Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface
Molecular dynamics simulations to study the behaviour of anionic (Sodium Dodecylsulfate, SDS) and nonionic (Monooleate of Sorbitan, SPAN80) surfactants close to a SiO₂ (silicon dioxide) surface were carried out. Simulations showed that a water layer was first adsorbed on the surface and then the sur...
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| Zitieren: | Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface / E. Nunez-Rojas, H. Dominguez // Condensed Matter Physics. — 2016. — Т. 19, № 1. — С. 13602: 1–8. — Бібліогр.: 30 назв. — англ. |
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nasplib_isofts_kiev_ua-123456789-1557812025-02-10T01:31:02Z Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface Чисельнi дослiдження поведiнки анiонних та неiонних сурфактантiв на границi роздiлу мiж двоокисом кремнiю (SiO₂) i водою Nunez-Rojas, E. Dominguez, H. Molecular dynamics simulations to study the behaviour of anionic (Sodium Dodecylsulfate, SDS) and nonionic (Monooleate of Sorbitan, SPAN80) surfactants close to a SiO₂ (silicon dioxide) surface were carried out. Simulations showed that a water layer was first adsorbed on the surface and then the surfactants were attached on that layer. Moreover, it was observed that water behaviour close to the surface influenced the surfactant adsorption since a semi-spherical micelle was formed on the SiO₂ surface with SDS molecules whereas a cylindrical micelle was formed with SPAN80 molecules. Adsorption of the micelles was conducted in terms of structural properties (density profiles and angular distributions) and dynamical behaviour (diffusion coefficients) of the systems. Finally, it was also shown that some water molecules moved inside the solid surface and located at specific sites of the solid surface. Виконано моделювання поведiнки анiонних (додецилсульфат натрiю, SDS) та неiонних (монолеат сорбiтану, SPAN80) сурфактантiв поблизу поверхнi SiO₂ (двооксиду кремнiю) методом молекулярної динамiки. Спостерiгаємо, що шар води спочатку адсорбується на поверхнi, а тодi сурфактанти приєднуються до цього шару. Бiльш того, спостережено, що поведiнка води поблизу поверхнi впливає на адсорбцiю суфрактантiв, оскiльки напiв-сферична мiцела утворюється на поверхнi SiO₂ з SDS молекулами, в той час, як цилiндрична мiцела формується у випадку молекул SPAN80. Адсорбцiя мiцел описана структурними властивостями (профiлi густини i кутовi розподiли), а також i в термiнах коефiцiєнтiв дифузiї. Показано, що деякi молекули води проникають всередину твердої поверхнi. HD acknowledges support from DGAPA-UNAM-Mexico and Conacyt-Mexico for sabbatical scholarships. The authors acknowledge support from DGTIC-UNAM for the supercomputer facilities. 2016 Article Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface / E. Nunez-Rojas, H. Dominguez // Condensed Matter Physics. — 2016. — Т. 19, № 1. — С. 13602: 1–8. — Бібліогр.: 30 назв. — англ. 1607-324X DOI:10.5488/CMP.19.13602 arXiv:1603.02168 PACS: 68.08De, 68.43.Hm, 68.43.Jk https://nasplib.isofts.kiev.ua/handle/123456789/155781 en Condensed Matter Physics application/pdf Інститут фізики конденсованих систем НАН України |
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| description |
Molecular dynamics simulations to study the behaviour of anionic (Sodium Dodecylsulfate, SDS) and nonionic (Monooleate of Sorbitan, SPAN80) surfactants close to a SiO₂ (silicon dioxide) surface were carried out. Simulations showed that a water layer was first adsorbed on the surface and then the surfactants were attached on that layer. Moreover, it was observed that water behaviour close to the surface influenced the surfactant adsorption since a semi-spherical micelle was formed on the SiO₂ surface with SDS molecules whereas a cylindrical micelle was formed with SPAN80 molecules. Adsorption of the micelles was conducted in terms of structural properties (density profiles and angular distributions) and dynamical behaviour (diffusion coefficients) of the systems. Finally, it was also shown that some water molecules moved inside the solid surface and located at specific sites of the solid surface. |
