Langnuir 1993,9, 3587-3593
3587
Adsorption of a Soluble Dye Polymer onto Spread Monolayers Kazuya Asano,+Kenjiro Miyano,' and Hiroki Ui Department of Applied Physics, Faculty of Engineering, University of Tokyo, Bunkyo-ku, Tokyo 113, Japan
Masatsugu Shimomura Research Institute for Electronic Science, Hokkaido University, Sapporo 060, Japan
Yoshihisa Ohta Tsukuba Research Laboratory, Japan Synthetic Rubber Co.,Ltd., 25 Miyukigaoka, Tsukuba 305, Japan Received May 10,1993. In Final Form: September 2, 1 9 9 9 We have studied in situ the adsorption of a soluble anionic dye polymer onto spread monolayers by means of reflection spectroscopy. We found that the adsorption is governed by the charge neutrality; the ratio of the adsorbed polymer unit and the cationic monolayer molecule is nearly 1:l for the surface density in the range of 50-77 &/monolayer molecule. On the other hand, no adsorption occurs onto an anionic monolayer. We have also investigated the adsorption kinetics of the polymer. The rate of adsorption can be reproduced very well with a simple diffusion simulation.
Introduction Adsorption of polyelectrolytes onto surfaces has been an attractive problem in many fields.'d The central issue in these studies is what form polymers take in the adsorbed state. For example, are they adsorbed flat or like a random coil? In other words, which is more important in the adsorption of polyelectrolytes, Coulombic energy or entropy? A particularly interesting surface to study in this respect is a monolayer, where a molecularly smooth and homogeneous boundary is realized with a variety of charge densities, signsof the charge, or the type of the counterions. It has a practical value as well. In the fabrication of LB (Langmuir-Blodgett) films,it has been known that some monolayers, which are difficult to deposit due to mechanical weakness, become easily transferable to a substrate, when they are backed with a polymer, in which an anionic polymer dissolved in the water is adsorbed electrostatically onto a cationic monolayer which is spread on the water s u r f a ~ e The . ~ ~conformation ~ of the polymer in the adsorbed state in LB films was studied with XPS (X-ray photoelectron spectro~copy)~*~ and X-ray diffraction.6 It was found that the number of anions of the polymer was equal to that of cations of the monolayer and that the thickness of the layer of the adsorbed polymer corresponded to that of the monomolecular layer. From these results, a conclusion was drawn that the polymer + Current address: Electronics Research Center, NKK Corp., Ayase, Kanagawa-ken 252, Japan. 0
Abstract published in Advance ACS Abstracts, November 1,
1993. (1)Coegrove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986,111,409. ( 2 ) Kawaguchi, M.; Hayaahi, K.; Takahashi, A. Macromolecules 1988, 21, 1016. (3) Papenhuijzen, J.; Van Der Schee, H. A.; Fleer, G.J. J. Colloid Interface Sci. 1986,104, 540. (4) Shimomura, M.; Kunitake, T. Thin Solid F i l m 1985, 132, 243. (5) Higaahi, N.; Kunitake, T. Chem. Lett. 1986; 106. (6) Takahara, A.; Morotomi, N.; Hiraoka, S.; Higashi, N.; Kunitake, T.; Kajiyama, T. Macromolecules 1989,22,617.
was adsorbed trainlike in the LB film. But few in situ measurements have been made on monolayers on the water surface. We have previously reported on the adsorption kinetics of a water-soluble polymer onto a spread monolayer.' The kinetics was measured in situ by means of the ATR (attenuated total reflection) method. We found that the polymer was adsorbed monomolecularly in the initial stage of the adsorption. In the second stage, however, the adsorbed layer increased to several times that of the monolayer and it was heterogeneous. But this behavior in the second stage may have resulted from the disturbance to the monolayer by the measuring technique. In the ATR method, it is unavoidable to contact the monolayer with a silver film evaporated on a prism. Adverse effects of mechanical or electrochemical origins may have influenced the monolayer and the polymer adsorption. In this study, therefore, we chose the reflection spectroscopy from a dyecontaining polymer, which enables us to measure in situ the polymer adsorption without disturbing the monolayer on water. The reflection spectroscopy is a powerful method to observe the surface phenomena.@ It is very sensitive to the state only near the surface but insensitive to the properties of the bulk phase. The measurements of the reflectivity and the reflection spectra from the dye adsorbed monolayer can provide information about both the adsorbed amount and the adsorbed state of the polymer.
