16 research outputs found
Specific Ion Effects on the Self-Assembly of Ionic Surfactants: A Molecular Thermodynamic Theory of Micellization with Dispersion Forces
The self-assembly of amphiphilic
molecules is a key process in
numerous biological and chemical systems. When salts are present,
the formation and properties of molecular aggregates can be altered
dramatically by the specific types of ions in the electrolyte solution.
We present a molecular thermodynamic model for the micellization of
ionic surfactants that incorporates quantum dispersion forces to account
for specific ion effects explicitly through ionic polarizabilities
and sizes. We assume that counterions are distributed in the diffuse
region according to a modified PoissonâBoltzmann equation and
can reach all the way to the micelle surface of charge. Stern layers
of steric exclusion or distances of closest approach are not imposed
externally; these are accounted for through the counterion radial
distribution profiles due to the incorporation of dispersion potentials,
resulting in a simple and straightforward treatment. There are no
adjustable or fitted parameters in the model, which allows for a priori
quantitative prediction of surfactant aggregation behavior based only
on the initial composition of the system and the surfactant molecular
structure. The theory is validated by accurately predicting the critical
micelle concentration (CMC) for the well-studied sodium dodecyl sulfate
(SDS) surfactant and its alkaline-counterion derivatives in mono-
and divalent salts, as well as the molecular structure parameters
of SDS micelles such as aggregation numbers and micelle surface potential
Molecular Thermodynamic Modeling of Reverse Micelles and Water-in-Oil Microemulsions
Surfactant aggregation plays an important
role in a variety of
chemical and biological nanoscale processes. On a larger scale, using
small amounts of amphiphiles compared to large volumes of bulk-phase
modifiers can improve the efficiency and reduce the environmental
impact of many chemical and industrial processes. To model ternary
mixtures of polar, nonpolar, and amphiphilic molecules, we develop
a molecular thermodynamic theory for polydisperse water-in-oil (W/O)
droplet-type microemulsions and reverse micelles based on global minimization
of the Gibbs free energy of the system. The incorporation of size
polydispersity into the theoretical formulation has a significant
effect on the Gibbs free energy landscape and allows us to accurately
predict micelle size distributions and micelle size variation with
composition. Results are presented for two sample ionic surfactant/water/oil
systems and compared with experimental data. By predicting the structural
and compositional characteristics of w/o microemulsions, the molecular
thermodynamic approach provides an important bridge between the modeling
of ternary systems at the molecular and the macroscopic level
Induced Charge Density and Thin Liquid Film at Hydrate/Methane Gas Interfaces
The hydrate/methane gas interface
is studied by molecular dynamics
simulations. Below the hydrate melting temperature a thin liquid film
forms with an associated surface charge density and electrostatic
potential. The thickness of the thin liquid film, the charge density,
and electrostatic potential at the hydrate/gas interface are established
at different subcooling temperatures for the first time. The hydrate
interface has mixed polarity, being predominantly positive. A comparison
is made with the ice/methane interface, which reveals similarities
and differences in the induced charge density. The thin liquid film
and the induced charge density have important implications for the
interfacial properties of methane hydrates
Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates
Adsorption of functional
molecules on the surface of hydrates is
key in the understanding of hydrate inhibitors. We investigate the
adsorption of a hydrocarbon chain, nonionic and ionic surfactants,
and ions at the hydrateâaqueous interface. Our results suggest
a strong connection between the water ordering around solutes in bulk
and the affinity for the hydrates surface. We distinguish two types
of water ordering around solutes: (i) hydrophobic hydration where
water molecules form a hydrogen bond network similar to clathrate
hydrates, and (ii) ionic hydration where water molecules align according
to the polarity of an ionic group. The nonionic surfactant and the
hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed
on the hydrate surface. Adsorption of ions and the ionic headgroups
on the hydrate surface is not favorable because ionic hydration and
the hydrogen bond structure of hydrates are incompatible. The nonionic
surfactant is adsorbed by the headgroup and tail while adsorption
of the ionic surfactants is not favorable through the head. Water
ordering is analyzed using the hydrogen bond and tetrahedral density
profiles as a function of the distance to the chemical groups. The
adsorption of solutes is studied through the free energy profiles
as a function of the distance to the hydrate surface. Salt lowers
the melting temperature of hydrates, disrupts hydrophobic hydration,
reduces the solubility of solutes in the aqueous solution, and increases
the propensity of solutes to be adsorbed on hydrate surfaces. Our
studies are performed by the unbiased and steered molecular dynamics
simulations. The results are in line with experiments on the effect
of salt and alkanes in hydrate antiagglomeration
Effect of Water on Deposition, Aggregate Size, and Viscosity of Asphaltenes
The aggregation and structure of
polar molecules in nonpolar media may have a profound effect on bulk
phase properties and transport. In this study, we investigate the
aggregation and deposition of water and asphaltenes, the most polar
fraction in petroleum fluids. In flow-line experiments, we vary the
concentration of water from 500 up to 175â000 ppm and provide
the evidence for clear changes in asphaltene deposition. Differential
interference contrast (DIC) microscopy and dynamic light scattering
(DLS) are used to measure the size of the aggregates. Rheological
measurements are performed to get fixed ideas on the structural changes
that water induces at different concentrations. This study demonstrates
the significant effect of water on asphaltene aggregation and deposition
and explores the molecular basis of waterâasphaltene interaction.
