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Surface Forces in Aqueous Polyvinylamine Solutions. 2. Interactions between Glass and Cellulose E. Poptoshev, M. W. Rutland,* and P. M. Claesson Department of Chemistry, Surface Chemistry, Royal Institute of Technology, SE-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, P.O. Box 5607, SE-114 86, Stockholm, Sweden Received July 20, 1999. In Final Form: October 19, 1999 The forces acting between one surface coated with Langmuir-Blodgett cellulose films and one glass surface have been measured using the MASIF surface force technique. This study is mainly concerned with the effects due to addition of cationic polyvinylamine and changes in ionic strength. The results have implications for the interactions between cellulose and mineral surfaces in the papermaking process. The cellulose surface is weakly negatively charged at pH 5.5-6.0. Polyvinylamine adsorbs to both glass and cellulose surfaces, which first causes charge neutralization and subsequently a charge reversal upon increasing the polymer concentration. The cellulose films swell upon immersion in aqueous solutions as evidenced by the appearance of a short-range steric force barrier. The dynamics of the swelling process was found to be dependent on the ionic strength of the solution. It was found that an increased ionic strength accelerates the swelling, which can be attributed to an increase in surface charge density and hence an increase in the short-range repulsion between individual cellulose chains. The results obtained in this study are consistent with those obtained for the interaction between two glass surfaces in polyvinilamine solutions as described in a previous report. However, the long-range (bridging) attraction observed between two glass surfaces immersed in the polyvinylamine solution was absent between one glass and one cellulose surface. We attribute this to the lower surface charge density of the cellulose surface.
Introduction series1
we reported on the In the first paper of this interactions between two identical glass surfaces across aqueous polyvinylamine solutions. The main aim of that study was to gain knowledge about polyelectrolyte adsorption on glass surfaces as models for surfaces with a moderately high negative charge density. Here, we extend our study to include an asymmetric system consisting of a moderately charged glass surface and a weakly negatively charged cellulose surface. This system also has a practical interest since cationic polyelectrolytes are added in order to reduce the repulsive electrostatic force barrier between cellulose fibers and additives (fillers, pigments, etc.) in papermaking processes. There are only a few works present in the literature that report on measurements of interactions between cellulose surfaces. The first attempt was due to Neuman et al.2 who measured forces between spin-coated cellulose layers on mica using the interferometric surface force apparatus.3 However, it was only partly successful because of the large swelling of their model surfaces, thus giving rise to a long-range steric force. Recent developments in the techniques of surface modification and surface force measurements allowed a large variety of surfaces to be studied. Interactions between cellulose micron sized particles were studied by Rutland et al.4 and Carambassis et al.5 using scanning probe microscopy. They found that * Corresponding author. (1) Poptoshev, E.; Rutland, M. W.; Claesson, P. M. Langmuir, submitted. (2) Neuman, R. D.; Berg, J. M.; Claesson, P. M. Nordic Pulp Paper Res. 1993, 8, 96. (3) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans. 1 1978, 74, 975. (4) Rutland, M. W.; Carambassis, A.; Willing, G. A.; Neuman, R. D. Colloids Surf. A 1997, 123-124, 369. (5) Carambassis, A.; Rutland, M. W. Langmuir 1999, 19, 5584.
