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Formation, Chemical Composition, and Structure of Polyelectrolyte-Nanoparticle Multilayer Films Therese Sennerfors,*,† Goran Bogdanovic,† and Fredrik Tiberg†,‡ Institute for Surface Chemistry, Box 5607, 114 86 Stockholm, Sweden, and University of Oxford, Physical & Theoretical Chemistry Laboratory, Oxford OX1 3QZ, United Kingdom Received February 25, 2002. In Final Form: May 13, 2002 The buildup and structure of multilayer films containing cationic polyelectrolyte and silica nanoparticles have been studied by means of ellipsometry, atomic force microscopy, and X-ray photoelectron spectroscopy. Emphasis was placed on the effect of ionic strength on the adsorption behavior and structure formation. Consecutive exposure of a silica substrate to low ionic strength solutions containing polyelectrolyte and nanoparticles resulted in the formation of stable adsorbed films with a reproducible stratified multilayer structure. The films formed in high ionic strength solutions were initially much thicker but also clearly less stable. A significant desorption was observed to take place in conjunction with the second exposure to silica nanoparticles. The effect of electrolyte concentration is discussed in terms of a salt-induced glassliquid transition, above which the relaxation rate in the adsorbed composite film increases and thereby hinders the formation of multilayer structures.
Introduction Multilayered films are attractive in a wide range of applications, including sensors,1,2 electronic and optical applications,3-6 and surface coatings.7,8 Several different techniques have been developed for multilayer film formation. One often-used technique is layer-by-layer (LBL) self-assembly, first described by Decher and coworkers.9-13 Surface-deposited multilayer films with stratified compositions are produced through sequential adsorption of oppositely charged components with intermediate rinsing steps. A distinct advantage of this method to Langmuir-Blodgett (LB) deposition and spin-coating techniques is that it is not limited to flat substrates. Multilayer film formation has been studied for a range of adsorbate systems, including combinations of cationic and anionic polyelectrolytes14-17 and combinations of polyelectrolytes and oppositely charged particles,18-21 clays, or other inorganic materials.22-25 * Corresponding author. E-mail: therese.sennerfors@surfchem. kth.se. † Institute for Surface Chemistry. ‡ University of Oxford. (1) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422-3426. (2) Harris, J. J.; Bruening, M. L. Langmuir 2000, 16, 2006-2013. (3) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (4) Schlenoff, J. B.; Laurent, D.; Ly, H.; Stepp, J. Adv. Mater. 1998, 10, 347-349. (5) Balasubramanian, S.; Wang, X.; Wang, H.; Yang, K.; Kumar, J.; Tripathy, S.; Li, L. Chem. Mater. 1998, 10, 1554-1560. (6) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224-2231. (7) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (8) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chsndross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; TRabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950. (9) Decher, G.; Hong, J. D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430-1434. (10) Decher, G.; Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327. (11) Decher, G. Science 1997, 277, 1232-1237. (12) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831-835. (13) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772-777.
