J. Phys. Chem. B 2006, 110, 19543-19551
19543
Effect of Nanoparticles on the Interfacial Properties of Liquid/Liquid and Liquid/Air
Surface Layers
Francesca Ravera,*,† Eva Santini,† Giuseppe Loglio,‡ Michele Ferrari,† and Libero Liggieri†
CNR, Institute for Energetics and Interphases, Genoa Department, Via De Marini 6, 16149 Genoa, Italy, and
Department of Organic Chemistry, UniVersity of Florence, Florence, Italy
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An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the
presence of a cationic surfactant. These properties are the result of different phenomena such as the particle
attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the
latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid
interfacial environment, all those phenomena are closely correlated. The equilibrium and dynamic interfacial
tensions of the liquid/air and liquid/oil interfaces have been measured as a function of the surfactant and
particle concentration. The interfacial rheology of the same systems has been also investigated by measuring
the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed
with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments
based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms
determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particlesurfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface
and by the surfactant redistribution between the particle and fluid interface.
1. Introduction
The study of colloidal particles of nanometric dimensions and
their interaction with fluid interfaces is a topic of increasing
interest, especially for their applicability in the field of foams
and emulsion.1-6
Nanoparticles are employed like surfactants, and often in
association with them, as stabilizing additives of such disperse
systems, in different fields of practical interest.7-10
Though many experimental11-17 and theoretical18-20 works
are available in the literature about these systems, the basic
mechanisms underlying the stabilizing effect of nanoparticles
is not completely understood yet. From one side the increased
stability is related to the presence of particles in the foam
lamellae or, in the case of emulsions, in the liquid film between
approaching droplets. In those cases, in fact, particle engulfment
exerts a remarkable resistance to thinning,21,22 as also observed
for micelles. Other mechanisms may be related to the formation
of liquid bridges between particles,23 in a configuration that may
be particularly stable. Finally, the macroscopic mechanical
properties of liquid interfacessinterfacial tension and dilational/
shear surface elasticityscan be strongly affected by the attachment of particles,24-26 significantly contributing to stabilization.
Similar effects have been observed in studies concerning
surfactant systems typically used for foam stability, where the
formation of surface condensed phases, or solidlike aggregates,
confers a strongly elastic character to the interfacial layer.27-29
From the theoretical point of view, the problem of solid
nanoparticles in fluid biphasic systems has been investigated,
especially concerning the thermodynamic aspects of the partitioning of particles between aqueous bulk and fluid interfaces.30-32
* Corresponding author. Fax +39.010.6475700. E-mail address: r.ravera@
ge.ieni.cnr.it.
† CNR.
‡ University of Florence.
The hydrophilic or lipophilic character of particles is the basis
of the affinity of them for the fluid interface. In fact, the freeenergy change, ∆EP, associated with the transfer of one spherical
particle of radius R from the bulk phase to a planar fluid
interface can be expressed in term of the solid-water contact
angle, ϑ, and of the interfacial tension, γ,33 i.e.
∆EP ) - πR2γ(1 ( cos ϑ)2
(1)
The sign in the bracket is “-” for transfer from water and “+”
from oil/air phase. Thus, the attachment at the interface is
strongly favored for particles presenting partial wetting, that
is, ϑ significantly different from 0 and π.
In the composite surfactant plus nanoparticle systems, the
particle surface, and in turns its wettability properties, can be
strongly influenced by the surfactant adsorption.
At present no suitable methods exist for contact angle
measurements on the nanometric scale. Thus, particle wetting
has been investigated on the basis of the thermodynamic
properties of particle layers.34,35
Moreover, especially for ionic surfactants, adsorption at the
particle interface can result in variations of the surface charge.36,37
As a consequence, more significant particle interactions may
appear in the interfacial layer, which also influences the
mechanical interfacial properties.31
It is important to note that the interfacial layers of these
systems are indeed, at the nanometric scale, a multiphase zone
that comprises three interfaces, one between the two fluid phases
and, if the nanoparticles are partially wetted, two solid interfaces.
The surface energy associated with each of these interfaces is
also determined by the characteristics of surfactant adsorption
on them.
