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Oct 1, 2010 - This is most evident after the adsorption of the fourth layer, where the adsorption of PSS and PAH is larger in each step when adsorbed ...
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Influence of Salt and Rinsing Protocol on the Structure of PAH/PSS Polyelectrolyte Multilayers Zsombor Feld€ot€o,† Imre Varga,‡ and Eva Blomberg*,†,§ †

Surface & Corrosion Science, Department of Chemistry, School of Chemical Science and Engineering, Royal Institute of Technology, Drottning Kristinas v€ ag 51, SE-100 44 Stockholm, Sweden, ‡Eotvos Lorand University, Institute of Chemistry, H-1518 Budapest, Hungary, and §Institute for Surface Chemistry, Box 5607, SE-114 86 Stockholm, Sweden Received June 9, 2010. Revised Manuscript Received September 2, 2010 A quartz crystal microbalance (QCM) and dual polarization interferometry (DPI) have been utilized to study how the structure of poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) multilayers is affected by the rinsing method (i.e., the termination of polyelectrolyte adsorption). The effect of the type of counterions used in the deposition solution was also investigated, and the polyelectrolyte multilayers were formed in a 0.5 M electrolyte solution (NaCl and KBr). From the measurements, it was observed that thicker layers were obtained when using KBr in the deposition solution than when using NaCl. Three different rinsing protocols have been studied: (i) the same electrolyte solution as used during multilayer formation, (ii) pure water, and (iii) first a salt solution (0.5 M) and then pure water. When the multilayer with PAH as the outermost layer was exposed to pure water, an interesting phenomenon was discovered: a large change in the energy dissipation was measured with the QCM. This could be attributed to the swelling of the layer, and from both QCM and DPI it is obvious that only the outermost PAH layer swells (to a thickness of 25-30 nm) because of a decrease in ionic strength and hence an increase in intra- and interchain repulsion, whereas the underlying layers retain a very rigid and compact structure with a low water content. Interestingly, the outermost PAH layer seems to obtain very similar thicknesses in water independent of the electrolyte used for the multilayer buildup. Another interesting aspect was that the measured thickness with the DPI evaluated by a single-layer model did not correlate with the estimated thickness from the model calculations performed on the QCM-D data. Thus, we applied a two-layer model to evaluate the DPI data and the results were in excellent agreement with the QCM-D results. To our knowledge, this evaluation of DPI data has not been done previously.

Introduction Layer-by-layer assembly of polyelectrolyte multilayers for producing novel surface coatings has become more and more popular ever since the method was introduced by Decher et al. in the early 1990.1,2 The method relies on the self-assembly of oppositely charged polyelectrolytes on a surface, and the growth depends on the respective intrinsic properties of the polyelectrolytes as well as on the experimental conditions. Polyelectrolyte multilayers are very versatile because of the fact that it is possible to create highly tailored polymer thin films with a nearly unlimited range of functional groups incorporated within the structure of the film creating an excellent base for surface modifications.3-6 This has resulted in a large number of possible applications such as chemical sensors,7 photodiodes,8 nonlinear optics,9 optical devices,10 and drug delivery.11 It has been recognized that charge inversion after each adsorption step is a necessity in order to build multilayers.12 The charge

inversion has been measured experimentally and has led to the conclusion that this is one requirement for layer-by-layer buildup using polyelectrolytes.13-15 However, charge reversal requires energy, thus it could be anticipated that the adsorption would stop when the surface charge is neutralized unless nonelectrostatic interactions also are present. Furthermore, it is possible to form multilayers at high salt concentrations where the electrostatic attraction is largely screened by the counterions.16 Thus, there must be contributing driving forces for multilayer buildup other than purely electrostatic forces. For instance, forces of hydrophobic origin17 and hydrogen bonding have been suggested.18 The highest contributing factor for multilayer formation is proposed to be a gain in entropy due to the release of counterions and waters of hydration from the dissolved polyelectrolyte chains (i.e., an ion-exchange process), where polymer-counterion association is replaced by polymer-polymer ion pairs.16,19-21 Model calculations have also indicated that to describe multilayer buildup one has to consider short-range interactions together

*Corresponding author. E-mail: [email protected]. (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) Sukhishvili, S. A. Curr. Opin. Colloid Interface Sci. 2005, 10, 37–44. (4) Cooper, C. L.; Dubin, P. L.; Kayitmazer, A. B.; Turksen, S. Curr. Opin. Colloid Interface Sci. 2005, 10, 52–78. (5) Sch€onhoff, M. Curr. Opin. Colloid Interface Sci. 2003, 8, 86–95. (6) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 430–442. (7) Yang, Y. J.; Jiang, Y. D.; Xu, J. H.; Yu, J. S. Thin Solid Films 2008, 516, 2120–2124. (8) Eckle, M.; Decher, G. Nano Lett. 2001, 1, 45–49. (9) Laschewsky, A.; Mayer, B.; Wischerhoff, E.; Arys, X.; Bertrand, P.; Delcorte, A.; Jonas, A. Thin Solid Films 1996, 285, 334–337. (10) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59–63. (11) Izumrudov, V. A.; Kharlampieva, E.; Sukhishvili, S. A. Biomacromolecules 2005, 6, 1782–1788.

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(12) Steitz, R.; Jaeger, W.; von Klitzing, R. Langmuir 2001, 17, 4471–4474. (13) Ladam, G.; Schaad, P.; Voegel, J. C.; Schaaf, P.; Decher, G.; Cuisinier, F. Langmuir 2000, 16, 1249–1255. (14) Blomberg, E.; Poptoshev, E.; Claesson, P. M.; Caruso, F. Langmuir 2004, 20, 5432–5438. (15) Caruso, F. Chem.-Eur. J. 2000, 6, 413–419. (16) v. Klitzing, R. Phys. Chem. Chem. Phys. 2006, 8, 5012–5033. (17) Kotov, N. A. Nanostruct. Mater. 1999, 12, 789–796. (18) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717–2725. (19) Bucur, C. B.; Sui, Z.; Schlenoff, J. B. J. Am. Chem. Soc. 2006, 128, 13690– 13691. (20) Laugel, N.; Betscha, C.; Winterhalter, M.; Voegel, J. C.; Schaaf, P.; Ball, V. J. Phys. Chem. B 2006, 110, 19443–19449. (21) Bharadwaj, S.; Montazeri, R.; Haynie, D. T. Langmuir 2006, 22, 6093– 6101.

