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Measurement of Interaction Forces between Lignin and Cellulose as a Function of Aqueous Electrolyte Solution Conditions Shannon M. Notley*,† and Magnus Norgren‡ Department of Applied Mathematics, Research School of Physical Sciences and Engineering, Australian National UniVersity, Canberra 0200 ACT, Australia, and Department of Natural Sciences, FSCN, Mid Sweden UniVersity, SE-851 70 SundsVall, Sweden ReceiVed June 28, 2006. In Final Form: September 23, 2006 The interaction between a lignin film and a cellulose sphere has been measured using the colloidal probe force technique as a function of aqueous electrolyte solution conditions. The lignin film was first studied for its roughness and stability using atomic force microscopy imaging and quartz crystal microbalance measurements, respectively. The film was found to be smooth and stable in the pH range of 3.5-9 and in ionic strengths up to and including 0.01 M. This range of ionic strength and pH was hence used to measure the surface force profiles between lignin and cellulose. Under these solution conditions, the measured forces behaved according to DLVO theory. The forcedistance curves could be fitted between the limits of constant charge and constant potential, and the surface potential of the lignin films was determined as a function of pH. At a pH greater than 9.5, a short range steric repulsion was observed, indicating that the film was swelling to a large extent but did not dissolve. Thus, lignin films prepared in this manner are suitable for a range of surface force studies.
Introduction Cellulose and lignin are the two major chemical components found in woody tree species accounting for about 75% of the mass of the tree.1 Cellulose is found in both crystalline and amorphous forms and is generally described as a long chain biopolymer of repeat β-(1,4)-D-glucan units. Lignin cannot be so clearly defined. The base molecular subunit of lignin is the phenylpropane unit; however, the way in which these monomers are linked are many and varied, leading to its irregular and highly branched amorphous structure.2 Cellulose is predominantly found in the secondary cell wall of the wood tracheid along with the bulk of the lignin. However, the highest concentration of lignin is found between the fibers in the middle lamella.2 When the fibers are liberated through either chemical or mechanical means, either species may be found at the surface of the fibers. In chemical pulping, cellulose dominates the surface chemistry; however, precipitation of lignin onto the fibers during the latter stages of pulping may alter this fact.3 Bleaching is often used to remove the surface lignin, which causes discoloration or darkening of the pulp, making it unsuitable for use in the subsequent manufacture of fine paper grades. Understanding interactions between lignin and cellulose is of significant interest in paper-making, not the least when it comes to considering the molecular interactions in a fiber-fiber joint. As paper-making is always undertaken in an aqueous environment, a range of surface forces may reasonably be expected to influence the final properties of products prepared from woodbased fibers. These fibers, with either cellulose or lignin rich surface chemistries, typically have significant charges due to hemicellulose and cleavage or oxidative reactions.4 Such charged * Corresponding author. E-mail:
[email protected]. † Australian National University. ‡ Mid Sweden University. (1) Wood and Cellulosic Chemistry; Hon, D. N., Shiraishi, N., Eds.; Marcel Dekker: New York, 1991. (2) Sjostrom, E. Wood Chemistry: Fundamental and Applications, 2nd ed.; Academic Press: New York, 1993. (3) Gellerstedt, G.; Al-Dajani, W. W. Nordic Pulp Paper Res. J. 2003, 18, 52. (4) Fras, L.; Laine, J.; Stenius, P.; Stana-Kleinschek, K.; Ribitsch, V.; Dolecek, V. J. Appl. Polym. Sci. 2004, 92, 3186.