Experiments 1. Samples. The solubledye polymer used in this experiment is shown in Figure 1. It was purchased from Aldrich and was used without furtherpurification. Ita molecularweight was about 80 OOO, which was determined by the light scatteringmeasurement (7) Miyano, K.; hano, K.; Shimomura, M. Langmuir 1991, 7, 444. (8) Omt,M.; Mbbius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986,85,4966. (9) Gruiger, H.; Mbbius, D.; Meyer, H. J . Chem.Phys. 1983,79,3701.
0 1993 American Chemical Society 0743-7463/93/2409-3587~04.0010
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3588 Langmuir, Vol. 9, No. 12, 1993 ~ F H - C H Z ~
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Figure 1. Water soluble polymer with a chromophore. The direction of the dipole moment suggested by the theoretical calculation is indicated with a dotted line. Sample 1 (cation, two chains)
Sample 2 (cation, four chains)
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enough. The reservoir also served for the water level control, since the distance between the monolayer and the optical fibers (about 10 mm) must be held constant. The level sensor was a piece of Mylar sheet, one end of which was held at a fixed height, forming a cantilever (not shown in Figure 3). The free end lay on the surface of water. A laser light reflected from aluminum foil placed on the Mylar sheet was monitored with a divided PD (photodiode). The variation of the water height caused the deflection of the cantilever, which in turn moved the laser light off the PD. The height of a partially immersed Teflon block was adjusted to keep the reflected light in the middle of the PD. In this manner, the water level was kept constant within f10 pm. The trough was equipped with a movable barrier and a Langmuir-type pressure sensor'. The barrier movement control and the readout of the pressure sensor were performed with a personal computer.
Sample 3 (anion,two chains)
(DLS-700),Otsuka Electronics)in water, through the Zi" plot.l0 In a 0.1 M NaBr solution,however,the same measurement yielded a value of about 630 OOO, which was, due to aggregation, much larger than the molecular weight in the water without salt. The monolayer-forming materials are shown in Figure 2. Samples 1and 3 were purchased from Sogo Chemicals. Sample 2 was synthesizedll in Kunitake Laboratory at Kyushu University. Samples 1 and 2 were chosen to provide cationic monolayers with different charge densities; the quaternary ammonium head group is known to be ionized in a wide pH range. Sample 3 was for an anionic monolayer in which, other than the charge difference, the monolayer properties are expected to be similar to those of sample 1. They are soluble neither in pure water nor in polymer solution. 2. Experimental System. A schematic diagram of the experimental setup is shown in Figure 3. It consisted of a Langmuir trough and a reservoir which were connected with two Tygon tubes: one of the tubes was a part of a microtube pump. The pump was used when it was needed to exchange the subphase in the trough with the polymer solution in the reservoir without disturbing the monolayer. The reflection spectrum from 350 to 750 nm was measured with an OMA (optical multichannel analyzer, Princeton Applied Research). The light source was a 50-W halogen lamp. Quartz optical fibers were used between the light source and the trough and between the trough and the OMA. The incident side was a bundle of 300 fibers of 100 pm diameter and the reflected side was a single fiber of 400 pm diameter (Fujikura). The exact dimension of the fibers is unimportant since the intensity of the reflected light is strong