Published on Web 10/01/2010

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with electrostatic interactions.22 Finally, it should also be noted that polyelectrolyte multilayer buildup is not an equilibrium process. It has been shown that the thickness of the deposited polyelectrolyte multilayer is dependent on the ionic strength of the deposition solution.23,24 Increasing electrolyte concentration in the deposition solution results in a more effective screening of the polyelectrolyte charge, which induces coiling of the polyelectrolyte chain and gradually more globular polyelectrolyte conformations. Hence, thicker multilayers are formed with increasing electrolyte concentration because of a larger thickness increment per layer.13 Other factors that influences the buildup of polyelectrolyte multilayers are pH (in the presence of titratable groups on the polyelectrolyte chain),25 the charge density of the polyelectrolytes,26,27 the quality of the solvent,28,29 and the electrolyte type.28,30 The type of salt has been shown to have a significant effect on the structure during the buildup of a layer-by-layer assembly. Salom€aki and co-workers30 refer to the so-called Hoffmeister series, which is a series of anions and cations classified by their ability to precipitate a given protein. The Hoffmeister ions are summarized by Leontidis.31 The anionic part of the series is ClO4- > SCN- > I- > NO3- > Br- > Cl- > CH3COO>HCOO- > F- > OH- > HPO4- > SO42-, where chloride is the median. To the left of chloride the ions are referred to as chaotropic ions. These ions are large with a high polarizability and a weak electric field, and the water of hydration is easily removed. The cosmotropic ions are to the right of chloride and are small with a relatively small polarizability and a high electric field at small distances, and it is harder to remove the water of hydration. The effect of the anions is greater than that of the cations, and it is the chaotropic ions that promote thicker multilayer buildup. For example, it has been found that in the case of poly(styrene sulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMA) the multilayer buildup increases more rapidly in the presence of Br- ions than in the presence of Clions at the same electrolyte concentration.30 This can be explained by a stronger attraction between the larger bromide ion and the polycation (PDADMA), which results in stronger chain coiling and an increase in thickness, comparable to the effect of increasing the ionic strength.16 Evidence that the structure of the multilayers in the first layers differs from that of the last deposited layers has been proposed by Ladam et al.13 They studied the effect of ionic strength (NaCl) on poly(allylamine hydrochloride) (PAH)/poly(styrene sulfonate) (PSS) multilayer systems by using streaming potential measurements and scanning angle reflectometry. This was described by different regions in the multilayer structure, where the first zone (the precursor zone, zone I) is affected by the properties of the surface. The second zone (zone II) forms the core of the multilayer structure. Here the charges are assumed to be compensated for by polyelectrolyte complexes via intrinsic charge compensation. Finally, the outer zone (zone III) is in direct contact with the solution, and the chains have a more looplike conformation. This (22) Shafir, A.; Andelman, D. Eur. Phys. J. E 2006, 19, 155–162. (23) Decher, G.; Schmitt, J. Prog. Colloid Polym. Sci. 1992, 89, 160–164. (24) L€osche, M.; Schmitt, J.; Decher, G.; Bouwman, W. G. Macromolecules 1998, 31, 8893–8906. (25) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213–4219. (26) Schoeler, B.; Kumaraswamy, G.; Caruso, F. Macromolecules 2002, 35, 889– 897. (27) Glinel, K.; Moussa, A.; Jonas, A. M.; Laschewsky, A. Langmuir 2002, 18, 1408–1412. (28) Dubas, S. T.; Schlenoff, J. B. Macromolecules 1999, 32, 8153–8160. (29) Poptoshev, E.; Schoeler, B.; Caruso, F. Langmuir 2004, 20, 829–834. (30) Salomaki, M.; Tervasmaki, P.; Areva, S.; Kankare, J. Langmuir 2004, 20, 3679–3683. (31) Leontidis, E. Curr. Opin. Colloid Interface Sci. 2002, 7, 81–91.

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is the zone in which the excess charge of the last layer is distributed, and the zone model proposes that this zone could be swollen in pure water because of the possible loops and tails extending out into the solution whereas the swelling of zone II would require increasing ionic strength to partially break the bonds between the oppositely charged polyelectrolytes.32 When PAH/PSS multilayers were formed at higher ionic strength and subsequently exposed to pure water, a swelling of the layer was observed and it was suggested that the swelling originated from the outer zone of the multilayer film.13 Swelling in water is an interesting feature that we have studied in great detail in the present study. The aim of our study was to elucidate the structure of PAH/PSS multilayers built on a hydrophilic silica surface. Furthermore, we also sought to examine how the structure and swelling in pure water was influenced by the choice of electrolyte. To study this, we employed a quartz crystal microbalance with dissipation (QCM-D) and a dual polarizing interferometer (DPI) to follow the buildup of PAH/PSS multilayers at high salt concentrations with two salts (NaCl and KBr). The buildup of the multilayer was performed without any drying step between the deposition steps. To envisage the structure of the PAH/PSS multilayers, a two-layer model was used in the analysis of the QCM data and a novel approach to analyzing the DPI data was used. The above-mentioned increase in thickness upon exposure to water will be discussed, as will the effect of the counterions used in the deposition solution.

Materials and Methods Materials. Poly(ethylene imine) (PEI) with a molecular weight of 25 000 g mol-1 was purchased from BASF. Poly(allylamine hydrochloride) (PAH, Mw = 70 000 g mol-1), poly(styrene sulfonate sodium salt) (PSS, Mw = 70 000 g mol-1), and sodium dodecyl sulfate (SDS) with a minimum purity of 98.5% where purchased from Sigma-Aldrich. Sodium chloride and potassium bromide, both puriss, were purchased from Sigma-Aldrich. Deconex 20NS (Bohrer Chemie) was obtained from Fischer Scientific. According to the manufacturer, this detergent is a very alkaline surfactant-free cleaning solution. All chemicals were used as received. Methods. Quartz Crystal Microbalance with Dissipation (QCM-D). To measure the adsorption and viscoelastic properties of the multilayer, a QCM-D from Q-sense (Gothenburg, Sweden) with a fundamental frequency of 4.95 MHz was employed. The instrument is thoroughly described in ref 33. Prior to measuring, the cell is cleaned in Deconex (Bohrer Chemie) for 1 h and then rinsed thoroughly with Milli-Q water and pure ethanol. The tubing is cleaned with 1 cmc of SDS for 1 h and then rinsed thoroughly with Milli-Q water. The temperature of the cell is set to 23 °C, which is controlled by four Peltier elements to an accuracy of (0.02 °C. The measurements were performed in a temperature-controlled environment. The initial information obtained from the measurement is the change in resonance frequency of the oscillator, Δf, and the change in the dissipation factor, ΔD, which is defined as D = Edis/(Est  2π) where Edis is the energy dissipated during one period of oscillation and Est is the energy stored in the oscillating system. The silica surfaces are cleaned by heating in a solution of H2O2 (12%), NH3 (17%), and water (71%) at a temperature of 60 °C and then rinsing in excess Milli-Q water and 99.5% ethanol before drying in a gentle jet of nitrogen. After the cleaning process, the surfaces are hydrophilic. Before the experiment starts, it is imperative to achieve a stable temperature (23 °C) and a baseline for each of the electrolyte (32) Schoeler, B.; Poptoschev, E.; Caruso, F. Macromolecules 2003, 36, 5258–5264. (33) Rodahl, M.; Hook, F.; Krozer, a.; Brzezinski, P.; Kasemo, B. Rev. Sci. Instrum. 1995, 66, 3924–3930.

