A. Micelle
Structure Relaxation in Aqueous Media
Particular emphasis is placed on
understanding how surfactants aggregate and form the micellar
structure. The surfactants are in an aqueous medium that is modeled
with empirical potentials. The concentration of surfactants in the
system is 0.23 M. The cmc of C12TAB surfactants in aqueous
media is 16 mM. Thus, the concentration is much above the cmc and
spherical micelles are expected. The three-dimensional periodic
boundary conditions extend 70 Å in each direction and are used to
mimic an infinite aqueous medium around the initial surfactant micelle.
The temperature of the systems is maintained at 300 K by application of
the velocity rescaling method to all the atoms in the system. All
structures are allowed to evolve to a lower-energy configuration under
equilibrium conditions. Total simulation time ranges from 0.65 to 1.3
nanoseconds. The simulations predict that the micelle structure in
water is compact and either spherical or elliptical in shape.
(1) Clusters
-
(a) Initial
configuration setup
A single cluster of 24 surfactants
was initially built in a spherical geometry. This cluster is replicated
and thus two clusters are placed 10 Å apart in aqueous media. In
the left cluster, the blue spheres represent the head groups N+(CH3)3
and the green spheres represents the tail molecules CH3/CH2.
In the right cluster, the red spheres represent the head groups N+(CH3)3
and the yellow spheres represents the tail molecules CH3/CH2.
The water molecules are not shown for clarity.
(b) Structural evolution
Initially, exchange of surfactants
occurs between the two clusters and eventually the two clusters
approach each other and eventually merge into one another to form a
single, larger micelle. The micelle thus formed has a spherical shape,
with all the head groups on the surface of the structure and the tails
randomly arranged inside the structure. The aggregate is thus densely
packed. No surfactants appear to move away from the micelle and no
water molecules find their way inside the micelle interior over the
course of the simulation. Thus the growth mechanism of micelle follows
Smoluchowski model where cluster-cluster coalesce and form a bigger
cluster.
(2) Spherical micelle
-
(a) Initial
configuration setup
To model the micelle structure of C12TAB
surfactants in water, 48 surfactants are initially placed such that
they are close to each other in a spherical fashion in an aqueous
medium. The blue spheres represent the head groups N+(CH3)3
and the green spheres represents the tail molecules CH3/CH2.
The water molecules are not shown for clarity.
(b) Structural evolution
Cationic and hydrophilic head
groups are outside the structure shielding the hydrophobic chains
inside the structure, while the surfactant tails are densely packed in
the micelle interior. The aggregate is thus a densely packed structure.
No surfactants separate from the main structure and no water molecules
find their way inside the micelle interior over the course of the
simulation. Interestingly, the micelle structure keeps transforming its
shape into spherical and elliptical shapes, which in agreement with
experimental AFM images.
(3) Monolayer
-
(a) Initial
configuration setup
A monolayer of 48 surfactants where
the all the head groups point in the same direction is initially built
and placed in aqueous media. The blue spheres represent the head groups
N+(CH3)3 and the green spheres
represents the tail molecules CH3/CH2. The water
molecules are not shown for clarity.
(b) Structural evolution
Head groups start repelling each
other due to Columbic repulsion. Tails start coming together due to
hydrophobic attraction, avoiding the water molecules around them. The
structure starts swelling on the head group side and narrowing on tails
side. Due to hydrophobic repulsion between tails and water molecules
and hydrophilic attraction between head groups and water molecules,
head groups start wrapping around the structure and start appearing on
the other side of all the head groups. Tails of surfactant begin to
randomize within the structure and eventually a spherical micelle
results at the end of the simulation. The aggregate is thus a densely
packed structure. No surfactants separate from the main structure and
no water molecules find their way inside the micelle interior over the
course of the simulation.
(4) Bilayer
-
(a) Initial
configuration setup
Bilayer of 48 surfactants where the
head groups point in alternating directions is initially built and
placed in aqueous media. The blue spheres represent the head groups N+(CH3)3
and the green spheres represents the tail molecules CH3/CH2.
The water molecules are not shown for clarity.
