a) b - École Polytechnique de Montréal

in swelling of the multilayer, and excess charge can be spread more efficiently into the
multilayer surface. Due to the increase in the salt concentration, the surface charge increases for
PSS/PDADMAC multilayers (Figure 2-42).
The surface charge depends on the last adsorbed layer, allowing a degree of control over surface
and interface properties. In this case, charge overcompensation is the key to multilayer
construction. The last-added polymer charge compensates the previous layer at the multilayer
surface by a factor of φ. Polymer charges within the bulk of the multilayer balance with a 1:1
stoichiometry-termed intrinsic charge compensation if φ=1 and equal φ for both positive and
negative polymers. Hence, a value of φ-1 represents an overcompensation factor. Normally, the
phenomenon of overcompensation of the surface charge occurs when polyelectrolyte chains
deposited on the multilayer surface compensate the previous layer charge while it has extra
charge to compensate the next deposited layer. In other words, overcompensation can be
described for a polyelectrolyte adsorbing on an oppositely charged surface as an overcharging
created during repeating of alternating positive and negative dipping. The overcompensation has
a maximum value at the multilayer surface and decreases towards the substrate surface. The level
of overcompensation at the multilayer/solution interface decreases in an exponential fashion
from the interface into the multilayer, representing a characteristic length of decay.
Figure 2-42. Surface charge in LbL process as a function of salt concentration(Schlenoff &
Dubas, 2001)
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