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Article |
| author |
Nunez-Rojas, E. Dominguez, H. |
| spellingShingle |
Nunez-Rojas, E. Dominguez, H. Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface Condensed Matter Physics |
| author_facet |
Nunez-Rojas, E. Dominguez, H. |
| author_sort |
Nunez-Rojas, E. |
| title |
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface |
| title_short |
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface |
| title_full |
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface |
| title_fullStr |
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface |
| title_full_unstemmed |
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface |
| title_sort |
computational studies on the behaviour of anionic and nonionic surfactants at the sio₂ (silicon dioxide)/water interface |
| publisher |
Інститут фізики конденсованих систем НАН України |
| publishDate |
2016 |
| url |
https://nasplib.isofts.kiev.ua/handle/123456789/155781 |
| citation_txt |
Computational studies on the behaviour of anionic and nonionic surfactants at the SiO₂ (silicon dioxide)/water interface / E. Nunez-Rojas, H. Dominguez // Condensed Matter Physics. — 2016. — Т. 19, № 1. — С. 13602: 1–8. — Бібліогр.: 30 назв. — англ. |
| series |
Condensed Matter Physics |
| work_keys_str_mv |
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2025-12-02T12:00:17Z |
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2025-12-02T12:00:17Z |
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1850397758905647104 |
| fulltext |
Condensed Matter Physics, 2016, Vol. 19, No 1, 13602: 1–8
DOI: 10.5488/CMP.19.13602
http://www.icmp.lviv.ua/journal
Computational studies on the behaviour of anionic
and nonionic surfactants at the SiO2 (silicon
dioxide)/water interface
E. Núñez-Rojas1, H. Dominguez2∗
1 Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, México, D.F. 09340
2 Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, México, D.F. 04510
Received October 15, 2015, in final form November 23, 2015
Molecular dynamics simulations to study the behaviour of anionic (Sodium Dodecylsulfate, SDS) and nonionic
(Monooleate of Sorbitan, SPAN80) surfactants close to a SiO2 (silicon dioxide) surface were carried out. Simula-
tions showed that a water layer was first adsorbed on the surface and then the surfactants were attached on that
layer. Moreover, it was observed that water behaviour close to the surface influenced the surfactant adsorption
since a semi-spherical micelle was formed on the SiO2 surface with SDS molecules whereas a cylindrical micelle
was formed with SPAN80 molecules. Adsorption of the micelles was conducted in terms of structural properties
(density profiles and angular distributions) and dynamical behaviour (diffusion coefficients) of the systems. Fi-
nally, it was also shown that some water molecules moved inside the solid surface and located at specific sites
of the solid surface.
Key words: computer simulations, SDS surfactant, SPAN80 surfactant, adsorption, Cristobalite
PACS: 68.08De, 68.43.Hm, 68.43.Jk
1. Introduction
Adsorption of surfactant molecules at solid-liquid interfaces has been investigated for years not only
for its relevance in science but also for its numerous industrial applications, such as detergency, crude oil
refining, treatment of waste water, adsorption on activated charcoal and even in pharmaceutical prepa-
rations [1–3].
In particular, self-assembly of surfactant molecules on solid surfaces has shown different issues from
those observed at liquid/vapor and at liquid/liquid interfaces. For instance, it has been observed that
interactions between hydrophobic tails, repulsions between headgroups and interactions between sur-
factant molecules with solid surface [4, 5] could change the isotherms at the critic micellar concentration
(CMC). Therefore, studies of surfactant aggregation will help us to obtain more physical insights of self-
assembly phenomena [6, 7].
