2486
Langmuir 1992,8, 2486-2490
Adsorption of Poly(styrenesu1fonate)onto an Ammonium Monolayer on Mica: A Surface Forces Study Peter Berndt, Kame Kurihara, and Toyoki Kunitake'vt Molecular Architecture Project, Research Development Corporation of Japan, Kurume Research Park, Kurume 830, Japan Received March 13,1992. In Final Form: July 1, 1992
The adsorptionprocess of a negatively charged polymer (poly(styrenesu1fonate))to a cationic monolayer surface was studied by means of direct surface forces measurement and surface pressure-area isotherms. The expanded state of the monolayer at the air-water interface is considerably reduced in the presence of the polymer in the subphase. Maximum contraction of the surface monolayer is attained at a polymer concentration of -1 mg/L, or 5 X lo+ M. The surface forces between the charged surfaces decrease by more than 1 order of magnitude at this polymer concentration. This indicatesnearly complete neutralization of the surface by adsorption of poly(styrenesulfonate). This fact and the absence of structural forces in the vicinity of the surface point to flat adsorption of the polymer onto the surface. In contrast, the surface forces reappear at a larger polymer concentration (approximately5 mg/Lj, probably due to additional adsorption of the polymer and recharging of the surface.
Introduction Adsorption processes of polymers to solid surfacesfrom various solvents have been studied intensively in the last decade. Strong interests on this subject are derived from a large variety of practical applications and the theoretical impact of understanding the often complex adsorption behavior of polymers. Numerous studies on the adsorption have been carried out with fairly simple polymers like poly(styrenesulfonate). An obvious advantage of using this homopolymer is its narrow molecular weight distribution, availability, and a ready comparison of the experimental results with theoretical considerations. Its adsorption to platinum metal was studied by ellipsometry, and a distinctive dependence of the thickness of the adsorbed layer on the sign of the applied potential was found.' Positive potential differencesled to a significantflattening of the adsorbed polymer layer. An electrophoresis study of colloidal monodisperse silica spheres clearly shows the inversion of the surface charge after addition of poly(styrenesulfonate).2 Its adsorption to mineral hematite (a main constituent of iron ore) was driven by electrostatic interaction^.^,^ This is illustrated by the observation that the adsorption behavior depends on the ionic strength of the medium and that adsorption of high molecular weight/high charge polymers is preferred. The adsorption of poly(styrenesulfonate)to monolayer surfaces has been studied recently by several groups. This polymer efficiently stabilizes monolayers with cationic head groups, decreasing their solubility in water and enhancing their mechanical stability.k8 This process is governed by electrostatic interactions. However, little is known about the structure of the poly(styrenesulfonate)/ monolayer complexes. Covering of an amphiphilic viol-
ogen monolayer adsorbed on a gold electrode with poly(styrenesu1fonate) leads to a surface that shows signs of anionic character, such as enhanced reduction of cationic oxidants and nonhydrophobic wetting behavior? Both of the works of Takahara et al.8and Miyano et al.9 indicate that equimolar amounts of poly(styrenesulfonate) are incorporated into Langmuir-Blodgett films of cationic monolayers from the aqueous polyion subphase. Theoretical studies of polymer adsorption onto charged surfaces1° predict that the polymer can be arranged in a flat manner on the surface in a good solvent, if both of the repulsive ionic interaction within the polymer chain and the attraction of the polymer by the surface are strong. However, these predictions have not been tested by modern physicochemical methods. Direct surface forces measurement is one of the recent techniques that possess a nanometer resolution. It has been employed for the investigation of charged polymer surface^.^^-^^ These investigations have been conducted mainly on the interaction of weakly charged surfaces (e.g. mica surface) and polymers (e.g. polylysine). To our knowledge, adsorption of a charged polymer onto a surface with high charge density has not been investigated with surface forcesmeasurements. In the present investigation, we attempt to answer questions concerning the mechanism of poly(styrenesu1fonate) adsorption onto ammonium amphiphile monolayers.
