Properties of Cationic Polyelectrolyte Layers

Journal of Dispersion Science and Technology, 30:969–979, 2009 Copyright # Taylor & Francis Group, LLC ISSN: 0193-2691 print=1532-2351 online DOI: 10.1080/01932690802646488

Properties of Cationic Polyelectrolyte Layers Adsorbed on Silica and Cellulose Surfaces Studied by QCM-D—Effect of Polyelectrolyte Charge Density and Molecular Weight ¨ sterberg, and Janne Laine Terhi Saarinen, Monika O Helsinki University of Technology, Department of Forest Products Technology, Helsinki, Finland

The adsorption of cationic polyelectrolytes (C-PAMs and PDACMAC) with different charge densities and molecular weights on silica and cellulose model surfaces was examined using a quartz crystal microbalance with dissipation, QCM-D. The conformation and viscoelastic properties of the adsorbed polyelectrolyte layers were studied by modelling the data with the Q-Tools program. When comparing the adsorption behavior on cellulose and silica a clear difference both in adsorption kinetics of polyelectrolytes and the viscoelastic properties of the formed polyelectrolyte film was observed. On cellulose the polyelectrolyte films were viscous and more dissipative at the beginning of the adsorption but when the adsorption proceeded the layers became more rigid and stiffer, in contrast to the behavior on silica. Keywords

Cellulose, polyelectrolyte adsorption, QCM-D, silica, viscoelastic properties

1. INTRODUCTION The adsorption behavior of polyelectrolytes has attracted a lot of attention recently because of the widespread industrial use of these substances for example in papermaking, in mineral processing, in waste water treatments, in paints and in cosmetics. In papermaking, polyelectrolytes are used as retention aids to promote retention by flocculation and to improve drainage, as fixing agents to promote aggregation of colloidal material to the fibers and as wet and dry strength additives to promote the strength of paper.[1] Following the increased closure of water loops and the growing use of recycled fiber, organic and inorganic components accumulate in process waters, making the interactions with polyelectrolytes more complicated. To be able to understand the Received 9 May 2008; accepted 14 May 2008. Part of the special issue, Surface and Colloid Chemistry Without Borders: An International Festschrift for Professor Per Stenius on the Occasion of His 70th Birthday. This work was funded by Tekes (Finnish Funding Agency for Technology and Innovation), Kemira Oyj, M-Real Oyj, Stora Enso Oyj and UPM-Kymmene. The authors would like to thank Tekla Tammelin, Dr. Tech., and Jani Salmi, M.Sc, for valuable discussions, technicians Ms. Marja Ka¨rkkaïnen and Ms. Ritva Kivela¨ for their excellent assistance with the laboratory work and Kemira Oyj for providing the polyelectrolytes. ¨ sterberg, Helsinki Address correspondence to Monika O University of Technology, Department of Forest Products Technology, P.O. Box 6300, FIN-02015 TKK, Finland. E-mail: monika.osterberg@tkk.fi

functions of different additives, the interactions between various chemical compounds and different surfaces have to be studied systematically. The adsorption of polyelectrolytes on cellulose fibers has been studied extensively, producing a fairly detailed picture of the effect of different polyelectrolytes on various paper properties.[2] However, the source of fibers strongly affects the results and the layer properties of adsorbed polyelectrolytes cannot be studied with bulk experiments. To gain a better understanding of the interactions between different components in aqueous solutions, some model systems have been developed. Model surfaces provide a means to study the chemical and morphological changes in certain desired and simplified conditions. Because of its easy availability, smoothness and homogeneity, silica has been widely used in studying the adsorption of polyelectrolytes.[3–8] These studies have been very valuable in understanding the adsorption of polyelectrolyte on negatively charged mineral surfaces. However, using silica as a model for the cellulose fiber surface is not ideal. The porous structure of the cellulose fibers and the amount and type of fines strongly affect for example the adsorbed amount of polyelectrolytes.[2] In addition, cellulose has a substantially lower charge density than silica. Hence, there has been growing interest in preparing smooth and wellcontrolled thin films from cellulose that can be used for fundamental research,[9] and studies have been conducted on the adsorption of polyelectrolytes on spin-coated[10–12] or Langmuir-Schaefer cellulose surfaces.[13]