For these microscopically nonhomogeneous interfacial layers,
a critical item concerns the formal definition of quantities
describing their mechanical properties on a macroscopic scale.
10.1021/jp0636468 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/14/2006
19544 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Ravera et al.
In spite of rigorous thermodynamic treatments being unavailable,
these macroscopic mechanical properties can be operationally
described by an effectiVe interfacial tension. The latter can be
defined as the quantity entering, instead of the ordinary
interfacial tension, in the mechanical equilibrium conditions of
a macroscopic interface. Thus, the effective interfacial tension
can be unambiguously measured by any of the tensiometric
methods exploiting the Laplace equation on a macroscopic scale,
for example, those based on the acquisition of drop/bubble
shape, under gravity effect.
In the following, for sake of brevity the terms “surface
tension” and “interfacial tension” will be utilized, meaning the
corresponding effective quantity, as defined above.
In this work a model composite liquid/liquid and liquid/air
interfacial layer has been investigated, obtained with a cationic
surfactant and a dispersion of colloidal silica nanoparticles.
The aim of this work is to deepen the knowledge about the
interaction between particles and fluid interfaces, in relation to
the surfactant adsorption on the solid surface, and to quantify
its effect on the macroscopic interfacial properties.
The effect of nanoparticles has been investigated both from
the equilibrium and kinetic point of view. The equilibrium study
has been mainly based on the measurement of the equilibrium
interfacial tension as a function of surfactant and particle
concentration.
To investigate the dynamic aspects of such systems, the
effective interfacial tension, γ, has been measured during the
equilibration of the composite interfacial layer, for different
particle-surfactant compositions.
The same systems have also been studied from the rheological
point of view to evaluate the response of the interfacial layer
to perturbations of its equilibrium state. To this aim, the
dilational viscoelasticity, , of these systems has been measured
in a low-frequency range. This quantity, for harmonic perturbations of the interfacial area, A, can be written as a complex
quantity, characterized by a module and a phase, Φ
)
dγ
) ||eiΦ
d ln A
(2)
can be directly evaluated by acquiring for each frequency the
response of the interfacial tension γ to a given harmonic surface
area A.
Such measurements from one side provide information about
the viscoelastic properties of the fluid interfaces, which are
expected to play an important role in the stability of films,
foams, and emulsions. In fact, in the case of surfactant-stabilized
systems,38 the dilational viscoelasticity expresses the capability
of the system to dampen external disturbances.
On the other side, it has been shown in some previous works
about surfactant systems39-41 that these measurements together
with suitable theoretical models represent an effective way to
get information about the kinetic process occurring in the mixed
particle/surfactant interfacial layer.
2. Materials and Methods
Most of the results reported hereafter concern the interface
with air or hexane of diluted dispersions of silica nanoparticles
in a cationic surfactant (CTAB) aqueous solution. These
dispersions are obtained by further dilution with the surfactant
solution of the commercial colloidal silica dispersion Levasil
200/30 (H. C. Starck/Bayern), kindly supplied by the producer.
Levasil 200/30 is a colloidal dispersion at 30.38 wt % of
spherical silica nanoparticles. These are characterized by a
narrow radius distribution around 15 nm and a specific BET
Figure 1. Typical drop images acquired during the interfacial tension
measurement.
area of 200 m2/g. Levasil 200/30 has been chosen among
different colloidal silica dispersions commercially available since
it is free from stabilizing additives and thus more suitable as a
model dispersion. In fact, stability is ensured by a manufacturing
process providing negatively charged nanoparticles, which is
at the origin of the pH 9.2 of the dispersion.
CTAB (hexadecyltrimethylammonium bromide) was purchased from Fluka and utilized without further purification. The
water utilized in this work was produced by a Millipore (Elix
plus MilliQ) purifier system. As usual for investigations
involving ionic surfactants, salt was added to the CTAB solution
to promote its adsorption. Thus, for all results here reported,
CTAB solutions always contain 1 mM of NaCl, unless otherwise
specified. NaCl, purchased from Merck, has been roasted at 600
°C for 24 h before use in order to eliminate any surfactant
impurity. The purity grade of the salt aqueous solution was
checked by measuring the surface tension, γ, over a long time.