Results and Discussion 1. *-ACurves. The P A curves of samples 1 and 2 are plotted in Figure 4. The temperature was 22-24 "C. The curves a were taken on pure water, and curves b on polymer solution (polymer concentration 106 M). Notice that sample 1 expands on polymer solution, while sample 2 contracts. In fact, in the pressure range of 5-30 mN/m, the T-A curves of the two are very close to each other. This implies that the polymer adsorption is strong enough to dominate the mechanical properties of the monolayer and that the average area occupied by an adsorbed polymer unit is between the corresponding molecular areas of samples 1 and 2. 2. Absorption Spectrum. The absorption spectrum of a water solution of the dye polymer is shown in Figure 5. The absorption peak is at 475 nm, and its molar absorption coefficient is 1.25 X 104 (L/mol)/cm. From the area of the absorption peak, the oscillator strength is estimated to be about 1. We performed a CNDO/S calculation12on a model dye molecule (2-hydroxynaphthalene-l-azobenzene) and found that the peak at 475 nm is attributed to a ~ - a transition * of azo group and a concomitant charge transfer transition from the s orbital of benzene ring t o the s* orbital of 2-hydroxynaphthalene. Its transition moment was calculated to be 7.8 D and oscillator strength 0.83, in reasonable agreement with the observation above. The values for the transition moment and the oscillator strength will be used for estimating the dipole-dipole interaction energy when the chromophores are oriented parallel to each other (cf. Figure 7).
(10) Berne, B. J.; Pecora, R. Dynamic Light Scattering; John Wiley & Sons, Inc.: New York, 1976. (11) Kimizuka, N.; Ohira, H.; Tanaka, M.; Kunitake, T. Chem. Lett. 1990, 29.
(12) del Bene, D.; Jaffe,H. H.J. Chem.Phys. 1968,48,1807. Quantum Chemistry Program Exchange (QCPE) No. 174. The geometry of the dye molecule was optimized under the restriction to be planar by means of MOPAC Version 6 (Stewart, J. J. P. QCPE Bull. 1989, 9, 10).
Figure 2. Monolayer forming amphiphiles.
Langmuir, Vol. 9,No. 12,1993 3589 1
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area / molecule (A' area / molecule (A ' Figure 4. P A curves: (1) sample 1; (2) sample 2. Line a is measured on pure water, b on polymer solution (polymer concentration
= 1W M).
1
Table I. Reflectivity Measurements at 485 nm and the Chromophore Tilt Angle Calculated from the Ratio of A&./= Using the Analysis of Reference 8. sample 1 sample 2
A
angle of incedence u (de@
a,(%) a,(%)
30
45
30
45
0.37 0.18 0.28 0.25 0.094 0.044 0.13 0.057 77.1 8.1 76.4 18.9 75.8 7.0 77.8 17.8 1.0 47.8 0.91 8.2 1.0 33 0.96 1.7
tiltangle0 (deg) demit9 0 The density of the chromophore was calculated from AR, and 0 using the formulagiven in Appendix. b Densityin terms of the number of chromophores per monolayer molecule. 400
500
600 wavelength (nm)
Figure 5. Absorption spectrum of the polymer solution in water (polymer concentration = 1W M).
