Langmuir 2006, 22, 7543-7551
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Adsorption of Randomly Annealed Polyampholytes at the Silica-Water Interface Y. Tran,*,† P. Perrin,† S. Deroo,‡ and F. Lafuma† Laboratoire de Physico-chimie des Polyme` res et des Milieux Disperse´ s (UMR 7615), ESPCI 10, Rue Vauquelin, 75231 Paris Cedex 05. France, and Rhodia Recherches, CRA. 52, Rue de la Haie Coq, 93308 AuberVilliers Cedex, France ReceiVed December 21, 2005. In Final Form: May 11, 2006
We have investigated the adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) with various molar compositions. The adsorption was performed from dilute aqueous solutions onto silicon substrates. The adsorbed layers were characterized by reflectivity techniques such as reflectometry, ellipsometry, and neutron specular reflection. As expected for annealed polyampholytes, the adsorption was found to depend strongly on the pH, with a maximum within the isoelectric domain of the polyampholyte. The monomer volume fraction profiles of the adsorbed layers were determined from neutron specular reflection measurements. In the isoelectric domain, the polyampholyte chains adopt a compact conformation, with a layer thickness of about 60 Å. The polyampholyte layer is as dense as the adsorbed layer of fully charged polyelectrolyte but much thicker. Finally, we found that changing the ratio of neutral units along the polyampholyte chain in the isoelectric domain had no significant effect on the concentration profile of the adsorbed layer.
Introduction The adsorption at the solid-liquid interface of polymers bearing electric charges has received considerable attention in the last two decades. This is not only due to the fascinating and challenging aspects of the topic from an academic point of view but also to the great diversity of industrial and technical applications.1 Indeed, polyelectrolytes are widely used as rheology modifiers in water purification, paints, and food products. The adsorption mechanism of polyelectrolytes on solid surfaces from aqueous solutions is now well understood (for reviews see ref 2 and 3). Considerable efforts have been devoted to describe the interfacial conformation of polyelectrolyte chains using a broad range of experimental techniques such as spectroscopy methods, reflectivity techniques, and surface force measurements to name but a few. It is established that polyelectrolytes adsorb in forming loops and long tails, containing several units which number depends on both chains and surface charge density. Polyelectrolytes adopt a flat conformation when the charge density is high and a more extended conformation at low charge density. Unlike polyelectrolytes, polyampholytes can bear both positive and negative charges on the same chain. The mechanism of polyampholyte adsorption is essentially governed by electrostatic interaction with the attraction between the charged surface and the oppositely charged units of the chain and the repulsion between the surface and the similarly charged units. It follows that adsorption strongly depends on the charge densities of both the surface and the polymer and on the charge distribution along the polyampholyte backbone. * To whom correspondence should be addressed. E-mail: yvette.tran@ espci.fr. Phone: 33 1 40 79 58 12. Fax: 33 1 40 79 46 40. † UMR 7615. ‡ Rhodia Recherches. (1) Dautzenberg, H.; Jagere, W.; Ko¨tz, J.; Philipp, B.; Seidel, C.; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Application; Carl Hanser Verlag: Munich, 1994. (2) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993. (3) Cohen Stuart M. A.; Kleijn J. M. Surf. Sci. Ser. 2001, 99, 281.
Recently, a review has been devoted to the properties of polyampholyte solutions, the interaction of polyampholytes with surfaces and polyelectrolytes.4 Most of the theoretical and experimental studies on polyampholyte adsorption have been summarized in this review. The theory reveals that the adsorption of the polyampholytes is due to the polarization of chains in the external electric field created by the charged surface.5-13 The model of single chain adsorption predicts three regimes of adsorption: pole, fence, and pancake.6 This model has been extended to the case of multichain adsorption.9 The structure and thickness of adsorbed layers depends on the charge distribution along the chain, solution concentration, and surface charge density. It has been demonstrated that the long-range of polyampholyte adsorption can lead to an adsorbed layer much thicker than the size of individual chains. Neutral polyampholytes with equal numbers of positively and negatively charged monomers (zero net charge) are able to adsorb on a charged surface due to the polarization-induced attractive interaction. Even polyampholytes with a non zero net charge can adsorb onto similarly charged surfaces. In this case, the adsorption stops at distances from the surface for which the Coulomb repulsion between polyampholytes and the surface becomes stronger than the polarization-induced attraction. The thickness of the adsorbed layer and the surface coverage decreases with increasing the net charge of the (4) Dobrynin, A. V.; Colby, R. H.; Rubinstein, M. J. Polym. Sci. Part B: Polym. Phys. 2004, 42, 3513. (5) Joanny, J.-F. J. Phys. II France 1994, 4, 1281. (6) Dobrynin, A. V.; Rubinstein, M.; Joanny, J.-F. Macromolecules 1997, 30, 4332. (7) Dobrynin, A. V.; Rubinstein, M.; Joanny, J.-F. J. Chem. Phys. 1998, 109, 9172. (8) Netz, R. R.; Joanny, J.-F. Macromolecules 1998, 31, 5123. (9) Dobrynin, A. V.; Obukhov, S. P.; Rubinstein, M. Macromolecules 1999, 32, 5689. (10) Dobrynin, A. V. Phys. ReV. E 2001, 63, 051802. (11) Dobrynin, A. V.; Zhulina, E. B.; Rubinstein, M. Macromolecules 2001, 34, 627. (12) Zhulina, E. B.; Dobrynin, A. V.; Rubinstein, M. J. Phys. Chem. B 2001, 105, 8917. (13) Zhulina, E. B.; Dobrynin, A. V.; Rubinstein, M. Eur. Phys. E 2001, 5, 41.
