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Polarized resonance Raman spectra of adsorbed thin layers at the glass-water interface
Polarized Resonance Raman Spectra of Adsorbed Thin Layers at the Glass-Water Interface TOHRU T A K E N A K A AND K A Z U FU MI YAMASAKI Institute for Chemical Research, Kyoto University, Uji, Kyoto-Fu, 611, Japan Received N o v e m b e r 2, 1979; accepted F e b r u a r y 15, 1980 R e s o n a n c e R a m a n m e a s u r e m e n t s have been made on adsorbed layers of a surface-active azo dye at the interface between a quartz glass and an a q u e o u s solution by the total reflection m e t h o d previously proposed. F r o m polarization m e a s u r e m e n t s of the spectra, the orientation of the adsorbed molecules was evaluated and results were discussed in connection with the a m o u n t of adsorption which was estimated from the band intensity of an electronic absorption spectra recorded by the multiple total reflection technique. It was found that the molecular orientation was improved with an increase of the a m o u n t of adsorption until m o n o l a y e r adsorption was completed, while it remained u n c h a n g e d after multilayer adsorption started. Addition of an electrolyte into the a q u e o u s solution led to a remarkable increase in the a m o u n t of adsorption and to a considerable i m p r o v e m e n t in molecular orientation. INTRODUCTION
was estimated from the band intensity of an electronic absorption spectra recorded by a technique of multiple total reflection at the interface (5). Effects of an electrolyte on the adsorption were also studied.
In a previous paper (1), we have proposed a method of total reflection for resonance Raman measurements of thin films at liquid interfaces and have studied the molecular orientation in monolayers adsorbed or spread at the liquid-liquid and gas-liquid interfaces (1-4). In the present work, the same technique has been applied to the study of thin layers of a surface-active anionic azo dye, Suminol Milling Brilliant Red BS (abbreviated as BRBS)
EXPERIMENTAL
CHs(CH2)II It
0
S05Na
N-C-CH s
S03Na
adsorbed at the interface between a quartz glass and an aqueous solution. From polarization measurements of resonance Raman spectra, the orientation of the BRBS molecules in the adsorbed layers was evaluated and the results were discussed in connection with the amount of adsorption which
The sample of BRBS was the same as that reported previously (2). Pure water was prepared by redistillation of distilled water which had been passed through an ion-exchange resin column. When necessary, a guaranteed reagent of sodium chloride was dissolved in a concentration of 1 x 10-2 M into the aqueous solution of BRBS. Figure 1 represents the apparatus of the total reflection method for recording resonance Raman spectra of adsorbed layers at the interface between the quartz glass and an aqueous solution. The apparatus consists of a truncated pyramidal prism made of quartz glass (Nippon Sekiei Glass Co., Type SG) and a branched glass tube equipped with a stopcock and ball joint for attachment to the vacuum manifold. The dimensions of the prism are 23 mm (at the lower
37
Journal of Colloidand Interface Science, Vol. 78, No. l, November 1980
0021-9797/80/110037-07502.00/0 Copyright © 1980by AcademicPress, Inc. All rights of reproduction in any form reserved.
38
TAKENAKA AND YAMASAKI
Roman
Slit
Polocoot
Glon-Thomso prism
Lens
Z
~
I
.,~,J
scattering
LI~---, ~ "
Loser
~ X
/
gloss
beom Aqueous solution
\
F ~ " Quor tz gloss
FIG. 1. Apparatus of the total reflection method for resonance Raman measurements of adsorbed layers at the interface between a quartz glass and an aqueous solution. side) and 38 m m (at the u p p e r side) in length and width, and 21 m m in height. The flat end of the b r a n c h of the glass tube was tightly pressed upon the optical plane of the quartz-glass prism through a silicone rubber O-ring. The aqueous solution of BRBS was placed in the tube being in contact with the prism. A horizontally propagating laser b e a m was incident upon a side face of the prism. After the refraction at the side face, the laser b e a m c a m e to the interface b e t w e e n the quartz glass and the aqueous solution with the angle of incidence 84 ° , and followed by the total reflection at the interface. The R a m a n radiation scattered by the interaction b e t w e e n the a d s o r b e d layers and the e v a n e s c e n t w a v e in the total reflection was collected by a c o n d e n s e r lens in the direction perpendicular to the plane of incidence of the exciting light and was led to the m o n o c h r o m a t o r . The 488.0-nm light of a Spectra Physics Model 164 Ar + laser was used for excitation with an output p o w e r of 80 roW. Since this wavelength lies within an electronic Journal of Colloid and Interface Science, Vol, 78. No. 1, November 1980
absorption doublet at 510 and 550 nm of BRBS (2), the r e s o n a n c e e n h a n c e m e n t of R a m a n intensity was achieved. The spectra were recorded on a Jasco Model R-500S R a m a n s p e c t r o p h o t o m e t e r . F o r polarized exciting light, a B a b b i n e t - S o l e i l ' s compensator and a G l a n - T h o m s o n prism were placed in the optical path of the laser b e a m , and for polarized R a m a n radiation a Polacoat filter was used. Since, b y the irradiation with the exciting light, the aqueous solution at the interface was apt to generate small air bubbles which interfered with the R a m a n m e a s u r e m e n t s , the aqueous solution was transferred to the round b o t t o m of the glass tube and thoroughly degassed in the usual m a n n e r prior to the m e a s u r e m e n t s . The R a m a n spectra were recorded keeping the aqueous solution in v a c u u m , after it was replaced on the quartz-glass prism. The lower output p o w e r of the laser b e a m (80 m W as mentioned above) was also essential to protect the interface f r o m thermal agitation. The a m o u n t of adsorption was evaluated from the band intensity of the electronic absorption doublet of BRBS r e c o r d e d by a technique of multiple total reflection at the g l a s s - s o l u t i o n interface (5). Figure 2 is a schematic diagram of the apparatus for this m e a s u r e m e n t . The quartz glass plate was 74
Quartz glass
\
t
/ J
U
Packing
Aqueous solution
FIG. 2. Apparatus of the multiple total reflection method for recording electronic absorption spectra of adsorbed layers at the interface between a quartz glass and an aqueous solution.
