60 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
modification of the charge density of the multilayers due to the ionic
condensation mechanism [210] and the importance of the entropic contribution
on the assembly [26, 206]. Highly charged polyelectrolytes present usually
intrinsic-like compensation, with the compensation occurring due to the ionic
pairing between the monomers of the chains in adjacent layer. This type of
compensation implies an important release of counterions to the solution that
leads to a strong increase of the entropy of the system. On the other hand, the
role of the counterion release is smaller for polyelectrolyte with reduced
charge density, thus the entropic contribution to the assembly becomes less
important, and an extrinsic-like compensation is preferred with a large number
of counterions trapped inside the polymer matrix. The transition from intrinsic
to extrinsic compensation are clearly observed in (PDADMAC + PSS)n with
the increase of the ionic strength [26, 31, 206]. This can be evaluated easily by
the increase of the number of charged monomers in positive and negative
layers as shown Figure 10.
Recently Lehaf et al. [211] have showed that the different types of
compensation are strongly correlated to differences on the layer structure. In
particular for (PDADMAC + PSS)n multilayers, the highly ionic content of
PDADMAC layers [26, 31] leads to a more swollen structure than that found
for PSS layers, which can be easily understood considering the strong osmotic
effect induced by the higher counterions concentration. These differences are
reflected on other aspects of the layer structure such as their roughness [212].
In addition to the ionic strength, it is worth mentioning that the different
parameters modifying the ionic equilibrium in the system can contribute in a
similar way to the modification of the compensation mechanism. Among the
aforementioned parameters are: solvent quality [209], pH [35], type of
polyelectrolyte or charge density of the chains [23, 138].
In summary, it is possible to conclude that the assembly of polyelectrolyte
multilayers is governed by an intincate balance between electrostatic factors
and different entropic contributions [134]. This provides the bases for the
multilayering even for those cases in which the electrostatic could hinder the
growth.
5.3. Multilayer Growth
The understanding of the different factors affecting the multilayer growth
is not well understood yet. Two different types of the dependence of the
adsorbed amount on the number of layers, N, (growth mechanisms) have been