10.1021/la053451b CCC: $33.50 © 2006 American Chemical Society Published on Web 08/04/2006
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polyampholyte. These predictions qualitatively agree with experimental findings.14-19 To our knowledge, only a few experimental studies were devoted to the adsorption of polyampholytes on planar surfaces. Gelatin-adsorbed layers on mica have been characterized by surface force measurements.15,16 It has been found that gelatin adsorbed at pH values below and above the isoelectric point (IEP). A flattened configuration was observed on the acid side where gelatin and mica were oppositely charged, whereas on the basic side where gelatin and mica were similarly charged, adsorption occurred with a more expanded configuration. At the isoelectric point, the adsorbed layer was compact due to intersegmental charge coupling. Kato et al.20 have measured by ellipsometry the adsorbed thickness on silica of polysulfobetaine, a zwitterion-type of polyampholyte. They found a decrease of the adsorbed amount and an increase of the thickness of the adsorbed layer with salt concentration. Recently, the interfacial properties of a model random polyampholyte adsorbed on silica and mica surfaces have been studied using ellipsometry, photoelectron spectroscopy (ESCA), and surface force measurements.21 The sample was a quasi neutral quenched polyampholyte containing acrylamide (AM), (acrylamido)methylpropanesulfonate (AMPS), and (methacryloyloxy)ethyltrimethylammonium (MADQUAT). Segregation between positively and negatively charged groups of the chain was observed when adsorbing onto negatively charged mica. The negatively charged groups are concentrated further away from the surface. For the same polyampholytes, Ozon et al.22 obtained the loop size distributions of the adsorbed polymer chains using atomic force microscopy (AFM). The gold surface charge was modified by mixing charged and neutral thiols in various proportions. They found that the higher the surface charge, the smaller the number of monomers in a loop, in good agreement with the theory. Adsorption of ampholytic diblock copolymers, poly(methacrylic acid)-block-poly((dimethylamino)ethyl methacrylate) (PMAAb-PDMAEMA), on silicon substrates has been studied using ellipsometry, scanning force microscopy (SFM), and grazing incidence small-angle X-ray scattering (GISAXS). The amount and the lateral structure of the adsorbed layer have been determined as a function of pH, copolymer chain length and composition, and salt concentration. In particular, as a function of pH, the adsorbed amount reaches a maximum at the isoelectric point of the polyampholyte.23-30 In this work, we have investigated the adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2(14) Blaakmeer, J.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1990, 140, 314. (15) Kawanishi, N.; Christenson, H. K.; Ninham, B. W. J. Phys. Chem. 1990, 94, 4611. (16) Kamiyama, Y.; Israelachvili, J. Macromolecules 1992, 25, 5081. (17) Neyret, S.; Ouali, L.; Candau, F.; Pefferkorn, E. J. Colloid Interface Sci. 1995, 176, 86. (18) Vaynberg, K. A.; Wagner, N. J.; Sharma, R.; Martic, P. J. Colloid Interface Sci. 1998, 205, 131. (19) Hone, J. H. E.; Howe, A. M.; Whitesides, T. H. Colloids Surf. A: Physicochem. Eng. Aspects 2000, 161, 283. (20) Kato, T.; Kawaguchi, M.; Takahashi, A. Langmuir 1999, 15, 4302. (21) Le Berre, F.; Malmsten, M.; Blomberg, E. Langmuir 2001, 17, 699. (22) Ozon, F.; di Meglio, J.-M.; Joanny, J.-F. Eur. Phys. J. E 2002, 8, 321. (23) Walter, H.; Harrats, C.; Mu¨ller-Buschbaum, P.; Je´roˆme, R.; Stamm, M. Langmuir 1999, 15, 1260. (24) Walter, H.; Mu¨ller-Buschbaum, P.; Gutmann, J. S.; Lorenz-Haas, C.; Harrats, C.; Je´roˆme, R.; Stamm, M. Langmuir 1999, 15, 6984. (25) Mahltig, B.; Walter, H.; Harrats, C.; Mu¨ller-Buschbaum, P.; Je´roˆme, R.; Stamm, M. Phys. Chem. Chem. Phys. 1999, 1, 3853. (26) Mahltig, B.; Gohy, J.-F.; Je´roˆme, R.; Bellman, C.; Stamm, M. Colloid Polym. Sci. 2000, 278, 502. (27) Mahltig, B.; Gohy, J.-F.; Je´roˆme, R.; Stamm, M. J. Polym. Sci. Part B: Polym. Phys. 2001, 39, 709.
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Figure 1. Structural units of the polymers: [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC). The studied polyampholytes are terpolymers with various monomer molar ratios.
methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) from dilute aqueous solutions onto silicon substrates. Different reflectivity techniques (reflectometry, ellipsometry, and neutron specular reflection) were used to characterize the adsorbed layers at the silica-water interface. These investigations have been performed for polyampholytes with various charge densities. In the first part, we have determined the isoelectric pH range of the polyampholytes using titration, turbidity, and viscosity measurements. In the second part, we have reported on the adsorbed amount of polyampholytes as a function of pH and discussed its influence on the adsorption isotherms. The concentration profiles of the polyampholyte adsorbed layers in the isoelectric range were determined from specular neutron reflection data. We have also studied the effect on the profiles of changing the neutral monomers ratio. Finally, we have compared polyampholyte and polyelectrolyte adsorbed layers. Experimental Section Materials and Sample Preparation. The polymers were synthesized and characterized in the Rhodia Research Center of Aubervilliers (Rhodia Recherches, France). The structural units of the polymers [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) are given in the Figure 1. Polyampholytes with various monomer compositions were synthesized by radical polymerization using sodium persulfate as initiator. The polymers were purified by precipitation from 10% w/w aqueous solution into acetone and dialyzed against deionized water (Millipore, resistivity ) 18.2 MΩ cm) for one week, before being freeze-dried. The main characteristics of the polymers, molecular weight, polydispersity index, and monomer composition (Table 1), were given by Rhodia. The monomer composition was also determined by acid/base titration. According to Rhodia, monomers are randomly distributed along the chain for all polyampholytes. We point out that the copolymers were obtained by free radical polymerization in water, which is a good solvent for all monomers. In the series of polymers (T0, T4, T6, and T8), the MAA to MAPTAC molar ratio is always equal to 1, and the proportion of DMAEMA increases gradually from T0 to T8: 0% for T0, 40% for T4, 60% for T6, and 80% for T8. Substrates for adsorption experiments are polished 〈100〉 silicon wafers with a native oxide layer of about 15 Å. To perform reflectometry measurements, the silicon substrates were thermally oxidized in pure and saturated oxygen at 950 °C to obtain an oxidized (28) Mahltig, B.; Mu¨ller-Buschbaum, P.; Wolkenhauer, M.; Wunnicke, O.; Wiegand, S.; Gohy, J.-F.; Je´roˆme, R.; Stamm, M. J. Colloid Interface Sci. 2001, 242, 36. (29) Mahltig, B.; Je´roˆme, R.; Stamm, M. Phys. Chem. Chem. Phys. 2001, 3, 4371. (30) Mahltig, B.; Gohy, J.-F.; Je´roˆme, R.; Buchhammer, H. M.; Stamm, M. J. Polym. Sci. Part B: Polym. Phys. 2002, 40, 338.
