JOURNAL OF PULP AND PAPER SCIENCE: VOL. 27 NO. 11 NOVEMBER

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1 Physiochemical Differences Between Dissolved and Precipitated Kraft Lignin Fragments as Determined by PFG NMR, CZE and Quantitative UV Spectrophotometry M. NORGREN, H. EDLUND and N.-O. NILVEBRANT A combination of analytical techniques, specifically capillary zone electrophoresis, 1 H pulsed-field gradient nuclear magnetic resonance selfdiffusion measurements and quantitative UV spectrophotometric measurements, was used to investigate physicochemical differences between dissolved and precipitated kraft lignin (KL) fragments, obtained from the same sample. Precipitation was induced by heating alkaline (poh 4) Indulin AT solutions, containing various concentrations of NaCl ( mol/l), at 75 C. Depending on the salt concentration in the samples, different amounts of KL were precipitated. The KL precipitated at the lowest NaCl concentrations was found to consist of the largest lignin fragments whereas, at high NaCl concentrations, the KL fragments in the supernatants were found to be of comparably lower mean molecular weights. From the outcome of the investigation, it was found that the combination of analytical techniques used provides the possibility of collecting important information about physicochemical characteristics related to the solution behaviour of industrial lignins. Une combinaison de techniques analytiques, et particulièrement l électrophorèse de zone sur colonne capillaire, les mesures d auto-diffusion de la résonance magnétique nucléaire sur gels en champ pulsé 1 H et des mesures par spectrophotométrie UV-visible quantitative, ont été utilisées pour analyser les différences physico-chimiques entre des fragments dissous et précipités de lignine kraft (KL), obtenus du même échantillon. La précipitation a été induite en chauffant des solutions alcalines (poh 4) Indulin AT, à diverses teneurs en NaCl (0,20-1,0 mol/l, à 75 C). Selon la teneur en sel des échantillons, des quantités différentes de KL ont été précipitées. Le KL précipité aux teneurs les plus faibles en NaCl comprenait les fragments de lignine les plus gros, tandis qu à de fortes teneurs en NaCl, les fragments de KL dans les liquides surnageants étaient d un poids moléculaire moyen comparativement inférieur. Selon les résultats de l analyse, nous avons trouvé que la combinaison des techniques d analyse a permis de recueillir des données importantes sur les caractéristiques physico-chimiques reliées au comportement des solutions de lignines industrielles. INTRODUCTION In recent years, knowledge of the very complex structure of native and industrial processed lignins has been greatly improved by the development of more sophisticated analytical M. Norgren and H. Edlund* Dept. Chem. Process Technol. Mid Sweden Univ. SE Sundsvall, Sweden (magnus.norgren@kep.mh.se) N.-O. Nilvebrant STFI Box 5604 SE Stockholm, Sweden *Now with: Dept. Chem. Eng Univ. Delaware Newark, DE, USA methods [1]. Among those, different nuclear magnetic resonance (NMR) spectrometric techniques have been found particularly useful [2 5]. By applying magnetic field gradients, the NMR technique also brings about new possibilities in determining hydrodynamic properties, such as the self-diffusion coefficient of the molecules and, moreover, of mixtures of different molecules in the same sample [6]. Sometimes even the molecular weight distribution may be determined in a polydisperse macromolecular sample [7,8]. When discussing lignins in general and kraft lignins specifically, one has to remember that they are polydisperse in many senses [9]. The often very wide molecular weight distribution certainly affects the physicochemical characteristics of lignin samples, also making many other properties vary. For instance, it is known that the number of phenolic groups per structural unit goes down rapidly when the molecular weight increases [10]. Furthermore, the apparent pk a value of kraft lignin (KL) has been shown to increase with the molecular weight due to an increased electrostatic counterion attraction, including hydrogen ions, as the size of the fragments increase [11]. The latter might also influence the overall solubility and the colloidal stability of aqueous alkaline lignin solutions. Rudatin et al. declared that the presence of large lignin macromolecules facilitates self-association of lower molecular weight lignin [12]. Like Lindström [13], they found that factors influencing the solution conditions, such as the ph value and ionic strength, were important to consider in controlling the stability of KLs. JOURNAL OF PULP AND PAPER SCIENCE: VOL. 27 NO. 11 NOVEMBER