Substrates range from flat surfaces to micron-sized particles. Caruso and co-workers have, for instance, used the sequential adsorption approach to produce multilayer films at surfaces of micron-sized particles, generating composite core-shell structures.26,27 These structures can, through further processing, be transformed to so-called hollow spheres.28-30 Sequential adsorption processes can consequently be used to coat various types of solid supports with functional multilayer structures. The properties of such a film can relatively easily be tailored for specific technical applications by varying the chemical and morphological nature of the different adsorbates. The main problem of using the sequential adsorption approach is that the structures formed usually are metastable at some film buildup stages and that we have little knowledge about the dynamics of the film relaxation during these stages. When films are (14) Lo¨sche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G.; Kjaer, K. Macromolecules 1998, 31, 8893-8906. (15) Lvov, Y.; Ariga, K.; Onda, M.; Ichinose, I.; Kunitake, T. Colloids Surf. A 1999, 146, 337-346. (16) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 81538160. (17) Pardo-Yissar, V.; Katz, E.; Lioubashevski, O.; Willner, I. Langmuir 2001, 17, 1110-1118. (18) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101-1110. (19) Kovtyukhova, N.; Ollivier, P. J.; Chizhik, S.; Dubravin, A.; Buzaneva, E.; Gorchinskiy, A.; Marchenko, A.; Smirnova, N. Thin Solid Films 1999, 337, 166-170. (20) Lvov, Y. M.; Rusling, J. F.; Thomsen, D. L.; Papadimitrakopoulos, F.; Kawakami, T.; Kunitake, T. Chem. Commun. 1998, 1229-1230. (21) Chen, K. M.; Jiang, X. P.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825-7834. (22) van Duffel, B.; Schoonheydt, R. A.; Grim, C. P. M.; De Schryver, F. C. Langmuir 1999, 15, 7520-7529. (23) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038-3044. (24) Ichinose, I.; Tagawa, H.; Mizuki, S.; Lvov, Y.; Kunitake, T. Langmuir 1998, 14, 187-192. (25) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941-3949. (26) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mohwald, H. J. Am. Chem. Soc. 1998, 120, 8523-8524. (27) Caruso, F.; Mohwald, H. Langmuir 1999, 15, 8276-8281. (28) Caruso, F.; Caruso, R. A.; Mohwald, H. Chem. Mater. 1999, 11, 3309-3314. (29) Caruso, F.; Schuler, C.; Kurth, D. G. Chem. Mater. 1999, 11, 3394-3399. (30) Caruso, F. Chem.sEur. J. 2000, 6, 413-419.
10.1021/la020204o CCC: $22.00 © 2002 American Chemical Society Published on Web 07/09/2002
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produced using oppositely charged components, each exposure generally results in the formation of a new adsorbed layer at the surface and in reversal of the surface charge, making it possible for a new layer to adsorb. However, interfacial mixing can inhibit the sequential reversing of the surface charge and, consequently, the buildup of a stratified multilayer structure. Kovacevic and co-workers have in a recent study come to the conclusion that the buildup of kinetically stable multilayer structures is facilitated by the components being in a glassy immobile state.31 Their investigation featured a system of two oppositely charged polyelectrolytes, which through a sequential adsorption from low ionic strength solutions generated stable multilayer films, whereas unstable films were produced at ionic strengths above a so-called glass transition ionic strength. In this work, we investigate the multilayer film formation process during sequential adsorption of cationic polymer and anionic silica nanoparticles onto a silica substrate. Special emphasis was put on the influence of added electrolyte. The dynamics of the film buildup was followed in situ by means of ellipsometry. These studies of the buildup of the films in the solution state were combined with measurements of the film structure and composition in the dry state. The chemical composition and structure of the different layers of the dried films were studied using X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM).