RAMAN SPECTRA OF ADSORBED LAYERS m m long, 20 m m wide, and 4 m m thick. The angle of incidence was 74 ° and the n u m b e r of reflections was 5. The quartz glasses were cleaned in chromic acid for several hours, and ultrasonically rinsed with redistilled w a t e r and with ethanol. The glasses were m a d e hydrophobic by immersing the cleaned ones for ca. 10 min in a 5% toluene solution of dimethyldichlorosilane, rinsing with toluene, ethanol, and redistilled w a t e r (6). Resonance R a m a n spectra of bulk aqueous solutions were obtained by using a 1-mm capillary tube.
1.0
*'
A. Adsorption of BRBS When the quartz-glass surface was not treated with dimethyldichlorosilane, and therefore hydrophilic, no bands of BRBS were detected either in the r e s o n a n c e R a m a n s p e c t r u m or in the electronic absorption spectrum. When the glass was treated to be h y d r o p h o b i c , on the other hand, the BRBS bands were found in both spectra. Thus it is apparent that the B R B S molecules are not a d s o r b e d on the hydrophilic surface, while they are a d s o r b e d on the h y d r o p h o b i c one, orienting the dodecyl groups of the BRBS molecules t o w a r d the glass and the sulfonic groups toward the aqueous phase. Therefore, all subsequent work was carried out for the h y d r o p h o b i c glass surface. The a b o v e - m e n t i o n e d facts also show that the observed bands are ascribed to the BRBS molecules a d s o r b e d at the interface and not due to those dissolved in the aqueous solution.
B. Electronic Absorption Spectra of Adsorbed BRBS Figures 3A and B r e p r e s e n t electronic absorption spectra of BRBS in the bulk aqueous solution (2 × 10 -4 M) and in solid film deposited on a glass plate, respectively.
A.Aqueous solution (2xJ°-4M)
~ ' ~
0.5
0
~
'
,
,
~
C. Adsorbed
0.5
foyer
g~ 0.2 4oo
RESULTS AND DISCUSSION
39
5oo WQvelength /
6;o
--
nm
F~G. 3. Electronic absorption spectra of BRBS (A) in the aqueous solution (2 x 10.4 M), (B) in the solid film deposited on a glass plate, and (C) in the adsorbed layer at the interface between the quartz glass and the aqueous solution (4 × 10 5 M). Figure 3C is the electronic s p e c t r u m of BRBS adsorbed at the g l a s s - s o l u t i o n interface obtained by the multiple total reflection technique. The similarity of the spectrum of the a d s o r b e d layer (Fig. 3C) to that of the solid film (Fig. 3B) suggests that the BRBS molecules in the thin layer are in an e n v i r o n m e n t s o m e w h a t similar to a crystal field. In Fig. 4 the areas of the BRBS absorption bands are plotted against the logarithm of the concentration of B RB S in the aqueous solution. The solid and b r o k e n lines show the results obtained for the aqueous solution without and with sodium chloride as an electrolyte, respectively. In the absence of sodium chloride, the curve illustrates the B E T - t y p e isotherm (7). The m o n o l a y e r adsorption was accomplished at an aqueous solution concentration of ca. 5 x 10 -5 M. Assuming the m o l a r extinction coefficient of BRBS in the m o n o l a y e r to be 3.1 x 10 r mole -a cm 2 as given by Ohnishi and Tsubomura (5), the amount of the m o n o l a y e r adsorption is ca. 1.4 x 10 -20 mole cm -2, corresponding to the occupied surface Journal of Colloid and Interface Science, Vol. 78, N o . 1, N o v e m b e r 1980
40
TAKENAKA AND YAMASAKI 1.5 With NaCI ( 10-2 M ) .~6
1.0
o
o
/
i io-~
o
i io-~
i 16 4
Concentration
of
i tot 3 BRBS /
i icT2 M
FIG. 4. Areas of the electronic absorption band of adsorbed BRBS against the logarithm of the concentration of BRBS in the aqueous solution with and without sodium chloride. area of ca. 120 A2 for each BRBS molecule. When sodium chloride was added to the aqueous solution in a concentration of 10 -2 M, the a m o u n t of adsorption•was rem a r k a b l y increased as seen in Fig. 4. This fact m a y be u n d e r s t o o d as due to the saltingout effect of the electrolyte and/or the shielding effect of cation and m a y largely affect the orientation• of the BRBS molecules in the adsorbed layers.
corded by the total reflection method shown in Fig. 1. The s p e c t r u m which r e s e m b l e d that of t h e bulk solution was obtained on the b a c k g r o u n d due to the quartz glass. Only a v e r y slight shift was o b s e r v e d in the frequency of the BRBS bands w h e n the material was transferred from the aqueous solution to the adsorbed layer.
