Colloidal Zr-based Clusters as Pickering Emulsion Stabilizers
A. Gossard, G. Toquer, J. Causse, A. Grandjean
ICSM-UMR 5257, CEA/CNRS/UM2/ENSCM, BP 17171, 30207 Bagnols sur Cèze, France
Abstract
The synergy between Pickering emulsion and a colloidal sol-gel process based on a
quaternary system [zirconyl nitrate + acetylacetone + ammonia + water] is investigated. By
adding dodecane into this system, we show how nanoparticle Zr-based clusters are competent
surface active to stabilize dodecane/water emulsions, exhibiting a typical Pickering emulsion
behaviour. We study the effects of the Zr-precursor concentration, the complexing agent
amount (acetylacetone) and the pH on the nanoparticles at the dodecane/water interface. In
particular, acetylacetone acts like a steric barrier promoting the nanoparticle dispersion and
for high precursor content, more particles are formed leading to a more efficient stabilization
of the emulsion. No significant evolution is observed and the polydispersity is not reduced
contrary to the case of the limited coalescence phenomenon. The effectiveness of the
nanoparticle Zr-based clusters to prevent coalescence is enhanced by droplets freezing which
is probably due to an irreversible bridging effect.
Introduction
In colloid science, the self-assembly of solid colloidal particles and their adsorption behaviour
at interfaces have attracted much interest. Contrary to the case of bulk colloidal suspensions,
which are often well explained through the DLVO (Derjaguin–Landau–Verwey–Overbeek)
[Derjaguin,
Landau,
L.D.,
1941,
14,
633;
Verwey,
E.J.W.,
Overbeek,
J.T.G.,
Theory…Colloids, Amsterdam, 1948] theory, the description of the mechanisms of
stabilization of the liquid−liquid interface by particles is mainly based on the decrease of the
interfacial energy. Indeed, the assembly of particles at interfaces implies the decrease of the
effective interfacial tension. Concerning nanoparticles (NPs), the energy of thermal
fluctuations causing NPs displacements from the interface is comparable to the interfacial
energy and gives rise to a particle-size–dependent self-assembly [Y. Lin, H. Skaff, T. Emrick,
A. D. Dinsmore, T. P. Russell, 2003 VOL 299 SCIENCE]. Interfacial assembly of
nanoparticles is driven by the minimization of the Helmholtz free energy and therefore the
adsorption enthalpy (ΔE) of particles at the liquid-liquid interface must be negative. In the
case of an oil-water-NPs system, the three contributions to the interfacial energy result from
the interfacial tension at NP-oil (γ NP/O), NP-water (γ NP/W) and oil-water (γ O/W) interfaces. The
adsorption enthalpy due to a single NP at the oil-water interface is given by:
∆𝐸 = −πR2 γ𝑂/𝑊 × [1 −
(γ𝑁𝑃/𝑊− γ𝑁𝑃/𝑂 )
γ𝑂/𝑊
2
]
(E1)
where R is the NP radius [P. Pieranski, Phys. Rev. Lett. 45, 569 (1980)]. Consequently, the
energy gain is smaller and the assembly is less stable for smaller nanoparticles than for larger
ones.
The residence time of the NPs at the interface is expected to increase with increasing NP size.
The effectiveness as emulsion stabilizers is however subtle since sedimentation or creaming
phenomenon, which occur with large NPs, have to be avoided. In this way, a decrease in
particle size may sometimes induce an enhancement of emulsion stability due to a decrease of
emulsion drop size [Binks and Whitby, 2005, Colloids Surf. A: Physicochem. Eng. Aspects
253, 105], [Binks and Lumsdon, 2001, Langmuir 17, 4540]. More precisely, larger particles
have more significant effect on the emulsion stability, and smaller particles have more
influence on the emulsion drop size [Wang, S., He, Y., Zou, Y., 2010, PARTICUOLOGY, 8,
390-393]. Interfaces stabilized by colloidal particles forming oil in water (O/W) or water in
oil (W/O) emulsions are known as Pickering emulsions [Pickering, S. U. Emulsions. J. Chem.
