Adsorption of a Strong Polyelectrolyte To Model Lignin Surfaces

Jun 25, 2008 - Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT,...
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Adsorption of a Strong Polyelectrolyte To Model Lignin Surfaces Shannon M. Notley*,† and Magnus Norgren‡,§ Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National University, Canberra, ACT, Australia, Department of Fibre and Polymer Technology, Royal Institute of Technology (KTH), SE-10044 Stockholm, Sweden, and Department of Natural Sciences, Fibre Science and Communication Network, Mid Sweden University, Sundsvall, Sweden Received December 6, 2007

The adsorption of a strong, highly charged cationic polyelectrolyte to a kraft lignin thin film was investigated as a function of the adsorbing solution conditions using the quartz crystal microbalance. The polyelectrolyte, PDADMAC, with a molecular weight of 100 kDa and one cationic charge group per monomer, was adsorbed to the anionically charged lignin film in the pH range 3.5-9.5 in electrolyte solution of 0.1 to 100 mM NaCl. At low pH, the adsorbed amount of PDADMAC was minimal, however, this increased as a function of increasing pH. Indeed, the surface excess increased significantly at about pH 8.5, where ionization of the phenolic groups on the lignin macromolecule may be expected. Furthermore, at this elevated pH, the adsorbed amount of PDADMAC decreased as the ionic strength of the solution increased above 1 mM. This is due to the competitive adsorption of counterions to the lignin surface and indicates that the adsorption of PDADMAC to lignin is of a pure electrosorption nature.

Introduction After cellulose, lignin is often considered to be the most abundant, naturally occurring biopolymer. Lignin is predominantly found in woody tree species with the highest concentration found between the wood fiber tracheids however the majority of lignin by weight is found in the fiber secondary wall.1 The chemical structure of lignin can be somewhat difficult to define. The base unit of the lignin polymer is a phenylpropane structure that may be linked together to other units through any of about 10 different chemical bonds, often in a heterogeneous and branched manner, leading to its amorphous and irregular macromolecular structure. Indeed, the manner in which the units are linked is often tree species dependent and complicating matters further are the possible ways in which lignin may be isolated, whether through various chemical or mechanical means.2 It has hence been a recent focus to use model lignin polymers in investigations of their surface properties.3–11 These may be prepared synthetically through polymerization of model monomer units or, alternatively, to use a model lignin polymer isolated from a natural source but carefully characterized in terms of its chemistry. Understanding the surface chemistry of lignin can be considered to be of great importance in many natural and industrial applications. For example, the wettability of fibers will be governed by the chemistry of the surface as well as the surface roughness component. This was recently shown to influence the conduction of water through the network of capillaries in living wood tissue.12 An understanding of the interfacial properties of lignin surfaces after the fibers have been liberated from wood is of great industrial importance to the pulp and paper industry. * To whom correspondence should be addressed. Tel.: +61 261257583. Fax: +61 261250732. E-mail: [email protected]. † Australian National University. ‡ Royal Institute of Technology (KTH). § Mid Sweden University.

Particularly when the fibers are pulped using mechanical means, the surface of the fiber is rich in lignin. However, also, during many chemical pulping processes, lignin may be only partially removed or precipitated onto the fiber surfaces. Therefore, the interfacial properties of lignin will often dominate over other wood biopolymeric components such as cellulose when fibers interact in an aqueous environment.9,13,14 A number of recent publications have detailed methods for the preparation of lignin films from various sources.3–11 In one study, a softwood kraft lignin was used to prepare smooth, continuous films of suitably homogeneous chemistry and structure for the subsequent use in determining fundamental surface forces interactions.8 Kraft lignin can be prepared from the spent black liquor left after pulping the wood fibers using a combination of high concentrations of hydroxide and hydrogen sulfide.2 This chemical breakdown is due to nucleophilic attack on the aryl-ether linkages, resulting in the cleavage of the lignin macromolecule into smaller polymeric units with the introduction of a significant number of free phenolic groups.1 Moreover, a small amount of carboxyl groups is also introduced into the structure mostly by aromatic ring cleavage. Thus, kraft lignin has a greater charge than naturally occurring lignin. The properties of these softwood kraft lignin films have been extensively studied in terms of their stability against a range of solution conditions which may be of interest in typical surface chemistry applications.8 For instance, it was shown that these films do not swell appreciably at pH less than 9 and remain stable even at relatively high salt concentrations of 0.1 M. Furthermore, from surface forces measurements, both the surface potential and hence surface charge could be determined over a range of pH and ionic strength solution conditions for these kraft lignin thin films.9 For many industrial uses, the lignin surface will be modified by the adsorption of polymers and polyelectrolytes in order to tune the surface interactions for a variety of applications. One such application is the use of cationic polyelectrolytes to

