Adsorption of Polyelectrolytes onto Charged Surfaces 43Ka
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where Γ and Γ∞ represent the adsorbed amount and the saturation adsorbed
amount, respectively, whereas a representes the activity of the adsorbing
species and K(Γ) is the apparent constant of the adsorption equilibrium,
showing a dependence on the adsorbed amount and including the different
interactions involved in the adsorption process. For small molecules such as
surfactant, the adsorption isotherms are easily accessible from experimental
results [89]. However for the adsorption of polyelectrolytes the situation is
more complex, especially due to the high number of surface – polymer
interactions that modify strongly the affinity of the polymer for the surface,
leading to values of K(Γ) several order of magnitude higher than that expected
for small molecules.
From a practical point of view, the access to the experimental isotherm
is rather difficult, thus rendering inaccessible the main thermodynamic
information of the adsorption process. In most of the cases, it is possible to
assume that the affinity polymer-surface is governed by the effective
molecular size of the macromolecules which depends on the chain length,
stiffness, charge density and non-electrostatic affinity to the surface, the
charge density of the surface, the ionic strength, pH and solvent quality. The
effect of these issues will be discussed in the following sections.
3. PARAMETERS CONTROLLING THE ADSORPTION
The adsorption of polyelectrolytes onto surfaces is controlled by a
complex interplay between different electrostatic and non electrostatic factors,
which include the interactions between the different components of the
system: polyelectrolyte – surface, polyelectrolyte – polyelectrolyte and
polyelectrolyte – solvent. The combination of the different interactions
determines both the structure and properties of the adsorbed layer, as well as
the total adsorbed amount and thickness. Therefore, the control of the
adsorption processes is a critical issue for the understanding of these systems,
being strongly dependent on different paramers which will be discussed in the
following [20, 21].