groups will influence the interaction energy between fibers with factors such as pH and ionic strength of the solution, which are important due to the predominantly weakly dissociable nature of the charges.5,6 Thus, a fundamental understanding of the magnitude of the apparent surface forces is necessary, not the least because this forms the basis for subsequent surface modifications used to impart improved properties to the fiber web through, for example, polymer and polyelectrolyte adsorption. Model studies of the interaction between cellulose and lignin, however, are difficult to perform on these natural systems due to the great structural and chemical heterogeneities of real wood fibers liberated under traditional pulping conditions. Hence, there has been an increasing interest in the development of model films and substrates of cellulose and lignin with controlled geometry for such studies.7-16 Cellulose films have received significant attention in recent literature, and extensive work has often focused on surface force applications.7,17-21 Model cellulose (5) Israelachvili, J. Intermolecular and surface forces, 2nd ed.; Academic Press: New York, 1991. (6) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114. (7) Neumann, R. D.; Berg, J. M.; Claesson, P. M. Nordic Pulp Paper Res. J. 1993, 8, 96. (8) Gunnars, S.; Wågberg, L.; Cohen-Stuart, M. A. Cellulose 2002, 9, 239. (9) Fa¨lt, S.; Wågberg, L.; Vesterlind, E. L. Langmuir 2003, 19, 7895. (10) Kontturi, E.; Thune, P. C.; Niemantsverdiet, J. W. Polymer 2003, 44, 3621. (11) Eriksson, J.; Malmsten, M.; Tiberg, F.; Honger Callisen, T.; Damhus, T.; Johansen, K. S. J. Colloid Interface Sci. 2005, 284, 99. (12) Lee, S. B.; Luner, P. Tappi J. 1972, 55, 116. (13) Micic, M.; Radotic, K.; Benitez, K.; Ruano, M.; Jeremic, M.; Moy, V.; Mabrouki, M.; Leblanc, R. M. Biophys Chem. 2001, 94, 257. (14) Constantino, C.; Dhanabalan, A.; Coota, M.; Pereira-da-Silva, M. R.; Curvelo, A.; Oliveira, O. N. J. Holzforschung 2000, 54, 55. (15) Pasquini, D.; Balogh, D. T.; Olivera, O. N. J.; Curvelo, A. A. S. Colloids Surf., A 2005, 252, 193. (16) Maximova, N.; Osterberg, M.; Laine, J.; Stenius, P. Colloids Surf., A 2004, 239, 65. (17) Holmberg, M.; Berg, J.; Stemme, S.; Odberg, L.; Rasmusson, J.; Claesson, P. J. Colloid Interface Sci. 1997, 186, 369. (18) Notley, S. M.; Petterson, B.; Wågberg, L. J. Am. Chem. Soc. 2004, 126, 13930. (19) Nigmatullin, R.; Lovitt, R.; Wright, C.; Linder, M.; Nakari-Setala, T.; Gama, M. Colloids Surf., B 2004, 35, 125. (20) Notley, S. M.; Wågberg, L. Biomacromolecules 2005, 6, 1586.
10.1021/la0618566 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/09/2006
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films have been used to demonstrate a range of surface forces including dispersion,18 electrostatic,20,21 and electrosteric interactions.22 Recently, it was shown that the model surface preparation procedure greatly influences the observed potential energy of interaction.21 Thus, careful consideration needs to be given to how the model film relates to the original, raw material and if the model surface can indeed be used as a substitute. It is likely that due to the various forms of naturally occurring cellulose, more than one type of model film should be used to appropriately describe all the potential interactions. A number of preparation techniques have been described in the literature for lignin films. Because of its rather difficult to define chemical nature, the source of lignin has varied considerably and has included material isolated from both softwood12 and hardwood as well as sugarcane bagasse.14 Furthermore, the manner in which the film has been formed has varied and has included simple evaporation techniques as well as the LangmuirBlodgett technique. Stable, continuous model lignin films have also been prepared through a spin-coating procedure.23 In these films, a softwood kraft lignin was used as the raw material source.2 The kraft process results in an increase in charged groups on the lignin macromolecules mainly due to the cleavage of the β-O-4 ether linkages during the cooking of the wood chips due to nucleophilic attack of the hydrogen sulfide ion under the highly alkaline conditions. Thus, lignin films prepared using this derivative are somewhat chemically modified but in essence retain similar moieties to that of natural lignin. ToF-SIMS analysis of lignin films prepared in this manner showed that the base guaiacyl residues were still present in the formed film.23 A range of different solvent systems for lignin was investigated with the ammonium hydroxide solution found to give the best films in terms of stability and roughness. Minimizing the roughness is particularly important for subsequent surface force applications, especially when measuring using the colloidal probe technique as significant roughness will introduce errors in the determination of the point of contact. Furthermore, the films need to be stable over a wide range of ionic strengths and pH in order for the films to be useful under relevant or industrial conditions. The aim of this study is to probe the molecular interactions between cellulose and lignin as a function of aqueous solution conditions using surface force measurements. By using welldefined model surfaces, the potential energy of interaction between cellulose and lignin may be determined, leading to a better understanding of the role molecular forces play in the formation of fiber-fiber bonds in many paper grades. Materials and Methods Materials. All chemicals used in this study, apart from the lignin samples, were of analytical grade and were supplied by SigmaAldrich without further purification. Milli-Q water was used in the preparation of all aqueous solutions. The salt used was NaCl. The pH value of the solutions was adjusted using an appropriate amount of either HCl or NaOH. The lignin derivative used in the study was an isolated kraft lignin from softwood (Picea abies) with a weight averaged molecular mass of Mw ) 5670 g mol-1 and a polydispersity index of Mw/Mn ) 1.54 according to pulsed field gradient 1H NMR self-diffusion measurements. Details of the isolation and purification procedures have recently been published.22 The silica wafers were supplied by Peregrine Semiconductors Pty Ltd. (Sydney, Australia) with a native (21) Notley, S. M.; Eriksson, M.; Wågberg, L.; Beck, S.; Gray, D. G. Langmuir 2006, 22, 3154. (22) Carambassis, A.; Rutland, M. Langmuir 1999, 15, 5584. (23) Norgren, M.; Notley, S. M.; Majtnerova, A.; Gellerstedt, G. Langmuir 2006, 22, 1209.