3. Orientationof theChromophoreof the Adsorbed Polymer. In order to quantify the amount of the adsorbed polymer onto the monolayer,the chromophore orientation in the adsorbed state has to be known. A standard techniqueto determine the orientation of the chromophore at the surface is the polarized reflection spectroscopy.8 Monolayers of sample 1 and 2 were spread on water containing 106 M of the dye polymer and compressed to areas of 55 and 77&/molecule, respectively. The reflection spectra before and after the monolayer was spread were recorded. No reflection peak corresponding to the chromophore absorption was noted from the surface before the monolayer was spread, hence the polymers were not adsorbed to the bare surface. The reflectivity peak was found at 485 nm after the monolayer wm spread, redshifted by 10nm from the absorption peak of the solution (cf. Figures 5 and 6). We suppose that the peak shift is due to the stronger interchromophore interaction in the adsorbed state because of the confined geometry. This point will be discussed later. The results of the reflectivity measurements at 485 nm for s- and p-polarized incident light at the incident angles of 30' and 45O are summarized in Table I. The ratio of the reflectivity change of the p-polarized light AR, to that of the s-polarized light AR, gives the tilt angle 6 of the transition dipole moment with respect to the surface normal.8 As was indicated in Figure 1, this is roughly parallel to the long axis of the chromophore, but nothing can be said about ita short axis, of course. Unfortunately, as is shown in Table I, the tilt angle thus determined is not unique (double valued in Figure 2 in ref 8); the chromophore either lies flat or stands up. From the surface
Table 11. Polarized Absorption at 485 nm through Single Layer LB Films of the Monolayer/Polymer Complex Transferred onto Glass Slides. sample 1 sample 2 0.0079 0.0061 OD ( 8 ) 0.0053 0.0044 OD (P) ATJAT, 1.49 1.38 tilt angle 0 (deg) 81 76 OD (n) (measured) 0.0061 0.0050 OD (n) (calculated) 0.0063 0.0049 a They aregiven interme of the opticaldensityfor s-and p - p o W light incident at 45O. The tilt angle 0 was calculated from the ratio of the transmittancechange ATJAT,,. The internal consistency of the model (Le. the tilt angle, the strength of the transition dipole moment,and the density)is confiied by comparing the absorbance of the LB f i i at the normal incidence;the measured OD and the calculated values based on the same model agree fairly well.
density estimated from AR, (the last column in Table I), however, it is clear that the geometry in which the chromophore lies flat gives consistent values. The angle between the chromophore and the surface normal is about 76.8'. The derivation of the surface density from AR, is given in Appendix. It is worth mentioning that, in Table I, the scatter of the tilt angles around 6 = 76.8O obtained under different experimental conditions is much less than that of the corresponding density. This is because, as was noted in ref 8, the latter depends on the details of the model whereas the former is fairly model-insensitive. One may add incidentally that the orientation of the chromophore in the LB film transferred onto a glass substrate showed a similar result. Here, polarized transmission measurementa were made at the incident angle of 4 5 O . The absorption peak was red-shifted by 10 nm from that of the solution as was the case in the reflection spectra. The results are summarized in Table 11. The ratio of the transmittance change of the a-polarized light AT, to that of the p-polarized light ATp again suggests the chro-
Asano et al.
3590 Langmuir, Vol. 9, No. 12, 1993 10 I (1) h
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0 wavelength (nm)
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Figure 6. Reflection spectra: (1)sample 1; (2)sample 2. Line a was measured at equilibrium after the total of four exchange cycles of the subphase, b at the end of the second cycle. For sample 2, line b coincides with a and is not drawn. Rw is the reflectivity of the bare water surface and AR is the change of the reflectivity on adsorption of the dye. mophore orientation parallel to the surface. Note that the tilt angle in the LB film is remarkably close to that on the water surface. This can be interpreted as the structural robustness of the ionically bound polymer monolayer: the observation which agrees with the reinforcingeffect of the adsorbed polymers.4~5The absorbance of the same LB film at the normal incidence suggests one chromophore adsorbed to one spread monolayer molecule. No macroscopic domains were found in the LB films confirming that chromophores are oriented randomly within the surface. 