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At present, stagnation point adsorption reflectometry (SPAR),[7,12,14] ellipsometry,[3,4,6,15] and quartz crystal microbalance with dissipation (QCM-D)[16–19] are the most commonly used methods to study polyelectrolyte adsorption kinetics and=or the thickness of adsorbed polyelectrolyte layers. When comparing the results obtained with these different methods it is important to remember that they measure different phenomena, which consequently will affect the results. SPAR is based on the changes in intensities of the parallel and perpendicular components of the incoming laser light reflected off a surface.[20] This technique determines only the adsorbed amount which can be calculated from the reflected intensity ratio. Ellipsometry is also an optical technique, which measures the changes in polarization of elliptically polarized light when it reflects on a sample surface. The instrument provides both the average optical layer thickness calculated from the ellipsometric angles as well as the adsorbed mass calculated from average thickness and refractive index. In contrast to the optical techniques, which are not sensitive to water associated with the adsorbed polymer layer, QCM is based on the changes in frequency of an oscillating crystal and it takes into account the total mass, including bound water. If water is closely bound to the adsorbed layer, it will affect the change in frequency. Hence, the observed adsorbed mass of loosely bound polymer layers is often much higher when using QCM-D than when using ellipsometry. This feature of the QCM-D can actually be used to estimate the amount of bound water in a film.[21,22] By monitoring the damping of the oscillation when the driving signal is switched off, the viscoelastic properties of the adsorbed layer can also be studied using QCM-D. This feature has only recently been utilized in polyelectrolyte studies.[16,17,19,23] Although many researchers have studied the adsorption of different polycations, the structure and properties of the adsorbed layers on various substrates in different conditions are not yet fully understood. Specifically, the effects of the charge density and molecular weight of polyelectrolytes on the adsorption on cellulose model surfaces have not to our knowledge been studied earlier. The present work emphasizes the structural and viscoelastic properties of the adsorbed polyelectrolyte layers. The conformational changes and viscoelastic properties of the adsorbed layers were studied by examining the slopes of the DD=Df figures and by modelling the data with the Q-Tools program. The effect of the molecular weight and charge density of the polyelectrolytes was studied systematically. The polyelectrolytes were adsorbed on two different substrates, silica and cellulose. The effect of the electrolyte concentration on the adsorption was also investigated.

2. MATERIALS AND METHODS 2.1. Materials The polyelectrolytes used were polydimethyldiallylammonium chloride (PDADMAC, Allied Colloids Ltd., Yorkshire, England) and cationic polyacrylamides (C-PAMs), kindly donated by Kemira Oyj, Finland, with different molecular weights and charge densities (Figure 1 and Table 1). C-PAMs were fractionated by filtration in a tangential flow ultrafiltration system (Filtron Technology Corp., USA), using membranes with different cut-offs to obtain definite molecular weight fractions. PDADMAC was ultrafractionated with an Amicon ultrafiltration unit (Amicon Inc., Beverly, MA, USA).[24] The charge densities were determined according to the procedure applied by Koljonen et al.[25] and the molecular weights by means of intrinsic viscosities as described by Dautzenberg et al.[26] The standard buffer in the QCM-D measurements was 1 mM NaHCO3 (pH 8). Deionized water, which had been further purified with a Millipore Synergy UV unit, was used. The inorganic electrolytes were of analytical grade. Fresh polyelectrolyte stock solutions were prepared by dissolving the polymer powder in buffer to a final polymer concentration of 2 g=l. The solutions were stored for a maximum of one week before use. Further dilutions were made 1 hour before the experiment to eliminate the effect of ageing of C-PAM solutions. The QCM-D crystals were AT-cut quartz crystals supplied by Q-Sense AB, Gothenburg, Sweden. The thickness of the crystals was 0.3 nm, the fundamental frequency (f0) 5 MHz and the sensitivity constant (C) 0.177 mg=m2Hz. The silica surfaces were prepared by vapor deposition by the supplier. Silica-coated crystals were cleaned in the following way: exposure to UV=ozone (Bioforce Nanosciences, Inc., USA) for 10 minutes, followed by 30 minutes exposure to 2% surfactant (RBS, R. Borghgraef s.a., Belgium) and finally rinsing with plenty of water and drying with nitrogen. Immediately prior to use the crystals were re-exposed to UV=ozone for 10 minutes. The ozone

FIG. 1. Molecular structures of (a) PDADMAC; (b) uncharged and (c) cationic unit of C-PAM.

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TABLE 1 Properties of the polyelectrolytes Polyelectrolyte

Charge density (meq=g)

Charge density (mol %)

Molecular weight 106 (g=mol)

0.9 1.0 2.8 6.1

7 9 33 100

0.21 2.43 0.33 0.39

C-PAMLC-LMW C-PAMLC-HMW C-PAMMC-LMW PDADMACHC-LMW

effectively removes hydrocarbon and organic contaminants and renders the surface hydrophilic. The cellulose surfaces were obtained by depositing trimethylsilyl cellulose (TMSC) onto polystyrene-coated gold crystals by using the horizontal Langmuir-Schaefer (LS) dipping technique, as described by Tammelin et al.[27] After desilylation the cellulose-coated crystals were allowed to swell in the appropriate electrolyte solution for at least 12 hours before experiments. 2.2. Quartz Crystal Microbalance with Dissipation The adsorption of polyelectrolytes was studied with a QCM-D from Q-Sense, Gothenburg, Sweden. Chamber D300, based on batch mode addition, was used in isotherm experiments and chamber E4 with a flow mode (first 500 ml=min following constant 50ml=min) in the other adsorption experiments. The basic principle of this instrument has been described by Rodahl et al.[28] and Hoö¨k et al.[29] The QCM-D method allows simultaneous measurement of the adsorbed amount and viscoelastic properties of the adsorbed layer. The crystal oscillates at a resonance frequency, f0, without adsorbate. When material adsorbs on the surface of the crystal, the frequency is lowered to f. If the adsorbed mass is evenly distributed, rigid and small compared to the mass of the crystal, Df ¼ f – fo is related to the adsorbed mass by the Sauerbrey equation.[30] DmSauerbrey ¼

