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Interactions between Two Polyelectrolyte Multilayers Investigated by the Surface Force Apparatus A Ä gnes Kulcsa´r,† Philippe Lavalle,† Jean-Claude Voegel,† Pierre Schaaf,‡ and Patrick Ke´kicheff*,‡ Institut Charles Sadron, C.N.R.S. UPR 22, 6, Rue Boussingault, 67083 Strasbourg Cedex, France Received July 25, 2003. In Final Form: November 19, 2003 The distance-dependent interaction between multilayers of alternating polycation (poly-L-lysine) and polyanion (poly-L-glutamic acid) deposited onto mica surfaces in aqueous solution is investigated with the surface force apparatus. At large separations, a long-range, weak attraction is observed. The interaction turns into repulsion as opposite multilayers overlap when the surface separation is reduced. The initially observed exponential force-distance law is ultimately overcome by a steep steric interaction at full compression of the films. The evolution of the force profiles as a function of time and upon subsequent unload-load sequences is monitored. The origin of the different interaction regimes arises from osmotic pressure and the formation of complexes between the oppositely charged chains.
Introduction The alternate deposition of polycations and polyanions on a charged surface leads to the buildup of films generally called polyelectrolyte multilayers.1,2 These films have received considerable attention during the past decade because of their potential applications in various fields, ranging from electroluminescent diodes to biomaterial coatings.3-11 Two major film buildup mechanisms have been reported on the basis of the ability of at least one of the polyelectrolytes to diffuse within the multilayer. Historically, the first reported multilayers correspond to systems where the polyelectrolytes from the solution interact only with the one or two latest deposited layers on the film. Polystyrene sulfonate/poly(allylamine hydrochloride) multilayers constitute one of the most prominent examples of such systems.12 The mass and thickness of these films usually grow linearly with the number of deposited bilayers, and the films present a layered nanostructure.13-15 More recently, films whose * To whom correspondence should be addressed E-mail:
[email protected]. † Institut National de la Sante ´ et de la Recherche Me´dicale, Unite´ 595, Centre de Recherche Odontologique, Universite´ Louis Pasteur, 11 Rue Humann, 67085 Strasbourg Cedex, France. ‡ Institut Charles Sadron. (1) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (2) Decher, G. Science 1997, 277, 1232-1237. (3) Chluba, J.; Voegel, J.-C.; Decher, G.; Erbacher, P.; Schaaf, P.; Ogier, J. Biomacromolecules 2001, 2, 800. (4) Boura, C.; Menu, P.; Payan, E.; Picart, C.; Voegel, J. C.; Muller, S.; Stoltz, J. F. Biomaterials 2003, 24, 3521. (5) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (6) Ladam, G.; Schaaf, P.; Decher, G.; Voegel, J.-C.; Cuisinier, F. J. Biomol. Eng. 2002, 19, 273. (7) Kakkassery, J. J.; Fermin, D. J.; Girault, H. H. Chem. Commun. 2002, 123, 1240. (8) Lukkari, J.; Salomaki, M.; Viinikanoja, A.; Aaritalo, T.; Paukkunen, J.; Kocharova, N.; Kankare, J. J. Am. Chem. Soc. 2001, 123, 6083. (9) Barker, S. L.; Ross, D.; Tarlov, M. J.; Gaitan, M.; Locascio, L. E. Anal. Chem. 2000, 72, 5925. (10) Barker, S. L.; Tarlov, M. J.; Canavan, H.; Hickman, J. J.; Locascio, L. E. Anal. Chem. 2000, 72, 4899. (11) Sukhorukov, G. B.; Donath, E.; Moya, S.; Susha, A. S.; Voigt, A.; Hartmann, J.; Mohwald, H. J. Microencapsulation 2000, 17, 177. (12) Decher, G. In Comprehensive Supramolecular Chemistry: Templating, Self-Assembly and Self-Organization; Sauvage, J. P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528.
