Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 165

other short-ranged attractive forces become important. The conformation of the hemi-
cellulose chain also changes. At high ionic strength the polyelectrolyte chain becomes
more coiled and as a consequence, more polymer can fit to adsorb onto the cellulose sur-
face. Due to these facts, the hemicellulose layer seems to be very strongly bound on the
cellulose surface. Both the shear viscosity and shear elastic modulus are high indicating
the formation of a strongly bound hemicellulose film (Figure 6.5). Furthermore, it can
be assumed that the formation of relatively thick hemicellulose layer (the hydrodynamic
thickness is estimated to be 9 nm at high ionic strength) is due to better packing of

the molecules for two reasons: (1) the more coiled conformation of the hemicellulose
chains and (2) the lower solubility of the hemicelluloses at higher ionic strength, which
promotes adsorption.
Hemicelluloses isolated from the peroxide bleached TMP adsorbed to a lesser extent
on cellulose compared to hemicellulose fraction isolated from the unbleached TMP.
This is as expected since the repulsion between charged carboxylic acid segments of
the hemicellulose chain is high and, thus, the polyelectrolyte molecules in solution take
a stiff and rodlike conformation (Fleer et al . 1993). When adsorbing on the surfaces
these hemicelluloses should tend to form a thin and flat layer, and according to the Voigt
based modellings, a relatively thin layer of hemicelluloses adsorbed on cellulose surface.
The final hydrodynamic thickness was approximately 2 nm, see Figure 6.6. Although
the adsorbed amount is lower compared to the adsorbed amount of the unbleached
hemicellulose fraction, the hemicellulose layer is relatively strongly bound on cellulose
surface (Figure 6.5). Especially the shear elastic modulus values are high and at the end
of the adsorption process the shear elastic modulus reaches nearly the same level as the
film formed from unbleached hemicellulose fraction at high ionic strength.
The adsorption behavior of anionic hemicelluloses on slightly anionic cellulose surface
and the effect of ionic strength on the adsorption can largely be explained by screen-
ing of intramolecular, electrostatic repulsive interactions. Consequently, the screened
repulsion leads to a more coiled conformation of the hemicellulose chain and decreased
double layer forces which enables closer contact between the hemicellulose chains and
the cellulose surface. The closer contact may facilitate the other important attractive
forces, such as van der Waals forces, to become more predominant (Israelachvili 1992).
Adsorption behavior of pure GGM and pure pectin clarifies and supports the idea of the
effect of electrostatics on the hemicellulose adsorption. More or less neutral and rela-
tively large GGM adsorbs on cellulose forming a layer with loops and tails pointing out
to the solution phase whereas the highly charged and small pectin molecules adsorbed
on cellulose forming a very thin and flat layer (Figure 6.1b). However, the main driving
force of adsorption is unclear. The dissolved hemicelluloses may prefer the contacts with
cellulose to contacts with solvent and they probably adsorb due to the similarities in the
molecular structure with cellulose. When polymer reaches sufficiently close contact to
cellulose the formation of hydrogen bonds between cellulose and hemicellulose chains
may be promoted. However, in aqueous environment the hydrogen bonding with water
is probably dominating.
Strongly bound hemicellulose layers, especially those forming loops and tails point-
ing out to the solution phase, can effectively sterically stabilize surfaces. Thus, the
hemicelluloses can be used as stabilizers. It is well known that hemicelluloses can sta-
bilize extractive colloids in TMP process waters preventing them from aggregating and

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