10 research outputs found
Molecular Dynamics Simulations Reveal Inhomogeneity-Enhanced Stacking of Violanthrone-78-Based Polyaromatic Compounds in <i>n</i>āHeptaneāToluene Mixtures
We
elucidated the effect of inhomogeneity in solutes on the aggregation
of our representative polyaromatic (PA) compounds through a series
of molecular dynamics simulations. Two kinds of solutes, a single
type of PA compounds and a mixture of four types of PA compounds,
were simulated in toluene, <i>n</i>-heptane, and heptol
(mixture of toluene and <i>n</i>-heptane). The geometries
of the resultant aggregates were quantified using gyradius ratios.
Our results revealed that in toluene, while a single type of PA compound
can only form short-cylinder-like aggregates, by having a solute mixture,
parallel stacking of PA cores is enhanced, leading to the formation
of one-dimensional (1D) rod-like structure. The enhanced stacking
is caused by collective arrangement of the PA molecules; i.e., PA
compounds of different types appear in an alternating manner in the
aggregate. In addition, while the aggregated geometries of a single
type of PA compounds were found to be affected by the composition
of the solvents, the existence of the 1D structure formed by mixture
seems to be insensitive to the solvents. On the other hand, the longest
range of stacking is achieved by having a small amount of toluene
(āgoodā solvent) in <i>n</i>-heptane (ābadā
solvent)
Molecular Dynamics Investigation on the Aggregation of Violanthrone78-Based Model Asphaltenes in Toluene
In
order to investigate the aggregation mechanisms of asphaltenes
in toluene, a series of molecular dynamics simulations were performed
on Violanthrone78-based model asphaltenes with different aliphatic/aromatic
ratios. Our simulation results show that the attraction between poly-aromatic
cores is the main driving force for asphaltene aggregation in toluene,
and that the extent of aggregation is independent of the aliphatic/aromatic
ratios. On the other hand, analysis of the aggregated structures indicates
that long side chains do hinder the formation of large direct parallel
stacking structures. In contrast with water as a solvent, toluene
exhibits attractive interactions with both the aliphatic and aromatic
regions of the asphaltenes, hence reducing the size and stability
of the asphaltene aggregates. Our findings help to elucidate, at a
molecular level, the different solubility behaviors of asphaltenes
in toluene and in water
Probing the Effect of Side-Chain Length on the Aggregation of a Model Asphaltene Using Molecular Dynamics Simulations
A series of molecular dynamics simulations
were performed to investigate
the effect of aliphatic side-chain length on the aggregation behavior
of a model asphaltene in water. We found that the extent of aggregation
has a nonmonotonic relationship with the side-chain length. Asphaltene
molecules with very short or very long side chains can form dense
aggregates, whereas those with intermediate chain lengths cannot.
Through analysis of the kinetics and driving forces of the aggregation,
the role of the side chains in affecting the aggregation behavior
was elucidated. Long side chains hinder the formation of parallel
stacking structures of the polyaromatic cores while also favoring
aggregation through hydrophobic association. The simulation results
reported here can be used to propose appropriate means to reduce the
extent of aggregation of asphaltene in the presence of water
A Molecular Dynamics Study of the Effect of Asphaltenes on Toluene/Water Interfacial Tension: Surfactant or Solute?
A series of molecular dynamics simulations
were performed to investigate
the effects of model asphaltenes on the toluene/water interfacial
tension (IFT) under high temperature and pressure conditions. In the
absence of model asphaltenes, the toluene/water IFT monotonically
decreases with increasing temperature, whereas, with the presence
of model asphaltenes, especially at high concentrations, such monotonic
dependence no longer holds. Furthermore, in contrast with the decreasing
trend of IFT with increasing model asphaltene concentration at low
temperature (300 K), increasing concentration at high temperature
(473 K) leads to increasing IFT. This relation can even be nonmonotonic
at moderate temperatures (373 and 423 K). Through detailed analysis
on the distribution of model asphaltenes with respect to the interface,
such complex behaviors are found to result from the delicate balance
between miscibility of toluene/water phases, solubility of model asphaltenes,
and hydrogen bonds formed between water and model asphaltenes. By
increasing the temperature, the solubility of model asphaltenes in
toluene is enhanced, leading to their transition from being a surfactant
to being a solute. The effect of pressure was found to be very limited
under all model asphaltene concentrations. Our results here present,
for the first time, a complete picture of the coupled effect of (high)
temperature and asphaltene concentration on IFT, and the methodology
employed can be extended to many other two-phase or multiphase systems
in the presence of interface-active chemicals
Probing the Adsorption of Polycyclic Aromatic Compounds onto Water Droplets Using Molecular Dynamics Simulations
A series
of molecular dynamics (MD) simulations were performed
to probe the adsorption behaviors of polycyclic aromatic compounds
(PACs) from <i>n</i>-heptane and toluene onto water droplets.