D. Orientation of BRBS in the Adsorbed Layers F r o m polarization m e a s u r e m e n t s of the resonance R a m a n spectra, we discuss the orientation o f the BRBS molecules in the adsorbed layers. We now consider the threep h a s e plane-bounded system shown in Fig. 6. The space-fixed axes X, Y, and Z are defined as shown in Figs. 1 and 6. As was discussed in the previous papers (1,2), when the exciting light is totally reflected at the interface in the X Y plane and the R a m a n radiation is o b s e r v e d along the Z axis, there are four possible geometries of polarization m e a s u r e m e n t s as a c o n s e q u e n c e of the combination of two polarization direc-
Aqueous solution
(2XIO-4M)
A
-
C. Resonance Raman Spectra of Adsorbed BRBS Figure 5A is the r e s o n a n c e R a m a n spectrum of the bulk aqueous solution of BRBS at a concentration of 2 × 10 -4 M. The observed bands are interpreted as a mixture of the azo and quinoid type of B R B S (8). The depolarization ratios m e a s u r e d for the strong- and medium-intensity b a n d s have b e e n found to be nearly 1/3 (2). Thus it can be said that these bands are due to totally s y m m e t r i c vibrations and for each vibration only one diagonal element of the R a m a n scattering tensor is resonance e n h a n c e d (9). Figure 5B represents the resonance R a m a n
spectrum of B R B S i n t h e adsorbed layer at the interface between the quartz glass and the 2 × 10 -4 M aqueous solution reJournal of Colloid and Interface Science, Vol. 78, No. 1, November 1980
Adsorbed layer
",4
]
Background i
i
1600
i
r
~
1400
r
1200
Wovenurnber / cm-I
FIG. 5. R e s o n a n c e R a m a n s p e c t r a of BRBS (A) in the a que ous solution (2 × 10-4 M) and (B) in the ads orbe d l a y e r at the interface b e t w e e n the quartz glass and the aqueous solution (2 × 10 -4 M).
RAMAN SPECTRA OF ADSORBED LAYERS
41
Yi
the thin layer and are given b y Eqs. [5], Phase 3,~3 ................... [6]y and [7] of Ref. (2) as a function of the Aqueous t solution i refractive indexes o f the three phases nl, I n2, and n3 and the angle of incidence tO. -:i~:~i~d.,~ii~L.!.:~i~~G:;~::i x In order to express the elements O~xx, O~xv, etc. in terms of the elements ~xx, o~x~, etc. which are b a s e d on the molecule-fixed axes E2 I Glass [ Phase I , n I x, y, and z, it is necessary to a s s u m e a type I of orientation of the BRBS molecules in FIG. 6. Three-phase plane-bounded system and the the a d s o r b e d layer. If we r e p r e s e n t the total reflection of the exciting light. c h r o m o p h o r e of the B R B S molecule as a rectangle as s h o w n in Fig. 7A, the two tions of the exciting light (parallel and sulfonate groups are located at a side o f the perpendicular to the plane of incidence) rectangle and the dodecyl group at a corner and two polarization directions of the of the opposite side. The molecule-fixed R a m a n radiation (parallel to the X and Y axes are defined as shown in Fig. 7A, fixing axes). The R a m a n intensities o b s e r v e d by the z axis parallel to the transition m o m e n t these polarization m e a s u r e m e n t s are given T of the electronic absorption band. I Therefore, the a b o v e - m e n t i o n e d nonzero diagonal by the following expressions: element of the R a m a n scattering tensor [l] should be ~z~. Since the~BRBS molecules 2, [2] . . . . . . . . . . . . . to be adsorbed on the hydrophobic glass surface, orienting the dodecyl Z,,x - , xlE l + , xlE l =, [3[ group toward the glass and the sulfonic z,l =+ =. [4] groups toward the aqueous p h a s e (Fig. 7A) as discussed above, it is reasonable to asH e r e , the subscripts ]1 and _L refer to the polarization of the exciting light and X and Y 1The electronic absorption bands of BRBS have not been fully ~studied yet; however, the doublet to that of the R a m a n radiation. ~xx, ~ x v , band at 510 and 550 nm may be assigned to the etc. are the elements of the R a m a n scatterlowest rr-Tr* transition with the transition moment ing tensor b a s e d on the space-fixed axes. parallel to the long axis of the chromophore from E x , Ev, and E z are the r e s p e c t i v e comthe analogy with the results on the corresponding ponents of the electric field amplitudes in band of trans-azobenzene (10, 11). I
-
-
Aqueous
Y ~
solution
5°2 so-, '3so °a - so3 - s% )~3i
T
z
<:522
---Z_/--
Aqueous
solot,oo
f Z
//__.
Glass
FIG. 7. Model of the molecular orientation of BRBS adsorbed at the interface between the quartz glass and the aqueous solution. T is the transition moment of the electronic absorption band. Journal of Colloid and Interface Science,
Vol.78, No. 1, November1980
42
TAKENAKA AND YAMASAKI 1.2 I.LX 1.0
Izy
-
\
Izx
•~ 0.6
I~¥
\~,.
0
I
I
io-6
16~
Without NoCl [
~o-4
:. I
io-3
0° - < 0 < 0 o -
8<- 0 < - 0 o + 6,
8,
0o+ 6<0-<90
sume that the z axis of the c h r o m o p h o r e is Uniaxially oriented with r e s p e c t to the Y axis with the angle O, the x and y axes freely rotating around the z axis (Fig. 7B). U n d e r this assumption, axx 2, axy e, etc. are given by (1) O~2Z,
[5]
c~xY = ' ~ x
= c~y = ½(½B
aZxx = 3a~x = 3/8(A - ~ B
VsC)c~z,
+ 1/sC)a~,
[9] [10]
A = cos (Oo - 8) - cos (0o + 6),
[11]
B =cos 3(Oo- 8)-cos
3(Oo + 8),
[12]
cos 5(O0 + 8).