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improve the bonding between wood fibers to increase the strength of many paper grades.15 Furthermore, polyelectrolytes are used to improve the retention of many inorganic filler particles to the lignin rich fiber surface. In both cases, knowledge of the adsorbed polyelectrolyte characteristics is important. Thus, understanding the adsorbed amount, adsorbed polyelectrolyte conformation and the adsorption kinetics is of great significance. The adsorption of cationic polyelectrolytes to model lignin films has yet to be fully studied. A recent publication showed the adsorption of polyelectrolyte complexes as well as the adsorption of both the anionic and the cationic polyelectrolytes to a model lignin film was highly solution condition dependent and experimental evidence pointed to the possible existence of nonionic interactions.10 It is, hence, of interest to continue to study these adsorption phenomena to probe the effects of a number of solution condition variables such as pH, ionic strength, and polyelectrolyte solution concentration. In this study, a strong, highly charged polyelectrolyte, polydiallyldimethylammonium chloride (PDADMAC) has been used to investigate the influence of the kraft lignin surface charge and probe the existence of any nonionic interactions between PDADMAC and lignin. This polyelectrolyte has been often used in various applications in the field of paper chemistry to modify noncovalently the fibers and thereby influence the properties of the paper formed from these fibers.15 Here, the quartz crystal microbalance (QCM) has been used to probe the adsorption and used to determine the adsorbed layer thicknesses of PDADMAC to the model lignin surfaces as a function of pH and ionic strength. The range of solution conditions studied is appropriate for real applications where cationic polyelectrolyte adsorption for surface modification is in use in the paper manufacturing.

Materials and Methods Polyelectrolytes. The strong, highly charged cationic polyelectrolyte poly(diallyldimethylammonium chloride) or poly(DADMAC) was used in this study. It was obtained from Sigma Aldrich and used without further purification. The polyDADMAC had a molecular weight of around 100 kDa. Solutions of poly(DADMAC) were prepared using Milli-Q water with sodium chloride used as the background electrolyte. The pH of the solutions was adjusted immediately prior to use with appropriate amounts of either HCl or NaOH. Typically, solutions in the concentration range of 10-200 ppm were prepared at least 24 h prior to use to ensure the polymers were sufficiently dissolved. Preparation of Lignin Model Surfaces. Thin kraft lignin films supported on silica substrates were prepared according to a previously described method.8–10 Silica coated, AT-cut quartz crystals (KSV, Finland) were used as the underlying substrate before the addition of a thin layer of lignin. Prior to spin-coating, the silica surface was treated with 1 M NaOH, rinsed with Milli-Q water and then rinsed with ethanol before a final light water plasma treatment. This ensured that the silica surface was clean and completely hydrophilic. A softwood kraft lignin, isolated from black liquor, was used in this study. This sample has been extensively analyzed previously in terms of its molecular weight and purity.8,9 Typically, 50 mg of the dry, purified kraft lignin powder was added to 4 mL of 1 M ammonium hydroxide solution and stirred for at least 24 h to ensure complete dissolution. This solution was then spin-coated onto the silica-coated quartz crystals (spinning speed 1500 rpm, for 60 s) to produce lignin thin films with a thickness in the range of 60-75 nm.8 Atomic Force Microscopy. AFM was used to image the surface and measure the surface roughness. Imaging was performed using noncontact mode with an Asylum Research MFP-3D (Asylum Research, U.S.A.) with a silica TESP tapping mode tip. The root-mean-square roughness of the surface was determined from at least 10 height images over a 1 µm2 area. Furthermore, images of at least 400 µm2 were taken to ensure that the lignin films were continuous over a large area.