thickness and mass increase exponentially with the number of bilayers were also reported.16,17 Hyaluronic acid/poly-L-lysine and poly-L-glutamic acid/poly-L-lysine (PLL/PGA) constitute two examples.18,19 It was shown that in both examples at least one of the polyelectrolytes diffuses “in” and “out” of the whole film during each bilayer deposition step. To describe this buildup mechanism, let us assume that both the polycation and polyanion diffuse “in” and “out” of the multilayer, as it is the case for the system PLL/ PGA.19 Let us assume that we are at a buildup step where the film was in contact with a PGA solution that was rinsed with buffer. At this stage of the construction, the film contains strongly bound PLL and PGA chains and also PGA chains that are weakly bound to the film network. We will call them “free” PGA chains. When this film is now brought in contact with the polycation solution, the PLL chains first directly interact with the outer polyanion layer. This leads to a charge reversal with the outer surface of the film becoming positively charged. This outer surface, thus, also becomes attractive for the “free” polyanion chains within the film that then diffuse “out” of the multilayer. As soon as they reach the outer part of the film, they are complexed by the polycations from the solution. These polyanion/polycation complexes can stick to the surface and form the new outer layer of the film. When all the “free” polyanion chains have diffused “out” of the multilayer, the polycation chains from the solution diffuse into the film, up to the deposition substrate. These polycations that diffuse in the film will not form strong interactions with the PLL/PGA complexes that constitute the core of the multilayer. They are thus “free” polycations. (13) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422-3426. (14) Ramsden, J. J.; Lvov, Y. M.; Decher, G. Thin Solid Films 1995, 254, 246-251. (15) Picart, C.; Ladam, G.; Senger, B.; Voegel, J.-C.; Schaaf, P.; Cuisinier, F. J. G.; Gergely, C. J. Chem. Phys. 2001, 115, 1086-1094. (16) Ruths, J.; Essler, F.; Decher, G.; Riegler, H. Langmuir 2000, 16, 8871-8878. (17) McAloney, R. A.; Sinyor, M.; Dudnik, V.; Goh, M. C. Langmuir 2001, 17, 6655. (18) Picart, C.; Mutterer, J.; Richert, L.; Luo, Y.; Prestwich, G. D.; Schaaf, P.; Voegel, J. C.; Lavalle, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12531. (19) Lavalle, Ph.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458
10.1021/la035355l CCC: $27.50 © 2004 American Chemical Society Published on Web 12/18/2003
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Figure 1. (A) AFM height mode images in liquid on mica substrates coated with (PLL/PGA)5-PLL multilayers. Maximum Z range is 20 nm. (B) Profilometric section: the horizontal black line corresponds to the zero value in Z, arbitrarily defined by the apparatus.
When the polycation solution is then rinsed with buffer, a portion of the “free” polycations present in the film diffuse “out”, but because of the electrostatic barrier, a fraction of these free chains also remain in the film, which allows for the buildup process to be continued. When the film is then placed in contact with the polyanion solution, a similar process as that just described takes place, the polyanions playing the role that the polycations have played in the previous buildup step and vice versa. The existence of this diffusion mechanism was directly demonstrated by means of fluorescence confocal laser microscopy for the system (hyaluronic acid/PLL).18 It is clear that the structure of a film resulting from such a process is close to that of the complexes formed in solution. Comparing the secondary structures of polypeptide multilayers with their complex analogues in solution demonstrates this feature. A better understanding of the structure of the films resulting from an exponential growth mechanism may be gained by investigating the interactions between two such films. Recent advances in force measurements using the surface force apparatus (SFA) permit such experimental studies where one probes the mechanical response of the films to compression. SFA will allow, in particular, information to be obtained on how far the multilayers extend and what is the nature of the outer layers that are in contact with the solution. Materials and Methods Force-distance profiles were measured using a homemade device based on the initial version of the Tabor-Israelachvili SFA.20 The new version of the apparatus allows enhanced spatial and time resolution and automation, as described in detail elsewhere.21 The instrument allows the force F between two mica surfaces (of mean radius of curvature R) to be measured to within 0.1 µN as a function of the determined surface separation D, which can be measured to a typical accuracy of 0.2 nm, using multiple beam interferometry.22 The normalized force F/R can be detected to within 0.005 mN/m, while the maximum reliably measurable force will depend on the mechanical compressibility of the entire system. Typically, surface deformations occur for applied loads larger than 8-15 mN/m and F/R becomes meaningless as a result of the deformation of the glue beneath the mica sheet; for that reason, data are only reported for smaller loads, where the measured values of F/R correspond to the free energy E per unit area of two equivalent flat surfaces, as given by the Derjaguin approximation (F/R ) 2πE).23 After the assembly of the apparatus and measurement of the contact position in air, the mica surfaces (glued onto curved silica (20) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1978, 74, 975. (21) Ke´kicheff, P.; et al. Manuscript to be submitted for publication. (22) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259. (23) Derjaguin, B. V. Kolloid Z. 1934, 69, 155.
