Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 166

accumulating on the fiber surfaces or on the process equipment surfaces even at the
addition of electrolyte (Hannuksela et al . 2004; Sundberg et al . 1996b). This way the
negative effects of wood extractives on paper properties can be diminished. Investiga-
tions dealing with wood extractive colloid interactions with TMP fine material showed
that hemicelluloses which were adsorbed on cellulose rich fibrillar fines prevented the
further adsorption of colloidal extractives (Johnsen et al . 2007; Mosbye et al . 2003).
Colloidal extractives widely adsorbed on fine material when the dissolved hemicellu-
loses were not present in the system and the adsorption increased with increasing ionic
strength. The similar behavior was observed when the adsorption experiments were
conducted with QCM-D using cellulose model surfaces as substrates for extractive col-
loid adsorptions (Tammelin et al . 2007). Extractive colloids significantly adsorbed on
pure cellulose surface but the adsorption was prevented when dissolved hemicelluloses
were allowed to sterically stabilize the cellulose surface, the colloids or both the cel-
lulose surface and the extractive colloids. The adsorption was prevented at high ionic
strengths as well.

6.6.2 Adsorption Behavior and Interaction Forces between Xylan and Cellulose

The results of the QCM-D adsorption experiments and the AFM force measurement
showed that despite the negative charges on both interacting partners, xylan adsorbed
onto cellulose in weakly alkaline solutions at low ionic strength. Addition of charges on
weakly charged cellulose surfaces by adsorption of xylan increases naturally electrostatic
repulsion, but steric repulsion is increased as well: charges also cause swelling of the
adsorbed layer. The domination of steric forces ( Osterberg et al . 2001) and electrostatic
repulsion (Claesson et al .

1995) at large distances between xylan-coated surfaces has

been reported previously.
The force measurement results also showed that the adhesion between cellulose sur-
faces is very low before and after adsorption of xylan. The QCM-D results showed
that the interaction between cellulose and xylan is weak, but according to the force
measurements the cellulose-xylan interaction is strong enough to prevent desorption
upon dilution (Paananen et al . 2003). These findings suggest that a combination of the
increase in inherent entropy increase associated with the release of solvent molecules
upon adsorption of xylan and weak van der Waals' attraction is the driving force of
the cellulose-xylan association, rather than formation of hydrogen bonds as has repeat-
edly been cited (Mora et al . 1986). This does not exclude, though, the importance of
hydrogen bonding in dry systems. Hence, the results indicated that the role of xylan in
increasing the paper strength (interfiber bonding) is probably to increase the contact area
between fibers, and this would be associated with processes taking place during drying.
The adsorption behavior of xylan at higher ionic strengths was not as expected; xylan
adsorption was not enhanced by increased electrolyte concentration. On the contrary,
less was adsorbed and the dissipation response was relatively high indicating soft and
water containing film of xylan on cellulose surface (Figure 6.2). Xylan reacted to the
changes in electrolyte concentration in a completely different way compared to dissolved
TMP hemicelluloses as shown in Figure 6.2.
The expected effect of electrolyte on the interaction forces between surfaces containing
charges is reducing the electrostatic repulsion with increasing electrolyte concentration,