Mediation of the Nanotribological Properties of ... - ACS Publications

Feb 18, 2009 - Niklas Nordgren,† Paula Eronen,‡ Monika O¨ sterberg,‡ Janne Laine,‡ and. Mark W. Rutland*,†. Department of Chemistry, Surfac...
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Biomacromolecules 2009, 10, 645–650

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Mediation of the Nanotribological Properties of Cellulose by Chitosan Adsorption ¨ sterberg,‡ Janne Laine,‡ and Niklas Nordgren,† Paula Eronen,‡ Monika O Mark W. Rutland*,† Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas va¨g 51, SE-100 44 Stockholm, Sweden, and Department of Forest Products Technology, Helsinki University of Technology, P.O. Box 3320, FIN-02015 TKK, Finland Received December 18, 2008; Revised Manuscript Received January 19, 2009

Cellulosic model surfaces functionalized with chitosan, a naturally occurring cationic biomacromolecule, by in situ adsorption have been studied with an atomic force microscope (AFM) in colloidal probe configuration. The interaction forces on approach and separation, as well as the nanotribological properties, were shown to be highly pH-dependent, and a significant difference in the behavior was seen before and after chitosan adsorption. In general, all forces on approach showed a highly repulsive interaction at shorter distances due to deformation of the probe. At high pH, before chitosan adsorption, a long-range electrostatic repulsion was observed, consistent with DLVO theory. However, at low pH no electrostatic contribution was found before adsorption, probably due to charge neutralization of carboxyl groups. After chitosan adsorption, repulsive forces acting over a much longer distance than predicted by DLVO theory were present at low pH. This effect was ascribed to chain extension of the chitosan species of which the magnitude and the range of the force increased dramatically with higher charge at low pH. In all cases, a typical saw-tooth patterned adhesion was present, with pull-off events occurring at different separations. The frequency of these events after chitosan adsorption was greatly increased at longer distances. Additionally, the adsorbed chitosan markedly reduced the friction, where the largest effect was a 7-fold decrease of the friction coefficient observed at low pH.

Introduction Chitosan is a cationic biopolymer obtained by alkaline deacetylation of chitin. Generally, chitosans have a degree of deacetylation above 75% at which point changes in solubility properties occur.1 The molecular structures of chitin, chitosan and cellulose are depicted in Figure 1, illustrating their chemical similarities. Chitin can be found in fungi, insects, and the shells of marine crustaceans and is, thus, abundantly available from seafood industry waste.2 Factors favoring the use of chitosan include biocompatibility and nontoxicity, which are essential requirements for numerous healthcare and pharmaceutical applications.3,4 As a cationic polyion it can also be employed in surface engineering within the oral cavity which has prompted studies of multilayer formations with glyco-proteins.5,6 Similarly, in papermaking, chitosan has gained interest during the past decades where successful studies have used chitosan as a wet or dry strength agent,7-9 coating additive,10 and surface sizing agent.11 As a polyion with the charge located directly adjacent to the backbone, chitosan differs from most commercial additives, which have the charge located on the substituents.12 Also, the similarity of the polysaccharide backbones of chitosan and cellulose means that there may be additional implications for the adsorption and adhesion processes in cellulosic fiber based systems. This information could for example be beneficial for flocculation of fibers and web formation during the papermaking process. It is, therefore, highly relevant to study the interactions of cellulose in the presence of chitosan at the single contact level, * To whom correspondence should be addressed. E-mail: mark.rutland@ surfchem.kth.se. † Royal Institute of Technology. ‡ Helsinki University of Technology.

