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 107

onto the surface of self-assembled monolayers. By measuring at multiple frequencies
and applying this model the adhering film can be characterized in detail: viscosity, elas-
ticity and thickness may be extracted even for soft films when certain assumptions are
made.

4.10 Application of SPR and QCM to Probe Adsorbed Films

4.10.1 Monitoring Adsorption and Desorption of Macromolecules

SPR and QCM techniques are useful to determine if a given molecule has affinity or
not with the respective metal/organic/polymeric substrate. They also enable elucida-
tion of how strong the affinity is by measuring the actual kinetics of adsorption and
desorption. For example, in a report about the uptake from an organic solution of
octadecyltrichlorosilane, which is of particular interest for the fabrication of microelec-
tromechanical system devices, the authors used quartz crystal microbalance data to fit
a Langmuir isotherm (Hussain, Krim et al . 2005). In this case the adsorption rate was
written as follows (Equation 4.12):

β
α

φ(t) = [1 − exp ( − αt) ] (4.12)

where φ is the fraction of free active sites on the surface, α = C b k af + k ar and β =
C b k af . C b is the concentration of adsorbate, while k af and k ar represent the constants of
adsorption and desorption. The parameters α and β can be obtained by fitting frequency
data. Furthermore, from the relation between α and C b , the values of k ar and k af and
the adsorption equilibrium constant ( K eq = k af /k ar ) was calculated as well as the free
energy of adsorption (Equation 4.13):
G =− RT ln K eq (4.13)

Lubricant degradation can also be measured via QCM. In order to monitor the
degrading process of lubricants at high temperature, Wang et al . (Wang, Mousavi
et al . 2004) used QCM at high temperatures (more than 200 ◦ C) to evaluate the
thermal stability of polyol ester lubricants. Figure 4.9 provides an example that
demonstrates how two lubricants showed different sensitivities to temperature. Here
the lubricants were held in a T-controlled chamber. The lubricants degraded gradually
when they were heated to very high temperature leaving solid residues on the tested
surfaces. The behavior of two commercial-grade pentaerythritol tetrapelargonate
based lubricants, represented by the codes 'EM' and 'AF' (corresponding to two
commercial lubricant compositions), are shown in this figure. During the first nine
hours, both EM and AF did not change with the thermal treatment indicating that both
lubricants were stable. However, after exposure to high temperatures for nine hours
the frequency of AF decreased rapidly while that of EM barely changed. This behavior
indicated that EM was much more stable than AF at the tested temperature of 200 ◦ C.
QCM can thus provide an integral picture of the thermal stability of lubricants in
real-time.