Notley and Norgren oxide layer of 4 nm as measured by ellipsometry. The silica surfaces were rinsed in 10% w/w aqueous NaOH for 30 s followed by rinsing with copious amounts of Milli-Q water before being blown dry with N2. As a final step, the surface was subjected to a mild water plasma treatment to ensure that the silica substrate was clean and wet. AFM imaging of the surfaces showed that these cleaning treatments did not appreciably increase the surface roughness of the silica substrate. Amorphous cellulose spheres were provided by MonoGel AB (Helsingborg, Sweden). The spheres were solvent exchanged to ethanol before a light heat treatment at 40 °C for 2 h to remove the solvent prior to attachment to the cantilever. Reverse imaging of the cellulose spheres using atomic force microscopy was performed to determine the surface roughness of the interacting area. These cellulose spheres have previously been successfully used as colloidal probes in surface force measurements due to their limited swelling.20 Methods. Lignin thin films were prepared using a previously described and optimized method.23 A 3% w/w solution of lignin in 0.75 M ammonium hydroxide was spin-coated onto oxidized silicon wafers or AT-cut quartz crystals at 1500 rpm for 1 min. This produced uniform films of lignin with a typical thickness of 80-90 nm as measured by ellipsometry. These films were then used in surface force measurements. The colloidal probe technique was used to measure the interaction potential between lignin thin films and cellulose spheres in aqueous electrolyte solution. A Multimode Scanning Probe Microscope (Veeco Ltd, Santa Barbara, CA) was used for all force experiments conducted in this study. A cellulose sphere (of size between 5 and 20 µm) was attached to the end of an AFM cantilever with a small amount of epoxy adhesive according to the method of Ducker et al.24 Standard, tipless, contact SiO2 cantilevers (MikroMasch, Tallinn, Estonia) were used for force measurements in this study. Using the thermal noise method, the cantilever spring constant was determined to be 0.60 N/m.25 Typical force-distance experiments have previously been described in detail.26 The measured deflection of the cantilever was converted to a force through application of Hookes’ Law after calibration of the optical sensitivity factor and knowledge of the spring constant. The interaction potential can be related to the observed force through normalization with the colloidal probe radius through the Derjaguin approximation. The force-distance data were typically fit to DLVO theory within the boundary limits of constant charge and constant potential for the solution of the nonlinear Poisson-Boltzmann equation.27 The procedure employed allowed fitting of the asymmetric surface potentials for the two different surfaces as a function of aqueous solution conditions. At least 20 force-distance curves were analyzed, and the data presented here are representative of the average of these curves. Force measurements were undertaken after a minimum of 1 h equilibration time. The quartz crystal microbalance (Q-Sense D300, Gothenburg, Sweden) was used to measure the stability and swelling of the lignin films as a function of pH. Lignin films were spin-coated onto silica coated AT cut quartz crystals. The resonance frequency and overtones were monitored as a function of time as well as the energy dissipated through the layer. Any change in the observed frequency can be related to mass uptake through the Sauerbrey equation,28 while the change in dissipation gives a qualitative indication of the layer viscoelastic properties.29 Higher energy dissipation implies a softer film or layer. The pH adjusted aqueous electrolyte solutions were flushed through the cell to observe changes in the lignin film properties.