4. SurfaceDensity of the AdsorbedPolymer. Given the orientation and the strength of the optical absorption of the dye chromophore, we can determine the surface density of the dye polymer with the reflection spectroscopPo@under a more controlled condition. The following was the experimental protocol: (1)The trough was filled with pure water and the reservoir with a polymer solution. A monolayer was then spread on the surface of water in the trough and compressed to 50 A2/moleculefor sample 1 or 77 A2/molecule for sample 2. At this density, monolayer molecules were tightly packed. (2) The subphases of the trough and the solution in the reservoir were exchanged and mixed by the microtube pump. The pump displaced 20 mL of the solution per min. The trough had a volume of 150 mL. The pump was turned on for 5 min and was off the subsequent 10 min. During the latter period, the water level was adjusted and the reflection measurement performed. This cycle was repeated 4 times after which we assumed that the final polymer concentration of 106 M was fully reached. Polymers were adsorbed with increasing polymer concentration in the trough. Because the chromophore lies nearly parallel to the surface, the reflectance change can be most conveniently and accurately measured at the normal incidence. Furthermore, with the optics oriented for near normal incidence, a larger compression ratio of the monolayer was attained. Due to the geometrical constraint, however, the minimum angle of incidence was at loo without a polarizer, which was the experimental condition emoloyed. The analysis was, therefore, done for the randomly polarized light. For one thing, the residual polarization was small. For another, the reflectance difference between the s- and p-polarized light is still small at this incident angle. For example, a dye layer that gives the reflectivity change of +8.7 5% relative to the reflectivity of bare water surface for s-polarized light would give a +9.0% change
Table 111. Surface Density of the Adsorbed Polymer Unit sample 1 sample 2 sample3 8.8-9.5 6.8-7.0 0 W R , (%) density of polymer unit (/lo0 A2) 1.8-1.9 1.4-1.5 0 0.90-1.0 1.1 0 no. of polymer units/ monolayer molecule
for p-polarization. Therefore, the overall error introduced by assuming the perfectly unpolarized light should be negligible. Measured reflection spectra are shown in Figure 6. The peak is shifted to longer wavelength by 10nm as was noted before. Note that the spectrum in the intermediate stage is the same as that in the final stage, which indicates that the peak shift is not caused by the interchain interaction. The surface density of the adsorbed dye was calculated from the reflectivity change using the formula described in Appendix and is shown in Table 111. The number of polymer units (anion)/monolayer molecule (cation) is almost 1for samples 1and 2 despite the disparity in the occupied area of the cations, whereas no adsorption occurred onto an anionic monolayer. This indicates that the Coulombic energy is dominant in the adsorption of polyelectrolyte. The polymer is likely to be adsorbed monomolecularly. Note that the number of densities per monolayer molecule obtained in Table I, in LB film, and in this subsection are all close to unity for samples 1and 2. 5. Peak Shift. In the reflection and transmission spectra of the dye adsorbed monolayers, the peak is shifted to longer wavelength by 10 nm compared to that of the absorption spectrum of the water solution. We suppose that this is due to the interaction between adsorbed dye chromophores, and try to explain it by the extended dipole model by Kuhn.13 From the results of the reflection spectroscopy that (1) the transition moment of the adsorbed dye polymer is oriented along parallel to the layer and that (2) the ratio of the adsorbed polymer unit and the monolayer molecule is nearly 1:1, we suggest that the polymer is adsorbed flat and that the dye unit sticks out alternately along the polymer backbone as sketched in Figure 7. In the calculation of the interaction between the transition dipoles by the extended dipole model, parameters were chosen as follows (see Figure 7): the length of extended dipole 1 is assumed to be 6.5 A, which is the (13)Czikklely, V.; Foretarling, H.D.;Kuhn, H.Chem. Phys. Lett. 1970, 6,207.
Adsorption of Dye onto Monolayers
Langmuir, Vol. 9, No. 12,1993 3591
A
0
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time(min.)
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80
Figure8. Time developmentof the reflectivitychange. Polymer concentration waa 1V M. Salt concentration aa follows: +, no salt added, 0 , l W M 0 , l PMIx, lo-*M; A,lo-' M. Amonolayer was spread at time = 0. In the intermediate period, the data can be fitted with lines.