Langmuir, Vol. 18, No. 16, 2002 6411 (df) and the refractive index (nf) of the adsorbed film. The variables nf and df were thereafter used to calculate the adsorbed amount (Γ) according to de Feijter’s formula.33
Γ)
nf - n0 df dn dc
Materials. The cationic polyelectrolyte used in this study is a copolymer of acrylamide (AM) and (3-methacrylamido propyl)trimethylammonium chloride (MAPTAC). This polyelectrolyte has a charge density of 4.2 mol % and a mean molecular weight, Mw, of 105 g mol-1. To remove impurities in the sample, the polymer was dissolved in double-distilled deionized water (Milli-Q Plus grade), filtered through a 0.2 µm membrane, and finally freeze-dried prior to use. The silica nanoparticles, kindly provided by EKA Chemicals, have a specific surface area of 223 m2/g, giving a calculated mean diameter of 12 nm. To minimize the effect of impurities, the particles were dialyzed for 1 week. All polymer and particle solutions were freshly prepared in KCl solution, and the pH was set to 5.6 before each measurement. All water used in the measurements was of Milli-Q Plus grade. The chemicals KCl and HCl were of analytic grade from Merck and used without further purification. The polyelectrolyte and nanoparticles were adsorbed onto polished Si/SiO2 wafers with a mean SiO2 thickness of approximately 300 Å. The wafers were cleaned in a two-step process, as described elsewhere.32 The wafers were dried in vacuum and treated in a plasma cleaner prior to each ellipsometry measurement. Methods. Ellipsometry. In situ null ellipsometry was used to study the buildup of the polyelectrolyte-nanoparticle multilayers. The instrument used was a Rudolf Research thin film ellipsometer, type 43603-200E, equipped with high-precision step motors. A xenon arc lamp was used as the light source. The measurements were performed at a wavelength of 4015 Å, with an angle of incidence of Φ ≈ 67.7°. Prior to every experiment, a four-zone measurement was performed in air and in the electrolyte solution used, to determine the complex refractive index of the bulk Si and the thickness and refractive index of the SiO2 layer. All adsorption measurements were conducted under continuous stirring at 300 rpm, in a thermostated cuvette at 25.0 °C. The polymer and nanoparticles were in sequence (with intermediate rinsing) injected in the cuvette, and the ellipsometric angles Ψ and ∆ were measured, giving the mean optical thickness
The parameter n0 in this formula is the refractive index of the ambient bulk solution. The refractive index increments, dn/dc, are 0.191 cm3/g for the polyelectrolyte and 0.061 cm3/g for the nanoparticles. The parameters were interpreted by means of an optical fourlayer model assuming planar interfaces and isotropic media. However, smooth layers are not always produced when adsorbing polymers and particles. This must be taken into consideration when evaluating the results. As mentioned above, the components adsorbed in the film do not have similar values of dn/dc. When estimating the surface excess, the additions of polymer are assumed to result in pure polyelectrolyte adsorption at the surface, while the particle additions are assumed to result in layers containing both particles and polyelectrolyte. A “mixed” dn/dc has been used to calculate the surface excess in these layers. The same approach as in ref 34 has been used. The refractive index increments, (dn/dc)mix, for the mixed layers are 0.073 and 0.064 cm3/g for the films produced in 1 and 50 mM KCl solution, respectively. Hence, we can only estimate a value of the surface excess. Another, possibly more precise, way to do this would be to use the average values of the refractive index and thickness of the film (see Figure 1a,b). However, calculating the surface excess in this way is not an easy task as both the volume fraction and the distribution of the individual components in the different layers are unknown. X-ray Photoelectron Spectroscopy. XPS is a surface-sensitive technique. The method was used in order to analyze the surface chemical composition of the film. Dry samples were placed in a high-vacuum chamber and irradiated with X-rays. During this process, photoelectrons are ejected from the surface. These electrons are detected and characterized by their specific kinetic energies. The XPS spectra for the multilayer films were measured using a Kratos AXIS HS X-ray photoelectron spectrometer (Kratos Analytical, Manchester, U.K.). The light source was a monochromator (Al X-ray), operating at 300 W (15 kV/20 mA). The analysis area of the samples was approximately 1 mm2. Detailed spectra for Si 2p, O 1s, N 1s, C 1s, K 2p, and Cl 2p were obtained with a pass energy of 80 eV. The electron takeoff angle was set to 90°, 30°, and 20°, corresponding to an approximate analysis depth of 10, 5, and 3 nm, respectively. To detect elements present in the surface layers, wide spectra were first run. This was followed by detailed spectra for each element. The relative surface composition was obtained from quantification of the detailed spectra. The photoelectron intensity of the N 1s peak was used as a relative measure of the polyelectrolyte content in the films and was measured after each adsorption step. All samples studied were produced in the ellipsometer, and the analysis was performed for different surfaces covered by films containing an increasing number of layers. All samples were stored in salt solution and dried just before the analysis. Atomic Force Microscopy. AFM imaging was performed on samples produced in the ellipsometer. Tapping Mode images were obtained in air using a MultiMode NanoScope IIIa AFM (Digital Instruments, Santa Barbara, CA) and Olympus OMCLAC160TS-W silicon probes. Large salt crystals were present at the surfaces of the films, especially for the films adsorbed at a high ionic strength. The AFM imaging was performed in areas between these salt crystals. Image analysis functions in the NanoScope 4.43r8 software were used. One image each at the 0.5 × 0.5, 2 × 2, and 10 × 10 µm scales were obtained for all surfaces. Infrequent features were excluded in the calculations
(31) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Submitted. (32) Sennerfors, T.; Fro¨berg, J. C.; Tiberg, F. J. Colloid Interface Sci. 2000, 228, 127-134.