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Table 1. Main Characteristics of the Investigated Polymers polymer sample
monomer molar ratio, DMAEMA:MAA:MAPTAC
molecular weight, MW (g/mol)
degree of polymerization, N
polydispersity index
PMAPTAC T0 T4 T6 T8
0/0/1 0/5.10/4.90 4.10/3.00/2.90 6.10/1.95/1.95 7.90/1.05/1.05
220000 390000 179000 222000 162000
1000 2370 1100 1390 1020
1.2 2.7 2.4 2.1 1.8
layer with a thickness of 500 Å. Prior to use, all substrates were carefully cleaned: they were immersed for 30 min in a hot mixture of H2SO4 (95% pure) and H2O2 (30% concentrated) (H2SO4: 70% v/v; H2O2: 30% v/v) (caution: piranha solution is extremely corrosiVe). The substrates were then thoroughly rinsed with deionized water and dried under an argon stream. The adsorption experiments were all performed from dilute aqueous polymer solutions (0.2 g/L). The pH of the solutions was adjusted by adding aliquots of HCl or NaOH solutions. For reflectometry experiments, the solution reached the silica surface by a siphon system with a flow rate of about 1 mL/min. After 5 h, the silica surface was rinsed with water at a pH similar to that of the polymer solution. This time is long enough to reach the adsorption equilibrium. In all cases, the values of the adsorbed amount remain constant. For ellipsometry and atomic force microscopy (AFM) measurements, the silicon wafers were dipped into the polymer solution for 5 h, rinsed with an aqueous solution at the same pH, and dried with argon. The silicon blocks used in neutron reflectivity experiments were maintained in contact with the polymer solution using a clamp system for at least 5 h prior to any measurement. Titration, Turbidity, and Viscosity Measurements. Acid/base titration was performed with a commercial apparatus (Titrando 809/ Metrohm) using 0.1 mol/L NaOH solution. Turbidity experiments were performed on polymer T0 solutions at concentrations of 0.2 and 2 g/L. The values of the optical transmittance were measured at a wavelength of 500 nm using a Hewlett-Packard 8453 spectrometer with a Helium lamp. Reduced viscosities of polymer solutions at a concentration of 2 g/L were measured using a low-shear rotational rheometer (Contraves Low Shear 30). The viscosity was found to be independent of shear rate in the investigated range of shear rates (from 20 to 30 s-1). Reflectometry and Ellipsometry. Light reflectometry experiments were performed at Rhodia Recherches (Aubervilliers, France). The principle of reflectometry in a stagnation-point flow cell has been reported in previous papers.31,32 A polarized laser beam (λ ) 632.8 nm) is reflected off the silicon wafer at the silicon-water Brewster angle (70°). The reflected light is split into its parallel and perpendicular components. The signal S is the intensity ratio of these two components. The adsorbed amount Γ(mg/m2) is proportional to the relative signal change ∆S/S0 (S0 is the initial signal before adsorption and ∆S is the change of the signal after adsorption) and the sensitivity factor AS Γ)
1 ∆S AS S0
(1)
AS depends on the thickness of the different layers adsorbed on the silicon wafers, the refractive index of the silicon substrate, the silica layer, the adsorbed layer, the solvent, and the refractive index increment of the adsorbed layer. A Sentech SE 400 commercial apparatus was used for ellipsometry experiments. The light source was a helium-neon laser (λ ) 632.8 nm), and the angle of incidence was set to 70°. All measurements were performed on dried samples obtained after adsorption, rinsing and drying steps. The thickness of the adsorbed layer γ (Å) was calculated from the ellipsometric angles ∆ and Ψ using a multilayer model for a homogeneous isotropic film. The adsorbed amount Γ
could be deduced from the measured thickness by Γ ) γδ where δ is the density of the polymer. The comparison between reflectometry and ellipsometry measurements was quite satisfactory since the values of the adsorbed amount obtained with both techniques were similar. The data presented in the paper result from the average of measurements obtained by reflectometry and ellipsometry. Atomic Force Microscopy (AFM). The topography of dry adsorbed layers of polyampholyte was investigated by atomic force microscopy (AFM). All measurements were performed with a commercial apparatus (Multimode Nanoscope III/Digital Instruments). The pictures were taken in the tapping mode to minimize any damage to the soft polymer layer caused by tip contact. Surfaces (1 µm × 1 µm) of the polymer layers obtained after adsorption, rinsing, and drying were scanned in air all over the sample. Specular Neutron Reflection. Neutron reflectivity experiments were performed on the reflectometer EROS at the Laboratoire Le´on Brillouin, CEA-Saclay (France). The experimental procedure and setup were described in a previous paper.33 The sample holder maintained the silicon block tightly clamped against a Teflon trough filled with adsorption solution. The incoming neutron beam passes through the silicon block before reflecting at the silicon-D2O interface. The specular reflection R is measured as a function of the wave vector transfer, k, perpendicular to the reflecting surface. k is defined as 4π/λ sin θ, where λ is the neutron wavelength and θ the glancing incident angle. In our case, reflectivity was measured at the incident angle of 1.4°, using neutrons of wavelength ranging from 3 to 22 Å. From neutron reflectivity measurements, the scattering length density profile perpendicular to the interface F(z) can be determined. In the kinematic approximation, the reflectivity R(k) is related to F(z) by 1 R(k) ) RF(k) 2 F∞
|∫
dF(z) exp(ikz) dz dz
|
2
(2)
The Fresnel reflectivity RF(k) is the reflectivity of the bare surface. As RF(k) decreases asymptotically as k-4, the normalized representation (R/RF versus k) will be shown in order to emphasize the contribution of the scattering length density profile of the adsorbed layer to the reflectivity signal. Different procedures have been developed to determine the profile F(z) using eq 2. In our case, F(z) is approximated to a model with a fixed number of layers of constant scattering length density, adjustable thickness and roughness (the interfacial roughness between two adjacent layers). The corresponding reflectivity curve is calculated by iterations to find the parameters that best fit the experimental data. Once the fitting parameters determined, the scattering length density profile F(z) is calculated. We then deduce the volume fraction profile φ(z) which integral γ ) ∫∞0 φ(z) dz is the thickness of the dry layer. This value is systematically compared to the values measured by ellipsometry γ(Å) and reflectometry Γ(mg/m2). To analyze quantitatively the volume fraction profile, it is convenient to define the mean thickness of the adsorbed layer, h, and the maximum thickness, hmax. hmax corresponds to the z value for which φ(z) < 0.01. h is given by34
∫ z φ(z) dz h)2 ∫ φ(z) dz ∞
(31) Dijt, J. C.; Cohen Stuart, M. A.; Fleer, G. J. AdV. Colloid Interface Sci. 1994, 50, 79. (32) Geffroy, C.; Labeau, M.-P.; Wong, K.; Cabane, B.; Cohen Stuart, M. A. Colloids Surf. A: Physicochem. Eng. Aspects 2000, 172, 47.