the cellulose surface is weakly negatively charged and swells in water. The AFM technique was also used by Alfano et al.6 to measure forces between silica and mica in the presence of the cationic coagulants used in papermaking. An alternative method to prepare cellulose surfaces suitable for surface force studies was used by Holmberg et al.7,8 They utilized Langmuir-Blodgett deposition of trimethylsilyl cellulose onto hydrophobized mica, followed by desilylation. Such surfaces have a molecular scale roughness as proved by AFM imaging7 and do not swell to the same extent as the spin-coated layers used earlier. It was shown that LB cellulose films are suitable substrates for surface force measurements and they were successfully used to model both fiber-fiber7 and fiber-filler8 interactions in various solution conditions. In the present paper, we employ the same LB technique for surface preparation but we use these surfaces to probe the interactions between cellulose and mineral surfaces in the presence of a rather novel polyelectrolyte, polyvinylamine. The study is thus relevant to areas such as polymer adsorption and interactions between differently charged surfaces. Materials and Methods Chemicals. Trimethylsilyl cellulose (TMSC) was synthesized in our laboratory according to the method described elsewhere. 7,8 For the LB deposition 10.3 mg TMSC was dissolved in 25 mL chloroform, which was purchased from Merck and further purified by distillation. Arachidic acid (AA) was obtained from Merck and used as received. Eicosylamine (EA) has been previously prepared (6) Alfano, J. C.; Carter, P. W.; Nowak, M. J.; Whitten, J. E. Nordic Pulp Paper Res. 1999, 14, 30. (7) Holmberg, M.; Berg, J.; Stemme, S.; O ¨ dberg, L.; Rasmusson, J.; Claesson, P. J. Colloid Interface Sci. 1997, 186, 369. (8) Holberg, M.; Wigren, R.; Erlandsson, R.; Claesson, P. M. Colloids Surf. A 1997, 129-130, 175.
10.1021/la990961v CCC: $19.00 © 2000 American Chemical Society Published on Web 12/16/1999
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Figure 1. Molecular structure of the vinylammonium monomer unit. according to Berg et al.9 Equal amounts of EA and AA were dissolved in a mixture of 95% chloroform and 5% absolute ethanol to obtain a solution with a total concentration of 2.7 mM. Polyvinylamine (PVAm) of average molecular weight 90 000 (molecular structure shown in Figure 1) was kindly provided by Dr. Ralf No¨renberg BASF AG, Ludwigshafen, Germany and precipitated in spectrographic grade ethanol.10 Sodium chloride was obtained from Merck and used without further treatment. The water was purified using RiOs-8 and Milli-Q+ 185 units from Millipore and finally passed through a 0.2 micrometer filter. The total organic content of the outgoing water did not exceed 10 ppb as indicated by a TOC monitor (A-10 from Millipore) connected to the Milli-Q unit. Surface Preparation. An automatic Langmuir-Blodgett balance system from KSV Instruments Finland was used for preparing model cellulose surfaces. A freshly cleaved piece of muscovite mica (about 1 cm2) was glued onto a stainless steel flat surface holder of the MASIF surface force apparatus using an epoxy resin, Epicote 1004. The hydrophobization was carried out by depositing a mixed monolayer of AA/EA onto the mica at a constant surface pressure of 30 mN/m. In the next step, 10 monolayers of TMSC were deposited at a constant surface pressure of 15 mN/m and a deposition rate of 5 mm/min. After the deposition process was completed the surface was exposed to humid HCl (positioned above a 10% HCl solution) for one minute in order to convert the TMSC to cellulose.11 Previous SFM studies have shown that such films are very smooth with rms roughness of 0.16 nm over a 1 mm2 area.7 More details about the surface preparation method can be found elsewhere.7,11 All procedures mentioned above were carried out inside a laminar flow cabinet in order to minimize the risk of airborne contamination. Once prepared, the surfaces were immediately mounted and sealed into the measuring cell of the MASIF instrument. Surface Force Measurements. Interaction forces between cellulose and glass surfaces were measured using the noninterferometric surface force apparatus (widely known as MASIF) developed by Parker.12 Elaborate description of the technique is given in numerous of works.12-14 Here only a brief outline is provided. Both surfaces are attached to piezoelectric materials. The lower surface (the flat cellulose surface in our case) is attached to a bimorph force sensor and the upper one (glass sphere) to a piezo tube. All the assembly is enclosed in a liquid cell (volume ca. 10 mL) and mounted on a translation stage. The distance between the surfaces is controlled by applying a triangular voltage wave to the piezo crystal. Simultaneously, the response of the bimorph force sensor (charge produced upon bending) is recorded. After the surfaces come into a hard wall contact, the linear motion of the piezo is directly transmitted to the bimorph and the deflection of the latter can be obtained from the constant compliance regime. The force is simply obtained from the deflection data together with the spring constant of the bimorph by using Hooke’s law. Glass surfaces were prepared by melting the end of a borosilicate glass rod in a butane-oxygen flame until a spherical droplet with radius ca. 2 mm was formed.12 After the end of each set of experiments the radius of curvature R was measured with a micrometer and the force scaled by this radius was related to the free energy of interaction per unit area, Gf, according to the (9) Berg, J. M.; Eriksson, L. G. T.; Claesson, P. M.; Bo¨rve, N. Langmuir 1994, 10, 1225. (10) Dautzenberg, H.; Jaeger, W.; Ko¨tz, J.; Philipp, B.; Seidel, Ch.; Stscherbina, D. Polyelectrolytes; Hanser Publishers:, 1994; Vol. II. (11) Schaub, M.; Wenz, G.; Wegner, G.; Stein, A.; Klemm, D. Adv. Mater. 1993, 5, 919. (12) Parker, J. L. Prog. Surf. Sci. 1994, 47, 205. (13) Parker, J. L.; Yaminsky, V. V.; Claesson, P. M. J. Phys. Chem. 1993, 97, 7706. (14) Claesson, P. M.; Ederth, T.; Bergeron, V.; Rutland, M. W. Adv. Colloid Interface Sci. 1996, 67, 119.