(33) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759-1772. (34) Sennerfors, T.; Tiberg, F. J. Colloid Interface Sci. 2001, 238, 129-135.
Experimental Section
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Figure 1. Layer thickness, d (b), and refractive index, n (O), for 30 ppm AM-MAPTAC and 25 ppm silica nanoparticles adsorbed from (a) 1 mM and (b) 50 mM KCl solution at pH 5.6. The sample cell is rinsed with KCl solution for 10 min, at 10 mL/min, between every addition of polyelectrolyte and nanoparticles. The adsorptions denoting (pe) polyelectrolyte and (np) nanoparticles and the rinsing (r) steps are indicated. of the surface roughness, as they pitch the roughness values in a nonrepresentative way.
Results and Discussion Wet Films. Adsorption.The buildup of multilayer films through sequential adsorption of cationic polyelectrolyte (AM-MAPTAC) and silica nanoparticles was monitored in situ using null ellipsometry. The time evolution of the thickness and the refractive index after sequential polyelectrolyte and nanoparticle additions and rinsing cycles is shown in parts a and b of Figure 1 for the 1 and 50 mM KCl solutions, respectively. Parts a and b of Figure 2 display the plateau adsorbed amount and thickness as a function of the number of layers in the films for the same systems. The layers formed after the polyelectrolyte additions are denoted 1, 2, and 3, and the layers formed after the particle additions are denoted 1′, 2′, and 3′. The first adsorption step seen in Figure 1 corresponds to the adsorption of cationic polyelectrolyte at the flat silica substrate. In the 1 mM KCl solution (Figure 1a), this leads to the formation of a 5-10 nm thick layer with a surface excess of about 0.8 mg/m2, which is quite typical for a polyelectrolyte on a low-charged surface. The polyelectrolyte layer is irreversibly adsorbed with respect to rinsing in pure solvent (KCl solution, 10 mL/min for 10
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min), in accordance with previous findings for polyelectrolytes.32,34,35 Subsequent to polyelectrolyte and rinsing phases, SiO2 particles were added and adsorbed onto the polyelectrolyte layer. The sample cell was again rinsed once plateau adsorption conditions had been established. Just as for the polyelectrolyte, no desorption could be detected during the rinsing phase. The processes of adsorbing polyelectrolyte and particles with intermediate rinsing stages were then repeated, resulting in the forming of a stratified multilayer film. It is apparent from the curves in Figures 1a and 2a that sequential adsorption of polyelectrolyte and nanoparticles, with intermediate rinsing steps, results in a successive buildup of a multilayer structure when adsorption is performed from a solution of low electrolyte strength. The polyelectrolyte adsorption steps lead to moderate increases in adsorbed amount of 0.5-1 mg/m2. However, only the first polyelectrolyte addition leads to a measurable increase in the layer thickness of the film. The nanoparticle additions, on the other hand, always result in extensive layer swelling. In Figure 1a, it can be seen that when the particles are added to the solution the refractive index of the film initially decreases before it reaches a plateau value. This local dip may be explained by an initial swelling of the layer as the particles start to adsorb. As more particles adsorb, the density of the film increases and the refractive index of the film stabilizes. As can be seen in Figure 2, the increases in adsorbed mass and thickness tend to be more pronounced for the later particle additions, pointing to the fact that more adsorption sites become available as the multilayer structure grows. The reason for this is not clear but may be due to the polymer chains adsorbing in later stages being less constrained by the underlying silica surface and therefore increasingly available for nanoparticle anchoring. Furthermore, the adsorption of a new layer may affect not only the outermost