Investigation by evanescent waves of the charge and conformation of

Parkville, Victoria 3052, Australia. Received March 9, 1992. In Final Form: July 16, 1992. A new evanescent wave spectroscopic technique which has the...
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0 Copyright 1992 American Chemical Society

OCTOBER 1992 VOLUME 8, NUMBER 10

Letters Investigation by Evanescent Waves of the Charge and Conformation of an Adsorbed Polyelectrolyte at the Silica/ Aqueous Solution Interface? Mathias Trau, Franz Grieser, Thomas W. Healy, and Lee R. White* School of Chemistry and Department of Mathematics, University of Melbourne, Parkville, Victoria 3052, Australia Received March 9, 1992. In Final Form: July 16, 1992 A new evanescent wave spectroscopictechnique which has the capacityto yield conformational (segment density profile) and charge distribution information for polyelectrolytes adsorbed at the solid/aqueous solution interface is reported. In a first step, to illustrate the capacity of this technique, we demonstrate that evanescent wave spectroscopic data may be used to quantitatively determine both the surface excess and the mean separation distance from the interface of charged and uncharged segments attached to the backbone of a model polyelectrolyte adsorbed at the silica/aqueous solution interface.

Introduction Determination of the conformation and charge of adsorbed polyelectrolytes is an area which remains a challengeboth experimentally and theoretically. Although there presently exists a relative abundance of theoretical work in the area, in particular calculations via the lattice mean field approach,1*2there still remains a great paucity of experimental data which can critically test these theoretical calculations. In view of this, we have developed a spectroscopic evanescent wave technique which has the capacity to quantitatively determine conformational and degree of charge information for adsorbed polyelectrolytes. The technique measures the attenuation of a totally internally reflected beam of light which results from the interaction of the evanescent wave, generated a t the silica/ aqueous solution interface, with absorbing chromophores attached to the backbone of the adsorbed polyelectrolyte. Conformational information is determined by varying the depth of penetration of the evanescent wave, controlled by reflection angle, and charge information is determined

* To whom correspondence should be addressed.

+ Presented at the 7th International Conference on Surface and Colloid Science, CompiBgne, France, July 7-13, 1991, as part of a Symposium entitled ‘Surface Characterization by Spectroscopy”. (1) Van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984,88,6661. ( 2 ) B(lhmer, M. R.;Evers, 0.A.; Scheutjens,M. H. M. Macromolecules 1990,23, 2288.

by varying the wavelength of the beam and probing an ionizable chromophore which has clearly distinct spectra for the ionized and neutral forms. Although we shallreport results here for a model polyelectrolyte, one which has been specifically synthesized to contain an ionizable probe (acridine) randomly distributed along the backbone, the technique is not restricted to such artifically tagged polymers. In principle any chromophore on the polymer backbone may be used, as long as the distribution of the chromophore on the backbone is either known or random. Our technique also has several distinct and important advantages over comparable evanescent wave induced fluorescence (EWIF) techniques (e.g. 3) which have been previously reported: (i) the problem of fluorophore quantum yield variation with distance from the interface4* is completely circumvented; (ii) fluorescence induced by surface and/or background scattered light (an annoying artifact which is extremely difficult to accurately remove from the data) is an effect which is completely absent in (3) Caucheteux,I.; Hervet, M.; Jerome, R.; Rondelez, F. J. Chem. SOC., Faraday Trans 1990,86, 1369. (4) (a)Lukosz,W.; Kunz, R. E. J.O p t . SOC.Am. 1977,67,1607-14. (b) Lukosz, W.; Kunz, R. E. J. Opt. Soc; Am. 1977,67, 1615-19. (5) Lukoez, W.; Kunz, R. E. Opt. Commun. 1977,20, 195. (6) Kuhn, H. J . Chem. Phys. 1970,53, 101. ( 7 ) Suci, P.; Hlady, V. Colloids Surf. 1990, 51, 89. (8)Rumbles, G.; Brown, A. J.; Phillips, D. J. Chem. SOC.,Faraday Trans. 1991,87, 825.