disks of radius ≈2 cm) are transferred from the apparatus into a sealed flow chamber to add the assembly of polyelectrolyte multilayers coating the mica surfaces. The operation of removing and repositioning the silica disks with coated mica back into the SFA yields an uncertainty of about 1 nm in the determination of the absolute zero separation. All operations are carried out in a laminar flow cabinet (class 100) installed in a clean room. The homemade small-volume flow chamber is designed to allow a desired solution to travel as a laminar flow just above the top of the curved mica surfaces in a direction parallel to the long axis of the cylinders. Solutions of alternatively positively and negatively charged polyelectrolytes are run through at a low flow rate (2 mL/min) for 15 min each. The first deposited layer is the cationic PLL because mica is negatively charged in aqueous solution. To prevent the formation of complexes in the bulk volume of the chamber and inlet tubings caused by the encounter of a new solution with the remaining solution of oppositely charged polyelectrolyte, the flow cell is rinsed with a buffer solution for 15 min between each deposition of polyelectrolyte. The method yields homogeneous coverage of the mica surface, as demonstrated by atomic force microscopy (AFM) for films consisting of at least five alternate pairs of cationic PLL and anionic PGA (Figure 1). All solutions are freshly prepared and filtered (0.22 µm) before use. Water is purified by a commercial Milli-Q Gradient system and degassed for 30 min. The MES-Tris buffer solution at pH 7.4 consists of 100 mM NaNO3 (Sigma, purity >99.99%), 25 mM 2-(N-morpholino)ethanesulfonic acid (MES; Sigma, purity >99.5%), and 25 mM Tris(hydroxymethyl)aminomethane (Tris; Sigma, purity >99.9%). Polyelectrolytes, both from Sigma (MWPLL ) 23 400; MWPGA ) 61 000), are dissolved at 0.1 mg/mL in filtered MES-Tris buffer solution. At the pH constant of 7.4, the polyelectrolytes remain ionized (pKPLL ) 9; pKPGA ) 5), which prevents the dramatic collapse or swelling changes in the initial film structure that would otherwise be due to charge variations upon the subsequent deposition of additional layers. All experiments were performed at 25.0 °C with mica surfaces coated with (PLL/PGA)5-PLL films, where 5 stands for the number of deposited layer pairs. AFM images were obtained in the contact mode in a liquid cell with a Nanoscope IV from Digital Instruments. Cantilevers with a spring constant of 0.01 N/m were used. Profilometric section analysis is achieved by cross sectioning the images in the Y orientation, perpendicular to the scanning direction.
Results Force distance profiles were measured directly after the film buildup and monitored over time. The contact position was changed several times to increase the reliability of the results. In particular, the adhesion values, range, and magnitude of the interaction profile are reliably determined only if pristine contact positions are used because the local structure of the multilayers is altered from the measurement as a result of the interpenetration of the coated films. Nevertheless, the occurrence and the qualitative nature of all the observed repulsive and
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Figure 2. Effect of subsequent approaches on the normalized force-distance profile for mica surfaces coated each with a film of (PLL/PGA)5-PLL multilayers. The repulsive force profile arises mainly from the increasing osmotic pressure as a result of the unfavorable entropy associated with compressing (confining) the loops and the tails of the chains between the surfaces when the two films interpenetrate. This exponential repulsion is reduced at large separations, evidencing a deviation due to the presence of a competing attraction. Upon compression (solid symbols), the local structure of the coated films is irreversibly affected, as indicated by both the hysteresis observed upon separation (empty symbols) and the changes in the magnitude and range upon subsequent approaches.
Figure 3. Reversibility of the attractive force-distance profile measured at large separations. Solid symbols correspond to the force on approaching the surfaces, while the open symbols are the data measured on separation. Here, the direction of the surface displacement was reversed before the pure exponential repulsive regime was attained.
