. The concentration of 1.0 10 5 M is below the cmc of AR138 at pH 6.2 (1.1 10 3 M) and at pH 14 (cmc is expected to be close to 2.5 10 4 M at pH 13.2) [13]. On the other hand, the concentration of 1.0 10 3 M is above the cmc at pH 14, and just near the cmc at pH 6.2. Aqueous solutions of AR138 were sealed into Pyrex capillary tubes with inner diameter of 1.5 mm and they are mounted vertically. Resonance Raman spectra of bulk solutions and adsorbed monolayers at the solution and air were measured by focusing the horizontal 488.0 nm laser beam (100 mW) from a Spectra-Physics model 2016 Ar+ laser into the center of the solution part and into the concaveshaped solution–air interface from the solution side, respectively. The scattered Raman signals were collected at the right angle to the incident beam and introduced, after reflections by two mirrors, vertically to the 100 mm slit of a Spex Triplemate with a Photometrics model PM512 CCD detector. The CCD detector was operated at 125 8C by a Photometrics Series 200 cryogenic camera system. The optical resolution was about 10 cm 1. The exposure time of 30–200 s was spent to record resonance Raman spectra. It has been known that at all concentrations at pH 6.2 and at concentrations above cmc in basic conditions like pH 13.2, the electronic absorption spectra of aqueous AR138 solution show a doublet at 514 and 542 nm (552 nm above cmc) typical of hydrazone Ib form [13,14]. At concentrations below cmc in basic conditions, the absorption maxima shift to a singlet peak of 458 nm typical of azo IIa form. An isosbestic point was observed at ca. 470 nm in the visible absorption spectra when the pH was varied, and hence the 488.0 nm excitation close to this isosbestic point is best suited to monitor both the azo and hydrazone forms of AR138 [13]. We also measured spectra using Ar+ 514.5 nm line which is more sensitive to the hydrazone Ib form, but they were essentially the same as those excited with 488.0 nm line and will not be presented in this paper.
3. Results and discussion Fig. 2 shows the typical resonance Raman spectra of AR138 observed from a 1.0 10 3 M bulk aqueous solution at pH 6.2 and from adsorbed monolayer at the interface between the solution and air. In the interface spectrum, signals come mainly from the interface and partly from the solution phase. Since the surface excess quantity is much more than the solute quantity in the solution [1], however, the information from the interface can be obtained easily. This condition is identical throughout all of the following figures. In principle, there is no difference in spectra between bulk and interface. These spectral features represented by the most intense peak at 1280 cm 1 are characteristic of the hydrazone Ib form depicted in Fig. 1 [11,13]. Marks of H as appendices to frequency readings represent those of the hydrazone form. Assignments of the observed peaks are mainly made after Machida’s work on azobenzene derivatives [15–18] and are given in Table 1. It has been known that the more micelle-rich bulk solutions at higher concentrations exhibit only spectra typical of the hydrazone Ib form [11]. At this neutral pH, therefore, the protonated hydrazone form prevails both in solutions and in Gibbs monolayers. At this stage, the reader might wonder if
Fig. 4. Resonance Raman spectra of AR138 from a 1.0 10 3 M bulk aqueous solution at pH 14 and from the interface between the solution and air.
J. Umemura, S.R. Park / Vibrational Spectroscopy 38 (2005) 29–32 Table 1 Observed peak frequencies in cm and azo forms of AR138a Hydrazone form Assignments
1606 1590 1575 1509 1500 1431 1409
Ring vibration Ring vibration Ring vibration Ring vibration – Ring vibration C–C stretching
1363 m
Frequency
Assignments
1433 m
Ring vibration
1389 m
–
1320 s 1302 w
N N stretching –
1229 m
Ring vibration
1146 m
C–N stretching
–
1280 vs 1225 m 1178 m
C–N stretching Ring vibration N–N stretching
1112 m
Ring vibration
a
and their assignments for hydrazone Azo form
Frequency m m m m sh sh s
1
31
m: medium; sh: shoulder; s: strong; vs: very strong; w: weak.
Fig. 2. Resonance Raman spectra of AR138 from a 1.0 10 3 M bulk aqueous solution at pH 6.2 and from the interface between the solution and air.