+ Permanent address: Department of Chemical Science and Technology, Faculty of Engineering, Kyushu University, Fukuoka 812, Japan. (1)Kawaguchi, M.; Hayashi, K.; Takahashi, A. Macromolecules 1988, 21,1016. (2)Okubo, T. Polym. Bsll. (Berlin) 1990,23,211. (3)Ramachandran, R.; Somasundaran, P. Colloids Surf. 1988,32,307. (4)Ramachandran, R.; Somasundaran, P. Colloids Surf. 1988,32,319. ( 5 ) Shimomura, M.;Kunitake, T. Thin Solid Films 1985,132,243. (6) Gomez, M. E.; Li, J.; Kaifer, A. E. Langmuir 1991,7 (8),1571. (7)Hyodo, K.; Kobayashi, N.; Kagami, Y. Electrochim. Acta 1991,36, 799. (8)Takahara, A.; Morotomi, N.; Hiraoka, S.; Higashi, N.; Kunitake, T.; Kajiyama, T. Macromolecules 1989,22 (2),617.
(9)Miyano, K.; Asano, K.; Shimomura, M. Langmuir 1991,7 (3),444. (10)Van der Schee, H. A.; Lyklema, J. J . Phys. Chem. 1984,88(26), 6661. (11)Argillier, J. F.;Ramachandran, R.; Harris, W. C.; Tirrell, M. J . Colloid Interface Sci. 1991,146 (l),242. (12)Hadziioannou, G.; Patel, S.; Granick, S.; Tirrell, M. J . Am. Chem. SOC.1986, 108 (ll),2869. (13)Dix, L. R.; Toprakcioglu, C.; Davis, R. J. Colloids Surf. 1988,31,
0743-7463/92/2408-2486$03.00/0
Experimental Section Materials. The preparation of fluorocarbon amphiphile 1 was described previously.16 Dioctadecyldimethylammonium bromide was purchased from Sogo Pharmaceutical Co. Sodium poly(styrenesu1fonate) (2) was a gift from Tokuyama Soda Ltd.
147. ~~
(14)Kawanishi, N.; Christenson,H. K.; Ninham, H. W. J.Phys. Chem. 1990,94 (II), 4611. (15)Marra, J.; Hair, M. L. J . ColloidInterface Sci. 1988,125(2),552. (16)Asakuma, S.; Okada, H.; Kunitake, T. J . Am. Chem. SOC.1991, 113 (5), 1749.
0 1992 American Chemical Society
Surface Forces Study of Polymer Adsorption
CFsC6i8CH=CHCoHi80-C CF3C$,8CH=CHC,H,80
Langmuir, Vol. 8, No. 10, 1992 2487
FIT
-C
I1
PSS (MwlxlO?
0
In our experimenta, we used the sodium salt of the polymer (different preparations with molecular weights of approximately 1 X lo.', 2 X lo.', and 5 X 106, as determined by gel permeation chromatography). Inorganic salta were reagent grade from Wako Chemicals. All commercial reagenta were used without further purification. Spreading solventa for monolayers were of spectragrade (Kishida Chemicals). Water was deionized and distilled twice by a Nanopure I1 and FI-streem 48D glass still system (Barnstead). The water had aspecificreaistance of more than 18MQ.cm (prior to distillation). Prior to surface forces measurements, freshly distilled water was deaerated with an oil-free rotary pump. Monolayer Studies. A computer-controlled film balance system FSD 50 (San-esu Keisoku) was used for measuring surface pressure as a function of molecular area (pressurearea isotherms). The trough size was 160 X 600 mm,the compression rate was 24 mm/min, and the temperature was maintained at 20.0 & 0.1OC. The solvents used for spreading monolayers were mixtures of ethanoVCH&lz/benzene (1:1:8)for the fluorocarbon amphiphile and methanol/CHCla (1:9)for diodadecyldimethylammonium bromide. Monolayerswere spread over aqueous solutions of poly(styreneaulfonate). The incubation time before the start of compression was varied in the range between 3 and 30 min and the spreading volume was varied from 80 to 200 r L in order to ensure equilibrium conditions. P A isotherms were not affected by these variations. Preparation of LB Films on Mica Surfaces. Deposition of monolayers onto mica sheets glued to silica lenses was performed in the vertical mode using a computer-controlled film balance (FSDSO) and lifter (FSD21) system (San-Esu Keisoku). First, dioctadecyldimethylammoniumbromide was transferred in the upstroke mode at a surface pressure of 45 mN/m and a deposition rate of 10 mm/min. After 30 min, a layer of fluorocarbonamphiphile was transferred in the downstroke mode onto the deposited layer of dioctadecyldimethylaonium bromide at a surface pressure of 30 mN/m and a deposition rate of 10mm/min. The transfer ratios of the amphiphiles were found to be 1.0 f 0.1 for dioctadecyldimethylammoniumbromide and 0.8-1.0 for the fluorocarbon