[13]
C =cos 5(0o-
8)-
Substituting Eqs. [8]-[10] into Eqs. [1]-[4], we have the R a m a n intensity ratios
1 3(15A - 10B + 3 C ) [ E x l 2 + 4(5B - 3 C ) ] E v l z
I, ky
4
Ij_x
1 15A - 10B + 3C
Ilr
4
-
-
where
L~c
(5B
°.
[8]
a ~ y = 1/sCc~z,
FIG. 8. Two Raman intensity ratios for the 1600-cm band against the logarithm of the concentration of BRBS in the aqueous solution with and without sodium chloride.
COS40
0o-
Multiplying Eqs. [5]-[7] by F(O)sinO and integrating the products about 0, we have
[
io-Z
Concentration of BRBs / M
O~2y =
[7]
F(O) = 0
J'JlY
(io -2 M)
0.2
a2xx = 3a~x = 3/~ sin40 a~z.
F(O) = 1
I,_xx
With NaCI
[6]
Further, we m a k e an assumption about 0 that the z axes are uniformly distributed b e t w e e n 0 = 00 - 6 and 0 = Oo + & Thus the distribution function can be defined as
I±v
o 0.8
g O.4
a2xr = a],x = a~v = 1/2 sin20 cos20 a~z,
3C)]ExL + 6CIE I
[14] [15]
5B - 3C
In the present case of the three-phase syst e m shown in Fig. 6, using p r o p e r values, nl = 1.46, na = 1.34, and tO = 84 ° , and making the convenient assumption that n~ = 1.43, f r o m Eqs. [5], [6], and [7] of Ref. (2) we obtain IExI2/IEr] z = 0.193.
[16]
Therefore, it is possible to discuss the orientation of the BRBS molecules in the adsorbed layers f r o m the m e a s u r e m e n t s of the two R a m a n intensity ratios. In Fig. 8, the two R a m a n intensity ratios JburnatofCoIIoid and'lnierfaCe Science,~Vol. 78, No. 1, November 1980
m e a s u r e d for the 1600-cm -a b a n d were plotted against the logarithm of the concentration of BRBS in the aqueous solution. The solid and b r o k e n lines r e p r e s e n t the results obtained in the absence and p r e s e n c e of sodium chloride, respectively. Similar results were also obtained for the 1420-cm -1 band. It is seen from Fig. 8 that the two R a m a n intensity ratios d e c r e a s e with increasing concentration of BRBS. Substitution of these values into Eqs. [14] and [15] leads to a conclusion that 8 -~ 35 ° and that t h e value of O0 decreases f r o m 53 to 46 °
RAMAN SPECTRA OF ADSORBED LAYERS
with increasing the concentration from 2 x 10-5 to 1 x 10- z M in the absence of sodium chloride. In the presence of sodium chloride, on the other hand, t% decreases from 53 to 39° with increasing the concentration from 1 × 10-6 to 2 x 10-5 M. This may be stated in a qualitative way as follows. The adsorbed BRBS molecules improve the orientation of the chromophores with increasing the concentration of BRBS and consequently with increasing the number of adsorbed BRBS molecules. This tendency becomes remarkable when the electrolyte is added to the aqueous solution. Since, as shown in Fig. 4, the number of the adsorbed molecules is largely increased by the addition of the electrolyte, the tendency of the molecular orientation can be ascribed to an increase of an interaction among the adsorbed molecules at the interface. Comparison of Figs. 4 and 8 suggests that the orientation of the BRBS molecules is improved with an increase of the amount of adsorption until monolayer adsorption is completed, while it remains unchanged after multilayer adsorption starts. From comparison of the t90 values between the present case of the adsorbed layers of BRBS at the glass-water interface and the previous case of the adsorbed monolayers at the oil-water interface (2, 3), it can be said that the degree of molecular orientation is more or less the same in both cases, although the types ofuniaxial orientation are somewhat different in each: In the case of spread monolayers at the a i r - w a t e r interface (4), on the other hand, it was f o u n d that the O0 value decreased from 45 to 32° with decreasing the surface area,
43
without the distribution of the angle tg, i.e., = 0°. Therefore, it may be concluded that the degree of molecular orientation is much lower in the adsorbed layers as compared with that in the spread layers. It is apparent from the present study that the total reflection method for resonance Raman measurements is a useful technique for studying the molecular orientation in thin layers not only at the liquid-liquid and gasliquid interfaces but also at the liquid-solid interface. Biological membranes and related systems at these interfaces may also be worthy subjects for this technique. ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan, to which the authors' thanks are due.
REFERENCES 1. Takenaka, T., and Nakanaga, T., J. Phys. Chem. 80, 475 (1976). 2. Nakanaga, T., and Takenaka, T., J. Phys. Chem. 81, 645 (1977). 3. Takenaka, T., Chem. Phys. Lett. 55, 515 (1978). 4. Takenaka, T., and Fukuzaki, H., J. Raman Spectrosc. 8, 151 (1979). 5. Ohnishi, T., and Tsubomura, H., Chem. Phys. Lett. 41, 77 (1976). 6. Honig, E. P., Hengst, J. H. Th., and Engelsen, D. den., J. Colloid Interface Sci. 45, 92 (1973). 7. Brunauer, S., Emmett, P. H., and Teller, E., J. Amer. Chem. Soc. 60, 309 (1938). 8. Machida, K., Kim, B. K., Saito, Y., Igarashi, K., and Uno, T., Bull. Chem. Soc. Japan 47, 78 (1974). 9. Mortensen, O. S., Chem. Phys. Lett. 3, 4 (1969). I0. Jaffr, H. H., Yeh, S. J., and Gardner, R. W., J. Mol. Spectrosc. 2, 120 (1958). 11. Beveridge, D. L., and Jaffr, H. H., J. Amer. Chem. Soc. 88, 1948 (1966).