Soc. 1907, 91, 2001−2021] since one century ago [W. Ramsden, Proc. Roy. Soc. 72 (1903)
156]. Due to large advances in the synthesis and characterization of colloidal particles,
Pickering emulsions have attracted a surge of interest [Aveyard and P. Binks Advances in
Colloid and Interface Science, 2003, 100, 503–546]. Unlike surfactants, particles irreversibly
adsorbed at the interface of emulsions, due to their high energy of attachment, have a shelf
life of months even years. There is then a renewed interest in these emulsions for different
kind of applications as drug delivery [Angelova, A.; Angelov, B.; Mutafchieva, R.; Lesieur,
S.; Couvreur, Acc. Chem. Res. 2011, 44, 147−156], catalysis [Crossley, S.; Faria, J.; Shen,
M.; Resasco, D. E. Science 2010, 327, 68−72] + [A. Desforges, R. Backov, H. Deleuze, O.
Mondain-Monval, Adv. Funct.Mater. 15 (2005) 1689–1695], biofuel processing [Drexler, S.;
Faria, J.; Ruiz, M. P.; Harwell, J. H.; Resasco, D. E. , Energy Fuels 2012, 26, 2231−2241],
food science [Dickinson, E. Curr. Opin. Colloid Interface Sci. 2010, 15, 40−49], electrode
materials [H.F. Zhang, I. Hussain, M. Brust, A.I. Cooper, Adv. Mater. 16 (2004) 27–30],
supports for separation [T.S. Dunstan, P.D.I. Fletcher, Langmuir 27 (2011) 3409–3415],
macroporous materials [B.P. Binks, , Adv.Mater. 14 (2002) 1824–1827].
The aim of the present study is to investigate the potential synergy between Pickering
emulsion and a colloidal sol-gel process. This latter is based on a free surfactant system
[zirconyl nitrate + acetylacetone + ammonia + water] which has been recently studied and
whose the gelation regimes are now well determined [public TSG]. From these previous
results, the formation of Pickering emulsion implying sols which are stable for a long time is
considered. In particular, we show how Zr-based NP clusters from a colloidal sol-gel route
synthesis are competent surface active to stabilize dodecane/water emulsions, exhibiting a
high stability against coalescence as the well-known Pickering emulsions. We investigate the
effects of the Zr-precursor concentration, the complexing agent amount (acetylacetone) and
the pH on the behaviour of Zr-based NPs at the dodecane/water interface. We probe thus from
interfacial tension measurements, the magnitude of the energy with which particles are
anchored to the interface and hence their effectiveness to prevent coalescence. The energy
barrier for their adsorption can be tuned from the physicochemical parameters (i.e. precursor
concentration, complexing agent amount, pH) impacting the NPs features (polydispersity and
dispersion). Finally, at a fixed applied shear rate, the optimal oil volume fraction impacting
the droplet size and thus the stability of the emulsion is discussed.
Experimental Part
Materials and sample preparation
The different operations and reactions are performed at room temperature in ambient
atmosphere.
ZrO(NO3)2, 6H2O (from Sigma-Aldrich) is used as the zirconium precursor, and several
deionized aqueous solutions with two zirconium concentrations (i.e., 0.1 M and 0.15 M) were
prepared. Acetylacetone (acacH) (from Sigma-Aldrich) is used as a complexing agent. Next,
the pH of these solutions was adjusted by the dropwise addition of ammonia 3M under
vigorous stirring to initiate the hydrolysis and condensation reactions. We observed the
spontaneous formation of a white precipitate, which re-dissolves after several minutes. A sol,
whose characteristics depend on the system parameters is formed. Then, dodecane 99% (from
Acros) is added dropwise into this sol. Ultra-Turax T25 (from IKA) homogeneizer is
simultaneously used to shear the added oil at a constant speed (13000 rpm) and using a single
rotor (S25N-10G, from IKA), which is equivalent to a constant shear rate. In this way, a
creamy-white solution is obtained corresponding to an emulsion which is stabilized by the
adsorption of the sol particles at the interface oil-water. The behavior and the stability of these
emulsions are strongly dependent of the different parameters of the system such as the Zrprecursor concentration, the pH of the sol and the oil amount.