0
∞
0
(3)
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Figure 2. (a) Net charge of homopolymers as a function of pH. This schematic representation is based on the dissociation constants of PDMAEMA and PMAA determined by acid/base titration. (b) Total charge (above the arrow) and net charge (below the arrow) of polyampholytes as functions of pH. The total charge f is the total fraction of positively charged f+ and negatively charged f- monomers (f ) f+ + f-). The net charge δf is the difference between the fraction of positively and negatively charged monomers (δf ) f+ - f-). The pH range of zero net charge corresponds to the isoelectric domain of the polyampholyte. (c) Fractions of positively charged monomers f+ and negatively charged monomers f- as functions of pH for T4, T6, and T8 terpolymers. f+ and f- profiles were deduced from acid/ base titration curves. This plot is complementary to the scheme presented in Figure 2b.
Results and Discussion Bulk Polyampholyte Properties: State of Charge and Isoelectric Domain. Figure 2 gives an idea of the state of charge of the homopolymers (Figure 2a) and polyampholytes (Figure (33) Tran, Y.; Auroy, P.; Lee, L. T. Macromolecules 1999, 32, 8952.
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2, panels b and c). The dissociation constants, which correspond to the pH of half-neutralization, were determined by acid/base titration. Figure 2a shows the dependence of the charge with the pH for both PMAA and PDMAEMA homopolymers. The + or signs mean that the homopolymer is positively or negatively charged and neutral means that the polymer is not ionized. The PMAPTAC homopolymer is positively charged over the whole pH range. The values of the dissociation constants found for PMAA (pKa ) 5) and PDMAEMA (pKa ) 8) homopolymers are in good agreement with values available from the literature. The value of the pKa of the PMAA homopolymer can vary from 4.8 to 5.5 and that of the PDMAEMA homopolymer ranges from 7.7 to 8.1.35,36 The dissociation constants of MAA and DMAEMA units were determined from the titration curves of the polyampholytes. The titration curves also allowed to deducing the monomer composition (see Table 1). The pKa values of MAA were 4.5 for T0 copolymers and about 4 for T4, T6, and T8 terpolymers. The pKa values of DMAEMA were around 8 for all terpolymers. As for homopolymers, the values of the dissociation constants of polyampholytes are again in agreement with literature. Merle has investigated the influence of the nearest-neighbor interactions on potentiometric curves of copolymers containing various ratios of MAA and DMAEMA.36 The dissociation constants depend on the distribution of the monomers along the chain. The pKa of the monomer units composing diblock copolymers (MAAb-DMAEMA) are the same as those he measured for the homopolymers. Moreover, he found that the pKa of the basic and acidic monomer units of an alternating polyampholyte (MAAalt-DMAEMA) are negatively and positively shifted by a value of 1.8 (pH unit) as compared to the values of the corresponding homopolymers. In addition, the neutralization range of the random copolymers was found to fall within the limits defined by the diblock and alternating copolymers.36 In Figure 2b, the dependence of the charge with the pH for T0, T4, T6, and T8 polyampholytes is indicated. This analysis from the dissociation constants qualitatively allowed us to give the total and net charges of the polyampholytes as a function of pH. The total charge f ) f+ + f- is the total fraction of positively charged f+ and negatively charged f- monomers. For example, f ) 1 means that all monomers are charged. The net charge δf ) f+ - f- is the difference between the fraction of positively and negatively charged monomers. The isoelectric domain is defined as the pH range where polyampholytes have a zero net charge. In our series of polyampholytes (T0, T4, T6, and T8), the proportion of DMAEMA varies from 0% for T0 to 80% for T8, whereas the MAA to MAPTAC molar ratio is always equal to 1 (Table 1). Consequently, the T4, T6, and T8 terpolymers have zero net charge above pH 8.0, which corresponds to the pKa of DMAEMA units. The isoelectric domain of the T0 polymer corresponds to a range of pH above the pKa of MAA units (4.5). The Figure 2c gives the evolution of the fractions of positively charged units f+ and negatively charged units f- as functions of pH for T4, T6, and T8 terpolymers. f+ and f- profiles were (34) Habicht, J.; Schmidt, M.; Ru¨he, J.; Johannsmann, D. Langmuir 1999, 15, 2460. (35) (a) Bekturov, E. A.; Bakauova, Z. Kh. Synthetic water-soluble polymers in solution; Hu¨thig & Wepf Verlag: Heidelberg, Germany, 1986. (b) Leyte, J. C.; Mandel, M. J. Polym. Sci. 1964, A2, 1879. (c) Pradny, M.; Seveik, S. Makromol. Chem. 1985, 186, 111. (d) van de Wetering, P.; Zuidam, N. J.; van Steenbergen, M. J.; van der Houwen, O. A. G. J.; Underberg, W. J. M.; Hennick, W. E. Macromolecules 1998, 31, 8063. (e) Lee, A. S.; Gast, A.; Bu¨tu¨n, V.; Armes, S. P. Macromolecules 1999, 32, 4302. (f) Gohy, J.-F.; Creutz, S.; Garcia, M.; Mahltig, B.; Stamm, M.; Je´roˆme, R. Macromolecules 2000, 33, 6378. (36) Merle, Y. J. Phys. Chem. 1987, 91, 3092.
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Figure 4. Adsorbed amount (filled circles and solid line) and optical transmittance (hollow symbols and dotted lines) of the T0 polyampholyte, as a function of pH. Optical transmittance is shown for 0.2 (hollow circles) and 2 g/L (hollow squares) concentrated solutions. The solid and dotted lines are guides for the eyes. The arrows below the graph indicate the pH range of positive or negative net charge of the silica surface S (S+ or S-) and the polyampholytes P (P+ or P-). P0 corresponds to the pH range of zero net charge. As in the Figure 3, the adsorbed amount has been determined by both reflectometry and ellipsometry.
