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Adsorption of Low Charge Density Polyelectrolytes to an Oppositely Charged Porous Substrate Andrew T. Horvath,† A. Elisabet Horvath,† Tom Lindstro¨m,‡ and Lars Wågberg* Royal Institute of Technology, Department of Fibre and Polymer Technology, SE-100 44 Stockholm, Sweden ReceiVed January 27, 2008. ReVised Manuscript ReceiVed March 11, 2008 The adsorption behavior of a low charge density cationic polyelectrolyte to cellulosic fibers has been studied. Cationic dextran served as a model polyelectrolyte, as it can be prepared over a range in molecular mass and charge density. The adsorption behavior of the cationic dextran was measured in electrolyte-free conditions using polyelectrolyte titration techniques. By fluorescent labeling the cationic dextran, the extent to which adsorption occurs inside the porous structure was further determined by fluorescent confocal laser scanning microscopy. Cationic dextran having a sufficiently low charge density adsorbed into the pores, although the extent the cationic dextran adsorbed was governed by the molecular mass. The adsorption behavior of the cationic dextran was also studied in various electrolyte concentrations. The adsorbed mass monotonically decreased with increasing electrolyte, as the electrostatic interaction with the substrate was more effectively screened. This behavior also suggests that the interactions between adsorbed polyelectrolyte chains, i.e. lateral correlation effects, are negligible for low charge density polyelectrolytes. Finally, the effect of having a preadsorbed layer of cationic dextran on the adsorption behavior was determined in electrolytefree conditions using fluorescent double staining techniques. The preadsorbed cationic dextran had almost no effect on the adsorption of low molecular mass fractions. Low molecular mass fractions directly adsorbed into the pore structure, as opposed to adsorbing to a free surface and diffusing into the pores. It was also shown that cationic dextran can be selectively adsorbed to different locations, such that the surface of a porous substrate can be treated uniquely from the bulk.
Introduction Polyelectrolyte adsorption to an oppositely charged substrate is generally electrosorptive in nature, such that the adsorption behavior is governed by a delicate balance of electrostatic interactions.1 If the substrate is porous in nature, these electrostatic interactions also determine whether the adsorbing polyelectrolyte can penetrate into the pore structure. This behavior has been observed for polyelectrolyte adsorption onto porous glass2 and cross-linked polyelectrolyte gels,3,4 for which the molecular properties of the polyelectrolyte influenced the adsorption behavior. Cellulose fibers are thought to behave like a hemicellulosic gel. Despite numerous investigations on the adsorption of cationic polyelectrolytes to cellulosic fibers,5–9 there has not been any direct experimental evidence showing the extent of adsorption into the porous fiber wall. Nonetheless, it has often been inferred that the polyelectrolyte adsorbs into the porous fiber wall, particularly low molecular mass polyelectrolytes. Recent investigations have cast doubt on this, especially for highly charged polyelectrolytes.10,11 *
[email protected] † Current address: Mondi Frantschach GmbH, 9413 St. Gertraud, Austria. ‡ STFI-Packforsk AB, Box 5604, SE-114 86 Stockholm, Sweden. (1) van de Steeg, H. G. M.; Cohen Stuart, M. A.; de Keizer, A.; Bijsterbosch, B. H. Langmuir 1992, 8, 2538. (2) Shirazi, M.; van de Ven, T. G. M.; Garnier, G Langmuir 2003, 19, 10829. (3) Bysell, H.; Malmsten, M. Langmuir 2006, 22, 5476. (4) Kabanov, V. A.; Skobeleva, V. B.; Rogacheva, V. B.; Zezin, A. B. J. Phys. Chem. B 2004, 108, 1485. ¨ dberg, L.; Wågberg, L.; Lindstro¨m, T. J. Colloid Interface (5) Tanaka, H.; O Sci. 1990, 134, 219. (6) Wågberg, L; Ha¨gglund, R. Langmuir 2001, 17, 1096. (7) Horvath, A. E.; Lindstro¨m, T.; Laine, J. Langmuir 2006, 22, 824. (8) Pelton, R. H. J. Colloid Interface Sci. 1986, 111, 475. (9) Shirazi, M.; van de Ven, T. G. M.; Garnier, G Langmuir 2003, 19, 10835. (10) Hubbe, M. A.; Rojas, O. J.; Lucia, L. A.; Jung, T. M. Cellulose 2007, 14, 655.
On the other hand, low charge density polyelectrolytes exhibit fewer electrostatic repulsions both along the backbone of the polyelectrolyte chain and between two adsorbing polyelectrolyte chains. Low charge density polyelectrolytes consequently adsorb in greater amounts and thicker layers, due to the reduced lateral correlation with neighboring adsorbed chains and a more flexible molecular conformation. However, there is little experimental evidence regarding the extent that low charge density polyelectrolytes penetrate into the pore structure. Therefore, it is of interest to determine the extent that low charge density polyelectrolytes penetrate into cellulosic fibers. Consequently, the effect of the polyelectrolyte charge density, molecular mass, and electrolyte concentration on the extent of adsorption into the fiber wall can also be established. Fluorescent labeling techniques have recently been developed to determine the location that polyelectrolytes adsorb into cellulosic fibers. Changes in the adsorption behavior may therefore be rationalized by the extent that the polyelectrolyte penetrates into the pore structure. Cationic dextran has been prepared to serve as a model low charge density polyelectrolyte for investigating adsorption over a wide range in molecular mass, charge density, and electrolyte concentration. Finally, some evidence exists to suggest a mechanism for how polyelectrolytes adsorb into a porous substrate. By labeling the polyelectrolytes with distinguishable fluorescent tags, the effect of having a preadsorbed layer on the adsorption of a second polyelectrolyte can be further studied. It can be determined if the cationic dextran adsorbs through the preadsorbed layer or by pushing the preadsorbed layer into the pores. This approach can be used to selectively tailor porous materials with multiple polyelectrolytes, by controlling the extent that each polyelectrolyte adsorbs. (11) Horvath, A. T. ; Horvath, A. E. ; Lindstro¨m, T. ; Wågberg, L. Langmuir, in press.