2 KL samples contain distributions of macromolecules showing polydispersity in a number of properties, for example number of potential charges and hydrodynamic radii. This introduces the possibility of using analytical methods, which are sensitive to small variations in the electrochemical characteristics of the macromolecules. By studying the behaviour of KL macromolecules in an electric field, using capillary zone electrophoresis (CZE), differences in migration time due to variations in surface charge densities between low and high molecular weight species have been observed [14]. One other important parameter is the influence of temperature on the KL solution behaviour. Macromolecules in solution are known to behave differently at room temperature than at elevated temperatures, e.g. dissociation, solubility and conformation may increase or decrease [15]. Recently, it was shown that, when the temperature is elevated in an alkaline KL solution, the degree of dissociation of free phenolic groups decreases [11]. The main reason for this may be explained by the temperature dependency of the ionization constant of water, which influences the dissociation of phenolic groups negatively as the temperature increases. The effect of temperature on the dissociation behaviour is described in Eq. (1): [ O ] K ( ) a T [ ] ( ) [ OH OH 1 ] (1) Kw T where and K a are the degree of dissociation and the dissociation constant, respectively. [ OH] is the concentration of a protonated acidic group and [ O ] the concentration of its corresponding base. [OH ] is the hydroxyl ion concentration in the system, which at higher concentrations can be considered as independent of the temperature-induced changes in the autoprotolysis of water (K w ). This certainly accredits the use of poh as a better description of the alkalinity of a solution at varying temperatures compared to the more commonly utilized ph value. From Eq. (1), it is found that, if the K a /K w ratio decreases, the net dissociation of a weakly acidic group also will decrease. The only possible way to maintain a constant degree of dissociation is then by increasing the hydroxyl ion concentration in the system. Furthermore, if the solution conditions get worse, e.g. if the poh, the ionic strength and/or the temperature increase, the stability of dissolved KL has been found to decrease [16]. However, since KL samples are polydisperse, it also would be natural to assume that the solution behaviour would vary among fragment species of different sizes. In the present study, a combination of analytical techniques, capillary zone electrophoresis (CZE), 1 H pulsed-field gradient (PFG) NMR and quantitative UVspectrophotometric analysis, has been utilized to investigate dissolved and precipitated KLs, obtained from the same sample source, treated under different solution conditions. EXPERIMENTAL Materials Commercially available KL (Indulin AT, Stockholm, Sweden, lot 123H0189) was supplied by Sigma-Aldrich. The molecular weight distribution in the KL was determined by high-performance size exclusion chromatography (HPSEC). The KL was acetylated by standard procedures [17]. The acetylated KL was dissolved in tetrahydrofuran (THF) and filtered through a 0.45 m membrane filter. In the analysis, a Waters 510 high-pressure liquid chromatography (HPLC) pump, a Waters 410 Differential Refractometer and a 20 m loop were used. The columns used were in the cut-off range of 100, 50 and 5 nm mounted in a series. The HPSEC system was calibrated by polystyrene standards. From the HPSEC analysis, the weightand number-averaged molecular weight average and the polydispersity index were determined to