Figure 1. Structure of the β(1-4)-linked glucan backbone with substituents on the C2 atom denoted X corresponding to (a) chitin, (b) chitosan, and (c) cellulose.

and compare the behavior with that previously observed for uncharged polysaccharides such as xyloglucan,13-15 also a polymer with the same backbone as cellulose. Xyloglucan is a water soluble polysaccharide (due to its side chain substituents) that acts as a matrix between the reinforcing cellulose microfibrils in the structural part of the primary cell wall in wood and plants.16 It has been established that physisorption of xyloglucan to cellulose increases adhesion and reduces friction at the contacts,14,15 a highly unusual finding, because increased adhesion in general leads to increased friction. The effects can be correlated with macroscopic findings where xyloglucan has been shown to increase paper strength (increased adhesion) and paper formation (lowered friction).17,18 More recently it was shown that these properties are preserved even when xyloglucan is chemically end-anchored to a solid substrate.13 It is therefore interesting to see whether the unique nanotribological properties are retained in the case of chitosan, which provides additional cationic functionality and can thus also contribute to adhesion and other interactions via electrostatic mechanisms.

10.1021/bm801467w CCC: $40.75  2009 American Chemical Society Published on Web 02/18/2009

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It is known that the structure of adsorbed chitosan layers on mica and the interaction forces acting between two such surfaces depend reversibly on the pH.19 Holmberg et al. measured interactions between Langmuir-Blodgett cellulose films and chitosan-coated surfaces using the interferometric surface force apparatus (SFA) with long-range attractive forces observed at pH 5.6, due to bridging.20 For cellulose, extensive studies with atomic force microscopy (AFM) utilizing the colloidal probe technique with regenerated cellulose spheres21 have been performed under various conditions, for example, changing electrolyte concentration,22-24 pH,24,25 and in the presence of surfactant.26 The normal19 and lateral (frictional)27 forces acting on adsorbed chitosan layers on mica and the effects of glycoprotein and surfactant addition28 have been studied previously. Although mica and silica are ideal substrates for surface force and friction measurements due to their high modulus and low roughness, they are highly charged in comparison to cellulose. It has previously been shown that the charge ratio between the adsorbing polyion and the substrate affects the state of the adsorbed film.29 The inherent porous structure of cellulose and the possibility of specific polysaccharide interactions between the backbones make it necessary to depart from the convenient model substrates (mica and silica) to characterize behavior in the more complex system.

Experimental Section Solutions containing 1 mg/mL chitosan (∼20% β(1-4)-linked N-acetyl D-glucosamine and ∼80% β(1-4)-linked D-glucosamine as specified by the manufacturer) were prepared in 1% w/w acetic acid in water. The chitosan had a weight-average molecular mass, Mw of 4.0 × 105. It was provided by Sigma-Aldrich, had been sourced from crab shell powder, and was used as received. The water used in all experiments was first pretreated by a Milli-RO Plus unit and then further purified by a Milli-Q Plus185 system and filtered through a 0.2 µm Millipak filter. The resistivity after the treatment was 18.2 MΩ cm, and the total organic carbon content of the water did not exceed 2 ppb. The chitosan (1 mg/mL) was allowed to adsorb at pH 4 to the cellulose surfaces in situ in the AFM liquid cell for either 1 or 8 h before performing force and friction measurements. The electrolyte concentration of the measuring media was adjusted with NaCl (>99.99% purity) and pH was adjusted using either CH3COOH or NaOH of analytical grade. Before measurements, the system was excessively flushed with the corresponding solution to remove bulk material and any loosely bound chitosan from the surfaces. Force and friction measurements were performed using a MultiMode Picoforce AFM with Nanoscope III controller (Veeco; Digital Instruments, U.S.A.) equipped with a closed loop scanner. The measurements were conducted following the procedures extensively described in an IUPAC report.30 In this study the colloidal probe technique,31 extended to include measurements with cellulose functionalized probes,21 has been utilized. The spheres (approximate diameters of 10 µm produced from regenerated cellulose by the viscose process (Kanebo, Japan)) were attached to the end of the cantilever using a diminutive amount (∼1 fL) of epoxy resin (Henkel Technologies). The cantilevers used were rectangular uncoated tipless silica cantilevers (CSC12/NoAl with approximate dimensions: length 250 µm, breadth 35 µm) from MikroMasch. To obtain accurate normal and torsional spring constants the cantilevers were calibrated using the AFM Tune IT v2.5 software (ForceIT, Sweden), based on thermal noise with hydrodynamic damping.32,33 The lateral photodetector sensitivity was calibrated using a procedure of tilting the photodetector.34 Measurements were performed in aqueous solutions at pH 3, 5.7, or 10. Typical force measurements were performed with ramp size 1 µm at a rate of 2 µm/ s, which is sufficiently low to avoid hydrodynamic effects.15 A second cellulose sphere was similarly glued to the lower substrate (a mica sheet)