From the experimental point of view, the surfactant adsorption on surfaces has been studied by dif-
ferent techniques, such as streaming potential methods [8], calorimetry [9], neutron reflection [10], ellip-
sometry [11], fluorescence spectroscopy [12] and atomic force microscopy (AFM) [13]. In fact, AFM has
proved to be a reliable technique to obtain information on the topology of surfactant aggregation since it
allows us to observe how surfactants are formed on surfaces. For instance, CTAB arrays in parallel stripes
on a graphite surface [14], SDS forms hemimicelles on a rough gold surface [15] and similar morphologies
have been seen for other surfactants on hydrophobic surfaces [16–18].
On the other hand, computer simulations have been very useful to study such complex systems. For
instance, Monte Carlo simulations have been used to provide information on structural transitions of sur-
∗Corresponding author. E-mail: hectordc@unam.mx. Present address: Department of Physics and Astronomy, University of
British Columbia, Vancouver, British Columbia, Canada. On sabbatical leave.
© E. Núñez-Rojas, H. Dominguez, 2016 13602-1
http://dx.doi.org/10.5488/CMP.19.13602
http://www.icmp.lviv.ua/journal
E. Núñez-Rojas, H. Dominguez
factant aggregation [19] while molecular dynamics simulations have been used to investigate aggregation
at atomistic scales [20, 21]. In previous papers we have reported the surfactant behaviour on different
surfaces [21–24]. It has been observed how graphite surfaces impose orientational order on the surfac-
tant tails [21, 24] and how different solid faces of a titanium dioxide produced different aggregates on the
surface [22, 23]. In the present work we are interested to extend the studies of surfactant aggregation on
a hydrophilic SiO2 (silicon dioxide) surface in order to compare the behaviour of surfactant molecules on
different solid substrates.
2. Computational method and model
Molecular dynamics simulations of anionic and nonionic surfactant molecules at silicon dioxide sur-
face (SiO2), in its Cristobalite form, were carried out for the present study. The surface was constructed
using an atomistic model with a surface orientation (001) (figure 1). The parameters used for a solid
surface were taken from reference [25].
Figure 1. (Color online) The unit crystal cell and a snapshot of the solid surface. Oxygen atoms are in red
and silicon atoms are in yellow.
For the anionic surfactant molecule, sodium dodecyl sulfate (SDS), a model of 12 united carbon atoms
attached to a headgroup (SO4) was simulated by using a force field already reported in the literature [21].
For the nonionic surfactant molecule, Monooleate of Sorbitan (SPAN80), there was used, a model with
17 united carbon atoms, three OH− and one ester groups in the head group. The force field reported in
reference [23] was used for this molecule.
In the case of simulations with the anionic molecules, the initial configuration was prepared from a
monolayer of 36 molecules in all-trans-configuration with the headgroups initially pointed to the solid
surface. Then, 2535 water molecules were added (using the SPC model [26]) to the system and 36 sodium
cations (Na+). In the case of simulations with nonionic molecules, 25 molecules were used with the same
number of water molecules.
A simulation box having dimensions X = Y = 43.7019 and Z = 150 Åwas used with the usual periodic
boundary conditions. The Z -dimension of the box was long enough to prevent the formation of a second
water/solid interface due to the periodicity of the system. Instead, a liquid/vapor interface was present at
one end of the box (z > 0) whereas at the other end of the box (z < 0) beyond the solid there was an empty
space. All simulations were carried out in the NVT ensemble with a time step of 0.002 ps using DL-POLY
package [27]. Bond lengths were constrained using SHAKE algorithm with a tolerance of 10−4, and the
temperature was controlled using the Hoover-Nose thermostat having a relaxation time of 0.2 ps [28] at
T = 298 K. Long-range electrostatic interactions were handled using the Particle Mesh Ewald method,
and the Van der Waals interactions were cut off at 10 Å. Finally, the simulations were run up to 40 ns
and configurational energy was monitored as a function of time in order to determine the moment the
systems have reached equilibrium. Then, the last 2 ns were collected for analysis.
13602-2
Anionic and nonionic surfactants at the SiO2/water interface
3. Results
In this section we present calculations of the surfactant molecules at the SiO2 surface. Studies on the
behaviour of the surfactant molecules and how they aggregate at the liquid/solid interface are discussed.