layer but all underlying layers in the film. The increases in the adsorbed amount and layer thickness (Figures 1b and 2) become more pronounced when the components are adsorbed from a solution of higher electrolyte strength (50 mM KCl). For the case of polyelectrolytes whose adsorption at least partially is driven by nonelectrostatic interactions, the reason for this is well understood and due to the screened interchain repulsion. The increased screening also leads to a reduced electrostatic attraction to the oppositely charged surface, thereby controlling the adsorption. When comparing the adsorption kinetics for the different ionic strengths, it was seen that the initial rate of both polyelectrolyte and nanoparticle adsorption is faster at higher ionic strength, indicating a transport-limited adsorption.36 A second addition of particles to the 50 mM solution results at first in increased surface excess and layer thickness as the particles adsorb onto the composite layer underneath. However, after a short induction time the surface excess starts to decay, while the thickness continues to increase. This observation suggests that polyelectrolyte-particle complexes begin to desorb as more particles are binding to the preadsorbed polyelectrolyte chains in the composite interfacial film. The result is an extended low-density polyelectrolyte-nanoparticle film clinging to the surface. Kovacevic et al.31 recently introduced the concept of a salt-induced glass-liquid transition (35) Bo¨hmer, M. R.; van der Zeeuw, E. A.; Koper, G. J. M. J. Colloid Interface Sci. 1998, 197, 242-250. (36) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 133-145.
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Figure 2. (a) Estimated surface excess, Γ, and (b) layer thickness, d, of the multilayer films as a function of the number of adsorption cycles in 1 mM (b) and 50 mM (O) KCl solution at pH 5.6. The layers formed after the polyelectrolyte additions are denoted by 1, 2, and 3, while 1′, 2′, and 3′ denote the layers formed after the nanoparticle additions. Further details are shown in the caption for Figure 1.
to explain an analogous effect observed during multilayer buildup using oppositely charged polyelectrolytes. They argued that the glassy state was a prerequisite for building multilayer structures, as these films always are inherently unstable at some stages of the repeated addition-rinsing cycles during the deposition procedure. This line of thinking may also be used for explaining the observed behavior for the system in the present study. The occurrence of a glassy state is facilitated by strong attractive interactions between the polymers and particles, resulting in chain and particle mobility barriers. A low background electrolyte strength should therefore amplify the glass-forming tendency of a polyelectrolyte-nanoparticle system dominated by electrostatic attraction between the components. Components in the “frozen” layer structure obtained under these conditions will not be free to mix and equilibrate with those in the solution phase. This makes the sequential buildup of a stable multilayer structure possible in the case of the low ionic strength 1 mM KCl solution. However, the components mix readily above the glass transition ionic strength, which in our study must lie between 1 and 50 mM salt. The liquid state seems to prevail in the interfacial region during sequential deposition from the concentrated 50 mM KCl solution. This results in efficient mixing of nanoparticles and polyelectrolytes in the surface film, so that when the nanoparticle concentration has become high enough polymer-nanoparticle complexes begin to desorb.
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Figure 3. Ratio of the photoelectron intensity (I/I1) of N 1s as a function of the number of adsorption cycles at an analysis depth of (b) 2 nm, ([) 3 nm, and (9) 10 nm for the dry films formed in (a) 1 mM KCl solution and (b) 50 mM KCl solution. Layers are denoted as in Figure 2.