Figure 3. Adsorbed amount (filled circles and solid line) and reduced viscosity (open circles and dotted line) of polyampholytes T6 (top) and T4 (bottom) as a function of pH. The solid and dotted lines are guides for the eyes. The arrows below the graph indicate the pH range of positive or negative net charge of the silica surface S (S+ or S-) and the polyampholytes P (P+ or P-). P0 corresponds to the pH range of zero net charge. The adsorbed amount has been determined by both reflectometry and ellipsometry. The data reported on the Figures give average values obtained by both techniques.
deduced from acid/base titration. These profiles are complementary to the scheme presented in Figure 2b, giving more details on the state of charge of the polyampholytes around the dissociation constants. Besides the acid/base titration, which allowed us to give the state of charge of the polyampholytes with pH, we have investigated the solution properties of the polyampholytes by viscosimetry (Figure 3) and turbidimetry (Figure 4). In general, the isoelectric domain corresponds to the viscosity minimum of the polymer solution, caused by the chain contraction. The change in chain conformation is due to the attraction between oppositely charged units with equal ratio. Outside the isoelectric domain, the viscosity is higher, which is explained by the electrostatic repulsion of same chain monomers with a nonzero net charge. In some cases, the chain contraction leads to more or less turbid solutions and eventually to the precipitation of the polyampholyte chains. Turbidity measurements are then relevant to determine the isoelectric domain. Since solutions of T4, T6, and T8 were transparent over the whole range of pH, viscometric measurements were performed in an attempt to determine the isoelectric domain. Because turbidity was observed for T0 solutions with increasing pH, the isoelectric domain was determined by measuring the optical transmittance of the solution. Figure 3 gives the reduced viscosity of the T4 and T6 polyampholyte solutions. The viscometric behavior of T8 solutions is similar to that of T4 and T6 solutions and is hence not given for the sake of clarity. The reduced viscosity decreases as the pH increases before reaching
a plateau value of 300 cm3/g at pH larger than about 9.0, which corresponds to the isoelectric domain. The viscometric results are hence in good agreement with the pKa based analysis summarized in Figure 2b. Figure 4 displays the turbidity measurements solution properties of the T0 polyampholyte as a function of pH. The polymer solution becomes slightly turbid for pH larger than 6.0, which corresponds to the isoelectric domain of T0, in qualitative agreement with the schematic representation of the Figure 2. The value of the optical transmittance remains constant and about equal to 93.5% for 2 g/L concentrated solutions above a pH value of about 7.0. However, for 0.2 g/L concentrated solutions, the transmittance increases from 93.5% at pH 7.0 to 97% at pH 11.0. These features could be explained by an increase of the ionic strength due to the pH adjustment of the solutions. The ionic strength due to the addition of NaOH is actually not negligible as compared to the concentration of charged units along the chain at a polyampholyte concentration of 0.2 g/L (0.2 g/L corresponds to 1.2 × 10-3 mol/L of positively and negatively charged monomers in solution). Similar behaviors for polyampholyte solutions were already reported in the literature. T0 is indeed not fully soluble (turbid solutions) within the isoelectric domain as it was shown for other random ampholytic copolymers in similar conditions.37,38 For instance, quenched polyampholytes with a balanced stoichiometry such as AMPS-MADQUAT random copolymer precipitate.39 Also, for randomly annealed polyampholytes containing DMAEMA and MAA units in equal molar proportions, Merle has observed the precipitation of polyampholyte solutions for pH between 4.5 and 7.2.40 In contrast to T0, the T4, T6, and T8 terpolymers solutions are not turbid within the isoelectric domain because of the presence of neutral monomers (DMAEMA), the amount of which is sufficiently high to conserve the solubility of the chains in water. The solubility properties of random terpolymers containing neutral units have been observed in somewhat different conditions. For instance, Patrickios et al.41 have shown that randomly annealed polyampholytes comprising (37) Candau, F.; Joanny, J.-F. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 7, p 5476. (38) Kudaibergenov, S. E. AdV. Polym. Sci. 1999, 144, 115. (39) Corpart, J.-M.; Candau, F. Macromolecules 1993, 26, 1333. (40) Merle, L.; Merle, Y. Macromolecules 1982, 15, 360. (41) Patrickios, C. S.; Hertler, W. R.; Abbott, N. L.; Hatton, T. A. Macromolecules 1994, 27, 930.
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DMAEMA, MAA, and MMA methyl methacrylate of 0.33/0.33/ 0.33 molar ratios are water-soluble over the entire pH range. In addition, the solubility of AM-NaAMPS-MADQUAT solutions has been observed even for samples with net charge close to zero.42 pH Dependence of Adsorption of the T4, T6, and T8 Terpolymers. The adsorption of the various polyampholytes has been investigated as a function of pH. Both reflectometry and ellipsometry were used to determine the adsorbed amount of the various polyampholytes (Figures 3 and 4). In the figures, the net charge carried by the silica surface (S+ or S-) and the polyampholytes (P+, P-, or P0) as a function of the pH are indicated by the arrows below the graphs. The isoelectric point of the silica surface is admitted to be equal to pH 3.8.23-30 Figure 3 gives the variation with pH of the adsorbed amount of the T6 and T4 polyampholytes. The pH profiles of the adsorbed amount are similar for T4, T6, and T8 (data not shown). At pH lower than about 4.0, the adsorbed amount is typically less than 1 mg/m2. In this pH region, adsorption is thus observed even if the polyampholytes and the surface are identically charged. However, it is widely admitted that the charge of polyampholytes with an overall positive net charge can be locally neutral. Consequently, the relatively weak adsorption (1 mg/m2) of T4, T6, and T8 could be driven by interactions between the neutral MAA monomer units and the positively charged surface provided that the MAA-surface interaction energy is larger than the electrostatic repulsions between the surface and the positively charged DMAEMA and MAPTAC units. This hypothesis is supported by the fact that PMAA homopolymer adsorbs on the positively charged surface at low pH values.23,30 Adsorption increases from pH 4.0 to reach a plateau value close to 4 mg/m2 at a pH of about 9.0. In that pH range, the surface and the polyampholytes are oppositely charged and the adsorption is likely to be promoted by electrostatic attraction between polymer positive segments (MAPTAC and DMAEMA units) and surface negative sites. Above pH 9.0, the adsorbed amount (4 mg/m2) remains constant. In other words, adsorption attains a maximum within the isoelectric domain of the polyampholytes. For instance, it is theoretically predicted that the surface coverage increases with decreasing the polyampholyte net charge.9 Polyampholyte chains with zero net charge adsorb by binding their positively charged monomers (MAPTAC) to the negative surface sites while keeping their negatively charged units (MAA) away from the surface. It is important to note that the DMAEMA neutral monomer units do not adsorb on the silica surface at pH above 9.0.30 Consequently, a new negatively charged surface on which other chains can adsorb is hence created. The adsorption equilibrium is reached when the long-range polarization-induced attraction of the chains to the charged surface balances the repulsion between monomers.9 Experimentally, the values of the maximum adsorbed amount (of order of 4 mg/m2) measured for T4, T6, and T8 are in a range of those measured for other random polyampholytes. For example, the maximum was reached at about 5.5 mg/m2 on mica substrates and 3.0 mg/m2 for silica surfaces for AM-NaAMPS-MADQUAT (92.5/3.75/3.75, i.e., the net charge is close to zero) quenched polyampholytes with MW ∼ 107 g/mol and 0.5 g/L concentrated solutions.21 To summarize, the dependence of the adsorbed amount with pH is similar for the T4, T6, and T8 terpolymers. This could be explained by the fact that the dependence of their net charge signs with pH is similar and that their molecular weights are comparable. Also, the results show that the amount of the (42) Ohlemacher, A.; Candau, F.; Munch, J.-P.; Candau, S. J. J. Polym. Sci. 1996, 34, 2747.