A stable value, γ ) 72.5 mN/m, was found at 20 °C.
Hexane was of spectrophotometric grade purity (MerkUvasol) and utilized without further purifications. The purity
degree of this solvent was also checked by measuring its
interfacial tension with the aqueous NaCl solution, which was
found to be stable over long time, with a value of γ ) 51.0
mN/m at 20 °C.
All interfacial tensions and dilational viscoelasticities reported
here have been measured by a drop shape tensiometer.42 This
technique can be successfully applied to study liquid/air and
liquid/liquid interfaces, with appreciable density difference,
under nearly mechanical equilibrium conditions. For the measurements here reported, a pendant drop of the aqueous dispersion
is formed at the tip of a Teflon capillary tube, with an internal
diameter of 0.7 mm, inside a glass cell containing air or hexane.
(Figure 1). The specific drop shape apparatus utilized (PATSinterface) implements an automatic feedback for the control
of the interfacial area during the experiments. Thus, the aging
of the interface can be accurately characterized by measuring
the dynamic interfacial tension of a drop with constant interfacial
area, obtained after a sudden and large expansion. These data
are also utilized to evaluate the equilibrium interfacial tension
values. The apparatus also allows for the measurement of the
dilational viscoelasticity by analyzing the interfacial tension
response to controlled harmonic perturbations of the interfacial area up to frequencies of 0.2 Hz. Further details about
the method, its capabilities and its limitations, are available
elsewhere.42
All glassware and parts of the instrument in contact with the
samples were carefully cleaned with different standard procedures, depending on the materials, to avoid any contamination.
Before each measurement the absence of contaminants was
checked by dynamic interfacial tension measurements on the
pure water/hexane or water/air interfaces.
Effect of Nanoparticles on Interfacial Properties
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19545
Figure 3. Equilibrium surface tension versus CTAB concentration:
(O) 1 wt % silica nanoparticle aqueous dispersion/air interface; (2)
CTAB solution/air interface, without nanoparticles. The solid line is
the best-fit Frumkin isotherm with the parameters reported in Table 1.
Figure 2. Dynamic interfacial tension during CTAB adsorption at
freshly formed solution/air (a) and solution/hexane (b) interfaces, for
different values of the CTAB concentration.
3. Results and Discussion
First of all the effect of the presence of nanoparticles has
been investigated on the water/air and water/hexane systems
without CTAB. For a concentration of silica particles of 1 wt
%, both systems maintain a constant value of the interfacial
tension, corresponding to that of the pure system. No appreciable
effect has been observed on the surface rheology as well.
This result is not surprising because, in the absence of CTAB,
the utilized particles are strongly hydrophilic. These are then
completely wetted and do not influence the properties of the
aqueous dispersion interface. Adsorption of CTAB on the
nanoparticles surface is instead expected to increase their
hydrophobicity, providing a driving force for their attachment
at the biphasic liquid/fluid interface.
A second preliminary characterization concerned the interfacial properties of the aqueous CTAB solution plus 1 mM
NaCl, versus air and hexane. To this aim the dynamic interfacial tension of solutions with different CTAB concentrations
have been measured during the aging of fresh interfaces.
The results reported in Figure 2 show that adsorption proceeds
very fast and the equilibrium is attained within a few seconds.
The corresponding equilibrium interfacial tensions versus
CTAB concentration are reported in Figures 3 and 4, where
the best-fit Frumkin isotherm has been also reported together
the experimental data. Such an adsorption model is commonly
used for this kind of surfactant system43 and provides the
following relationships between the interfacial tension, the
Figure 4. Equilibrium interfacial tension versus CTAB concentration: (O) 1 wt % silica nanoparticle aqueous dispersion/hexane
interface; (2) CTAB solution/hexane interface, without particles. The
solid line is the best-fit Frumkin isotherm with the parameters reported
in Table 1.
