Multiscale Approaches and Perspectives to Modeling Aqueous Electrolytes and Polyelectrolytes - Multiscale Molecular Methods in Applied Chemistry - page 283

The most important facts we infer from our studies of the simple coarse-grained
PEM model are that:

l Without any surface-monomer short range interaction there is no stable surface
coverage.
l Using only electrostatic interactions and purely repulsive monomer-monomer
interactions for the excluded volume, no multilayering occurs.
l A short range hydrophobic interaction between all polymeric monomers as well
as a monomer-surface attraction is needed to achieve multilayering.
l The precise interaction values need to be fine tuned in order to obtain a stable
layering.
This situation is, of course, far from satisfactory. We therefore turned to atomis-
tic simulations of all-atom (AA) poly(styrene sulfonate) (PSS) and poly(diallyldi-
methylammonium) (PDADMA) systems [ 160 ]. The polymers had a low degree of
polymerization which was 12 at most. Our main target was to investigate some
properties such as the dielectric constant and water structure that cannot be
addressed using CG models. Moreover, in a second recent effort, we looked at
PSS monolayer simulations for hydrophilic/hydrophobic surfaces and different
surface charge [ 161 ].
Our AA molecular dynamics simulations on PSS-PDADMA-water-salt (NaCl)
mixtures have provided us with another significant insight: when NaCl is added
to the PSS-PDADMA-water mixtures of composition similar to real PEMs, the salt
ions tend to form clusters inside the mixture [ 187 ], which can explain why the
addition of moderate amounts of salt has been observed not to modify significantly
the structural properties of the PE complexes. In such PE complexes, intrinsic
(polyions pair with polycations) and extrinsic (polyions pair with salt ions) charge
compensations have been found to co-exist, although the intrinsic mechanism
has been observed to be dominant [ 160 ]. The addition of NaCl decreases the weight
of the intrinsic mechanisms in the PE complexes. But even at the highest simulated
NaCl concentrations ( C NaCl ¼ 1 mol/L), the intrinsic charge compensation mecha-

nism is still the dominant one. Furthermore, our study of such mixtures has allowed
us to determine the relative scale of the interaction energy of the ion pairs in the
mixture: Na

PDADMA [ 160 ].
The relative scale of the interaction energy can be very useful to model properly
the interactions between ion pairs in refined CG numerical simulations and theoret-
ical approaches. The analysis of the water structure showed us that PE mixtures are
percolated through leading to a homogeneous distribution of waters.
Even though the slow dynamics of water in PEMs has been experimentally
studied [ 190 ], no theoretical or numerical works have been reported yet. Some
recent simulation studies [ 68 , 75 , 191 ] have provided an important insight about the
dielectric constant of water in concentrated electrolyte solutions where electrostatic
interactions can be almost twice as strong as in pure water. In the work of Qiao et al.
[ 160 ] the slow dynamics of water inside PSS/PDADMA systems was investigated
via the static dielectric constant (i.e., static relative permittivity) and the water
diffusion coefficient (see Table 1 ). Error estimates are determined by block

Cl > PSS Na > PDADMA Cl PSS