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solutions applied during the experiment (e.g., water, 0.5 M KBr, or 0.5 M NaCl in the present experiments). The first layer adsorbed before multilayer buildup is PEI,34 which was adsorbed from a solution consisting of 1000 ppm PEI and an electrolyte concentration of 0.5 M. (Depending on the experiment, the electrolyte is either KBr or NaCl.) After the first layer, the buildup starts by alternatively adsorbing PSS and PAH until the desired number of layers is deposited. Each adsorption step is stopped after 20 min. The concentration of both polyelectrolytes is 1000 ppm, and depending on the experiment, the polyelectrolytes are dissolved in either 0.5 M KBr or NaCl. After each adsorption step, a rinsing step follows with either the electrolyte solution or with pure Milli-Q water or sequentially with both. To investigate the effect of ionic strength on the formed multilayer, the concentration of the rinsing electrolyte solution (KBr or NaCl) was increased in a stepwise manner, starting with a 1 mM solution that was followed by the injection of 10 and 100 mM electrolyte solutions. The obtained data is evaluated by using the Sauerbrey equation35 Δm ¼ -

Fq tq Δfn nf0

ð1Þ

where Δm denotes the sensed mass or apparent mass, Fq is the specific density of the quartz crystal (2648 kg/m3), tq is the thickness of the quartz crystal (3.3  10-4 m), n is the overtone number, f0 is the fundamental frequency of the crystal in air (4.95 MHz), and Δfn is the change in resonance frequency for overtone n.36 Equation 1 is a good approximation when changes in the dissipation values are low, less than 10-6/5 Hz for Δf.36,37 However, if eq 1 is used despite the fact that the dissipation value is high then the sensed mass is always underestimated. We also note that the sensed mass includes the material adsorbed to the surface and the solvent associated with it. When the dissipation is well above the given limit for the validity of eq 1, the Voight38,39 model can be employed for the evaluation of the experimental data. The model is a general solution of a wave equation describing the dynamics of two-layer viscoelastic polymer materials of arbitrary thickness deposited on solid quartz surfaces in a fluid environment. The evaluation is done with Q-tools purchased from Q-sense, and we model the data with a two-layer model. This means that we define the outermost polyelectrolyte layer (L2) of the multilayer as one layer and the remaining layers as a single layer (L1). To use the model, the following parameters must be known: the solution density and viscosity, the L1 layer density, the viscosity, the shear and thickness, and the density of the outermost layer. The density of the solution is that of water at room temperature (Table 1). The thickness of the L1 layer is estimated by employing the Sauerbrey equation. The density of all solutions used is measured at 23 °C with a DSA 5000 density and sound velocity analyzer from Anton Paar (Austria). The kinematic viscosity of all solutions used was measured at 23 °C with a Lauda PVS 1 (processor viscosity system) (Dr. R. Wobster, GMBH & Co. KG, Lauda-K€ onigshofen, Germany) using a 0C Ubbelohde dilution viscometer from Scott Instruments GmbH (Mainz, Germany). The absolute viscosity was calculated by multiplying the density by the kinematic viscosity. The capillary constant was calculated from repeated measurements on Milli-Q water at 25 °C using a value of 0.89  10-3 mPa s for the viscosity of water.40 We note that the measured (34) Trybala, A.; Szyk-Warszynska, L.; Warszynski, P. Colloids Surf., A 2009, 343, 127–132. (35) Sauerbrey, G. Z. Phys. 1959, 155, 206–222. (36) Hook, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796–5804. (37) Brewer, S. H.; Glomm, W. R.; Johnson, M. C.; Knag, M. K.; Franzen, S. Langmuir 2005, 21, 9303–9307. (38) Johannsmann, D. Macromol. Chem. Phys. 1999, 200, 501–516. (39) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59, 391–396. (40) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics, 87th ed.; CRC Press: Boca Raton, FL, 2007.

17050 DOI: 10.1021/la102351f

Table 1. Parameters Used in the Voight Calculations of the QCM Dataa parameters to fit

L2 visc (kg/ms) L2 shear (Pa) L2 thick (m)

Min

Max

0.001 1000 1.00  10-10

0.2 1.00  109 1.00  10-5

fixed parmeters Fld dens (kg/m3) 1000 Fld visc (kg/ms) 0.001 1050 L1 dens (kg/m3) L1 visc (kg/ms) 0.02 L1 shear (Pa) 1  109 L1 thick (m) from the Sauerbrey equation 1050 L2 dens (kg/m3) a L1 is the inner layer, and L2 is the outermost layer.

densities and viscosities are all measured in bulk solution, and it was assumed that these values are also good estimates for the film properties. Dual Polarization Interferometry (DPI). To determine the optical thickness and the refractive index of the multilayer buildup, dual polarization interferometry (DPI) (AnaLight Bio200, Farfield Group Ltd., Crewe, U.K.) is used.41 DPI is an evanescent wave technique that is based on the observation of the phase shift of a laser beam traveling through a waveguide. A detailed description of the instrument can be found elsewhere.42 Because the measured phase shift is determined by the polarization of the laser beam (TE or TM) as well as by the adsorption layer formed on the waveguide, the method allows the real-time characterization of the adsorption taking place on the waveguide surface. The waveguide surface is silicon oxynitride, which is similar to the silica surfaces used in the QCM-D measurements. Furthermore, each experiment starts with the deposition of PEI and the template surface becomes a PEI surface, and because PEI usually adsorbs in a flat, thin layer, we assume that that the surfaces between the two techniques provide information on the formation of identical multilayer structures. Before the experiment is started, the flow system is cleaned with 2% surfactant-free deconex followed by 50% ethanol for 1 h and finally with water for approximately 24 h. The chips are also cleaned by washing them for 10 min in a mixture of 1:1 hydrochloric acid (fuming 36%) and methanol, followed by a thorough rinse with Milli-Q water before immersing the surfaces in 2% surfactant-free deconex for 15 min and then finally rinsing and storing them in Milli-Q water for approximately 1 day. This cleaning procedure provides reproducible hydrophilic probe surfaces. The cleaned chips are mounted into the apparatus after drying with a gentle jet of nitrogen gas. All solutions used in the DPI experiments were degassed to prevent the formation of air bubbles in the system. Because only the water-rinsing protocol was measured by DPI, the running electrolyte solution in this case was water. Under the assumption that a homogeneous adsorption layer is formed on the waveguide surface, the measured DPI signals (TE and TM) can be unambiguously resolved into the thickness and refractive index of the immobilized layer. From these data, the adsorbed mass (Γ) can also be determined in principle using the de Feijter equation43 nf - nbulk ð2Þ Γ ¼ df dn=dc (41) Cross, G. H.; Reeves, A.; Brand, S.; Swann, M. J.; Peel, L. L.; Freeman, N. J.; Lu, J. R. J. Phys. D: Appl. Phys. 2004, 37, 74–80. (42) Swann, M. J.; Peel, L. L.; Carrington, S.; Freeman, N. J. Anal. Biochem. 2004, 329, 190–198. (43) de Feijter, J. A.; Benjamins, J.; Veer, F. A. Biopolymers 1978, 17, 1759– 1772.