Notley and Norgren Quartz Crystal Microbalance. The adsorption of PDADMAC to the model lignin surfaces was measured using QCM. A KSV Z300 QCM (KSV, Finland) which is able to measure multiple overtones as well as monitor the impedance was used. At the heart of the QCM, a quartz crystal is excited into resonance with the frequency dependent upon a number of factors including the crystal dimensions and properties. Furthermore, the resonance frequency will be influenced by the properties of the fluid above the crystal surface. As material is adsorbed by the surface, the resonance frequency will decrease, which may be described theoretically by employing the Sauerbrey relationship, assuming that the adsorbed layer is rigid, relatively thin, and not water-rich.16,17 In practice, for virtually all cases of polymer adsorption, some water will be entrained within the layer up to the plane of shear and, as such, QCM cannot give a “dry” adsorbed mass, only a sensed mass that may be compared with other techniques.18–21 However, by measuring the adsorption at multiple overtones as well as measuring the impedance change, the visco-elastic properties of the adsorbed layer may be modeled leading to an estimate of the layer thickness.22–24 Thus, the adsorbed layer conformation as a function of solution conditions may be implied from this data. In this study, all adsorption of PDADMAC to the kraft lignin model surface was undertaken at a temperature of 25 °C. Typically, data from the third overtone is displayed, owing to its superior stability and reproducibility over the fundamental resonance frequency. Furthermore, PDADMAC solution concentrations were kept relatively dilute to minimize the influence of the change in solution viscosity and density on the resonance frequency of the quartz crystal.25

Results The lignin thin films deposited onto the quartz crystals by spin-coating were imaged using AFM prior to use in adsorption measurements. Figure 1 shows noncontact mode height images of the surfaces over 1 µm2 and 100 µm2. As can be seen in this image, the lignin films are smooth and continuous over a large area. The surface roughness (root-mean-square) was determined to be in the range of 1-2 nm for the 1 µm2 images. This is in agreement with previous studies which also investigated the film continuity using optical methods and imaging ellipsometry.8 Figure 2 shows a typical example of the adsorption of PDADMAC to the kraft lignin covered QCM crystal at pH 8.5 and an ionic strength of 1 mM. In this case, the change in frequency, ∆f, as a function of time as the PDADMAC is adsorbed is displayed. A total of 5 mL of the PDADMAC solution was injected into the QCM measurement chamber and the frequency and impedance measured continuously throughout the experiment. Importantly, the third, fifth, and seventh overtones are shown in Figure 2 with the change in the respective frequencies normalized by the overtone number. As can be observed, the adsorption as a function of time for the three overtones shows excellent agreement such that the change in frequency may be converted to a surface excess using the Sauerbrey equation.15 The fundamental frequency, however, does not agree with the overtones and as such this data has been omitted. This is due to the well-known issues of energy trapping due to the presence of an O-ring in the instrumental design. Figure 2 shows that the adsorption of PDADMAC to the kraft lignin film rapidly comes to equilibrium within 20 min under these solution conditions. It is, hence, of interest to convert the measured change in frequency to an adsorbed amount using the Sauerbrey equation. Figure 3 shows the change in frequency as well as the adsorbed mass, which in this case has been labeled a “sensed mass” as there is an unknown contribution of water entrained within the adsorbed PDADMAC layer. Also, as can be seen in Figures 3 and 4, at a time of approximately 350 s, there is a slight discontinuity in the

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Figure 1. AFM noncontact mode height images of the kraft lignin thin films deposited onto quartz crystals. The left image is 1 µm × 1 µm (z scale 10 nm) and the right image is 10 µm × 10 µm (z scale 50 nm). The surface roughness was determined from these images and was in the range of 1-2 nm over 1 µm2 for more than 10 images.