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Figure 5. AFM pictures of dried layers of T0 adsorbed from solutions at various pH: (a) pH ) 5 and pH ) 10; (b) pH ) 7.
nonadsorbing DMAEMA monomers does not change the adsorption of the chains with pH within the investigated 4080% composition range. pH Dependence of Adsorption of the T0 Copolymer. The adsorption profile of T0 with respect to pH shows a relatively weak adsorption (less than 1 mg/m2) at pH lower than 4.0 (Figure 4). The adsorbed amount increases up to pH 7.0 to attain a maximum of about 4.5-5 mg/m2. Consequently, the maximum is again reached within the isoelectric domain of the polyampholyte. The value of the maximum adsorbed amount, about 5 mg/m2, compares well with values reported in the literature. For instance, from gelatin (MW ∼ 100 000 g/mol) solution at a concentration of 100 ppm adsorbed onto mica, Kamiyama et al. obtained about 5 mg/m2 in low ionic strength conditions (10-4 M NaCl).16 It is worth noticing that values of the maximum adsorbed amount much higher than 5 mg/m2 were reported for diblock copolymers (PDMAEMA-b-PMAA) adsorbed onto silicon substrates: 9 mg/m2 for MW ) 62 000 g/mol at pH 5.3 (with pHIsoElectricPoint ) 5.9),23,24 about 15 mg/m2 for MW ) 63 000 g/mol at pH 7.4 and pH 8.8 (pHIEP ) 9.3),27 14 mg/m2 for MW ) 62 000 g/mol at pHIEP ) 9.3,26,27 and 25 mg/m2 for MW ) 15 000 g/mol near pHIEP ) 8.5.28 However, these results could only be explained by considering the adsorption of aggregates already formed in the bulk polymer solutions. Figure 5 shows AFM topography pictures of dry adsorbed layers of T0 at three pH values (pH ) 5.0, 7.0, and 10.0). The lack of contrast at pH 5.0 and pH 10.0 is characteristic of laterally homogeneous adsorbed layers (Figure 5a). Since ellipsometry measurements gave a layer thickness of 12 and 45 Å at pH values of 5.0 and 10.0 respectively, the root-mean-square (RMS) roughness, which is less than 3 Å, is weak. Similar AFM images were observed for adsorbed layers of terpolymers T4, T6, and T8 over the entire range of pH. Figure 5b shows the topography of the T0 sample at pH 7. In contrast to Figure 5a, the adsorbed layer is built of large structures of quite round shape with a
Adsorption of Randomly Annealed Polyampholytes
Langmuir, Vol. 22, No. 18, 2006 7549
Figure 6. Monomer volume fraction profile of T0, T4, T6, and T8 polyampholytes layers at pH 10. The corresponding reflectivity curve of T0 adsorbed layer is given in Figure 7.
diameter of 300 ( 100 Å and a height of 100 ( 40 Å. These large structures could be explained by the adsorption of aggregates. This could be consistent with the fact that the lowest value of the optical transmittance was indeed observed at pH 7.0. However, the measured value (94%) is not particularly low (Figure 4) as compared to that reported for PDMAEMA-b-PMAA polymers (around 80%).24,27,28 Also, the adsorbed amount measured at pH 7.0 (order of 4 to 5 mg/m2) is not consistent with the values measured for these copolymers (larger than 9 mg/ m2).24,27,28 At the present time, we cannot give a satisfactory explanation of the AFM observations at pH 7.0. The analogy between the structures of polyampholytes formed in bulk and adsorbed to the surface has been investigated extensively.24-30 Our purpose is not to give a detailed characterization of the lateral structures of random polyampholyte adsorbed layers. However, the AFM study allows to selecting a series of topographically homogeneous samples for the neutron reflectivity study. As a consequence, the neutron reflectivity concentration profiles of the T0, T4, T6, and T8 copolymers will be compared within their isoelectric domains (i.e., the adsorption amount is maximum) at pH 10.0. Under these conditions, only in-plane uniform (to the AFM scale: σRMS < 5 Å) adsorbed layers were investigated by neutron specular reflection. According to the above remarks, the T0 sample prepared at pH 7.0 is not investigated using neutron reflectivity. Volume Fraction Profile of Polyampholyte Adsorbed Layers. The volume fraction profiles of the adsorbed layers of the T0, T4, T6, and T8 polyampholytes (within the isolectric domain and at pH 10.0) are given in the Figure 6, and the corresponding neutron reflectivity curves are presented in the Figure 7. The concentration profiles of all polyampholytes are quite similar. They suggest the formation of compact adsorbed layers with a surface polymer volume fraction φ ≈ 0.7 and a maximum extension of the chains, hmax, of about 100 Å from the surface. The mean layer thickness, h, is 60 Å. The integral of the volume fraction profiles (γ ) ∫∞0 φ(z) dz) corresponds to the thickness of the dry adsorbed layers obtained with ellipsometry γ(Å) and the adsorbed amount measured by reflectometry Γ(mg/m2). Consequently, the measurements show that the effect on the surface chain conformation of the proportion of neutral units (DMAEMA monomer units at pH 10.0) for DMAEMA molar ratios lower than 80% is not significant. The experimental results are in good agreement with numerical simulations. In our experimental conditions, adsorption was performed on the silica surface, whose charge density is controlled
Figure 7. Reflectivity curves of the T0 polyampholyte (O) and PMAPTAC polyelectrolyte ([) adsorbed layers at pH 10. Open circles and full diamonds correspond to the experimental data. Best fits are shown by solid lines. The logarithmic variations of both reflectivity R and normalized reflectivity R/RF as functions of the wave vector k are presented. As the Fresnel reflectivity RF decreases asymptotically as k-4, the normalized representation log10 R/RF versus k allows to highlighting the difference of the period and the amplitude of the oscillations between the two curves. The corresponding volume fraction profiles that best fit the reflectivity experimental data are shown in Figure 8.