Figure7. (top)Model for calculating interaction energybetween extended dipoles. The x-axis is the polymer backbone. *ex are located at the center of benzene and naphthalene rings, respectively (cf. Figure 1). (bottom)A proposed model of the adsorbed polymer seen at an angle from above the monolayer. The line in the middle is the polymer main chain. The chromophore is depicted by the Lshaped plates. Arrows indicate the transition dipole momenta. distance between the centers of the benzene and naphthalene rings. This length is estimated based on the fact that, in the case of cyanine dyes, the extended dipole length is nearly equal to the electron conjugation length. The extended dipole charge xe is determined to be 0.25e from the relation xel = M,where M is the transition dipole moment calculated as 7.8 D by means of the CNDO/S method as described before. The distance between the neighboring extended dipoles is 6 A, which is the length of four C-C bonds in the polymer backbone. The distance between the backbone and the center of the extended dipole is estimated to be 8 A from the molecular model (Figure 1). The fitting parameter is p, the angle between the backbone and the extended dipole. We assumed that in the water solution, the dye chromophores are oriented randomly such that the interchromophore interaction is effectively zero; the interaction energy was thus taken relative to the absorption peak of the water solution. From the observed red shift of 10 nm, cp was calculated to be 63O. This value is not so far from 4 5 O , which can be read from a simple molecular model as the angle between the polymer backbone and the line connecting the center of the benzene ring and the naphthalene ring, the direction of the transition dipole moment, when the chromophores are pressed flat as is shown in Figure 7. The local pH near the monolayer may differ from the bulkvalue:14 the higher pH is expectednear the ammonium head group. However, the possible pH change as the cause for the red shift (e.g. protonation of the azo group) was ruled out by studying the spectra of the solutions with various pH strength. No shift of the peak was observed in the range of pH < pK, of the hydroxyl group of the dye polymer, and a blue shift was observed in pH > pK,. The pK, was found to be in the range of 11to 12 from titration. 6. Kinetics. In the experiments in subsections 3 and 4, steady state was reached before the reflection measurements. Therefore, the kinetics of polymer adsorption was not clear. In order to understand the kinetics, measurements were made employing the following (14) Chi, L.F.;Dhathathreyan,A.; MBbiue,D.Langmuir 1990,6,1360.
procedure: (1)We filled both the trough and the reservoir with a same solution so that the concentration was welldefined (polymer concentration was 106 M). A varying amount of NaBr was added to the solution in order to see the effect of the counterions on the adsorption kinetics (NaBr concentration: 0, lV, 103, 1k2,and 10-1 M). (2) A monolayer of sample 1was spread in trough 1. It was not compressed this time. The molecular area was about 80 if2;molecules were loosely packed. (3) The time development of the reflectivity change was measured at each salt concentration. Results are plotted in Figure 8. One might expect that the absorption proceeds as t112since the polymers have to diffuse to the monolayer. However, the time dependence turned out to be closer to t213. For want of the theoretical model that explains this exponent, we abandoned the data analysis using the analytic form. Rather, we chose to understand the process via a numerical simulation, which will be described shortly. Although the exact form is not clear, all data in Figure 8 show a similar trend. In the initial stage of the adsorption, the adsorbed amount increases abruptly. This indicates that polymers near the surface are adsorbed without delay when the monolayer is spread. In the second stage, the adsorbed amount increases linearly with time as is shown by the lines drawn through the data points as a guide to the eye. This linear time dependence was somewhat unexpected but we were led to explain it by the polymer diffusion under a steady concentration gradient, which will be described later. In the third stage, the adsorbed amount saturates. In this stage the monolayer is completely covered. In order to understand the role of the polymer diffusion in the adsorption process, one needs an independent knowledge of the diffusion constant. We, therefore, performed a dynamic light