Figure 2. The adsorption of PDADMAC to a kraft lignin film measured using the QCM. Data for three overtones, the third, fifth, and seventh are displayed, normalized by the overtone number.

otherwise steadily decreasing frequency due to some unknown nonequilibrium effect, which may be due to polymer reconformation at the interface. Such a surface effect is likely to more evident at lower overtones as the shear wave extends into solution to a lesser degree. Simultaneously, with the measurement of the change in frequency, the change in impedance of the quartz crystal due to the adsorption of PDADMAC is also determined. The impedance is a measure of the increase in resistance of the oscillating crystal due to the coupling of a nonrigid surface layer. This change in impedance can therefore be used to determine the visco-elastic properties of the adsorbed polyelectrolyte layer using an appropriate model. Furthermore, by combining data for the change in frequency and impedance using multiple overtones, an estimate of the layer thickness may be given. Figure 4 shows the change in impedance as a function of the adsorption of PDADMAC to the lignin film at pH 8.5 and an ionic strength of 1 mM. As can be seen from this data, the apparent resistance of the crystal increases rapidly as the polyelectrolyte is adsorbed. Figures 3 and 4 demonstrate that PDADMAC strongly adsorbs to kraft lignin surfaces. This is as expected when the

Figure 3. Adsorption of PDADMAC to the kraft lignin film at pH 8.5 and an ionic strength of 1 mM. The change in frequency was converted to a “sensed mass” using the Sauerbrey equation. Data from the third overtone is shown. Closed symbols ∆f and open symbols adsorbed mass.

nature of the surface charge is considered. In a previous study using these lignin thin films, the surface potential was determined from colloidal probe microscopy measurements.9 From this, and in conjunction with the known chemical properties of similarly isolated lignin, the surface charge could be determined. The previous surface forces study showed that there was an increasing surface potential as the pH of the solution increased. This may be expected as the dominant charged species on the lignin polymers at lower pH values are carboxyl groups introduced due to oxidation of aromatic ring moieties during the kraft pulping process.1 As these carboxyl groups possess a weak negative charge, with a pKa of about 4-5, it is reasonable to assume that these charges will dissociate to a greater degree at elevated pH. Furthermore, if the hydroxide concentration is increased above pH 8.5, dissociation of the free phenolic groups with a pKa of about 10 in the lignin also starts to occur.26 Thus, the adsorption of a cationic polyelectrolyte to these lignin thin films will, hence, be highly influenced by the solution conditions, in particular, the pH of the adsorbing solution.

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Figure 4. Change in frequency and impedance of the lignin-covered quartz crystal as a function of the adsorption of PDADMAC at pH 8.5 and an ionic strength of 1 mM. Closed symbols ∆f and open symbols ∆R.

Figure 6. Sensed mass and polyelectrolyte layer thickness of PDADMAC adsorbed to lignin thin films measured using QCM at a constant PDADMAC solution concentration of 100 ppm and ionic strength of 1 mM as a function of pH. Closed symbols adsorbed mass and open symbols thickness.

Figure 5. Sensed mass of PDADMAC adsorbed to lignin thin films measured using QCM at pH 9.5 and a background electrolyte concentration of 1 mM as a function of initial polyelectrolyte solution concentration. Closed symbols are the adsorbed mass. The thickness calculated is also shown as the open symbols.

Figure 7. Calculated surface potential as a function of pH at different ionic strengths for a lignin model film based on data from refs 9 and 26.