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Figure 1. Typical example of the changes in frequency and dissipation during multilayer buildup. The adsorption of the polyelectrolytes was done in 0.5 m KBr, and they were subsequently rinsed with pure Milli-Q water. The upper and lower curves correspond to the shifts in frequency and dissipation, respectively. Data from three overtones (normalized by the overtone number) are shown. where df is the film thickness and nf and nbulk are the refractive indices of the film and the bulk solution, respectively, that were determined from the collected DPI signals. However, as indicated by the above equation, the determination of the adsorbed mass requires the refractive index increment (dn/dc) of the adsorbed film. Because this quantity is not known for the PSS/PAH complex film, the DPI data was not used for the calculation of the adsorbed mass. To model the change in the DPI signals (TE and TM) in the case of the formation of various two-layer structures, DPI simulation software provided by Farfield Group Ltd. was used. The software allowed the definition of the structure of the waveguide chip, which was set identical to the structure of the waveguide chips used in the experiments (FB80). Furthermore, the structure of the adsorption layer was defined by giving the number of sublayers (two) on top of the waveguide and setting the thickness and refractive index of each sublayer.

Results As a typical example, the measured changes in frequency and dissipation by employing the QCM-D technique are shown in Figure 1. The upper curves denote the change in frequency, and the lower ones show the measured change in dissipation for three different overtones. The experiment starts by establishing a baseline in water and in the applied salt solutions, and then PEI is adsorbed onto the silica surface. The decrease in frequency clearly shows that adsorption takes place. The adsorption is allowed to proceed for 20 min and is stopped by injecting either 0.5 M salt solutions or pure water, which rinses out the nonadsorbed polyelectrolytes from the measuring chamber. The experiment is continued by alternatively adsorbing PSS and PAH until the desired number of layers has been reached. As indicated in Figure 1, the frequency significantly decreases with each polyelectrolyte injection, demonstrating that multilayer buildup takes place. The dissipation has approximately the same value when polyelectrolyte adsorption is taking place; however, it increases considerably when the system is rinsed with water, which indicates significant changes in the structure of the multilayer. This phenomenon will be discussed further below. Effect of the Rinsing Solution. Polyelectrolyte adsorption was performed in 0.5 M salt in all experiments. However, to investigate the effect of the rinsing procedure of the multilayer buildup, we used three different rinsing protocols between the consecutive polyelectrolyte adsorption steps. In the first case, we Langmuir 2010, 26(22), 17048–17057

Figure 2. 9, b corresponds to the adsorption step from 0.5 M KBr for PSS and PAH, respectively. (, 2 corresponds to the adsorption step from 0.5 M NaCl for PSS and PAH, respectively. The filled star symbol is the first adsorption step with PEI from either NaCl or KBr electrolyte solution (0.5 M). The unfilled stars are the corresponding rinse with electrolyte solution. Unfilled symbols represent the rinsing step with the (polyelectrolyte free) electrolyte solution. (a) Show the sensed mass and (b) show the dissipation. The inset in a) is the D/F plot where b is the adsorption of PAH and 9 is the adsorption of PSS.

used the same salt solution to terminate the polyelectrolyte adsorption as that used during the adsorption step (i.e., 0.5 M). In the second case, rinsing the multilayer with pure water terminated the polyelectrolyte adsorption. In the third protocol, we first used salt solution (13 min) and then pure water (3 min) to terminate the polyelectrolyte adsorption. The sensed mass calculated by the Sauerbrey equation (eq 1) as a function of the number of layers obtained by using the first rinsing protocol (0.5 M salt solution) is depicted in Figure 2a for both salts (KBr and NaCl). The values of sensed mass were calculated at the end of the adsorption and rinsing periods. As indicated in the Figure, the buildup process is nearly linear in both electrolyte solutions. However, when KBr is used as the electrolyte solution, the multilayer buildup increases faster than in the presence of NaCl. This is most evident after the adsorption of the fourth layer, where the adsorption of PSS and PAH is larger in each step when adsorbed from a KBr solution than when adsorbed from a NaCl solution. Another interesting observation is that when the outermost polyelectrolyte layer is PAH the sensed mass decreases to a much larger extent upon rinsing with the electrolyte solution in the case of KBr than in the case of NaCl. This effect is also clearer after the fourth layer. When the outermost layer is PSS, no significant differences can be observed between the effects of the two electrolytes on rinsing. Finally, from the Figure it is evident that after the rinsing step a larger amount of PSS is adsorbed in each step compared to the amount of PAH adsorbed, independent of the type of salt. DOI: 10.1021/la102351f

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Figure 3. 9, b corresponds to the adsorption step from 0.5 M KBr

for PSS and PAH, respectively. (, 2 corresponds to the adsorption step from 0.5 M NaCl for PSS and PAH, respectively. The filled star symbol is the first adsorption step with PEI from either NaCl or KBr electrolyte solution (0.5 M). The unfilled stars are the corresponding rinse with pure water. Gray symbols represent the rinsing step with the pure Milli-Q water. (a) Show the sensed mass and (b) show the dissipation.

