Hemicelluloses at Interfaces: Some Aspects of the Interactions - The Nanoscience and Technology of Renewable Biomaterials - page 152

technique is used to study the effect of adsorbed xylan on the forces between cellulose
surfaces.
The QCM-D data were modelled using the Voigt-based model for viscoelastic solid
(Voinova et al . 1999). In this way estimations for the layer thickness development
during the adsorption process could be attained and in addition, the shear viscosity and
shear elastic modulus of the formed hemicellulose layer could be estimated.
By combining the results of the adsorption behavior of different hemicelluloses with
direct force measurements and a thorough investigation of the viscoelastic properties
of the adsorbed hemicellulose films the aim was to gain a deeper understanding of the
formation of the hemicellulose films on cellulose, its properties and structure as well
as how it affects the interactions present in the system. In addition, it was clarified
how parameters such as ionic strength and the source of the hemicellulose change the
properties of the adsorbed film and the interaction forces. We hope that the enhanced
understanding of the properties of different hemicellulose films facilitates the use of
hemicelluloses in novel applications.

6.3 Theoretical Basis for Interpreting QCM-D and AFM Data

6.3.1 QCM-D Data

QCM-D technique enables in-situ adsorption studies at solid/liquid interface (Rodahl
et al . 1995). Without adsorbate the piezoelectric quartz crystal oscillates at a resonant
frequency f 0 which is lowered to f when material adsorbs on the surface of the crystal. If
the adsorbed mass is evenly distributed, rigidly attached, fully elastic and small compared
to the mass of the crystal, the shift in the resonant frequency is related to the adsorbed
mass by the Sauerbrey equation (Ho ok et al . 1998):

Cf
n

m =− (6.1)

where m is the adsorbed mass per unit surface, f = f − f 0 is the frequency shift, n

is the overtone number ( n = 1, 3, 5, 7) and C is a constant that describes the sensitivity

of the device to changes in mass. 0 . 177 mg m − 2 Hz − 1

For the crystals used, C ≈

(Edvardsson et al . 2005).
The resonant frequency of the crystal depends on the total oscillating mass, including
water coupled to the oscillation. By measuring several frequencies and the dissipation
it becomes possible to determine whether the adsorbed film is rigid or water-rich (soft)
which is not possible by looking only at the frequency response. If the adsorbed material
is not fully elastic, frictional losses occur that lead to a damping of the oscillation with
a decay rate of amplitude that depends on the viscoelastic properties of the material.
With the QCM-D instrument the change in the dissipation factor, D = D − D o , when
material is adsorbed can be measured. D o is the dissipation factor of the pure quartz
crystal immersed in the solvent and D is the dissipation factor when material has been
adsorbed. D is defined by

E diss
2 πE stor

D = (6.2)