Tools to Probe Nanoscale Surface Phenomena in Cellulose Thin Films: Applications in the Area of Adsorption and Friction - The Nanoscience and Technology of Renewable Biomaterials - page 108

0

EM

− 500

− 1000

− 1500

2000

−

AF

− 2500

0 2468 0 2

Time (hour)

14 16 18 20 22 24

Figure 4.9 Time-dependent frequency change of QCM for EMandAF adsorbed onQCM
crystalat200 ◦ C.Reprintedwithpermissionfrom,Mousavietal.Copyright(2004)American
ChemicalSociety.

4.10.2 Conformation of Adsorbate Layers Revealed by the QCM-D

Indirect information about the conformation of adsorbed layers can also be derived
from QCM experimental data. For rigid, ultrathin, and evenly distributed adsorbed
layers, the Sauerbrey equation (Sauerbrey 1959) describes successfully the proportional
relationship between the adsorbed mass ( m) and the shift of the QCM crystals' resonance
frequency ( f) . Under these conditions, the dissipation value is a constant. It doesn't
change with time or with increasing adsorbed mass. On the other hand, if the adsorbed
material exhibits a viscoelastic behavior, such as that exhibited by layers of proteins,
substantial deviations from the Sauerbrey equation can occur. Using D - f plots one
can eliminate time as an explicit parameter and as concluded in previous studies (Rodahl
and Kasemo 1996a; Hook, Kasemo et al . 2001; Edvardsson, Rodahl et al . 2005), the
absolute slopes and their gradients can provide information about the kinetic regimes
and the conformational changes of the polymer. The magnitude of the slope provides
an indication on the conformation of the adsorbed layer, lower values indicate a softer
layer. If more than one slope exists it can be concluded that different conformation
states of the adsorbed layer are present during the adsorption process.
Figure 4.10 shows QCM results (shifts in frequency) for a cellulose-coated sensor
after injection of a high charge density polyampholyte solution using a 1 mM NaCl
background electrolyte solution (130

µ l/min flow rate). For comparison, the case of a

silica surface is also included.
Figure 4.10 illustrates that shifts in frequency upon polyampholyte adsorption on silica
were two times larger than those measured in the case of cellulose films. Also, it is
interesting to note that for both substrates, silica sand cellulose, a small change in the