The observed changes in dissipation during adsorption and rinsing are illustrated in Figure 2b. The dissipation values are small during both adsorption and rinsing. However, the dissipation values measured when PAH is the outermost layer are larger than the values measured in the case of PSS. When the multilayer is rinsed with the electrolyte solution, the change in dissipation always decreases, indicating a compaction of the layer structure. Finally, it should be noted that the observed dissipation values are usually slightly higher in KBr than in NaCl. The inset of Figure 2a depicts the dissipation as a function of frequency during the adsorption of both polyelectrolytes. The values are subtracted by the initial values of the frequency and dissipation in the adsorption step. As shown in the Figure, the adsorption of both polyelectrolytes is represented by a constant slope, indicating that the adsorption is not accompanied by structural changes in the multilayer structure. The trend in the inset is the general feature for all QCM-D measurements with both salts. The results obtained by using the second rinsing protocol (water rinse) can be seen in Figure 3a,b. Although several features of the multilayer buildup show similarities to the previous case, the different rinsing protocol also gives rise to major differences. As before, the multilayer buildup is found to be nearly linear in the presence of both salts (Figure 3a). However, in this case it is much more pronounced that the buildup of the multilayer is faster and results in a higher sensed mass when adsorbed from 0.5 M KBr than from NaCl. Another important difference between the rinsing protocols was observed when the outermost layer was 17052 DOI: 10.1021/la102351f

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PAH. In this case the sensed mass slightly increased upon rinsing instead of decreasing as was observed when rinsing with 0.5 M KBr or NaCl. By calculating the sensed mass per deposit layer, it is found that the increase in sensed mass is significantly higher for PAH than for PSS when KBr is used as the electrolyte. In NaCl, the increase in the sensed mass in each step is about the same for PAH and PSS, where the increase in sensed mass for PSS is nearly the same as in KBr. The change in dissipation during the buildup of the multilayers is shown in Figure 3b. From the Figure, it can be seen that the dissipation change is small during the adsorption of the polyelectrolyte layers, indicating a compact layer structure. The most interesting observation is that when the system is rinsed with pure water the dissipation value showed more than an order of magnitude increase when PAH is facing the bulk solution. In contrast, when PSS is facing the bulk solution a slight decrease in the dissipation change is observed. It should be noted that the observed large change in dissipation when PAH is in the outermost layer is independent of the number of bilayers in the multilayer. However, it is somewhat larger when the multilayer is formed from a KBr solution than from a NaCl solution. It is noted that this large increase in dissipation values is not accompanied by a similarly large decrease in frequency (i.e., increase in the sensed mass). This means that in water strong mechanical coupling is established between the surface layer and the bulk because the decrease in ionic strength leads to strong electrostatic repulsive forces within the layer and hence its swelling. To obtain further information about the effect of rinsing, multilayer buildup was also performed by the third rinsing protocol (first rinsing in 0.5 M salt and then in pure water). The results of these measurements are represented in Figure 4a,b. Most of the observed phenomena are similar to those found in previous experiments. The multilayer buildup is nearly linear, and it increases faster in 0.5 M KBr than in 0.5 M NaCl (Figure 4a). When PSS is the outermost layer, both the initial rinse with the salt solution and the second rinse with water give rise to small decreases in the sensed mass and the dissipation (Figure 4b). This is somewhat similar to the previous experiments in which a small desorption of PSS is observed when the layer is rinsed with 0.5 M salt and a larger desorption is observed when the layer is rinsed with pure water. However, when PAH is the outermost layer the multilayer behaves differently in the two applied inert electrolytes. In KBr, the sensed mass slightly decreases when the multilayer is rinsed with a 0.5 M KBr solution and then it increases when the rinsing is continued in water. Although, the increase in water seems to be slightly lower than when the layer is directly rinsed in pure water. Another difference is that the sensed mass per deposited layer is not the same in the two different salts. In KBr, the increase in adsorption is slightly higher for PAH than for PSS and the sensed mass per layer increases with the number of layers. On the contrary, in NaCl the increase in adsorption is higher for PSS and the sensed mass is the same for each deposited layer. The change in dissipation during the rinsing steps shows both similarities and differences compared to the previous experiments (Figure 4b). The dissipation value decreases slightly during the rinsing step with salt, as it does for the first rinsing protocol. When consecutive rinsing is done with water, the dissipation values increase significantly, as in the case of the second rinsing protocol (rinsing directly with water). It should be noted that the dissipation values observed in water are independent of the number of deposited bilayers in the case of KBr; however, the absolute values are 30-35% smaller than when salt rinsing did not precede water rinsing. Langmuir 2010, 26(22), 17048–17057

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Figure 4. 9, b corresponds to the adsorption step from 0.5 M KBr

for PSS and PAH, respectively. (, 2 corresponds to the adsorption step from 0.5 M NaCl for PSS and PAH, respectively. The filled star symbol is the first adsorption step with PEI from either NaCl or KBr electrolyte solution (0.5 M). The unfilled stars are the corresponding rinse with electrolyte solution. Unfilled symbols represent the rinsing step with the corresponding salt. Gray symbols represent the rinsing step with the pure Milli-Q water. (a) Show the sensed mass and (b) show the dissipation.

When the multilayer buildup is performed in 0.5 M NaCl solution, the sensed mass slightly decreases during the rinse with salt solution, but contrary to the KBr case, it further decreases when additional rinsing is done with water. In the meantime, the small initial dissipation value decreases during the rinse with NaCl solution and increases in water, but the increase becomes proportional to the number of deposited bilayers (Figure 4b). Also, the absolute value of the dissipation change during the last rinsing step is much smaller than when the layer was not rinsed with salt solution as the first step. Although the increase in sensed mass per deposited layer is somewhat affected by the rinsing protocol, the sensed mass after each deposition step is nearly the same for all cases (i.e., a similar buildup for all multilayers). This is true for both KBr and NaCl (Figures 2a, 3a, and 4a). The QCM-D measurements provide information about the polyelectrolyte multilayer deposited on a sensor surface by probing its mechanical properties. This in turn gives the total sensed mass, the adsorbed amount including the mass of solvent trapped in the layer, and the viscoelastic properties of the layer. To gain deeper insight into the effect of rinsing on the multilayer buildup, an optical method (dual polarization interferometry, DPI) was also used to monitor the multilayer formation when it was performed by the second rinsing protocol (water rinse). The optical thickness and the refractive index of the multilayer structure were determined by means of a single-layer model (details in the Materials and Methods section), and the results are depicted in Langmuir 2010, 26(22), 17048–17057

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Figure 5. The thickness (a) and refractive index (b) obtained from the DPI as a function of number of layers deposited from a 0.5 M salt solution on a silica surface starting with PEI and continuing by alternatively adsorbing PSS and PAH. 9, b corresponds to the adsorption step from 0.5 M KBr for PSS and PAH, respectively. (, 2 corresponds to the adsorption step from 0.5 M NaCl for PSS and PAH, respectively. The filled star symbol is the first adsorption step with PEI from either NaCl or KBr electrolyte solution (0.5 M). The unfilled stars are the corresponding rinse with pure Milli-Q water. Gray symbols represent the rinsing step with the pure MilliQ water.