scattering measurementlo (DLS-800,Otsuka Electronics) from the polymer solutions with various salt concentrations. Much to our disappointment, however, it turned out that the intensity of the scattered light from the solution without salt was not strong enough to gain reliable statistics. On the other hand, at the salt concentration of 10-' M, the polymers were aggregated roughly 8 times as large as the polymers without salt as was revealed by the Zimm plot, providing enough scattered light. In this case, the diffusion constant was found to be D = 1.3 X 10-7 cm2Is. The linear increase of the adsorbed polymer with time in the second stage can be understood in terms of the polymer diffusion under a steady concentration gradient from the bulk phase, where the polymer concentration is C, to the surface, where the concentration is zero. One can argue that, at this stage, the polymers near the surface
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3592 Langmuir, Vol. 9, No. 12, 1993
have nearly 100% probability of being adsorbedjustifying the boundary condition of zero concentration. If this is the case, the polymer flow is expressed by the equation (Fick’s first law)
I 2
I
(w=DC
dt 6 where N is the adsorbed amount, D is the diffusion constant, and 6 is the depth of diffusion layer. For instance, 6 is calculated to be 0.16mm for 0.1 M salt solution, a reasonable value for a diffusion 1 a ~ e r . lA~similar kinetic measurement of dye adsorption onto a monolayer has already been explained’s in terms of a Nernst diffusion layer. The faster adsorption for the lower salt solution is qualitatively reasonable, since the diffusion constant of less aggregated polymer should be larger. However, quantitative description is not possible at this stage due to the dearth of data. The theoretical analysis3predicts that, in equilibrium, the amount of the adsorbed polymer increases with the added salt concentration. It is because the repulsive Coulombic interaction between the chromophore anions is screened by salt ions allowing random coils and aggregations. This trend is seen for the solutions with the salt concentration of and 10-1 M, although the saturation was not reached. However, the final amount of adsorption even for the 10-l M solution would be rather low considering the high degree of aggregation in the solution. Further studies on the highly screened systems are clearly needed. It is to be pointed out that, in the final stage, the ratio of adsorbed polymer unit and the monolayer molecule is 1.3:levenon the subphase without salt. This is in contrast to the results in subsections 3 and 4. The cause of this difference is not clear. The relatively loose packing of the monolayer charge is not likely to be the cause. Otherwise, a similar chromophore density should have been seen for sample 2 at 77 A2per monolayer molecule. One possibility may be traced to the way the monolayer was spread. The initial adsorption of the polymer is very rapid when a concentrated monolayer is spread on the polymer solution, so that many conformational defects may be trapped in the adsorbed layer leading to a thicker film. Because of the strong Coulombic interaction, further conformational relaxation after being adsorbed can hardly occur in the polymers; it takes many hours even for neutral polymers.17 7. Simulation of Adsorption Kinetics. In order to elucidate the adsorption kinetics, we simulated the adsorption process of the polymer diffusion using a simple two-dimensional lattice model of 256 X 64 lattices. The simulation’s rules were as follows: (1)The top row of the lattice corresponded to the boundary between the monolayer and the solution. The lowest three rows of the lattice were considered as the bulk phase in which the polymer density was kept constant. The left and right end columns were connected (cyclic boundary condition). (2) 256 particles were distributed randomly at the start of the simulation. One particle occupied one site. (3)A particle moved randomly to a nearest neighbor lattice site per cycle. (4) A particle in the top row was adsorbed to the “monolayer” with the probability proportional to the number of the unoccupied monolayer sites. When a particle was adsorbed, it was removed from the simulation (16) Anderson, T. N.; Eyring,H. In Physical Chemistry,An Advanced Treatise, Eyring, H., Ed.;Academic: New York, 1970;Vol. MA. (16)MObiw, D. In Langmuir-Blodgett Film; Roberts, G., Ed.; Plenum: New York, 1990. (17) Frantz, P.; Granick, 5.Phys. Rev. Lett. 1991, 66, 899.
0
time (a.u.)