0143-1463/92/2408-2349$03.00/00 1992 American Chemical Society

2350 Langmuir, Vol. 8, No. 10,1992

this technique; (iii) as mentioned above, the technique does not necessarily require a fluorescent (or absorbing) probe to be artificially attached to the polymer backbone. Item i is a particularly grave restriction for the EWIF technique because it requires that any fluorophorepolymer system to be studied needs to be fully characterized with respect to the photophysics of the fluorescent moiety. For example, solvent environmental effects may alter the emission yield of the fluorophore, and therefore fluorescencefrom different environmentalsites within the polymer will greatly complicate the site distribution analysis. The experimental determination of adsorbed polymer conformation is an extremelydifficult task which has been previously attempted via a wide variety of techniques: e.g. neutron scattering/reflectivity,gJOellipsometry,llJ2 photon correlation spectros~opy,~ nuclear magnetic resonance,13 electron spin resonance,14 and capillary flow techniques.15 Of these, only the neutron scattering/ reflectivity techniques provide a method for rigorously determining the segment density profile of the adsorbed polymer. Although ellipsometry, in principle, can also give such information,16it is generally limited by sensitivity constraints to measuring only the surface excess of the adsorbed polymer,12J7 with the exception of highly reflective substrates (e.g. metals) where an “ellipsometric thickness”, a measure of the extension of the polymer normal to the interface, can also be measured.ll We see the evanescent wave technique reported here as an important adjunct and, eventually, as an alternative to the neutron scattering/reflectivity techniques: our technique uses relatively simple and low cost equipment to determine essentially the same information as well as provide the additional capacity to measure charge distribution in the adsorbed polymer layer-or indeed, the distribution of multiple chromophores attached to the polymer backbone.

Apparatus The apparatus is shown schematically in Figure 1. All of the components displayed in the diagram fit inside the cavity of a standard UV-visible spectrophotometer (Varian,Cary 2215). The incoming beam from the spectrophotometer is passed through a collimating lens (designed to collimate the beam to a maximum divergence of 0.25O),a Glan-Thomas polarizer (which can be set for either s-, or p-polarization states), a collimating slit, and a silica waveguide (where, depending on the chosen angle of incidence, the beam is totally internally reflected between 5 and 20 times) before being passed through to the spectrophotometer detector via a series of mirrors. The angle of incidence for all of the total internal reflections in the waveguide is accurately controlled by a precision rotor which can rotate the waveguide and both mirrors around the central pivot point, shown in Figure la, to within a precision of 1arc minute. This geometry allows accurate control of angle of incidencewhile preserving the natural path of the beam to the spectrophotometer detector at all angle settings. Figure l b illustrates the flow-through mechanism by which an aqueous phase from a remote reservoir is brought into (9) Cosgrove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986,111,409. (10) Cosgrove, T.; Phipps, J. S.; Richardson, R. H. In Surface X-ray and Neutron Scattering; Springer Proceedings in Physics; Zabel, H. I., Robinson, I. K., Eds.; Springer: Berlin, 1992; Vol. 61. (11)Kawaguchi, M.; Hayashi, K.; Takahashi, A. Colloids Surf. 1988, 31, 73. (12) Malmsten, M.; Lindman, B. Langmuir 1990,6, 357. (13) Cosgrove, T.; Ryan, K. Langmuir 1990,6, 136. (14) Fox, K. K.; Robb, I. D.; Smith, R. J. Chem. SOC.,Faraday Trans 1 1974, 70, 1186. (15) Gramain, P. L.; Myard, Ph. Macromolecules 1981, 14, 180. (16) Charmet, J. C.; de Gennes, P. G. J. Opt. SOC.Am. 1983,73,1777. (17) Trau, M. Ph.D. thesis; University of Melbourne, 1992.