(b) Structural evolution
Head groups start repelling each
other due to columbic repulsion. Tails start coming together due to
hydrophobic attraction in the presence of water molecules around them.
The structure starts swelling from the head group side at both ends of
bilayer. Due to hydrophobic repulsion between tails and water molecules
and hydrophilic attraction between head groups and water molecules,
head groups start wrapping around the structure and start appearing all
around the surface of the structure. Tails of surfactant begin to
randomize within the structure and eventually, a spherical micelle
results at the end of the simulation. The aggregate is thus a densely
packed structure. No surfactants separate from the main structure and
no water molecules find their way inside the micelle interior over the
course of the simulation. The evolution of the bilayer to spherical
micelle appears faster than the monolayer.
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B. Absorption
of Micelle on Surfaces
The focus is on how micelles change
shape at high concentrations in aqueous media and in the presence of
hydrophilic and hydrophobic surfaces. The periodic boundary conditions
are around the size of the substrate and are used to mimic an infinite
surface with micelles adjacent to each other with spacing in infinite
directions. The temperature of the systems is maintained at 300 K by
application of the velocity rescaling method to all the atoms in the
system except the surface atoms which are held rigid. In the presence
of a hydrophilic surface of silica, the structure evolves into a flat
elliptical shape with heads of surfactants attracted toward the surface
due to hydrophilic interaction, in agreement with experimental
findings. In the presence of a hydrophobic surface of graphite, the
structure evolves into a hemi-spherical shape with tails of surfactants
lying on the surface due to hydrophobic interaction, also in agreement
with experimental findings.
(1) Hydrophilic Surface -
Silica
-
(a) Initial
configuration setup
A micelle of 48 surfactants that
was relaxed in aqueous media was used. The aggregate was placed 6
Å from the negatively charged silica surface with dimensions of
60 Å on each side within the plane of the surface and a slab
thickness of 5 Å. The blue atoms represent the head groups N+(CH3)3
and the green atoms represent the tail molecules CH3/CH2.
Surface atoms (Si and O) are represented by yellow (Si) and red (O).
The water molecules are not shown in these figures for clarity.
(b) Structural evolution
The simulation predicts that the
round micelle adsorbs onto the silica surface without any connections
to adjacent micelles through the applied periodic boundary conditions,
which is consistent with experimental data. As the simulation evolves,
the head groups are columbically attracted to the oppositely charged
sites on the silica surface and the structure flattens into an
elliptical shape.
(2) Hydrophobic Surface -
Graphite
-
(a) Initial
configuration setup
The simulation was carried out with
a monolayer of 48 surfactants placed on a graphite surface. The
monolayer was placed 6 Å from the hydrophobic graphite surface
(single sheet) with dimensions of 60 Å on each side within the
plane of the surface. The blue atoms represent the head groups N+(CH3)3
and green atoms represents the tail molecules CH3/CH2.
The gray atoms represent C of graphite surface. The water molecules are
not shown in these figures for clarity.
(b) Structural evolution
This simulation was designed to
study the discrete self-aggregation of surfactants formed on graphite
surfaces and to better understand experimental data from AFM. Results
AFM technique indicates that C12TAB surfactants form
hemi-cylindrical micelles on the hydrophobic graphite surface with
chains of surfactants lied down on surface due to strong hydrophobic
attraction. The simulation predicts that the surfactants in the
monolayer start adsorbing with chains of surfactants lying down on the
surface due to their strong hydrophobic attraction, leaving head group
standing up towards the water molecules. Finally, the adsorbed
structure takes the shape of hemi-cylindrical micelle.
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C.
Indentation of Micelles
The aim is to study at what force
micellar structures break apart during indentation of micelle-covered
surfaces with a proximal probe microscope tip. The temperature of the
systems is maintained at 300 K by application of the velocity rescaling
method to all the atoms in the system. The simulated indentation of the
micelle/silica system causes the micelle to break apart at an
indentation force about 1 nN and form a surfactant monolayer. The
predicted force curve is in excellent agreement with experimental
measurements. The simulated indentation of micelle/graphite system
causes breakage of micelle at an indentation force of about 1.25 nN,
which is slightly above the force predicted to break the micelle
structure on silica (1 nN). This difference can be explained by a
stronger interaction (hydrophobic) between the absorbed structure and
the graphite substrate.