Furthermore, for a given set of solution conditions (alkalinity, ionic strength, temperature) where the surface may be expected to be close to fully charged, the adsorbed amount of PDADMAC will be dependent upon the initial concentration of polyelectrolyte in solution. This is, of course, up to where equilibrium is reached. Figure 5 shows the sensed mass of PDADMAC adsorbed to the lignin surface as well as the estimated layer thickness from calculations using the frequency and impedance changes of multiple overtones at pH 9.5 and an ionic strength of 1 mM. At PDADMAC solution concentrations below 5 ppm, almost no change in the resonance frequency is observed implying adsorption levels close to the resolution limit of the QCM technique or less. However, as the concentration of the polyelectrolyte solution is increased, the observed surface excess increases up to an equilibrium value of 4.5 mg-2 for concentrations above 100 ppm. The properties of the polyelectrolyte, PDADMAC, are virtually unaffected by changes in solution pH as the quartenary amine groups are always charged at any pH value. Thus, changes in the adsorbed amount of PDADMAC to lignin thin films must only be dependent upon the dissociation of charges on the lignin itself if it is assumed that the PDADMAC only interacts with the lignin surfaces through electrostatic interactions. Thus, the adsorbed amount was determined as a function of solution pH at a constant initial polyelectrolyte solution concentration of 100

ppm and at a constant ionic strength of 1 mM. This data is shown in Figure 6. As can be seen from Figure 6, there is a slight increase in the adsorbed amount of PDADMAC in the pH region 5-7. This is consistent with the increase in ionization of the limited number of carboxylic groups on the lignin surface. Previous surface forces studies have shown that the surface potential and surface charge both increase significantly above the pKa of the carboxyl groups and hence the greater surface excess of PDADMAC in this pH range observed here is in agreement.9 However, at pH greater than 7, there is a rapid increase in the adsorbed amount at this ionic strength. This is most likely due to dissociation of phenolic groups on the lignin macromolecules. Furthermore, the layer thickness rapidly increases in parallel with the increased adsorbed amount as may be expected. It must be borne in mind that the ratio between free phenolic groups and carboxyl groups in a typical kraft lignin is about 10, thus, a considerable increase in the charge of the lignin film is expected to arise due to phenolic dissociation in the interval pH 8.5-9.5.9 Figure 7 shows the calculated variation in surface potential for a model lignin film as a function of ionic strength and pH considering the ionization of dissociable chemical groups. As can be seen in this figure, the surface potential decreases significantly with ionic strength at a given pH assuming that the surface charge density remains constant.27 However, with increasing pH, the surface potential rapidly increases through the pH region 4-6

Polyelectrolyte Adsorption to Lignin

Figure 8. Sensed mass and polyelectrolyte layer thickness of PDADMAC adsorbed to lignin thin films measured using QCM at a constant PDADMAC solution concentration of 100 ppm and pH of 9.5 as a function of ionic strength. Closed symbols are adsorbed mass and open symbols are layer thickness.

with the dissociation of carboxyl groups and then subsequently again at pH greater than 9. Hence, by considering the surface potential of the lignin films as a function of pH and ionic strength, the adsorbed amounts of polyDADMAC agrees qualitatively. The ionic strength of the solution from which the polyelectrolyte was adsorbed is a key variable in determining the surface excess. Figure 8 shows the sensed mass as a function of ionic strength for the adsorption of PDADMAC at elevated pH. It may be expected that there are two competing phenomena that will influence the adsorbed amount; those being the change in polyelectrolyte solution conformation from an extended rod to more globular conformation leading to a decreased polyelectrolyte “footprint” with increasing ionic strength and second, the increase in competition for adsorption sites on the lignin film between the polyelectrolyte molecules and adsorbed counter-ions. Figure 8 shows that a maximum in the adsorbed amount was measured at an ionic strength of 1 mM with a rapid decrease observed at higher background electrolyte concentrations. The estimated thickness of the adsorbed PDADMAC layers also decreased as a function of ionic strength.

Discussion The three major influences of the adsorption of a polyelectrolyte at an interface are the properties of the polyelectrolyte including charge density and type and the properties of the surface including once again the charge and finally the solution conditions under which the adsorption occurs. Understanding then, how the charge of the surface and on the polyelectrolyte varies with pH and ionic strength is hence of great importance. Figure 6 showed that there was an increase in the adsorbed amount as the solution pH was increased. This is expected when the charge of the surface is considered.27,28 In a previous surface forces study, the charge density of the lignin thin films was determined as a function of pH and ionic strength.9 At a background electrolyte concentration of 1 mM, the kraft lignin surface charge density ranged from 1 charge/140 nm2 at pH 3.8 to 1 charge/67 nm2 at pH 8.5. In this current study, the adsorption of PDADMAC was investigated in the pH range from 3.5 to 9.5. Thus, the higher adsorbed amount of polyelectrolyte at 9.5 than 8.5 implies a greater surface charge density at the higher pH than in the previous study if the same adsorption stoichiometric ratio remains unchanged. This is most certainly

