A molecular dynamics investigation of tetrahedral liquids and aqueous solutions in bulk (SiO2 ) and confinements (SiO2, H2O, water-octanol mixtures)
Water is an essential material in our everyday life and is the solvent mostly used for chemical and bi- ological reactions. In particular, water in living cells are surrounded by macromolecules and behaves as a confined fluid. Confined water does not only occur in biology but also in geology for e...
| Summary: | Water is an essential material in our everyday life and is the solvent mostly used for chemical and bi-
ological reactions. In particular, water in living cells are surrounded by macromolecules and behaves
as a confined fluid. Confined water does not only occur in biology but also in geology for example in
porous rocks, or in technology as for example nanofluidic devices. For all these systems, an understand-
ing of the transport mechanism of water is necessary. Water molecules are highly polar and can form
hydrogen-bonded networks. They can also share hydrogen bonds with other molecules. In particular,
aqueous mixtures of water confined within nanoscale geometries of hydrophilic surfaces is subject to
several competing interactions: liquid-liquid interaction between the liquid species composing the mix-
ture and surface-liquid interactions.
In the first part of the work, the behavior of water molecules in partially filled pores is investigated while
comparing conventional silica models. We show that water droplets can undergo several configurations
in the pore when the silica surface described by the model is not highly hydrophilic. For the expected
highly hydrophilic silica surface, the pore surface is always completely wetted by water, agreeing with
experimental observations. We have as well evaluated the contact angle of a water droplet on top of
a silica slab and finds out that the interaction becomes more hydrophilic with increasing slab thickness
and saturates around 2.5 − 3 nm, consistent with experimentally found value.
Having verified the silica model capturing adequately the wetting mechanism of water in silica surfaces,
we have then examined the behavior of water/octanol mixture in silica confinement. Octanol is a long
chain alcohol and resembles lipid macromolecules which are main components of biological membranes.
It is a peculiar choice because its long hydrocarbon chain plays an important role in the competitive pro-
cess of interaction near the pore surface. We show indeed evidence of a bilayer-like structure in pore
when the pore size is approximately 2 times larger as the octanol chain length. We show that the wa-
ter/octanol ratio does not significantly affect the orientation of octanol or water molecules near the pore
surface. Whereas, the amount of water molecules accumulating near the pore surface increases with
water content. We interpret these findings by referring to the relative facility of water molecules to fit
near the pore surface in comparison to octanol molecules. Combined with the fact that when water
reach the pore wall, it almost gets stack in the surface and its motion considerably slows down due to
the effect of the restriction imposed by the fixed atoms of the wall together with the restriction arising
from the slower mobility of octanol near the pore.
The ensuing part consists in understanding the dramatic dynamical slowdown of glass forming liquids.
To approach the problem, we study liquid silica in bulk and in neutral confinement. Alike water, sil-
ica molecules are polar molecules which can build tetrahedral networks due to their directed bonds.
They exhibit analogous anomalous features such as a density maximum upon cooling, dynamical tran-
sitions such as the fragile-to-strong transition and possibly thermodynamic transitions. Specifically, the
fragile-to-strong transition in liquid silica is followed by a structural change to lower density and higher
tetrahedral order. In order to clear up the effect of Coulomb electrostatic interactions on the structure
and dynamics of silica, we modify the bond polarity by varying the partial charges attached to silicon
and oxygen.
We find out that density, tetrahedral order and structural relaxation times decrement when the bond
polarity is reduced. Interestingly, the density maximum and the fragile-to-strong transition shift to lower
temperatures and disappear when the partial charges are decremented below roughly 75 % of their
standard value. We as well show that the temperature-independent energy barriers associated to strong
dynamics at low temperature and to the activated dynamics at high temperature both decrease faster
than linearly with charges. We then demonstrate that the fragile-to-strong transition is closely related to
structural changes happening between the first and the second neighboring shell which influence both
a lowering of density and an increase in local ordering. We have further applied the recently intro-
duced relations to describe the intermediate fragile dynamics of silica. This new model decomposes the temperature-dependent activation barrier into a single particle contribution (that is the high-temperature
activation energy) and a temperature-dependent collective contribution (which should pictured dynam-
ical correlations). It describes qualitatively well intermediate fragile dynamics of silica variants although
universal relations expected from the out-coming model parameters are not quantitatively reproduced
for silica.
Finally, the dynamics slowdown of silica variants in neutral confinements have been addressed. We
would like to check whether one can see a trend in the possible growing mechanism of correlated
clusters. And in particular, we would like to sort out the effect of bond polarity. Assuming that the
emergence of correlated clusters upon cooling can be pictured by typical length scales related to the
size of the clusters, we have accessed structural, elastic and dynamical length scales from our analysis in
neutral confinements. We show that these length scales increase upon cooling. Another important aspect
is the effect of the fragile-to-strong crossover on the temperature variation of length scales. We perform
an in-depth analysis of the temperature-dependent length scales by comparing the trend between silica-
like liquids and additionally by relating length scales with bulk-like structural relaxation times. We find
out no simple relations between studied length scales and activation energy barriers. We postulate that
length scales characterizing strong dynamics increase slower with time scale in comparison with length
scales describing fragile dynamics. |
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