10.1021/la800274w CCC: $40.75 2008 American Chemical Society Published on Web 05/29/2008
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Experimental Section Pulp. A never-dried, unbleached kraft pulp (Stora Enso, Skoghall, Sweden) oxygen delignified to a κ number of 18 and refined to 22° SR was used for adsorption measurements. Fine material was removed with a Cellecofilter having 100 µm screening slots, such that only intact fibers remained. The total charge of the pulp was measured to be 91 µequiv/g by conductometric titration.12 The surface charge, measured by polyelectrolyte titration using polyDADMAC (from Ciba, Yorkshire, UK, Mw ) 9.2 × 105 Da and ε ) 6.19 mequiv/g),13 was measured to be 3.7 µequiv/g. The pulp was washed into its sodium form though a standard procedure, such that all counterions are exchanged to sodium ions.14 Polyelectrolyte Preparation. Native dextran (from Leuconostoc mesenteroides bacteria) served as a substrate molecule for subsequent chemical modifications. Several molecular mass fractions, ranging from Mw ) 1.0 × 104 to 2.0 × 106 Da, were obtained from Pharmacia Ltd. (Uppsala, Sweden) and used without further purification. Cationic modifications were made with 2,3-epoxypropyltrimethylammonium chloride (ETAC) in water under basic conditions.15 In a typical reaction, 30 g of native dextran was dissolved in 150 mL of an aqueous 5% solution of potassium hydroxide. ETAC was added to the solution under constant stirring at a ratio typically ranging between 0.05 and 1.1 g of ETAC/g of dextran, with the degree of substitution depending on the ETAC addition. This ratio must be adjusted for each particular molecular mass fraction in order to produce a similar charge density. The solution was then heated to 40 °C for 6 h, after which it was allowed to cool under ambient conditions and neutralized with hydrochloric acid. The solution was then dialyzed against deionized water to remove any impurities and freeze-dried for characterization and subsequent modification. Two fluorescent labels were used to label cationic dextran: fluorescein isothiocyanate (FITC) and sulforhodamine B acid chloride. These fluorescent labels were chosen as they have distinctly different emission spectra, such that they can be distinguished when adsorbed to the same fiber. The native dextran was labeled with FITC following a method developed by de Belder and Granath,16 and the cationic dextran was labeled with FITC analogously. In a darkened room, 1 g of cationic dextran was dissolved in 10 mL of dimethyl sulfoxide containing a few drops of pyridine. FITC was added, keeping a ratio of 1 fluorescent label for 100 glucose units, followed by 20 mg of dibutyltin dilaurate. The solution was then heated under constant stirring for 2 h at 95 °C, after which the solution was cooled to ambient conditions. The solution was then dialyzed against deionized water to remove any excess FITC and then freeze-dried for characterization and use. Size exclusion chromatography and potentiometric titrations were used to show that the polyelectrolyte properties were not significantly changed by the labeling procedure. Labeling cationic dextran with sulforhodamine B acid chloride follows a general protocol for labeling biological macromolecules.17 Cationic dextran was dissolved at a concentration of 2 mg/mL in a 0.1 M sodium carbonate/bicarbonate buffer adjusted to pH 9. Sulforhodamine B acid chloride was dissolved separately in N,Ndimethylformamide (DMF) at a concentration of 2 mg/mL, being protected from light at all times. In darkened conditions, the sulforhodamine B acid chloride solution was slowly added at a ratio of 1 fluorophore to 150 glucose units under gentle mixing, letting the mixture react for 1 h at room temperature. The reaction was ceased by dilution with deionized water, and the solution was likewise dialyzed to remove any excess fluorescent label and then freezedried. It should be noted that ester formation of sulfonyl chlorides (12) Katz, S.; Beatson, R. P.; Scallan, A. M. SV. Papperstidning 1984, 87, R48. ¨ dberg, L.; Glad-Nordmark, G. Nord. Pulp Paper Res. J. (13) Wågberg, L.; O 1989, 4, 71. (14) Wågberg, L.; Bjo¨rklund, M. Nord. Pulp Paper Res. J. 1993, 8, 399. (15) Zhang, J.; Pelton, R.; Wågberg, L.; Rundlof, M. J. Pulp Paper Sci. 2001, 27, 145. (16) de Belder, A. N.; Granath, K. Carbohydr. Res. 1973, 30, 375. (17) Hermanson, G. T. Bioconjugate Techniques, 1st ed.; Academic Press: San Diego, CA; 1996.
