Multiscale Approaches and Perspectives to Modeling Aqueous Electrolytes and Polyelectrolytes - Multiscale Molecular Methods in Applied Chemistry - page 279

We will conclude this part of the review by mentioning that if one is interested in
solvent mediated interactions in polyelectrolytes, i.e., to study dynamical behavior,
then one immediately faces a problem since we have used so far an “implicit”
solvent. The obvious thought of including an explicit solvent is only viable for very
small system sizes, since the amount of solvent particles needed quickly reaches the
range of millions and more particles even for small simulation boxes, and the
needed computational time quickly becomes prohibitive. A possible way out is to
use a mesoscale solver for solvent dynamics such as the Lattice-Boltzmann (LB)
method, stochastic rotational dynamics (SDR), dissipative particle dynamics
(DPD), or Stokesian dynamics, and couple the coarse-grained PE model to the
background fluid in such a way that the PE can exchange momentum with the fluid
background and vice versa, such that correct hydrodynamical interactions are
generated [ 137 , 138 ]. This becomes necessary for electrophoretic applications
[ 139 ] where the dynamics of the PE under applied fields is studied [ 140 - 143 ].
The usage of simple Langevin (or Brownian) dynamics destroys any hydrodynamic
interactions, and, although several studies of dynamical aspects of PE behavior
have appeared [ 144 - 146 ], they can be valid only as long as hydrodynamical
interactions do not play any role.
4.1 Polyelectrolyte Multilayers
Polyelectrolyte multilayers (PEMs) denote an interesting class of materials that are
formed by alternating layers of oppositely charged PEs. In the early 1990s Decher
et al. [ 147 , 148 ] demonstrated the feasibility of building such a type of multilayers
using the so-called Layer-by-Layer (LbL) technique. In order to build up a PEM
one sequentially exposes a normally negatively charged substrate to a cationic PE
solution followed by a rinsing step in order to dispose the supernatant ions and PEs.
Then the substrate is dipped into an aqueous solution of anionic PEs, always
followed by a rinsing step. Repeating this simple procedure can result in a build-
up of hundreds of alternating charged layers. Films made up by PEMs exhibit
unique properties which make them suitable candidates for many different uses:
matrix materials, bio-coatings, selective membranes, chromatography, optical
materials and devices, micro- and nanocarriers, or biocides, just to name a few.
There have been only a few attempts to describe theoretically the electrostatic
self-assembly of PEMs, and often they rely on several serious assumptions that are
hard to test experimentally [ 149 - 151 ]. Recently simple models based on mean-field
descriptions have been developed [ 152 - 155 ] that reproduced better some of the
experimental observations regarding the stability and the different growth regimes
for PEMs. The film thickness of PEMs normally grows linearly with the number of
layers, but exponentially growing PEMs are also known. Strong correlations that
exist between oppositely charged polyions provide a formidable challenge to the
theoretical description of PEMs.