CDf n

½1

where Dm is the adsorbed mass per unit surface, n is the number of the overtone used in the measurement (in the present case n ¼ 3) and C is a constant (0.177 mg=m2Hz). Equation (1) takes into account only the closely adhered polymer layer and also the water inside the polymer layer. The thickness of the adsorbed layer, dSauerbrey, can be calculated by combining the estimated mass of the adsorbed layer with the assumed density of the polymer layer, q ¼ 1.15 g=cm3. dSauerbrey ¼

DmSauerbrey q

½2

The adsorption isotherms were repeated at least twice on both surfaces. The separate polyelectrolyte adsorptions

were repeated several times, and, despite some scatter in the data, all repetitions seemed to follow the same trend. The effect of the ionic strength on the adsorption behavior and the properties of the adsorbed layer are strongly dependent on the way QCM-D measurements are performed. In this study, the quartz crystal was equilibrated in the respective electrolyte solution for 12 hours (cellulose) or 1 hour (SiO2) before injecting the polyelectrolyte solution into the chamber. In this way the effect of viscosity changes in the solvent and possible swelling=deswelling effects, due to changes in ionic strength, were eliminated. The polyelectrolytes were also equilibrated in the respective electrolyte solutions for 1 hour before the adsorption experiments. 2.3. Interpretation of Viscoelastic Properties When the source driving the oscillation of the crystal is cut off, the amplitude of the oscillation decays due to dissipation of energy in the crystal, the adsorbed layer and the surrounding solution. The decay rate depends on the viscoelastic properties of these materials. Dissipation is characterized by the dissipation factor D, which is defined by D¼

Ediss 2pEstor

½3

where Ediss is the dissipated energy during one cycle and Estor is the total energy stored in the oscillation. With the QCM-D the change in dissipation factor, DD ¼ D D0 is measured, where D0 is the dissipation factor of the pure crystal in solvent before adsorption and D is the dissipation factor after adsorption. In this way, a qualitative measure of the relative stiffness or conformation of the adsorbed layer may be determined. High dissipation values reflect a thick adsorbed layer with loops and tails, while a thin and rigid layer vibrates with the crystal, indicating a low dissipation factor. Using a Voigt model, it is possible to determine the viscous and shear modulus components and also the thickness of the adsorbed layer. The interpretation of the viscoelastic properties of the adsorbed layer in this work is based on the model presented by Voinova et al.[31] In this model,

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the adsorbed layer is represented by a single Voigt element. The model is very simple: the quartz crystal is assumed to be purely elastic and the surrounding solution is assumed to be purely viscous and Newtonian. The adsorbed film is expected to have uniform thickness and density. It is also assumed that there is no slip between the adsorbed layer and the quartz crystal and that the viscoelastic properties are independent of frequency. The results were fitted to the Voigt model at several overtones (third, fifth, seventh, ninth, eleventh, and thirteenth) using the Q-Tools program (Q-Sense Ab, Gothenburg, Sweden), assuming constant fluid density (1.05 g=cm3), fluid viscosity (1.3 103 Nsm2) and density of the adsorbed layer (1.15 g=cm3).

3. RESULTS AND DISCUSSION 3.1. Adsorption Isotherms To determine the optimum polyelectrolyte concentrations to be used in the subsequent measurements, the change in frequency as a function of added polyelectrolyte

was measured. These adsorption isotherms at the third overtone are shown in Figure 2. The adsorption of high-charge polyelectrolytes (C-PAMMC-LMW and PDADMAC) on silica (spheres and triangles in Figure 2a) shows that a plateau is reached already after the first addition and that the total change in frequency is low (24 Hz and 6 Hz, respectively). The change in dissipation for these polyelectrolytes follows the same trend as the change in frequency (Figure 2b), and the final DD is well below 1 106 for both polyelectrolytes. These polyelectrolytes settle in a flat conformation instantly covering the surface. The change in frequency of medium- and high-charge polyelectrolytes on cellulose was slightly higher than that on silica, because the polyelectrolyte conformation on the low-charge cellulose surface is different from that on the high-charge silica. The other explanation is that the polyelectrolyte may be able to penetrate into the cellulose film (Figure 2c). In the same way as with silica, the change in frequency was lower when adsorbing highcharge PDADMAC than when adsorbing medium-charge C-PAM. Although the Df increased slightly with

FIG. 2. Adsorption isotherms of polyelectrolytes in 1 mM NaCl on SiO2 and cellulose surfaces. (a) frequency shift on SiO2; (b) dissipation shift on SiO2; (c) frequency shift on cellulose; and (d) dissipation shift on cellulose. The additions have been made at 20-minute intervals. f0 ¼ 5 MHz, n ¼ 3. The polyelectrolyte properties can be found in Table 1.