HorVath et al. with alcohols is subject to nucleophilic displacement, such that care must be taken to ensure that amine-containing molecules are not present.18 Polyelectrolyte Characterization. Molecular mass distributions were measured for both native and cationic dextrans before fluorescent labeling using size exclusion chromatography (SEC). Measurements were taken at room temperature using a Progel-TSK column. A buffer of 0.3 M acetic acid and 0.3 M sodium acetate was used as the eluent in order to screen electrostatic interactions between the cationic dextran and the column. The buffer also acts to screen electrostatic repulsions along the dextran backbone, negating the electrostatic contribution to the molecular conformation. Molecular mass distributions were obtained from fairly monodisperse polyethylene oxide standards (Tosoh Corp.) using broad molecular mass calibration standard methods.19The charge density was determined from polyelectrolyte titrations using a Mu¨tek particle charge detector (PCD 03, Mu¨tek Analytical). On the basis of streaming current measurements, a standard polyethylene sodium sulfonate (Pes-Na) solution (BTG Mu¨tek GmbH, Herrsching, Germany) was used to titrate a cationic dextran solution of known concentration to the charge equivalence point. As the charge density of the Pes-Na solution was known, the charge density of the cationic dextran could be calculated from the consumption of the titrant. Dynamic light scattering measurements were made on a Nano-ZS Zetasizer (Malvern Instruments) for the unlabeled cationic dextran in order to determine the hydrodynamic diameter, DH, of the cationic dextran in different electrolyte concentrations. Adsorption to Untreated Fibers. Adsorption measurements were conducted following a standard procedure developed by Winter et al.20 An excess of cationic dextran was added to a pulp suspension that had been diluted to a consistency of 5 g/L in the desired electrolyte solution. The suspension was vigorously shaken for 30 min, which has been shown to be sufficient for reaching pseudoequilibrium for high molecular mass polyelectrolytes.21,22 The fibers were then filtered from the suspension, with the filtrate later being used to determine the adsorbed amount of cationic dextran. Fibers that were to be imaged could not be oven-dried, due to an irreversible closure of the fiber wall and lumen, known as hornification.23 Therefore, fibers treated with fluorescent-labeled polyelectrolytes must be freezedried to be imaged properly. Adsorption to Presaturated Fibers. Adsorptions to presaturated fibers were made in a two-step procedure. First, a high molecular mass cationic dextran (2.0 × 106 Da) was adsorbed in a method similar to that previously described. If an excess amount was added to the suspension, only the outermost surface will become saturated by the high molecular mass cationic dextran. Whereas the filtrate was again used for polyelectrolyte titration, the fiber fraction was thoroughly rinsed with deionized water and diluted again to a consistency of 5 g/L. A second adsorption can then be made using the same procedure with another cationic dextran. The second cationic dextran was dosed to the fiber suspension based on the adsorbed mass that was known to adsorb to the bare fibers. An adsorption time of 30 min was used for the second adsorption, which was shown to be sufficient for reaching pseudoequilibrium. The fibers were thereafter filtered from solution, with the filtrate being used to determine the adsorbed amount of the second cationic dextran by polyelectrolyte titration. In order to distinguish the adsorption of each dextran using fluorescent confocal laser scanning microscopy (CLSM), each cationic dextran must be labeled with distinguishable fluorescent tags. These fibers were also freeze-dried in order to be imaged. Polyelectrolyte Titration. The measurement of the unadsorbed cationic dextran in the filtrate was performed analogous to the (18) Scouten, W. H.; van den Tweel, W.; Delhaes, D.; Kranenberg, H.; Dekker, M. J. Chromatogr. 1986, 76, 289. (19) Swerin, A.; Wågberg, L. Nord. Pulp Paper Res. J. 1994, 9, 18. ¨ dberg, L.; Lindstro¨m, T. J. Colloid Interface (20) Winter, L; Wa˚gberg, L.; O Sci. 1986, 111, 537. (21) Lindstro¨m, T.; So¨remark, C J. Colloid Interface Sci. 1976, 55, 305. ¨ dberg, L.; Lindstro¨m, T. Colloids Surf. 1987, (22) Wågberg, L.; Winter, L.; O 27, 305. (23) Lindstro¨m, T.; Carlsson, G. SV. Papperstidn. 1982, 85, R146.
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Langmuir, Vol. 24, No. 13, 2008 6587 Table 1. Properties of the Cationic Dextran charge density (mequiv/g)
a
b
c
polyelectrolyte
molecular mass (Da)
polydispersity index
hydrodynamic diameter (nm)
before labeling
after labeling
CatDex A CatDex B CatDex C CatDex D Cat Dex E Cat Dex F CatDex G CatDex H Cat Dex I Cat Dex J CatDex K
2.0 × 106 5.0 × 105 5.0 × 105 5.0 × 105 2.5 × 105 1.1 × 105 4.0 × 104 2.0 × 104 1.0 × 104 1.0 × 104 1.0 × 104
3.85 2.23 1.99 1.92 2.22 1.39 1.55 1.45 1.52 1.41 1.78
85.3 39.8 43.6 44.1 17.4 13.8 9.3 7.2 5.7 5.9 6.3
0.35 0.53 0.35 0.18 0.38 0.37 0.28 0.38 0.29 0.56 0.83
0.33 0.49 0.31 0.13 0.32 0.33 0.26 0.35 0.28 0.53 0.80
a
Supplied by the manufacturer.
b
Measured by size exclusion chromatography. c Measured in 10-1 M NaHCO3 before fluorescent labeling.
measurement of the polyelectrolyte charge density. A standard PesNa solution of known charge density was used to titrate the filtrate to the charge equivalence point. The concentration of the cationic dextran was then calculated from the titrant consumption, as the charge density of the cationic dextran had already been determined. The adsorbed amount was calculated as the difference between the amount of cationic dextran initially added and the amount remaining in solution. These measurements were used to construct adsorption isotherms, depicted as the adsorbed amount of polyelectrolyte as a function of equilibrium concentration of the unadsorbed cationic dextran. Extrapolating the isotherm to zero equilibrium concentration indicates the adsorbed mass that is needed to saturate the fiber. Microscopy. Fluorescent CLSM images were taken with a BioRad Radiance 2000 confocal system mounted on a Nikon Eclipse 800 microscope. A krypton-argon laser was used for excitation at 488 and 568 nm. Freeze-dried fibers were placed on a glass slide in a few drops of immersion oil. The fibers were covered with a glass coverslip and the immersion oil was allowed to settle overnight, such that the oil would penetrate the fiber cell wall. Images of the fibers were taken using a 100× N.A. 1.4 oil-immersion lens (Nikon) in a darkened room. Thin optical sections were collected along the fiber length, and fluorescent intensity profiles were taken across the fiber at several positions to account for the heterogeneous nature of the pulp fibers.
