44 Sara Llamas, Laura Fernández-Peña, Ana Mateos-Maroto et al.
3.1. Charge Density of the Polyelectrolytes
It can be expected that the nature of the polyelectrolyte studied can play a
central role on their ability for the adsorption [90]. Among the chemical
aspects that governs the adsorption of polyelectrolytes onto solid surfaces are
probably the chemical functionality, and consequently the charge density of
the polymers the most important factors, because they play a central role on
the control of the polymer-surface interactions [13, 22, 23].
The charge density is a critical factor that determine the conformation and
flexibility of the chains, as well as the hydrophobic-hydrophilic balance [13,
23]. In polyelectrolytes presenting low charge density, the role of the
intermonomer repulsions is small and the chain adopt a coiled conformation in
solution, thus leading to the formation of fuzzy adsorbed layers in which each
single chain is attached by a small number of monomer to the surface, and the
layer structure presents many loops and tails protruding to the solution. On the
contrary, in polyelectrolytes with high charge density, the electrostatic
repulsions between the charged monomers lead to the appearance of extended
conformation for the polyelectrolyte chains. Thus, the adsorption of the chains
occurs parallel to the surface, leading to the formation of thin and flat adsorbed
layers. Therefore, it can be concluded that the thickness of the adsorbed layer
is inversely proportional to the polymer charge [21, 36, 51, 91-94]. Figure 1
schematized the two limit cases for the adsorption of polyelectrolytes.
Rojas et al. [51] and Samoshina et al. [93, 94] showed, independently, the
enhancement of the adsorption of copolymers with different weight ratio of
acrylamide and (3-(methacrylamido)propyl)trimethylammonium chloride
(AM-MAPTAC) onto negatively charged surfaces as the charge density of the
copolymers was reduced due to the increase of the number of segments
protruding to the solution.
a bFigure 1. Different structures of the polyelectrolyte adsorbed layer depending on the
charge density of the chain. (a) High charge density polyelectrolytes. (b) Low charge
density polyelectrolytes.