Characterizations
pH and conductivity measurements
The pH of sols and solutions were measured (just before adding oil) using a glass combination
electrode connected to a Mettler Toledo SevenMulti pH-meter and calibrated at pH 2, pH 4
and pH 7. Conductivity measurements were performed (after adding oil) using the same
device in conductivity meter mode with Pt/Pt electrodes and calibrate at 84 μS.cm-1,1413
μS.cm-1 and 12.88mS.cm-1.
Tensiometer
Interfacial tension between the low density liquid (dodecane) and the high density liquid
(aqueous solution or sol) were measured using a Krüss K100 Tensiometer using the du Noüy
ring method. Briefly, a Pt-ring is moved from one phase to another. Interfacial tension can be
obtained measuring the maximum force acting on the ring during this operation.
Optical microscope
Optical microscopy of emulsions was performed using a Zeiss Axio Imager A1 microscope
coupled with an AxioVision software.
Result and Discussion
Sol description
The colloidal sol-gel transition based on zirconyl nitrate solution systems has been recently
investigated. [publi TSG]. The mechanisms involved in the formation of a ZrO2-x(OH)2x,yH2O
sol have been understood using Small Angle X-rays Scattering coupled with Raman
spectroscopy. The precursor ZrO(NO3)2 is dissolved into de-ionised water and a specific
complexing agent, acacH (regularly used for Zr-based system [Guinebretière et al., JofNonCrystalline Solids, 1992, 147&148, 542-547; Ekberg et al., Jof Solution Chemistry, 2004, 33,
47-79; Peyre et al. 1997]), is added. This complexing agent is used to prevent fast
precipitation of oxy-hydroxide aggregates when the pH is increased, allowing a precise
control of the system kinetic. AcacH is also known for its ability to stabilize Zr-based colloids
acting as a chelating agent [Peyre 1997].
After the dissolution of zirconyl nitrate salts in water, cyclic tetramers ([Zr4(OH)8(H2O)16]8+)
are formed [Clearfield 1956; Clearfield 1964]. These cyclic tetramers self-organize and stack
on top of one another into cylindrical objects which possess a length depending on the initial
precursor concentration. Furthermore, it can be noticed that the addition of the complexing
agent (acacH) does not significantly modify the cylindrical size objects [publi TSG].
By increasing the pH, condensation of the salt precursors lead to a dispersion of amorphous
zirconium oxy-hydroxides nanoparticles whose stability depends on the physicochemical
parameters of the system (pH, zirconium concentration and acacH concentration). The size of
these particles is difficult to precisely evaluate, particularly due to their high polydispersities
[publi TSG]. From a certain value of pH (≈3 depending on the precursor concentration),
stable clusters of nanoparticles are formed into the sol due to apparent attractive interactions.
A polydisperse system is therefore expected with independent nanoparticles of a few
nanometers coexisting with clusters of several hundred nanometers.
The study of the colloidal sol-gel transition of the system [ZrO(NO3)2, 6H2O + H2O + acacH
+ NH4OH] allowed the determination of gelation regimes as a function of the system
parameters [public TSG]. From these results, we choose systems implying sols stable for a
long time enough, namely systems without a sol-gel transition or having a very high gelation
time, for the formation of Pickering emulsions. In this way, the impact of the gelation on our
measurements is then negligible. Thus, the following relevant systems are:
-
[Zr]=0.1M and pH  5.5 with a K=[acacH]/[Zr] ratio of 0.5 and 1
-
[Zr]=0.15M and pH  5 with a K=[acacH]/[Zr] ratio of 0.5
Both influences of the initial precursor concentration and the complexing agent amount on the
emulsion behavior have an impact onto the Pickering emulsion feature and are shown below.