M w = 5500 gmol -1, M n = 1060 gmol -1 and M w /M n = 5.2, respectively. Sample Treatment A stock solution containing 1.0 gl -1 Indulin AT was prepared by dissolving 1.0 g KL in 30 ml 0.1 mol/l NaOH in a 1 L flask. After a few minutes, water was added in portions of 50 ml, up to about 0.5 L. The KL was allowed to dissolve overnight under magnetic stirring. Finally, the rest of the water was added and a few drops of 1 mol/l HCl or 6 mol/l NaOH adjusted the ph. Thereafter, 30 ml of the stock solution together with the desired amount of salt was poured into different cuvettes. Different amounts of salt were added ([NaCl] = mol/l), the hydroxyl ion concentration in the samples was adjusted to poh 4 (equals ph 10 at room temperature; see previous section for the motivation for the use of the poh scale) and the samples were stored at 75 C for 64 h. Thereafter, they were equilibrated at room temperature and centrifuged at 4000 g for 10 min. The supernatants were gently decanted and the precipitates were dissolved in 0.1 mol/l NaOH. Before the samples were put in vials and freeze-dried they were acidified, centrifuged and the precipitates were washed three times with water to remove the salt excess. UV-Spectrophotometric Determination of Lignin Concentrations The concentration of KL in the initial and supernatant solutions was determined by UV spectrophotometric measurements at 280 nm on a Shimadzu UV-160A double-beam spectrophotometer. All samples to be measured were prepared in mol/l NaOH (constant ph). The extinction coefficient of Indulin AT was determined to be = 19.4 L g -1 cm -1.No detectable absorbency effects due to the sodium chloride content could be found at the wavelength of analysis. The degree of precipitation, ß, was defined as ß = (1-C sup )/C init, where C sup and C init are the KL concentration in the supernatant (after centrifugation) and the initial stock solution, respectively. Capillary Zone Electrophoresis The CZE was performed on a Waters Capillary Ion Analyzer with a 45 cm x 50 µm i.d. fused silica capillary. A voltage of 20 kv was applied and the temperature was kept at 25 C in the separation compartment. The electrolyte used in the experiments was a 100 mmol/l glycine sodium hydroxide solution, adjusted to ph 12 [18]. Pyridine (0.1% V/V) was used as a neutral marker. The electropherograms were registered and calculated by the EZchrom software (Scientific Software, Illinois, USA). 1 H PFG NMR Self-Diffusion Measurements The different Indulin AT samples were weighed into 1 ml vials and 0.1 mol/l NaOD was added. By rotating the vials, the KL was dissolved. After 12 h, 5 mm NMR tubes were filled up to a height of 1 cm with the sample solutions. Echo decays were obtained through measurements on a Bruker DPX 250 MHz NMR spectrometer equipped with a Bruker diffusion probe, capable of providing 12 Tm -1 at 40 A. The pulsed field gradient spin-echo (PGSE) sequence [6] was applied and the method of gradient incrementation at constant diffusion time was used. Typical settings; = 40 ms, = 1.2 ms, g [0.1, 2.4] Tm -1 and 64 scans. A delay between subsequent scans of 10 s was applied to provide for sufficient T 1 relaxation. NaOD and D 2 O used in the NMR study were supplied by Cambridge Isotope Laboratories (Massachusetts, USA). RESULTS AND DISCUSSION KL Macromolecular Differences as Determined by CZE In CZE, molecular variations in electrical charge and size can be used to separate and detect different macromolecules in a sample [19]. In the present investigation, the same KL stock sample solution was treated at poh 4 and 75 C and the NaCl concentration was tuned to induce fractionation of the sample in various precipitated and soluble fractions. All the precipitated KL samples and the supernatant at the highest NaCl concentration (1.0 mol/l) were redissolved at ph = 12 and analyzed by CZE. From the electropherograms obtained (Fig. 1), it is found that the precipitated KL in the sample containing the highest salt concentration, [NaCl] = 1.0 mol/l, shows a comparably long migration time, 10 min 32 s (Table I). As the ionic strength decreases, the migration time and the peak width of the redissolved KL macromolecules also decrease. The CZE analysis of the supernatant at the highest salt concentration (1.0 mol/l) reveals an even longer mean migration time, 11 min 37 s, and a much larger peak width. In a distribution of macromolecules, there always are some high as well as low molecular weight fragments, containing both higher and lower amounts of ionic groups 360 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 27 NO. 11 NOVEMBER 2001

