Forces between cellulose beads

In this study the AFM colloidal probe technique was used to investigate the forces between cellulose beads in aqueous solutions of simple electrolyte and xylan. Particular attention was paid to the behaviour of the cellulose beads. The adsorption kinetics and characteristics of adsorbed xylan on cellulose was studied with a quartz crystal microbalance with dissipation (QCM-D). 

The force measurements were done by the colloidal probe technique in contact mode using a NanoScope III MultiMode AFM (Digital Instruments, California) equipped with a fluid cell and a scanner E, vertical engagement, using an 0-ring. When measuring forces between cellulose beads, the bead attached to the cantilever was placed directly on top of a bead on the sample support. The position of the bead was checked using the optical microscope of the AFM instrument. 

The forces between cellulose beads and mica in water and in 1 mM NaCl at pH 10 were measured. The behaviour of the beads was monitored for at least 6 h, using different loading forces and varying time gaps between consecutive force curves. In order to get representative force curves, at least three different spots on the mica were selected and several force curves were taken on each spot. The measuring velocity was 50 nm/s and the ramp size was 500 nm. 

Investigation of cellulose systems in closer detail requires the choice of representative cellulose model surfaces for the experiment. A spin-coated cellulose surface on mica was the first model surface used in studies of forces in papermaking systems (70). This work was followed by other SFA studies using Langmuir-Blodgett (LB) films of cellulose (11-14). These films are noticeably smoother and more stable than spin-coated surfaces. In studies using the atomic force microscope (AFM) colloidal probe technique (Ducker et al. (75)), interaction forces were measured either between two cellulose beads (16,17) or between cellulose beads and spin-coated cellulose surfaces (18,19). 

The behaviour of the cellulose beads during force measurements was studied by measuring the forces between a cellulose bead and a flat macroscopic mica surface. This system was chosen since the properties of mica in aqueous solutions are well known. The fact that mica is hard and can be cleaved to reveal a perfectly smooth and clean surface just prior to the measurements also makes the interpretation of the results easier. 

Figure 2, The force, normalised by the radius of curvature, as function of surface separation, between a cellulose bead and a mica surface in 1 mM NaCl at pH 10. The forces measured on approach Ih (n), 2h (o) and 6h (A) after introducing the sphere into water are shown. Each curve was recorded several times at different spots on the mica surface since the curves were very reproducible only one curve for each time is shown.

The forces between two cellulose spheres across a solution containing xylan were measured for different xylan concentrations (10, SO, and 100 mg/1) in 1 mM NaCl, pH 10 (reference). When the compliance behaviour of the cellulose beads was taken into account and the forces were scaled by the effective radius of curvature of the immersed beads, the force measurements became very reproducible. This is illustrated in Figure 5, where approach curves in reference solution from three separate force experiments are shown. 

Figure 6. TAe forces measured on approach (a) and separation (b) between two cellulose beads across xylan solutions at different xylan concentrations (0-100 mgA). The forces after changing the solution in the chamber back to the reference solution are also shown (A). The lines are the best fit to the DLVO theory assuming constant charge using the parameters in Table L...

The swelling also affects the normalisation of the forces, which depends on whether one uses radii that are determined for wet or dry beads. In our experiments the adhesive forces observed between two cellulose beads in electrolyte solution at pH 10 are lower than the forces reported previously (17). A possible explanation is that in the normalisation of the force we used the radius of the beads determined after at least 2 h incubation in electrolyte solution (and overnight in pure water). This radius is 5-20 % larger than the radius of dry... 

The recovery of the cellulose surfaces between consecutive force measurements was found to require at least five minutes. This is related to the previous publications (77,78) that reported on the effect of the approach speed and relaxation of the cellulose chains, as well as hydrodynamic effects involved. The slow recovery of the cellulose beads between force runs may also explain why we in this work observed adhesion between cellulose surfaces at pH 10 in contrast to previous observations (77). 

CeIluiose>cellulose interaction forces were measured in a reference solution (1 mM NaCl, pH 10) and in xylan solutions (10,50 and 100 mg xylan/1 in 1 mM NaCl, pH 10) starting from the reference and continuing with growing xylan concentration. All measurements were done at pH 10 to ensure that xylan was soluble. After changing the xylan concentration the system was allowed to stabilize for 1 h before measurements. Different loading forces were used and the time gap between consecutive force curves was varied from 0.5 to 10 min. The reproducibility of the force curves was checked by shifting the cantilever horisontally in less than 1 pm steps between measurements. Thus, the force curves were centered at slightly different spots on the lower bead, but still on the central area of the beads. 

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