Letters \Mirror

1

Collimating

Incoming Beam

Detector Polarizer

/

Pivot Point

I

Waveguide

4 .. v

cont:ict area o-nnx Stainless steel cell holder

Figure 1. Schematic diagram of the evanescent wave apparatus: (a) displays all of the optical components present in the cavity of the spectrophotometer; (b) displays the waveguide housing and the flow-through mechanism by which the aqueous phase is brought into contact with the waveguide surface; (c) displays the aqueous phase/waveguide contact area which is sampled by evanescent waves generated by the total internal reflections shown in part a. contact with the waveguide and Figure ICshows the area of contact between the aqueous phase and the silica waveguide. With this arrangement evanescent wave spectra may be collected for any angle of incidence in exactly the same manner as standard transmission spectra.

Theory The basic principle of this technique is to measure the attenuation (absorbance) of a totally internally reflected spectrophotometer beam which results from the interaction of the evanescent wave, generated at the silica/ adsorbed polyelectrolyte/aqueous solution interface, with absorbing chromophores attached to a polyelectrolyte backbone. A solution of Maxwell’s equations for this systeml8J9gives the profile of the evanescent wave in the aqueous phase and allows a rigourous derivation (with no assumptions about the structure of the polymer layer) of an expression for the absorbance, Abs(A,B),measured by the spectrophotometer at any particular wavelength, A, and for any chosen angle of incidence, d17

where n(8) represents the effective number of reflections the light beam experiences inside the silica waveguide for any chosen angle of incidence (this can be determined either by ray tracing or via a simple calibration technique20),q ( A ) represents the extinction coefficient of the absorbing chromophore, P, attached to the polymer backbone, I(B)/cos 8 represents the intensity of the evanescent wave at the reflection interface and is derived from a solution of Maxwell’s equations for this system, pp(z) is the concentration of absorbing chromophores at a distance z normal to the interface, and d,(8) represents the penetration depth of the evanescent wave into the (18)Harrick, N. J. Internal Reflection Spectroscopy; Interscience: New York, 1967. (19) Born, M.; Wolf, E. Principles of Optics; Pergamon: New York, 1983. (20) The calibration technique involves recording ATR spectra, Aba(A,@, for asystem where pp(z) is known (e.g., a bulk solution of free probe) and solving eq 1for n(8). Consistent results have been obtained” using a wide variety of water-soluble probes and this is the n(e) data which is used here.

Letters

Langmuir, Vol. 8, No. 10, 1992 2351 0.71

"

"

"

'

2r--7?Fl

t

1.6

5 3 0 235 2 4 0 245 2 5 0 255 260 265 2 7 0

230 235 240 245 2 5 0 255 2 6 0 265 2 7 0

Wavelength (nm)

Wavelength (nm)

Figure 2. ATR spectra of polyelectrolyte EPI-26 adsorbed onto

silica at a pH of 3.0 and a bulk concentration of 100 ppm after 1 h adsorptiontime. Each spectrumcorrespondsto one reflection angle setting, 0; with 0 ranging from 67.35' (top spectrum) to 77.94' (bottom spectrum). The spectra between these two were collected at angle settings of 0 = 68.Olo, 68.67', 69.34', 70.00', 70.66 ", 71.33', 71.99", 72.65', 73.32', 73.98', 74.64', 75.30', 75.96O, 76.62', and 77.28', respectively. A moment analysis of this data, once deconvoluted into the component spectra of the species P and PH+, at wavelengths of 250, 253, and 257 nm provides the following data: rp+pH+ = (1.13 0.05) X lo4 mol m-2 (=1.10 0.05 mg m-2of polymer), Z p + p ~ += 20.1 A 2.3 nm; r p = (4.0 0.2) X lo-' mol m-2,Zp = 22.7 2.0 nm; r p H + = (7.1 f 0.3) X lo-' mol m-2,ZPH+ = 18.7 2.6 nm.

Figure 3. (a) Solution spectrum of the polymer EPI-26 collected

at the same time as the ATR surface spectra in Figure 2 (this displays a distinctdifference between the degree of ionization of polymer in solution as compared to polymer on the surface). (b and c) Component P and PH+ spectra which result from a deconvolution of spectrum a.