Journal of Colloidand Interface Science, Vol. 78. No. 1, November1980
was estimated from the band intensity of an electronic absorption spectra recorded by a technique of multiple total reflection at the interface (5). Effects of an electrolyte on the adsorption were also studied.
In a previous paper (1), we have proposed a method of total reflection for resonance Raman measurements of thin films at liquid interfaces and have studied the molecular orientation in monolayers adsorbed or spread at the liquid-liquid and gas-liquid interfaces (1-4). In the present work, the same technique has been applied to the study of thin layers of a surface-active anionic azo dye, Suminol Milling Brilliant Red BS (abbreviated as BRBS)
EXPERIMENTAL
CHs(CH2)II It
0
S05Na
N-C-CH s
S03Na
adsorbed at the interface between a quartz glass and an aqueous solution. From polarization measurements of resonance Raman spectra, the orientation of the BRBS molecules in the adsorbed layers was evaluated and the results were discussed in connection with the amount of adsorption which
The sample of BRBS was the same as that reported previously (2). Pure water was prepared by redistillation of distilled water which had been passed through an ion-exchange resin column. When necessary, a guaranteed reagent of sodium chloride was dissolved in a concentration of 1 x 10-2 M into the aqueous solution of BRBS. Figure 1 represents the apparatus of the total reflection method for recording resonance Raman spectra of adsorbed layers at the interface between the quartz glass and an aqueous solution. The apparatus consists of a truncated pyramidal prism made of quartz glass (Nippon Sekiei Glass Co., Type SG) and a branched glass tube equipped with a stopcock and ball joint for attachment to the vacuum manifold. The dimensions of the prism are 23 mm (at the lower
37
Journal of Colloidand Interface Science, Vol. 78, No. l, November 1980
0021-9797/80/110037-07502.00/0 Copyright © 1980by AcademicPress, Inc. All rights of reproduction in any form reserved.
38
TAKENAKA AND YAMASAKI
Roman
Slit
Polocoot
Glon-Thomso prism
Lens
Z
~
I
.,~,J
scattering
LI~---, ~ "
Loser
~ X
/
gloss
beom Aqueous solution
\
F ~ " Quor tz gloss
FIG. 1. Apparatus of the total reflection method for resonance Raman measurements of adsorbed layers at the interface between a quartz glass and an aqueous solution. side) and 38 m m (at the u p p e r side) in length and width, and 21 m m in height. The flat end of the b r a n c h of the glass tube was tightly pressed upon the optical plane of the quartz-glass prism through a silicone rubber O-ring. The aqueous solution of BRBS was placed in the tube being in contact with the prism. A horizontally propagating laser b e a m was incident upon a side face of the prism. After the refraction at the side face, the laser b e a m c a m e to the interface b e t w e e n the quartz glass and the aqueous solution with the angle of incidence 84 ° , and followed by the total reflection at the interface. The R a m a n radiation scattered by the interaction b e t w e e n the a d s o r b e d layers and the e v a n e s c e n t w a v e in the total reflection was collected by a c o n d e n s e r lens in the direction perpendicular to the plane of incidence of the exciting light and was led to the m o n o c h r o m a t o r . The 488.0-nm light of a Spectra Physics Model 164 Ar + laser was used for excitation with an output p o w e r of 80 roW. Since this wavelength lies within an electronic Journal of Colloid and Interface Science, Vol, 78. No. 1, November 1980
absorption doublet at 510 and 550 nm of BRBS (2), the r e s o n a n c e e n h a n c e m e n t of R a m a n intensity was achieved. The spectra were recorded on a Jasco Model R-500S R a m a n s p e c t r o p h o t o m e t e r . F o r polarized exciting light, a B a b b i n e t - S o l e i l ' s compensator and a G l a n - T h o m s o n prism were placed in the optical path of the laser b e a m , and for polarized R a m a n radiation a Polacoat filter was used. Since, b y the irradiation with the exciting light, the aqueous solution at the interface was apt to generate small air bubbles which interfered with the R a m a n m e a s u r e m e n t s , the aqueous solution was transferred to the round b o t t o m of the glass tube and thoroughly degassed in the usual m a n n e r prior to the m e a s u r e m e n t s . The R a m a n spectra were recorded keeping the aqueous solution in v a c u u m , after it was replaced on the quartz-glass prism. The lower output p o w e r of the laser b e a m (80 m W as mentioned above) was also essential to protect the interface f r o m thermal agitation. The a m o u n t of adsorption was evaluated from the band intensity of the electronic absorption doublet of BRBS r e c o r d e d by a technique of multiple total reflection at the g l a s s - s o l u t i o n interface (5). Figure 2 is a schematic diagram of the apparatus for this m e a s u r e m e n t . The quartz glass plate was 74
Quartz glass
\
t
/ J
U
Packing
Aqueous solution
FIG. 2. Apparatus of the multiple total reflection method for recording electronic absorption spectra of adsorbed layers at the interface between a quartz glass and an aqueous solution.