Multilayer films formed through layer-by-layer selfassembly by immersing substrates into solutions containing polymer and particle components often show a linear relation between the measured thickness for the dry film and the number of adsorption cycles. This is especially pronounced after numerous deposition cycles.18,19 Comparison between our results and the results obtained using the alternating immersion method is difficult, since the arrival at steady-state adsorption conditions is not confirmed when using the latter approach. In the present study, the structure adopted by the polyelectrolyte and silica nanoparticles at the interface was investigated further after drying of the films. The properties of these layers will be discussed in the following section. Dry Films. Surface Chemical Analysis. The surface composition of the layers was studied by means of XPS at three analysis depths: 3, 5, and 10 nm. When making compositional analysis of the different layers, the photoelectron intensity of the N 1s peak in the XPS spectra was used to identify the polyelectrolyte in the different layers. The intensity of Cl 2p illustrates the amount of salt present in the films, and as expected, the intensity of Cl 2p is much higher in the film formed at the higher ionic strength. Figure 3 shows the ratio (I/I1) between the photoelectron intensity of N 1s in the individual layers and the intensity in the first layer, as a function of the number of layers in
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the film. As can be seen, the ratio is much lower for the particle layers (1′, 2′, etc.) compared to when the outer layer consists of polyelectrolyte (1, 2, etc.). These results clearly suggest a stratified structure where the latest added component dominates at the surface of the dried film. Almost no variation is seen in the intensity ratios obtained when using different analysis depths for the films deposited from solutions of low electrolyte strength, indicating that the compositions are fairly constant within one surface layer. However, for the stronger 50 mM electrolyte solution we observe a large variation between intensity ratios obtained for different X-ray penetration depths. This suggests an uneven composition of the layers, possibly due to partial mixing of layers. For the 50 mM system, we also note that the first particle addition completely extinguishes the nitrogen signals measured using analysis depths of less than 5 nm. This shows that an almost complete particle coverage is obtained when the silica nanoparticles are first adsorbed from the solution of high ionic strength. Due to the desorption and mixing that evidently takes place in conjunction with the second silica particle addition, an increase in the relative nitrogen signals is seen for layer 2′. The nitrogen intensity ratio for the polyelectrolyte layers, displayed in Figure 3, decreases with increasing number of adsorption cycles, irrespective of ionic strength. This is in agreement with the results obtained for the wet film where layer 1 was observed to be thicker than layers 2 and 3 (Figure 2), where the change in thickness was almost negligible. The smaller ratios seen at the second and third polymer additions are consistent with the adsorbed polymer chains being, to a significant degree, integrated in the particle layers. Hence, it appears that the results from the nitrogen intensity ratios of the dry films and the ellipsometry data on wet structures are supportive and give a similar picture of the structure buildup in these composite films. The decreased nitrogen intensity ratios observed for the later particle additions are also in good accord with the larger increments in surface excess and layer thickness observed in the ellipsometry measurements. Surface Structure. AFM imaging was performed on the dry films in order to investigate the structure of the films and the degree of surface coverage. The imaging was done on different scales ranging from 0.5 × 0.5 to 10 × 10 µm2. Representative images are displayed in Figures 4 and 5. Both the topography and phase mode images are shown. The latter are shown because of their improved contrast. No attempt is made to interpret the phase information. In Figure 4a-c, the particles adsorbed from solutions of low electrolyte strength appear in small aggregates with a typical size of 10-50 nm, most likely surrounded by a matrix of cationic polymer. The particles are firmly attached at the surface as evidenced by the fact that imaging is possible and reproducible. Furthermore, the nanoparticle aggregates are randomly distributed across the whole surface. The in-plane structure seen in this work can be compared with the heterogeneous structures, with large particle domains, observed in studies by other authors of layer-by-layer deposition using similar twocomponent systems.18,19 Further addition of the components in the work reported in refs 18 and 19 resulted in a continued preferential adsorption of the particles onto the particle domains. In contrast to that, our study shows that further addition of particles resulted in the particles filling the gaps in the surface plane (see Figures 4 and 7). As can be seen in Figure 4a,b, the change in surface structure when a layer of polyelectrolyte is adsorbed onto a particle layer is negligible. This is in accordance with
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Figure 4. Atomic force microscopy images of films with the outer layers (a) 1′, (b) 2, and (c) 3 produced at low ionic strength (1 mM KCl solution). Images to the left are in height mode, and images to the right are in phase mode. The gray scale contrast of the left-hand images spans over 130 nm.
