Xylan adsorption measurement

Within one series of experiment the effect of increasing xylan concentration was always the same. On the other hand, the absolute strength of the interaction was clearly different when repeating measurements with new surfaces. This may be due to the kinetics of xylan adsorption. Variations in adsorption times could lead to different adsorbed amount of xylan and thus different magnitudes of repulsion. 

To verify the conclusions about the adsorption of xylan on cellulose drawn from the force measurements the adsorption kinetics of xylan on cellulose was studied using QCM-D. Figure 7 shows the change in frequency, 4 and dissipation, AD as a function of time when xylan is adsorbed from a 100 mg/1 solution in 1 mM NaCl at pH 10 on LB-cellulose. Figure 8 shows a plot of AD vs Af for the xylan adsorption. 

The force curves were recorded 1-2 h after changing the xylan concentration. The QCM-D measurements suggested that the adsorption reached a plateu value only after about 3 h. No firm conclusions can be drawn about the nature of the interactions between the xylan-coated surfaces based on these experiments. The effect of electrolyte concentration should be investigated to clarify whether the forces are purely electrostatic or not. This is done in an ongoing study. 

In previous force measurements we found that the interactions between xylan-coated mica surfaces were dominated by steric forces (8). In that work we argued that due to the stiffness of the xylan molecule, fitting to other theories of polymer interaction is not straightforward. The adsorbed xylan layo was ratho thick in that study. However, in another investigation (32) it was found that electrostatic repulsion dominated the interactions between xylan-coated mica surfaces at large distances. In that case the adsorption of xylan was very limited and the layer thin. These results agree with our conclusions that steric factors become dominating when the adsorption of xylan increases. 

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