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due to that dissociation of phenolic groups in the lignin starts to occur above pH 8.5. It is interesting to note that under all solution conditions investigated in this study that if the number of adsorbed charges is considered in comparison to the surface charge density then significant charge reversal may have occurred. However, it must also be considered that some proportion of the sensed mass will be due to entrained water within the adsorbed PDADMAC layer. Previous studies have shown, though for highly charged polyelectrolytes, such as PDADMAC, that adsorb in essentially flat conformations at the interface, that the surface excess determined using QCM closely approximates the adsorbed amounts measured using other techniques such as optical reflectometry. Here, the adsorbed layer thicknesses determined from the change in resonance frequency and the impedance of the quartz crystal are of the order of 1-5 nm. Thus, it is expected that the proportion of solvent sensed in these experiments would be minimal. The stability of similarly prepared kraft lignin thin films supported on silica was previously reported.8 That study showed that at ionic strengths up to 0.1 M, the films essentially stayed intact with only a minimal reduction in thickness. Furthermore, no significant degradation of the films was observed at pH less than 9.5. Hence, the stability of these lignin films provides an opportunity to probe the presence or otherwise of nonionic interactions between various polyelectrolytes and the kraft lignin surface. Figure 8 shows the sensed mass as a function of the background electrolyte concentration of the adsorbing solution of PDADMAC at high pH. The adsorbed amount of PDADMAC to the lignin surface at pH 9.5 decreased at ionic strengths above 1 mM. In the absence of nonionic interactions, this is expected according to the Scheutjens-Fleer theory of charged polymer adsorption to an oppositely charged surface. Indeed, in the ionic strength range investigated in this study, there was no evidence of nonionic interactions and the decrease in adsorbed amount of polyDADMAC with increasing ionic strength is consistent with theory and previous studies.29,30 However, to fully probe the possibility of nonionic interactions, much higher electrolyte concentrations than 0.1 M is required. This is not possible here due to the instability of the lignin films at higher solution ionic strengths.10 This could simply be due to the fact that the stability of the kraft lignin films at higher electrolyte concentration than 0.1 M is not sufficient to probe their presence. At all ionic strengths investigated in this study, the extension of the PDADMAC away from the lignin film interface is minimal according to the modeled data in Figure 8, which agrees with predictions based on the Scheutjens-Fleer theory.31,32 The polyDADMAC molecules adopt slightly less extended conformations with charge screening, however, on the whole, the stiffness of the polyelectrolyte backbone is such that the volume occupied by the molecule is much greater than what may be expected of a purely random coil. This also helps to explain the relatively fast kinetics of adsorption, that is, the time taken to reach equilibrium is short. The polyDADMAC molecules retain the extended conformation with adsorption limiting the possibility for other chains to adsorb due to geometric considerations as well as the rapid effective charge neutralization and subsequent charge reversal. Under these solution conditions, such flat adsorbed layer conformations have been predicted from theory31–33 and simulation34 and also have been experimentally observed using surface forces measurements for highly charged polyelectrolytes including in the adsorption of polyDADMAC to a charged interface.20,35–39 Previous studies comparing highly charged polyelectrolyte adsorption to an oppositely charged