the information from the interface is really obtained or not. However, it will become obvious at the last stage of this presentation. Fig. 3 shows the resonance Raman spectra of AR138 observed from a 1.0 10 5 M bulk aqueous solution at pH 6.2 and from an adsorbed monolayer at the interface between the solution and air. In the bulk solution, AR138 molecules are molecularly dispersed as monomers since the concentration is appreciably lower than its cmc of 1.1 10 3 M. These spectral features are also the same as in Fig. 1, and they are ascribed to those of the hydrazone Ib form. In short, at neutral pH of 6.2, the AR138 takes the protonated hydrazone form, irrespective of the molecular states as monomer, micelle, or Gibbs adsorbed monolayer at the interface. Fig. 4 depicts the resonance Raman spectra of AR138 observed from 1.0 10 3 M bulk aqueous solution at pH
14 and from adsorbed monolayer at the interface between the solution and air. In the bulk solution, peaks ascribable to the hydrazone Ib form (marked by H as appendices to frequency readings) appear as well as those ascribable to the azo IIb form (marked by A as appendices). This situation is slightly different from that of the bulk solution at pH 13.2 where the hydrazone peaks are predominant [11], but is reasonable if we consider the fact that intensity ratio of the hydrazone 1280 cm 1 band to the azo 1320 cm 1 band decreases with increasing pH in the bulk phase [13]. Therefore, the azo and hydrazone forms coexist in the bulk solution at this concentration and pH. Assignments of the azo peaks are also given in Table 1. In the spectrum at the solution–air interface in Fig. 4, the relative intensities of azo peaks decreases from those in the bulk solution. The selectivity of the interface region is not so good as the total reflection method at the CCl4–aqueous solution interface [12], but the data in Fig. 4 clearly
Fig. 3. Resonance Raman spectra of AR138 from a 1.0 10 5 M bulk aqueous solution at pH 6.2 and from the interface between the solution and air.
Fig. 5. Resonance Raman spectra of AR138 from a 1.0 10 5 M bulk aqueous solution at pH 14 and from the interface between the solution and air.
32
J. Umemura, S.R. Park / Vibrational Spectroscopy 38 (2005) 29–32
demonstrate that the information from the Gibbs monolayer can be obtained by this experimental setup and that the protonated hydrazone form is preferred in the Gibbs monolayer. The resonance Raman data at pH 14 and 1.0 10 5 M are given in Fig. 5. In the bulk solution, barely discernible peaks from the noisy background are only those of the azo form. The other several data that were not shown here support this conclusion. This is in conformity with the fact that in this condition AR138 molecules are molecularly dispersed as monomers in the azo form [13]. At the interface, however, hydrazone peaks are observed clearly and more prominently than azo peaks. The data at basic pH of 14 in Figs. 4 and 5 indicate that the protonated hydrazone form is more stable in the micelle and in the Gibbs monolayer at the solution–air interface. This result is consistent with the previous data in the bulk solution [13] and at the soution–CCl4 interface [11]. The promotive force of this azo-hydrazone tautomerism is in reducing the Coulombic repulsion forces, which are prevailed between charged azo IIb species dispersed molecularly in the solution, by protonation in the aggregated states as micelle or Gibbs monolayer.
4. Conclusions The resonance Raman signal from Gibbs monolayer adsorbed from aqueous solution of AR138 to solution–air interface could be obtained by illuminating the interface encapsulated into a Pyrex capillary tube. Gibbs monolayers adsorbed from basic solutions of pH 14 convert their tautomeric forms from the azo form into the protonated
hydrazone form, by reducing the Coulombic repulsion between charged azo forms.
References [1] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley, New York, 1978. [2] K. Tajima, M. Muramatsu, T. Sasaki, Bull. Chem. Soc. Jpn. 43 (1970) 1991. [3] Y.S. Tung, T. Gao, M.J. Rosen, J.E. Valentini, L.J. Fina, Appl. Spectrosc. 47 (1993) 1643. [4] Y. Ren, M.S. Shoichet, T.J. McCarthy, H.D. Stidham, S.L. Hsu, Macromolecules 28 (1995) 358. [5] T. Kawai, H. Kamino, K. Kon-No, Langmuir 14 (1998) 4964. [6] T. Kawai, J. Umemura, T. Takenaka, Chem. Phys. Lett. 162 (1989) 243. [7] D.N. Batchelder, C. Cheng, W. Mu¨ ller, B.J.E. Smith, Makromol. Chem., Makromol. Symp. 46 (1991) 171. [8] G.A. Schick, M.R. O’Grady, Thin Solid Films 215 (1992) 218. [9] T. Nakanaga, T. Takenaka, J. Phys. Chem. 81 (1977) 645. [10] T. Takenaka, Chem. Phys. Lett. 55 (1978) 515. [11] H. Takahashi, J. Umemura, T. Takenaka, J. Phys. Chem. 87 (1983) 739. [12] T. Takenaka, J. Umemura, in: J.R. Durig (Ed.), Applications of Infrared and Raman Spectroscopies to the Study of Surface Chemistry, Vibrational Spectra and Structure, 19, Elsevier, Amsterdam, 1991 (Chapter 5). [13] H. Takahashi, J. Umemura, T. Takenaka, J. Phys. Chem. 86 (1982) 4660. [14] St. Stoyanov, T. Iijima, T. Stoyanov, L. Antonov, Dyes Pigments 27 (1995) 237. [15] K. Machida, B.-K. Kim, Y. Saito, K. Igarashi, T. Uno, Bull. Chem. Soc. Jpn. 47 (1974) 78. [16] H. Terada, B.-K. Kim, Y. Saito, K. Machida, Spectrochim. Acta A31 (1975) 945. [17] K. Machida, H. Lee, A. Kuwae, J. Raman Spectrosc. 9 (1980) 198. [18] Y. Saito, B.-K. Kim, K. Machida, T. Uno, Bull. Chem. Soc. Jpn. 47 (1974) 2111.