Results Figure 1 shows representative AFM tapping mode images of the lignin films used in this study. The surface roughness of the (24) Ducker, W. A.; Senden, T. J.; Pashley, R. M. Nature 1991, 353, 239. (25) Hutter, J. L.; Bechhoefer, J. ReV. Sci. Instrum. 1993, 64, 1868. (26) Senden, T. J. Curr. Opin. Colloid Interface Sci. 2001, 6, 95. (27) Chan, D. Y. C.; Horn, R. G. J. Chem. Phys. 1985, 83, 5311. (28) Sauerbrey, G. Z. Phys. 1959, 155, 206. (29) Rodahl, M.; Ho¨o¨k, F.; Krozer, A.; Brzezinski, P.; Kasemo, B. ReV. Sci. Instrum. 1995, 66, 3924.
Surface Forces between Cellulose and Lignin
Figure 1. AFM tapping mode height image of a lignin film used in this study. The rms roughness was calculated to be 1.1 nm over this 1 µm2 image. Typically, five images were recorded over different areas of the film; furthermore, multiple films were used in the surface force measurements.
films was evaluated to be 1.1 nm over a 1 µm2 image. The lignin films are continuous over length scales of greater that 2 mm and very smooth on the micrometer scale, which is of great importance when measuring surface forces. High surface roughness can lead to asperity contacts between the film and the sphere, resulting in difficulty in reaching the hard-wall compliance necessary to accurately scale the force-distance curves with surface forces measurements using the colloidal probe technique. Previously, it was shown that films prepared from the same raw material and in the same manner were smooth over an area of 100 µm2, which is significantly greater than the typical interaction area in a surface force measurement using the colloidal probe technique.23 These lignin films were then used in subsequent surface force measurements. The surface potential of the cellulose spheres used in this study has previously been measured against silica and was -3 mV.21 While only the magnitude of the surface potential can be determined in this way, knowledge of the raw material implies that the cellulose sphere carries an anionic charge.4 Furthermore, it has been shown that the cellulose spheres essentially behave as solid materials, with no observation of steric layers extending away from their interface, which has been observed with other types of spheres.22 Surface forces between cellulose and lignin were measured as a function of varying ionic strength and solution pH. Previously, it has been demonstrated that the lignin films are stable up to ionic strengths of 100 mM and pH values up to 9.23 Above this pH, the lignin films may be expected to significantly degrade due to the increase in solubility of lignin due to the increasing dissociation of phenolic groups.30 Thus, forces in the ionic strength range of 0.1-10 mM and pH 3.5-8.5 were typically measured. However, to test the stability of these films, qcm measurements were undertaken as a function of pH. Figure 2 shows the change in frequency and dissipation as the solution pH is increased. Initially, Milli-Q water adjusted to pH 7 was introduced into the chamber. The injection spikes were due to changes in the pressure that the crystal experiences while fluid is flowing into the chamber. Very little change in the film is observed up to pH 9. However, at pH 9.5, the significant decrease in frequency of approximately (30) Gunnarson, G.; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1980, 84, 3114.
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Figure 2. Change in frequency (closed diamonds) and dissipation (open squares) as a function of pH for a lignin film subjected to aqueous electrolyte solutions with increasing pH. The spikes upon injection are due to slight pressure changes on the crystal due to the flow of solution into the measurement chamber. Injection at time ) 0 s was pH 7, t ) 300 s was pH 8.5, t ) 600 s was pH 9, and t ) 900 s was pH 9.5.
Figure 3. Normalized force-distance curve for the interaction of cellulose sphere and kraft lignin film at a pH of 8.5 in a background electrolyte of NaCl with a concentration of 0.1 mM. The data were fit to DLVO theory in the limits of constant charge (upper fit) and constant potential (lower fit). The fitting parameters were ψcell ) -3 mV, ψlig ) -75 mV, and κ-1 ) 30 nm. Inset shows the same data on a log-linear scale to demonstrate the exponential decay.