For each system, also the role of the pH has been studied.
Macroscopic observation of the emulsions
In a first step, the macroscopic behavior of the defined systems has been studied taking into
account the variation of two parameters: the pH of the initial sol and the added volume
fraction of dodecane. These parameters are critical and their study allows the establishment of
an indicative phase diagram for any composition. Different states have been observed and are
summed up into the figure 1. Some examples of these evolutions are also shown on the
pictures in figure 2.
Figure 1 : Indicative diagram of the emulsion
behaviour as a function of the pH and the oil
volume fraction for (a) [Zr]=0.1M, K=0.5 (b)
[Zr]=0.15M, K=0.5 and (c) [Zr]=0.1M, K=1
(a)
(b)
Figure 2: Examples of emulsion behaviour for (a) [Zr]=0.1M, K=0.5, fvol(dodecane)=0.6 at
different pH and (b) [Zr]=0.1M, K=0.5, pH=5 at different oil volume fractions
First of all, the figure 2a (photos) shows that for low pH (pH < 2), no stable emulsion is
formed and the system is quickly biphasic after several minutes. In the aqueous phase, at this
low pH, the Zr-based species are still under molecular form (cyclic tetramers
[Zr4(OH)8(H2O)16]8+) [Publi TSG], unable to stabilize the emulsion.
From pH ≈ 2, in all cases, we observe the formation of a triphasic system composed of a
stable viscous creamy-white phase, corresponding to an emulsion, between an organic and an
aqueous phase. This triphasic system is due to an oil excess released from the system in the
upper part on the one hand and to a creaming phenomenon in the O/W emulsion on the other
hand. At these low pHs, only few particles are already formed in the aqueous phase (Zr-based
sol) which is not significant enough to promote efficient stabilization of the emulsion for any
oil volume fraction. Therefore, the concentration of “active” Zr-based particles at the oilwater interface is not high enough and a part of the dodecane is released. Next, a phase
separation occurs between the anchored oil droplets and the aqueous phase. This is due to the
creaming phenomenon corresponding to a migration and accumulation of the stabilized
droplets at the surface of the aqueous phase by gravity. We have observed that the thickness
of the layer of the emulsion, in this triphasic system, increases with the pH at a fixed oil
volume fraction which is linked to an increase of the amount Zr-based particles with
increasing pH.
Increasing pH and depending on the different parameters (i.e., oil volume fraction, precursor
concentration and K= [acacH]/[Zr] ratio), three cases are observed:
-
A triphasic system similar to the one described above
-
A biphasic system that could be either due to dodecane excess or creaming
phenomenon.
-
A stable emulsion
It is noteworthy that efficient emulsion stabilization was obtained into the three systems
studied (see phase diagrams on figure 1). The oil volume fraction and the pH of the sol have a
direct influence on the state of the system. This is due to the relation existing between the
quantity of oil/water interface and the amount of particles into the sol available for the
interface stabilization.
For a fixed oil volume fraction, low pHs lead to small amount of particles unable to stabilize
the droplets giving rise to unstable emulsions, whereas increasing pH allows the formation of
a stable emulsion due to the presence of more and bigger particles.
For a fixed pH and for low oil volume fraction, we observe either a triphasic system, or a
biphasic system due to creaming. For high oil volume fraction, the system is either a stable
emulsion or a biphasic system due to the excess of oil. So, in all cases for a fixed pH, the
emulsion is more stable as the oil volume fraction is enlarged. This is due to the increase of
the emulsion viscosity occurring when the internal phase content is high. This tends to slow
down the creaming phenomenon. The oil volume fraction upper limit is reached when the oil
droplets close to packing parameter are exceeded (73%vol). This latter represents the
maximum inner phase content in an emulsion assuming oil drops as monodisperse spheres.