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interface using QCM and X-ray photoelectron spectroscopy also demonstrated that the polymer adsorbed with a flat conformation with only minimal water entrained within the layer.38 This is in agreement with the results presented here, where modeling of the QCM data predicts that the extension of the polyDADMAC away from the lignin interface under all solution conditions is less than 7 nm. The adsorbed layer conformation of a cationic polyelectrolyte adsorbed onto lignin surfaces has important practical applications in the pulp and paper industry. In many paper grades, the fiber surface chemistry will be predominantly lignin with those prepared from mechanical pulping mechanisms such as newsprint the best example. One important application, where cationic polyelectrolytes find use, is in promoting the joint strength between individual fibers in a paper sheet. It is thought that an extended cationic polymer layer away from the interface can aid in improving adhesion through molecular interdiffusion and mixing of chain segments as capillary forces drive the overlapping fiber surfaces together.15,40 For this to be effective, the polymeric molecules must at least extend to sufficient degree to overcome the inherent roughness of the fiber surfaces. Furthermore, the overlap of highly charged polymeric layers is entropically unfavorable due to the effectively higher concentration of counter-ions between the surfaces over that in the bulk leading to a repulsive interaction energy of osmotic origin. Hence, polyDADMAC fails on both these counts when the adsorbed layer thickness and molecular charge is considered.

Conclusions In this study, the adsorption of a cationic polyelectrolyte, PDADMAC to an anionic kraft lignin film was investigated. The quartz crystal microbalance was used to determine the surface excess and estimate the polyelectrolyte layer thickness as well as to monitor the adsorption kinetics. In the solution pH range investigated, from pH 3.5-9.5, PDADMAC adsorbed irreversibly to the anionically charged lignin thin film. As the pH was increased, the adsorbed amount of PDADMAC increased, with a rapid rise in the sensed mass observed above pH 8.5 where it may be expected that the ionization of the phenolic groups on the lignin macromolecule begins. Thus, this rapid increase in surface charge density gave rise to an increased number of potential binding sites for the cationic polyelectrolyte. At a constant pH of 8.5, the adsorbed amount of PDADMAC was observed to decrease as the ionic strength of the solution increased. This is due to competitive adsorption of the small counter-ions. This observation is in agreement with theories of polyelectrolyte adsorption to oppositely charged surfaces and demonstrates that any nonionic contribution to the adsorption is minimal. However, for this to be stated conclusively for PDADMAC adsorption to a kraft lignin surface, ionic strengths greater than 0.1 M need to be probed, but in this study, the stability of the films to exposure at higher ionic strength solutions is poor. Acknowledgment. S.M.N. acknowledges financial support from the Co-operative Research Centre SmartPrint. M.N. gratefully acknowledges the European Union, Objective 1 (EU Structural Funds Contract Y3041-66-2007) for financial funding. Go¨ran Gellerstedt and Andrea Majtnerova, KTH, are thanked for providing the softwood kraft lignin sample.