Results Polyelectrolyte Properties. Cationic dextran served as a model low charge density polyelectrolyte, as it can be prepared over a wide range in molecular mass and charge density. Dextran is a linear molecule that is available in discrete molecular mass fractions up to Mw ) 2.0 × 106 Da. Chemical modifications can be made through well-developed methods to introduce quaternary ammonium groups along the backbone of the dextran chain. Thus, the structure of the native dextran is maintained while the charge density can be chemically controlled by the cationic modification. A maximum charge density of 0.9 mequiv/g was reached by the cationic modification, presumably being limited by steric and electrostatic effects, which was suitable for a low charge density polyelectrolyte. Moreover, quaternary ammonium groups are considered to be “strong” cationic groups, such that the charge density remains constant regardless of pH. The properties of the cationic dextran can be seen in Table 1. Two important points can be seen in Table 1 regarding the structure of the cationic dextran. Excluding the highest molecular mass fraction, Cat Dex A, the polydispersity index of each cationic dextran has been kept relatively low. This is important for limiting an overlap in the molecular mass distributions, which could be problematic for comparing low molecular mass fractions (i.e., Mw e 1.1 × 105 Da). Another significant factor is the charge density of the cationic dextran before and after fluorescent labeling. Only minimal differences were measured in the charge
Figure 1. Saturation adsorption as evaluated from adsorption isotherms for high (Mw ∼ 5.0 × 105 Da) and low (Mw ∼ 1.0 × 104 Da) molecular mass fractions of cation dextran having different charge density.
density due to the fluorescent labeling, suggesting that the structure of the cationic dextran is not effected by the presence of the fluorescent label, as expected. Effect of Molecular Properties on the Adsorption Behavior in Electrolyte-Free Conditions. The saturation adsorption of low molecular mass (Mw ) 1.0 × 104 Da) and high molecular mass (Mw ) 5.0 × 105 Da) cationic dextran in electrolyte-free conditions is presented in Figure 1. The saturation adsorption represents the amount needed to cover the available surface, as determined by extrapolating the adsorption isotherm for a particular cationic dextran to an equilibrium concentration of zero.24 The adsorbed mass is lowered as the charge density increases, most probably as less cationic dextran is needed to compensate the available fiber charges. Whereas surface charge compensation is a plausible explanation for the changes in adsorption behavior of the 5.0 × 105 Da cationic dextran, the reasons for the more pronounced changes in the adsorption behavior of the 1.0 × 104 Da cationic dextran are not obvious. Specifically, it is difficult to assess if charge compensation occurs as Cat Dex I (Mw ) 1.0 × 104 Da and ε ) 0.29 mequiv/g) may access more charges by adsorbing into the pores. Fluorescent CLSM was therefore used to study the adsorption of the fluorescent labeled low molecular mass cationic dextran, and the images and corresponding fluorescent intensity (24) Lindstro¨m, T. Fundamentals in Papermaking, Transactions of the 9th Fundamental Research Symposium held at Cambridge, Vol. 1; Mech. Eng. Pub. Ltd.: London, England; 1989.
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Figure 2. CLSM images and the corresponding intensity profiles for a fiber cross-section depicting the adsorption of cationic dextran (Mw ) 1.0 × 104 Da) having different charge density under electrolyte-free conditions: (a) Cat Dex I having ε ) 0.29 mequiv/g, (b) Cat Dex J having ε ) 0.56 mequiv/g, and (c) Cat Dex K having ε ) 0.83 mequiv/g.
profiles are presented in Figure 2. In order to correspond to the adsorbed mass needed to saturate the available surface, the images were taken at an equilibrium concentration corresponding to slightly above zero in the adsorption isotherms. The fluorescent CLSM images indicate that the changes in the adsorption behavior of the 1.0 × 104 Da cationic dextran can be explained by the extent that the cationic dextran adsorbs into the porous wall. The adsorption of Cat Dex K, having the highest charge density, is limited to the outermost surface. Cat Dex J, having a lower charge density, penetrates into the pores, although the maximum in the fluorescent intensity still occurs at the outermost surface. Thus it appears as though a transition occurs for which the cationic dextran has access to the pore structure. When the charge density is sufficiently low, as is the case for Cat Dex I, the cationic dextran adsorbs uniformly throughout the entire fiber, such that a gradient in the fluorescent intensity does not occur. Effect of Molecular Mass on the Adsorption Behavior in Electrolyte-Free Conditions. The adsorption of low and high molecular mass cationic dextran presented in Figure 1 indicates that the molecular mass also influences the adsorption behavior. Although the charge density had a noticeable effect on the adsorption of the low molecular mass cationic dextran, this effect was not as significant for the higher molecular mass fraction. The effect of molecular mass was further investigated by measuring the adsorbed mass of cationic dextran having a similar charge density in electrolyte-free conditions. A charge density of ∼ 0.30 mequiv/g was chosen as a noticeable difference already exists between the low and high molecular mass fractions. Figure 3 presents the adsorption isotherms of several molecular mass fractions of cationic dextran. An important feature of Figure 3 is that the adsorption isotherms of each molecular mass fraction are distinguishable from each
Figure 3. Adsorption isotherms for cationic dextran having similar charge density (ε ∼ 0.30 mequiv/g) but varying in molecular mass.
other. We have previously shown for a high charge density polyelectrolyte that the adsorption behavior only differed for the lowest molecular mass fractions (Mw < 1.0 × 104 Da).11 However, it is evident from Figure 3 that the adsorption of the low charge density cationic dextran is affected by the molecular mass in a different manner. The adsorption isotherm for each molecular mass fraction falls onto separate curves, with the lower molecular mass fractions adsorbing more than the higher molecular mass fractions. The effect of the molecular mass was again rationalized using fluorescent CLSM, with the images and corresponding intensity profiles presented in Figure 4. It again becomes apparent that the changes in adsorbed mass are dependent on the extent that each cationic dextran adsorbs into the pore structure. The adsorption of Cat Dex A, having Mw ) 2.0 × 106 Da, is typically restricted to the outermost surface. However, image (a) of Figure 4 also shows that adsorption occurred within the lumen, i.e. the hollow inner surface of the
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Figure 4. CLSM images of the fiber length-section and the intensity profiles for the adsorption of several molecular mass fractions of cationic dextran having similar charge density (ε ∼ 0.30 mequiv/g) but different molecular mass: (a) Cat Dex A having Mw ) 2.0 × 106 Da, (b) Cat Dex E having Mw ) 2.5 × 105 Da, and (c) Cat Dex I 1.0 × 104 Da.