In <i>n</i>-heptane, the simulations revealed distinct adsorbed
structures of PAC molecules on the water droplets with different sizes.
In the system with a small water droplet (radius 1.86 nm), the adsorbed
PAC aggregate renders a straight one-dimensional (1D) structure; contrarily,
in the presence of a large water droplet (radius 3.10 nm), a bent
1D structure of nonzero curvature is formed. Such size effect is a
result from the delicate balance between the deformation energy required
for adsorption and the available attractions between the PAC and water
molecules. While the adsorbed structures are sensitive to the size
of the water droplet at relatively low PAC concentration in <i>n</i>-heptane, the size effect in toluene is only prominent
when the concentration of PAC molecules is sufficiently high
Probing the Effect of Salt on Asphaltene Aggregation in Aqueous Solutions Using Molecular Dynamics Simulations
The
presence of salts in different processes of oil production
has attracted wide attention because of its effects on asphaltene
aggregation, stability, interactions of emulsions, etc. In this work,
molecular dynamics simulations were employed to study the effect of
salts on aggregation of model asphaltenes. Four types of polyaromatic
compounds possessing key structural features of continental-type asphaltenes
were dispersed into NaCl solutions of different concentrations. These
models have the same polyaromatic core but different lengths for the
side chains. In the two models with relatively long side chains, the
hydrophobic association among side chains is the main driving force
for aggregation. The effect of salt on aggregation is therefore closely
tied to its influence on the hydrophobic interaction: the salt ions
promote the hydrophobic interaction at a low salt concentration while
suppressing it at a high salt concentration. For the model with an
intermediate side chain length, the hydrophobic interaction between
side chains becomes less dominant and the salt has mutual influences
on the coreācore, chaināchain, and coreāchain
interactions. For the model with the shortest side chains, although
the coreācore and coreāchain interactions are more important,
the side chains still play a role in aggregation when the salt is
present. Our results provide new insights into the fundamental understanding
of the influence of salts on the aggregation and interaction behaviors
of polyaromatic compounds in an aqueous environment
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 ĻāĻ
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 ĻāĻ 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 ĻāĻ 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., ĻāĻ 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
Competitive Adsorption of Naphthenic Acids and Polyaromatic Molecules at a TolueneāWater Interface
The
early-stage competitive co-adsorption of interfacially active
naphthenic acids (NAs) and polyaromatic (PA) molecules to a tolueneāwater
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 tolueneāwater
interface. At
low concentrations of PA (ā¼2.3 wt %) and NA (ā¼0.4 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 tolueneāwater
interface. At high NA concentrations, the adsorption of PA molecules
(ā¼5.75ā17.25 wt %) was greatly hindered by the presence
of small NA molecules (ā¼1.6ā4.8 wt %) due to the solvation
of PA nanoaggregates in the bulk. Adsorption mechanisms of PA and
NA molecules at tolueneāwater 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
Reduction of Water/Oil Interfacial Tension by Model Asphaltenes: The Governing Role of Surface Concentration
In
this work, pendant drop techniques and molecular dynamics (MD)
simulations were employed to investigate the effect of asphaltene
concentrations on the interfacial tension (IFT) of the oil/water interface.
Here, oil and asphaltene were represented by, respectively, common
organic solvents and Violanthrone-79, and two types of concentration,
i.e., bulk concentration and surface concentration, were examined.
Correlations between the IFTs from experiments and MD simulations
revealed that surface concentration, rather than the commonly used
bulk concentration, determines the reduction of oil/water IFTs. Through
analyzing the hydrogen bonding, the underlying mechanism for the IFT
reduction was proposed. Our discussions here not only enable the direct
comparison between experiments and MD simulations on the IFTs but
also help with future interfacial studies using combined experimental
and simulation approaches. The methodologies used in this work can
be extended to many other oil/water interfaces in the presence of
interfacially active compounds
Mechanistic Understanding of the Effect of Temperature and Salinity on the Water/Toluene Interfacial Tension
In
this work, a series of pendant drop measurements and molecular
dynamics (MD) simulations were performed to investigate the effects
of temperature and salinity on the interfacial tension (IFT) of water/toluene
binary systems. Both experimental measurements and theoretical simulations
demonstrated that elevating temperature decreased the IFT, while adding
salts resulted in an increment of IFT. Furthermore, it was found that
the presence of model asphaltene compound could alleviate the effects
of temperature and salinity on the IFTs. That is, in the presence
of the model asphaltene compound, the decrement effect of elevating
temperature as well as the increment effect of adding salts was reduced.
Through detailed analysis of the simulated systems, the underlying
mechanisms for the effects of temperature and salinity on the IFTs
were clarified for cases with and without the presence of the model
asphaltene. The results reported here can help to modulate the IFT
values of oil/water interfaces in petroleum processing