attractive regimes appear to retain their features, even if they do evolve over time and upon subsequent approach/ separation cycles performed on the same position. A force-distance profile is composed of four distinct regimes: the first one at short separations (20-50 nm) is a steep regime, called a steric wall, and is marked with “A” in Figure 2. A second regime at intermediate separations, marked “B” in Figure 2, represents the exponentially repulsive part of the force. A third regime, denoted “C” in Figure 2, between 150-200 nm reveals the transition from a repulsive force to an attractive force. At large separations up to 550-600 nm, an attractive well (denoted “D”) reveals a fourth regime (Figure 3). Comparison with the bare mica contact position indicates the thickness of the coating on the surfaces. Indeed, the primary minimum observed for adhesive mica in air or in water can no longer be achieved, this being prevented by a steep repulsive force at small separations (marked
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with “A” in Figure 2). There is a marked change in the repulsive force profile toward this steeper regime at short separations, which may be ascribed as an onset of the steric wall. Monitored over time, for periods of up to 6 days, all values lay between 20 and 50 nm, suggesting a thickness of two juxtaposed single films increasing from about 10-15 nm each (first day) to about 25 nm (second and subsequent days). This value represents the thickness of the compressed film, the “solid” -like core of the multilayers. The complete thickness of the film could be estimated as 1/2 of the distance of the onset of the repulsive force. From Figure 2, for that particular measurement one estimates a value for the thickness of the film of 100 nm. At the beginning of the experiment, this value was 70 nm, and in subsequent days, it increased up to 120150 nm. Upon unloading, the surfaces separate themselves, first in small irregular outward jumps before the pull-off force is sufficient to wrench the surfaces apart (4-7 mN/m). Remarkably, the measured adhesion, which remains almost unchanged over a week-long period, does not present a strong dependence on the resting time at contact under an applied load (up to a few hours) nor on the number of cycles (load-unload) previously performed at the same contact position. Only the position from which the surfaces start to separate is shifted. Together with the doubling of the onset of the steric wall over the first day, which indicates the thickness of the core of the film, both features indicate a swelling-like evolution of the coated film. The increase of the film thickness is consistent with AFM imaging of the coated surfaces. When observed at room temperature in a liquid cell with an AFM microscope in the contact mode, the (PLL/PGA)5-PLL multilayers appear dense and homogeneous, with an almost complete coverage of the mica surface (Figure 1). The thickness of the multilayer-coated mica film has been determined directly by scratching the surface with the cantilever. Immediately after the multilayers build up, the thickness is about 5-8 nm. After 1 day, it has almost doubled and it will ultimately remain constant at about 14-15 nm when measured a week later. At the film surface, “clumps” were observed. In all the samples, at least 30 bumps could be counted at any time per 5 × 5 µm2. These clumps have an average lateral size of 100-150 nm and a height of 15 nm sticking out of the multilayer surface. No visible rearrangement in the morphology of the layers can be detected over a week using this technique, despite a doubling in the film thickness over the first 24 h. Beyond the steep repulsion observed at very short separations due to the steric walls, the interaction decreases steadily as the separation between the mica surfaces increases (marked with a “B” in Figure 2). At these intermediate surface separations, the interaction is exponentially repulsive, as can be observed from the semilogarithmic scale of Figure 2. An electrical double layer cannot be attributed to the origin of the repulsion for several reasons. First, the measured decay lengths lie in the 20-30 nm range, an order of magnitude larger than the Debye screening length expected at the ionic strength of the electrolyte solution. Second, reversible force profiles are not observed upon compression and on separation. Subsequent load-unloadreload sequences on the same contact area yield increasingly smaller repulsive ranges with weaker compliances. Even long annealing times (days) failed to fully recover the pristine situation. This indicates that the local structure of the multilayers adsorbed on both mica surfaces is irreversibly affected once interpenetrated. The extension to which the chains and tails protrude into the
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solution can be inferred from load-unload sequences where the minimum separation distance for the approaching surfaces is well beyond the thickness of the fully compressed films. Figure 3 shows that down to a certain separation such force cycles remain reversible, suggesting that the opposite chains do not interact, while for compression/separation cycles with increasingly smaller turning points in separation, the force-distance profile becomes more and more irreversible. A threshold separation of about 150 nm can be assumed from the observation of increasing hysteresis in the force profile. This corresponds to a 70 nm range of pending chains protruding from each multilayered film. From all these observations, the long-range, exponentially decaying repulsive force encountered at intermediate separations is, thus, attributed to steric rather than to strictly electrostatic interactions of the polyelectrolyte entangling chains of the interpenetrating multilayers. Deviation from this pure exponential repulsion eventually occurs at larger separations, indicating the presence of a competing attractive