Biomacromolecules 2005, 6, 3057-3066
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Direct Force Measurements between Cellulose Surfaces and Colloidal Silica Particles Igor L. Radtchenko, Georg Papastavrou, and Michal Borkovec* Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, Sciences II, 30, Quai Ernest-Ansermet, Geneva 1211, Switzerland Received June 2, 2005; Revised Manuscript Received July 19, 2005
The interaction of cellulose layers with colloidal silica particles was investigated by direct force measurements with the atomic force microscope (AFM). Upon approach, repulsive forces were found between the negatively charged silica particles and the cellulose surface. The forces were interpreted quantitatively in terms of electrostatic interactions due to overlap of diffuse layers originating from negatively charged carboxylic groups on the cellulose surface. The diffuse layer charge density of cellulose was estimated to be 0.80 mC/m2 at pH 9.5 and 0.21 mC/m2 at pH 4. The forces upon retraction are characterized by molecular adhesion events, whereby individual cellulose chains desorb from the probe surface. The retraction profiles are dominated by well-defined force plateaus, which correspond to single-chain desorption forces of 35-42 pN. We surmise that adsorption of cellulose to probe surfaces is dominated by nonelectrostatic forces, probably originating from hydrogen bonding. Electrostatic contributions to desorption force could be detected only at high pH, where the silica surface is highly charged. Introduction Interaction of colloidal particles with cellulose surfaces is relevant in various industrial applications, most notably in the papermaking1-6 or the textile sector.7-9 For papermaking, this aspect is of importance during the preparation of the slurry containing pulp (i.e., cellulose fibers) and the filler particles (e.g., silica, clay, calcium carbonate).1-4 The fillers are added to improve the optical properties and the printability of the paper. At the same time, however, they are not always easily retained and they tend to decrease the paper strength. For these reasons, various retention aids (e.g., cationic polyelectrolytes, starch) are being added to the papermaking slurry to enhance the paper strength though attractive particle-cellulose interactions.1,5,6 In the textile sector, one is interested to modify the properties of cotton fabrics (i.e., cellulose) by deposition of colloidal particles on their surface.7-11 The fact that the wear resistance of textiles can be improved through the application of natural rubber particles has been known since the early 1930s.8 More recently, latex particles have been used to increase the lubricity and smoothness of fabrics,10 while hydrophobic particles have been employed to enhance water repellency and self-cleaning properties of textiles.11 Finally, controlled deposition of activated particles (e.g., containing perfumes, insect repellents) on fabrics imparts textiles higher functionality.7,9 Again, in all these applications it is essential to control the interactions between the particles and the cotton surface, through either additives or surface modifications. To study the interactions between colloidal particles and cellulose surfaces in a reproducible manner, several tech* Corresponding author: phone 0041 22 702 6405; fax 0041 22 702 6069; e-mail
[email protected].
niques to prepare well-defined cellulose coatings on solid supports have been proposed. One popular method is the deposition of (trimethylsilyl)cellulose to mica by the Langmuir-Blodgett (LB) technique, followed by cellulose regeneration by acid hydrolysis.12 Spin-coating has also been used in the preparation of model cellulose surfaces on solid substrates, either by direct coating by cellulose from solution13-15 or suspensions16 or by coating by (trimethylsilyl)cellulose with subsequent acid hydrolysis.17 The LBcellulose films are amorphous and rather smooth, but they do swell substantially in solution. On the other hand, the spin-coated films have a higher degree of crystallinity and a smaller tendency toward swelling, but they can be unstable and relatively rough, especially after heating. The latter aspect was clearly demonstrated by atomic force microscope (AFM) imaging.18 This study has shown that cellulose films undergo an irreversible structural charge upon heat treatment at 105 °C, which was interpreted as originating due to hydrogen bonding in the crystalline regions. Nevertheless, such spincoated films are most interesting as model systems, since they were shown to behave similarly to cellulose fibers, particularly when carboxymethylated.14 With such surfaces at hand, interactions between cellulose surfaces and colloidal particles have been studied by direct force measurements, either by the surface forces apparatus (SFA)1,12 or by the AFM technique colloidal probe both in the sliding friction19 and the normal force mode.1,2,4,20,21 To investigate the interactions between cellulose surfaces, colloidal spheres of several micrometers in diameter have been attached to the AFM cantilever.2,4,19-21 This point was judged important in order to investigate interactions between cellulose surfaces but also to interpret the interaction forces in the framework of classical theory of Derjaguin, Landau,
10.1021/bm050371d CCC: $30.25 © 2005 American Chemical Society Published on Web 08/23/2005
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Verwey, and Overbeek (DLVO) for symmetric systems.22-24 In this fashion, one is able to estimate the contributions of repulsive diffuse layer overlap forces of electrostatic origin, van der Waals dispersion forces, and additional non-DLVO steric interactions. In most situations, repulsive diffuse layer interactions and short-range steric forces have been observed,4,18,21 while attractive van der Waals interactions have been reported for cellulose films anchored to cationic polyelectrolyte layers.20 The repulsive scenario seems also typical for interactions in the asymmetric system of a colloidal silica particle and an LB-cellulose film.12 In the repulsive situation, the approach and retract force curves are normally reversible. This point appears surprising, since cellulose molecules have been shown to form bridges between the surface and the AFM cantilever leading to random attractive forces upon the