increasing polyelectrolyte concentration, only the first addition of polyelectrolyte increased the DD (Figure 2d). As expected, the polyelectrolytes with low charge density adsorbed on the SiO2 surface to a much higher extent than the high-charge PEs. The change in dissipation was also high, indicating a thick and loose layer (diamonds and squares in Figures 2a and b). The adsorption of lowcharge C-PAMs increased with increasing polyelectrolyte concentration, until it started to level off at 0.03 g=l. For similar polyelectrolytes, a similar adsorption maximum has been found in the literature. The adsorption of lowcharge C-PAMs was clearly lower on cellulose than on SiO2 (Figures 2a and c). Because of the difference in charge density, less polyelectrolyte was required to neutralize the cellulose surface than the SiO2 surface. Generally, the dissipation curves of the low-charge C-PAMs showed similar trends as the frequency curves (Figure 2d). Moreover, the change in dissipation was equal for both high- and low-Mw C-PAM, although the change in frequency was higher for the low-Mw polymer.

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The adsorption isotherms were very reproducible except for the high-Mw C-PAM. One reason for this may be that the high-Mw polyelectrolyte can adopt different conformations on the surfaces and the sample could have been more polydisperse than the others. 3.2. Effect of Ionic Strength, Substrate, and Polyelectrolyte Properties on Adsorption In Figure 3, the change in frequency and dissipation after 1 hour’s adsorption on SiO2 and cellulose surfaces at different electrolyte concentrations are compared for 32 separate experiments (polyelectrolyte addition 0.5 g=l). All frequency and dissipation curves reached a maximum with an increase in the ionic strength. Depending on the polyelectrolyte and the surface used, the maximum occurred between 10 and 100 mM NaCl. The change in frequency and dissipation upon adsorption on SiO2 is illustrated in Figures 3a and b. The medium- and high-charge PEs adsorb in a rather flat conformation at a low ionic strength, which is evident from

FIG. 3. Change in frequency and change in dissipation as a function of ionic strength for adsorption of 0.5 g=l cationic polyelectrolytes (a) frequency shift on SiO2; (b) dissipation shift on SiO2; (c) frequency shift on cellulose; and (d) dissipation shift on cellulose. f0 ¼ 5 MHz, n ¼ 3, t ¼ 60 minutes.

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low Df and DD values. When the ionic strength increased (10 and 100 mM NaCl), the PEs adopted a more coiled conformation due to the reduced repulsion between the charged polymer segments. Thus, more polyelectrolyte molecules can fit onto the surface, causing the change in frequency and dissipation to increase. A further increase in ionic strength to 1 M NaCl leads to a decrease in Df and DD, indicating that less polyelectrolyte is adsorbed on the surface. The change in frequency and in dissipation of lowcharge and low-Mw C-PAM remained almost unchanged at low ionic strengths (Figure 3a and b) and increased only slightly in 100 mM salt concentration. When both the charge density and the molecular weight are low, the conformation of the polyelectrolyte is rather insensitive to the salt concentration up to 100 mM NaCl. On the other hand, the change in frequency and dissipation of C-PAM with higher Mw increased with increasing ionic strength according to the adsorption theory of polyelectrolytes on a smooth surface.[20,32–34] For this polyelectrolyte the salt concentration did affect the coil conformation, allowing more polyelectrolyte to adsorb with an increase in the electrolyte concentration. At a very high salt concentration the polyelectrolytes behave more or less as neutral polymers and the attraction between the silica surface and polymers decreases. At 1 M NaCl all curves coincide. The substrate had an influence on the adsorbed amount and the layer properties. Comparing the adsorption on cellulose (Figures 3c and d) with the adsorption on SiO2 (Figures 3a and b), a very different behavior can be noted. The change in frequency and dissipation of low-charge C-PAMs was significantly greater at all ionic strengths on silica than on cellulose, whereas the higher charged polyelectrolytes adsorbed more on cellulose. On cellulose, the Df and DD of low-charge and low-Mw C-PAM slightly decreased with an increase in ionic strength. A slight adsorption maximum is found at 10 mM NaCl for the low-charge-density and high-Mw polyelectrolyte, after which the adsorption decreases with increasing electrolyte concentration. Thus, at low ionic strength (1 mM NaCl) Df and DD clearly decreased with increasing molecular weight, while at 100 mM NaCl the change in frequency decreased somewhat but the change in dissipation was slightly higher. The attraction between the low-charge C-PAMs and the low-charge cellulose is probably weak and the salt addition fully screens this attraction. For the high-charge polyelectrolytes the adsorption maximum is found at 100 mM NaCl, as on silica. Figure 4 shows how the change in the charge density of the polyelectrolyte affects the adsorbed mass on both silica and cellulose surfaces. The properties of the polyelectrolytes used are found in Table 1. The adsorbed mass on silica decreased when the charge density of the PE increased. At low salt concentration the most

FIG. 4. Adsorbed mass as a function of charge density of polyelectrolyte for adsorption of 0.5 g=l cationic polyelectrolytes on SiO2 and cellulose determined using the Sauerbrey Equation (1). The polyelectrolytes used were C-PAMLC-LMW, C-PAMMC-LMW and PDADMACHC-LMW. f0 ¼ 5 MHz, n ¼ 3, t ¼ 60 minutes.

