Adsorption of Polyelectrolytes onto Charged Surfaces 51thus it is possible to rewrite Equation (30) as
// 12
12A e ttA e
(32)
Figure 4a shows an example corresponding to the adsorption of a
polycation (poly(diallyldimethylammonium chloride), PDADMAC) onto a
negatively charged substrate. The experimental results are well described by
the Equation 32. Two different steps are differentiated from the experimental
results, being the adsorption at the end of the first step around 60% of the total
amount of the final polyelectrolyte adsorbed. Further evidences of the bimodal
character of the experimental data are found from the separated analysis of the
two processes showed in Figure 4b and 4c [13].
4.2. Adsorbed Amount and Structural Aspects
The equilibrium adsorption of polyelectrolyte and its organization in the
layer also depends on the complex balance of interactions involved in these
systems. The adsorbed chains can be present different types of segments
depending on their spatial configuration in relation to both the polymer chain
and the surface: loops, tails and trains. In general, it is expected that
the adsorbed amount and the structure of the layers show a strong dependence
on the relative proportion of these three types of segments, which is
simultaneously associated with the different parameters governing the
polyelectrolyte adsorption [13, 26, 35]. The increase of the effective charge of
the chains increases the inter- and intrachain electrostatic repulsions, thus
leading to the formation of flat layers with a small number of adsorbed chains
due to the electrostatic repulsions. This favours the formation of layers formed
by isolated chains. The decrease of the charge density by the modification of
the solution parameter favours the decrease of the repulsive interactions, and
layers with a fuzzier structure can be expected, which increases the number of
segments protruding to the solution and adsorbed chains. Similar effect is
obtained when the adsorption of polymers formed by blocks of different
charge density is studied [125]. The increase of polyelectrolyte concentration
increases the number of groups able to interact with the surface, thus
modifying the structure and number of polyelectrolyte chains involved in the
formation of the layers [25, 93].