a) b - École Polytechnique de Montréal

2.4.1.1.7 Effect of Salt on Multilayers
The presence of salt has a subtle effect. It screens the monomer-monomer repulsive interactions,
leading to enhancement of adsorption, because the interaction between the non-charged surface
and the chain does not have an electrostatic component.
Multilayers are fabricated using salt-containing or salt-free solution. Salt concentration has also
been taken into account as the most important factor in multilayer thickness ( Decher, 1997;
Decher & Schmitt, 1992). In salt-containing solutions, the salt ions of opposite charge
(counterions) are drawn to the charged object and form a loosely bound counterion cloud around
it. They effectively reduce or screen the charge of the object. The effective electrostatic
interaction between two charges in the presence of salt ions is governed by Debye-Huckle law.
There are two scenarios for charge balance in a system with and without salt, as shown in Figure
2-41.
a)
b)
Figure 2-41. Deposition of layer in LbL process a) in the absence of salt and b) in the
presence of salt
In the salt-free case, a positively charged polyelectrolyte is balanced by a negatively charged
one. In an alternative mechanism, for a salt-containing system, charged polyelectrolytes are
balanced by salt counterions derived from the bathing solution and construct multilayers.
Thereby, multilayers containing salt ions are thicker, less interpenetrating, and chains have a
high degree of mobility, leading to less stable structures.
Schlennof et al.( Dubas & Schlenoff, 2001a; Schlenoff & Dubas, 2001) performed sequential
adsorption of polyelectrolytes in the presence of various concentrations of salt (NaCl) from 0 to
1M. They reported that for strong polyelectrolytes, addition of further salt to the solution results
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