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Figure 2. Normalized force profiles on approach between two untreated cellulose spheres (closed symbols) and after being subjected to chitosan for 8 h (open symbols). The displayed results are averages taken from 10 individual force curves. Measurements were conducted in aqueous solutions at pH 10 (triangles) and pH 3 (squares). The solid lines are fits to DLVO theory with a Debye length of 30 nm (calculated value, not a fitting parameter) and Hamaker constant of 8 × 10-21 J. The solid line represents the constant surface charge boundary condition, and the broken line the boundary of constant surface potential. The surface potential obtained for neat cellulose at pH 10 is -12 mV.

and the axis of interaction was obtained by scanning the colloid probe over the lower sphere. The effective interaction radius of curvature, R, between the two cellulose spheres was calculated by

R)

R1R2 R1 + R2

(1)

where R1 and R2 are the radii of the two cellulose spheres. To estimate a value of the surface potential, the normal force was fitted using DLVO theory.35,36 The van der Waals attraction between two cellulose surfaces was estimated with a Hamaker constant of 8 × 10-21 J,37 and the nonlinear Poisson-Boltzmann equation was solved numerically using the algorithms of Bell et al.38 and Devereux et al.39 Friction was measured as in earlier work13,14 as a function of increasing and decreasing loads with scan size 2 µm and scan rate 1 Hz.

Results and Discussion Forces on Approach. Figure 2 summarizes the normalized forces on approach as a function of apparent separation between two cellulose spheres at pH 10 and pH 3 both before and after being subjected to overnight adsorption (8 h) of chitosan (1 mg/ mL) at pH 4. The forces vary by more than 2 orders of magnitude at some distances, indicating dramatically different surface character. At pH 10, for the bare cellulose two clear force regimes are identified, as observed earlier22,25 the form of the force curve is almost identical as in that work. At longer range, a double-layer force exists, originating from a small number of charged moieties, usually assumed to be carboxyl groups, on the cellulose surface. The measured force could be fitted adequately with DLVO theory as described above using a calculated Debye length of 30 nm and yielded a surface potential of -12 mV. In this case the plane of charge was taken to be at the point where the force deviated from DLVO theory, at 42 nm. As pointed out earlier,14,21 the plane of charge is rather poorly located as the charge is distributed over a swollen cellulose layer, and thus the calculated surface potentials should