Figure 5a,b. The changes in the thickness of the multilayer closely mimic the changes in sensed mass observed by QCM-D when using the same rinsing protocol. The multilayer buildup was found to be linear in the presence of both salts, but in the presence of KBr, the multilayer buildup was faster than in NaCl. Furthermore, when the outermost PSS layer was rinsed with water the measured thickness decreased slightly. However, when PAH was rinsed with water the thickness of the layer increased slightly. Figure 5b shows the refractive index of the multilayer assembly as a function of number of layers. Because the refractive index is proportional to the average density of the deposited layer, it provides some information about the compactness of the formed structure. At the beginning of the multilayer buildup, the layer density is relatively low, as indicated by the low refractive index. With an increasing number of polyelectrolyte layers the refractive index steadily increases and levels off at a rather high value (∼1.50). It is interesting that the refractive index of the polyelectrolyte multilayer seems to become independent of the inert electrolyte used during polyelectrolyte deposition after the formation of the first bilayer. When the multilayer is rinsed with water, the refractive index increases when PSS is the outermost layer but decreases when PAH is deposited at the end, indicating that the layer contains more solvent. It is important to note that the change in refractive index observed upon rinsing decreases as the number of deposited layers increases. DOI: 10.1021/la102351f

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Thickness of the Outermost Layer. Using the measured QCM-D data, we calculated the thickness of the multilayers during the adsorption and rinsing steps for the buildup in both electrolyte solutions by using the Sauerbrey relation when the dissipation value is low (Figure 6a,b). When the dissipation values are very high, the Sauerbrey relation is not valid and therefore the thickness of the outermost PAH layer during rinsing with pure water is evaluated by using the Voight model. The simulations via the Voight model were performed using a two-layer structure, where one layer represents the outermost polyelectrolyte (PAH) layer with low viscosity and the other layer represents a very rigid structure formed by all underlying layers. The obtained layer thicknesses are illustrated in Figure 6a,b for adsorption from 0.5 M NaCl and 0.5 M KBr, respectively. From the Figures, it is clear that the multilayers grow in a linear fashion in both electrolytes. However, as expected by considering the sensed mass, the layer thickness becomes somewhat larger during the buildup in KBr than in NaCl. The outermost PAH layer swells significantly, to a calculated layer thickness of between 25 and 30 nm, when the system is rinsed with pure water. Furthermore, it is observed that the outermost PAH layer slightly increases with the number of layers. Interestingly, the PAH layer seems to obtain a very similar thickness in water independent of the electrolyte used during buildup. To elucidate whether the large amount of swelling in the outermost layer of PAH during the rinse with pure water was an effect of salt ions or structural changes in the layer, 10-layer (PSS is the outermost layer) and 11-layer (PAH is the outermost layer) multilayer structures built up with the second rinsing protocol (water rinse) were rinsed with an electrolyte solution with increasing ionic strength ranging from 1 to 100 mM. Figure 7a,b demonstrates the changes in sensed mass and dissipation, respectively, with increasing electrolyte concentration From the Figure, it can be observed that when PSS is facing the bulk solution and is rinsed with water prior to the addition of salt there is no change in the sensed mass or dissipation, indicating that structural changes do not take place within the layer during the rinsing steps (Figure 7a,b). The same effect is seen for both electrolytes used, KBr and NaCl. However, when PAH is facing the bulk solution a decrease in the sensed mass is observed at the highest electrolyte concentrations (Figure 7a), about 18% for KBr and about 9% for NaCl. Furthermore, a large decrease in the dissipation values for PAH with increasing electrolyte concentration is found for both types of electrolytes. At the highest electrolyte concentration, the dissipation values have decreased to the same value as during adsorption, indicating a compaction of the layer. In Figure 7c,d, the thickness of the outermost PAH layer simulated with the Voight model is shown for KBr and NaCl, respectively. The largest reduction in layer thickness is observed at electrolyte concentrations of 10 and at 100 mM where the layer has become compact, only a few nanometers thick.

Discussion Effect of Rinsing Solution on the Buildup of PAH/PSS Multilayers. The sensed mass as a function of the number of adsorbed layers built with three rinsing protocols is depicted in Figures 2a, 3a, and 4a. As shown in the Figures, the multilayer buildup of PAH/PSS is linear and independent of the applied (44) Hubsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J. C.; Schaaf, P. Langmuir 2004, 20, 1980–1985. (45) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 4458–4465.

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Figure 6. The calculated layer thickness of multilayer build-up where the electrolyte solution is 0.5 M NaCl (a) and 0.5 M KBr (b), respectively.

rinsing method. This observation is in good agreement with previous literature studies13,44-46 and is usually interpreted in terms of the restricted mobility of the polyelectrolyte chains within the multilayer.47 The multilayer buildup is faster in potassium bromide solution than in sodium chloride solution, which is in good agreement with previous results from Salom€aki et al. where the multilayer buildup is discussed in relation to the Hoffmeister series.30 The bromide ion is larger and has a greater polarizability with a weaker electric field, and the hydration shell is more easily removed than for the chloride ion. This means that bromide ions can adopt better to a surrounding medium than can chloride, which in this case is the polyelectrolyte. Therefore, the interaction between the polyion and bromide ions is stronger, which leads to weaker intrachain repulsion and more coiling and thicker layers. Furthermore, in line with this reasoning, conductivity measurements show that larger anions have a stronger attraction to polycations, which results in a larger amount of chain coiling because of decreased intrachain repulsion as evidenced by the reduction in solution viscosity.48 The anions have a much greater effect than the cations because the anions have a much larger difference in polarizability than do typical cations.16,30 Structure of the PAH/PSS Multilayer. Our measurements indicate that the applied rinsing protocols do not have a significant effect on the overall buildup of the multilayer. There is only a few percent difference in the sensed mass of the multilayer. (46) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422–3426. (47) Salomaki, M.; Vinokurov, I. A.; Kankare, J. Langmuir 2005, 21, 11232– 11240. (48) Ghimici, L.; Dragan, S. Colloid Polym. Sci. 2002, 280, 130–134.

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Figure 7. (a) Sensed mass and (b) dissipation as a function of electrolyte concentration. (9, b) Multilayer in KBr for bulk-facing PSS and PAH, respectively. ((, 2) Multilayer in NaCl for bulk-facing PSS and PAH, respectively. (c) Change in thickness of a PAH layer facing the bulk, calculated with the Voight model, when the concentration of KBr is increased. (d) Change in thickness of a PAH layer facing the bulk, calculated with the Voight model, when the concentration of NaCl is increased.