3 1 neutral marker I = 1.0 mol/l I = 1.75 mol/l I = 1.50 mol/l Intensity (a.u.) mol/l (p) 0.50 mol/l (s) 0.75 mol/l (s) 1.0 mol/l (s) I = 1.20 mol/l Migration time (min) Fig. 1. CZE electropherograms showing macromolecular differences between kraft lignin samples obtained at various degrees of precipitation (Table I). Pyridine was used as a neutral EOF marker k G Fig. 2. Intensity data determined by 1 H PFG self-diffusion measurements on some sample supernatants (s) and one of the precipitates (p) obtained at various NaCl concentrations. The log-normalized intensity, I, is plotted ( a. u. arbitrary units) versus k G ( 2 2 g 2 )( /3). k G reflects the strength of the applied field gradient pulses. than the average molecules. This, of course, will induce peak broadening in the electropherograms. Differences in KL Size Distribution Determined by Self-Diffusion Measurements In Fig. 2, the intensity decays from 1 H PFG NMR self-diffusion measurements on some of the sample supernatants and one of the precipitates are shown. The decays may be recognized as data points belonging to non-linear curves. The latter reflect the distributions of self-diffusion coefficients, which may be expected in polydisperse samples [7]. As the slope of the different decay curves becomes steeper, the mean self-diffusion increases, which of course indicates that the size of the KL fragments decreases. By fitting a suitable distribution function to the intensity decays, it is thus possible to determine the complete distribution of diffusion coefficients [7,8,10]. In Eq. (2), the log-normal distribution function used in the evaluation of the intensity decays in the present study is shown. 1 Ik ( G ) 0 D 2 2 ln( D) ln( D ) exp m (2) 2 exp( k D) dd G where, I equals the normalized signal intensity and k G ( 2 2 g 2 )( /3) reflects the strength of the applied field gradient. The nuclear magnetogyric ratio of the studied nucleus is defined by, is the gradient pulse length, g is the gradient strength and is the time between the pulses. D represents the diffusion coefficient of a molecular species belonging to the lognormal distribution as long as I >0.D m is the fitted mass-weighted median diffusion coefficient and is a fitted constant, reflecting the distribution width. The log-normal approach is found to be successful in describing the distributions of self-diffusion coefficients in the samples (Fig. 3). As the electrolyte concentration increases, the echo intensities plotted in Fig. 2, obtained from measurements on the sample supernatants, show both increasingly steeper intensity decays and curvatures, which indicates faster mass-weighted median self-diffusion, but also broader distributions of self-diffusion coefficients. This is because the amount of KL fragments in the supernatants, showing low selfdiffusion coefficients, is decreasing as the sample solutions are treated with increasing amounts of NaCl. The result implies that the stability of large KL macromolecules is more strongly influenced by the ionic strength than the stability of lower molecular weight fragments. Furthermore, in this case the correlation between self-diffusion coefficients (molecular weight) and migration time differs from that recently determined by Sjöholm et al. [14]. By using CZE and size exclusion chromatography analysis on KL samples, they found that the migration time of KL fragments was increasing as the number-averaged molecular weight average (M n ) increased. There is one main difference between the two studies, which also may explain the different results. In the present study, the analyses are performed on fractions derived by tuning the salt concentration in the same stock sample solution, whereas Sjöholm et al. examined