The figure 1 shows the phase diagrams corresponding to the three systems studied by varying
the Zr precursor concentration and the amount of added complexing agent. When the zirconyl
ions concentration is increased from 0.1M to 0.15M, the stabilization of the emulsion occurs
at lower pHs (figure 1a and figure 1b). Indeed, for higher precursor concentration, at fixed
pH, more particles are formed leading to a more efficient stabilization of the emulsion. The
influence of the [acacH]/[Zr] ratio (K) is also noticeable (figure 1b and figure1c). By
increasing this latter, the Zr-based particles aggregation is reduced thanks to a higher
stabilization of the colloids [PEYRE 1997]. This is due to the greater degree of complexation
of the chemically reactive sites by acacH, acting like a steric barrier. Consequently, increasing
K value, while Zr concentration is kept constant, leads to a less colloidal polydispersity. This
means that the specific surface of interfacial active Zr-based colloids increases, favouring a
better coverage of the oil drops. In this way, we observe a better stabilization of the emulsion
for [acacH]/[Zr]=1 than for [acacH]/[Zr]=0.5 from pH≈2.5.
Determination of the emulsions type
These emulsions seem to be plainly stabilized by the presence of the Zr-based colloids and
two cases can be distinguished: either an oil-in-water or a water-in-oil emulsion, depending
on the contact angle θ between the aqueous phase, the oil phase and the particle (measured
through the aqueous phase) [Aveyard et al., Adv in Colloid Interface Science,2003 , 100-102,
503-546]. The type of the emulsion can be simply determined from conductivity
measurements [Zhao et al., Chemical Engineering Science, 2013, 87, 246-257; Binks,
Langmuir, 2000, 16, 2539-2547; Binks, Langmuir, 2000, 16, 3748-3756; Binks, Langmuir,
2001, 17, 4540-4547; Binks, PCCP, 2000, 2, 2959-2967]. Indeed, for a dispersion of water in
oil, the conductivity of the emulsion is very low due to the apolar feature of the continuous
phase (order of magnitude of μS.cm-1), while for dispersion of oil in water, the conductivity is
much higher (order of magnitude of mS.cm-1), water being very conductive.
Conductivity measurements have been performed on the system in which emulsions are stable
or slightly creamed (pH ≥ 3). The table 1 summarizes the conductivity values measured.
Conductivity (mS/cm)
fvol(dodecane)
[Zr]i=0.1M and K=0.5
[Zr]i=0.15M and K=0.5
[Zr]i=0.1M and K=1
pHsol=3
pHsol=4
pHsol=5
pHsol=3
pHsol=4
pHsol=5
pHsol=3
pHsol=4
pHsol=5
0.4
4
3.1
2.9
5.6
5.3
7.6
2.4
3.6
2.7
0.6
2.5
2.4
2.1
4.1
3
5.6
2
2.2
2.4
0.8
1.9
2
1.6
2.5
2
2.5
1.8
1.3
1.7
Table 1: Summary of the conductivity values as a function of the system composition and at
different pH and oil volume fraction
The different values of conductivity are all of the order of magnitude of mS.cm-1. The
aqueous phase is therefore the continuous phase and the oil is dispersed into droplets
anchored by the Zr-based nanoparticles. We can thus conclude that all these emulsions are
directs (i.e. dispersion of oil droplets in water). In addition, none emulsion inversion [Binks
and Lumsdon Langmuir, 2000, 16, 2539-2547; Binks and Lumsdon, Langmuir, 2000, 16,
8622-8631; Binks, Philip and Rodrigues, Langmuir, 2005, 21, 3296-3302; Binks and
Rodrigues, Angewandte, 2005, 44(3), 441-444] occurs whatever the oil volume fraction, the
particle concentration or the pH. The contact angle θ (measured through the aqueous phase)
between the aqueous phase, the oil phase and the particle, would remain under 90° due to the
hydrophilicity of the ZrO2-x(OH)2x,yH2O nanoparticles leading to High Internal Phase
Emulsions (HIPE) when the oil volume fraction is upper than the water one [Silverstein,
Progress in Polymer Science, 2014, 39, 199-234; Ikem et al., Angewandte, 2008, 47, 82778279; Menner et al., Chem. Commun., 2007, 4274-4276].