TABLE 1: Adsorption Isotherm Parameters for CTAB Plus
1 mM NaCl, Obtained as Best-Fit Values Assuming the
Frumkin Model
solution/air
solution/hexane
Γ (µ mol/L)
a (mol/L)
H
3.91
6.16
2.38 ×
1.26 × 10-6
0.9 × 10-3
4.14
10-5
bulk surfactant concentration (c), and the surface coverage
(θ ) Γ/Γ∞),
c)a
θ
e2hθ
1-θ
γ ) γ0 + RTΓ∞[ln(1 - θ) - hθ2]
(3)
(4)
where h is the Frumkin interaction parameter, Γ and Γ∞ are the
actual and the saturation adsorptions, and a is the LangmuirSzyszkowski constant, which describes the surface activity of
the adsorbing species. The best fit values of these parameters
for the measured γ-c equilibrium data are reported in Table 1.
The best fit values of h show that interaction is negligible for
19546 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Ravera et al.
Figure 5. Equilibrium surface tension versus the particle concentration
for silica nanoparticle aqueous dispersion/air interface, at CTAB
concentration of 5 × 10-4 M.
Figure 7. Surface tension of the supernatant solution obtained after
centrifugation of silica nanoparticle dispersions with (a) a different
particle concentration and initial CTAB concentration of 5 × 10-4 M
and (b) 1 wt % particle concentration and different initial CTAB
concentrations.
Figure 6. Equilibrium surface tension versus the particle concentration
for silica nanoparticle aqueous dispersion/hexane interface, at CTAB
concentration of 5 × 10-4 M.
the CTAB molecules adsorbed at water/air, while significant
repulsive interactions exist between the surfactant molecules
adsorbed at water/hexane. This difference is probably due to a
more effective influence of NaCl in the case of the water/air
interface, resulting in a larger neutralization of the ionic
interaction.
Effect of Silica Nanoparticle Concentration. The equilibrium interfacial tension versus air and hexane of the silica
dispersions in solutions with fixed CTAB concentration (5 ×
10-4 M) has been measured for different values of particle
concentration, CP. The results are reported in Figures 5 and 6,
respectively.
These observed interfacial tensions can be the result of two
opposite effects of the nanoparticles: (i) the depletion of the
surfactant solution due to the adsorption of CTAB on the
particles interface, which leads to an increase of the interfacial
tension with the particle content, and (ii) the attachment of
particles at the interfacial layers, which instead decreases the
interfacial tension. This latter effect should increase (up to a
given extent) with the CTAB concentration, CCTAB, which results
in larger particle hydrophobicity, and indeed, it is not present
for pure particle dispersions.
The data reported in Figures 5 and 6 show a monotonic
increase of the interfacial tension both for the dispersion/air and
dispersion/hexane interface. Thus it is impossible from these
data alone to discriminate between the above two effects.
It is then necessary to investigate the depletion of the
surfactant solution due to CTAB adsorption on the nanoparticle
interface. To this aim, dispersions with different silica concentrations were prepared by diluting the Levasil in solutions with
a rather large CTAB concentration (8 × 10-4 M). After
preparation, the dispersions were allowed for equilibration
during 6 h. The nanoparticles were then separated by ultracentrifugation (Beckman L-60 centrifuge). The centrifugation
time (16 h), as well as the angular velocity (30 000 rpm) and
the centrifuge test tubes, have been chosen in order to warrant
the nearly complete precipitation of the particles. The supernatant was then sampled and its surface tension was measured
in order to evaluate the concentration of the CTAB still available
in solution by comparison with the CTAB γ-c isotherms.
Similar processing of CTAB solutions has allowed any segregation of the free surfactant molecules to be excluded. Thus, any
depletion observed after these tests can be certainly attributed
to CTAB adsorption on the particle interface. The results of
these tests are summarized in Figure 7 and show that for
nanoparticle concentrations larger than 0.4 wt %, the surface
tension of the supernatant approaches that of pure water. This
means that, in these cases, the nanoparticles nearly completely
remove the surfactant from the bulk aqueous phase. Thus, for
nanoparticles concentrations below 0.4 wt %, the interfacial
tension of the dispersion is determined by a mixed layer
composed by attached nanoparticles and surfactant adsorbed at
the liquid interface. Above this concentration, there is no more
free surfactant available in the bulk, and the decrease of the
Effect of Nanoparticles on Interfacial Properties
Figure 8. Dynamic surface tension during the equilibration of freshly
formed 1 wt % silica nanoparticle aqueous dispersion/air interface, for
different CTAB concentrations.