Despite the minor effect on the overall multilayer buildup, the rinsing protocol has a major effect on the properties of the outermost layer. The most notable observation is the large difference in the measured dissipation values during the rinsing of a multilayer terminated by PAH (Figures 2b, 3b, and 4b). This is especially surprising because previous results indicated that the PAH/PSS multilayer is not sensitive to changes in the electrolyte concentration (NaCl) below 2 M.16 Neutron reflectivity measurements showed that the thickness of the multilayer and even its internal structure remain intact when it is exposed to an electrolyte solution. This may at first seem to be in sharp contrast to our findings that imply significant structural differences due to different rinsing protocols. However, it is important to note that the structural differences between the rinsing protocols are observed only when the outermost layer is PAH. This implies that the observed differences in this study are determined by the interfacial properties of the last adsorbed layer. This conclusion is supported by the dissipation versus frequency graphs of the polyelectrolyte adsorption steps that indicated frequency-independent dissipation during adsorption (Figure 2a). Thus, the major channel of energy loss of the oscillating sensor is not the viscous deformation of the multilayer, which should increase with adsorption (decreasing frequency), but the energy dissipation to the environment of the sensor through mechanical coupling of the outermost part of the multilayer to the liquid phase. The latter interaction is independent of the adsorbed amount but strongly depends on the structure of the surface layer (e.g., roughness or the presence of polymer chains protruding into the bulk phase).49 During the adsorption steps, (49) Macakova, L.; Blomberg, E.; Claesson, P. M. Langmuir 2007, 23, 12436– 12444.

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the dissipation is also independent of the number of adsorbed polyelectrolyte layers and the applied rinsing protocol ((1-3)  10-6), hence it can be concluded that the PAH/PSS multilayer has a compact, rigid internal structure and the differences in the observed dissipation are determined by the interfacial properties of the last adsorbed layer. The compact internal structure of the PAH/PSS multilayer indicates a high degree of complexation between the polyelectrolyte chains (intrinsic charge compensation). Earlier studies using infrared spectroscopy have found that PAH/PSS forms multilayers with a very small number of counterions within the multilayer structure, hence yielding a compact internal structure.50 An additional effect that can contribute to the compact internal structure of the PAH/PSS multilayers is the recent observation made by Raman spectroscopy that the amine groups in PAH and the oxygen in the sulfonate groups in PSS are able to form hydrogen bonds.51 This extra interaction may contribute to the exceptional stability of the PAH/PSS interaction sites. This may also be an explanation of the high Young’s moduli (300 MPa) of PAH/PSS capsules measured with atomic force microscopy combined with reflection interference contrast microscopy.52 Despite the compact buildup, there are some differences in the properties of the multilayer structure depending on which polyelectrolyte is adsorbed in the outermost layer. Although dissipation is low during adsorption for both polyelectrolytes, the dissipation value is always somewhat larger in the PAH case (50) Jaber, J. A.; Schlenoff, J. B. Langmuir 2007, 23, 896–901. (51) Estrela-Lopis, I.; Iturri Ramos, J. J.; Donath, E.; Moya, S. E. J. Phys. Chem. B 2010, 114, 84–91. (52) Picart, C.; Senger, B.; Senupta, K.; Dubreuil, F.; Fery, A. Colloids Surf., A 2007, 303, 30.

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(Figures 2b, 3b, and 4b). This indicates that PAH adopts a less dense, thicker adsorption layer. The applied rinsing protocol gives rise to important differences in the properties of the multilayer structure. Using the first rinsing protocol (Figure 2a,b), when the adsorption step is terminated with 0.5 M salt, the dissipation decreases and is accompanied by a slight decrease in the sensed mass, independent of the number of adsorbed layers. It seems that some weakly bound polyelectrolytes are removed. This effect is greatest when the bulk-facing polyelectrolyte is PAH and in KBr. This is in line with our previous statements where adsorption from a 0.5 M KBr solution gives the thickest polyelectrolyte layers and contains the largest amount of hydrated water. Similar decreases in dissipation and sensed mass are observed in the second rinsing protocol (Figure 3a,b) for the polyelectrolyte PSS. The interpretation is similar to that in the previous case. However, PAH in the outermost layer behaves very differently when water is used to terminate the adsorption. The sensed mass increases slightly, and the dissipation increases by an order of magnitude. Clearly, in this case the outermost layer swells because of the decrease in ionic strength and results in higher intra- and interchain repulsion. The large increase in dissipation is present in both types of electrolytes (KBr and NaCl), where the increase is higher in KBr where PAH forms larger coils because of the larger size of the bromide ions.30 Despite the large increase in dissipation, only a slight increase in the sensed mass is observed when the outermost PAH layer is rinsed with water. The reason for this observation is that Sauerbrey’s equation (eq 1) breaks down for noncompact structures, resulting in an underestimation of the sensed mass. When the adsorption is terminated by a salt solution and then rinsing with water, the situation differs from the above case. The salt rinsing step behaves in the same manner as previously. However, the effect of the following water rinse on a bulk-facing PAH layer shows differences (Figure 4a,b). In the case where KBr is used, the sensed mass increases slightly and the dissipation increases as before, although the increase is only 2/3 of that found for the water rinsing protocol. The NaCl case shows a decrease in sensed mass that becomes smaller with increasing layer number. The dissipation seems to increase with increasing layer number. The largest value for the last PAH layer is only 50% of that for the previous case. We suggest that during the salt rinse the less tightly bound chains can desorb. In the NaCl case, where coiling and interdigitation are smaller, the first salt rinse is sufficient to remove most of the dangling chains, which leads to no dissipation increase in the consecutive water rinse. With increasing layer number, the interface could become rougher, which favors the interdigitations of the polyelectrolytes, thus the desorption of the loose chains become less perfect. This could be the reason that the dissipation increases with increasing layer number, but it is still smaller than when only water is used for rinsing. In the KBr case, the coiling and interdigitation are larger and there is no complete desorption of loose chains during the initial rinse with the KBr solution. The interpretations of the QCM-D experiments are in excellent agreement with the DPI results (Figure 5a,b). The refractive index determined from the measured DPI signals by using a single-layer model monotonously increases when either the series of PSS or the PAH-terminated multilayers are considered individually. The increase in the refractive index decreases with increasing layer number and levels off at a value of approximately 1.5, which resembles the refractive index of a pure polymer film.53 This implies that the multilayer has an increasing dense inner part and an outer part that has a higher water content (optically less dense). (53) Picart, C. Curr. Med. Chem. 2008, 15, 685–697.

