Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 151

cellulose (Hannuksela et al . 2002). Adsorption increases with decreasing deacetylation
and decreasing amounts of galactose side groups.
In our previous publications (Tammelin et al . 2007; Johnsen et al . 2006) we have
further clarified on a molecular level the interactions in the presence of hemicelluloses
using quartz crystal microbalance with dissipation monitoring (QCM-D). We were able to
show why the hemicelluloses derived from the waters of spruce consuming TMP process
can sterically stabilize extractive colloids. The hemicelluloses adsorbed extensively on
the extractives forming a layer with loops and tails pointing out to the solution phase
and thus they were able to form the steric hindrance around the colloids. The results
fully supported the conclusions drawn by Sundberg et al . (1994a; 1996a). These results
also showed that a few nanometers thin film of hemicelluloses significantly changed the
properties of cellulose and extractives surfaces.
The major hardwood hemicellulose is glucuronoxylan. Xylan has been shown to
have affinity towards fibers (Yllner and Enstrom 1956) and at the end of pulp cooking
some dissolved xylan is readsorbed to fibers (Mitikka-Eklund 1996). Mora et al . (1986)
have concluded that xylan preferentially readsorbs to xylan rather than to cellulose, and
Henriksson and Gatenholm (2001) have suggested several types of association between
cellulose and xylan. In early studies by Marchessault et al . (1967) it has been shown that
the orientation of xylan molecules is parallel to the fiber axis and may hence affect the
mechanical properties of individual pulp fibers. That xylan molecules align themselves
on cellulose was later theoretically calculated by Kroon-Batenburg et al . (2002). Other
studies also propose that xylan on the fiber surfaces improves paper strength (Buchert
et al . 1995; Schonberg et al . 2001). FTIR spectroscopy experiments by Akerholm and
Salmen (2001) have indicated that xylan associates with mannan more closely than with
cellulose. Based on CP/MAS 13C NMR experiments Teleman et al . (2001) have con-
cluded that the supermolecular structure of xylan is highly dependent on the immediate
environment.
Most studies dealing with cellulose-hemicellulose interactions have been bulk exper-
iments and there are only a few direct measurements of the interaction forces in the
presence of xylan. The forces between xylan-coated mica surfaces have been studied
by Neuman et al . (1993), Claesson et al . (1995) and Osterberg et al . (2001) using the
surface force apparatus (SFA, Israelachvili and Adams 1978). Mica is a highly anionic
mineral surface and the adsorption of xylan to mica was minor and mainly driven by
the low solubility of the xylan. In our previous publication (Paananen et al . 2003) the
adsorption of xylan to cellulose model surfaces and their effect on the forces between
cellulose surfaces was studied using the atomic force microscopy (AFM) colloidal probe
technique developed by Ducker et al . (1991). We found that xylan adsorbed onto
cellulose and steric forces dominated over double-layer forces (Paananen et al . 2003).
The goal of this work was to link together our previous scattered studies (Tammelin
et al . 2006a; Paananen et al . 2003) dealing with hemicellulose interactions with differ-
ent fibrous components and to update the results conducted on cellulose with viscoelastic
modellings. The QCM-D instrument (Rodahl et al . 1995) was used to study the adsorp-
tion of dissolved hemicellulose fractions isolated from unbleached and peroxide-bleached
spruce thermomechanical pulp (TMP) as well as pure O -acetyl-galactoglucomannan,
pure pectin and pure xylan onto cellulose model surface. The AFM colloidal probe