9 research outputs found
Molecular Dynamics Study of the Diffusivity of a Hydrophobic Drug Cucurbitacin B in Pseudo-poly(ethylene oxide‑<i>b</i>‑caprolactone) Micelle Environments
Isobaric–isothermal
molecular dynamics simulation was used
to study the diffusion of a hydrophobic drug Cucurbitacin B (CuB)
in pseudomicelle environments consisting of polyÂ(ethylene oxide-<i>b</i>-caprolactone) (PEO-<i>b</i>-PCL) swollen by
various amounts of water. Two PEO-<i>b</i>-PCL configurations,
linear and branched, with the same total molecular weight were used.
For the branched configuration, the block copolymer contained one
linear block of PEO with the same molecular weight as that of the
PEO block used in the linear configuration but with one end connecting
to three PCL blocks with the same chain length, hereafter denoted
PEO-<i>b</i>-3PCL. Regardless of the configuration, the
simulation results showed that the diffusivity of CuB was insensitive
to the water concentration up to ∼8 wt % while that of water
decreased with an increasing water concentration. The diffusivity
of CuB (10<sup>–8</sup> cm<sup>2</sup>/s) was 3 orders of magnitude
lower than that of water (10<sup>–5</sup> cm<sup>2</sup>/s).
This is attributed to the fact that CuB relied on the wiggling motion
of the block copolymers to diffuse while water molecules diffused
via a hopping mechanism. The rates at which CuB and water diffused
into PEO-<i>b</i>-PCL were twice those in PEO-<i>b</i>-3PCL because the chain mobility and the degree of swelling are higher
and there are fewer intermolecular hydrogen bonds in the case of PEO-<i>b</i>-PCL. The velocity autocorrelation functions of CuB show
that the free volume holes formed by PEO-<i>b</i>-3PCL are
more rigid than those formed by PEO-<i>b</i>-PCL, making
CuB exhibit higher-frequency collision motion in PEO-<i>b</i>-3PCL than in PEO-<i>b</i>-PCL, and the difference in frequency
is insensitive to water concentration
Prediction of the Active Layer Nanomorphology in Polymer Solar Cells Using Molecular Dynamics Simulation
Active layer nanomorphology is a
major factor that determines the
efficiency of bulk heterojunction polymer solar cells (PSCs). Synthesizing
diblock copolymers in which acceptor and donor materials are the constituent
blocks is the most recent method to control the structure of the active
layer. In the current work, a computational method is proposed to
predict the nanomorphology of the active layer consisting of a diblock
copolymer. Diblock copolymers have a tendency to self-organize and
form well-defined nanostructures. The shape of the structure depends
on the Flory–Huggins interaction parameter (i.e., χ),
the total degree of polymerization (<i>N</i>) and volume
fractions of the constituent blocks (φ<sub>i</sub>). In this
work, molecular dynamics (MD) simulations were used to calculate χ
parameters for two different block copolymers used in PSCs: P3HT-<i>b</i>-polyÂ(S<sub>8</sub>A<sub>2</sub>)-C<sub>60</sub> and P3HT-<i>b</i>-polyÂ(n-butyl acrylate-<i>stat</i>-acrylate perylene)
also known as P3HT-<i>b</i>-PPerAcr. Such calculations indicated
strong segregation of blocks into cylindrical structure for P3HT-<i>b</i>-polyÂ(S<sub>8</sub>A<sub>2</sub>)-C<sub>60</sub> and intermediate
segregation into cylindrical structure for P3HT-<i>b</i>-PPerAcr. Experimental results of P3HT-<i>b</i>-polyÂ(S<sub>8</sub>A<sub>2</sub>)-C<sub>60</sub> and P3HT-<i>b</i>-PTP4AP,
a diblock copolymer having very similar structure to P3HT-<i>b</i>-PPerAcr, validate our predictions
Effect of Inorganic Salt Contaminants on the Dissolution of Kaolinite Basal Surfaces in Alkali Media: A Molecular Dynamics Study
Owing to the increasing
popularity of using waste disposal as a
source material, inorganic salts such as CaCl<sub>2</sub> and MgCl<sub>2</sub> are frequently present in geopolymerization typically taking