fiber, which is unexpected. Although this behavior is not necessarily representative, it is attributed to the cationic dextran penetrating through regions in the cell wall that possibly were damaged in the refining process. It is difficult to assess if this behavior also occurs for the lower molecular mass fractions, which penetrate into the pores. Image (b) of Figure 4 shows that adsorption within the fiber cell wall can occur for molecular mass fractions as high as 2.5 × 105 Da. The gradient in the fluorescent intensity profile suggests that adsorption primarily occurs near the fiber exterior. However, some of Cat Dex E has adsorbed throughout the fiber cell wall, indicated by the substantial fluorescent intensity at the fiber lumen. This could be explained by the polydispersity, as the lower molecular mass chains may penetrate into the fiber cell wall while the higher molecular mass chains are limited to the fiber exterior. Nonetheless, it is evident from image (c) of Figure 4 that the polydispersity is not a factor for sufficiently low molecular mass fractions, as the 1.0 × 104 Da fraction adsorbs uniformly throughout the fiber cell wall. The Effect of Electrolyte Concentration on the Adsorption Behavior. Adsorption isotherms were constructed in various electrolyte concentrations for several high molecular mass fractions of cationic dextran. Figure 5 presents the saturation adsorption, determined by extrapolating the isotherms to an equilibrium concentration of zero, in each electrolyte concentration. The behavior of each cationic dextran was similar in that the saturation adsorption was found to decrease with increasing electrolyte concentration. However, the electrolyte concentration at which the adsorption ceased appears to be influenced by the charge density, such that the adsorption of a lower charge density cationic dextran becomes sensitive to the electrolyte at lower concentrations. This can be particularly noted by comparing Cat Dex B and Cat Dex D, for which the adsorption of the lower charge density cationic dextran ceases at an electrolyte con-
Figure 5. Adsorption of high molecular mass cationic dextran as a function of NaHCO3 concentration.
centration 10 times lower than the higher charge density counterpart. A similar decrease in the saturation adsorption with increasing electrolyte concentration has been noted for the adsorption of low charge density polyelectrolytes to cellulosic fibers.8,9 However, this behavior is notably different than the adsorption of high charge density polyelectrolytes, for which the saturation adsorption substantially increases as the electrolyte concentration increases before eventually ceasing at high electrolyte concentrations.7 As the high charge density polyelectrolytes were found to penetrate into the pore structure at high electrolyte concentrations,11 it is also of interest to determine the extent to which the
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Figure 6. Cross-sectional CLSM images for the adsorption of Cat Dex A in different NaHCO3 concentrations: (a) adsorption in 10-5 M NaHCO3, (b) adsorption in 10-3 M NaHCO3, and (c) adsorption in 10-2 M NaHCO3.
cationic dextran adsorbs into the fiber cell wall. The fluorescent CLSM images for the adsorption of Cat Dex A in various electrolyte concentrations are presented in Figure 6. It is interesting to note that the extent that Cat Dex A adsorbs into the fiber cell wall changes with the electrolyte concentration. However, this change in the adsorption behavior does not quite reflect the changes in the saturation adsorption data presented in Figure 5. Although the adsorbed mass remains relatively constant for electrolyte concentrations below 10-2 M NaHCO3, the images in Figure 6 show that Cat Dex A penetrates into the fiber wall already at 10-3 M NaHCO3. Moreover, the CLSM images suggest that Cat Dex A adsorbs to a lesser extent in the fiber wall as the electrolyte concentration is increased to 10-2 M NaHCO3. This behavior at 10-2 M NaHCO3 was further investigated for the other high molecular mass fractions, and the fluorescent CLSM images are presented in Figure 7. The CLSM images for the adsorption in 10-2 M NaHCO3 qualitatively agree with the adsorption data presented in Figure 5. The effect of the charge density on the adsorption behavior is also evident. The cationic dextran having the lowest charge density, Cat Dex D, is effectively being screened from the fiber exterior at 10-2 M NaHCO3 . Therefore the fluorescent intensity in image (a) of Figure 7 is close to zero. The adsorption behavior is different for Cat Dex A, which has a slightly higher charge density. The electrostatic interaction with the fiber surface is not effectively screened at 10-2 M NaHCO3 and adsorption is relatively unaffected, as Cat Dex A still adsorbs. However, the electrolyte does disrupt the balance of electrostatic interactions such that Cat Dex A is able to adsorb into the fiber cell wall. This is no longer the case for Cat Dex B, which has an even higher charge density. Although the high charge density leads to electrostatic interactions with the fiber exterior that are not effectively screened at 10-2 M NaHCO3, the charge density is sufficiently high enough to prevent Cat Dex B from adsorbing
into the fiber cell wall. Thus the difference in molecular mass between Cat Dex A and B is negligible in comparison to the difference in charge density. Effect of a Preadsorbed Layer. The effect of having a preadsorbed layer on the adsorption of the cationic dextran was also investigated. An excess of Cat Dex A was first added to the fiber suspension in electrolyte-free conditions, as it was shown to only adsorb to the fiber exterior. Lower molecular mass fractions were then added to the fiber suspension, and the saturation adsorption of each molecular mass fraction is plotted in Figure 8. The adsorption is presented in terms of the adsorbed number of charges to account for differences in the charge density. The adsorption of each cationic dextran onto untreated fibers is also presented for comparison. A definite similarity exists for the adsorption of cationic dextran onto both untreated fibers and fibers containing a preadsorbed polyelectrolyte layer. It is clear from Figure 8 that the high molecular mass fraction does not adsorb to a substantial amount when the fiber exterior is blocked by a preadsorbed layer, presumably as the preadsorbed layer has already adsorbed to compensate the available fiber charges. As the molecular mass is decreased, the cationic dextran adsorbs to a greater amount. A transition occurs below a molecular mass of 2.5 × 105 Da, such that the lower molecular mass fraction adsorbs in much greater amounts. For example, 70 µequiv/g of the 1.0 × 104 Da cationic dextran adsorbs to fibers containing a preadsorbed layer, whereas 108 µequiv/g adsorb to untreated fibers. Considering that 17.5 µequiv/g is already contained within the preadsorbed layer, the total amount of adsorbed charges is comparable to adsorption to untreated fibers. Therefore, it is plausible that the preadsorbed layer does not effect adsorption, which is in accordance with earlier investigations using different molecular mass fractions of high charge density polyDADMAC.6
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Figure 7. CLSM images taken in the fiber length and intensity profiles for the adsorption of high molecular mass cationic dextran in 10-2 M NaHCO3: (a) Cat Dex D, (b) Cat Dex B, and (c) Cat Dex A, for which the image and intensity profile corresponds to image c of Figure 6 taken for the fiber length as opposed to the fiber cross-section.