Figure 2. Surface force measured on first approach between a flat cellulose surface and a glass sphere in air (RH ≈ 30%). Derjaguin approximation15
F/R ) 2πGf All the procedures regarding solution preparation and solution exchange during the measurements were identical to those reported in the previous paper.1 Force distance curves were analyzed using DLVO theory16,17 for the case of interaction between dissimilar surfaces using the computer software developed by J. Fro¨berg, based on the algorithm of Bell and Peterson.18 To reduce the number of fitting parameters, the decay length was not fitted but calculated from the electrolyte concentration, i.e., 30.4 nm in 0.1 mM NaCl and 9.6 nm in 1 mM NaCl. It was shown previously1 that addition of polyelectrolyte does not influence the decay length of the double layer force.
Results and Discussion Interactions in Air. Figure 2 shows the interaction force between a flat cellulose surface and a glass sphere on first approach in air (relative humidity ≈ 30%) together with the expected van der Waals force. A nonretarded Hamaker constant of 5.9 × 10-20 J was used for the fitting as estimated for the cellulose-air-silica system.19 The measured force appears to be in very good agreement with the predictions down to a separation of about 5 nm where the gradient of the attraction exceeds the bimorph spring constant and the surfaces “jump” to the next stable region. Careful inspection of Figure 2 shows that surfaces jump to a position about 1.5 nm from contact which can be attributed to a certain molecular level roughness on the cellulose surface. On separation (not shown) a strong adhesion force was detected, in fact so strong that it exceeded the measuring range of the MASIF, i.e., by the end of the force run the surfaces remained adhesive contact and had to be separated by using a stepping motor. However, when subsequent measurements between the same surfaces at the same position (not shown here) were attempted it was found that the forces on both approach and separation were purely repulsive and somewhat irreproducible. This is an indication that the strong adhesion during the first contact led to a local destruction of the LB film layered structure and increased the surface (15) Derjaguin, B. Kolloid Z. 1934, 69, 155. (16) Derjaguin, B.; Landau, L. Acta Physiochem. 1941, 14, 633. (17) Verwey, E. G. W.; Overbeek, J. T. G. The Theory of the Stability of Liophobic Colloids; Elsevier: Amsterdam, 1948. (18) Bell, G. M.; Peterson, G. C. J. Colloid Interface Sci. 1972, 41, 542. (19) Bergstro¨m, L.; Stemme, S.; Dahlfors, T.; Arwin, H.; O ¨ dberg, L. Cellulose 1999, 6, 1.
Glass/Cellulose Interactions in Aqueous Polyvinylamine Sols
Figure 3. Surface forces between cellulose and glass in 0.1 mM aqueous NaCl solution. Filled circles, after 30 min of incubation; unfilled squares after 24 h of incubation. Solid lines represent fits to DLVO theory for constant surface potential (lower line) and constant surface charge (upper line) limits. The broken line represents a fit with Y1 ) 95 mV and Y2 ≈ 0 mV and Debye length 30 mn. The inset shows forces measured on separation on a linear scale. Arrows indicate the position of outward jump.