significant change in the adsorbed mass can be observed when increasing the charge density of the cationic polyelectrolyte from 0.9 to 2.8 meq=g on SiO2. As a result, the adsorbed mass decreases from 4.4 g=m2 to almost zero (calculated from Equation (1)). A further increase in the charge density of the PE no longer affected the adsorbed mass. At a high salt concentration the adsorbed mass was higher at all charge densities, but the adsorption still clearly decreased with an increase in the charge density of the polyelectrolyte. On cellulose, the adsorbed mass did not significantly change when increasing the charge density of the cationic


polyelectrolyte from 0.9 to 2.8 meq=g at low salt concentration. A further increase in charge density to 6.1 meq=g slightly decreased the adsorbed mass, because less PE is needed to compensate for the surface charge. Compared to the adsorption of PE on fibers, a similar trend has been observed when adsorbing different C-PAMs with various charge densities at low ionic strength.[35] At a higher salt concentration the adsorption first increases, followed by a decrease in the adsorbed mass with an increase in the charge density of the PE. Salt probably effectively screens the attraction between the low-charge PE and the lowcharge cellulose surface. At a higher PE charge density (2.8 meq=g) the salt presumably cannot screen all charges, so more PE can adsorb on the surface. When the charge density of the PE further increases, the adsorption decreases because of the compensation of surface charges in the same way as for SiO2. Since the polyelectrolytes had slightly different molecular weights (with a maximum variation from 0.21 106 to 0.39 106 g=mol, see Table 1), the effect of molecular weight on the results shown in Figure 4 should be taken into account. At low electrolyte concentrations the molecular weight had only a minor effect on the adsorption (see Figure 3) on both surfaces. Hence, we can be confident that the molecular weight does not affect the low ionic strength results illustrated in Figure 4. However, the molecular weight affected the adsorption of C-PAM on silica at high electrolyte concentration. Since the adsorption increased with increasing molecular weight of the polyelectrolyte, the effect of molecular weight on adsorption is the opposite to what we see in Figure 4. Since the adsorption decreases with increased charge density (and slightly increased molecular weight), the charge density of the polyelectrolyte affects the adsorption on silica more than the molecular weight. Given that the molecular weight did not affect the adsorption on cellulose even at a high electrolyte concentration, we can assume that the effects seen in Figure 4 are dominated by the changes in charge density also for the adsorption on cellulose. Compared to the results obtained using ellipsometry,[3,4] XPS[13] or stagnation point adsorption reflectometry,[7,12,14] the adsorbed mass observed was generally slightly higher. The main reason for this difference is the coupled water included in the mass calculated with QCM-D, although slight differences in the Mw and CD of the polyelectrolytes used obviously also affect the results. To illustrate the differences in adsorption kinetics for the various systems, Figure 5 shows the adsorption of two polyelectrolytes, C-PAMLC-HMW and C-PAMMC-LMW, on silica and cellulose during the first 15 minutes. At low salt concentration, both C-PAMs adsorbed immediately on silica, instantly covering the surface. The only difference was the higher Df for the LC-HMW

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FIG. 5. Change in frequency as a function of time for adsorption of C-PAMLC-HMW and C-PAMMC-LMW in 1 and 100 mM NaCl on SiO2 and cellulose surfaces. f0 ¼ 5 MHz, n ¼ 3. Polyelectrolyte solution was added at t 2 minutes.

polyelectrolyte. At a higher ionic strength, the adsorption was slightly slower, which can be observed as a rounding of the curves. The curves, however, level off within six minutes. The adsorption kinetics on cellulose was rather different. The adsorption process is clearly slower on cellulose since all curves are rounded. The effect of salt on the kinetics was not as obvious on cellulose as on silica. Instead, a more pronounced difference is seen between the adsorptions of different polyelectrolytes. The adsorption of LC-HMW polyelectrolyte levels off after about six minutes, while the MC-LMW polyelectrolyte still adsorbs after 15 minutes, possibly due to the penetration of the polyelectrolyte into the cellulose film.

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3.3. Conformation of the Adsorbed Layers The adsorption process was further examined by plotting the change in dissipation as a function of the change in frequency (called a Df plot and shown in Figure 6). As time is removed from the data, the slope of the curve is related to the overall viscoelasticity of the adsorbed layer. A more irregular or viscous layer is able to dissipate more energy than a rigid layer, so the layer has a higher dissipation per unit of mass (frequency). The steepness of the slope of the Df plot describes the softening or packing of the layer structure during the adsorption process. A curved and irregular graph can be interpreted as conformational changes during the adsorption process. The cellulose surfaces were equilibrated in the electrolyte solution before polyelectrolyte adsorption. Hence, the effect of background electrolyte can be neglected in the conformation studies. However, adsorption of polyelectrolyte may also affect the conformation of the cellulose film, and as a consequence, some of the observed changes may be a combination of the properties of the cellulose film and the adsorbed polyelectrolyte.