place in alkali media. Such contaminants influence the dissolution
of the aluminosilicate source materials and consequently the properties
of the geopolymers made. This work is particularly aimed at elucidating
the dissolution mechanism of a well-known clay, namely, kaolinite,
in alkali media with the presence of two aforementioned aqueous medium
contaminants using molecular dynamics (MD) simulation. A series of
MD simulations was carried out on model kaolinite, with its tetrahedral
and deprotonated octahedral surfaces exposed to the alkali solutions
containing neat Na<sup>+</sup> or neat K<sup>+</sup> cations at two
concentrations, 3 and 5 M. Different concentrations of CaCl<sub>2</sub> and MgCl<sub>2</sub> contaminants (i.e., 0.1, 0.3, and 0.5 M) were
added to such alkali solutions. Atomic density profiles show that
all cations, including those from the contaminants adsorbed on the
two basal surfaces, intensify the dissociation of the aluminate groups
from the deprotonated octahedral surface. The dissociation mechanism
is somewhat similar to that of the alkali media without contaminants,
in which cations weaken the interaction between the aluminum and bridging
oxygen atoms. The number of the aluminate groups dissociated decreased
with increasing contaminant concentration. In fact, at the highest
contaminant concentration used (i.e., 0.5 M), the number of dissociated
aluminate groups was even lower than that of the system without contaminants.
This observation was attributed to the fact that most of chloride
anions remained in the bulk solution at 0.1 M but an increasing amount
of chloride ions started to cluster around the cations at 0.5 M, thereby
screening the interaction between cations and the deprotonated octahedral
surface. As a result, the screening effect reduced the number of aluminate
groups dissociated from the surface. Structural analyses of the deprotonated
octahedral surface indicated that the crystallinity of the surface
decreased with increasing simulation time and alkali solution concentration.
No dissolution of the tetrahedral surface was observed for all systems
studied
Molecular Dynamics Study of the Role of Water in the Carbon Dioxide Intercalation in Chloride Ions Bearing Hydrotalcite
Molecular dynamics
simulation was used to study the role of water
in the intercalation of CO<sub>2</sub> with a model Mg–Al–Cl-hydrotalcite
mineral at ambient pressure and temperature. The ClayFF force field
was used along with a model Mg–Al–Cl-hydrotalcite containing
different amounts of water (H<sub>2</sub>O) and carbon dioxide (CO<sub>2</sub>) molecules in its interlayer spacing. It was observed that
high CO<sub>2</sub> content, say 3.85 mmol g<sup>–1</sup>,
could be achieved at low water concentrations or even without the
presence of water. However, high water concentrations (e.g., 2 H<sub>2</sub>O molecules/hydrotalcite unit cell, the maximum allowed water
concentration observed experimentally) could also yield similar CO<sub>2</sub> content, but in this case, the presence of water led to a
significant interlayer spacing expansion (from 23.0 Ã… (no water)
to 28.5 Ã…). The expansion was likely due to the change in the
orientation distribution of the CO<sub>2</sub> molecules. Analyzing
the orientation of CO<sub>2</sub> molecules revealed that they preferred
to orientate parallel to the mineral surface at low water concentrations.
However, as water concentration increased, CO<sub>2</sub> molecules
exhibited a wider range of orientations with a significant fraction
of them orienting more or less perpendicular to the mineral surface,
especially at high CO<sub>2</sub> contents. The observed change in
the orientation of CO<sub>2</sub> was attributed to the dipole interaction
between H<sub>2</sub>O and CO<sub>2</sub> molecules and the reduced
interaction between CO<sub>2</sub> and the hydroxyl groups on hydrotalcite.
