48 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
segments protruding to the liquid and with a close packing due to the decrease
of the electrostatic repulsion between the attached chains. This adsorption
regime in which the increase of ionic strength leads to the increase of the
thicknes of the adsorbed layer is called screening-enhaced adsorption regime.
However, beyond a threshold value of ionic strength, the adsorption enters into
a screening-reduced adsorption regime in which the minimization of the
electrostatic interactions leads to the hindering of the adsorption [96, 101]. The
modification of the pH of the solutions present a similar effect than that of the
ionic strength due to their effect on the ionic balance of the system [102, 103].
The modification of the ionic strength can be reached using different types
of salts, being the adsorption different depending on the nature of the salts in
the solution. This can be associated with the ability of the different counterions
to bind to the polymer chains [104]. In most of the cases, the adsorption is not
affected by the nature of the cation [105], especially when we deal with
monovalent counterions. However, the nature of the anion affects strongly to
the thickness and properties of adsorbed layer [33, 106], being possible to
associated the decrease of thickness of the layers with the nature of the anions
to the Hofmeister series: Br- > NO 3 - > ClO 3 - > Cl-^ > BrO 3 - > HCOO- > F. The
adsorption dependence on the anion nature is explained considering that the
hydration of the chains, and consequently their swelling degree and size
depends on the ability of counterions to bind to the polyelectrolyte chains.
Thus, for counterions with high tendency to bind to the chains a coiled
conformation of these latter can be expected, and the adsorption occurs in a
coiled conformation. This leads to the increase of the thickness and roughness
of the adsorbed layer. It is worth mentioning that the changes in the polarity of
the solvent modify the effect of counterions in the adsorption process [107].
Finally, the quality of the solvent can be also tuned with temperature. The
influence of the temperature can be more important that the corresponding to
the variables described above [36]. However, the influence of temperature on
the adsorption of polyelectrolyte onto surfaces has been less studied than the
ionic strength or pH.
In addition to the electrostatic interactions, non electrostatic interactions
must be taken into account when the adsorption of polyelectrolyte is
considered. These interactions allow the adsorption onto the solid surface of
some polyelectrolytes, even when both have similar charge [22, 108]. Among
the non electrostatic contributions is possible to highlight dispersion and
solvation/desolvation forces. In some cases, the adsorption of the
polyelectrolyte chains occurs by hydrophobic or specific interactions of the
chain with the solid substrate [22]. Furthermore, the role of the entropic