18 Hz indicates a mass uptake due to swelling of the lignin film. Furthermore, the dissipation increases, confirming the softer nature of the lignin film. It is interesting to note that after the initial swelling of the film upon injection of the aqueous solution at pH 9.5, the frequency once again increases until equilibrium is reached some 10 min later. This overshooting behavior will be discussed in more detail later. Figure 3 shows the force-distance interaction between cellulose and lignin at pH 8.5 and an ionic strength of 0.1 mM. The interaction is monotonically repulsive and can be satisfactorily fit using DLVO theory within the limits of constant charge and constant potential. Using a cellulose surface potential of -3 mV, the surface potential of the lignin film was determined to be -75 mV. The decay of the interaction behaves as expected from DLVO theory with a fitted Debye length of 30 nm measured. A small jump into contact is also noticeable from a surface separation of 3 nm. Normalized force-distance curves for the interaction between cellulose and lignin as a function of pH at an ionic strength of 0.1 mM are shown in Figure 4. The interaction is repulsive at all pH values for this particular ionic strength; however, the magnitude of this repulsive force decreases with decreasing pH. This decrease is somewhat expected as the dissociation of the
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Figure 4. Normalized force-distance curves for the interaction of a cellulose sphere and lignin film as a function of pH at an ionic strength of 0.1 mM. Fitting parameters are κ-1 of 30 nm and ψ0 ) -5, -60, and -75 mV for pH 3.5, 5.8, and 8.5, respectively.
Figure 5. Normalized force-distance curves for the interaction of a cellulose sphere and lignin film as a function of ionic strength at pH 8.5. Closed squares are 0.1 mM NaCl, and open diamonds are 1 mM NaCl. Inset shows the same data but on a log-linear scale to demonstrate the exponential decay of the normalized force with distance.
dominant charged species in the lignin film in this pH interval, that being carboxyl groups, is highly influenced by changes in solution pH. Furthermore, it may be expected that the surface potential of the cellulose sphere will also decrease somewhat with decreasing solution pH due to a small number of carboxyl groups. All curves in Figure 4 could be fitted with DLVO theory between the limits of constant charge and constant potential. The carboxyl group dissociation will also be influenced by the ionic strength of the solution.4,6 Furthermore, it may be expected that the measured surface potential for a given surface charge decreases with increasing electrolyte concentration as demonstrated by considering the Grahame equation.5 As such, the interaction between cellulose and lignin as a function of ionic strength was measured at pH 8.5 and is shown in Figure 5. The measured force-distance curve on approach shows a significant decrease in the electrostatic repulsion as the ionic strength is increased. The curves can be fit with DLVO theory. As expected, the electrostatic repulsion decays more rapidly with an increase in salt concentration due to charge screening effects. In Figure 6, the results from a theoretical attempt to calculate the decay of the electrostatic potential of the kraft lignin film is shown. Surface potentials are calculated by using the PoissonBoltzmann cell model31 in planar symmetry at the actual pH values chosen for the measurements shown in Figure 4. The calculations are based on experimentally obtained data of the (31) Norgren, M.; Lindstrom, B. Holzforschung 2000, 54, 519.
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Figure 6. Calculated electrostatic potential as a function of distance from the lignin model surface at various pH values. At pH 8.5, the influence of change in background concentration of a monovalent salt is also shown.
number of phenolic groups per structural unit30,32 and carboxylic groups33 in a softwood kraft lignin of representative molecular mass in comparison to the present lignin sample. Thus, the numbers of phenolic groups and carboxylic groups are set to 7.6 and 0.8, respectively, per kraft lignin molecule at a molecular mass of 5700 g mol-1. The dissociation behavior of the lignin film is modeled by assuming pKa ) 4.8 for carboxylic groups and pKa ) 10.2 for the phenolates in the kraft lignin at a temperature of 298 K. From the calculations, it is found that the degrees of dissociation of the carboxylic groups at the surface of the lignin film are 0.05, 0.67, and 0.99 at 0.1 mM of a monovalent salt for the three pH values reported. The degree of phenolic dissociation is zero at the same conditions. Although the calculated electrostatic surfaces potentials in Figure 6 generally are a little higher than determined from the force measurements in Figures 4 and 5, they are of the same magnitude. The calculations clearly demonstrate how the absolute value of the electrostatic potential increases by increasing pH and decreases by increased salt concentration in the system. These trends correlate very well with the results in Figures 4 and 5 and support that the strength of the repulsive interaction observed at a given background electrolyte concentration is solely due to the degree of dissociation of carboxylic groups in the kraft lignin film under these conditions. Figure 2 showed that lignin films prepared using this method were stable at pH 9 and below with relatively little swelling. In a further test of the lignin film stability, surface forces were measured between the cellulose sphere and the lignin film at pH 9.5 with a background electrolyte concentration of 0.1 mM. Figure 7 shows a typical interaction profile. The data in Figure 7 can be satisfactorily fit within the limits of constant charge and constant potential at separations greater than 5 nm. However, a short range steric force is observed at separations less than 5 nm. This is most probably due to swelling of the lignin film at a pH greater than 9 due to a beginning dissociation of the phenolic groups in the lignin resulting in a softer, water rich film that is easily compressible. From the theoretical calculations, it is found that the degree of dissociation of phenolic groups in the lignin film is approximately 0.5% at pH 9.5 and a background electrolyte (32) Norgren, M.; Lindstrom, B. Holzforschung 2000, 54, 528. (33) Argyropoulos, D. S. J. Wood Chem. Technol. 1994, 14, 45.