roughness. For this reason, measurements in air were not performed in each set of experiments before introducing a solution into the measuring cell (unlike the case between two glass surfaces1). Interactions across NaCl Solutions. The interactions measured on approach between cellulose and glass in 0.1 mM aqueous NaCl solution after 30 min and 24 h of incubation are shown in Figure 3. The inset shows corresponding force curves measured on separation. The forces on approach were analyzed using DLVO theory for the case of interaction between surfaces with uneven potentials with constant surface charge (upper line) and constant surface potential (lower line). Both the potentials at the glass and cellulose surfaces were used as fitting parameters, though typical values for the magnitude of the potential at the glass surface previously found1 were used as a guide. For the present case we assumed a value of -63 mV at the glass surface1 and -20 mV at the cellulose layer. This value is in agreement with that found on cellulose spheres.4 At large separations, down to approximately 15 nm, the interaction seems to be well described by the Poisson-Boltzmann theory using the constant surface charge boundary condition. In the above analysis it was assumed that the cellulose surface bears a small negative charge which is in contradiction to what was found in earlier works7 on LB cellulose films. However, the magnitude and the slope of the observed double layer force suggest that there should be a small negative charge associated with the cellulose surface. It should be noted that for the case of unequal surface potentials, with one surface having a very low potential, the slope of the double layer force depends on the magnitude of the low potential surface at fixed Debye length. This point is illustrated in Figure 3 where we have included a separate theoretically calculated force curve (dashed line) assuming the same Debye length of 30 nm, but with surface potentials on the glass and cellulose surface of -95 mV and close to 0 mV, respectively. The disagreement between experiment and theory in this case is obvious. In fact, our interaction profiles after 30 min are very similar to those obtained by Holmberg et al.8 for the interaction between LB cellulose films and colloidal silica. The authors assumed that their cellulose surface
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is uncharged, but there were no fits provided in order to support this statement. There are a few facts that are worth paying attention to when classical DLVO theory is applied to these systems. First, the interaction has been assumed to take place between rigid bodies with the plane of charge positioned at the cellulose surface. This is rather unlikely to be the case, since cellulose surfaces swell in water (see below), which gives rise to a diffuse layer of charges on the cellulose chains. Second, the MASIF technique does not allow the thickness of the cellulose layer to be measured. In our analysis we assumed that the plane of charge coincided with the zero separation, as defined by the constant compliance region. Third, it can be seen from Figure 3, and in fact for all the cases treated here, that the constant charge boundary condition predicts a stronger repulsion than observed whenever the separation is less than 1520 nm. A similar behavior has been observed between two (charged) cellulose spheres in electrolyte solution.4 A nonDLVO attractive force contribution (bridging, hydrophobic attraction) cannot be considered here since both surfaces are rather hydrophilic and do not bear any adsorbed polymer layers. Based on this we have come to the conclusion that the interaction takes place between charge regulating surfaces, i.e, both the surface potential and the surface charge density vary with surface separation. The charge regulation capabilities of silica surfaces have been thoroughly described in the literature.20 On the other hand the surface charge of cellulose originates from dissociation of surface -COOH groups21 (present due to oxidation) which can regulate their charge when approached by the glass surface. All the reasons mentioned above make the absolute values of the potentials extracted from DLVO fitting somewhat uncertain. However, the main conclusion that our cellulose surface is weakly negatively charged remains valid. The long-range electrostatic force remains virtually unchanged after incubation in the aqueous 0.1 mM NaCl solution for 24 h. However, the short-range force has changed dramatically, and it is now purely repulsive all the way to contact. We argue that the additional repulsion is a result of swelling of the cellulose film, which leads to an increase of the amount of cellulose chains extending into solution. This would generate a steric force contribution in addition to the electrostatic repulsion. The force measured on separation also changes with time (inset in Figure 3). After 30 min of incubation a pull-off force of 6.5 mN/m was measured. Prolonged incubation resulted in a significant decrease in the pull-off force, and after 24 h the magnitude was only about 0.5 mN/m. This decrease is a consequence of the swelling of the cellulose film. Consecutive force runs on the same position were reproducible, unlike the case in air, hence the above effect cannot be explained by a local film destruction due to the presence of a large pull-off force. The interactions across 1 mM NaCl solution are shown in Figure 4. The two curves represent separate experiments performed on freshly prepared surfaces and incubated for 30 min. Considering the level of surface modification involved, the reproducibility is satisfactory. The constant charge limit describes the interaction more adequately than the constant potential limit with some deviations seen below 15 nm. This is consistent with the observations made at the lower salt concentration. The surface potential on the glass surface, deduced by fitting (20) Chapel, J.-P. Langmuir 1994, 10, 4237. (21) Marsh, J. T. Textile Science. An Introductory Manual, 4th ed.; Chapman & Hall: London, 1958.
