9 research outputs found
Electric Interfacial Layer of Modified Cellulose Nanocrystals in Aqueous Electrolyte Solution: Predictions by the Molecular Theory of Solvation
The X-ray crystal structure-based
models of I<sub>Ī±</sub> cellulose nanocrystals (CNC), both pristine
and containing surface
sulfate groups with negative charge 0ā0.34 <i>e</i>/nm<sup>2</sup> produced by sulfuric acid hydrolysis of softwood
pulp, feature a highly polarized ācrystal-likeā charge
distribution. We perform sampling using molecular dynamics (MD) of
the structural relaxation of neutral pristine and negatively charged
sulfated CNC of various lengths in explicit water solvent and then
employ the statistical mechanical 3D-RISM-KH molecular theory of solvation
to evaluate the solvation structure and thermodynamics of the relaxed
CNC in ambient aqueous NaCl solution at a concentration of 0.0ā0.25
mol/kg. The MD sampling induces a right-hand twist in CNC and rearranges
its initially ordered structure with a macrodipole of high-density
charges at the opposite faces into small local spots of alternating
charge at each face. This surface charge rearrangement observed for
both neutral and charged CNC significantly affects the distribution
of ions around CNC in aqueous electrolyte solution. The solvation
free energy (SFE) of charged sulfated CNC has a minimum at a particular
electrolyte concentration depending on the surface charge density,
whereas the SFE of neutral CNC increases linearly with NaCl concentration.
The SFE contribution from Na<sup>+</sup> counterions exhibits behavior
similar to the NaCl concentration dependence of the whole SFE. An
analysis of the 3D maps of Na<sup>+</sup> density distributions shows
that these model CNC particles exhibit the behavior of charged nanocolloids
in aqueous electrolyte solution: an increase in electrolyte concentration
shrinks the electric interfacial layer and weakens the effective repulsion
between charged CNC particles. The 3D-RISM-KH method readily treats
solvent and electrolyte of a given nature and concentration to predict
effective interactions between CNC particles in electrolyte solution.
We provide CNC structural models and a modeling procedure for studies
of effective interactions and the formation of ordered phases of CNC
suspensions in electrolyte solution
Cellulose Aggregation under Hydrothermal Pretreatment Conditions
Cellulose, the most abundant biopolymer
on Earth, represents a
resource for sustainable production of biofuels. Thermochemical treatments
make lignocellulosic biomaterials more amenable to depolymerization
by exposing cellulose microfibrils to enzymatic or chemical attacks.
In such treatments, the solvent plays fundamental roles in biomass
modification, but the molecular events underlying these changes are
still poorly understood. Here, the 3D-RISM-KH molecular theory of
solvation has been employed to analyze the role of water in cellulose
aggregation under different thermodynamic conditions. The results
show that, under ambient conditions, highly structured hydration shells
around cellulose create repulsive forces that protect cellulose microfibrils
from aggregating. Under hydrothermal pretreatment conditions, however,
the hydration shells lose structure, and cellulose aggregation is
favored. These effects are largely due to a decrease in celluloseāwater
interactions relative to those at ambient conditions, so that celluloseācellulose
attractive interactions become prevalent. Our results provide an explanation
to the observed increase in the lateral size of cellulose crystallites
when biomass is subject to pretreatments and deepen the current understanding
of the mechanisms of biomass modification
Theoretical Modeling of Tunneling Barriers in Carbon-Based Molecular Electronic Junctions
Density functional theory (DFT) is
applied to three models for
carbon-based molecular junctions containing fragments of graphene
with covalent edge-bonding to aromatic and aliphatic molecules, with
the graphene representing a sp<sup>2</sup> hybridized carbon electrode
and the molecule representing a molecular layer between two electrodes.
The DFT results agree well with experimental work functions and transport
barriers, including the electronic coupling between molecular layers
and graphitic contacts, and predict the compression of tunnel barriers
observed for both ultraviolet photoelectron spectroscopy (UPS) and
experimental tunneling currents. The results reveal the strong effect
of the dihedral angle between the planes of the graphene electrode
and the aromatic molecule and imply that the molecules with the smallest
dihedral angle are responsible for the largest local current densities.