amphiphile. Deposition of fluorocarbon amphiphile alone was possible, but resulted in slightly lower transfer ratios (about 0.8). After deposition, the lenses were transferred under water and mounted into the water-filled surface forces apparatus. Direct Surface Forces Measurements. Surface forces measurementawerecarriedoutemployingaMark4(ANUTECH, Australia) apparatus. After ita introduction by Israelachvili in 1971,17it has been used by many research groups for detailed studies on forces between surfaces in liquids. Detailed descriptions of the apparatus itaelf, the measurement procedures, and the mathematical formalism employed can be found in references.la1e In a brief description, the distance between the surfaces mounted as crossed cylinders are measured by multiple beam interferometry with an accuracy of up to 0.5 nm. The respective f o r m are determined with a detection limit of 10-7 to 10-8N from the deflection of a double-cantilever spring (on which one of the surfaces is mounted). In accordance with the Deryaguin approximation (eq 11, measured forces F were normalized by the ~~~
~~
(17)braelachvili, J. N. Nature 1971,229,851 (18)CWtemon, H. K.; braelachvili, J. N.; Pashley, R. M. SPE Rereruoir Eng. 1987,5, 166. (19) braelachvili, J. N. J. Colloid Interface Sci. 1973,44 (2), 259.
lo0
L
0
0.5
1
1.5
2
2.5
Area (cim2/molecule)
Figure 1. Surface pressurearea ( P A ) isotherms of fluorocarbon kphiphile 1on pure water and on subphases containing 2 mg/L of poly(styrenesulfonate) (PSS)at 20 OC. Three polymer samples of different molecular weight (M,)were used. The two curves for M, 1 x lo.' and M, 5 x 1@ represent different experimenta with different solutions and troughs, illustrating data reproducibility. geometric mean radius R of the surfaces, yielding a value proportional to the free energy EOof interaction of the surfaces.
FIR = 2rE0 The zero value for the force is set as the force at large distances between the surfaces where no interactions are expected. With the accuracy required for the force detection limit, it is possible to cover a range of ca. 300 nm from the position chosen as zero distance. For measurements in electrolyte solutions, this is in most cases sufficient for an exact estimation of the zero force value. However, the range of detectable force extends much farther in pure water. I n this case, the measured force-distance dependence must be considered to be the difference between the real force and the force at a distance arbitrarily chosen as the distance of the zero force value. In our measurements, electrolyte or polyelectrolyte solutions were added sequentially. After each addition, forces were measured immediately as well as after incubation periods of 1040 h, in order to assure equilibrium conditions. Nevertheless, for high molecular weight poly(styrenesulfonate) at high concentrations, an equilibrium could not be attained even after prolonged incubation periods. Model Calculations. Electrostatic repulsion between charged surfaces in 1:l electrolyte solution was simulated employing the classical Poisson-Boltzman equation
where $ ( x ) is the surface potential in dependence on the distance x , e is the elementary charge, c is the dielectric constant, k is the Boltzmann constant, T i s the absolute temperature, and K is the Debye length as defined by eq 4. A simple numerical procedure (as first described by Chan, Pashley, and Whitem) was used for the calculation.
Results and Discussion Monolayer Studies. Amphiphile 1 contains two identical fluorocarbon chains as hydrophobic tails. It spreads out over water to give a surface monolayer with a distinct expanded phase. Upon compression, phase transition to a condensed phase is observed, as shown in Figure 1.The collapse pressure is higher than 65 mN/m. Introduction of NaaSOc into the subphase with concentrations of up to 1mM had no effect on the pressurearea isotherms (data not shown). When poly(styrenesulfonate) is present in (20) Chan, D. Y. C.; Paehley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77 (l),283.