Figure 9. Dynamic surface tension during the equilibration of freshly
formed 1 wt % silica nanoparticle aqueous dispersion/hexane interface,
for different CTAB concentrations.
interfacial tension can be completely attributed to the attachment
of nanoparticles at the liquid interface. From the data reported
in Figures 5 and 6 it is evident that the effect of the particles
on the interfacial tension is much larger for the dispersion/
hexane than for the dispersion/air interface. Indeed, as will be
clarified in the following, two mechanisms can be responsible
for this effect. With the particles partially coated by the surfactant, the exposition of the alkylic tails provides a more partial
wetting character and then a larger affinity with the water/hexane
interface. Another reason could be that the transport of surfactant
from the solid interfaces of attached particles to the fluid
interface is more favorable for water/hexane than for water/air.
Effect of CTAB Concentration. Studies versus CTAB
concentration for given values of particle concentration are
necessary to better understand the role of the adsorption on silica
in the particle/interfacial layer interactions.
To this aim, the dynamic interfacial tension during the aging
of fresh 1 wt % dispersion/air (Figure 8) and dispersion/hexane
(Figure 9) interfaces have been measured for different CTAB
concentrations. Comparing these signals with those reported in
Figure 2, it is evident that the adsorption kinetics without
nanoparticles is very fast with respect to the equilibration of
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19547
the composite nanoparticle plus surfactant system, which
requires a time 3 orders of magnitude larger.
It is important to recall that, under these CTAB and particle
concentration ranges, no free surfactant in the bulk phase is
expected. Then the increase of equilibration time cannot be
attributed to a reduced CTAB concentration, as predicted by
diffusion-controlled adsorption.
Besides the increase of the characteristic times, the data
present some particular features proving that the kinetic mechanisms of such composite systems are essentially different from
those of simple surfactant adsorption.
As shown in Figures 8 and 9, for the intermediate concentrations (from 2 to 8 × 10-4 M for dispersion/air and from 2 to
5 × 10-4 M for dispersion/hexane), the dynamic interfacial
tension evolution is almost the same during the first 100 s. Only
at larger times do these signals split, reaching the different
equilibrium values. Thus, the interfacial tension seems to vary
according to two consecutive relaxation processes, the first one
independent from the surfactant concentration. On the contrary,
for the lowest concentration, as well as for the largest one in
water/hexane, such a double kinetic process is not appreciable
and the dynamic interfacial tension evolution is different already
from very short time. This behavior is reproducible in its main
features. A full explanation for these observations is not
available so far, but some hypothesis about the mechanisms of
equilibration of such composite systems can be argued on the
basis of the fact that in the investigated dispersions CTAB is
initially present only as adsorbed onto the silica surface. Thus,
one could relate to the diffusion of particles and to their
attachment/accumulation to the liquid/fluid interface, the first
observed relaxation step of the interfacial tension, which is, in
fact, independent from CTAB concentration. The consequent
process, leading to the establishment of the different equilibrium
values, could instead be the result of the rearrangement of the
composite interfacial layer, with a possible redistribution of the
surfactant between the solid/liquid and liquid/fluid interfaces.
More in detail, owing to CTAB adsorption, the particles
become more hydrophobic and tend to populate the interfacial
layer. This first process is driven essentially by the particle
Brownian diffusion in the bulk and it is not expected to depend
too much on the CTAB concentration. This picture is also
supported by the estimation of the characteristic time, τp, for a
particle motion of a length d, comparable with the droplet size.
Indeed, the diffusion coefficient for Brownian motion of the
nanoparticles in the dilute dispersion can be estimated as
Dp )
RT
6πηrN
(5)
where R is the gas constant, T the absolute temperature, η the
viscosity of the liquid, r the particle radius, and N the Avogadro
number, providing in this specific case Dp ≈ 1.6 × 10-6 cm s-2.