KL samples extracted from black liquors and pulps obtained from different kraft cooks and various cooking times. Moreover, the variations in M n between the different samples were generally small and deviations from TABLE I DIFFERENCES BETWEEN KRAFT LIGNIN SAMPLES AT VARIOUS NaCl CONCENTRATIONS AS DETERMINED BY CZE AND UV-SPECTROPHOTOMETRIC MEASUREMENTS Sample [NaCl] (mol/l) a Migration Time (min) Peak Width b (min) Precipitate Precipitate Precipitate Precipitate Supernatant a Degree of precipitation. b Measured at half peak height. the proposed trend were observed at higher M n [14]. Experimental Observations Versus Theoretical Considerations The results obtained by the CZE and 1 H PFG NMR analysis may be explained by the molecular weight dependent dissociation behaviour of KL [10,11]. The apparent pk a value of KL increases with the molecular weight and one should be aware that this relationship also strongly influences the true surface charge density of the KL fragments of different sizes. This is a fact if 0, the number of free phenolic groups per KL surface area, would be considered constant, as has been shown earlier [10]. Considering equivalent charge densities when the phenolic groups are fully dissociated, at ph 12 (room temperature) high molecular KL macromolecules will show a lower true surface charge density than fragments of lower molecular weight, as is illustrated in Fig. 4. By considering the arguments above, it is found that the reduced colloidal stability, experimentally observed for higher molecular weight fragments, can be explained, at least qualitatively, by Eq. (3). At lower surface charge densities, the relationship between the surface charge density and the critical coagulation concentration JOURNAL OF PULP AND PAPER SCIENCE: VOL. 27 NO. 11 NOVEMBER

4 Arbitrary units mol/l (p) mol/l (s) 0.75 mol/l (s) 1.0 mol/l (s) (mc m ) Fig. 3. Log-normal distributions of self-diffusion coefficients obtained by applying Eq. (2) to the intensity data presented in Fig. 2. The curve showing the lowest mass-weighted median self-diffusion coefficient is determined on the precipitate. The kraft lignin macromolecules in the supernatants show increasingly faster self-diffusion, indicating a decrease in molecular weight, as the ionic strength in the sample solutions increases. The notations are equivalent to those in Fig. 2. The curves are not normalized. Fig. 4. Calculated surface charge densities of kraft lignin fragments at a constant 0 of 30 mcm -2, but two different radii. The solid and the broken line represent R = 1 and 2 nm, respectively. The calculations are performed at an ionic strength of 0.01 mol/l, various ph values and 25 C. (CCC) of an electrolyte can be described as [20]: kt r CCC B ( 13 0 ) (3) ( ze) 2H 23 / 121 where CCC is given in molecules per m 3, k B is Boltzmann s constant and T is the temperature. The dielectric constant of the medium is denoted as 0 r, z is the valence of the counterion, is the elementary charge and H 121 the effective Hamaker constant. In this discussion, a uniform Hamaker constant for all KL fragments within the distribution is assumed. Since is lower for large KL fragments than for the smaller ones at equivalent ph values (Fig. 4), Eq. (3) implies decreasing CCCs, and thus reduced stability, as the fragments become larger. Transferred to the kraft process, this means that, if the hydroxide concentration decreases to about poh = 2 at higher temperatures (>150 C), at first the largest KL fragments will self-aggregate and finally precipitate. In conventional kraft pulping, the conditions described above are often met [16,21]. Fortunately, in today s modified kraft pulping processes, the hydroxide concentration in the digester is never lower than 0.01 mol/l. Consequently self-aggregation and precipitation of KL exhibiting similar physical characteristics as the presently investigated KL sample will probably not occur here. Still, from a physicochemical point of view, adsorption of macromolecules (the larger, the more) usually prevents self-aggregation [20] and experiments concerning KL adsorption on fibre material at relevant conditions should and will be further