Berndt et al.
2488 Langmuir, Vol. 8, No.10, 1992 100000
;
E
'
t L
0 01
0.1
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poly(styrenesu1fonate)
100
the subphase, a condensed phase is formed at much lower surface pressures, and the maximal area where surface pressure can be detected is reduced. Similarly, Shimomura and Kunitakes observed facile formation of condensed phases after polyanion adsorption to monolayers of azobenzene-containing ammonium amphiphiles. This resulted in monolayer stabilization and permitted deposition of the monolayer by the LB technique. Apparently, the polyanion restricts the lateral diffusion of molecules and improves the molecular packing in the monolayer. Ae shown in Figure 1, the monolayer condensation is pronounced with polymers of higher molecular weights. This fact supports the presence of multiple point interactions between polymer and amphiphile, which is enhanced with increasingmolecular weights. Condensation of the monolayer is most pronounced with the polymer of molecular weight of 5 X 106. Therefore, we used this polymer to probe the effect of the polymer concentration on the pressure-area isotherm. Figure 2 plots the surface area occupied by one amphiphilemoleculeat several values of the surface pressure as a function of the polymer concentration. The data show that the maximum contraction is achieved at polymer concentrations as low as 1 mg/L (=2nM) for all pressure values plotted. The monolayer contains approximately 50 nmol of the amphiphile, so that saturation is reached at a molar ratio of 1:40 (amphiphile/polymer sulfonate groups). Forces between Monolayers of an Ammonium Amphiphile. Figure 3 demonstrates force-distance relationships between layers of the fluorocarbon ammonium amphiphile in water and in KBr solutions. Ae expected for a charged monolayer,long range electrostatic repulsive force is observed. Model calculationslead to the conclusion that the apparent surface potential is in the range of 150 mV in 8 mM KBr. Applying the Grahame equation
2re
(?!E) 2kT
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\ A
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Distance (nm)
1I )
Figure 2. Effect of poly(styrenesulfonate) (M,5 X 106) concentration on the area occupied by one molecule of the fluorocarbon amphiphile at three surface pressure values.
uo = 'KkTsinh
10
(3)
we can deduce a reasonable surface charge density uo of ca 1.5 nm2/charge. Typical values for fully ionized surfaces are about 0.8 nm2/chargefor solid surfaces (glass, clays)?* >0.9 nm2/chargefor black lipid membranes,and >2 nm2/ charge for biological (cell) membranes.22 The charge density deduced from data in 1 mM KBr solution is about 3 nm2/chargeand seemsto be too low for (21) Israelachvili, J. N. Intermolecular and surface forces. With Application8to colloidaland biologicalsystems,3rd ed.;Academic Prees: London, 1989; p 296. (22) Cevc, G. Biochim. Biophys. Acta 1990,1031 (3), 311.
Figure 3. Forces between surfaces covered with monolayer of the fluorocarbon amphiphile in water and in aqueoue KBr: 1, force in pure water; 2, force in 1 mM KBr; 3,force in 8 mM KBr. The solid lines show the theoretical electrostatic repulsion for a surface 100 mV in 1 mM 1:l electrolyte (A) and for a surface with a potential of 150 mV in 8 mM 1:l electrolyte (B),respectively.