interaction (marked with “C” in Figure 2). Remarkably, this long-range attraction, which spans over about 300 nm along a shallow well, is fully reversible as the surfaces approach or separate (Figure 3). Nevertheless, if the interpenetrating films have been disrupted upon subsequent unload-load cycles the position of the well but not its shape (i.e., the range of the attraction is unchanged) moves concomitantly with the decreasing range of the repulsion observed at shorter separations. This feature is consistent with the idea that at large separations the interaction is composed of two competing repulsive and attractive components. No major change is observed in the range of the attraction as time passes except that the well deepens from a maximum depth of about -0.04 mN/m (first day after the films build up) to -0.15 mN/m (second and subsequent days). Discussion The SFA allows the direct measurement of the interaction between two surfaces immersed in a solution. Using our procedure of coating mica substrates with multilayers of alternate polyelectrolytes, a macroscopic surface, smooth at the nanometer scale, is obtained. Two main properties are obtained from a force-distance profile, the long-range interaction and the adhesion at contact between the films. Both features appear to be related to the local structure of the multilayers and to its evolution over time and with mechanical constraints. Films composed of alternate multilayers of PLL and PGA are known to grow exponentially. The exponentially growing mechanism is related to the “in” and “out” diffusion of at least one of the polyelectrolytes through the whole film during the buildup of each bilayer, as summarized in the Introduction. In particular, Lavalle et al. have shown that PLL chains diffuse through the (PLL/ PGA)n films.19 The swelling of the film observed over the first 24 h after buildup suggests a slow rearrangement of the structure as a result of the diffusion of polyelectrolyte chains. The film can be imagined as composed of three parts: close to the deposition surface, a dense, complexed “solid” -like film of up to 15 nm thickness; then a more loose part forms the “brush” with loose tails and loops consisted essentially of polycations, because PLL was the last deposited layer; and finally, the sparsely distributed “clumps” lying on the most outer part of the film comprises both negatively and positively charged polyelectrolytes, as they are due to diffusion of both types of macromolecules.
Here, the ionic strength is too high to invoke electrostatics as the origin of the observed long-range repulsion. Rather, an osmotic origin is proposed. The repulsive forces between the two opposite approaching multilayers are due to the unfavorable entropy associated with compressing (confining) the loops and the tails of the chains between the surfaces. Force profiles having the same origin have been measured between two surfaces covered with polymers in a good solvent24-26 and with adsorbed polyelectrolytes.27-32 Theories of steric interactions in good solvents are rather complex33,34 because these forces depend on the coverage of polymer on each surface, on whether the polymer is weakly or strongly attached to the surfaces, and on the structure of the adsorbed polymer chains. The observed weaker and shorter ranged repulsive forces following the first approach/retraction cycle could be due to irreversible chain unfolding as well. All mean field and scaling theories developed in recent years33-37 predict an exponential law, with a decay length related to the thickness of the adsorbed layer. The expressions of the prefactor terms (amplitude) and of the relations between the decay lengths and the layer thickness are slightly different. However, in the case of high surface coverage with grafted polymer, the Alexander-de Gennes theory34,35 predicts a force law with λ decay length expressed as
F(D)/R ≈ 200πkBTλΓ3/2 exp(-D/λ)
(1)
where Γ is the number of adsorbed segments per unit area and L ) πλ is the “brush” thickness. Another variant of eq 1, still valid for the Alexander-de Gennes model, is given by38
F(D) 16πkBTL 2L ) 7 R D 35s3
5/4
[( )
+5
(2LD )
7/4
- 12
]
(2)
where s is the average distance between two anchoring points. Note that power laws have been used to describe the interaction between adsorbed polyelectrolytes at a low ionic strength.31,38 This latter expression fits better our measurements at large separations but is less good at shorter ones, as illustrated in Figure 4. However, the model of Alexander-de Gennes should not be used at high applied loads (short separations), that is, when loops are strongly interpenetrating. The exponentials were fitted only on the pure repulsive part and not on the regime where the repulsive force competes with the attractive (24) Luckham, P. F.; Klein, J. Macromolecules 1985, 18, 721. (25) Luckham, P. F.; Klein, J. J. Colloid Interface Sci. 1987, 117, 149. (26) Tauton, H. J.; Toprakcioglu, C.; Fetters, L. J.; Klein, J. Macromolecules 1990, 23, 571. (27) Luckham, P. F.; Klein, J. J. Chem. Soc., Faraday Trans. 1 1984, 80, 865. (28) Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; MacNaughtan, W.; Chapman, D. Colloids Surf. 1987, 25, 263. (29) Kamiyama, Y.; Israelachvili, J. Macromolecules 1992, 25, 5081. (30) Kjellin, U. R. M.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci. 1997, 190, 476. (31) Anthony, O.; Marques, C. M.; Richetti, P. Langmuir 1998, 14, 6086. (32) Lowack, K.; Helm, C. A. Macromolecules 1998, 31, 823 (33) Scheutjens, J. M. H. M.; Fleer, G. J. Adv. Colloid Interface Sci. 1982, 16, 1882. (34) de Gennes, P.-G. C. R. Acad. Sci. (Paris) 1985, 300, 839. (35) de Gennes, P.-G. Adv. Colloid Interface Sci. 1987, 27, 189. (36) Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988, 21, 2610. (37) Semenov, A. N.; Bonnet-Avalos, J.; Johner, A.; Joanny, J.-F. Macromolecules 1997, 30, 1479. (38) Robelin, C. Ph.D. Thesis, Universite´ Paris VI, Paris, France, 2000.