In addition, the results are consistent with the proposal that the
orbitals which mediate tunneling are those with significant electron
density in the molecular layer. These conclusions should prove valuable
for understanding the relationships between molecular structure and
electronic transport as an important step toward rational design of
carbon-based molecular electronic devices
Supramolecular Interactions in Secondary Plant Cell Walls: Effect of Lignin Chemical Composition Revealed with the Molecular Theory of Solvation
Plant biomass recalcitrance, a major
obstacle to achieving sustainable
production of second generation biofuels, arises mainly from the amorphous
cell-wall matrix containing lignin and hemicellulose assembled into
a complex supramolecular network that coats the cellulose fibrils.
We employed the statistical-mechanical, 3D reference interaction site
model with the KovalenkoāHirata closure approximation (or 3D-RISM-KH
molecular theory of solvation) to reveal the supramolecular interactions
in this network and provide molecular-level insight into the effective
ligninālignin and lignināhemicellulose thermodynamic
interactions. We found that such interactions are hydrophobic and
entropy-driven, and arise from the expelling of water from the mutual
interaction surfaces. The molecular origin of these interactions is
carbohydrateāĻ and ĻāĻ stacking forces,
whose strengths are dependent on the lignin chemical composition.
Methoxy substituents in the phenyl groups of lignin promote substantial
entropic stabilization of the ligno-hemicellulosic matrix. Our results
provide a detailed molecular view of the fundamental interactions
within the secondary plant cell walls that lead to recalcitrance
Adsorption of Bitumen Model Compounds on Kaolinite in Liquid and Supercritical Carbon Dioxide Solvents: A Study by Periodic Density Functional Theory and Molecular Theory of Solvation
The
geometry of phenanthridine, benzothiophene, tetralin, and naphthalene
representative of the heterocyclic, naphthenic, and aromatic components
of bitumen adsorbed on kaolinite is optimized using density functional
theory and periodic boundary conditions in gas phase. These bitumen
model compounds preferentially adsorb on the aluminum hydroxide surface
of kaolinite with energy decreasing in the order phenanthridine >
naphthalene > tetralin ā¼ benzothiophene. The adsorption
of
phenanthridine is strengthened by hydrogen bonding between the pyridinic
N atom and an axial hydroxyl group of kaolinite, while the rest of
the molecules adsorb through van der Waals interactions. The mechanism
of solvation in CO<sub>2</sub> and the effect of liquid and supercritical
CO<sub>2</sub> on the adsorption thermodynamics are studied using
the three-dimensional reference interaction site model theory with
the closure approximation of Kovalenko and Hirata (3D-RISM-KH) molecular
theory of solvation at 293ā333 K and 10ā30 MPa. The
CO<sub>2</sub> solvent interacts with the aluminum hydroxide surface
of kaolinite by hydrogen bonding, with the pyridinic N atom of phenanthridine
by electrostatic interactions, and with the rest of the bitumen model
compounds by hydrophobic interactions, as inferred from the 3D site
density distribution functions of CO<sub>2</sub>. The moleculeākaolinite
potentials of mean force in CO<sub>2</sub> show that the adsorption
of naphthalene and tetralin on kaolinite is substantially weakened
as the pressure is increased and the temperature is decreased. Benzothiophene
adsorption is the least sensitive to CO<sub>2</sub> temperature and
pressure changes. In liquid CO<sub>2</sub> at 30 MPa and 293 K, the
hydrocarbon molecules are weakly adsorbed and can be desorbed by CO<sub>2</sub>, while the heterocycles would remain adsorbed, suggesting
an approach for extraction of deasphalted bitumen from oil sands.
While the most favorable thermodynamic conditions for desorption are
in liquid CO<sub>2</sub>, the kinetic barrier for desorption is the
most sensitive to small changes in the temperature and pressure in
supercritical CO<sub>2</sub>, indicating that supercritical conditions
are important for desorption rate control. These results suggest that
the investigated bitumen components can be selectively desorbed from
kaolinite by controlling the temperature and pressure of the CO<sub>2</sub> solvent and agree with experimental reports on heavy oil
recovery. These insights are valuable for the development of improved
techniques for extraction of bitumen from oil sands and deasphalting
of bitumen using liquid and supercritical CO<sub>2</sub>
Adsorption of Indole on Kaolinite in Nonaqueous Media: Organoclay Preparation and Characterization, and 3D-RISM-KH Molecular Theory of Solvation Investigation
Current
oil sand mining operations in the Athabasca basin are predominantly
aqueous-based. Extracts containing large amounts of fines lead to
the formation of stable organoclay suspensions in froths giving lower
yields and greater tailing wastes and making the development of more
efficient extraction methods desirable from both economical and environmental
perspectives. We examine an indole-kaolinite system as a model for
these oil fines and their resistance to washing in nonaqueous solvents.