a fully dissociated layer. Competitive ion adsorption to an amphotheric surface (see ref 21 for an extensive description) or other effects not accounted by the GouyChapman approximation for the case of a simple electrolyte should be mentioned as possible reasons for this discrepancy. The data obtained for pure water phase can be fitted by using an apparent surface potential of 70 mV, as discussed below. However, the Debye length (48 nm) that can be deduced by fitting to an exponential function is less than expected for the electrolyteconcentration in pure water (>430 nm, as given by eq 4, where pi is ion concentration and zj the charge of ion i in solution). (4)
We believe that the inconsistency with theory is at least in part due to experimental reasons rather than to a real change in the surface charge. As explained in the Experimental Section, the force in solutions of low ionic strength can be estimated only relative to an arbitrary chosen zero value that is usually considerably (tene to hundreds of "/m) larger than zero. Consequently, the force values in water are lower limits, and therefore, the real surface potential must be higher than the value of 70 mV. Stoichiometric Adsorption of Poly(styrenesu1fonate) at Low Concentrations. Adsorption of acharged polymer to an oppositely charged surface should alter the charge on the surface. These alterations can be detected by surface forces measurement conveniently, and the mechanism and manner of polymer adsorption can be clarified. Care has to be taken for the choice of an experimental system allowing appropriate monitoring of the changes induced by surface polymer interactions. Carrying out experiments with aqueous solutions of the polyelectrolyte in the absence of simple electrolyta makes it possible to observe electrostatic interactions at distances far enough to avoid interference from nonelectrostatic forces. Polymer concentrations were chosen with reference to the monolayer studies described above. Thus, for the polymer with molecular weight of 5 X 105,the concentration was adjusted to 0.01, 0.05, 0.08, (no effect on the -A isotherm), 0.7 (90% saturation of the effect on the PA isotherm), and 7 mg/L (full collapsing effect) by successive
Langmuir, Vol. 8, No. 10,1992 2409
Surface Forces Study of Polymer Adsorption 10000
i
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Distance (nm)
Figure 4. Force-distancedependence for surfacescovered with fluorocarbon amphiphile 1 in pure water (1)and in aqueous solutionscontaining0.7mg/L poly(styrenesulfonate)(2)and 7.0 g/L poly(8tyreneeulfonat.e)(3). The molecular weight of the polymer is 5 X 1V. Lines are drawn to guide the eye.
addition of polymer stock solution into the aqueous phase in the surface forces apparatus. The resulting surface forces data are presented in Figure 4. When added in a concentration not greater than 0.05 mg/L (2 X lo-" M), the poly(styrenesulfonate) has no effect on the force distance dependence (data not shown). A decrease in the Debye length from ca. 60 to 25 nm can be observed after addition of 0.088 mg/L (0.17 nM) of high molecular weight poly(styrenesulfonate) (data not shown). Such a decrease in the Debye length should be expected, since after addition of the polymer, the solution contains ca. 0.40 NMsulfonate groups and an equivalent amount of sodium ions. It should be noted that the total amount of added electrolyte (85 pmol) is less than the amount of the amphiphile deposited onto the surfaces.23 A more drastic effect was observed as the amount of polyelectrolyte became close to that of the amphiphile in the deposited layer. At a polymer concentration of 0.7 mg/L (1.4X 1O-g M,equivalent to the addition of 0.7 nmol of polymer into the aqueous phase), the force decreased dramaticallyto a barely detectable extent. Over the whole range of separations from 5 to 100nm,the force decreased more than 1order of magnitude and did not exceed 100
Nm. Similar results were obtained for the polymer with a smaller molecular weight of 1 X 104, as shown in Figure 5. Addition of 1.1 mg/L (0.11 pM) of poly(styrenesulfonate) into the aqueous phase caused decreases in the force between the monolayer surfaces by 1 order of magnitude over the whole range from 5 to 150 nm. A similar effect of the polymer is observed for the molecular weight range of 104 to 5 X 105 in the same concentration range of ca. 1 mg/L (5 X lV M). The decrease in the force between the surfaces by polymer adsorption can be interpreted in terms of the Poisaon-Boltzmann formalism in which the parameters of the electrostatic interactions are restricted to the net charge density on the surface, the valency of the chargee, and the dielectric permittance of the mediume2' It is (23) The area of this silica lenses is approximately 4 emz, and the depition wm carried out at 0.6 nmZ/molecule with a transfer ratio of clooe to 100%. A simple calculation leads to an amount of 1.1 nmol of amphiphile transferred into the surface force apparatus. On the other hand, the volume of liquid in the apparatus is ca. 0.5 L; therefore, the amount of poly(styrenesulfonate)added equals 0.5 L X 0.17 nM = 0.085
nmol.
(24) In modem electroetatic theories, ion-ion interactions in the aqueous phase, spatial ion distributionon the surface, and several other factors are accounted for (see ref 22 for a review).