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Figure 4. Exponential (eq 1; dashed lines) and Alexander-de Gennes (eq 2; continuous lines) fits on the two force-distance profiles shown in Figure 2 (first approach, squares; third approach, circles). The fitting parameters have an error of about 1% for the exponential law for both sets of force profiles, whereas in the case of the Alexander-de Gennes fit, the error amounts to about 12% for the “brush” length (102 ( 13 nm at the first approach reduced to 90 ( 11 nm at the third one) and to about 35% for the prefactor of eq 2, which gives an average distance between two extended tails or loops on the surface of about 10 nm.
force (from the shortest separations up to 170 nm for the squares and up to 140 nm for the circles). The difference between the overall force-distance profiles, measured on approach and on separation of the surfaces, indicates that compression perturbs the structure of the coated layers and that a constant surface coverage might not be retained during measurements. The increasing hysteresis, observed upon load-unload sequences as films increasingly interpenetrate, suggests that the forced mechanical encounter of polycation and polyanion chains yields to the formation of complexes within the films. Therefore, in addition to bridging effects, the increasing density of complexes formed between oppositely charged chains is a likely explanation to the adhesion behavior observed at short separations when the films are fully compressed. Similarly, bridging between loops and tails of the loose part of the films appears to be a most unlikely explanation for the observed attraction at large separations. First, it is too long-range for chains to be linked at such extensions. Second, the attraction is fully reversible on approach and on separation of the surfaces. Depletion mechanisms may also be ruled out, even if some polyelectrolyte chains diffuse out from the multilayers: the attractive well, although shallow, is too deep to result from such a little bulk density
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of the coils. The long-range nature of the attraction rules out an electrostatic origin due to surface charge fluctuations39 because it would be rapidly screened by the high ionic strength of the electrolyte solution. The origin of the attraction at large separations is, thus, thought to arise for other reasons, which are related to the formation of complexes. The existence of these complexes is inherent in the (PLL/PGA)n films (even in the absence of a mechanical applied load) because polyelectrolyte chains diffuse in and out through the film during its multilayer buildup. On average, there are 500-1000 clumps per contact area in the SFA. Therefore, they will reduce the orientational fluctuations of the neighboring chains. In addition, when confined between rigid surfaces, some of the orientational fluctuations are further restricted. The reduction of fluctuations near the rigid walls cause an enhancement of the order parameter, which may induce the attractive background according to the theory of Marcˇelja and Radic´.40 As the separation between the walls decreases, the number of prohibited fluctuation modes increases and a depletion-type attraction results. Quantitative resolution of this question requires further investigations. Film extension, as observed with a SFA, can be defined as 1/2 the separation distance of the onset of the repulsive forces. This is on the order of 120-150 nm 1 week after the film buildup, while by AFM, it appears to be 15 nm under the same conditions. This discrepancy can be explained as follows: with an AFM, the film is imaged by the deflection of a cantilever with an applied force of about 0.5 nN on the sample. This corresponds to a normalized force/radius value of 10 mN/m because the tip has a radius of curvature of about 50 nm. Such a value is measured close to the solid limit in the SFA experiments, where the separation distance is of 20-30 nm. The AFM and SFA experiments are, thus, consistent in this respect. This illustration, however, shows that only the “solid” morphology of the film is imaged with an AFM microscope and that any “looser” parts of the film would be out of reach by this technique. Moreover, AFM measurements may compress the clumps as they do with the whole film, leading to a decreased apparent size. Acknowledgment. This work was supported by CNRS and INSERM and by a Marie Curie Individual Fellowship. Jean Iss is gratefully acknowledged for technical help in the SFA measurements. LA035355L (39) Ke´kicheff, P.; Spalla, O. Phys. Rev. Lett. 1995, 75, 1851. (40) Marcˇelja, S.; Radic´, N. Chem. Phys. Lett. 1976, 42, 129.