retract.2,25,26 Stretching of individual molecules, which leads to characteristic peaks in the retract curve, were observed for cellulose25,27 and between spin-coated cellulose surface and cellulose microparticles.2 For single xyloglucan molecules, on the other hand, plateaus in the retraction forces have been reported and interpreted as originating from the desorption of single molecules.26 Such single-molecule adhesion should also be the rule in systems involving cellulose surfaces, and their understanding is most relevant to address the interactions between cellulose and colloidal particles, as well as their release from such substrates. For these reasons, we have used colloidal probe AFM to investigate the interactions of two different preparations of cellulose surfaces with colloidal silica particles. The advantage of the technique is that surface forces can be measured to good accuracy, but at the same time one is sensitive to single-molecule adhesion. We can further estimate the electric surface potentials in the asymmetric systems, since the surface properties of the silica probe particle could be determined independently. This approach further overcomes the necessity to assume the identical surface properties of the cellulose surfaces on the substrate and the probe in order to estimate the surface potentials.4 We indeed find that repulsive electrostatic diffuse-layer forces dominates longrange interactions during approach, but upon retraction adhesion becomes important due to single-molecule desorption events. Experimental Section Cellulose Surfaces. We used two methods for preparing cellulose coatings on surfaces of oxidized silicon wafers cut to 10 mm squares. The wafers were heated for 120 min at 1000 °C and we obtained silicon oxide layers approximately 100 nm thick, as checked by ellipsometry in air. High-purity cellulose powder from Sigma was used throughout. (A) Cellulose A. Cellulose was dissolved at a concentration of 1 g/L in a solution of 90 g/L LiCl in N,Ndimethylacetamide (DMAc) at 90 °C for 2 h under occasional stirring.28 This solution was spin-coated on the silicon wafer mounted to a rotating disk and spun at 4000 revolutions per minute (rpm) for 5 min. The wafers were subsequently dried in the oven at 90 °C for 2 h. Afterward substrates were rinsed
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in deionized water (Milli-Q, Micropore) for 10 min to remove the residual LiCl from the film. Finally, the film was reheated in the oven at 90 °C for 1 h to enhance its adhesion to the substrate. (B) Cellulose B. A solution of 20 g/L cellulose and 90 g/L lithium chloride in DMAc was heated to 80 °C, and 20 mL of hexamethyldisilazane (HMDS) was slowly added under reflux in a nitrogen atmosphere. The mixture was cooled, and the crystallization of (trimethylsilyl)cellulose (TMSC) was induced by methanol addition. The crystallized TMSC was filtered, dissolved in tetrahydrofuran, and recrystallized from methanol.29 TMSC was dissolved at a concentration of 5 g/L in toluene, and the solution was spincoated at 4000 rpm on the wafers for 5 min. At that point, the film was already dry. To regenerate the spin-coated TMSC, the wafers were placed for 2 min in a closed container, whose atmosphere was saturated with the vapor of hydrochloric acid (2 M). Subsequently, they were dried in air.30 The surface topography of the dried cellulose surfaces was determined in air by tapping-mode AFM (Nanoscope IIIa, Veeco) with silicon tips (Ultrasharp, µ-Masch, Estonia). The dry cellulose films were imaged in air. Colloidal Probes. The colloidal probes used for the direct force measurements were mounted on tipless cantilevers (CSC-12, µ-Masch, Estonia). As colloidal probes we used silica particles with diameters in the range of 4.8-6.8 µm (Bangs Laboratories). To minimize contamination, the entire preparation was carried out in a clean room. Attachment of the corresponding colloidal particles to the end of the cantilevers was done with a micromanipulator (STM-3, Maerzhaeuser, Wetzlar) and an optical microscope equipped with a long focal distance objective. The particles were attached with a UV-curable glue (Norland Optical Cement No. 68), which was hardened by illuminating the sample in the microscope with a mercury lamp. The silica probes were cleaned prior to measurement for 5 min by exposing them to air plasma (PDC-32G-2, Harrick Scientific). Spring constants of the cantilevers were normally determined from the spectral power density of cantilever fluctuation due to the thermal noise31 and checked against the value estimated from the top-view dimensions of the cantilever and its material properties as described by Sader et al.32 The latter method was judged to be less reliable, but nevertheless the results did not deviate more than 20% from the ones obtained by the thermal noise method. For the different cantilevers, the spring constants were in a range of 30-80 mN/m and within the specifications of the manufacturer. Force-Distance Curves. A molecular force probe (MFP3D, Asylum Research, CA) was used for the force measurements, with Petri dishes as open fluid cells. The wafers were mounted to the bottom of the Petri dishes, which were filled with 5-7 mL of solution. Polished fused silica substrates (Schott-Guinchard, Switzerland) cleaned with a hot 4:1 mixture of concentrated sulfuric acid and 35% hydrogen peroxide were used to measure the surface properties of the silica probes. The pH was adjusted by KOH and HCl, whereby either pH 4.0 or 9.5 was chosen. The salt level was adjusted by addition of KCl. Within each experiment, the
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force-distance curves were measured on the surface at four different spots and on each spot 150 times at a constant approach-retract velocity of 0.6 µm/s. The experiments were carried out at room temperature of 22 ( 2 °C. The forcedistance curves were calculated from of the cantilever deflection and the displacement of the piezo. The force F is determined from the deflection and the spring constant with an overall sensitivity of about 20 pN. This value is mainly dictated by the thermal noise of cantilever. The separation distance D is obtained from the displacement of the piezocrystal and cantilever deflection. A linear fit of the constant compliance region yields zero separation with an accuracy of