(1) Silica Indentation
-
(a) Initial
configuration setup
A micelle of 48 surfactants that
was relaxed in aqueous media was used. The aggregate was placed 6
Å from the substrate (bottom silica surface) with dimensions of
60 Å on each side within the plane of the surface and a slab
thickness of 5 Å. 7 Å from the micelle, the indentor (top
silica surface) was placed and lowered at a constant velocity of 25
m/sec (0.00025 A/fs). The blue atoms represent the head groups N+(CH3)3
and the green atoms represent the tail molecules CH3/CH2.
Surface atoms (Si and O) are represented by yellow (Si) and red (O).
The water molecules are not shown in these figures for clarity.
(b) Mechanical
properties
Several AFM experiments have been
carried out to study the mechanical properties of adsorbed micelles at
liquid/silica interfaces. Two hypotheses are looked into. The first is
that the tip breaks the structure (the mechanical strength of the
micelle is measured to be about 1.5 nN). The second is that the
adsorbed micelle structure just slips away from the location between
tip and the surface, which allows the tip to be attracted to the
surface. The experimental data does not provide any information on what
is occurring at tip-surface distances of 5 Å to 26 Å. To
shed more light on these interpretations, an MD simulation is used to
indent the adsorbed micelle structure on silica with another silica
surface as an indentor.
The simulations predict that the
micelle structure breaks apart when the distance between the indentor
and the surface is 26 Å. This result is in good agreement with
experimental findings. The simulations also conclusively show that the
tip indentation process breaks the micelle structure (see peaks B, C,
D). The force felt by the indentor is calculated with respect to the
distance between the indentor (top silica surface) and the substrate
(bottom silica surface). Experimental data shows that the force
required to break the structure is 1.5 nN while MD simulations predict
the force required to break apart the micelle is 1 nN. These results
are in excellent qualitative agreement. The difference in the
quantitative values can be explained by the smaller AFM tip (diameter
of about 5 nm) used in simulations relative to the much larger
experimental tips.
The MD simulation of the
indentation process presented also shows the presence of small peaks in
the force curve after the micelle structure breaks apart. The presence
of peak C is explained by the fact that once the structure is broken
the surfactant monomers are still between the tip and the surface and
are not able to escape. Consequently, after the micelle breaks apart
the force increases.
(2) Graphite Indentation
-
(a) Initial
configuration setup
The configuration is a monolayer of
48 surfactants placed on a graphite surface. The monolayer was placed 6
Å from the substrate (bottom graphite single sheet) with
dimensions of 60 Å on each side within the plane of the surface.
7 Å from the monolayer, the indentor (top graphite single sheet)
was placed and lowered at a constant velocity of 50 m/sec (0.00050
A/fs). The blue spheres represent the head groups N+(CH3)3
and green spheres represents the tail molecules CH3/CH2.
The gray spheres represent C of graphite surface. The water molecules
are not shown in these figures for clarity.
(b) Mechanical
properties
The indentation results predict
that the micelle structure breaks when the distance between the
indentor and the graphite surface is 22.5 Å as shown in peak A.
This distance is smaller than in the case of indentation of micelle on
silica, 26 Å. This difference can be explained by the difference
in height between hemi-cylindrical structure of micelle on graphite and
elliptical structure of micelle on silica. Indentation results indicate
that the micelle structure breaks; the structure is not slipping away
from the region between indentor and substrate. The micelle structure
breaks at an indentation force of 1.25 nN, which is slightly above the
force predicted to break the micelle structure on silica, 1 nN. This
difference can be explained by a stronger interaction (hydrophobic)
between the absorbed structure and the graphite substrate. The highest
repulsion felt by the indentor due to trapped surfactants is
approximately 13 nN, as shown in peak B. This is much higher than the
repulsion, 2.25 nN, felt by silica indentor due to surfactants trapped
between indentor and silica substrate, as shown in peak D. This can be
explained by the stronger interaction of surfactant tail adsorbed on
graphite surface hydrophobically.
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We provide open source code for MD simulations: Micelle MD code
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