1
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Distance (nm)
Figure5. Force-dietancedependencefor surfaces covered with fluorocarbonamphiphile1in pure water (1)and aqueoussolutions containing 1.1 mg/L poly(styrenesulfonate) (2)and 3.76 mg/L poly(styrenesulfonate)(3). The molecular weight of the polymer is 1 x 104. Lines are drawn to guide the eye. obvious that our observation reflects a decrease in the net surface charge density. Using the data for 0.17 nM poly(styrenesulfonate) (Figure 4),we estimate that the residual apparent surface potential is less than 20 mV. At a Debye length of ca. 25 nm and a surface potential of 20 mV, the charge density can be calculated using the Grahame equation (3)as 1 charge per 300 nm2. Since the charge density of the amphiphile monolayer on mica is 1charge per 1.5 nm2 (see above), only one of200surfacecharges isleft after polymer adsorption; therefore, more than 99VJof the initial surface charge are masked. Mechanism of Surface Charge Neutralization. In order to explain nearly complete disappearance of the surface net charge, the number of negative charges brought to the surface by the polymer must be exactly equal to the number of positive chargeson the surface, e.g. one molecule of the 104 M, polymer (50charges/molecule) bound to 50 amphiphile molecules. The complete disappearance of charge also requires that the polymer must be bound in a configurationthat preventa adsorption of excess polymer (the discharged state of the surface is stable longer than 24h). Poly(8tyrenesulfonate) has been found to behave like a neutral polymer in a 0 solvent in a 3M KCl solution.26 The hydrodynamicradius (r) of the polymer (M,1.77 X 105) was deduced to be 9.5 nmaZ5Such a polymer coil would bear ca. 800 charges and ita projection onto the surface would cover *r2 = 280 nm2. This area is equivalent to only 200 charges on the amphiphile surface. Therefore, complete covering of the surface area with the polymer in a random coil conformation would lead to creation of 600 opposite charges per adsorbed polymer molecule. This is contrary to what we observed. Also, if a polymer layer which corresponds to the diameter of the random coil polymer (2 X 9.5 nm) would be formed, ita thickness would certainly be detectable with the surface forces measurement. Our data given in Figure 4 clearly indicate that the thickness of the adsorbed layer of the polymer with Mw of 5 X 105 is in the range of 1.5-2.5 nm and that of the adsorbed polymer of 1 X 104 M, is less than 1 nm. These data strongly point to flat and stoichiometric adsorption of poly(styrenesulfonate) on the ammonium monolayer surface. Figure 7A illustrates flat adsorption of polyanions onto the cationic monolayer surface. We can find some support for the proposed mode of adsorption in the current literature. First, poly(styrene(25) Wang,L.; Yu, H. Macromolecules 1988, 21 (12), 3498.
Berndt et al.
2490 Langmuir, Vol. 8, No. 10,1992
sulfonate)in aqueous solution with low ionic strength does not assume a globular or random coil structure. Early measurements by Earnest et showed a substantial expansion of the polymer coil with increasing contents of the sulfonate groups. Quasielastic light ~cattering,~5 small angle neutron scattering,27 and neutron spin echo28experiments yield evidence that the polymer conformation in water is best described by a prolate ellipsoid (with an axis proportion of 1.66 nm X [number of subunits X 0.25 nml) or a as nearly stiff rod, intercepted by coil-like perturbations (wormlike coil). The rods may associate into large clusters of several hundreds of nanometers.29 An extended conformationmust be advantageousfor flat adsorption. The adsorption behavior of poly(styrenesulfonate)onto various interfaces is also consistent with our model shown in Figure 7A. Multilayered LB films of a fluorocarbon amphiphile deposited in the presence of poly(styrenesulfonate) in the subphase have a composition equivalent to a 1:l polyion complex.8130 This proves, that a 1:l amphiphile-polymer arrangement is possible. In their recent work, Miyano, Asano, and Shimomurag used attenuated total reflection technique to study adsorption kinetics of poly(styrenesulfonate) onto a monolayer of a cationic amphiphile (dioctadecyldimethylammonium bromide). Adsorption of 0.2 pM of the polymer (1X 104M,) occurred in two distinguishable