One can then estimate τp ) xd/Dp ≈ 102 s, which is compatible with the observed characteristic time of the first relaxation
process.
The attachment of particles at the fluid interface, if their
density is low, does not imply necessarily a large change in the
interfacial tension. However, starting from this situation, the
CTAB molecules adsorbed on the silica surface can transfer
into the liquid/air or liquid/liquid interface. This second transfer
process, which occurs coupled with the first one, can be the
mechanism responsible of the interfacial tension decreasing.
The process is more complex when the particle density in
the interfacial layer increases. In fact in this case, depending
on the kind of adsorbed layer on the solid surface, particle-
19548 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Ravera et al.
particle interactions can become important, influencing more
significantly the interfacial tension.
For a better understanding of these processes, it is necessary
to get information about the initial surface state of the nanoparticle in the aqueous solution, defined by the adsorption of
CTAB on the silica, which determines the level of hydrophobicity of the particles. Being determined by the initial CTAB
concentration in the bulk and by the available silica surface area,
the value of this adsorption is the same for the two systems
investigated: dispersion/air and dispersion/hexane. However,
it can have a different effect depending on the interaction
between these modified particles with the interfacial layer, which
can determine a different tendency of the particles to populate
it. Moreover, the process of CTAB redistribution inside the
interfacial layer between the solid interfaces and the fluid
interface is driven by the characteristics of the CTAB surface
activity at the specific fluid interface.
The analysis of the equilibrium interfacial tension versus the
CTAB concentration can provide further information. The
equilibrium interfacial tension data of the dispersion/air and the
dispersion/hexane interfaces obtained from the kinetics reported
above are also plotted in Figures 3 and 4 as a function of the
CTAB concentration. Owing to the adsorption of CTAB on the
particles, all those equilibrium values are significantly larger
than the corresponding interfacial tensions of CTAB solutions.
For the dispersions/air interface, Figure 3 shows a weak
decrease of the surface tension with the CTAB concentration,
which becomes steeper at a concentration larger than 5 × 10-4
M. For the dispersion/hexane interface (Figure 4), instead, the
interfacial tension decreases with continuity in the whole
investigated concentration range.
Increasing the CTAB concentration at fixed particle content
implies the increase of the surface coverage of silica particle
and, as a consequence, of their hydrophobicity. This means a
larger affinity of the particles for the fluid interface. Thus, while
the quantity of particles at the interface increases with the CTAB
concentration, such particles contain a larger amount of surfactant that can be released to the fluid interface. Thus the weak
decrease of the equilibrium interfacial tension can be due, in
both cases, to a slight increase of the amount of surfactant
transferred to the fluid interface from the particle surface. The
change of the slope above a certain CTAB concentration can
instead be explained by assuming that particles start to interact
above a certain coverage degree of the interfacial layer. In that
case, the interfacial tension is no more determined only by the
CTAB adsorption at the fluid interface but also by the particleparticle interactions.
Thus, from the above considerations it is clear that the state
of the composite interfacial layer is mainly determined by the
surface state of the silica particles in the aqueous bulk and their
interaction with the fluid interface. To quantitatively define such
state, the adsorption ΓS has been estimated according to
ΓS )
∆C
aSxpwF
(6)
where ∆C is the difference between the initial CTAB concentration in the bulk and that of the depleted solution, xpw is the
weight fraction of particles in the dispersion, F the density of
the dispersion, and aS the silica surface area. For the latter, one
can assume the BET value provided by the producer for Levasil
200/30. The calculated values of ΓS are plotted in Figure 10
versus the corresponding values of the equilibrium interfacial
tension. All reported data concern concentration arrangements
providing a vanishing amount of free CTAB in the bulk.
Figure 10. Interfacial tension for silica nanoparticle dispersion/air and
dispersion/hexane interfaces as a function of the adsorption of CTAB
on the silica particle, as calculated from eq 6. All data refer to conditions
with no free CTAB in the aqueous bulk phase.