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be treated with a certain caution. For example, when setting the plane of charge to zero apparent separation, an higher potential is returned.22,25 Nevertheless, as expected, the value of the fitted surface potential (-12 mV) at pH 10 is higher than -8 mV in 0.1 mM NaCl at pH 5.6 obtained in previous work utilizing the same batch of cellulose spheres.15 A potential of -9 mV has been observed between other types of cellulose model surfaces at pH 8.540 and, in some cases, slightly higher potentials have also been observed.23 At shorter separation the forces can no longer be described by DLVO theory and there has been some debate as to their nature. This debate is summarized in an earlier article14 and is not further elaborated here. Suffice it to say that in early publications we attributed this force to an (electro-)steric interaction, whereas the balance of evidence seems to indicate that the force in fact originates from deformation of the swollen cellulose spheres. We also note that the distinction between these two cases is not sharp. In contrast, the interaction at pH 3 is characterized by shorter repulsion (compression) and the complete absence of a doublelayer interaction. Both of these observations are consistent with a negligible surface charge. The point of zero charge (pzc) of cellulose is between pH 2-2.541 so the surface potential at pH 3 is expected to be very small. Similar behavior has been observed for other cellulose systems at pH 3.540 and even as high as pH 5.6.20 Also, the degree of swelling of the cellulose surface has previously been related to the pH and surface charge.25,42 A lower charge generally leads to reduced swelling which is reflected by a decrease in the onset of deformation from 42 nm (pH 10) to about 32 nm (pH 3). Measurements at pH 5.7 (0.1 mM NaCl) were also performed, as have been reported earlier.14 They have not been displayed herein to preserve clarity of the figure but are discussed in previous work.14,15 After adsorption of chitosan (8 h at pH 4) a longer range force component was observed at pH 3. The long-range repulsion cannot be described by DLVO theory since the decay length is much longer than the calculated Debye length of 30 nm for the system. This is characteristic of an electrosteric force.43,44 No strict double-layer repulsion is observed at longer range. We ascribe this to variations in the length of the adsorbed chains and high surface roughness at the interface. The repulsive force is very long-ranged and strong, reflecting the relatively large molecular weight and the high charge on chitosan at pH 3, respectively. The range of the force is large in terms of surface forces, but the contour length of the molecule is of the order of a micrometer, so this result is not surprising, and is reminiscent of earlier measurements between silica surfaces using a different cationic polyion of high molecular weight.45 This force reflects an adsorbed film of polyions, with significant extension of tails and loops into solution due to the affinity of the charged polymer for the aqueous phase. Because the system has been extensively rinsed, the molecules are irreversibly bound, implying that there is a significant binding energy. The charge of the cellulose surface is also very low at the adsorbing pH, which suggests a nonelectrostatic adsorption mechanism. Such a mechanism is likely to involve hydrogen bonding, either due to specific polysaccharide recognition, based on the repeating backbone having the same structure, as observed for certain other biopolymers46 or alternatively due to hydrogen bonds between amine groups on the chitosan and carboxyl groups on the cellulose surface (with associated entropically favored water release).

Figure 3. Normalized force profiles on retraction between two cellulose spheres after being subjected to chitosan for 8 h. Measurements were conducted in aqueous solutions at pH 10 (open triangles) and pH 3 (open squares). Two single force curves after being 10 s in surface contact are displayed for each condition. The insets display the frequency of pull-off events as a function of distance before and after 8 h of chitosan adsorption for measurements at pH 3 (left) and pH 10 (right).

At pH 10 the adsorbed chitosan layer exhibit a very low charge since the pKa value of the glycosamine segments is 6.3-7.47 The cellulose is intrinsically more negatively charged at pH 10, but this charge appears to be compensated by residual charges on the chitosan, because it is clearly seen that the compressive force component matches that of the electroneutral (neat cellulose at pH 3) case almost exactly. Although not a requirement for the interpretation of the results, in the present study it may be noted that the exact adsorbed amount of chitosan is not known. Adsorption studies to model cellulose surfaces consisting of nanofibrillar cellulose48 using QCM-D suggest an irreversible adsorbed amount of up to 0.6 mg/m2.49 The adsorbed amount of chitosan (0.6 mg/m2) on cellulose is rather high if assuming a purely electrostatic adsorption mechanism compared to the reported values of 1.2 mg/m2 for mica,50 which exhibits a very high surface charge. For chitosan adsorption onto low charged silica at pH 4, the adsorbed amount determined by QCM-D was 0.4 mg/m2 (with 76% water incorporated).51 Although the authors conclude that the adsorption is mainly driven by electrostatics they note that it also contains a nonelectrostatic contribution. Further, the fact that a significant amount of chitosan irreversibly adsorbs to the lower charged cellulose, strengthens the evidence of a nonelectrostatic adsorption mechanism being present for the polysaccharides. Forces on Separation. As the chitosan bearing surfaces are separated (Figure 3), the interaction is also strongly pH dependent; it continues to be repulsive at pH 3, but at pH 10 it is adhesive. In both cases the separation curves exhibit multiple pull-off events arising from the stretching and disruption of bridging domains of individual chitosan molecules or bundles thereof. Similar characteristic behavior has previously been observed in single molecular force spectroscopy52-56 as well as in colloidal probe studies13-15 of biological macromolecules. At pH 10, the pull-off events occur strictly at negative forces with no repulsion present and a well defined hard-wall contact indicating a less swollen layer due to the insolubility of chitosan at high pH. This observation can be correlated with macroscopic findings where adsorbed chitosan was shown to yield the highest

