Langmuir 1996,11, 3617-3619
3617
Distribution of Charge in Wood Pulps Jorge M. B. Fernandes Diniz Bolseiro da JNICT, Rua da Carregueira 7, 2425 Monte Real, Portugal Received November 23, 1994. I n Final Form: May 9, 1995@ A n analysis of titration curves obtained for synthetic ion-exchange resins, where the location of acid groups is therefore known, has shown that discontinuity points in the titration curve andor in its first derivative are obtained when most of the acid groups are not on the surface but in the bulk of the resin beads. The intrinsic charge on wood pulp fibers, originating from the ionization of carboxylic acid groups during an alkalimetric titration, has most often been associated with surface charge. Titration curves with discontinuity points obtained for samples of a thermomechanical wood pulp have raised the question of whether the distribution of charge in wood pulps might have a mixture of contributions of bulk and surface charge. Specific surface areas determined by negative adsorption of a set of wood pulps have supported the concept that, for any wood pulp, bulk charge corresponds to a significant part of the total charge detected. Furthermore, for a much studied wood pulp, after the conversion of all carboxylic acid groups at the surface into carboxylate groups by ion exchange with concentrated sodium chloride solutions,it was found that this wood pulp still contained 30% of the initial acid content.
1. Introduction The determination of the acid content of a wood pulp can lead to the knowledge of the intrinsic charge of the wood pulp fibers, which results from the ionization of the acid groups attached to these fibers. The ionizable groups on cellulosic fibers are carboxylic acid groups, sulfonic acid groups, catecholic groups, phenolic groups, and hydroxyl gr0ups.l The carboxylic and sulfonic acid groups are not only the major contributors to the charge on the wood pulp fiber but also the only ones normally determined. It is accepted that sulfonic acid groups are introduced during sulfite pulping treatments of wood pulps, but the carboxylic groups either originate from noncellulosic components in wood or are created during the pulping and bleaching operations.' Using a n experimental technique in potentiometry, originally developed for colloidal systemq2where charge is plotted as a function of the pH of a n external aqueous medium, charge-pH isotherms have been applied by Herrington and c o - ~ o r k e r s , ~ in- ~order to describe the surface charge distribution of wood pulps throughout the pH range of a n alkalimetric titration. In the adaptation of the original work the assumption was made that all of the acid content titrated corresponded to acid groups attached to the surface of the wood pulp fibers. Potentiometric studies of two synthetic ion-exchange resins, Amberlite IRC-50 (H)and Dowex 50W-X8, already describedl have shown that for Dowex 50W-X8 (a resin containing only sulfonic acid groups) titration curves, obtained with additions of titrant every 5 min, presented no discontinuity points either in the titration curve or in its first derivative. Unlike Dowex 50W-X8,Amberlite IRC50 (H) (a resin containing only carboxylic acid groups) showed an array of discontinuity points, which gradually disappeared as additional time was given after the addition of titrant. The first discontinuity points to disappear were Abstract published in Advance A C S Abstracts, September 15, 1995. (1)Lindstrom, T. In Paper Chemistry; Roberts, J. C., Ed.; Blackie: Glasgow, 1991; pp 25, 28. (2) Tadros, Th. F.;Lyklema, J. J . Electroanal. Chem. 1968,17,267. (3) Herrington, T. M.; Midmore, B. R. J . Chem. SOC.,Faraday Trans. 1 1984,80, 1525. (4)Budd, J.; Herrington, T. M. Colloids Surf. 1989, 36,273. (5)Herrington, T. M.; Petzold, J. C. Colloids Surf. 1992, 64, 97. (6)Herrington, T.M. InPapermaking raw materials, their interaction with the productionprocess and their effectonpaperproperties; Punton, V., Ed.; Mechanical Engineering Publications Ltd.: London, 1985; p 165. (7) Fernandes Diniz, J. M. B. Colloids Surf. A 1994, 88, 191 @
the firstly obtained ones in the alkalimetric titration. The total disappearance of discontinuity points was only obtained for Amberlite IRC-50 (HI when the titrant was added after a waiting time of not less than 25 min. Figure 1illustrates the gradual disappearance of discontinuity points from the titration curve of Amberlite IRC-50 (HI. Successful titrations o f h b e r l i t e IRC-50 (H)took a period of about 14.5 h. Since Dowex 50W-X8 resin corresponds to a copolymer of divinylbenzene and ethylstyrene treated a t a later stage with either sulfuric or chlorosulfonic acid, one can envisage that the sulfonic acid groups attached to the former polymeric network are created a t the surface of the resin beads. Amberlite IRC-50 (H), on the other hand, is a copolymer of methacrylic acid and divinylbenzene. The synthesis of this resin clearly suggests that the methacrylic acid groups are distributed both at the surface and in the bulk of the resin beads. Therefore the relatively long period of waiting for equilibrium to take place in the titration of Amberlite IRC-50 (H) can be interpreted as dependent on the diffusion ofprotons from the bulk to the surface of the resin beads. The greatest problem in the application of this potentiometric technique to a suspension of cellulose fibers was the attainment ofequilibrium, as recognized by Herrington and M i d m ~ r e .These ~ authors also report that more rapid titrations led to erroneous peaks and troughs in the differential titration plot of cotton linters. When this aspect is taken into consideration, the experimental technique suggested by these authors was designed in order to allow a complete titration of a wood pulp to take about 4 h.3 Notwithstanding this experimental evidence, Herrington and co-workers continued to interpret the intrinsic charge of wood pulps as surface ~ h a r g e . ~ - ~ Specific surface areas of wood pulps were determined in this work from the measurement of the negative adsorption of co-ions. The method used in this work strictly follows that of Van den Hul and L ~ k l e m a . ~ , ~ This method is based upon the diffuse double layer theory and was originally developed for colloidal suspensions. Experimentally, the method consists of potentiometrically titrating chloride ions in a n aqueous solution of sodium chloride (blank solution) and the supernatant of a (8)Van den Hul, H. J.;Lyklema, J. J . ColloidInterface Sci. 1967,23, 500. (9) Lyklema, J.;Van den Hul, J. In Proceedings o f the International Symposium on Surface Area Determination; Everett, D. H., Ottewill, R. H., Eds.; Butterworths: London, 1970; p 341.
0743-746319512411-3617$09.00/0 0 1995 American Chemical Society
Letters
3618 Langmuir, Vol. 11, No. 10, 1995
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Volume of 0.3316 M NaOH (cm ) Figure 1. Representation of titration curves for Amberlite IRC-50 (H) resin in a medium of 0.01 mol dm-3 HCl and 0.100 mol dm-3 NaCl obtained with different waiting times between the additions of titrant.
suspension of a wood pulp in the same aqueous solution of sodium chloride. Due to the presence of carboxylate groups at the surface of pulp fibers, chloride ions are repelled from the inner solution inside the fibers to the outer aqueous solution. The difference in the chloride ion concentration measured leads t o t h e calculation of the excluded volume, AV. Surface area values (expressed as m2g-l) were obtained at 25.0 "C, through t h e slope in the plot of AV (expressed as cm3 g-l) as a function of [1/(1644 l/ci)l (expressed as cm3 m-2), where cLis the chloride ion concentration of the blank solution (expressed as mmol ~ m - ~ Different ). chloride ion concentrations were used t o generate t h r e e to five experimental points. The slope of a linear correlation of the type (r = axY was found through a discrete least-squares approximation, where the slope, a, w a s determined through the following mathematical equation:1° m
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2. Experimental Section Materials. The water used in these experiments was described e l ~ e w h e r e .This ~ water will be referred to as "ultrapurified water" throughout. Four commercialwood pulps were studied, MoDoCrown pulp, Stora-32 pulp, Organocell fluff, and Corner Brook pulp. MoDoCrown pulp is a bleached sulfite, wood pulp from Picea abies (L.)produced by MODOPaperAB in Ornskoldsvik,Sweden.Stora32 pulp is a bleached sulfate wood pulp from a mixture of pine and spruce trees, produced by the Stora Group in Sweden. Organocell fluff is an organosolv wood pulp, in bleached and unbleached form, from Picea abies (L.) produced by Bayerische Zellstoff GmbH in Kelheim, Germany. Corner Brook pulp is a thermomechanical wood pulp, in bleached and unbleached form, produced by Corner Brook Pulp and Paper Limited in Newfoundland, Canada. Apparatus. Temperature controlwas provided by a Hetofrig thermostatic bath maintained at 25.0 & 0.1 "C. Potentiometric titrations were performed under a constant flow of 99.999% pure nitrogen, using a Metrohm670 titroprocessorand a Metrohm 665 Dosimat. For charge determination a Metrohm combination pH glass electrode (ref. 6.0203.100)was used. For area determination a chloride electrode supplied by Kent Industrial Measurements Ltd. Model 8004-2 was used. The chloride electrodeassembly consists of a chloride electrodeand a mercury/ mercurous sulfate reference electrode. The chloride electrode ~
(10)Burden, R. L.; Faires, J. D. Numerical Analysis, 4th ed.; PWSKent Publishing Company: Boston, 1989;Chapter 8,p 429.