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With increasing layer number, the optical properties becomes dominated by the compact inner part, resulting in a decreased change in the average refractive index during adsorption and rinsing with pure water. It should also be noted that the PSSterminated multilayers always have a larger average refractive index than the corresponding PAH-terminated ones, indicating a thicker and less dense outer layer in the case of the outermost PAH layers. By calculating the water content in the film using the sensed mass obtained by QCM and the adsorbed amount as well as the refractive index determined by DPI, we report that it is evident that the PAH/PSS multilayer film adopts a very compact, rigid structure with a small amount of water in the film (around 20-30%) for both KBr and NaCl. This is in line with previous studies where a water content of 30% has been obtained for PAH/ PSS multilayers in NaCl.53,54 To confirm the plausibility of the conclusions made above, model calculations were made for the QCM-D as well as for the DPI results. The model in both cases divides the multilayer into two parts. The first (inner) layer consists of all but the last polyelectrolyte layer. It is assumed to have a rigid, dense (low water content) structure that is insensitive to changes in the liquid phase. The second (outer) layer consists of the last bulk-facing polyelectrolyte layer that is sensitive to changes in the liquid phase. This structure is consistent with our results and literature results.13,55-57 The inner layer density is set to be high, the reason being that during buildup the internal structure is rigid and compact. Furthermore, the parameters (Table 1) that will be fitted to the data and the given parameters are chosen to be as wide as possible to ensure that all possibilities are taken into account. The results are depicted in Figures 6a,b and 7c,d, respectively. The calculated thickness is fairly independent (the difference is 5 nm between the lowest and highest calculated thicknesses) of the layer number, indicating that the swelling is of the same order of magnitude regardless of the thickness of the inner layer. Furthermore, when the salt concentration increases, the thickness of the bulk-facing PAH layer will start to decrease, as can be observed in Figure 7c,d. The increase in thickness obtained by DPI upon rinsing with water (Figure 5a) is significantly smaller than the thickness provided by the QCM-D model calculations. Because DPI evaluations are based on a single-layer model, we also performed more advanced calculations to check the consistency of the results of the QCM-D model calculations with the DPI data. In this case, the assumed two-layer structure involves four parameters (thickness and refractive index for the inner and outer sublayers) and DPI provides only two signals, thus a unique layer structure cannot be calculated. To overcome this problem, the effect of the swelling of the outer layer on the DPI signal is calculated and compared to the measured signals. It was assumed in the model calculations that in the initial state (before the water rinse) both the internal layer and the outer PAH layer are in a compact state having a high refractive index (1.5). The thickness of the outer sublayer was assumed to be small in its compact state (3 nm). Finally, it was assumed that the swelling of the outer PAH layer takes place at a constant adsorbed amount and the resulting DPI signal was calculated for a series of swollen PAH layers (PAH layer thickness). The calculated values are depicted in Figure 8. (54) Sch€onhoff, M.; Ball, V.; Bausch, A. R.; Dejugnat, C.; Delorme, N.; Glinel, K.; v. Klitzing, R.; Steitz, R. Colloids Surf., A 2007, 303, 14–29. (55) Nazaran, P.; Bosio, V.; Jaeger, W.; Anghel, D. F.; von Klitzing, R. J. Phys. Chem. B 2007, 111, 8572–8581. (56) Jomaa, H. W.; Schlenoff, J. B. Macromolecules 2005, 38, 8473–8480. (57) Farhat, T.; Yassin, G.; Dubas, S. T.; Schlenoff, J. B. Langmuir 1999, 15, 6621–6623.

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layer is rinsed with water. Other factors that may be involved are the strong versus weak polyelectrolyte character of PSS and PAH, the hydrophobicity of the polyelectrolyte, counterion binding effects, differences in chain flexibility, the enhanced interaction of the aromatic group in PSS, the larger volume charge density of PAH (charge per unit volume due to the smaller monomer unit), or a combination of these factors. However, the determination of the reason requires further investigations.

Figure 8. Relative DPI signals compared to their values characteristic of the compact outer layer as a function of the refractive index, where ( is the trans electric (TE) signal and 9 is the trans magnetic (TM) signal. 2 denotes the thickness of the outer (PAH) layer as a function of the refractive index.

Figure 9. Change in the DPI signal as a function of time. The black lines are the trans electric (TE) signal, and the red lines are the trans magnetic (TM) signal. (1) Adsorption of PSS. (2) Rinsing step with Milli-Q water. (3) Adsorption of PAH. (4) Rinsing step with MilliQ water.

As the outer layer starts to swell, one of the two DPI signals increases while the other decreases. This trend proceeds until the refractive index starts to approach the refractive index of water. In this swelling range, the layer thickness of the outer layer diverges and both DPI signals decrease compared to the initial values characteristic of the compact layer. In Figure 9, the measured DPI signals are depicted before and after rinsing with water. Rinsing of the PAH layer results in decreases in both experimental DPI signals. This indicates that the swelling of the layer is in the regime where the thickness diverges, and thus the PAH thickness must be well above 10 nm. This result is in excellent agreement with the QCM-D results confirming the reliability of the assumed layer structure. It is interesting that the large amount of swelling in the outermost layer is observed only when PAH is facing the bulk, and it is evident that the effect it not completely due to the increased inter- and intrachain electrostatic repulsion when the

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Conclusions In this study, QCM-D and DPI have been used to illuminate the structure of PAH/PSS multilayer films in two different salts, KBr and NaCl. Three different rinsing protocols were used during multilayer formation. It could be concluded that the applied rinsing protocols do not have a significant effect on the overall buildup of the multilayer and the sensed mass of the multilayer differs by only a few percent when the buildup is terminated with salt and/or water. However, KBr produces thicker layers, and this can be explained by the effect of the counterions (the anions) compared to NaCl. The bromide ion is larger and has greater polarizability and thus stronger interactions with PAH, which leads to weaker intrachain repulsion, and PAH adopts a more coiled structure in solution and therefore a thicker layer is obtained. Furthermore, from the obtained data it can be concluded that the structure of the PAH/PSS multilayer contains a very compact, rigid core with a small amount of water incorporated into the film independently of the electrolyte and rinsing procedure. The compact structure can be explained by a high degree of complexation between the polyelectrolytes as well as hydrogen bonds between the amines in PAH and oxygen in PSS as observed in recent studies.50,51 However, when pure water is used to rinse and PAH is the outermost layer, a large amount of swelling in the outermost PAH layer is observed. From both QCM-D and DPI, using a two-layer model in the analysis, it is obvious that only the outermost layer swells (to a thickness of 25-30 nm) and the underlying layers retain their compact structure. For the QCM data, the Voight model was used, and to our knowledge, the twolayer model for the evaluation of DPI data has not been used previously. The thickness of the swollen outer PAH layer increases slightly with the number of deposited layers, and interestingly, the PAH layer seems to obtain a very similar thickness in water independent of the electrolyte used for the buildup. The swollen PAH layer contracts again when it is subjected to an electrolyte solution, indicating that the uptake and release of counterions are reversible. Acknowledgment. Z.F. and E.B. acknowledge the Swedish Research council (VR) for financial support. I.V. gratefully acknowledge the financial support from the 7th European Community RTD Framework Program via Marie Curie European Reintegration Grant PE-NANOCOMPLEXES (PERG02-GA2007-2249) as well as from the Hungarian Scientific Research Fund (OTKA H-07A 74230). I.V. is a Bolyai Janos fellow of the Hungarian Academy of Sciences.

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