Furthermore, we can observe that, for fixed pH and Zr and AcacH concentration values, the
conductivity of the emulsion decreases when the oil volume fraction increases. This
phenomenon is quite expected due to the presence of more and more organic species into the
emulsion.
Adsorption of the Zr based colloids on the water/dodecane interface
Interfacial tension measurements have been performed to assess the Zr-based particles ability
to adsorb at the dodecane/water interface. It is well known that particles adsorption on a
liquid/liquid interface induces lower interfacial tension values due to the decrease of the
interface free energy. Figure 3 shows the evolution of dynamic interfacial tension for various
systems. The curve evolutions are linked to the size of the objects adsorbed on the interface.
For small objects (i. e. molecules), kinetics are fast and the interfacial tension values converge
rapidly towards the equilibrium. This is mostly the case for samples of lowest pH supporting
the fact that Zr-based species are still under molecular form (pH ≤ 2).

pH ≤ 2 – K=0.5
The equilibrium interfacial tension value is reached quickly. This equilibrium value allows the
assessment of the interfacial activity of the dispersed species in water. For example,
acetylacetone (acacH) or zirconyl salts impact weakly the interfacial tension value decreasing
from 50 mN.m-1 in the pure dodecane/water system to respectively 44.3 mN.m-1 and 38.0
mN.m-1. This shows that cyclic tetramers, [Zr4(OH)8(H2O)16]8+ formed from zirconyl salts
dissolution, or acetylacetone molecules have only a slight effect on the interfacial tension.
While the decrease is substancial, the effect is not strong enough to stabilize emulsions (see
figure 1). However, the interfacial adsorption is enhanced as zirconyl salts and acacH are
associated since interfacial tension values go down to 26.4 - 30.2 mN.m-1 depending on the
precursor concentration or pH.
Water
Water+acacH
Water+Zr
pHi - [Zr]=0.1M
pH2 - [Zr]=0.1M
pH3 - [Zr]=0.1M
pH4 - [Zr]=0.1M
pH5 - [Zr]=0.1M
pHi - [Zr]=0.15M
pH2 - [Zr]=0.15M
pH3 - [Zr]=0.15M
pH4- [Zr]=0.15M
pH5 - [Zr]=0.15M
[Zr]=0.1M 50
Interfacial Tension (mN/m)
40
35
30
25
20
15
10
5
K=0.5
50
45
Interfacial Tension (mN/m)
Water
Water+acacH
Water+Zr
pHi - K=0.5
pH2- K=0.5
pH3 - K=0.5
pH4 - K=0.5
pH5 - K=0.5
pHi - K=1
pH2 - K=1
pH3 - K=1
pH4 - K=1
pH5 - K=1
45
40
35
30
25
20
15
10
5
0
0
0
5
10
15
20
0
5
10
15
20
Time (min)
Time (min.)
Figure 3: Evolution of dodecane/water dynamic interfacial tension for various pH. Effect of
the acacH concentration for systems with [Zr]=0.1M – Effect of [Zr] concentration for
systems with K=0.5

2 < pH ≤ 5 – K=0.5
As the pH increases, the interfacial tension kinetics evolution are slower; several minutes are
required to reach an equilibrium value assuming the fact that adsorbed objects are bigger than
for lower pHs. This occurs for the systems with pH values from 3 to 5. This phenomenon is in
agreement with the formation of Zr-based hard colloids whose the activity at the
dodecane/water interface induces a drastic interfacial tension decrease. Therefore, at these
pHs, the objects formed are bigger explaining the enhancement of the emulsion stability as
shown in the figure 1. The fact that bigger nanoparticles promote the emulsion stability is
widely known (ref Aveyard).