*

*

* *

aqueous phase and is defined by d,(e) = h/(4?r(nI2sin2e - n;)ll2)

(2)

where nl and n2 are the respective refractive indices for silica and the aqueous phase. Provided P is distributed either randomly or with a known distribution along the polymer backbone, the segment density profile of the polymer can be inferred directly from pp(z). In the case where P is randomly distributed, as for the polymer reported here, pp(z) corresponds exactly to the polymer segment density profile.

Data Processing The apparatus allows us to measure attenuated total reflectance (ATR) spectra of the polymer layer, Abs(X,B), for a variety of evanescentwave penetration depths simply by varying the angle of incidence (see Figure 2). Equation 1 shows that Abs(X,B) (once normalized with respect to n(B)tp(h)I(8)/cos0) is a Laplace transform of the chromophore concentration profile, pp(z). In principle therefore, it should be possible to extract pp(z) from the experimental data by performing an inverse Laplace transform of Abs(X,0) cos e/n(e)cp(x)I(e). In practice however, accurate inversion of Laplace transforms is an extremely difficult process which requires virtually noisefree data to distinguish between different types of profiles which possess almost identical Laplace transforms. An alternative approach is to rewrite eq 1 in terms of the moments for the chromophore distribution

. .I

1 z3 + (3) dp:@ 2d,(8I2 6d,(8)3 whereA(X,B) = Abs(X,B) cos e/n(e)tp(x)I(e),r is the surface excess of polymer, and Zn is the nth moment of the distribution and is defined as A(h,B) = r

(1

--2+-z2--

2"

= 'J" dz 2" pp(z)

ro

Wavelength (nm)

Figure 4. Transmission spectra of acridineacylhydrazine (free

probe) collected at pH values of 1.90,2.54,3.32,3.53,3.72,4.04, 4.57,5.14,5.53, and 6.32. The spectra of the ionized, PH+,and neutral, P, forms are clearly distinct and display an isosbestic point at X = 253 nm.

standard linear regression technique.21 Given a certain level of experimental noise, such a fitting will allow rigorous extraction of the maximum number of moments possible from the experimental data; the maximum number of moments extractable will critically depend on the level of experimental noise. Clearly, the more momenta that can be extracted from the data, the more information we can infer about the structure of the polymer layer and, hence, the better we can reconstruct the complete pp(z) function. When the chromophore P is chosen such that its absorption spectrum is different in the ionized (PH+)and neutral (P) states, eq 1 is modified by replacing tp(X)pp(z) with tp(X)pp(z) + c p ~ + ( X ) p p ~ + (Provided ~). that these spectra differ to the extent that a combined spectrum can be separated into its individual components (e.g., see Figures 3 and 4) the above moment analysis can be performed for each species individually, yielding information about the distribution of both chromophores(e.g., see Figure 2). This is the approach we adopt here to obtain both conformational and charge information about the adsorbed polyelectrolyte layer. Polymer

(4)

In such a representation 2 represents the meah separation distance of the chromophore from the interface, z2 represents the mean squared distance, and so on. Equation 3 can now be fitted to the experimental data using a

The polyelectrolyte used in these experiments (EPI26) was specifically synthesized to contain an ionizable (21) Wentworth, W. E.J . Chem. Educ. 1965,42,96.

Letters

2362 Langmuir, Vol. 8,No. 10, 1992 spectroscopic probe (acridine) randomly distributed along a water-soluble polymer chain (polyacrylamideldiacetone acrylamide copolymer). Acridine was specifically chosen because its spectrum is clearly distinct from ita conjugate acid, the acridinium ion (see Figure 4). The polymer chain has the following schematic formula:

95 h

N

‘E

aE x

“E:

v

?