Figure 6a shows that the high-charge PEs immediately cover the high-charge silica surface at low ionic strength, which can be seen as a cluster in measurement points, not a real curve in the Df plot. These layers have a very flat conformation. The adsorption of the low-charge CPAMs on silica has two distinct regions. The dissipation factor is clearly not linearly dependent on the frequency shift. Initially, the polyelectrolyte adsorption causes a rather limited change in dissipation, suggesting a quite rigid conformation. In the first region, the slope of the curves is approximately 6.6 108. As the adsorption proceeds, the Df slope increases slowly, suggesting a more viscous layer. In the second region, the dissipation changes while the frequency stays more or less constant and the curve is almost vertical. The slope of this part is 17.6 108. This type of behavior has previously been seen for adsorption of a low-charge polyelectrolyte on gold.[36] It was suggested that the last polyelectrolytes reaching the surface adsorb in a more extended conformation compared to the first molecules. The same explanation seems reasonable in this case too.

FIG. 6. Change in dissipation versus change in frequency for adsorption of 0.5 g=l polyelectrolytes in (a) 1 mM NaCl on SiO2; (b) 100 mM NaCl on SiO2; (c) 1 mM NaCl on cellulose; and (d) 100 mM NaCl on cellulose. f0 ¼ 5 MHz, n ¼ 3, t ¼ 60 minutes.


The increase in the electrolyte concentration noticeably changed the adsorption behavior of the low-molecularweight, low-charge-density C-PAMLC-LMW (Figure 6b). Roughly three different regions were found. In the first region, the dissipation increased almost linearly with decreasing frequency. After that the frequency started to level off, but the dissipation still continued to increase. In region three both the frequency and the dissipation decreased. This can be interpreted in the following way: first the polyelectrolyte adsorbs very fast on silica. This is followed by desorption of some excess loosely bound polyelectrolyte chains from the surface. Simultaneously, the other chains still settle and expand on the surface, which can be seen as an increase in dissipation. Finally, the dissipation also starts to decrease, which is probably due to the desorption of some outermost layers of the adsorbed polyelectrolyte film. Thus, both the frequency and the dissipation decrease after the first polyelectrolytes meet the surface, but the frequency decreases faster than the dissipation. PDADMAC (HC-LMW) still formed a rather rigid layer compared to the other polyelectrolytes at high electrolyte concentration on SiO2, but the change in conformation during adsorption could be divided into two regions (Figure 6b). First, there is a more or less linear relationship between DD and Df. In the second region, the slope is almost horizontal and the Df changes slightly, whereas the dissipation stays constant, suggesting no viscoelastic changes in layer properties after the initial adsorption. The other two polyelectrolytes (C-PAMLC-HMW and C-PAMMC-LMW) adsorb with a roughly linear relationship between DD and Df, suggesting that the conformation of the polyelectrolyte layer does not change during the adsorption process. Similar adsorption (DD vs. Df) behavior was observed on cellulose for all polyelectrolytes (Figures 6c and d). The adsorption behavior on cellulose is the opposite to the behavior on SiO2. At low electrolyte concentration two distinct regions were found on cellulose. In the first region, a steep increase in dissipation suggests a viscous layer. When the adsorption proceeds, the curves level off. The slopes of the first regions were approximately 12.5 10 8 and the slopes of the second region 3.4 10 8. There are two possible reasons for this: either the polyelectrolyte can penetrate into the porous cellulose film, making it more viscous, or the first polyelectrolytes that reach the surface attach to the cellulose surfaces only at a few anchoring points, giving rise to a very viscous layer. As the adsorption proceeds, the chains collapse, more polymer adsorbs and the layer becomes more rigid. This latter behavior has been found on cellulose fibers and seems more plausible.[37] The low-charge polyelectrolytes have a very low affinity to the cellulose surface. The steep slope of the D f plot indicates that only a few anchoring points of the chains are present on the surface and the polyelectrolyte chains extend out into the solution.

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At a high salt concentration (Figure 6d) the low-charge polyelectrolytes have a very low affinity to the surface, which is evident from the very low Df but rather high DD, suggesting a very extended conformation. The highand medium-charge polyelectrolytes adsorb in a high electrolyte concentration to a greater extent than a low electrolyte concentration. Both the final DD and Df values are higher than at a low salt concentration, but the shapes of the curves are roughly similar. The D f plots in Figure 6 imply that conformational changes occur for some systems during the adsorption process. In order to further investigate the time-dependence of the rheological properties of the adsorbed polyelectrolyte layers the systems were modelled using the Q-Tools program. In Figure 7, the viscosity and shear modulus of some polyelectrolytes (C-PAMLC-HMW and PDADMACHC-LMW) are shown as a function of time. The systems were chosen because they allow fitting to all overtones and because they show clear differences. Both the viscosity and shear modulus were higher for PDADMAC

FIG. 7.

Viscosity and shear modulus as a function of time for adsorption of C-PAMLC-HMW and PDADMACHC-LMW in 100 mM NaCl on SiO2 and cellulose surfaces determined using Voigt model.