Figure 5. Atomic force microscopy image of layer 1′ produced at high ionic strength (50 mM KCl solution). The left image is in height mode, and the image to the right is in phase mode. The gray scale contrast of the left-hand image spans over 130 nm.
the polyelectrolyte forming a thin film covering the particles on the surface. When imaging the surfaces, it was also found that the films containing particles in the
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Figure 6. Surface roughness of the dry films produced in (b) 1 mM KCl solution and (O) 50 mM KCl solution, calculated from topographic analysis of 2 × 2 µm surface images.
Figure 7. Surface coverage of the dry films produced in (white) 1 mM KCl solution and (gray) 50 mM KCl solution, calculated from topographic analysis of 2 × 2 µm surface images.
outer layer (layers 1′ and 2′) were somewhat more difficult to image, possibly due to the lack of polyelectrolyte binding the particles to the surface. Another observation was that the thicker the film, the more difficult it is to image. This points to the fact that thicker films are more compliant. In a comparison of the structures formed in solutions of low and high ionic strength (Figures 4a and 5 for layer 1′), it is clear that complete particle coverage is obtained at high ionic strength whereas a partial coverage is obtained at low ionic strength. This difference may simply be due to screening of the double layer allowing denser packing at the high ionic strength, in agreement with the greater adsorption from the high ionic strength solution. These results also confirm the interpretation of zero nitrogen signal seen in Figure 3b. To quantify the differences between the acquired images, the roughness and particle surface coverage are shown in Figures 6 and 7 for all dried surfaces over 2 × 2 µm scans. The results are in line with the earlier discussions. However, the quantities obtained for layers 2′ and 3 possibly contain errors, stemming from small tip artifacts during scanning. It can be seen that the rootmean-square (rms) roughness increases with the number of deposited particle layers when deposited from both high and low electrolyte bulk concentrations. For low electrolyte concentrations, the particle surface coverage increases with each added particle layer, as mentioned before. The 0.5 × 0.5 µm images of surfaces produced in a 1 mM solution in Figure 4 serve as illustrations for this. Profile analysis of the height images confirms that the regions surrounding the particle aggregates are as smooth as the underlying substrate before and after polymer addition. The polymer additions subsequent to the first particle adsorption step have no significant effects on the rms roughness or the apparent particle coverage, in line with what was found in the study of the wet films using
ellipsometry. The small effects seen are probably related to the earlier-mentioned changes in the improved mechanical integrity of the composite layers obtained when polymer is adsorbed onto the particles in the last step of the film formation process. Conclusion Multilayer films containing cationic polyelectrolyte and anionic silica particles have been studied by means of ellipsometry, AFM, and XPS. It was found that the ionic strength of the solution largely affects the formation, stability, and structure of the multilayer film. Kinetically stable multilayer films were relatively easily produced in a solution of low electrolyte concentration. However, adsorption of the components from a solution of higher electrolyte concentration resulted in an initially thicker but also less stable film. The destabilizing effect of salt is attributed to an electrolyte-induced glass-liquid transition above which the components in the multilayer film are able to equilibrate with their surrounding. The work also showed that the particles, in the form of small surface aggregates, were randomly distributed over the surface. The particle coverage increased as a function of the number of layer depositions when the film was produced in a solution of low ionic strength. Deposition from the higher ionic strength solution resulted, already in conjunction with the first particle addition, in a full surface coverage. Finally, XPS data were used to confirm the stratified nature of the adsorbed multilayers. These data also indicated that the layered structure vanished when the sequential deposition was performed at high ionic strength. Acknowledgment. Dr. Boris Zhmud is gratefully thanked for valuable discussions throughout the work. LA020204O
