Adsorption of Polyelectrolytes onto Charged Surfaces 59The appearance of charge overcompensation phenomena has been observed
for both planar substrates and colloidal templates using some physical
magnitude providing an estimation of the surface charge of the layers.
Thus, an alternance between positive and negative charge is found as
polycation or polyanion are forming the last layer, respectively. The level of
charge overcompensation is extremely dependent on the particular pair of
polyelectrolyte studied and is not affected by the assembly condition [35, 207,
208]. The overcompensation degree decreases in an exponential like fashion
from the surface to the inner regions of the layer. Figure 8 shows the charge
inversion of a multilayer formed by the alternate deposition of PDADMAC,
and the sodium salt of poly(4-styrene sulfonate), PSS deposited onto flat solid
substrates as was obtained using a Kelvin probe [26].
Despite the existence of the charge inversion phenomena, the neutrality,
beyond the Debye length, must be guaranted in polyelectrolyte multilayers
[26, 61]. In order to reach the zero net charge condition in polyelectrolyte
multilayers, two different mechanisms which are strongly dependent on the
entropic control of the building process of multilayers can occur. The former
one implies the direct complexation of polyelectrolytes, existing a perfect
matching between the adsorbed charges in adjacent layers, thus the formation
of interpolyelectrolyte complexes with stoichiometry 1:1 happens. This is the
so called intrinsic compensation. The formation of this type of complexes is
related to the counterion release during the multilayer formation with an
important decrease of the free energy due to the importance of the entropy.
Thus, the entropy associated with the counterion release is the main driving
force of the multilayer assembly. The second mechanism involves the
neutralization of the net charge due to the incorporation of counterions into the
polymer matrix, thus it is possible the appearance of complexes with a wide
range of stoichiometries. This latter is referred as extrinsic compensation,
being the role of the entropy rather limited in this mechanism. Figure D
schematizes the two compensation mechanisms.
In most of the multilayers, the predominant compensation mechanism is
the extrinsic one [23, 24, 26, 35, 138], whereas the intrinsic mechanism occurs
mostly in polyelectrolyte with high effective charge density [26]. However, the
prediction of the type of compensation is difficult because it depends on both
the specific polyelectrolyte pair and the assembly conditions [26, 61, 209]. The
careful control of the type of compensation is essential for the multilayer
properties, being the ionic strength of the solutions a critical parameter to tune
the nature of the compensation occurring in the multilayers. This is explained
considering the critical role of the ionic equilibrium in the multilayers on the