RAMAN SPECTRA OF ADSORBED LAYERS m m long, 20 m m wide, and 4 m m thick. The angle of incidence was 74 ° and the n u m b e r of reflections was 5. The quartz glasses were cleaned in chromic acid for several hours, and ultrasonically rinsed with redistilled w a t e r and with ethanol. The glasses were m a d e hydrophobic by immersing the cleaned ones for ca. 10 min in a 5% toluene solution of dimethyldichlorosilane, rinsing with toluene, ethanol, and redistilled w a t e r (6). Resonance R a m a n spectra of bulk aqueous solutions were obtained by using a 1-mm capillary tube.
1.0
*'
A. Adsorption of BRBS When the quartz-glass surface was not treated with dimethyldichlorosilane, and therefore hydrophilic, no bands of BRBS were detected either in the r e s o n a n c e R a m a n s p e c t r u m or in the electronic absorption spectrum. When the glass was treated to be h y d r o p h o b i c , on the other hand, the BRBS bands were found in both spectra. Thus it is apparent that the B R B S molecules are not a d s o r b e d on the hydrophilic surface, while they are a d s o r b e d on the h y d r o p h o b i c one, orienting the dodecyl groups of the BRBS molecules t o w a r d the glass and the sulfonic groups toward the aqueous phase. Therefore, all subsequent work was carried out for the h y d r o p h o b i c glass surface. The a b o v e - m e n t i o n e d facts also show that the observed bands are ascribed to the BRBS molecules a d s o r b e d at the interface and not due to those dissolved in the aqueous solution.
B. Electronic Absorption Spectra of Adsorbed BRBS Figures 3A and B r e p r e s e n t electronic absorption spectra of BRBS in the bulk aqueous solution (2 × 10 -4 M) and in solid film deposited on a glass plate, respectively.
A.Aqueous solution (2xJ°-4M)
~ ' ~
0.5
0
~
'
,
,
~
C. Adsorbed
0.5
foyer
g~ 0.2 4oo
RESULTS AND DISCUSSION
39
5oo WQvelength /
6;o
--
nm
F~G. 3. Electronic absorption spectra of BRBS (A) in the aqueous solution (2 x 10.4 M), (B) in the solid film deposited on a glass plate, and (C) in the adsorbed layer at the interface between the quartz glass and the aqueous solution (4 × 10 5 M). Figure 3C is the electronic s p e c t r u m of BRBS adsorbed at the g l a s s - s o l u t i o n interface obtained by the multiple total reflection technique. The similarity of the spectrum of the a d s o r b e d layer (Fig. 3C) to that of the solid film (Fig. 3B) suggests that the BRBS molecules in the thin layer are in an e n v i r o n m e n t s o m e w h a t similar to a crystal field. In Fig. 4 the areas of the BRBS absorption bands are plotted against the logarithm of the concentration of B RB S in the aqueous solution. The solid and b r o k e n lines show the results obtained for the aqueous solution without and with sodium chloride as an electrolyte, respectively. In the absence of sodium chloride, the curve illustrates the B E T - t y p e isotherm (7). The m o n o l a y e r adsorption was accomplished at an aqueous solution concentration of ca. 5 x 10 -5 M. Assuming the m o l a r extinction coefficient of BRBS in the m o n o l a y e r to be 3.1 x 10 r mole -a cm 2 as given by Ohnishi and Tsubomura (5), the amount of the m o n o l a y e r adsorption is ca. 1.4 x 10 -20 mole cm -2, corresponding to the occupied surface Journal of Colloid and Interface Science, Vol. 78, N o . 1, N o v e m b e r 1980
40
TAKENAKA AND YAMASAKI 1.5 With NaCI ( 10-2 M ) .~6
1.0
o
o
/
i io-~
o
i io-~
i 16 4
Concentration
of
i tot 3 BRBS /
i icT2 M
FIG. 4. Areas of the electronic absorption band of adsorbed BRBS against the logarithm of the concentration of BRBS in the aqueous solution with and without sodium chloride. area of ca. 120 A2 for each BRBS molecule. When sodium chloride was added to the aqueous solution in a concentration of 10 -2 M, the a m o u n t of adsorption•was rem a r k a b l y increased as seen in Fig. 4. This fact m a y be u n d e r s t o o d as due to the saltingout effect of the electrolyte and/or the shielding effect of cation and m a y largely affect the orientation• of the BRBS molecules in the adsorbed layers.
corded by the total reflection method shown in Fig. 1. The s p e c t r u m which r e s e m b l e d that of t h e bulk solution was obtained on the b a c k g r o u n d due to the quartz glass. Only a v e r y slight shift was o b s e r v e d in the frequency of the BRBS bands w h e n the material was transferred from the aqueous solution to the adsorbed layer.
