15 research outputs found
Transferable Force Field for Carboxylate Esters: Application to Fatty Acid Methylic Ester Phase Equilibria Prediction
In this work, a new transferable united-atoms force field
for carboxylate
esters is proposed. All Lennard-Jones parameters are reused from previous
parametrizations
of the AUA4 force field, and only a unique set of partial electrostatic
charges is introduced for the ester chemical function. Various short
alkyl-chain esters (methyl acetate, ethyl acetate, methyl propionate,
ethyl propionate) and two fatty acid methylic esters (methyl oleate
and methyl palmitate) are studied. Using this new force field in Monte
Carlo simulations, we show that various pure compound properties are
accurately predicted: saturated liquid densities, vapor pressures,
vaporization enthalpies, critical properties, liquidâvapor
surface tensions. Furthermore, a good accuracy is also obtained
in the prediction of binary mixture pressureâcomposition diagrams,
without introducing
empirical binary interaction parameters. This highlights the transferability
of the proposed force field and gives the opportunity to simulate
mixtures of industrial interest: a demonstration is performed through
the simulation of the methyl oleate + methanol mixture involved in
the purification sections of biodiesel production processes
New Molecular Simulation Method To Determine Both Aluminum and Cation Location in Cationic Zeolites
The knowledge of
aluminum distribution in zeolites is a difficult
task due to limitations in experimental measurements. In the present
paper, we propose a new methodology to simultaneously determined aluminum
atoms distribution as well as the extraframework cation location in
a given experimental structure of the framework and thus allows comparison
of different synthesis routes. Aluminum mean distribution is obtained
over a great number of configurations that are generated during the
course of the simulations at finite temperature. The obtained aluminum
atom repartition is in agreement with the experimental and model data
available. The consequences of aluminum distribution on solid properties
such as extraframework Na<sup>+</sup> cation location have been analyzed
and successfully compared with the available information for different
zeolite topologies. The proposed methodology can be used as a powerful
complementary tool for aluminum location on X-Ray or neutron experimental
structure determinations
Heterometallic MetalâOrganic Frameworks of MOFâ5 and UiO-66 Families: Insight from Computational Chemistry
We
study the energetic stability and structural features of bimetallic
metalâorganic frameworks. Such heterometallic MOFs, which can
result from partial substitutions between two types of cations, can
have specific physical or chemical properties used for example in
catalysis or gas adsorption. We work here to provide through computational
chemistry a microscopic understanding of bimetallic MOFs and the distribution
of cations within their structure. We develop a methodology based
on a systematic study of possible cation distributions at all cation
ratios by means of quantum chemistry calculations at the density functional
theory level. We analyze the energies of the resulting bimetallic
frameworks and correlate them with various disorder descriptors (functions
of the bimetallic framework topology, regardless of exact atomic positions).
We apply our methodology to two families of MOFs known for heterometallicity:
MOF-5 (with divalent metal ions) and UiO-66 (with tetravalent metal
ions). We observe that bimetallicity is overall more favorable for
pairs of cations with sizes very close to each other, owing to a charge
transfer mechanism inside secondary building units. For cation pairs
with significant mutual size difference, metal mixing is globally
less favorable, and the energy signifantly correlates with the coordination
environment of linkers, determining their ability to adapt the mixing-induced
strains. This effect is particularly strong in the UiO-66 family because
of high cluster coordination number
Structure and Dynamics of Solvated Polymers near a Silica Surface: On the Different Roles Played by Solvent
Whereas it is experimentally
known that the inclusion of nanoparticles
in hydrogels can lead to a mechanical reinforcement, a detailed molecular
understanding of the adhesion mechanism is still lacking. Here we
use coarse-grained molecular dynamics simulations to investigate the
nature of the interface between silica surfaces and solvated polymers.
We show how differences in the nature of the polymer and the polymerâsolvent
interactions can lead to drastically different behavior of the polymerâsurface
adhesion. Comparing explicit and implicit solvent models, we conclude
that this effect cannot be fully described in an implicit solvent.
