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 111

Deviations between the signals in QCM and SPR experiments may indicate that the
film is viscoelastic or that there is some coupled water in the adsorbed layer. By carefully
considering the nature of each measurement it is possible to decouple the viscoelastic
properties and the contributions from coupled water in the film. Below a more detailed
explanation about the role of coupled water is presented. Water can be used as a
boundary lubricant as the fluidity of the hydration layers nanoconfined between two
surfaces significantly differs from the behavior of the water in the bulk (Raviv, Laurat
et al . 2001; Zhu and Granick 2001; Raviv and Klein 2002; Leng and Cummings 2005).
The water coupled with lubricant polymers has the same function, i.e. to protect the
contact surfaces and minimize abrasion. Measurement of the coupled water is not an
easy task since it is difficult to distinguish the coupled from the bulk water. Below
are two alternative ways to decoupled the contribution from water via QCM and SPR
measurements.
The first approach is to substitute water solvent with D 2 O, as reported by Hook and
others (Hook, Kasemo et al . 2001; Craig and Plunkett 2003; Notley, Eriksson et al .
2005). D 2 O substitution increases the density and shear viscosity of the bulk liquid and
coupled water by 25%, respectively but presumably it doesn't change any

kinetic and equilibrium state. Therefore, from the slight difference in frequency from
experiments conducted in normal and heavy water, the coupled water fraction can be
obtained through Equation 4.14 (Craig and Plunkett 2003).

∼ 10% and ∼

f s − f d

S fraction =

f s 1 − f d 1

(4.14)

ρ d
ρ p

ρ s
ρ p

− −

Subscript s, d, p represents solvent, deuterated water and polymer respectively. In some
cases where ρ p = ρ s , Equation (4.14) can be simplified to Equation (4.15):

f s − f d
f s 1

S fraction = (4.15)

ρ d
ρ p

−

Since the difference is very small, only polymers adsorbing in large quantities or carrying
large amounts of coupled water can be analyzed with this approach.
The second method to decouple hydration from bulk water is by combining QCM and
optical methods, for example SPR or ellipsometry (Hook, Kasemo et al . 2001). The
change in resonant frequency ( f) of the QCM crystal depends on the total oscillating
mass which includes the coupled water. In the case of SPR or ellipsometry water coupled
with adsorbed molecules doesn't affect the refractive index hence they are not detected
by these optical techniques. Therefore by subtracting the mass determined from SPR or
ellipsometry measurements from that obtained from QCM measurements the contribution
of coupled water can be revealed. Figure 4.13 is an example used here to demonstrate the
combination of QCM and SPR techniques. The polymer tested was a cationic polyamide
(5% cationic groups), with molecular weight ca . 3 million. The surface used in this
experiment was a negatively charged silica surface. The experimental results indicate
that there was around 25% of water in the adsorbed polymer layer.