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Figure 4. Friction measurements between (a) two neat cellulose spheres (closed triangles) and (b) after adsorption of chitosan for 8 h (open triangles). Measurements were conducted in aqueous solution at pH 10. The slope of the solid lines represents the friction coefficients, calculated as (a) µ ) 1.12 and (b) µ ) 0.88. The inset shows the corresponding normal approach force profiles on a linear scale.

paper wet-web-strength at pH 10.7 The larger electrosteric repulsion at pH 3 is consistent with the results seen in the forces on approach and is most likely arising from an extended chitosan layer with high charge. As seen from Figure 3, despite the large repulsion, there were nonetheless pull-off events observed, some of which occurred at a positive absolute force. The forces on separation for neat cellulose prior to chitosan adsorption (not shown) were consistent with previously reported results.15 An obvious qualitative difference between the adhesion behavior can be seen directly from the force curves in Figure 3. However, by their nature, the positions of the pull-off events are somewhat random and vary between force curves (unlike the repulsive contributions, which are reproducible). This renders the assignment of a “pull-off force” difficult. However, a simple statistical analysis has been performed on 10 force curves for each pH by counting the number of local minima associated with pull-off events and displaying them as a function of their corresponding separation (see insets of Figure 3). Clearly the presence of the adsorbed chitosan yielded an overall increase in the frequency of pull-off events at larger distances up to about 450 nm. Once again, this is long ranged, but well within the nominal contour length in the micron range. Furthermore, a polydispersity in the molecular weight is normal for such biological samples so such long ranged events are to be expected. Limited bridging involving cellulose chains is observed for neat cellulose, which is longer ranged at pH 10 reflecting the more expanded conformation. However, more extensive bridging is observed in the presence of chitosan. (While not reflected in the insets, depths of the minima are also deeper for the case of chitosan.) An equivalent range of the adhesive force has also been found for chitosan adsorbed onto mica.28 Also, both the magnitude and the range of the adhesion was shown to increase with longer time in contact, which has previously been seen for xyloglucan,13,15 reflecting the fact that the adhesion is a dynamic quantity and is limited by finite molecular relaxation times at the surface. Friction. The lateral forces as a function of increasing and decreasing normal load were measured between the neat and the chitosan treated cellulose spheres at pH 10 (Figure 4), pH

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Figure 5. Friction measurements between (a) two neat cellulose spheres (closed squares) and (b) after adsorption of chitosan for 8 h (open squares). Measurements were conducted in aqueous solutions at pH 3. The slope of the solid lines represent the friction coefficients, calculated as (a) µ ) 1.29, and (b) µ1 ) 0.13 (dashed line) and µ2 ) 0.56 (solid line). The inset shows the corresponding normal approach force profiles on a linear scale.

5.7 (not shown for clarity), and pH 3 (Figure 5). In all cases, on both loading and unloading, the frictional behavior follows a slightly modified version of Amonton’s law