by the dissociation of the silanol groups. The surface charge is characterized by the Gouy-Chapman length
λGC )
1 2πlBσ
(4)
The Bjerrum length, lB, has a value of 7.14 Å for water at room temperature and σ is the surface charge density. Recently, the surface charge density of silica has been determined as a function of pH at different KCl concentrations.43 According to this work, the charge density corresponding to our experimental conditions (pH ) 10 and low electrolyte concentrations) is about 0.5 charge/ nm2 and eq 4 gives a value of 4.5 Å for the Gouy-Chapman length. Furthermore, theoretical predictions on polyampholyte adsorption show that the fence regime, located between the charge densities σ2 and σ3, describes adequately our conditions.6,9 With N ) 1000 (Table 1), the estimated values of the charge density, σ2, are about 0.039 and 0.027 charge/nm2 for f ) 0.2 and 1 (Figure 2), respectively. Irrespective of f (0.2 < f < 1), the value of σ2 does not change significantly if a value of 2000 (Table 1) is taken for N instead of 1000. Similarly, σ3 is about 2 and 4.7 (43) Samoshina, Y.; Nylander, T.; Shubin, V.; Bauer, R.; Eskilsson, K. Langmuir 2005, 21, 5872.
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charge/nm2 for f ) 0.2 and 1, respectively. The characteristics of our adsorbed layers can then be compared to those determined by Monte Carlo simulations.44 In their work, Khan et al. have studied the adsorption of neutral polyampholytes onto charged surfaces as a function of surface charge density, polymer chain length, and charge distribution, for alternating, random, and diblock polyampholyte chains.44 The mean thickness of the adsorbed layer of randomly fully charged chains (f ) 1) has been obtained for different surface charges. For high surface charge densities (between λGC ) 1.12 and 11.2 Å), the mean thickness varies from 30 to 80 Å with a degree of polymerization ranging from 20 to 160, respectively. By extrapolating their results to N ) 2000, we found a value of about 90 Å for the mean thickness, in qualitative agreement with the value of 60 Å measured for the T0 polyampholyte. The effect of the ratio of neutral monomers on the chain has also been discussed by Khan et al.44 In that case, the numerical results for N ) 160 and two specific ratios, 0% and 90%, were reported. For high surface charges (1.12 Å < λGC < 11.2 Å), they found the same adsorbed amount for both ratios. This is in good agreement with our experimental results since the adsorbed amounts of the T4, T6, and T8 copolymers are similar. Also, calculations show that fully (f ) 1) and partially (f ) 0.1) charged chains have nearly identical layer thicknesses, in agreement with our experimental observations for which f was larger than 0.2. Actually, because the chains adsorb onto the surface by orienting their positively charged monomers to the negatively charged surface and keep their negatively charged units away from the surface, chains with low charge fractions (f lower than typically 0.1) are expected to give more extended layers. Consequently, adsorption of weakly charged polyampholytes (f < 0.1) would certainly display interesting features. Results on the adsorption of polysulfobetaine, a zwitterion type of alternating polyampholyte, on silica surfaces were also reported in the literature.20 The thickness of the adsorbed layer measured by ellipsometry is 84 Å for an adsorbed amount of 1.75 mg/m2 for a 2 g/L polymer concentrated solution, under low ionic strength conditions. Consequently, the polysulfobetaine layer is less compact or more dilute than the layers formed by our series of polyampholytes. Comparison with Polyelectrolyte Adsorbed Layers. We have also compared the volume fraction profile of the T0 polyampholyte adsorbed layer to the profile of the PMAPTAC homopolymer layer at the same pH. In this context, it is also useful to analyze the density profile of the T0 layer with pH since the variation of the charge fraction of the polyampholyte chains with pH could induce a change in the polymer interfacial configuration. Figure 7 displays the neutron reflectivity curves of the adsorbed layers of both the T0 polyampholyte and PMAPTAC at pH 10. The logarithmic variations of both reflectivity R and normalized reflectivity R/RF as functions of the wave vector k are presented in the figure. Actually, as the Fresnel reflectivity RF decreases asymptotically as k-4, the log10 R/RF versus k representation, which highlights the oscillations of the reflectivity curve, often helps to amplify the difference between the experimental curves. It should also be noticed that PMAPTAC adsorbed layer is thinner than any other layers investigated by neutron reflectivity in the present study. The corresponding volume fraction profiles that best fit the reflectivity data are reported in the Figure 8 for both T0 and PMAPTAC. The two profiles are strikingly different. The polyelectrolyte adsorbed layer is flat as compared to that of the polyampholyte. (44) Khan, M. O.; A° kesson, T.; Jo¨nsson, B. Macromolecules 2001, 34, 4216.
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Figure 8. Monomer volume fraction profiles of the T0 polyampholyte (solid lines) and PMAPTAC polyelectrolyte adsorbed layers (dashed lines) at pH 10.
The two layers are equally dense near the surface (φ ≈ 0.7), but the extension of the polyampholyte is more important than that of the polyelectrolyte. The mean thickness of the polyelectrolyte layer (h ) 15 Å) is four times smaller than the thickness of the polyampholyte layer. The thin adsorbed layer formed by the polyelectrolyte can be explained by the high polyelectrolytesurface affinity. The adsorption mechanism of polyelectrolyte chains is governed by the balance between electrostatic attraction to the oppositely charged surface and short-range monomermonomer repulsion.45,46 The adsorption of polyampholyte chains is due to the polarization of chains in the external electric field created by the surface charge. This polarization is illustrated by a redistribution of the charges inside the polyampholyte coil in such a way that the repelled charges are away from the surface and the attracted charges are closer to the surface.4-13 Consequently, the adsorbed amount and adsorbed layer thickness of polyampholytes are larger than that of polyelectrolytes for similar surface charge density. The monomer volume fraction profiles of T0 at pH 5 and pH 10 are given in the Figure 9. As shown by AFM, the adsorbed layers are in-plane homogeneous in such pH conditions (Figure 5). Clearly, the concentration profile is more flattened at pH 5 than at pH 10 and bears resemblance with the PMAPTAC profile (Figure 8). The polyampholyte layer is almost twice as thin at pH 5 (h ≈ 25 Å) than at pH 10 (h ≈ 60 Å). The adsorbed amount at pH 5.0 (Γ ) 1.4 mg/m2) is accordingly lower than that at pH 10.0 (Γ) 5 mg/m2), as measured by reflectometry. Actually, T0 can be considered as a partially charged polyampholyte with a positive net charge at pH 5. The MAA monomers might be only partially ionized at pH 5. Therefore, the T0 chains contain neutral, positively, and negatively charged units in this condition of pH. In addition, the surface is not as charged at pH 5 as at pH 10.0.43 It results that the T0 adsorbed layer at pH 5 is less extended than the layer at pH 10.0 but more extended than the PMAPTAC layer. In other words, the adsorption condition of T0 at pH 5.0 corresponds to a situation intermediate to that of the fully charged polyelectrolyte and the fully charged polyampholyte with zero net charge. Previous experimental studies of the adsorption of fully charged polyelectrolytes onto oppositely and highly charged surfaces (45) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Phys. ReV. Lett. 2000, 84, 3101. (46) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Macromolecules 2001, 34, 3421.