Figure 8. Saturation adsorption of cationic dextran varying in molecular mass in electrolyte-free conditions onto fibers containing a preadsorbed layer of high molecular mass (Mw ) 2.0 × 106 Da) cationic dextran.
It is important to determine how the cationic dextran adsorbs in the presence of a preadsorbed layer. Although it is evident that the cationic dextran does adsorb, it is unclear whether it adsorbs through the preadsorbed layer or adsorbs to the fiber exterior by forcing the preadsorbed layer to migrate into the fiber cell wall. By using fluorescent labels with distinct emission spectra, it is possible to distinguish the preadsorbed layer from the adsorbing cationic dextran using fluorescent CLSM. This so-called double staining technique was employed using FITC, which emits a green fluorescence, and sulforhodamine B acid chloride, which emits a red fluorescence. The images are presented in Figure 9, with the preadsorbed layer being labeled with FITC. The fluorescent CLSM images are in agreement with the adsorption behavior presented in Figure 8. The lack of red
Figure 9. CLSM images of fiber length-sections for the adsorption of cationic labeled dextran, labeled with sulforhodamine B acid chloride to be red, onto fibers containing a preadsorbed layer of (Mw ) 2.0 × 106 Da) cationic dextran, labeled with fluoroscein isothiocyanate to be green: (a) Cat Dex A, (b) Cat Dex C, (c) Cat Dex E, and (d) Cat Dex I.
fluorescence in image (a) of Figure 9 indicates that the 2.0 × 106 Da cationic dextran does not adsorb. The 5.0 × 105 Da cationic dextran adsorbs in small amounts on the fiber exterior, which is believed to be an effect of an overlap in the molecular mass
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distributions of the preadsorbed layer and the 5.0 × 105 Da fraction, such that the lower molecular mass chains in the preadsorbed layer could become displaced. For cationic dextran fractions having Mw e 2.5 × 105 Da, the adsorbing cationic dextran penetrates into the fiber cell wall, seemingly unaffected by the preadsorbed layer. Another subtle characteristic for the adsorption of the lower molecular mass cationic dextran is that the preadsorbed layer is also unaffected by the adsorption, indicating that the second adsorbing cationic dextran was drawn into the fiber wall rather than forcing the preadsorbed layer to diffuse into the fiber cell wall. It should be noted that this behavior differs from the work of Kabanov et al.4 using cross-linked polyelectrolyte gels, for which a “relay-race” type mechanism was observed in which the preadsorbed layer diffused into the pores structure.
Discussion Effect of Polyelectrolyte Properties on Adsorption Behavior. In electrolyte-free conditions, the charge density can have a large effect on the adsorbed mass. Polyelectrolyte adsorption strives to compensate the available fiber charges, where one polyelectrolyte charge adsorbs for each available fiber charge. This is generally accepted for the adsorption of high charge density polyelectrolytes to cellulosic fibers, which have been shown to adsorb at a 1:1 charge stoichiometry.20 However, charge compensation does not appear to be consistent for the adsorption of low charge density cationic dextran. It is evident from Figures 4 and 9 that the adsorption of Cat Dex A is restricted to the fiber exterior. In electrolyte-free conditions, Cat Dex A adsorbs at ∼50 mg/g or ∼ 17.5 µequiv/g. Considering that the surface charge of the pulp was measured to be 3.7 µequiv/g, overcompensation of the fiber charges already occurs in electrolyte-free conditions. This is somewhat in contradiction to the previous theoretical modeling,1,25 where overcompensation, i.e. more than one polyelectrolyte charge adsorbs for each accessible fiber charge, was shown to only occur in the presence of electrolyte, albeit through different mechanisms. The effect of the charge density on the molecular conformation of the cationic dextran offers a possible explanation for overcompensation. The charge density introduces electrostatic repulsions along the polyelectrolyte backbone that act to “stiffen” the molecular conformation. These electrostatic repulsions are significant for high charge density polyelectrolytes, and the polyelectrolyte adsorbs in a flat conformation. For low charge density polyelectrolytes, the distance between charges along the polyelectrolyte backbone is greater, such that the electrostatic repulsions are not as significant. This would allow the low charge density cationic dextran to adsorb in a loop-and-tail conformation, which would lead to overcompensation. The lateral correlation between adsorbing polyelectrolyte chains is another factor that must also be considered. Compared to high charge density polyelectrolytes, the electrostatic repulsion between adsorbing polyelectrolyte chains is much lower. As polyelectrolyte adsorption is fast, the adsorbing chains can adsorb in a closer proximity such that the chains could be sterically hindered from relaxing on the fiber surface by the other adsorbed chains. Thus, the structure of the adsorbed layer could be different for low charge density polyelectrolytes, which could allow overcompensation to occur without the presence of electrolyte. (25) Dobrynin, A. V.; Deshkovski, A.; Rubinstein, M. Macromolecules 2001, 34, 3421. (26) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch. ; Stscherbina, D. Polyelectrolytes: Formation, Characterization and Application; Hanser Publisher: Munich, Germany; 1994.