Also, it was observed that water molecules formed extensive hydrogen
bond networks. All of the above findings seem to explain the contradicting
results reported in the literature that water is needed under certain
conditions to increase the amount of CO<sub>2</sub> captured by hydrotalcites.
Here, we showed that high amounts of CO<sub>2</sub> can be intercalated
with the presence of water
Underwater Adhesion of a Stimuli-Responsive Polymer on Highly Oriented Pyrolytic Graphite: A Single-Molecule Force Study
Exploration
aiming to understand how a single polymer chain interacts
with interested solid–water interface and its dependence on
the environmental stimulus is critical, as it is important for a variety
of scientific research and practical applications, such as underwater
adhesives or smart systems for controlled attachment/detachment. Here,
by single molecule force spectroscopy (SMFS), results on underwater
adhesion of a single stimuli-responsive polymer chain on a model hydrophobic
surface are reported. Salt concentration was used as an effective
stimulus for the transition of the stimuli-responsive polymer from
hydrophilic (solvophilic) hydrated state to hydrophobic (solvophobic)
state in SMFS experiment. The probed equilibrium single-molecule adhesion
force that changed from 45 to 76 pN with increasing salt concentration
were free of other interactions that are responsible for cohesions,
indicating that solvent quality is critical in determining the strength
of single-molecule interfacial adhesion. Moreover, the derived simple
quantitative thermodynamic model by combining single-molecule detachment
mechanics with hydration free energy illustrated that higher single-molecule
adhesion force was resulted from higher solvation/hydration free energy.
The experimental study and theoretical analysis presented in this
work quantitatively revealed the role of hydrophobic and more general
solvophobic interactions in single-molecule level and shed light on
the molecular structure optimization for desired applications, such
as underwater adhesives or smart interfaces
Probing Single-Molecule Adhesion of a Stimuli Responsive Oligo(ethylene glycol) Methacrylate Copolymer on a Molecularly Smooth Hydrophobic MoS<sub>2</sub> Basal Plane Surface
Molybdenum
disulfide (MoS<sub>2</sub>) has been receiving increasing
attention in scientific research due to its unique properties. Up
to now, several techniques have been developed to prepare exfoliated
nanosize MoS<sub>2</sub> dispersions to facilitate its applications.
To improve its desired performance, as-prepared MoS<sub>2</sub> dispersion
needs further appropriate modification by polymers. Thus, understanding
polymer–MoS<sub>2</sub> interaction is of great scientific
importance and practical interest. Here, we report our results on
molecular interactions of a biocompatible stimuli-responsive copolymer
with the basal plane surface of MoS<sub>2</sub> determined using single
molecule force spectroscopy (SMFS). Under isothermal conditions, the
single-molecule adhesion force of oligoÂ(ethylene glycol) methacrylate
copolymer was found to increase from 50 to 75 pN with increasing NaCl
concentration from 1 mM to 2 M, as a result of increasing hydrophobicity
of the polymers. The theoretical analysis demonstrated that single-molecule
adhesion force is determined by two contributions: the adhesion energy
per monomer and the entropic free energy of the stretched polymer
chain. Further data analysis revealed a significant increase in the
adhesion energy per monomer with a negligible change in the other
contribution with increasing salt concentration. The hydrophobic attraction
(HA) was found to be the main contribution for the higher adhesion
energy in electrolyte solutions of higher NaCl concentrations where
the zero-frequency of van der Waals interaction were effectively screened.