We highlight the crucial role of polymer solvation for the adsorption
of the polymer chain on the silica surface, the significant dynamics
of polymer chains on the surface, and details of the modifications
in the structure solvated polymer close to the interface
Structure and Dynamics of Water Confined in Imogolite Nanotubes
We
have studied the properties of water adsorbed inside nanotubes
of hydrophilic imogolite, an aluminum silicate clay mineral, by means
of molecular simulations. We used a classical force field to describe
the water and the flexible imogolite nanotube and validated it against
the data obtained from first-principles molecular dynamics. With it,
we observe a strong structuration of the water confined in the nanotube,
with specific adsorption sites and a distribution of hydrogen bond
patterns. The combination of number of adsorption sites, their geometry,
and the preferential tetrahedral hydrogen bonding pattern of water
leads to frustration and disorder. We further characterize the dynamics
of the water, as well as the hydrogen bonds formed between water molecules
and the nanotube, which is found to be more than 1 order of magnitude
longer than waterâwater hydrogen bonds
Experiment and Theory of Low-Pressure Nitrogen Adsorption in Organic Layers Supported or Grafted on Inorganic Adsorbents: Toward a Tool To Characterize Surfaces of Hybrid Organic/Inorganic Systems
We report experimental nitrogen adsorption isotherms
of organics-coated
silicas, which exhibit a low-pressure desorption branch that does
not meet the adsorption branch upon emptying of the pores. To address
the physical origin of such a hysteresis loop, we propose an equilibrium
thermodynamic model that enables one to explain this phenomenon. The
present model assumes that, upon adsorption, a small amount of nitrogen
molecules penetrate within the organic layer and reach adsorption
sites that are located on the inorganic surface, between the grafted
or adsorbed organic molecules. The number of accessible adsorption
sites thus varies with the increasing gas pressure, and then we assume
that it stays constant upon desorption. Comparison with experimental
data shows that our model captures the features of nitrogen adsorption
on such hybrid organic/inorganic materials. In particular, in addition
to predicting the shape of the adsorption isotherm, the model is able
to estimate, with a reasonable number of adjustable parameters, the
height of the low-pressure hysteresis loop and to assess in a qualitative
fashion the local density of the organic chains at the surface of
the material
Stress-Based Model for the Breathing of MetalâOrganic Frameworks
Gas adsorption in pores of flexible metalâorganic
frameworks (MOF) induces elastic deformation and structural transitions
associated with stepwise expansion and contraction of the material,
known as breathing transitions between large pore (<b>lp</b>) and narrow pore (<b>np</b>) phases. We present here a simple
yet instructive model for the physical mechanism of this enigmatic
phenomenon considering the adsorption-induced stress exerted on the
material as a stimulus that triggers breathing transitions. The proposed
model implies that the structural transitions in MOFs occur when the
stress reaches a certain critical threshold. We showcase this model
by drawing on the example of Xe adsorption in MIL-53 (Al) at 220 K,
which exhibits two consecutive hysteretic breathing transitions between <b>lp</b> and <b>np</b> phases. We also propose an explanation
for the experimentally observed coexistence of <b>np</b> and <b>lp</b> phases in MIL-53 materials
Investigating the Pressure-Induced Amorphization of Zeolitic Imidazolate Framework ZIF-8: Mechanical Instability Due to Shear Mode Softening
We provide the first molecular dynamics
study of the mechanical
instability that is the cause of pressure-induced amorphization of
zeolitic imidazolate framework ZIF-8. By measuring the elastic constants
of ZIF-8 up to the amorphization pressure, we show that the crystal-to-amorphous
transition is triggered by the mechanical instability of ZIF-8 under
compression, due to shear mode softening of the material. No similar
softening was observed under temperature increase, explaining the
absence of temperature-induced amorphization in ZIF-8. We also demonstrate
the large impact of the presence of adsorbate in the pores on the
mechanical stability and compressibility of the framework, increasing
its shear stability. This first molecular dynamics study of ZIF mechanical
properties under variations of pressure, temperature, and pore filling
opens the way to a more comprehensive understanding of their mechanical
stability, structural transitions, and amorphization
Hydrothermal Breakdown of Flexible MetalâOrganic Frameworks: A Study by First-Principles Molecular Dynamics
Flexible
metalâorganic frameworks, also known as soft porous
crystals, have been proposed for a vast number of technological applications,
because they respond by large changes in structure and properties
to small external stimuli, such as adsorption of guest molecules and
changes in temperature or pressure. While this behavior is highly
desirable in applications such as sensing and actuation, their extreme
flexibility can also be synonymous with decreased stability. In particular,
their performance in industrial environments is limited by a lack
of stability at elevated temperatures and in the presence of water.
Here, we use first-principles molecular dynamics to study the hydrothermal
breakdown of soft porous crystals. Focusing on the material MIL-53Â(Ga),
we show that the weak point of the structure is the bond between the
metal center and the organic linker and elucidate the mechanism by
which water lowers the activation free energy for the breakdown. This
allows us to propose strategies for the synthesis of MOFs with increased
heat and water stability
Remarkable Pressure Responses of MetalâOrganic Frameworks: Proton Transfer and Linker Coiling in Zinc Alkyl Gates
Metalâorganic frameworks demonstrate
a wide variety of behavior
in their response to pressure, which can be classified in a rather
limited list of categories, including anomalous elastic behavior (e.g.,
negative linear compressibility, NLC), transitions between crystalline
phases, and amorphization. Very few of these mechanisms involve bond
rearrangement. Here, we report two novel piezo-mechanical responses
of metalâorganic frameworks, observed under moderate pressure
in two materials of the zinc alkyl gate (ZAG) family. Both materials
exhibit NLC at high pressure, due to a structural transition involving
a reversible proton transfer between an included water molecule and
the linkerâs phosphonate group. In addition, the 6-carbon alkyl
chain of ZAG-6 exhibits a coiling transition under pressure. These
phenomena are revealed by combining high-pressure single-crystal X-ray
crystallography and quantum mechanical calculations. They represent
novel pressure responses for metalâorganic frameworks, and
pressure-induced proton transfer is a very rare phenomenon in materials
in general