Surface Forces between Cellulose and Lignin
Figure 7. Normalized force-distance curve for the interaction between cellulose sphere and lignin film at pH 9.5 and an ionic strength of 0.1 mM. Solid line and dotted line show the limits of constant charge and constant potential, respectively. The fitting parameters were ψcell ) -3 mV, ψlig ) -80 mV, and κ-1 ) 30 nm. The fits were offset by 4 nm to account for the observed steric interaction force.
concentration of 0.1 mM. If the ionic strength is raised to 1 and 10 mM, respectively, the corresponding degree of phenolic dissociation is 1.7 and 5.3%. This clearly indicates that under these conditions, the lignin film is very close to instability. Moreover, from a theoretical viewpoint, the swelling observed in Figure 7 and a subsequent dissolution of the lignin could be expected in this pH region.
Discussion Lignin films prepared according to a previously optimized technique were used in the surface force measurements presented here. The films were shown by atomic force microscopy tapping mode imaging in air to be very smooth. Hence, it was reasonable to assume that because of the relatively low roughness of the lignin surfaces that they would show potential in subsequent surface force measurements. The lignin films were first subjected to a range of solution conditions to determine their stability. Figure 2 shows that the films are stable in pH adjusted Milli-Q water solutions up to and including pH 9.5 before they dissolved. However, significant swelling was seen at pH 9.5. Thus, in this range of pH values, it can reasonably be expected that the films will be stable in solutions of higher ionic strength as any charges will be screened resulting in a lower effective swelling pressure. Interestingly, the qcm data in Figure 2 show that the initial swelling of the lignin film at pH 9.5 is followed by some relaxation of the layer. This swelling behavior of polymer layers due to changes in solution pH has previously been observed with the qcm by Notley et al.34 The overshooting behavior of the swelling kinetics as seen for the change in pH from 8.5 to 9.5 is attributed to a cooperative physical cross-linking of the lignin gel-like material through hydrogen bonding between the available phenolic groups on the macromolecular lignin structures. Such overshooting has been routinely observed for the swelling of many types of hydrogels.35 Changes in the layer or film structure could not, however, be observed in the colloidal probe force data as the relatively short time scale of this overshooting phenomena could not be reproducibly probed due to the inherent difficulty in performing force measurements rapidly after changes in the solution conditions. Previously published data showed that the cellulose spheres used in this study have a relatively low charge. Thus, the high (34) Notley, S. M.; Biggs, S.; Craig, V. S. J.; Wågberg, L. Phys. Chem. Chem. Phys. 2004, 6, 2379. (35) Diez-Pena, E. Q.-G.; I.; Barrales-Rienda, J. M. Macromolecules 2003, 36, 2475.