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Volume of titrant (cm ) Figure 2. Titration curves of a suspension of an unbleached thermomechanical wood pulp in aO.O1O mol dm-3 CaClz aqueous solution obtained with different waiting times between additions of titrant.
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Figure 3. Representation of the linear correlation, the slope of which corresponds to the specific surface area of the pulp fibers of Stora-32 wood pulp. has a sensing tip consistingof a silver billet electrolyticallycoated with silver chloride. The billet is mounted in a chemically resistant high density polythene body. The titroprocessor used allows equivalence points to be determined by a numerical differentiation method of the titration curve, thus avoiding the use of indicators. The accuracy of the potentiometric technique was assessed by adapting it to study the dissociation of methanoic acid and glucuronic acid.ll The results obtained for the pK,values of these acids agreed with IUPAC published values. Charge Determination. The equivalent to one dry sheet of wood pulp was equilibrated with 1.0 dm3 of HC1 with pH 1.5 for 1 h.12 All ion-exchange equilibria took place in undisturbed media. From the acidic suspension the pulp was filtered, copiously rinsed with ultrapurified water, and filtered again. The pulp was subsequently divided into several samples and dried in an oven until a constant weight was obtained. The samples of wood pulp which were titrated had a mass of approximately 1g in oven-dry form. The samples of wood pulp were potentiometrically titrated in 0.10 mol dm-3 NaCl in order to allow the determination of the acid content. The titrant was 0.0201 mol dm-3 NaOH. The acid content, normally expressed as mmol kg-', is readily convertible to charge by multiplying it by 0.09648531, yielding a result expressed as C g-l. Area Determination. The equivalent to one dry sheet of wood pulp was equilibrated with 1.0 dm3 of 1.0 mol dm-3 NaCl (11)Fernandes Diniz, J. M. B.; Herrington, T. M. J. Chem. Eng. Data 1993,38, 109. (12)Fernandes Diniz, J. M. B.; Pethybridge, A. D. Holzforschung 1995,49,81.
Langmuir, Vol. 11, No. 10, 1995 3619
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charge (C g-l) surface area (m2g-l)
Table 1. Charge and Surface Area of Paper Wood Pulps Organocell TMP MoDoCrown bleached unbleached bleached unbleached 8.77f 0.19 4.96 f 0.35 10.6 f 1.0 3.74f 0.08 3.62f 0.04 107 f 12 85 f 18 75 f 18 78 f 7 60 f 12
twice in succession, in order to convert all of the carboxylic acid groups at the surface of the wood pulp fibers into carboxylate form. From this suspension the pulp was filtered, copiously rinsed with a -,0.01mol dm-3 NaCl solution, and filtered again. The pulp was subsequently divided into several samples and dried in an oven until a constant weight was obtained. The samples of wood pulp had a mass of approximately 1g in oven-dry form. Sodium chloride aqueous solutions were prepared with the following concentrations: 0.00406,0.00583,0.00907,0.01375, and 0.02741 mol dm-3. The method of pulp preparation for surface area determination differs from that of Herrington and Midmore13 in the drying process of the samples of wood pulp prior to the application of the method and the concentration of the sodium chloride solution to completely ion exchange the acid groups at the surface of the pulp fibers into the carboxylate form.