3.1. Density profiles
In order to determine where the surfactant molecules arrayed in the system, mass Z -dependent den-
sity profiles for the headgroups and the tails were calculated, i.e., normal to the liquid/solid interface.
From figure 2 we observed that water molecules (dotted line) were not only adsorbed but also ab-
sorbed by the solid surface which is located at a position of Z =−23 Å in the figure. In fact, the first water
profile peak (to the right of the surface) indicated strong adsorption, i.e., a water layer on the surface. The
other water peaks (to the left of the surface) suggested that few particles were inside the solid surface.
The presence of water molecules inside SiO2 surfaces has been also observed in real experiments [29, 30].
On the other hand, SDS density profiles showed a strong first peak for the polar group (≈ 4 Å from the
surface in figure 2 (a) suggesting that the surfactant was well adsorbed on the surface. Moreover, it was
noted that the peak was located to the right of the adsorbed water layer. The result showed that the sur-
factant molecules formed a micellar structure adsorbed on the water layer on the solid surface. It was
also possible to observe that the hydrocarbon chains (solid line) were sited between polar-groups (dashed
line) along Z direction.
In the case of simulations with SPAN80 molecules [figure 2 (b)], the water density profile also showed
that some of those molecules went deep into the substrate. Besides, unlike SDS system, here strong peaks
were observed for the water profiles. The SPAN80 profiles are also shown, the first peak (≈ 5 Å from the
surface) corresponded to the hydrocarbon chains and it was very close to the polar group, nevertheless
the hydrocarbon chains were surrounded by the polar groups. These profiles suggested the formation of
-30 -20 -10 0 10
Z(A)
0
0.5
1
1.5
2
ρ(
Z)
(g
r/c
c)
-30 -20 -10 0 10
Z(A)
0
0.5
1
1.5
2
ρ(
Z)
(g
r/c
c)
a
b
Figure 2. Density profiles calculated along the Z -direction of the SiO2 solid surface. (a) SDS surfactant
and (b) SPAN80 surfactant. Dotted lines represent water, dashed lines show a surfactant headgroup and
continuous lines show surfactant tails. The solid surface position is indicated by the red line.
13602-3
E. Núñez-Rojas, H. Dominguez
Figure 3. (Color online) Snapshots for SDS (a) and for SPAN80 (b and c) micelles on a SiO2 surface. Spher-
ical and cylindrical shapes are depicted for the SDS and SPAN80, respectively. Tail groups are in green
and head groups in yellow.
a micelle on the surface.
In figure 3, snapshots of the final SDS [figure 3 (a)] and SPAN80 [figures 3 (b) and 3 (c)] are shown. In
those figures it was possible to observe the micellar structure mentioned above. In the case of the SDS,
the micelle structure had a spherical-like shape (snapshot in X − Z and Y − Z looked alike) whereas for
SPAN80 the micelle had a cylindrical-like structure [see figures 3 (b) and 3 (c)].
In order to verify the structure of the micelles on the surfaces, particle density profiles in the other
−23 −13 −3 7 17
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X(A)
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d
e
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Figure 4. Particle density profiles of the surfactant headgroups (dashed lines) and tails (solid lines) for the
SDS (left-hand) and SPAN80 (right-hand) systems. The panels indicate the profiles in the X , Y and Z axis.
13602-4
Anionic and nonionic surfactants at the SiO2/water interface
two directions, X and Y , were calculated. In figures 4 (a) and 4 (b), head groups density profiles (dashed
line) and tail groups (solid line) of SDS molecules are shown. There were observed similarities between
the two particle profiles, ρ(X ) and ρ(Y ), where it was noted that polar groups surrounded the tail groups.
In both tail group profiles, a small sharp peak can be seen. In this case, SDS molecules are well adsorbed
on the SiO2 substrate with some surfactants attached on the surface by the head group [see figure 3 (a)].
