Adsorption of Polyelectrolytes onto Charged Surfaces 47surface interactions. These interactions does not influence the direct
electrostatic adsorption. However, their contribution in the enhancement
or worsening of the adsorption can be important, thus modifying the
compensation balance. The role of the specific interactions with the surface
can be critical in some cases [22] as evidenced the results in Figure 3 in which
the adsorption of a cellulosic polycation (JR400) and a neutral poly
(ethyleneglycol) (PEG) onto surfaces with different negative charge densities
ranging from 0% to 100% is shown. The results evidenced that an
enhancement of the adsorption of the polycation occurs as the charge density
of the surface is increased due to the preminent role of the electrostatic
interactions [22].
3.4. Nature of the Solvent
The adsorption process of polyelectrolytes onto solid surface involves
several processes occurring at the liquid – solid interface. Therefore, the
comprehensive understanding of the adsorption phenomena requires the study
of the role of both surface and liquid phase. The nature of the solvent is
essential because it can modify the balance of interactions occurring in the
polymer, thus affecting to the polymer conformation that has a critical impact
on the adsorption properties as was discussed above. This can be understood
introducing the concept of the solvent quality. The modification of the solvent
quality can be easily reached by the mixture of solvent with different polarity
or adding inert salt. The impoverishment of the solvent quality leads to an
increase on the adsorption due to a decrease of the strength of the electrostatic
interaction, giving more importance to dispersion forces or hydrogen bonds
[99, 100]. Thus, the polymer tends to separate from the solution, increasing its
tendency to adsorb onto the solid surface as a thick layer. In general, for
polyelectrolytes, mixtures of water and alcohol are used to ensure the
solubility of the polymers.
The solvent quality is also related to the effective charge density of the
polymers that is associated with the polymer conformation. This can be
modified changing the ionic strength of the solutions. In general, the increase
of the ionic strength leads to the increase of the thickness of the adsorbed layer
due to the transition of the polyemer chains from rod to coil conformations.
Therefore, the effect of the increase of the ionic strength can be considered
similar to a decrease of the effective charge density of the polymer and the
polyelectrolyte chains become loosely attached to the surface, presenting many