Adsorption of Polyelectrolytes onto Charged Surfaces 453.2. Concentration of the Solutions
The adsorption of polyelectrolytes onto negatively charged surfaces is
expected to increase with the concentration up to a plateau beyond which
further increases of the adsorption do not present any significant effect on the
adsorbed amount [1, 13, 25]. Thus, the choice of the most adequate polymer
concentration for the specific application is critical, being difficult to
generalize for different polymers because different polymers present different
adsorption isotherms, which are strongly dependent on the charge density and
solubility of the polymers [95]. Diluted polymer solutions leads to the
formation of layers in which the chains interacts with the surface for many
attachment point. This cannot occur in those cases in which the concentration
of polymer is high due to the establishment of a competence between the
different polymer chains, thus a high number of chains is attached to the
surface by a small number of points. Therefore, the increase of polyelectrolyte
concentration affects to the structure of the adsorbed layerd, leading to the
formation of less organized layers, fuzzy layers, with many segments
protruding to the solution [23, 25]. Figure 2 shows a example of the adsorption
isotherm of polyallylamine (PAH) with a molecular weight around 1 kDa from
water onto a negatively charged surface [1].
Adapted from reference [1], Copyright (2011), with permission from Elsevier.
Figure 2. Adsorption isotherms for PAH (molecular weight 1 kDa) from water onto a
negatively charged surface. The solid symbols represent the experimental data and the
line is the corresponding fit to the Langmuir isotherm.