Then, there is a possibility that a SDS molecule could remain anchored with its tail moving in one region
only. This could explain the small sharp peak observed in both profiles, i.e., there are a bit more tail
groups in that region. On the other hand, from the Z -particle density profile in figure 4 (c), a big head
group peak could be seen close to the solid surface (see also figure 2), i.e., there was an excess of head
groups attached to the surface. Moreover, the ρ(Z ) indicated that the size of the micelle in the Z -direction
was smaller than in the X −Y directions. Therefore, the structure can be described as a sphere deformed
along the perpendicular direction of the solid surface.
This calculation was also carried out for the SPAN80 molecules [figures 4 (d)–4 (f)]. In figure 4 (d),
the density profile along X -axis is shown where a molecular aggregation is observed. In figure 4 (e),
the profiles indicated a uniform distribution of the molecules along the Y -axis whereas in figure 4 (f)
it was again possible to observe aggregation of the molecules next to the surface. Here, it can be seen
that headgroups partially surrounded the tail groups. Then, these profiles revealed that the molecules
structured themselves as a cylindrical micelle along the Y direction. These results suggested that the
effect of the solid surface in the micelle formation was minimum.
3.2. Water orientation at the SiO2 surface
As it was observed above, water density profiles showed strong peaks close to the SiO2 surface sug-
gesting that those molecules might have some structure close to the solid. Therefore, studies of howwater
molecules were oriented in the solid were carried out. The analyses were conducted over the molecules
in the adsorbed layers only (defined by the peaks of figure 2).
-1 -0,5 0 0,5 1
cos(θ)
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
Pr
ob
ab
ili
ty
-1 -0,5 0 0,5 1
cos(θ)
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
a b
Figure 5. (Color online) Angular probability of the water dipole vector with the vector normal to the
interface. (a) For the SDS and (b) for the SPAN80 surfactant systems.
In figure 5, angular distributions of water molecules are shown for the SDS and SPAN80 systems.
The orientation of the water molecules was measured as the angle between the bisector vector of the
OH bonds and the vector normal to the interface (see figure 5). In both systems, SDS and SPAN80, there
was observed a privileged orientation of the water molecules, i.e., they were nearly perpendicular to
the surface. In fact, there was noted a larger number of water molecules pointing perpendicular to the
surface for the SPAN80 system [high peak in figure 5 (b)] than for the SDS system (small peak). Moreover,
the water angle distribution presented a small peak when water interacted with SPAN80 [figure 5 (b)]
and a wider distribution when water interacted with SDS.
13602-5
E. Núñez-Rojas, H. Dominguez
−22 −11 0 11 22
X (A)
0
2
4
6
at
o
m
d
is
tr
ib
u
ti
o
n
0
2
4
6
at
o
m
d
is
tr
ib
u
ti
o
n
−22 −11 0 11 22
Y (A)
a) b)
c) d)
Figure 6. Distribution of oxygen atoms in water (solid lines) and silicon atoms of the SiO2 surface (dashed
lines). Panels (a) and (b) atom positions for the SDS surfactant in the X and Y axis. Panels (c) and (d) atom
positions for the SPAN80 surfactant in the X and Y axis.
In figure 6, the distribution (in the first adsorbed water layer) of water molecules in the X and Y
directions, i.e., in the X −Y plane, is plotted. Figures 6 (a) and 6 (b) show the water oxygens positions in
the solid surface for the SDS system. In the same figure 6, the positions of the silicon atoms of the SiO2
surface are also shown in the same X −Y directions. In all cases it was observed that water molecules
(solid lines) were located only in specific sites. In fact, they were placed on the silicon atoms of the solid
surface (indicated by the dashed lines in figure 6). Figures 6 (c) and 6 (d) are the same plots for the SPAN80
system. From the above results we noted that water oxygens were adsorbed above silicon sites.