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(HC-LMW) when adsorbed on SiO2 than when adsorbed on cellulose, indicating a more compact and rigid layer. The modelled viscosity and shear modulus of PDADMAC increased as a function of time on both surfaces. On silica the increase was rather steep at the beginning of the adsorption and after 20 minutes the viscosity and shear modulus reached a plateau. The modelled viscosity increased from 2.1 to 2.6 10 3 Nsm 2 and the shear modulus from 32 to 47 104 Nm 2. On cellulose the viscosity and shear modulus increased at a uniform rate during the adsorption from 1.7 to 2.1 10 3 Nsm 2 and the shear modulus from 26 to 36 104 Nm 2. On the other hand, the viscoelastic properties of the C-PAMLC-HMW remained rather constant during the adsorption process. Similar behavior can also be noted in the D-f plots in Figure 6. The slope of the low-charge and high-Mw C-PAM did not change significantly at high ionic strength on either surface during the adsorption process, whereas the slope of PDADMAC clearly decreased as the adsorption proceeded, indicating a more compact layer. The thickness of the adsorbed polyelectrolyte layers was estimated using both the Sauerbrey equation (Equation (2)) and the Voigt model. Both approaches have their limitations. For non-rigid adsorbed layers the Sauerbrey relation is not valid. High dissipation values mean that the soft film cannot function as a fully coupled oscillator. The outer layers of the film, far from the surface, do not follow the oscillation of the crystal. Thus, the Sauerbrey relation between Df and Dm underestimates the thickness of the film; the higher the dissipation, the greater the underestimation. By measuring the Df and DD at several overtones it is also possible to estimate the film thickness using the Voigt model. The accuracy of the estimation improves and the noise-to-signal ratio decreases when several overtones are used in modelling. In the model, constant fluid density and viscosity are assumed as well as constant density of the adsorbed films. These assumptions are not all correct, since especially the high-molecular-weight polyelectrolytes may affect solvent viscosity and the density of the films may vary as well. The Voigt thickness could only be estimated when the adsorbed layer was thick enough (D 2 10 6). Figure 8 shows the estimated layer thickness using Equation (2). The Sauerbrey thickness follows the changes in frequency presented in Figure 3, because the equation describes the linear relation between frequency changes and the change in adsorbed mass. For comparison, also Voigt thicknesses are shown for some systems (indicated by smaller symbols in the figure). As expected, the estimated thickness using the Voigt model is higher than the Sauerbrey thickness, especially for dissipative layers. Neither of these estimations is probably fully correct, but they can well be used for comparison between the different polyelectrolyte=substrate systems.

FIG. 8. Thickness of the adsorbed polyelectrolyte layers as a function of ionic strength on SiO2 and cellulose surfaces determined using Sauerbrey equation (Equation (1), large symbols) and Voigt model (Equation (2), small symbols).

The calculated thickness of all adsorbed polyelectrolyte layers increased with an increase in ionic strength from 1 to 100 mM NaCl on SiO2 (Figure 8). This is as expected, since the polyelectrolytes have a more coiled conformation in solution at higher ionic strength, and, consequently, they adsorb in a looser conformation on the surface.[20,33] The layer thicknesses of the low-charge C-PAMs were higher than the layer thicknesses of the medium- and high-charge polyelectrolytes at all electrolyte concentrations on SiO2, which is also in accordance with theory.[20] Interestingly, the differences between the different polyelectrolyte layers adsorbed on cellulose are not as pronounced. The Sauerbrey thickness varied between 1.3 nm and 2.5 nm for all polyelectrolytes at 1–10 mM NaCl. In 100 mM NaCl the differences were slightly larger but now the trend was the opposite of that observed with silica. The high-charge polyelectrolytes formed thick layers while the low-charge polyelectrolytes hardly adsorbed at all.