5 4

0

I

I

c=o I

I

CH3

The synthesis was carried out via two steps: (i) a random copolymer of polyacrylamide/diacetone acrylamide (80mol 7% acrylamide, 20 mol 7% diacetone acrylamide (DAAM)) was prepared and characterizedvia a standard procedure;22 (ii) acridine was grafted to this copolymer by reacting acridineacylhydrazine (prepared via a standard procedure23) with the DAAM moieties, randomly distributed along the polymer backbone. The grafting reaction was carried out by heating a dilute solution of the copolymer and acridineacylhydrazine in DMSO solvent to 60 OC and maintaining this temperature for 5 h. Reaction progress was monitored by TLC (using ethyl acetate as a mobile phase) and the final product precipitated into stirring acetone and was vacuum dried. The polymer was further purified by redissolving into a minimum amount of water and reprecipitating into acetone. UV spectroscopy and H1 NMR were used to confirm the presence of grafted acridine. From the UV spectrum, the total amount of grafted acridine was quantified as 12.3mol 7%. The average molecular weight of the polymer was estimated from the synthesis conditionsz2to be approximately 100 OOO.

-

I

I

I

I

I

0

50

60

70

80

90

I

100

dp (nm) Figure 5. A plot of normalized absorbance, A&@), versus penetration depth, d,, as calculated from the data in Figure 2 at the isosbestic wavelength (A = 263 nm) (+): (a) the beet fit obtainable using only one moment in the fitting equation (eq 3); (b) the best fit obtainable using two momenta. Clearly, at least two momenta must be used to obtain a reasonable fit of eq 3 to the data.

Results and Discussion Figure 2 shows an example of the type of raw data which is collected via this technique. Each spectrum in Figure 2 represents an ATR spectrum of the adsorbed polyelectrolyte which has been collected at a particular angle of incidence. In this example the polyelectrolyte (EPI-26) was adsorbed onto the silica waveguide at a pH of 3.0 and at a polyelectrolyte concentration of 100 ppm. All of the spectra shown in Figure 2 were collected 1 h after the polyelectrolyte solution was introduced into the ATR cell holder shown in Figure lb-because the adsorption kinetics for polyelectrolytes such as these are so slow (typically it takes more than 10 h for an equilibrium concentration to be reached), such kinetic data are extremely easy to obtain. Because the P and PH+ component spectra contain an isosbestic point at X = 253 nm (see Figure 41, a moment analysis obtained by using eq 3 can be immediately performed at this wavelength to determine the spatial distribution of P and PH+together; i.e., p p + p ~ + ( z )= pp(z) + PPH+(Z). A plot of A(X,8) versus d, for X = 253 nm is shown in Figure 5 along with the Waveguide results of a one moment (Figure 5a) and two moment The waveguides used in this study were all prepared (Figure 5b) fit. Given the curvature expressed in the data, from Supracilglass (vitreous silica) supplied by H.A. Groiss it is clear that a reasonable fit can only be obtained if at Ltd. Plate dimensions were 5 cm X 2 cm X 0.2 cm with least two moments are used in the fitting function; this the short edges cut precisely to an angle of 70’ (see Figure indicates that the experiment is indeed sensing the 1). The large reflecting faces were polished to extreme structure of the adsorbed polymer layer. The two momenta smoothness via a standard polishing t e c h n i q ~ e .An ~ ~ ~ ~ extracted ~ by this analysis are I’p+pH+ = (1.13 f 0.05) X examination of these surfaces by atomic force microscopy 10-6mol m-2 (=1.10 f 0.05 mg of polymer) and Z p + p ~ + (Nanoscope 11)revealed a surface roughness of less than = 20.1 f 2.3 nm which are both extremelyreasonablevalues 2 nm (peak to trough) with a periodicity of 40 nm. On a for this type of polymer adsorbed onto silica at a pH of larger horizontal length scale the surface undulations were 3.0. Unfortunately, the present level of noise in A(X,B) of the order of 15 nm per 1.5 pm. After being polished, which is currently produced on our apparatus prohibita these surfaces were first mildly etched in a 1.5% (w/v) accurate extraction of any higher moments of the profile NH4HFz solution for 2 h, to remove residual material from (e.g. Z2, z3, etc.). Equipment modification, however, is the polising process, and then further cleaned by treating currently underway to reduce this experimental noise level with a hot ammonical peroxide solution (307% (w/v) HzO2 and, in principle, there is no reason why such higher aqueous solution to which was added one-tenth its volume moments should not be extractable from this type of of concentrated NHd for 5 min to remove any surface experiment in the future. adsorbed organic material. Information about the individual distribution of P and PH+ can also be obtained by decomposing each of the (22) McCormick, C. L.; Chen, G. S.J.Polym. Sci. 1984,22, 3633. combined spectra into their componentP and PH+spectra (23! Albert, A. The Acridines; their preparation, physical, chemical (e.g. see Figure 3) and performing the above moment and brologrcal properties and u e s , 2nd ed.; William Clowes and Sons: London, 1966. analysis for P and PH+ separately. This can be done at (24) Trau, M.;Murray,B. S.;Grant, K.; Grieser,F. J.Colloid Interface any wavelength where there is significant absorption; Sci. 1992, 148, 182. however, it is most sensitive at h , which occurs at 250 (25) Holland, L. The Properties of Glass Surfaces;Chapmanand Hill: nm for P and 257 nm for PH+. Results for such an analysis London, 1964. ~~

