Adsorption of Polyelectrolytes onto Charged Surfaces 49contributions can become in many case the main driving force of the
adsorption process [109, 110]. The non electrostatic contributions to the
adsorption can be also modified with different parameters, e.g., an increase of
the charge density of the polyelectrolyte increase the role of the entropic
contribution due to the increased importance of the counterions release. The
entropic contribution can be reduced by an increase in the ionic strength [26].
Moreover, the flexibility of the polyelectrolyte chains affects to the entropic
contribution, thus conditioning the level of conformational entropy retained in
the chains, thus disordered layers present higher entropic contributions to the
adsorption [41].
4. EXPERIMENTAL STUDIES OF THE ADSORPTION
ON POLYELECTROLYTE ONTO SOLID SURFACES
The characterization of the adsorption process of polyelectrolytes onto
surfaces requires the analysis of different aspects: adsorption kinetics,
adsorbed amount, structure and properties of the adsorbed layer.
4.1. Adsorption Kinetics
The control of the adsorption kinetics is also associated with the intricante
balance of interactions discussed above [111]. Several experimental and
theoretical studies have dealt with the adsorption of polyelectrolytes onto solid
surfaces. However, most of the studies are limited to a phenomenological
approach, and there is a lacked understanding on the real physico–chemical
bases underlying the adsorption kinetics of these systems [41, 111, 112]. There
is a general agreement in the fact that the adsorption processes are divided in
three steps. The first one is related to the mass transport phenomena from the
solution to the surface. This step can be governed by diffusion and/or
convection. The second step is related to the attachment of the chains to the
surface, which can be influenced by the presence adsorption barriers of
electrostatic or steric nature. The last step is related to the reorganization of the
adsorbed chains that relax at the surface allowing the adsorption of new chains
[41, 113, 114]. The reorganization allows the maximization of the number of
contacts polymer – surface, occurring in time-scales several order of
magnitude larger the two first steps [115, 116]. From the experimental point of