It is also noteworthy that dramatic breaks occur in the dynamic interfacial tension evolution
with time for systems containing Zr-based colloids (pH = 3 to 5). These breaks generally lead
to the interfacial lamellae rupture during the measurement due to the very low interfacial
tension values (e.g. [Zr]=0.15M – pH=5). Therefore, for these systems, there is no possibility
to finely measure the equilibrium interfacial tension because of such low values. We assume
that this phenomenon is due to interfacial adsorption of large clusters made of aggregated Zrbased nanoparticles. The effect of the adsorption of these clusters at the water/dodecane
interface is then huge, lowering interfacial tension values close to 0. Such low interfacial
tension values are therefore responsible for the very efficient stabilization of O/W emulsion
(see figure 1). Here, the Zr-based nanoparticles are so much efficient as emulsion stabilizers
that the system remains stable even if the oil ratio is very high such as in high-internal phase
emulsion (HIPE).

Influence of precursor concentration
Figure 3 also shows the effect of various system parameters on the evolution of
dodecane/water interfacial tension. For instance, the effect of precursor concentration is
assessed, showing that no big changes in the dynamic interfacial tension evolution are
observed for low pHs while higher values of pHs lead to noticeable changes. When pH ≥ 3,
colloids are made of Zr-based hard nanoparticles and the dynamic interfacial tension kinetic is
slower. It is therefore noteworthy that for systems containing 0.15M of Zr, the curve break
due to the adsorption of Zr-based clusters occurs at longer time values. This effect might be
due to the increased size of Zr-based clusters and to the drop of mobility induced. This leads
to a longer time needed for the cluster to move to dodecane/water interface.

Influence of K = [acacH]/[Zr] ratio
As previously explained, acetylacetone is a widely known complexing agent used to control
the sol-gel reaction kinetic [Guinebretière et al., JofNon-Crystalline Solids, 1992, 147&148,
542-547; Ekberg et al., Jof Solution Chemistry, 2004, 33, 47-79] and also to improve the Zrbased colloids stabilization [PEYRE 1997] by avoiding their aggregation. Using this molecule
allows to make a complex with zirconium, lowering the system reactivity. The figure 3 shows
how the amount of acacH influences the dynamic interfacial tension evolution, with Zr
concentration kept constant. Therefore, in this case, the specific surface of interfacial active
particles is increased allowing a better coverage of the oil drop and consequently a better
stabilization of the emulsion. The evolution of dynamic interfacial tension for K=1 systems is
in agreement with an emulsion behavior. The values are systematically lower than for the
K=0.5 systems for all pHs > 2. Furthermore, the kinetics is faster for higher K values
supporting the fact that the colloid mobility is higher. We assume that although smaller
colloids are known to be less favorable towards stabilizing emulsions [ref aveyard], here the
size decrease is balanced with the increase of interfacial active colloid specific surface.
Microscopic observation of the emulsions
Optical microscopy pictures of the formed emulsions have been performed in order to
characterize the size of the stabilized droplets and their evolution as a function of different
parameters: the pH value, the initial precursor concentration, the aging time of the emulsion
and finally the oil volume fraction. Using these pictures, we expect to observe the well-known
phenomenon of limited coalescence, characteristic of the Pickering emulsions [Destribats,
Adv. Funct. Mater., 2012, 22, 2642-2654; Binks, Langmuir, 2005, 21, 3296-3302; Arditty,
Eur.Phys.Journal, 2003, 11, 273-281]. If the amount of particles is not sufficient to cover the
totality of the oil-water interfaces, the emulsion droplets coalesce in order to minimize the oilwater interfacial area. Limited coalescence leads also to the formation of bigger droplets with
a narrow size distribution (sometimes until monodisperse systems). Three parameters have
been observed by optical microscopy: the roles of the sol pH, the initial precursor
concentration and the aging time of the emulsion. Some examples of pictures are given in
figure 4, 5 and 6.