Figure 9. Comparison between simulation and measured data. The abscissa for each set of data from Figure 8 has been rescaled to fit the simulated curve.
and the number of unoccupied monolayer sites was decreased by one. The last rule was based on the consideration that, near the monolayer, the Debye screening length is short so that the polymers do not feel the electrostatic field unless they effectively touch the monolayer. The probability of the adsorption reflects the strength of the Coulombic attraction. The result of the simulation for the adsorption is shown in Figure 9 together with the data in Figure 8. Since there is no absolute time scale in the simulation of a diffusion process, the abscissa is arbitrary. Therefore, the time scale for the data was adjusted to fit the calculated curve. A total of 32 individual runs were averaged to remove random noise. The simulated curve reproduces the measured data very well in all stages of the adsorption. The analysis of the simulation gave the following picture: In the fmt stage, polymers near the surface are adsorbed immediatelyafter spreading the monolayer, and the amount of the adsorbed polymers increases suddenly. In the next stage, a linear density gradient of the polymer is established, and the adsorbed amount increases linearly with time. This gives justification of using eq 1. In the final stage, the monolayer is fully covered and no adsorption occurs further. The result of the simulation did not sensitively depend on the strength of the attraction of the monolayer. Although the model is extremely simple, the good fit (essentially a universal curve) to the data in Figure 9 in the entire adsorption process strongly suggests that the polymer diffusion governs the rate of adsorption. Conclusions With the reflection spectroscopy from the dye adsorbed monolayer, we found that the adsorption of the polyelectrolyte is governed by the charge neutrality: the ratio of the adsorbed polymer unit and the cationic monolayer molecule is nearly 1:1, whereas no adsorption occurs onto an anionic monolayer. This result implies that the Coulombic interaction energy is dominant in the adsorption of the polyelectrolyte, while entropy has little effect. The finding supports the theoretical predictions3 and thus polymers are likely to lie flat to the monolayer. It was further found that the transition moment of the adsorbed dye is oriented almost parallel to the layer. This suggests the possibility to control the internal configuration of a polymer using charged interfaces. We have also investigated the adsorption kinetics of the polymer and found that a simple model which takes only the polymer diffusion in water into accountis adequate to reproduce the observed adsorption rate. Acknowledgment. We have benefited very much for the suggestions by Dr. A. Tomioka. Otsuka Electronics
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Co., Ltd. is acknowledged for the use of the light scattering apparatus. We thank Professor T. Kunitake for providing us with sample 2. This work was supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture and a Grant for International Joint Research Project from NEDO, Japan.
Appendix Following the formulation by Oritt et a1.F we calculated the number density of the chromophore using as follows: The oscillator strength f associated with an absorption peak of a solution, whose spectrum is A(w), can be expressed as f = kJA(w)dw = kg A(oo)
(AI)
where c is the speed of light, m and e are the mass and the charge of electron, respectively, eo is the dielectric constant of vacuum, and NAis Avogadro's constant. A(wo) is the molar absorption coefficient at the absorption peak wo and g is the geometrical factor that characterizes the spectrum A(w). Assuming that the transition moment
along only one direction is dominant, the transition dipole moment P is given by
Given the transition moment (A2) in terms of g and A(w)(which is 1.25 (L/mol)/cmin our case),and assuming that the spectral shape of the reflection due to the adsorbed chromophore hR,(w) is the same as A b ) , we can express the integrated reflectance, after some calculation, as JhR,(w) dw = g hR,b0) =
(7.17 x 10-21)g-&$
___
- (cos2d))u (A3) ..
where a is the angle of incidence, r, is the reflection coefficient from the bare subphase at this incidence angle, u is the surface density of the chromophore, and 0 is the orientation of the chromophore with respect to the layer normal. The angular bracket implies the statistical average. The factor g drops out from both sides of (A3), and Q is deduced from h R , ( w ~ ) . Although the absorption peak of the dye solution (475 nm) occurs at a different frequency from the reflection peak of the adsorbed dye (485 nm), the spectral shapes are quite similar (cf. Figures 5 and 6). We, therefore, ignored the spectral shift of 10 nm and used as hR,(wo) its peak value in evaluating Q from eq A3.