steps: a thin, uniform layer was adsorbed in the first step, then the thickness and nonuniformity of the layer increased for several hours until a final equilibrium plateau was attained. The apparent thickness of the layer adsorbed in the first stage was calculated to be 0.6 nm-implicating a flat, monolayerlike structure. Although their method can reveal only an average thickness and uneven adsorption cannot be completely denied, the coincidence with our direct observation is obvious. Additional Adsorption of Poly(styrenesu1fonate) onto the Neutralized Surface. Increased concentrations of poly(styrenesulfonate) at about 1-2 mg/L ((5-10) X lo4 M) lead to increases in the force to values comparable to or even larger than those found for a surface of fluorocarbon amphiphile alone (Figures 4 and 5, curve 3). The origin of this force is of electrostatic nature. This was proven by alteration of the forces upon addition of simple electrolyte in the aqueous phase. Figure 6, curve 3, shows the force distance dependence for a system containing 18.7 mg/L (ca. 35 X lo4 M) of poly(styrenesulfonate) together with 450 pM Na2S04.31 This curve can be expressed by a combination of a Debye length equivalent to 4 W 5 0 0 pM of a 1:2 electrolyte and a fairly high surface potential of more than 50 mV. Recharging of the surface is produced by additional adsorption of the polymer as shown in Figure 7B. This leads to net charge formation at the surface and causes build-up of an electrostatic double layer responsible for the force observed. The theoretical curve for the electrostatic interaction fits the experimental data satisfactorily only in the range beyond 5 nm. At closer distances an additional repulsive (26)Earnest, T.R. J.; Higgins, J. S.; Handlin, D. L.; MacKnight, W. J. Macromolecules 1981, 14, 192. (27) Ragnetti, M.; Oberthur, R. C. Colloid Polym. Sci. 1986,264, 32. (28)Kanaya, T.;Kaji, K.; Kitamaru, €2.; Higgins, J. S.; Farago, B. Macromolecules 1989, 22 (3, 1356. (29)Matauoh, H.;Schwahn, D.; Ise, N. Macromolecules 1991, 24, 4221. (30)Higashi, N.; Kunitake, T. Chem. Lett. 1986, 105. (31)Addition of poly(8tyrenesulfonat)leads toa decrease in the Debye length analogous to the addition of simple electrolyte. However, due to the nonzero ion eize, the theory does not exactly predict the observed Debye length.
.
‘ *
a
*
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Figure 6. Force-distance dependence for surfaces covered with fluorocarbon amphiphile 1 in solutions containing 3.75 mg/L poly(styrenesulfonate)(l),18.7mg/L poly(styrenesulfonate)(2), and 18.7 mg/L poly(styreneeulfonate)and 0.45 m M NazSO, (3). Poly(styrenesulfonate)of M, 1 X l(r was used in this experiment. The solid lines show the theoretical electrostatic repulsion for a surface with a potential of 80 mV in 0.08 mM 1:l electrolyte (A) and for 50 mV in 0.8 mM 1:l electrol* (B).
B:
Figure 7. Schematic illustration of adsorption of poly(styrenesulfonate) to an oppositely charged surface. For amphiphile surface in pure water or in simple electrolyte solutions, diesociation of charged groups leads to buildup a classicaldouble layer. (A) On the initial stage of adsorption, the polymer forms stoichiometric ion pairs and the layer becomes electroneutral. (B)At higher polyion concentrations, a restructuring process of the adsorbed polymer builds a new double layer by additional binding of the polymer.
force appears to be present. A comparison of curve 2 (the force in the presence of the polymer alone) and curve 3 in Figure 6 shows that the additional force is not sensitive to the ionic strength and, therefore, probably not of electrostatic origin. We may identify this force as the structural force caused by the adsorption of a thicker polymer layer. Concluding Remarks In the present investigation,we examined the adsorption process of charged polymers onto a surface bearing the opposite charge. The experimentaldata obtained provides strong evidence for a flat adsorption of the polymer which neutralized the surface charge. This result opens a wide range of possible applications not only for stabilization of monolayers but also for molecular design of novel selfassembling structures. Acknowledgment. We thank Mr. Yasuo Itami for providing the fluorocarbon amphiphile. 2,9080-79-9;MesN+[(CH*)l,Registry No. 1, 143121-94-2; C H ~ I ~ B I 3700-67-2. -,