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Table 1. Parameters Obtained from Fitting DLVO Theory to the Measured Forces
a
PVAm concentration [ppm]
NaCl concentration [mM]
surface potential glass [mV]
surface charge density glass [mC/m2]
surface potential cellulose [mV]
surface charge density cellulose [mC/m2]
0 1 2 5 10 0 1 2 5 10
0.1 0.1 0.1 0.1 0.1 1 1 1 1 1
-63 16 23 28 -40 (-60)a 20 (30) 25 (40) 28 (47) 31 (53)
1.8 0.4 0.5 0.7 3.3 (6.0) 1.5 (2.3) 1.9 (3.3) 2.1 (4.0) 2.4 (4.9)
-20 15 15 15 -10 8 12 14 15
0.5 0.3 0.3 0.3 0.8 0.6 0.9 1.0 1.1
The figures in parenthesis are taken from ref 1 and shown for comparison.
Figure 4. Surface forces between cellulose and glass in 1 mM aqueous NaCl solutions. Different symbols represent two separate experiments. Solid lines represent fits to DLVO theory for constant surface potential (lower line) and constant surface charge (upper line) limits.
theoretically calculated force curves to the measured points, was somewhat lower than the one previously obtained from measurements of the forces acting between two glass surfaces in the same salt concentration1 -60 mV compared to -40 mV. However, the glass surface potential fell well within the range reported for other force and electrokinetic studies.22,23 Another observation was that the rate of swelling of the cellulose surface increased when the salt concentration was increased. In both sets of experiments carried out in 1 mM NaCl the cellulose film was found to be already completely swollen after 30 min and no further changes occurred during incubation for up to 36 h. No adhesion was detected at this salt concentration, even when measurements were performed only 15 min after immersing the surfaces in the solution, which is the shortest possible time needed to bring the equipment into working condition (i.e., electrical drift adjusting, finding the contact, etc.). We propose that the accelerated swelling dynamics can be explained by the increased surface charge density (see Table 1). This leads to an increased repulsion between individual cellulose chains, as well as between charges along each backbone, even though the higher ionic strength solution efficiently screens electrostatic forces. The reason may be that not many ions can penetrate into the compact part of the cellulose layer and thus they do not screen the interaction between these charges very efficiently. It should be noted (22) Larson, I.; Drummond, C. J.; Chan D. Y. C.; Grieser, F. Langmuir 1997, 13, 2109. (23) Ducker, W. A.; Senden, T. J. Langmuir 1992, 8, 1831.
Figure 5. Surface forces between cellulose and glass in aqueous PVAm solutions also containing 0.1 mM NaCl. Solid lines are fits to DLVO theory with constant surface charge boundary conditions. The inset shows the interaction across 1 ppm PVAm on a linear scale.
that the cellulose Langmuir-Blodgett films swell much less than the spin-coated cellulose employed earlier.2 The swelling affects the surface forces only in the last 5-15 nm (indicated by the range of the electrosteric force), which makes it possible for the longer range double layer force to be clearly observed and analyzed. Interactions across Solutions Containing Polyvinylamine. Interactions in the presence of PVAm were studied for the bulk concentration range 1-10 ppm at two ionic strengths following the same protocol as in ref 1. The interactions in PVAm solutions at a fixed salt concentration of 0.1 mM are illustrated in Figure 5. Only fits using the constant charge boundary condition are included in order to make the graph easier to read. The inset shows the interaction across 1 ppm PVAm on a linear scale. Addition of only 1 ppm of polymer was enough to completely neutralize the negative surface charge on both surfaces as indicated by the absence of any long-range force. The same result was previously obtained using two glass surfaces.1 The only force observed in the 1 ppm case is a repulsion with a range of about 7 nm. This force is of steric origin and is due to compression of the layers, both the cellulose layer and the adsorbed PVAm layer. We note that no attraction was observed between the cellulose and the glass across a 1 ppm PVAm solution, and that the interaction profile did not show any change with time. In a previous paper1 we discussed the long-range attraction seen between glass surfaces in very dilute PVAm solutions in terms of bridging of polyelectrolyte chains. Clearly, no
Glass/Cellulose Interactions in Aqueous Polyvinylamine Sols
Figure 6. Surface forces between cellulose and glass in aqueous PVAm solutions also containing 1 mM NaCl. Solid lines are fits to DLVO theory with constant surface charge boundary conditions.