3. Results and Discussion Figure 2 represents potentiometric titration curves for the unbleached form of Corner Brook pulp, a thermomechanical wood pulp, in 0.010 mol dm-3 CaC12, studied with different waiting times between each addition of titrant. Previously the author performed potentiometric titrations of other wood p u l p ~ ' ~(sulfate J~ wood pulps, sulfite wood pulps, and organosolvwood pulps) in 0.010 mol dm-3 CaClz with 5 min of waiting time between successive additions of titrant that showed perfectly continuous titration curves and no discontinuity points. It is clear from this graph that it is not the case with this unbleached thermomechanical pulp. The requirement of a longer interval between additions of titrant also required for the bleached form of the thermomechanical pulp indicates that the carboxylic acid groups are mainly situated in a n inner portion of the noncellulosic material of the wood pulp. The time required for the protons to be detected in solution is therefore dependent on the diffusion process of these ions from the inner portion of the noncellulosic material of the wood pulp (hemicellulose region) through the external portion of the noncellulosic material (lignin region) into the solution. Figure 3 presents the graph used to determine the specific surface area by negative adsorption of a much studied woodpulp, Stora-32.15 The specific surface area found for this wood pulp was 110 i 8 m2 g-l. This value is 13%lower than the value proposed by Herrington and Midmore,ll which is 126 f 10 m2 g-l, because the pulp samples in this study were dried prior to the application of the method, as shown by the comparison of specific surface area values between never-dried and once-dried woodpulps." Table 1presents values ofmaximum charge (equivalent to carboxylic acid content) and specific surface areas in solution, determined by negative adsorption, for a set of five other wood pulps. The intrinsic charge values were obtained by direct potentiometric titration of the wood pulps referred to in 0.10 mol dm-3 NaC1. Both the uncertainties of charge and specific surface area were calculated for a 90%confidence interval, using the Student (13) Herrington,T.M.; Midmore,B.R.J.Chem. SOC.,Faraday Trans. 1 1984,80, 1539. (14)Femandes Diniz, J. M. B.Holzforschung, in press. (15) Hardman, T. M. Research on Cellulose Pulps at Reading University 1978-1988;Department of Chemistry, University of Reading, 1989.
t-distribution.16 The wood pulp samples were dried and weighed prior to the application of each method. Since a higher concentration of surface charge (carboxylate ions) determines a greater repulsion of chloride ions from the electric double layer, and therefore a greater value of specificsurface area, one would expect that higher surface charges would correspond to higher values of specific surface charge. Since this correlation is not found for the charge and specific surface area values tabulated, this fact suggests that the carboxylic acid groups titrated, and therefore the maximum charge in wood pulps, correspond to bulk and surface contributions. In order to test whether a chemically pulped and bleached pulp still had a significant amount of bulk charge, a portion of Stora-32 pulp was ion exchanged to the sodium form by equilibration twice in succession in 1.00 M NaCl solutions for 24 h. When a sample of Stora-32 pulp, having had the surface carboxylic groups converted to the sodium form, was potentiometrically titrated with NaOH, the results showed 30% of the acid content this pulp had when prepared in acid form.
4. Conclusions The set of experiments performed shows that most of the carboxylic acid content in thermomechanical wood pulps is located in an inner region of the noncellulosic components of wood pulps (hemicellulose domain). Chemical pulping processes destroy the original structure of the layers in the wood cells, removing most of the external noncellulosic component ofwood pulps (lignin)and a part ofthe hemicellulose domain. Contrary to what is believed' chemical-pulping processes will reduce the total number ofcarboxylicacid groups present in a wood pulp. However, when the pulping process is thermomechanical the original structure of the wood cells is maintained. Therefore protons will have to diffuse through the lignin layer, and this process will require a considerably greater time for chemical equilibrium to take place. The total acid content will be much higher (2 or 3 times greater) than in chemically pulped wood pulps because the hemicellulose domain is left intact. An underlying conclusion from these results is the fact that acid groups can be alkalimetrically titrated from within the inner portion of pulp fibers, thus not all the charge created in the pulp fibers through acid ionization corresponds to surface charge. Therefore descriptions of surface ~ h a r g e or ~ -surface ~ potential6 are clearly misapplied when used to identify the total charge with surface charge.
Acknowledgment. The author thanks Junta Nacional de Investiga@o Cientifica e Tecnol6gica-Programa Ci6ncia, Lisbon, Portugal, for the financial support of this research program and is indebted to the Chemistry Department of the University of Reading for the use of their equipment. The author also wishes to express gratitude to MODOPaper AB,the Stora Group, Bayerische Zellstoff GmbH, and Corner Brook Pulp and Paper Limited for the courtesy of supplying the Commercial pulps under study. LA940932E (16)Chatfield,C.Statistics for Technology, 3rd ed. (revised);Chapman and Hall: London, 1983; Chapter 6,pp 128-131.