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The extent that the cationic dextran adsorbs into the fiber cell wall appears to be controlled by both the charge density and the molecular mass. Similar to the adsorption of high charge density poly(AM-co-DADMAC),11 adsorption can be restricted to the fiber exterior if the charge density is sufficiently high. It can be seen in Figure 2 that this is even relevant for the adsorption of low molecular mass cationic dextran. However, the extent that the 1.0 × 104 Da cationic dextran adsorbs into the fiber wall increases as the charge density is lowered. It is unclear as to whether this behavior is due to an increased flexibility of the polyelectrolyte chain or a lower electrostatic interaction with the fiber pore wall. An effect of molecular mass is observed when the charge density is sufficiently low, i.e. adsorption is not restricted to the fiber exterior. Whereas the adsorption of high molecular mass cationic dextran is still restricted to the fiber exterior, as indicated in Figure 4, the lower molecular mass cationic dextran is able to adsorb into the fiber cell wall. It is also interesting to note that the extent the cationic dextran adsorbs into the fiber cell wall appears to be directly related to the molecular mass. Specifically, cationic dextran adsorbs further into the fiber cell wall as the molecular mass is decreased. This effect is also seen in Figure 8, for adsorption to untreated fibers as well as fibers containing a preadsorbed layer of cationic dextran. A transition in the adsorption behavior can be seen for Mw < 2.5 × 105 Da, such that the adsorbed mass increases more dramatically as the molecular mass is decreased. Above Mw g 2.5 × 105 Da, the molecular size of the cationic dextran is larger than the fiber pore size, which is ∼30-40 nm in diameter. A Mw < 2.5 × 105 Da is sufficiently small enough to enter the fiber pores. However, all the lower molecular mass fractions should therefore be sufficiently small enough to enter the pores in the fiber cell wall. Moreover, the similar charge density suggests that the electrostatic interactions with pore wall will be equivalent. The differences in the adsorbed mass, and presumably in the extent of adsorption into the fiber cell wall, are the effect of the molecular mass on the kinetic process governing the polyelectrolyte movement through the pores. The Effect of Electrolyte on the Balance of Electrostatic Interactions. The effect of adding electrolyte is not straightforward in regard to the extent that the cationic dextran adsorbs in the fiber cell wall. This is evident in the adsorption of the high molecular mass cationic dextran. It was established in Figure 4 that the adsorption of the highest molecular mass fraction was restricted to the fiber exterior in electrolyte-free conditions. The cationic dextran was able to penetrate into the pores as the electrolyte concentration increases to 10-3 M NaHCO3, as indicated in Figure 6. However, Figure 6 also suggests that further increasing the electrolyte concentration no longer facilitated adsorption into the pores, rather the adsorption again becomes restricted to the fiber exterior. It appears that the relative importance of the driving force compared to the molecular properties shifts above a certain electrolyte concentration, such that the cationic dextran no longer adsorbs into the pore structure. Therefore it is necessary to rationalize the specific electrostatic interactions that govern polyelectrolyte adsorption. The extent that the cationic dextran can adsorb into the fiber cell wall depends on (1) the molecular properties of the cationic dextran and (2) the driving force needed to promote adsorption, which are both dependent on the electrolyte concentration. The effect the electrolyte has on the molecular conformation of a polyelectrolyte is well-established.26 Figure 10 presents the effect of electrolyte on the cationic dextran using the hydrodynamic diameter (DH) as a measure of the molecular size.
Polyelectrolyte Adsorption to Substrate
Figure 10. The hydrodynamic diameter, DH, of high molecular mass cationic dextran as a function of the NaHCO3 concentration at pH ∼7.8.
Polyelectrolytes are relatively unaffected by low electrolyte concentrations, and therefore, the chains are molecularly “stiff” due to the repulsions between electrostatic charges along the polyelectrolyte backbone as well as excluded volume effects. As the electrolyte concentration increases, the electrostatic repulsions along the polyelectrolyte backbone decrease and the conformation of the polyelectrolyte gradually relaxes. The polyelectrolyte eventually can take the conformation of an uncharged polymer by further increasing the electrolyte concentration such that the electrostatic repulsions become completely screened. The electrolyte also acts to screen electrostatic interactions between the polyelectrolyte and fiber charges in the confines of the pore volume. Electrolyte reduces these interactions with the pore walls that would be expected to hinder movement within the pores. Despite the cationic dextran becoming more flexible, i.e. a further decrease in DH, and the electrostatic interactions with the pore being further reduced, the cationic dextran did not adsorb throughout the fiber wall when the electrolyte concentration was increased to 10-2 M NaHCO3 . It can be seen in Figure 5 that adsorption of the cationic dextran ceases in moderate electrolyte concentrations. It is also important to note that less electrolyte is needed to effectively screen cationic dextran having lower charge density. This is an important factor when considering the extent that the cationic dextran adsorbs into the fiber cell wall. Although the cationic dextran becomes more flexible by increasing the electrolyte concentration, the cationic dextran does not adsorb into the fiber cell wall if the electrostatic driving force is also sufficiently screened. Therefore, Cat Dex A does not adsorb into the fiber cell wall at 10-2 M NaHCO3, as seen in image (c) of Figure 6, as the interactions with the charges located in the fiber cell wall are essentially screened. This effect is exhibited best by cationic dextran having a lower charge density, particularly Cat Dex D. Despite the electrostatic repulsions within the cationic dextran being completely screened at 10-3 M NaHCO3, the electrolyte has effectively screened the electrostatic driving force and Cat Dex D no longer adsorbs at all, much less into the fiber wall. Thus, it appears that the high molecular mass cationic dextran can adsorb into the fiber wall if (1) the electrolyte concentration is not too high and (2) the charge density is not too low. Lateral Correlation Effects. A subtle effect of the electrolyte on adsorption behavior can be seen in Figure 5. Specifically, the adsorbed mass monotonically decreases as the electrolyte concentration increases. This behavior has also been noted for other low charge density polyelectrolytes8,9 and is indicative of pure electrosorption. However, it has also been shown that the adsorption of high charge density polyelectrolytes to cellulosic fibers initially increases with increasing electrolyte concentration.7,11 Thus there is a significant difference in how the electrolyte