The prepared organoclays show indole loading exclusively on the external
surface of the clay. Micron-scaled vermicular structures, similar
to natural kaolinite, are observed. Their formation is believed to
be driven by strong adsorbateāadsorbate interactions. Indole
is the primary adsorbate, as solvent adsorption is shown to be minimal
based on both experimental and computational results. Isotherms are
constructed and parameters calculated from linear regression analysis
fitted to the BrunauerāEmmettāTeller equation. Monolayer
quantities calculated match well to the theoretical amount calculated
from surface areas measurements. Washing the organoclays with both
toluene and isopropanol results in a 50% decrease of loaded organic
material, leaving a monolayer equivalent of organic matter. The statistical-mechanical
3D-RISM-KH molecular theory of solvation is employed to perform full
sampling of solvent orientations around a kaolinite platelet and gain
insights into the preferred orientation and adsorption thermodynamics
of indole on kaolinite in toluene and heptane solvents. In its preferred
orientation, indole is hydrogen-bonded to one or two O atoms at the
aluminum hydroxide surface of kaolinite. The calculated solvation
free energy and potential of mean force for adsorption of indole and
solvents on kaolinite in solution yield the increasing adsorption
strength order of heptane < toluene < indole on the aluminum
hydroxide surface. Multilayer adsorption profiles are predicted based
on the integrated three-dimensional distribution functions of indole,
toluene, and heptane
Computational Study of the Effect of Dispersion Interactions on the Thermochemistry of Aggregation of Fused Polycyclic Aromatic Hydrocarbons as Model Asphaltene Compounds in Solution
Density functional theory (DFT),
MĆøllerāPlesset second-order
perturbation theory (MP2), and semiempirical methods are employed
for the geometry optimization and thermochemistry analysis of ĻāĻ
stacked di-, tri-, tetra-, and pentamer aggregates of the fused polycyclic
aromatic hydrocarbons (PAHs) naphthalene, anthracene, phenanthrene,
tetracene, pyrene, and coronene as well as benzene. These aggregates
(stabilized by dispersion interactions) are highly relevant to the
intermolecular aggregation of asphaltenes, major components of heavy
petroleum. The strength of ĻāĻ stacking interaction
is evaluated with respect to the Ļ-stacking distance and thermochemistry
results, such as aggregation enthalpies, entropies, and Gibbs free
energies (Ī<i>G</i><sup>298</sup>). For both Ļ-stacking
interplanar distances and thermochemistry, the ĻB97X-D functional
with an augmented damped <i>R</i><sup>ā6</sup> dispersion
correction term and MP2 are in the closest agreement with the highly
accurate spin-component scaled MP2 (SCS-MP2) method that we selected
as a reference. The Ī<i>G</i><sup>298</sup> values
indicate that the aggregation of coronene is spontaneous at 298 K
and the formation of pyrene dimers occurs spontaneously at temperature
lower than 250 K. Aggregates of smaller PAHs would be stable at even
lower temperature. These findings are supported by X-ray crystallographic
determination results showing that among the PAHs studied only coronene
forms continuous stacked aggregates in single crystals, pyrene forms
dimers, and smaller PAHs do not form ĻāĻ stacked
aggregates. Thermochemistry analysis results show that PAHs containing
more than four fused benzene rings would spontaneously form aggregates
at 298 K. Also, round-shaped PAHs, such as phenanthrene and pyrene,
form more stable aggregates than linear PAHs, such as anthracene and
tetracene, due to decreased entropic penalty. These results are intended
to help guide the synthesis of model asphaltene compounds for spectroscopic
studies so as to help understand the aggregation behavior of heavy
petroleum
Computational and Experimental Investigations of the Role of Water and Alcohols in the Desorption of Heterocyclic Aromatic Compounds from Kaolinite in Toluene
Nonaqueous
extraction is an attractive alternative to the currently
employed warm water process for extraction of bitumen from oil sands,
as it could use less energy and water. Hydroxylated cosolvents, such