The results illustrate that oligoÂ(ethylene glycol) methacrylate copolymer
is a promising polymer for functionalizing MoS<sub>2</sub> and that
one can simply change the salt concentration to modulate the single-molecule
interactions for desired applications
Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a Toluene–Water Interface
The
early-stage competitive co-adsorption of interfacially active
naphthenic acids (NAs) and polyaromatic (PA) molecules to a toluene–water
interface from the bulk toluene phase was studied using molecular
dynamics (MD) simulation. The NA molecules studied had the same polar
functional group but different cycloaliphatic nonpolar tails, and
a perylene bisimide (PBI)-based molecule was used as a representative
PA compound. The results from our simulations suggest that the size
and structural features of NA molecules greatly influence the interfacial
activity of PA molecules and partitioning of NA molecules at the toluene–water
interface. At
low concentrations of PA (∼2.3 wt %) and NA (∼0.4 wt
%) molecules, NA molecules containing large cycloaliphatic rings (e.g.,
four rings) or with a very long aliphatic tail (e.g., carbon chain
length of 14) were observed to impede the migration of PA molecules
to the interface, whereas small NA molecules containing two cycloaliphatic
rings had little effect on the adsorption of PA molecules at the toluene–water
interface. At high NA concentrations, the adsorption of PA molecules
(∼5.75–17.25 wt %) was greatly hindered by the presence
of small NA molecules (∼1.6–4.8 wt %) due to the solvation
of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and
NA molecules at toluene–water interfaces were clarified through
a detailed analysis on the interactions among different species in
the system. The
results obtained from this work provide insights into designing appropriate
chemical demulsifiers or co-demulsifiers for breaking water-in-oil
emulsions of great industrial applications
Role of Naphthenic Acids in Controlling Self-Aggregation of a Polyaromatic Compound in Toluene
In this work, a series of molecular
dynamics simulations were performed
to investigate the effect of naphthenic acids (NAs) in early stage
self-assembly of polyaromatic (PA) molecules in toluene. By exploiting
NA molecules of the same polar functional group but different aliphatic/cycloaliphatic
nonpolar tails, it was found that irrespective of the presence of
the NA molecules in the system, the dominant mode of π–π
stacking is a twisted, offset parallel stacking of a slightly larger
overlapping area. Unlike large NA molecules, the presence of small
NA molecules enhanced the number of π–π stacked
PA molecules by suppressing the hydrogen bonding interactions among
the PA molecules. Smaller NA molecules were found to have a higher
tendency to associate with PA molecules than larger NA molecules.
Moreover, the size and distribution of π–π stacking
structures were affected to different degrees by changing the size
and structural features of the NA molecules in the system. It was
further revealed that the association between NA and PA molecules,
mainly through hydrogen bonding, creates a favorable local environment
for the overlap of PA cores (i.e., π–π stacking
growth) by depressing the hydrogen bonding between PA molecules, which
results in the removal of some toluene molecules from the vicinity
of the PA molecules
Single-Molecule MoS<sub>2</sub>–Polymer Interaction and Efficient Aqueous Exfoliation of MoS<sub>2</sub> into Single Layer
Single molecule
force spectroscopy (SMFS) was utilized to
study single-molecule interactions between synthesized polymers and
MoS<sub>2</sub> surfaces that provide the guidance to determine candidate
polymers for efficient aqueous exfoliation of bulk MoS<sub>2</sub> into single-layer nanosheets. The technique allowed the direct quantification
of the adhesion force of single polymer molecules with both basal
and edge surfaces of MoS<sub>2</sub>. Compared with two studied neutral
polymers, highly water-soluble cationic polyÂ(vinylbenzyl trimethyl
ammonium chloride) (PVBTA) was shown to be more promising for MoS<sub>2</sub> exfoliation as a result of strong single-molecule interactions
with both basal and edge surfaces of MoS<sub>2</sub>. In 1 mM NaCl
solution of pH around 5.5, the measured single-molecule adhesion forces
on basal and edge surfaces were around 59 and 51 pN, respectively.
Such strong adhesion force led to high performance of PVBTA in exfoliating
MoS<sub>2</sub> bulk material into single-layer sheets. Compared with
reported approaches in the literatures, an order of magnitude higher
concentration of single-layer MoS<sub>2</sub> sheets in water was
prepared using an order of magnitude less treatment time in this study.
This research demonstrated that SMFS is a powerful tool to select
appropriate surface-active polymers for effective exfoliation of MoS<sub>2</sub>, which could be applied to a variety of practical applications,
such as water adhesion and dispersing