investigated. In order to understand the KL solution behaviour it is important to find accurate methods able to characterize the system. From the present investigation, it is shown that the differences in physicochemical properties of dissolved and precipitated KL can be measured by these techniques. CONCLUSIONS By using a combination of CZE, 1 H PFG NMR and UV-spectrophotometry, quantitative estimates of physicochemical properties that influence the solubility characteristics of kraft lignin macromolecules were made. The combined techniques were utilized to demonstrate decreasing colloidal stability of KL macromolecules at increasing hydrodynamic size, as the temperature and the concentration of a monovalent salt were increased in the system. The results clarify the possibility of precipitating separate fractions of KL macromolecules having somewhat different physicochemical properties, by changing the solution conditions (ph/poh not included). The latter also shows the importance of treating KL as a diversified material when discussing solubility and stability limits. ACKNOWLEDGEMENTS This work was supported by Mid Sweden University, STFI and the Centre of Amphiphilic Polymers from Renewable Resources, CAP. H. Edlund gratefully acknowledges The Swedish Foundation for International Co-operation in Research and Higher Education, STINT, for a postdoctoral fellowship. REFERENCES 1. ADLER, E., Lignin Chemistry Past, Present and Future, Wood Sci. Technol. J. 11: (1977). 2. LUNDQVIST, K., 1 H NMR Spectral Studies of Lignins, Nordic Pulp Paper Res. J. 1:4 16 (1992). 3. EDE, R.M. and RALPH, J., Assignment of 2D TOCSY Spectra of Lignins: The Role of Lignin Model Compounds, Magn. Reson. Chem. 34(4): (1996). 4. AMMALAHTI, E., BRUNOW, G., BARDET, M., ROBERT, D. and KILPELAINEN, I., Identification of Side-Chain Structures in a Poplar Lignin using Three-Dimensional HMQC- HOHAHA NMR Spectroscopy, J. Agric. Food Chem. 46(12): (1998). 5. ARGYROPOLOUS, D.S., Quantitative Phosphorus 31 NMR Analysis of Lignins, A New Tool for the Lignin Chemist, J. Wood Chem. Technol. 14(1):45-63 (1994). 6. STILBS, P., Fourier Transform Pulsed-Gradient Spin-Echo Studies of Molecular Diffusion, Prog. Nucl. Magn. Res. 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5 Lignin Mobility Distributions Obtained by Capillary Zone Electrophoresis, J. Wood Chem. Technol. 20(2): (2000). 15. HIEMENZ, P.C., The Thermodynamics of Polymer Solutions in Polymer Chemistry The Basic Concepts, Marcel Dekker, Inc. New York, Ch. 8 (1984). 16. NORGREN, M., Some Aspects on the Physical Chemistry of Kraft Lignins in Aqueous Solutions, Licentiate Thesis, Mid Sweden Univ., Sundsvall and Lund (2000). 17. MÖRCK, R., YOSHIDA, H. and KRINGSTAD, K.P., Fractionation of Kraft Lignin by Successive Extraction with Organic Solvents. I. Functional Groups, 13 C-NMR Spectra and Molecular Weight Distribution, Holzforschung 40:51 60 (1986). 18. SJÖHOLM, E., NILVEBRANT, N.-O. and COLMSJÖ, A., Characterisation of Dissolved Kraft Lignin by Capillary Zone Electrophoresis, J. Wood Chem. Technol. 13(4): (1993). 19. LI, S.F.Y. Capillary Electrophoresis: Principles, Practice and Applications, Elsevier Science Publishers, Amsterdam (1992). 20. EVANS, D.F and WENNERSTRÖM, H., The Colloidal Domain, VCH Publishers, Inc., New York (1994). 21. RYDHOLM, S.A., Chemical Pulping in Pulping Processes, Interscience, New York, Ch. 9 (1965). REFERENCE: NORGREN, M., EDLUND, H. and NILVEBRANT, N.O., Physiochemical Differences between Dissolved and Precipitated Kraft Lignin Fragments as Determined by PFG NMR, CZE and Quantitative UV Spectrophotometry, Journal of Pulp and Paper Science, 27(11): November Paper offered as a contribution to the Journal of Pulp and Paper Science. Not to be reproduced without permission from the Pulp and Paper Technical Association of Canada. Manuscript received September 7, 2000; revised manuscript approved for publication by the Review Panel June 26, KEYWORDS: AxLKALI LIGNINS, PRECIPITATION, ELECTROPHORESIS, NUCLEAR MAGNETIC RESONANCE, SPECTROSCOPY, PHOTOMETRY, SODIUM CHLORIDE, CONCENTRATION, MOLECULAR WEIGHT, SOLUTIONS. JOURNAL OF PULP AND PAPER SCIENCE: VOL. 27 NO. 11 NOVEMBER