(a)
(b)
Figure 4: Optical microscopic images of emulsion for the system [Zr]=0.1M, K=0.5,
fvol(dodecane)=0.4. Influence of the pH of the sol: (a) pHsol=3 (b) pHsol=5
(a)
(b)
Figure 5: Optical microscopic images of emulsion for the system K=0.5, fvol(dodecane)=0.4,
pHsol=4. Influence of the initial precursor concentration: (a) [Zr]=0.01M (b) [Zr]=0.1M
(a)
(b)
Figure 6: Optical microscopic images of emulsion for the system [Zr]=0.1M, K=0.5,
fvol(dodecane)=0.4, pHsol=4. Influence of the aging time: (a) t=0 (b) t=5days
The different emulsions are composed of polydisperse droplets with sizes between a few
microns and a few tens of microns. Surprisingly, no significant evolution is observed for any
parameters variation and the polydispersity is not reduced contrary to the case of the limited
coalescence phenomenon. This droplet freezing might be due to an irreversible bridging effect
[Giermanska-Kahn, Langmuir, 2005, 21, 4316-4323; Destribats et al., Langmuir, 2013, 29,
12367-12374; Xu et al., Langmuir, 2007, 23, 4837-4841]. Indeed, the formed Zr-based
particles tend to aggregate due to the condensation reaction of the sol-gel transition [public
TSG] which can promote particle bridging. Clusters of drops could therefore be induced. This
would lead to a rigid system supporting the fact that some oil drops are frozen without any
possibility to promote the limited coalescence phenomenon. (figure 4, 5 and 6).
The effect of the oil volume fraction has been investigated in order to compare the size
and distribution of the droplets. For a fixed composition of the initial sol, [Zr]=0.1M / K=0.5 /
pHsol=4, we performed optical microscopy on emulsion with oil volume fraction of 0.4, 0.6
and 0.8 (see figure 7).
(a)
(b)
(c)
Figure 7: Optical microscopic images of emulsion for the system [Zr]=0.1M, K=0.5,
pHsol=4. Influence of the oil volume fraction : (a) fvol(dodecane)= 0.4 (b)
fvol(dodecane)= 0.6 and (c) fvol(dodecane)= 0.8
As for the previous studied parameters, no significant influence is observed supporting
the hypothesis of a bridging effect into the emulsion leading to a frozen system. The droplet
diameters are highly polydisperse in a size range from several micrometers to tens of
micrometers. We can assume that the initial droplet size is depending of the shear rate applied
with the Ultra-Turax device as already observed in literature in case of bridging effect
[Sturzenegger et al, Soft Matter, 2012, 8, 7471].
Conclusions
Zr-based nanoparticles from colloidal sol-gel reaction are shown to effectively stabilize
dodecane-in-water emulsions. We have established phase diagrams by varying the main
physicochemical parameters as pH, complexing agent content, Zr precursor concentration and
dodecane volume fraction in order to define the existence domains of Pickering emulsions.
We point out the dual role of the acacH complexing agent which allows us to control the
colloidal grow-up kinetic and also to enhance the nanoparticles dispersion. By increasing it
content, the Zr-based particles aggregation is reduced, acacH acting like a steric barrier. We
show from dodecane/water interfacial tension measurements that the dynamic interfacial
tension kinetic is clearly slower due to the colloids anchored to the interface. Their
effectiveness to prevent coalescence is also enhanced by droplets freezing which might be due
to an irreversible bridging effect. We argue that this study based on synergy between
Pickering emulsion and a sol-gel process is a useful and efficient guide to explore and
formulate numerous free-surfactant emulsions.