bridging attraction is observed between cellulose and glass coated with PVAm, which most probably is due to the lower charge density of the cellulose surface. In a regime of electrostatically driven adsorption, the lower charge density would lead to a lower adsorbed amount, i.e., there are not very many extending polymer tails that may form bridges to the PVAm coated glass surface. Further, the tails extending from the glass surface will not be strongly attracted to the cellulose since the polymer-cellulose surface affinity is low once all the charges in the cellulose layer have been compensated by adsorbed polyelectrolytes. This assumption is supported by results from Monte Carlo simulations,24 showing that the range and magnitude of the attractive force between polyelectrolyte coated surfaces decreases with decreasing surface charge density. When the PVAm concentration is increased above 1 ppm, a repulsive double layer force develops which is clear evidence of a charge reversal (see Figure 6). At separations below 10 nm, the repulsive steric force remains unchanged. One may expect that PVAm should adopt a more extended conformation at the cellulose surface, leading to a stronger repulsive force at short separations, but clear evidence for such an effect was not found. When fitting theoretically calculated DLVO forces to the experimental curves we used the same surface potential at the glass surface (Table 1) as in ref 1. This gave a surface potential at the cellulose surface of 15 mV. Although it can be expected that the cellulose potential will increase slightly with the increase of PVAm concentration, the limitations of the PoissonBoltzmann theory in this case make it impossible to track such small changes. However, it is clear that the adsorption of polyvinylamine causes a charge neutralization, followed by charge reversal upon increasing the polymer concentration on both glass and cellulose surfaces. The forces between glass and cellulose across PVAm solutions were also determined at higher ionic strength (1 mM). In this case a pronounced charge reversal was already observed at 1 ppm PVAm. This result is consistent with the findings using two glass surfaces as reported in the first paper of this series. The influence of ionic strength on the adsorption and surface forces was discussed in that (24) Dahlgren, M. A. G.; Waltermo, Å.; Blomberg, E.; Claesson, P. M.; Sjo¨stro¨m, L.; Åkesson, T.; Jo¨nsson, B. J. Phys. Chem. 1993, 97, 11769.
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Figure 7. Surface forces between a PVAm coated cellulose surface and a bare glass surface in 0.1 mM NaCl solution. Open circles (upper curve), forces measured on approach; filled squares, forces measured on separation.