~~

~~

Langmuir, Vol. 8,No. 10, 1992 2363

Letters Table I. Calculated Surface Excess, r, and Mean Thickness, #, Values for Adsorbed Polyelectrolyte EPI-26 at Various Solution Conditions and Adsorption Times time (h)

solution conditions

r (mg m-2)

i (nm)

1" 24 29 48 66 120 142

pH = 3.0 pH = 3.0 pH = 4.1 pH = 4.1,O.Ol M KCl pH = 5.2,O.Ol M KCl pH = 2.3,O.Ol M KCl rinse with acid (pH = 1.6) rinse with acid for 1 week

1.10 f 0.05 1.32 f 0.04 1.81 f 0.06 3.55 f 0.43 4.44 f 0.13 1.38 f 0.04 1.02 f 0.03 0.72 f 0.03

20.1 f 2.3 20.9 f 1.8 21.1 f 1.8 24.0 f 1.7 18.7 f 1.6 23.7 f 1.5 25.8 f 1.6 23.8 f 2.2

a This table entry is the only one not obtained at equilibrium. All other entries were obtained after sufficient time was allowed for the adsorbed layer to approach equilibrium.

for the data in Figure 2 give the following values: r p = (4.0 f 0.2) x 10-7 mol m-2, ZP = 22.7 f 2.0 nm; r P H + = (7.1 f 0.3) X 10-7mol m-2,ZPH+ = 18.7 f 2.6 nm. These values give information about the degree of charge and the charge distribution in the adsorbed polyelectrolyte layer: the z values show that the positively charged segments, PH+, of the polymer are on average closer to the silica surface than the neutral segments, P (this is not terribly surprising given that at a pH of 3.0 one would expect a fully hydroxylated silica surface to be slightly negative2% and the I' values allow us to calculate the total degree of ionization of the adsorbed polyelectrolyte (aswface = r P H d (rpH+ r p ) = 0.64). This value is significantly less than the corresponding degree of ionization of the polyelectrolyte in solution (asolution = 0.78),as measured via a standard transmission spectrum of the aqueous phase (Figure 3a) a t the same time as the ATR surface spectra were collected in Figure 2. This large difference between a,rufaee and asolution, which is commonly observed for many solution conditions, clearly shows the significant effect that the presence of the silica surface has on the PKa of ionizable groups attached to the adsorbed polyectrolyte backbone; the PKa of such groups will be affected by the field resulting from charges present at the silica/water interface26and also from a different charge distribution in the adsorbed polymer layer resulting from a conformational change upon adsorption. The ability to determine information about the degree of ionization and charge distribution in the adsorbed polyelectrolyte layer is a unique feature of the evanescent wave technique27and is one which we believe has great potential for the study of polyelectrolyte adsorption. As mentioned previously, the above experiment and analysis can be performed for any solution conditions (e.g. different pH or salt concentrations) which may be of interest and which would be expected to have a significant effect on the conformation and charge of the adsorbed polyelectrolyte layer. Table I shows the results (calculated r and Z values) for a large range of solution conditions to which the adsorbed layer was exposed. It is interesting to notice that small changes in solution conditions give rise to measurable changes in r and Z ; this suggests that the experiment is sensitive to subtle changes in the structure of the adsorbed polyelectrolyte layer which are induced by changes in the surrounding solution conditions. It is also interesting to observe that the reverse process of polyelectrolyte adsorption, polyelectrolyte desorption, is tediously slow and is not completed even after a week of rinsing with acid.