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References and Notes (1) Sjostrom, E. Wood Chemistry: Fundamental and Applications. 2nd ed.; Academic Press; New York, 1993. (2) Wood and Cellulosic Chemistry; Shiriashi, N., Hon, D. N.-S., Eds.; Marcel Dekker: New York, 1991. (3) Lee, S. B.; Luner, P. Tappi J. 1972, 55, 116. (4) Constantino, C.; Dhanabalan, A.; Coota, M.; Pereira-da-Silva, M. R.; Curvelo, A.; Oliveira, O. N. J. Holzforschung 2000, 54, 55. (5) Micic, M.; Radotic, K.; Benitez, K.; Ruano, M.; Jeremic, M.; Moy, V.; Mabrouki, M.; Leblanc, R. M. Biophys. Chem. 2001, 94, 257. (6) Maximova, N.; Osterberg, M.; Laine, J.; Stenius, P. Colloids Surf. A 2004, 239, 65–75. (7) Pasquini, D.; Balogh, D. T.; Olivera, O. N. J.; Curvelo, A. A. S. Colloids Surf. A 2005, 252, 193. (8) Norgren, M.; Notley, S. M.; Majtnerova, A.; Gellerstedt, G. Langmuir 2006, 22 (3), 1209–1214. (9) Notley, S. M.; Norgren, M. Langmuir 2006, 22 (26), 11199–11204. (10) Norgren, M.; Gardlund, L.; Notley, S. M.; Htun, M.; Wagberg, L. Langmuir 2007, 23, 3737–3743. (11) Tammelin, T.; Osterberg, M.; Johansson, L. S.; Laine, J. Nord. Pulp Pap. Res. J. 2006, 21, 444–450. (12) Kohonen, M. M. Langmuir 2006, 22, 3148–3153. (13) Notley, S. M.; Pettersson, B.; Wagberg, L. J. Am. Chem. Soc. 2004, 126 (43), 13930–13931. (14) Notley, S. M.; Eriksson, M.; Wagberg, L.; Beck, S.; Gray, D. G. Langmuir 2006, 22 (7), 3154–3160. (15) Lindstrom, T.; Wagberg, L.; Larsson, T. Transactions of 13th Fundamental Research Symposium of the Pulp and Paper Fundamental Research Society, Cambridge, U.K., 2005; Pulp and Paper Fundamental Research Society: Cambridge, U.K., 2005; pp 457-562. (16) Sauerbrey, G. Zeiyschrift Physic 1959, 155, 206. (17) Ho¨o¨k, F.; Rodahl, M.; Brzezinski, P.; Kasemo, B. Langmuir 1998, 14, 729–734. (18) Hoeoek, F.; Kasemo, B.; Nylander, T.; Fant, C.; Sott, K.; Elwing, H. Anal. Chem. 2001, 73, 5796. (19) Craig, V. S. J.; Plunkett, M. J. Colloid Interface Sci. 2003, 262, 126– 129. (20) Notley, S. M.; Biggs, S.; Craig, V. S. J.; Wågberg, L. Phys. Chem. Chem. Phys. 2004, 6, 2379–2386. (21) Notley, S. M.; Eriksson, M.; Wågberg, L. J. Colloid Interface Sci. 2005, 292, 29–37. (22) Johannsmann, D. Macromol. Chem. Phys. 1999, 200, 501–516. (23) Bandey, H. L.; Martin, S. J.; Cernosek, R. W.; Hillman, A. R. Anal. Chem. 1999, 71, 2205–2214. (24) Voinova, M. V.; Rodahl, M.; Jonson, M.; Kasemo, B. Phys. Scr. 1999, 59, 391. (25) Kanazawa, K. K.; Gordon, J. G. Anal. Chim. Acta 1985, 175, 99. (26) Norgren, M.; Lindstrom, B. Holzforschung 2000, 54, 519–527. (27) Hunter, R. J. Introduction to Modern Colloid Science; Oxford University Press: New York, 1993. (28) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: New York, 1991. (29) Shubin, V.; Linse, P. J. Phys. Chem. 1995, 99, 1285–1291. (30) Shubin, V. J. Colloid Interface Sci. 1997, 191, 372–377. (31) Fleer, G. J., Cohen Stuart, M. A., Scheutjens, J. M. H. M., Cosgrove, T., Vincent, B. Polymers at Interfaces; Chapman and Hall: New York, 1993. (32) Netz, R. R.; Andelman, D. Phys. Rep. 2003, 380, 1–95. (33) Bohmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288–2301. (34) Dahlgren, M. A. G.; Waltermo, A.; Blomberg, E.; Claesson, P. M.; Sjostrom, L.; Akesson, T.; Jonsson, B. J. Phys. Chem. 1993, 97, 11769–11775. (35) de Meijere, K.; Brezesinski, G.; Kjaer, K.; Mohwald, H. Langmuir 1998, 14, 4204–4209. (36) Rojas, O. J.; Ernstsson, M.; Neumann, R. D.; Claesson, P. M. Langmuir 2002, 18, 1604–1612. (37) Notley, S. M.; Biggs, S.; Craig, V. S. J. Macromolecules 2003, 36 (8), 2903–2906. (38) Plunkett, M. A.; Claesson, P. M.; Ernstsson, M.; Rutland, M. W. Langmuir 2003, 19, 4673. (39) Notley, S. M. Phys. Chem. Chem. Phys. 2008, 10, 1820–1826. (40) Wågberg, L. Nord. Pulp Pap. Res. J. 2000, 15, 586–597.

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