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electrostatic repulsion observed in the force measurements at low ionic strength and high pH between the cellulose sphere and the lignin film must be due to a relatively higher charge on the lignin polymers than on the cellulose sphere. Fitting of the normalized force-distance curve between the limits of constant charge and constant potential gave a surface potential for the lignin film of -75 mV at pH 8.5. Again, the sign of the potential cannot be determined from force measurements; however, this was inferred from knowledge of the charged groups. By converting the surface potential using the Grahame equation5 to a relative charge per area gives one charge per 67 nm2. This is perhaps a greater charge density than expected for lignin. During the kraft cooking procedure, which involves depolymerization of lignin by cleavage of ether linkages due to nucleophilic attack by hydrogen sulfide ions under alkaline conditions, a significant number of carboxyl groups are introduced possibly originating from oxidations of aliphatic hydroxyl groups and aromatic ring cleavage. Indeed, in comparison to more native lignin samples that may have a carboxylic content of much less than one in 100 phenylpropane groups,2 in softwood kraft lignin, typically about 5% of the monomers have carboxylic groups attached.33 The measured force interactions between cellulose sphere and lignin film show the expected trends for the dominating charged species being carboxyl groups. In almost all cases, the forcedistance data were satisfactorily fitted with DLVO theory with the correctly predicted decay (Debye length) for the given ionic strength. Furthermore, the measured surface potential decreased, as expected with decreasing pH due to lesser dissociation of the carboxyl groups. Interestingly, under aqueous conditions of high salt and low pH, where repulsive forces due to the overlap of the respective electrical double layers are minimized, no jump into contact due to dominating attractive dispersion forces was observed. This is perhaps not so surprising considering that both cellulose sphere and lignin film will somewhat swell under aqueous conditions leading to a reduction in the apparent van der Waals interaction force as the effective Hamaker constant will be greatly diminished.18 Another possibility is that the sodium ions have a strong specific ionic interaction with the carboxyl groups of the lignin film leading to a short range hydration force. Ion specificity in kraft lignin precipitation concerning both cations and anions has earlier been reported.36 Probing specific ionic interactions with lignin model films is currently under investigation. It is unlikely, however, that the small scale roughness of the sphere or film influences the lack of measurable dispersion interactions. Figure 3 shows that a small jump into contact is observable at a higher pH (greater charge dissociation), supporting the possibility of hydration forces in this system over roughness effects. Steric interactions could be observed between lignin film and cellulose film at elevated pH and low ionic strength. Charge repulsions within the film cause added swelling pressures. Indeed, the qcm data showed that at pH 9.5, significant swelling is observed. This observed steric force was short range and outside of this regime; the decay of the force was well-fitted with DLVO theory, indicating that the origin of the force was electrosteric in nature. Previous studies have shown that similar films are unstable at higher pH, and the qcm and colloidal probe measurements described in this paper support this view.23 Understanding the apparent surface forces between kraft lignin and cellulose has many practical implications, not the least in paper-making. While the lignin sample used here may not strictly be considered well-suited to describing mechanical pulp due to its relatively high charge, in many chemical pulping situations, (36) Norgren, M.; Edlund, H. Nordic Pulp Paper Res. J. 2003, 18, 400.
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relatively high charged lignin polymers may precipitate onto wood fiber surfaces.37 This effectively leaves the fiber with a higher anionic charge that will influence the potential energy of interaction with other species such as cellulose and inorganic particles such as silica or calcium carbonate. The apparent charge and surface forces present in cellulose and lignin systems will also influence polymer and polyelectrolyte adsorption. Polyelectrolytes are used for a range of purposes in the production of many paper and board grades including strength improvement and aiding in retaining fine inorganic filler particles. It is hence of vital importance to understand surface interactions between cellulose and lignin over a range of aqueous solution conditions.
Conclusion Lignin films prepared from a softwood kraft lignin source were successfully used to probe the interaction with cellulose in aqueous electrolyte solutions. The films were stable in a pH range of 3.5-9 as shown with a combination of qcm and colloidal probe measurements. Typically, the interaction between cellulose and lignin was dominated by electrostatic forces. The low surface potential of the cellulose sphere was in stark contrast to that of the lignin film, which had a higher surface potential, particularly at elevated pH. This was attributed to the introduction of an (37) Norgren, M.; Edlund, H.; Wagberg, L.; Annergren, G. Nordic Pulp Paper Res. J. 2002, 4, 370.
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increased number of carboxyl groups onto the lignin molecular structure due to the kraft pulping procedure. The dissociation of these charged groups was both pH and ionic strength dependent, which influenced the observed electrostatic interaction potential. The force-distance curves could be fit to DLVO theory in the limits of constant charge and constant potential. However, a short range steric interaction was observed for the interaction at pH 9.5, a solution condition known to cause significant swelling of the lignin film as shown through qcm measurements. Thus, these smooth films of lignin may be used as an ideal model surface for subsequent measurements of a range of surface force interactions over industrially relevant conditions potentially in the presence of a variety of additives such as polymers, polyelectrolytes, surfactants, and inorganic particles. Acknowledgment. S.M.N. acknowledges financial support from the Cooperative Research Centre for Functional Communication Surfaces (CRC SmartPrint). M.N. gratefully acknowledges The Alf de Ruvo Memorial Foundation for a postdoctoral fellowship. Helpful discussions with Vince Craig and Tim Senden, ANU, are acknowledged. Go¨ran Gellerstedt and Andrea Majtnerova, KTH, are thanked for providing the softwood kraft lignin sample. LA0618566