Ffriction(Fload) ) Ffriction(0) + µFload

(2)

where Ffriction(Fload) is the total friction force, Fload is the applied load, µ is the friction coefficient, and Ffriction(0) is the friction force at zero applied load. The friction coefficient at pH 10 is only slightly reduced after 8 h of chitosan adsorption (Figure 4). Both before and after adsorption a small adhesion was present, manifested as a nonzero friction at zero applied load. This phenomenon has also previously been observed for end-grafted nonionic xyloglucan brushes.13 In addition, the intercept of the extrapolated fit yields very similar values to the adhesion results at pH 10, displayed in Figure 3. The difference in the intercept seen before and after adsorption is consistent with the higher adhesion observed in the normal force measurements on retraction with adsorbed chitosan. The negative intercept is thus consistent with the adhesion acting as an effective extra load.57 At pH 3, the lubricating effect of the chitosan layer is significantly enhanced and a dramatic reduction in the friction data is observed (Figure 5). Two low friction regimes can be discerned after adsorption; a very low friction regime at low applied loads, µ1, and another at higher loads, µ2, with a somewhat larger gradient. The transition between the regimes occurs at an applied load which correlates with the trend in the force curve profiles. When the normalized load (Fload/2πR) in the first regime (µ1, Figure 5, dashed line) is compared to the normalized force curves on approach (inset of Figure 5) it becomes clear that the applied loads are in the electrosteric part of the force curve where the repulsive forces induced by the extended layer act as an efficient lubricant. The higher friction coefficient (µ2, Figure 5, solid line) is obtained in, or close to, the constant compliance region (defining hard-wall contact) or at least in the range of the sphere compression.

Nanotribological Properties of Cellulose

Figure 6. Friction coefficients obtained from measurements between cellulose spheres in aqueous solutions at pH 3, 5.7 (0.1 mM NaCl), and 10. Results shown were obtained before, after 1 h, and after 8 h of chitosan adsorption. The two different friction regimes obtained at pH 3 after chitosan adsorption are distinguished by µ1 (dashed border) and µ2 (solid border).

To investigate the stability and reversibility of the chitosan functionalized layer, the normal forces on approach were compared before and after friction measurements. Small differences were observed before and after friction measurements indicating that some disruption of the chitosan layer occurred. Nevertheless, the electrosteric force at pH 3 is still long-ranged and its magnitude large in comparison to the neat substrate, which suggests that most of the chitosan is retained. Interestingly, at pH 10 the electroneutrality was, in some cases, compromised after the shearing, yielding a slightly charged surface with a lower potential than for the neat cellulose. Figure 6 summarizes the obtained friction coefficients from measurements made under various pH conditions, between cellulose spheres, before and after 8 h of chitosan adsorption. The results are averages of multiple friction curves (such as those in Figures 4 and 5) for two independent experiments. In addition, the friction coefficients were investigated after 1 h adsorption times and these results are also presented in Figure 6. Clearly the shorter adsorption time produces a less lubricating layer than the 8 h adsorption. The lubricating ability is strongly pH dependent with drastically reduced friction at pH 3 but only moderate friction mediation at the higher pH values. A measurable difference in friction coefficients of the neat cellulose surfaces is also observed as a function of pH. The large scatter in the pH 3 data precludes the observation of a trend, but the difference between pH 5.7 and 10 is considered significant. At pH 3, the surface is uncharged, as indicated by the lack of double-layer forces (see above). Thus, the effect of surface roughness and possible fibril orientation play a significant role leading to greater variation between friction measurements. As the pH increases, a surface charge is built up that leads to a more swollen layer containing water that is capable of providing a small friction reduction. At even higher pH, the effective modulus of the spheres is reduced, leading to slightly more conformable surfaces with, consequently, increased viscous dissipation. It should be stressed that these trends are relatively small. Nevertheless, the results correlate well with macroscopic measurements performed between single kraft pulp fibers at pH 3 and 10, where it was also found that the friction coefficient is only slightly higher at elevated pH.58 A more dramatic effect, at least at low pH, on the friction comes from the adsorption of chitosan. Here the elastic,