as alcohols, that compete for the adsorptive clay surfaces and help
release bitumen components could help improve bitumen recovery. The
water naturally present in oil sand also affects oilāmineral
interactions. Electronic structure methods and the statistical-mechanical
3D-RISM-KH molecular theory of solvation as well as experimental desorption
measurements are employed to study the effects of water and aliphatic
alcohol cosolvents in toluene solvent on the desorption of fused pyridinic
heterocycles (ArN) from kaolinite. The geometries of phenanthridine
and acridine (representative of pyridinic heterocycles of petroleum
asphaltenes) adsorbed on the kaolinite clay surface are optimized
in periodic boundary conditions using density functional theory. The
3D-RISM-KH method is employed to calculate the solvation free energy
and potential of mean force for adsorption of the heterocycles on
kaolinite in pure and alcohol-containing toluene. The potentials of
mean force show that the adsorption of the fused pyridines on kaolinite
is stronger in pure toluene than in toluene mixed with aliphatic alcohol.
Analysis of the mechanism of desorption of phenanthridine and acridine
from kaolinite in toluene containing alcohol reveals that the alcohol
stabilizes both the pyridinic moiety and kaolinite platelet by hydrogen
bonding, thus disrupting the ArNĀ·Ā·Ā·HOāAlĀ(kaolinite)
hydrogen bond. A mechanism for retention of toluene on kaolinite is
also highlighted. Experimental studies of the desorption of fused
pyridines from an ArNākaolinite aggregate show that in water-saturated
toluene the rate of desorption of the phenanthridine from kaolinite
is twice as high as that in dry toluene. The experimental and computational
results show that water and aliphatic alcohols in toluene help desorb
pyridinic heterocycles from kaolinite, a clay mineral abundant in
the oil sands. The presented insights are valuable for understanding
the molecule-clay interactions in solution and relevant to improving
the nonaqueous extraction of bitumen from oil sand
MoleculeāSurface Recognition between Heterocyclic Aromatic Compounds and Kaolinite in Toluene Investigated by Molecular Theory of Solvation and Thermodynamic and Kinetic Experiments
Molecular recognition interactions
between kaolinite and a series of heterocyclic aromatic compounds
(HAC) representative of the N- and S-containing moieties in petroleum
asphaltene macromolecules are investigated using the three-dimensional
reference interaction site model with the KovalenkoāHirata
closure approximation (3D-RISM-KH) theory of solvation and experimental
techniques in toluene solvent. The statistical-mechanical 3D-RISM-KH
molecular theory of solvation predicts the adsorption configuration
and thermodynamics from the 3D site density distribution functions
and total solvation free energy, respectively, for adsorption of HAC
and toluene on kaolinite. Spectrophotometric measurements show that,
among the HAC studied, only acridine and phenanthridine adsorb quantitatively
on kaolinite. For these pyridinic HAC, the adsorption results fitted
to the Langmuir isotherm in the monolayer domain suggest a uniform
monolayer of HAC molecules. The 3D-RISM-KH studies predict that the
aluminum hydroxide surface of kaolinite is preferred for HAC adsorption
due to strong hydrogen bonding with the pyridinic N atoms, while the
rest of the HAC adsorb weaker. Adsorption on the silicon oxide side
is weak and delocalized, as evident from the 3D solvation free energy
density. Toluene sites effectively compete with non-hydrogen bonding
HAC, such as fused thiophenes, for the kaolinite surface. The adsorption
enthalpy and phenanthridine-acridine loading ratio are calculated
and correlated with the experimentally determined Langmuir constant
and adsorption loading. This combined experimental and computational
modeling approach is aimed to provide insight into the specific interactions
among clays, bitumen, and solvents so as to help accelerate the development
of environmentally friendly and efficient desorption systems for nonaqueous
extraction of bitumen from Oil Sands, an important unconventional
petroleum reserve