Our aggregate size measurements show that while asphaltene molecules
increase the solubilization of water, there is no increase in the
aggregate size. Our aggregation size measurements are different from
the reports in the literature
Controlling Nonpolar Colloidal Asphaltene Aggregation by Electrostatic Repulsion
While aromatic chemicals are applied to petroleum oil
systems to
thermodynamically prevent asphaltene precipitation, amphiphilic dispersants
can truncate the precipitation process and create stable suspensions
of asphaltene colloids in the submicrometer size range. Bulk sedimentation
and dynamic light scattering have shown that stabilizing dispersants
inhibit colloidal asphaltene aggregation at approximately the same
concentration as is needed to effectively slow bulk sedimentation.
At the same time, these same types of dispersants can alter the electrostatic
properties of colloidal asphaltenes in nonpolar suspensions. While
electrostatic stabilization has been linked to aggregation dynamics
in several types of colloidal systems, both aqueous and nonpolar,
the complete linkage between electrostatic interactions and aggregation
inhibition has yet to be shown in colloidal asphaltene suspensions.
In this work, we present dynamic light scattering and electrophoresis
measurements in colloidal asphaltene suspensions, using three different
petroleum fluids and a dispersant which truncates asphaltene precipitation
and colloidal aggregation by enabling uniform electrostatic charging
at the colloidal asphaltene surface
Hydrophobic Hydration and the Effect of NaCl Salt in the Adsorption of Hydrocarbons and Surfactants on Clathrate Hydrates
Adsorption of functional
molecules on the surface of hydrates is
key in the understanding of hydrate inhibitors. We investigate the
adsorption of a hydrocarbon chain, nonionic and ionic surfactants,
and ions at the hydrateâaqueous interface. Our results suggest
a strong connection between the water ordering around solutes in bulk
and the affinity for the hydrates surface. We distinguish two types
of water ordering around solutes: (i) hydrophobic hydration where
water molecules form a hydrogen bond network similar to clathrate
hydrates, and (ii) ionic hydration where water molecules align according
to the polarity of an ionic group. The nonionic surfactant and the
hydrocarbon chain induce hydrophobic hydration and are favorably adsorbed
on the hydrate surface. Adsorption of ions and the ionic headgroups
on the hydrate surface is not favorable because ionic hydration and
the hydrogen bond structure of hydrates are incompatible. The nonionic
surfactant is adsorbed by the headgroup and tail while adsorption
of the ionic surfactants is not favorable through the head. Water
ordering is analyzed using the hydrogen bond and tetrahedral density
profiles as a function of the distance to the chemical groups. The
adsorption of solutes is studied through the free energy profiles
as a function of the distance to the hydrate surface. Salt lowers
the melting temperature of hydrates, disrupts hydrophobic hydration,
reduces the solubility of solutes in the aqueous solution, and increases
the propensity of solutes to be adsorbed on hydrate surfaces. Our
studies are performed by the unbiased and steered molecular dynamics
simulations. The results are in line with experiments on the effect
of salt and alkanes in hydrate antiagglomeration
Nucleation of Methane Hydrates at Moderate Subcooling by Molecular Dynamics Simulations
Methane hydrates are crystalline
structures composed of cages of
hydrogen-bonded water molecules in which methane molecules are trapped.