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affects the adsorption of low and high charge density polyelectrolytes to cellulosic fibers. This difference can be rationalized by comparing two models for polyelectrolyte adsorption: (1) self-consistent mean-field theories and (2) scaling approaches. van de Steeg et al.1 used self-consistent mean-field theories and concluded that an increase in adsorption with increasing electrolyte, i.e. charge overcompensation, is only possible if a nonelectrostatic interaction exists between the polyelectrolyte and the surface (i.e., χs > 0). However, lateral correlation effects between the polyelectrolyte chains were not incorporated into the model. Dobrynin et al.25 later used scaling approaches to argue that charge overcompensation occurs without a nonelectrostatic interaction if later lateral correlation effects were included. Whereas self-consistent mean-field theories accurately depict the adsorption behavior of low charge density polyelectrolytes, they are limited in describing the adsorption behavior of high charge density polyelectrolytes. However, the difference in adsorption behavior between low and high charge polyelectrolytes can be explained by scaling approaches. The lateral correlation between high charge density polyelectrolyte chains is significant at low electrolyte concentrations. Increasing the electrolyte concentration will thereby screen these interactions, allowing more chains to adsorb closer together. As the cationic dextran had a low charge density, the adsorbing chains could pack close together even in electrolyte-free conditions. Therefore, the effect of the electrolyte on the lateral correlations was negligible, such that the adsorption behavior was primarily dictated by screening the interactions between the cationic dextran and the fiber charges. Nonetheless, self-consistent mean-field theories are applicable for electrosorption; i.e., nonelectrostatic interactions do not exist. Polyelectrolytes in a Porous Media. There is little experimental evidence concerning how a polyelectrolyte behaves in a porous media. Although a cellulosic fiber is not such an ideal porous substrate, the fundamental behavior should be indicative of a substrate having a more homogeneous pore structure. Polyelectrolytes initially adsorb to the available fiber exterior, presumably as adsorption into the fiber pores is kinetically limited by the molecular conformation and interactions with the fiber charges. Nonetheless, low molecular mass cationic dextran does adsorb into the fiber wall, raising the question of how it enters the fiber pores. The images in Figure 9 suggest that the low molecular mass cationic dextran adsorbs through the cationic dextran already adsorbed to the fiber exterior. This has several implications on the adsorption behavior. First, the cationic dextran does not need a free surface to adsorb to. This would imply that the cationic dextran does not adsorb to a free surface and then reptate into the fiber pores, creating a free surface for more cationic dextran to adsorb. Second, the entropic driving force, due to the loss of counterions, for adsorption must be significant in order to overcome the entropy loss for the cationic dextran entering the pore. Furthermore, the driving force is sufficient enough to pull the cationic dextran through the cationic barrier existing at the fiber exterior. This cationic barrier is significant for the cationic dextran, as overcompensation already occurs in electrolyte-free conditions. Finally, a kinetic effect can be seen for different molecular mass fractions of cationic dextran, as higher molecular mass fractions did not penetrate to the same extent. As entropic effects dominate polyelectrolyte adsorption, it should not be excluded that the molecular mass is not relevant. Although the high molecular mass cationic dextran does not adsorb to the same extent as the lower molecular mass fractions at 30 min, it is not known if this is purely a kinetic difference. Continuing work is being focused on resolving the time scales for adsorption
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into the pore structure. However, for adsorption times of 30 min, it can be seen that high molecular mass cationic dextran is effectively bound by the fiber charges.
Conclusions The adsorption of cationic dextran to cellulosic fibers is dependent on the molecular mass and charge density. Despite the cationic dextran being prepared as a low charge density polyelectrolyte, the adsorption of the highest charge density cationic dextran was still limited to the fiber exterior. However, adsorption occurred into the fiber cell wall if the charge density was sufficiently low. A notable effect of the molecular mass was observed at these low charge densities. Whereas the adsorption of high molecular mass cationic dextran was limited to the fiber exterior, lower molecular mass fractions penetrated the fiber cell wall to greater extents. The effect of electrolyte was to decrease adsorption. The lateral correlations between adsorbed cationic dextran chains is negligible, and such adsorption decreased as only the interactions with the fiber charges was screened. The effect of the electrolyte on the extent the cationic adsorbs into the fiber cell wall is somewhat governed by the molecular
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conformation. At low electrolyte concentrations, the electrostatic repulsions along the polyelectrolyte backbone become screened. The cationic dextran becomes more flexible and could penetrate into the fiber cell wall. Despite the increasing flexibility at high electrolyte concentrations, adsorption no longer occurred throughout the fiber cell wall. This was due to the electrolyte screening the electrostatic driving force to the fiber charges. Finally, it was shown that the adsorbed cationic dextran is tightly bound to the fiber charges. Subsequent cationic dextran additions adsorbed through the preadsorbed layer, rather than pushing the adsorbed layer into the fiber pores. This also indicates that low charge density polyelectrolytes adsorb directly into the fiber pores, rather than diffusing. Acknowledgment. The authors would like to thank Mrs. Anni Hagberg for her diligent efforts with the fluorescent confocal laser scanning microscopy. Financial support from the Biofibre Material Centre (BiMaC) at KTH and STFI-Packforsk AB is gratefully acknowledged. LA800274W