For all the cases here investigated, ΓS has been found to be
lower than the saturation values both for the fluid interfaces
(see Table 1) and for the silica surface.36,37 Thus particles are
covered by an unsaturated CTAB monolayer, with polar heads
on the solid and hydrophobic tails toward the water. This feature
lends to the particles a hydrophobic and a lipophilic character
at the same time, which may lead to significant differences in
the effects that particles with the same CTAB coverage have
on the two fluid interfacial layers: dispersion/air and dispersion/
oil. Since the reported data refer to a vanishing amount of free
CTAB in the bulk, Figure 10 allows for such comparison. In
fact, one can notice that the evident change of the slope of the
interfacial tension occurs at larger ΓS for the dispersion/hexane
interface compared to dispersion/air. Assuming that this change
of the slope is due to the onset of particle-particle interaction,
one can conclude that for the dispersion/hexane interface either
the onset of such interaction requires a larger particle density
or the attachment of particles is reduced with respect to the
water/air interface.
Dilational Rheology. The dilational rheological properties
of the composite nanoparticle/surfactant interfacial layers have
been investigated by measuring the dilational viscoelasticity,
, versus frequency. To evidence the role of nanoparticles, like
in previous sections, the systems have been investigated with
and without nanoparticles and for different surfactant concentrations.
Preliminary tests on 1 wt % dispersions, without surfactant,
have shown that no dilational viscoelastic effects were introduced by the silica particles alone. This result, together with
those obtained for the interfacial tension, definitely prove that
these silica nanoparticles do not interact with the fluid interface
unless they are modified by the presence of surfactant.
The module of the dilational viscoelasticity, ||, has been then
measured for dispersion/air systems at 1 wt % of particle
concentration and different values of CTAB concentration, in
the range of area perturbation frequency from 0.01 to 0.2 Hz.
The measured values are reported in Figure 11 together to those
obtained for the corresponding CTAB solutions without particles. These latter (Figure 11a) show at first a slight decrease
with the increasing CTAB concentration and then a weak
increase for c ) 8 × 10-4 M. Though the observed small
variation of || in the frequency range investigated does not
allow for an accurate interpretation, the observed trend is
compatible with what typically expected for surfactant solutions,
Effect of Nanoparticles on Interfacial Properties
Figure 11. Module of the dilational viscoelasticity versus frequency
of (a) CTAB solution/air interfaces for different CTAB concentrations
and (b) 1 wt % silica nanoparticle aqueous dispersion/air interface at
the same CTAB concentrations.
where the mechanism driving the adsorption equilibration is the
diffusion in the bulk.
When the same surfactant solutions are investigated in the
presence of 1 wt % of nanoparticles (Figure 11b), the trend
with the concentration of || is more significant and also its
value appreciably increases especially for the higher concentrations.
The module of versus frequency measured for the CTAB
solution/hexane and the dispersion/hexane interfacial layers are
reported in Figure 12.
In this case the principal effect of the nanoparticles, i.e., the
increasing of || for a given CTAB concentration, is even more
evident. In fact, the solution/hexane interface presents very small
values of || for all the CTAB concentration studied (Figure
12a). Moreover, as is evident from Figure 12b, || increases
with the CTAB concentration.
To explore further the effect of particles on rheological
properties, it is useful to compare the results obtained for the
two systemsssolutions and 1 wt % particle dispersionswhere
CTAB concentration have been chosen in order to obtain the
same value of the interfacial tension.
In Figures 13 and 14, the data corresponding to γ ) 30 and
33 mN/m, respectively, have been reported, showing that the
values of || for the dispersion interfacial layers are appreciably
larger in the whole frequency range.
Considering the value of the interfacial tension before the
onset of particle-particle interactions as mainly due to the
J. Phys. Chem. B, Vol. 110, No. 39, 2006 19549
Figure 12. Module of the dilational viscoelasticity versus frequency
of (a) CTAB solution/hexane interfaces for different CTAB concentrations and (b) 1 wt % silica nanoparticle aqueous dispersion/hexane
interface and the same CTAB concentrations.
Figure 13. Module of the dilational viscoelasticity versus frequency
of systems having the same interfacial tension (30 mN/m): (b) 5 ×
10-5 M CTAB solution/hexane interface and (4) 1 wt % silica
nanoparticle plus 5 × 10-4 M CTAB concentration aqueous dispersion/
hexane interface.