4. CONCLUSIONS In this work the conformational changes and viscoelastic properties of polyelectrolyte layers adsorbed on cellulose and silica were studied by means of the QCM-D method. Cationic polyelectrolytes with different charge densities and molecular weights were systematically adsorbed at several ionic strengths on silica and cellulose model surfaces. The adsorption of PEs was significantly different depending on the surface used. The low-charge polyelectrolytes adsorbed more on silica, while the adsorption of high-charge PEs was higher on cellulose. In addition, the differences in layer thicknesses between the different polyelectrolytes were not as pronounced at low ionic strengths on cellulose as on silica. An increase in the salt concentration increased the adsorbed mass and significantly increased the layer thickness of high-charge polyelectrolytes. The adsorption kinetics on cellulose was clearly slower than on silica. At the beginning of the adsorption the polyelectrolyte layers were more viscous on cellulose, but when the adsorption proceeded, the layers became more rigid, in contrast to the behavior on silica. These findings are important for clarifying the mechanisms of polyelectrolyte adsorption on solid surfaces and for making use of the information on polyelectrolyte adsorption in various industrial applications. REFERENCES [1] Neimo, L. (1999) Papermaking chemistry, Papermaking science and technology Book 4; Jyva¨skyla¨, Finland: Fapet Oy. [2] Wa˚gberg, L. (2000) Nord. Pulp Pap. Res. J., 15: 586–597. [3] Shubin, V. and Linse, P. (1995) J. Phys. Chem., 99: 1285–1291. ¨ dberg, L. and Sandberg, S. (1995) Langmuir, 11: 2621–2625. [4] O [5] Hoogeveen, N.G., Cohen Stuart, M.A., and Fleer, G.J. (1996) J. Colloid Interface Sci., 182: 133–145. ¨ dberg, L. (1999) Colloids Surf. A, 157: [6] Stemme, S. and O 307–313. [7] Wa˚gberg, L., Pettersson, G., and Notley, S. (2004) J. Colloid Interface Sci., 274: 480–488. [8] Samoshina, Y., Nylander, T., Shubin, V., Bauer, R., and Eskilsson, K. (2005) Langmuir, 21: 5872–5881. ¨ sterberg, M. (2006) Chem. [9] Kontturi, E., Tammelin, T., and O Soc. Rev., 35: 1287–1304. [10] Lefebvre, J. and Gray, D.G. (2005) Cellulose, 12: 127–134. [11] Fa¨lt, S., Wa˚gberg, L., Vesterlind, E.L., and Larsson, P.T. (2004) Cellulose, 11: 151–162. [12] Geffroy, C., Labeau, M.P., Wong, K., Cabane, B., and Cohen Stuart, M.A. (2000) Colloids Surf., A, 172: 47–56.

979

[13] Rojas, O.J., Ernstsson, M., Neuman, R.D., and Claesson, P.M. (2000) J. Phys. Chem. B, 104: 10032–10042. [14] Wa˚gberg, L. and Nygren, I. (1999) Colloids Surf. A, 159: 3–15. [15] Samoshina, Y., Diaz, A., Becker, Y., Nylander, T., and Lindman, B. (2003) Colloids Surf., A, 231: 195–205. [16] Munro, J.C. and Frank, C.W. (2004) Macromolecules, 37: 925–938. [17] Tammelin, T., Merta, J., Johansson, L.S., and Stenius, P. (2004) Langmuir, 20: 10900–10909. ˚ sberg, P., Bjo¨rk, P., Hoö¨k, F., and Ingana¨s, O. (2005) [18] A Langmuir, 21: 7292–7298. [19] Notley, S.M., Eriksson, M., and Wa˚gberg, L. (2005) J. Colloid Interface Sci., 292: 29–37. [20] Fleer, G.J., Cohen Stuart, M.A., Scheutjens, J.M.H.M., Cosgrove, T. and Vincent, B. (1993) Polymers at Interfaces; London: Chapman & Hall. ¨ sterberg, M., and [21] Ahola, S., Salmi, J., Johansson, L.S., O Laine, J. (2008) Biomacromolecules, 9: 1273–1282. [22] Craig, V.S.J. and Plunkett, M. (2003) J. Colloid Interface Sci., 262: 126–129. [23] Merta, J., Tammelin, T., and Stenius, P. (2004) Colloids Surf. A, 250: 103–114. [24] Laine, J., Buchert, J., Viikari, L., and Stenius, P. (1996) Holzforschung, 50: 208–214. [25] Koljonen, K., Mustranta, A., and Stenius, P. (2004) Nord. Pulp Paper Res. J., 19: 495–505. [26] Dautzenberg, H., Jaeger, W., Ko¨tz, J., Philipp, B., Seidel, C., and Stscherbina, D. (1994) Polyelectrolytes: Formation, Characterization, and Application; New York: Hanser Press. ¨ sterberg, M., and Laine, L. [27] Tammelin, T., Saarinen, T., O (2006) Cellulose, 13: 519–535. [28] Rodahl, M., Hoö¨k, F., Krozer, A., Brzezinski, P., and Kasemo, B. (1995) Rev. Sci. Instrum., 66: 3924–3930. [29] Hoö¨k, F., Rodahl, M., Brzezinski, P., and Kasemo, B. (1998) Langmuir, 14: 729–734. [30] Sauerbrey, G. (1959) Physik, 155: 206–222. [31] Voinova, M., Rodahl, M., Jonson, M., and Kasemo, B. (1999) Phys. Scr., 59: 391–396. [32] van de Steeg, H.G.M., Cohen Stuart, M.A., de Keizer, A., and Bijsterbosch, B.H. (1992) Langmuir, 8: 2538–2546. [33] Bo¨hmer, M.R., Evers, O.A., and Scheutjens, J.M.H.M. (1990) Macromolecules, 23: 2288–2301. ¨ dberg, L., and Park, S.B. (1997) J. [34] Tanaka, H., Swerin, A., O Pulp Paper Sci., 23: J359–J365. [35] Lindstro¨m, T. and Wa˚gberg, L. (1983) Tappi J., 6: 83–85. [36] Plunkett, M.A., Claesson, P.M., and Rutland, M.W. (2002) Langmuir, 18: 1274–1280. ¨ dberg, L., Lindstro¨m, T., and Aksberg, R. [37] Wa˚gberg, L., O (1988) J. Colloid Interface Sci., 123: 287–295.