3.3. Mobility of surfactant molecules
In order to describe the attachment of the surfactant molecules to the solid surface, the diffusion
coefficient of the molecular aggregates was also calculated. The diffusion coefficients were calculated
in each direction by measuring the square mean displacements of the surfactant atoms and using the
Einstein relation. For the SPAN80 surfactant, the diffusions were 0.126× 10−9 m2/s, 0.095× 10−9 m2/s
and 0.085×10−9 m2/s in the X , Y and Z directions, respectively. For the SDS surfactant, the diffusions
were 0.125× 10−9 m2/s, 0.135× 10−9 m2/s and 0.078× 10−9 m2/s in the X , Y and Z directions, respec-
tively. In both systems, the diffusions in the perpendicular direction (Z -axis) was lower than in the plane.
However, the values for the SPAN80 were higher than for the SDS surfactant, suggesting that the second
surfactant had a higher mobility than the first one. It is worthy to mention that these diffusions were
much lower than the diffusions found for the same molecules on a Titanium dioxide surface [22, 23].
4. Conclusions and discussion
A series of Molecular Dynamics simulations were carried out in order to describe the behaviour of
two different surfactant molecules interacting with a silicon dioxide solid surface. In the case of the an-
ionic surfactant (SDS), there was observed a spherical micelle formation on a layer of water molecules
13602-6
Anionic and nonionic surfactants at the SiO2/water interface
previously adsorbed on the solid. The micelle was described by density profiles and they showed a defor-
mation of the micelle next to the adsorbed layer of water molecules. This deformation can be explained
in terms of the SDS charged headgroups interactions with water molecules on the solid surface. On the
other hand, the nonionic surfactant (SPAN80) did not showmuch influence by the solid surface. It formed
a cilyndrical micelle next to the adsorbed water layer. In this case, there was observed a thicker water
layer between the surfactant and the solid surface. The influence of the surfactant on the surface was
characterized by water molecules in the surface. Dipole water orientation, in the solid surface, was more
tilted for the SDS molecules than for the SPAN80 suggesting a stronger SDS-surface interaction and con-
sequently more intensive adsorption of those molecules on the surface. Based on the previous results
(diffusion coefficients), it was noted that both surfactants showed less affinity with the SiO2 surface than
with other surfaces such as graphite and titanium dioxide.
Acknowledgements
HD acknowledges support from DGAPA-UNAM-Mexico and Conacyt-Mexico for sabbatical scholar-
ships. The authors acknowledge support from DGTIC-UNAM for the supercomputer facilities.
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E. Núñez-Rojas, H. Dominguez
Чисельнi дослiдження поведiнки анiонних та неiонних
сурфактантiв на границi роздiлу мiж двоокисом кремнiю
(SiO2) i водою
E. Нуньєс-Рохас1, Г. Домiнгес2
1 Хiмiчний факультет, Автономний унiверситет Метрополiтана-Iцтапалапа, Мехiко, Мексика
2 Iнститут матерiалознавства, Нацiональний автономний унiверситет Мексики, Мехiко, Мексика
Виконано моделювання поведiнки анiонних (додецилсульфат натрiю, SDS) та неiонних (монолеат сорбi-
тану, SPAN80) сурфактантiв поблизу поверхнi SiO2 (двооксиду кремнiю) методом молекулярної динамiки.
Спостерiгаємо, що шар води спочатку адсорбується на поверхнi, а тодi сурфактанти приєднуються до
цього шару. Бiльш того, спостережено, що поведiнка води поблизу поверхнi впливає на адсорбцiю су-
фрактантiв, оскiльки напiв-сферична мiцела утворюється на поверхнi SiO2 з SDS молекулами, в той час,
як цилiндрична мiцела формується у випадку молекул SPAN80. Адсорбцiя мiцел описана структурними
властивостями (профiлi густини i кутовi розподiли), а також i в термiнах коефiцiєнтiв дифузiї. Показано,
що деякi молекули води проникають всередину твердої поверхнi.
Ключовi слова: комп’ютерне моделювання, SDS сурфактант, SPAN80 сурфактант, адсорбцiя, кристобалiт
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Introduction
Computational method and model
Results
Density profiles
Water orientation at the SiO2 surface
Mobility of surfactant molecules
Conclusions and discussion
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