The nucleation mechanisms of crystallization are not fully resolved,
as they cannot be accessed experimentally. For methane hydrates most
of the reported simulations on the phenomena capture some of the basic
elements of the full structure. In few reports, formation of crystalline
structures is reached by imposing very high pressure, or dynamic changes
of temperature, or a pre-existing hydrate structure. In a series of
nanoscale molecular dynamics simulations of supersaturated waterâmethane
mixtures, we find that the order of the crystalline structure increases
by decreasing subcooling. Crystalline structures I and II form and
coexist at moderate temperatures. Crystallization initiates from the
spontaneous formation of an amorphous cluster wherein structures I,
II, and other ordered defects emerge. We observe the transient coexistence
of sI and sII in agreement with experiments. Our simulations are carried
out at high methane supersaturation. This condition dramatically reduces
the nucleation time and allows simulating nucleation at moderate subcooling.
Moderate temperatures drive hydrates to more ordered structures
Contact Angle, Liquid Film, and LiquidâLiquid and LiquidâSolid Interfaces in Model OilâBrineâSubstrate Systems
Oilâwaterâsubstrate
wettability is of prime importance
in most branches of science and technology, from biology and nanomaterials
to geology and petroleum science. Wetting is a three-phase interaction
phenomenon as expressed in Youngâs equation. Microscopically
wetting is from the fluidâsubstrate interactions and surfaces
are designated as lyophilic and lyophobic (fluid-wet and nonwet).
Here we investigate the microscopic mechanisms of wettability changes
by salt concentration in oilâwaterâmineral substrate
systems. A model oil droplet (<i>n</i>-decane) placed in
an aqueous electrolytic solution next to a solid substrate surface
(muscovite mica) is simulated. A thin water layer between oil and
the substrate regulates the oilâsubstrate interaction. We find
that at zero and low salt concentrations, the oil adsorption on the
hydrophilic mineral substrate is stabilized by a thin layer of water
giving rise to a nonzero contact angle (partial oil wetting). As the
salt concentration increases ionic adsorption and the water layer
thickness increase reducing the oilâsubstrate wettability.
Ions adsorb unsymmetrically on the substrate and promote water adsorption
into the water layer. Ionic adsorption is higher away from the droplet
than under the droplet. Our contact angles by molecular dynamics simulations
are in agreement with experimental measurements
Tunable Substrate Wettability by Thin Water Layer
In oilâwaterâmineral
substrate systems, we show that the contact angle can be tuned by
ionic structures in the water layer confined between an oil droplet
and the substrate. We perform molecular dynamics simulations of a
complex oil droplet in a NaCl aqueous solution on a mica surface;
the oil is a mixture of <i>n</i>-decane and surfactant molecules.
The surfactant head contains an OH group and an aromatic ring. A thin
water layer between the oil droplet and the substrate and ionic stratification
regulate the wetting behavior. The concentration of salt ions in the
thin film is nonmonotonic; it first increases, then decreases, and
starts increasing again as the salt concentration in the bulk increases.
On the other hand, the surfactant head adsorption in the thin film
first increases as the bulk salt concentration increases. Then, it
decreases with further increase in the bulk salt concentration. The
change of contact angle with salt concentration also shows a nonmonotonic
behavior; the contact angle is first nearly constant to a low salt
concentration of 0.1 wt % NaCl. Then, it decreases sharply as the
salt concentration increases from 0.1 to 1.1 wt % NaCl. A reverse
trend in contact angle follows with further salt concentration increase.
The nonmonotonic trend unlike the monotonic trend of interfacial tension
with salt concentration is in line with recent measurements of contact
angle of oilâbrineâsubstrate systems. A sharp increase
of surfactant head adsorption in the thin film, the decrease of ion
adsorption, and the minimum of contact angle are all related. This
is the first report of such correlations with change of wetting in
the brineâcomplex oilâmineral substrate predicted from
molecular simulations