32 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
2. THEORETICAL STUDIES ON THE ADSORPTION
OF POLYELECTROLYTE ONTO SOLID SURFACES
The theoretical analysis of the adsorption of polymers onto solid surfaces
is a field widely developed in the last fifty years, thus providing an
extraordinary support to the advancement on the understanding of the
molecular bases underlying the adsorption processes [21, 52] However, on the
best of our knowledge, there are no any theoretical model allowing for a
quantitive explanation of the entire complexity of the adsorption process of
polyelectrolytes [53]. This can be due in part because most of the theoretical
studies considers the adsorption of the polyelectrolyte chains onto solid
surfaces as a reversible process, being this view far from the experimental
scenario found for most of the real systems studied. The adsorption of
polymers onto surfaces can be considered, in general, as a quasi-irreversible
process at least in the scales of time normally accesible for the experiments
[54]. An additional limitation for a comprehensive theoretical description of
the polymer adsorption onto solid surfaces is raised from the different length
scales involved in the study of the adsorption of polyelectrolytes, being this
essential on the adequate description of systems involving polymers [47].
The theoretical analysis of the adsorption of polyelectrolytes onto
surfaces, both solid and soft ones, has been developed mainly following two
different approaches [9, 16, 17, 21]. The former one is based in the application
of a self-consistent method that provides simultaneously solutions for the
Edward and the Poisson-Boltzmann equations, thus obtaining information
about the fluctuations of the polymer chains close to the surface and the local
electrostatic potential. The second approach, based in a scaling analysis,
explains the adsorption-desorption of polyelectrolyte chains onto the solid
surface as a phase transition process between compressed states and
underformed ones. It is worth mentioning that the results obtained using the
different approaches differ significantly due to the different physico-chemical
considerations used on the development of such models. Among these
different the fact that the self-consistent fields neglect the role of the
conformational changes during the adsorption process can be considered
probably as the most significant difference.
The use of the mean-field theory to describe the adsorption of
polyelectrolytes can be supported in different assumption, the first attempt
to provide a detailed explanation to the different processes involved in the
adsorption of polyelectrolytes considered the formation of laterally