D. Orientation of BRBS in the Adsorbed Layers F r o m polarization m e a s u r e m e n t s of the resonance R a m a n spectra, we discuss the orientation o f the BRBS molecules in the adsorbed layers. We now consider the threep h a s e plane-bounded system shown in Fig. 6. The space-fixed axes X, Y, and Z are defined as shown in Figs. 1 and 6. As was discussed in the previous papers (1,2), when the exciting light is totally reflected at the interface in the X Y plane and the R a m a n radiation is o b s e r v e d along the Z axis, there are four possible geometries of polarization m e a s u r e m e n t s as a c o n s e q u e n c e of the combination of two polarization direc-
Aqueous solution
(2XIO-4M)
A
-
C. Resonance Raman Spectra of Adsorbed BRBS Figure 5A is the r e s o n a n c e R a m a n spectrum of the bulk aqueous solution of BRBS at a concentration of 2 × 10 -4 M. The observed bands are interpreted as a mixture of the azo and quinoid type of B R B S (8). The depolarization ratios m e a s u r e d for the strong- and medium-intensity b a n d s have b e e n found to be nearly 1/3 (2). Thus it can be said that these bands are due to totally s y m m e t r i c vibrations and for each vibration only one diagonal element of the R a m a n scattering tensor is resonance e n h a n c e d (9). Figure 5B represents the resonance R a m a n
spectrum of B R B S i n t h e adsorbed layer at the interface between the quartz glass and the 2 × 10 -4 M aqueous solution reJournal of Colloid and Interface Science, Vol. 78, No. 1, November 1980
Adsorbed layer
",4
]
Background i
i
1600
i
r
~
1400
r
1200
Wovenurnber / cm-I
FIG. 5. R e s o n a n c e R a m a n s p e c t r a of BRBS (A) in the a que ous solution (2 × 10-4 M) and (B) in the ads orbe d l a y e r at the interface b e t w e e n the quartz glass and the aqueous solution (2 × 10 -4 M).
RAMAN SPECTRA OF ADSORBED LAYERS
41
Yi
the thin layer and are given b y Eqs. [5], Phase 3,~3 ................... [6]y and [7] of Ref. (2) as a function of the Aqueous t solution i refractive indexes o f the three phases nl, I n2, and n3 and the angle of incidence tO. -:i~:~i~d.,~ii~L.!.:~i~~G:;~::i x In order to express the elements O~xx, O~xv, etc. in terms of the elements ~xx, o~x~, etc. which are b a s e d on the molecule-fixed axes E2 I Glass [ Phase I , n I x, y, and z, it is necessary to a s s u m e a type I of orientation of the BRBS molecules in FIG. 6. Three-phase plane-bounded system and the the a d s o r b e d layer. If we r e p r e s e n t the total reflection of the exciting light. c h r o m o p h o r e of the B R B S molecule as a rectangle as s h o w n in Fig. 7A, the two tions of the exciting light (parallel and sulfonate groups are located at a side o f the perpendicular to the plane of incidence) rectangle and the dodecyl group at a corner and two polarization directions of the of the opposite side. The molecule-fixed R a m a n radiation (parallel to the X and Y axes are defined as shown in Fig. 7A, fixing axes). The R a m a n intensities o b s e r v e d by the z axis parallel to the transition m o m e n t these polarization m e a s u r e m e n t s are given T of the electronic absorption band. I Therefore, the a b o v e - m e n t i o n e d nonzero diagonal by the following expressions: element of the R a m a n scattering tensor [l] should be ~z~. Since the~BRBS molecules 2, [2] . . . . . . . . . . . . . to be adsorbed on the hydrophobic glass surface, orienting the dodecyl Z,,x - , xlE l + , xlE l =, [3[ group toward the glass and the sulfonic z,l =+ =. [4] groups toward the aqueous p h a s e (Fig. 7A) as discussed above, it is reasonable to asH e r e , the subscripts ]1 and _L refer to the polarization of the exciting light and X and Y 1The electronic absorption bands of BRBS have not been fully ~studied yet; however, the doublet to that of the R a m a n radiation. ~xx, ~ x v , band at 510 and 550 nm may be assigned to the etc. are the elements of the R a m a n scatterlowest rr-Tr* transition with the transition moment ing tensor b a s e d on the space-fixed axes. parallel to the long axis of the chromophore from E x , Ev, and E z are the r e s p e c t i v e comthe analogy with the results on the corresponding ponents of the electric field amplitudes in band of trans-azobenzene (10, 11). I
-
-
Aqueous
Y ~
solution
5°2 so-, '3so °a - so3 - s% )~3i
T
z
<:522
---Z_/--
Aqueous
solot,oo
f Z
//__.
Glass
FIG. 7. Model of the molecular orientation of BRBS adsorbed at the interface between the quartz glass and the aqueous solution. T is the transition moment of the electronic absorption band. Journal of Colloid and Interface Science,
Vol.78, No. 1, November1980
42
TAKENAKA AND YAMASAKI 1.2 I.LX 1.0
Izy
-
\
Izx
•~ 0.6
I~¥
\~,.
0
I
I
io-6
16~
Without NoCl [
~o-4
:. I
io-3
0° - < 0 < 0 o -
8<- 0 < - 0 o + 6,
8,
0o+ 6<0-<90
sume that the z axis of the c h r o m o p h o r e is Uniaxially oriented with r e s p e c t to the Y axis with the angle O, the x and y axes freely rotating around the z axis (Fig. 7B). U n d e r this assumption, axx 2, axy e, etc. are given by (1) O~2Z,
[5]
c~xY = ' ~ x
= c~y = ½(½B
aZxx = 3a~x = 3/8(A - ~ B
VsC)c~z,
+ 1/sC)a~,
[9] [10]
A = cos (Oo - 8) - cos (0o + 6),
[11]
B =cos 3(Oo- 8)-cos
3(Oo + 8),
[12]
cos 5(O0 + 8).
[13]
C =cos 5(0o-
8)-
Substituting Eqs. [8]-[10] into Eqs. [1]-[4], we have the R a m a n intensity ratios
1 3(15A - 10B + 3 C ) [ E x l 2 + 4(5B - 3 C ) ] E v l z
I, ky
4
Ij_x
1 15A - 10B + 3C
Ilr
4
-
-
where
L~c
(5B
°.