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extended, swollen and load bearing layer at pH 3 contains water59 and acts as a “biomimetic lubricant”.60 If the two regimes are considered separately then the friction reduction is even more dramatic with coefficients µ1 ) 0.15 ( 0.018 for the first and µ2 ) 0.54 ( 0.073 for the second regime. Thus, measurements at pH 3 correspond to a friction reduction of up to a factor of 7 depending on the proximity of the chitosan bearing cellulose surfaces. The low charge density of the cellulose surface contributes to a high extension of the positively charged chitosan species since the majority of cationic groups are not neutralized by negative surface sites. Thus, we observe a more long-range force than observed earlier for chitosan adsorbed to highly charged anionic mica.19,27 This electrosteric interaction provides highly hydrated “cushion-like” surfaces yielding good interfacial lubrication. The more than 3-fold increase of the friction coefficient in the higher friction regime likely arises from the forementioned cushion of water being squeezed out, promoting physical contact of chain-chain interactions resulting in reduced lubricity at the interface. A similar observation of load dependent friction for chitosan was also made by Kampf et al.27 with an average effective friction coefficient of 0.07 for chitosan coated mica surfaces. This value is smaller than measured in the present study which mainly reflects the much smoother mica substrates employed - roughness of cellulose surfaces has been demonstrated to have a clear effect on friction coefficients.14 At pH 5.7 (0.1 mM), only a small effect on the friction coefficient can be seen for chitosan in contrast to earlier work with a nonionic (1f4)-β glucan (xyloglucan),13-15 which showed considerable friction reduction under the same conditions. This observation, in combination with the large friction reduction seen at pH 3, suggests that the lubrication mechanism of chitosan arises from increased layer solubility59 and conformational changes induced by the charge.

Conclusions Several phenomena elucidating the biolubrication properties of chitosan on cellulose have been observed. The existence of a long-range electrosteric force measured on approach at pH 3 (after rinsing) confirmed the retention of chitosan to the cellulose surface. At this low pH, chitosan is highly positively charged whereas the neat cellulose was considered to be uncharged or containing few residual charges. This strongly suggests that the adsorption process involves a nonelectrostatic interaction. The nature of the adsorbed layer showed pH dependence with a more “collapsed” conformation at higher pH. This was also manifested in the adhesion being more prominent at high pH due to bridging or tethering at the interface. In all cases, the forces on retraction showed multiple pull-off events, of which the range and magnitude were increased by the adsorbed chitosan. The most significant friction mediation was found at pH 3, displaying two distinctly different regimes depending on the applied load. The low friction seen in the first regime (seven times lower than between the neat cellulose spheres) is thought to originate from the strongly hydrated chitosan forming an interface between two “cushion-like” layers with the trapped water acting as lubricant. In the second regime, at increased pressure, the water in the “cushion” is gradually expelled resulting in a higher friction coefficient due to chain-entanglement and other physical interactions. Interestingly, chitosan and xyloglucan seem to have similar effects on cellulose, such as increased adhesiveness, despite the different chemistries and the fact that the latter is nonionic. Conversely, only a small friction reduction was found

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for chitosan under the same solvent conditions as for xyloglucan (0.1 mM at pH 5.7). This indicates that the lubrication of chitosan arises from increased solubility and conformational changes induced by the higher charge at pH 3. Further investigations, including other polysaccharides and polyions using similar biomimetic systems or employing single molecular force spectroscopy, are needed to fully unravel the intricate nature of the specific interactions involved. Acknowledgment. This work was supported by the Swedish Foundation for Strategic Research (SSF) through a “Chemistry for Life Sciences” project grant and through Biomime, the Swedish Center for Biomimetic Fiber Engineering (http:// ¨ ., and J.L. acknowledge support www.biomime.org/). P.E., M.O from the Innoforest-project financed by the Finnish Technology Development Center (TEKES) and the Finnish forest industry. The AFM was financed by a generous grant from the Knut and Alice Wallenberg foundation. M.R. is a fellow of the Swedish Research Council. Dr. Johan Fro¨berg and Prof. Per Claesson are thanked for kindly providing the program for DLVO fitting and enlightening discussions, respectively. We acknowledge Jerker Nordgren for provision of graphics.

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