paper and only two main points are recapitulated here. First, an increase in ionic strength increases the surface charge density prior to polyelectrolyte adsorption, which makes the electrostatically driven adsorption more favorable. Second, the increased screening of the electrostatic forces reduces the free energy penalty for recharging the surfaces. Thus nonelectrostatic surface-polymer forces become more important and charge reversal occurs more easily. Our results are consistent with other studies25,26 that report an increased adsorption of cationic polyelectrolytes on cellulose fibers with increasing salt concentration. This effect was found to be especially pronounced for low molecular mass, high charge density polymers.26 From Table 1, it can be seen that at higher salt concentration, the potentials attributed to the glass surface are lower than those found earlier, but (as discussed above) the use of two surface potentials as fitting parameters introduces some uncertainty in the absolute values. Interactions between PVAm Coated Cellulose and Uncoated Glass. That PVAm does adsorb on cellulose was particularly clearly demonstrated in experiments using a cellulose surface which had been exposed to a solution containing 1 ppm PVAm and 0.1 mM NaCl. The measuring chamber was then drained and flushed several times with polyelectrolyte free 0.1 mM NaCl solution, before a fresh glass surface was immersed in this solution. The forces were then measured immediately in order to avoid an eventual transfer of PVAm from the cellulose surface to the glass surface through transport across the solution. The results are shown in Figure 7. The interaction on approach is long ranged and attractive down to about 5 nm. It decays roughly exponentially with distance with a decay length of approximately 20 nm. This is lower than expected for an attractive double layer force between oppositely charged surfaces; moreover the cellulose surface is assumed to be nearly uncharged (see inset in Figure 5) and in this case Poisson-Boltzmann theory predicts a repulsion. However, Monte Carlo simulations performed on asymmetric systems27 with one surface neutralized by adsorbed polyions suggest an attractive electrostatic force due to an asymmetric counterion distribution. At shorter separations bridging is also expected to contribute to the (25) Wågberg, L.; O ¨ dberg, L. Nordic Pulp Paper Res. 1989, 2, 135. (26) Pelton, R. H. J. Colloid Interface Sci. 1986, 111, 475. (27) Sjo¨stro¨m, L.; Åkesson, T. J. Colloid Interface Sci. 1996, 181, 645.
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observed attraction since in the present case all extending PVAm chains from the cellulose surface will be strongly attracted to the negatively charged glass surface. An even more interesting force-distance profile is observed on separation. Instead of the usual adhesive behavior with a jump out from contact occurring when a negative force is applied, we now observe a slow decrease of the attractive force when the separation is increased. The attraction persists to a separation of more than 200 nm! This phenomenon can be explained by the specific layered structure of the cellulose LB film. After the surfaces have been brought into contact many of the PVAm chains will be adsorbed to both surfaces. When separated, the PVAm molecules can act as anchors, thus pulling and disentangling whole cellulose layers underneath. It is unlikely that the polyelectrolyte alone can be responsible for inducing such forces. The estimated contour length of PVAm is about 200 nm, but analogous experiments with mica surfaces28 and even higher molecular weight polyelectrolyte have shown normal adhesive behavior i.e., when a sufficiently large negative force is applied the surfaces jump directly some distance apart. This is also the case between one PVAm coated mica surface and an uncoated glass sphere.29 On the other hand, in an experiment with a cellulose surface and a model ink, a similar long ranged bridging adhesion is observed when polymer additives were not present.30 Unlike mica, the layers in cellulose LB films are not chemically bonded but held together only by dispersion forces. It was already mentioned that strong adhesion observed in air damages the film locally. On the other hand adhesion of 6.5 mN/m measured in liquid was not enough to disrupt the film. Here, repeated measure(28) Dahlgren, M. A. G. J. Colloid Interface Sci. 1996, 181, 654. (29) Poptoshev, E. To be published. (30) Carambassis, A.; Rutland, M. W. To be published.
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ments could be done as well obtaining good reproducibility, which is a sign that despite the large stretching the film remained intact. Conclusions By using the Langmuir-Blodgett technique thin molecularly smooth films of cellulose on hydrophobized mica can be prepared. Such films swell only 5-15 nm in water, which allows long-range electrostatic forces to be measured. Contrary to other works performed using the same substrate, we found that the cellulose surface bears a small negative charge. Cationic polyvinylamine adsorption leads to neutralization of this charge and when the concentration is increased above 1 ppm a charge reversal occurs. The interaction on approach between a PVAm coated cellulose surface and a bare glass surface is long ranged and attractive in accordance with Monte Carlo simulations reported in the literature. On separation, a very longrange attractive force is present due to stretching of the LB film. However, the film remained intact and repeated measurements can be done at the same position with good reproducibility. This shows the robustness of the LB film despite its layered structure. Acknowledgment. E.P. acknowledges financial support from the Bo Rydins Foundation for scientific research. The polyvinylamine sample was kindly provided by Dr. Ralf No¨renberg, BASF AG, Ludwigshafen, Germany. Dr. J. Fro¨berg is acknowledged for providing the software used for fitting DLVO theory to the experimental curves. The authors would like also to tank Dr. Lars Wågberg and Mats Rundlo¨f from SCA-Research Sundsvall for valuable discussions during this investigation. LA990961V