[8]
a ~ y = 1/sCc~z,
FIG. 8. Two Raman intensity ratios for the 1600-cm band against the logarithm of the concentration of BRBS in the aqueous solution with and without sodium chloride.
COS40
0o-
Multiplying Eqs. [5]-[7] by F(O)sinO and integrating the products about 0, we have
[
io-Z
Concentration of BRBs / M
O~2y =
[7]
F(O) = 0
J'JlY
(io -2 M)
0.2
a2xx = 3a~x = 3/~ sin40 a~z.
F(O) = 1
I,_xx
With NaCI
[6]
Further, we m a k e an assumption about 0 that the z axes are uniformly distributed b e t w e e n 0 = 00 - 6 and 0 = Oo + & Thus the distribution function can be defined as
I±v
o 0.8
g O.4
a2xr = a],x = a~v = 1/2 sin20 cos20 a~z,
3C)]ExL + 6CIE I
[14] [15]
5B - 3C
In the present case of the three-phase syst e m shown in Fig. 6, using p r o p e r values, nl = 1.46, na = 1.34, and tO = 84 ° , and making the convenient assumption that n~ = 1.43, f r o m Eqs. [5], [6], and [7] of Ref. (2) we obtain IExI2/IEr] z = 0.193.
[16]
Therefore, it is possible to discuss the orientation of the BRBS molecules in the adsorbed layers f r o m the m e a s u r e m e n t s of the two R a m a n intensity ratios. In Fig. 8, the two R a m a n intensity ratios JburnatofCoIIoid and'lnierfaCe Science,~Vol. 78, No. 1, November 1980
m e a s u r e d for the 1600-cm -a b a n d were plotted against the logarithm of the concentration of BRBS in the aqueous solution. The solid and b r o k e n lines r e p r e s e n t the results obtained in the absence and p r e s e n c e of sodium chloride, respectively. Similar results were also obtained for the 1420-cm -1 band. It is seen from Fig. 8 that the two R a m a n intensity ratios d e c r e a s e with increasing concentration of BRBS. Substitution of these values into Eqs. [14] and [15] leads to a conclusion that 8 -~ 35 ° and that t h e value of O0 decreases f r o m 53 to 46 °
RAMAN SPECTRA OF ADSORBED LAYERS
with increasing the concentration from 2 x 10-5 to 1 x 10- z M in the absence of sodium chloride. In the presence of sodium chloride, on the other hand, t% decreases from 53 to 39° with increasing the concentration from 1 × 10-6 to 2 x 10-5 M. This may be stated in a qualitative way as follows. The adsorbed BRBS molecules improve the orientation of the chromophores with increasing the concentration of BRBS and consequently with increasing the number of adsorbed BRBS molecules. This tendency becomes remarkable when the electrolyte is added to the aqueous solution. Since, as shown in Fig. 4, the number of the adsorbed molecules is largely increased by the addition of the electrolyte, the tendency of the molecular orientation can be ascribed to an increase of an interaction among the adsorbed molecules at the interface. Comparison of Figs. 4 and 8 suggests that the orientation of the BRBS molecules is improved with an increase of the amount of adsorption until monolayer adsorption is completed, while it remains unchanged after multilayer adsorption starts. From comparison of the t90 values between the present case of the adsorbed layers of BRBS at the glass-water interface and the previous case of the adsorbed monolayers at the oil-water interface (2, 3), it can be said that the degree of molecular orientation is more or less the same in both cases, although the types ofuniaxial orientation are somewhat different in each: In the case of spread monolayers at the a i r - w a t e r interface (4), on the other hand, it was f o u n d that the O0 value decreased from 45 to 32° with decreasing the surface area,
43
without the distribution of the angle tg, i.e., = 0°. Therefore, it may be concluded that the degree of molecular orientation is much lower in the adsorbed layers as compared with that in the spread layers. It is apparent from the present study that the total reflection method for resonance Raman measurements is a useful technique for studying the molecular orientation in thin layers not only at the liquid-liquid and gasliquid interfaces but also at the liquid-solid interface. Biological membranes and related systems at these interfaces may also be worthy subjects for this technique. ACKNOWLEDGMENT This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Japan, to which the authors' thanks are due.
REFERENCES 1. Takenaka, T., and Nakanaga, T., J. Phys. Chem. 80, 475 (1976). 2. Nakanaga, T., and Takenaka, T., J. Phys. Chem. 81, 645 (1977). 3. Takenaka, T., Chem. Phys. Lett. 55, 515 (1978). 4. Takenaka, T., and Fukuzaki, H., J. Raman Spectrosc. 8, 151 (1979). 5. Ohnishi, T., and Tsubomura, H., Chem. Phys. Lett. 41, 77 (1976). 6. Honig, E. P., Hengst, J. H. Th., and Engelsen, D. den., J. Colloid Interface Sci. 45, 92 (1973). 7. Brunauer, S., Emmett, P. H., and Teller, E., J. Amer. Chem. Soc. 60, 309 (1938). 8. Machida, K., Kim, B. K., Saito, Y., Igarashi, K., and Uno, T., Bull. Chem. Soc. Japan 47, 78 (1974). 9. Mortensen, O. S., Chem. Phys. Lett. 3, 4 (1969). I0. Jaffr, H. H., Yeh, S. J., and Gardner, R. W., J. Mol. Spectrosc. 2, 120 (1958). 11. Beveridge, D. L., and Jaffr, H. H., J. Amer. Chem. Soc. 88, 1948 (1966).
Journal of Colloidand Interface Science, Vol. 78. No. 1, November1980