Adsorption of Randomly Annealed Polyampholytes
Figure 9. Monomer volume fraction profiles of the T0 polyampholyte adsorbed layers at pH 10 (solid lines) and pH 5 (dotted lines).
have also shown flat conformation. Cosgrove et al.47 have determined the segment density profiles from small angle neutron scattering of sodium poly(styrene sulfonate) (74 000 g/mol) adsorbed layer on positively charged polystyrene latex particles. The volume fraction of the adsorbed layer, measured in 0.5 M of sodium chloride, is larger than 0.85 near the surface and drops down rapidly to values near zero at a distance of about 10 Å from the surface. This is consistent with the profile determined by neutron reflectivity for the PMAPTAC adsorbed on silica (Figure 8). In our case, the PMAPTAC adsorbed layer is slightly thicker as expected for longer chains. It is also relevant to compare the thicknesses of adsorbed layers determined by neutron reflectivity to that measured by surface force apparatus (SFA). From SFA measurements on mica surfaces, one can determine the steric layer thickness, hS, and the compressed layer thickness, hC. hS corresponds to half the distance separating the mica surfaces while hC is the separation distance at high loads. The compressed layer thickness, which can be 10 times smaller than the steric thickness, compares reasonably well to the mean thickness (h) measured by reflectivity techniques. More specifically, hC is expected to be slightly lower than h since hC is the adsorbed layer thickness under compression at high loads. Reflectivity techniques are mostly sensitive to the loop fraction of the adsorbed layer and to the inner part of the layer whereas surface force measurements are rather sensitive to the presence of polymer long tails. Thus, it is not surprising that the steric thicknesses of polyampholyte layers measured by surface force apparatus (SFA) are much higher than that measured by neutron reflectivity. As a matter of fact, values of hS and hC of about 1500 and 250 Å were reported respectively for AM-NaAMPS-MADQUAT polyampholytes (molar ratio 87.07/4.73/8.20) adsorbed on mica.21 (47) Cosgrove, T.; Obey, T. M.; Vincent, B. J. Colloid Interface Sci. 1986, 111, 409. (48) Dahlgren, M. A. G.; Waltermo, A° .; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; A° kesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769. (49) Rojas, O. J.; Claesson, P. M.; Muller, D.; Neuman, R. D. J. Colloid Interface Sci. 1998, 205, 77 and references therein. (50) Kjellin, U. R. M.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476. (51) Claesson, P. M.; Fielden, M. L.; Dedinaite, A.; Brown, W.; Fundin, J. J. Phys. Chem. B 1998, 102, 1270.
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In this case, the value of hC is rather high as compared to the mean thicknesses determined by reflectivity. This is probably due to the high molecular weight of the polyampholyte (MW ∼ 107 g/mol) and to the low total charge and net charge (f ) 0.13 and δf ) 0.035). The values obtained for fully and partially charged polyelectrolytes are closer to our values found by reflectivity techniques. The adsorption of PMAPTAC on charged mica surfaces has been investigated by Dahlgren et al.48 Surface force measurements were performed at PMAPTAC (MW ) 100 000 g/mol) concentration of 10 ppm and at low salt concentration (10-4 M). The authors have measured a compressed layer of 10-20 Å, in good agreement with our observations. The adsorption of partially charged polyelectrolytes has been extensively studied, in particular for copolymers of acrylamide and cationic monomers (ref 49 and other references therein). It has been found that the thickness and the compressibility of the adsorbed layer increase with decreasing polyelectrolyte charge density. For example, a very low-charge-density cationic polyelectrolyte such as AM-MAPTAC random copolymer of 100/0.98 molar ratio has a steric layer thickness about 500 Å and a compressed layer thickness between 50 and 75 Å.49 For the same AM-MAPTAC copolymer of 90/10 molar ratio, the compressed layer is found at 50 Å.50 The force curves with a 30% charged polyelectrolyte were quite similar to those observed with the fully charged sample.51 It is quite consistent with the difference that we observed between the PMAPTAC adsorbed layer and the T0 layer at pH 5.0.
Conclusion The adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) at various molar compositions was investigated from dilute aqueous solutions onto silicon substrates. For all samples, a maximum of adsorbed amount was found in the isoelectric domain of the polyampholyte, with a value on order of 4-5 mg/m2. The concentration profiles of the polyampholyte adsorbed layers were determined in the isoelectric domain. The adsorbed layer is rather dense near the surface with a monomer volume fraction of about 0.7 and extends up to 100 Å from the surface. The polyampholyte layers are almost four times thicker than the PMAPTAC polyelectrolyte adsorbed layer for a similar surface density. We also found that the amount of neutral units on the polyampholyte chain (up to 80%) had no significant effect on the concentration profile. In general, the results compare well with numerical simulations and previous experimental studies. This work also shows that it could be interesting to study the adsorption of weakly charged polyampholytes (with f typically lower than 0.1) to highlight the role of neutral monomer units on the conformation of adsorbed polyampholyte chains. We also could envisage to determining the charge distribution anisotropy by with neutron reflectivity. It means that the density profiles of negatively and positively charged units could be distinguished by using polyampholyte chains containing protonated and deuterated monomers. Acknowledgment. The financial support of the Rhodia Research company (Centre de Recherches d’Aubervilliers) is greatly acknowledged. Authors are grateful to Laboratoire Le´on Brillouin, CEA-Saclay, for neutron reflectivity experiments. LA053451B