CTAB adsorption at fluid interface, these comparisons prove
that the increase of || is not simply due to a decrease of CTAB
19550 J. Phys. Chem. B, Vol. 110, No. 39, 2006
Figure 14. Module of the dilational viscoelasticity versus frequency
of systems having the same interfacial tension (33 mN/m): (b) 2 ×
10-5 M CTAB solution/hexane interface and (4) 1 wt % silica
nanoparticle plus 2 × 10-4 M CTAB concentration aqueous dispersion/
hexane interface.
adsorption but also to another mechanism involving the particles
attached at the fluid interface.
Thus, all these investigations, even if obtained in a limited
frequency range, show that for a given surfactant concentration
the presence of nanoparticles has the effect of increasing ||.
Since the reported data concern again a situation where the free
CTAB in the bulk is vanishing, the relaxation mechanisms
responsible for the viscoelastic response of these interfacial
layers are very different from those typical for surfactant
solutions.
The Brownian transport of particles from the bulk to the
interface, argued on the basis of the relaxation of dynamic
interfacial tension after large expansions of the interfacial area,
is most likely ineffective here. In fact, for energetic reasons,
the attachment of the particles at the fluid interface is essentially
an irreversible process. Moreover, the characteristic periods of
the applied small area perturbations are smaller than the
characteristic time for the Brownian transport of particles to
the interface. Therefore, under these conditions, this transport
is probably negligible, and the only processes effective in
determining the viscoelastic response are those involving the
reorganization of the mixed particle-surfactant layer. The latter
depend on CTAB concentration and involve different phenomena, which call for further investigations: the exchange of
surfactant between the particle and the fluid in the interfacial
layer and the particle-particle interaction at the interface.
4. Conclusion
The results of this experimental work show how dispersed
nanoparticles in an aqueous phase can modify the interfacial
properties of the liquid/air or liquid/liquid systems if their surface
is modified by the presence of an ionic surfactant.
Mixed particle/surfactant interfacial layers have been characterized through measurements of the effective interfacial
tension and surface rheology.
The features of the data corresponding to the kinetics of
reequilibration of these systems, as well as the equilibrium data
versus the CTAB concentration, can be interpreted by assuming
different mechanisms.
Already the equilibrium data obtained as a function of
particles and CTAB concentration have shown that a fundamental role is played by the adsorption of CTAB on the solid
Ravera et al.
surface of silica nanoparticles, which determines the affinity of
these particles for the fluid interfaces and the onset of particle
transport from the bulk to the interfacial layer.
From the features of the effective interfacial tension during
the equilibration of these composite systems it has been possible
to conclude that there are different kinetic mechanisms driving
this process.
Besides the Brownian motion of particles in the bulk, an
important role is also played by the internal reorganization of
the mixed particle/surfactant layer at the interface and by the
surfactant redistribution between the attached particles and the
fluid interface.
The results obtained by the study of the interfacial rheology
are in agreement with this picture, and the measured dilational
viscoelasticity corroborate the hypothesis of dynamic processes
introduced by the attachment of CTAB-modified nanoparticles
to the fluid interfaces.
Concluding, it is possible to assert that the tensiometric
characterizations of this kind of composite systems can give
information about the mechanisms occurring at the interfacial
layers.
However, to better quantify the interaction of nanoparticles
at the liquid/liquid and liquid/air interfaces and the role of
surfactants in such interactions, an extension of this study is
necessary.
To this aim, future work will concern the same kind of
measurements for different compositions of the dispersion, i.e.,
changing the surfactant and particle concentrations, and the
investigation of the surface rheology in a wider frequency range
with other more appropriate methods.44,45
Acknowledgment. The work has been supported by the
European Space Agency under the MAP project “Fundamental
and Applied Studies in Emulsion Stability-FASES” (AO-99052)”. The authors wish to thank Dr. Dimitri Grigoriev (MaxPlank Institut für Kolloid und Grenzflächenforschung) for
fruitful discussions.
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