+

(26) James, R. 0.In Advances in Ceramics-Vol. 21: Ceramic Powder Science; Messing, G. L., Mazdiyasni, K. S., McCaulley,J. W., Haber, R. A., Eds.; American Ceramic Society: Westerville, OH, 1987; p 349. (27) (a)Murray,B.S.;Godfrey,J.S.;Grieser,F.;Healy,T. W.;Lovelock, B.; Scales, P. J. Langmuir 1991, 7, 3057. (b) Murray, B. S.; Grieser, F.; Healy, T. W.; Scales, P. J. Langmuir 1992,8, 217.

In principle, it should be possible to resolve each of the

r and Z values in Table I into Fp, r P H + , Zp, and ZPH+ in the same way in which we analyzed the data in Figure 2. At long adsorption times (usually greater than 24 h) however, an additional scattering component begins to appear in the ATR spectra which is not present in the polyelectrolyte solution spectrum. The effect which this has is to slightly increase the measured value of and to obscure the degree of ionization information. We believe that this effect is due to a slow clustering of either polymer segments or probe, P, molecules in the adsorbed polymer layer, eventually forming scattering sites which contribute to the absorption measured by the spectrophotometer (such clustering of probe molecules is commonly observed in other interfacial systems%). Because such scattering generally has a simple l / X 4 dependence and occurs in regions of the ATR spectrum where P and PH+ are nonabsorbing, this effect can be easily removed from raw ATR spectra where it 0 ~ c u r s . l ~The details of this treatment however, we shall leave to a future publication.29

Conclusion We have tried to demonstrate the potential of a simple evanescent wave spectroscopic technique for measuring segment density profile information, p ( z ) , for multiple chromophores attached to the backbone of an adsorbed polymer. To begin with, in an effort to illustrate the capacity of the technique, we have demonstrated that the technique is currently able to rigorously determine (without any assumptions about the structure of the polymer layer) the surface excess, I', and the mean separation distance from the interface, 2, of charged and uncharged segments attached to the backbone of a model adsorbed polyelectrolyte. With improvement of the apparatus, to reduce experimental noise and to improve sensitivity toward chromophores with small extinction coefficients, it is hoped that this technique will be able to provide even more information on such model polyelectrolytes as well as improve the ability to study polymers which have not been specificallytagged with a high extinction coefficient chromophore.

Acknowledgment. We thank Mr. Rodney Parr (IC1 Research Group) and Dr. San Thang (CSIRO Division of Chemical and Polymers) for their tremendous help with the synthesis of the model polyelectrolyte. Mr. Richard Mathys is thanked for his excellent design ideas and for construction of a large portion of the apparatus. Dr. Colin Barraclough is also warmly thanked for extremely helpful discussions concerning the theoretical analysis of our experimental data. M.T. acknowledges the receipt of an Australian Postgraduate Research Award and an IC1 (Australia) Postgraduate Scholarship for this work and that reported in ref 24. Financial assistance from the Australian Research Council, the Advanced Mineral Products Centre (University of Melbourne), and IC1 (Australia) Specialty Chemicals Group is also gratefully acknowledged. Registry No. P, 143266-96-0; SiOz, 60676-86-0. (28) Urquhart, R. S.; Hall, R. A.; Thistlethwaite, P. J.; Grieser, F. J. Phys. Chem. 1990, 94,4173. (29) Trau, M.; White, L. R. Manuscript in preparation.