Stability evaluation

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New Pickering Emulsions Stabilized by Bacterial Cellulose Nanocrystals
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DOI: 10.1021/la200971f · Source: PubMed
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2. Published: May 23, 2011
r 2011 American Chemical Society 7471 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
ARTICLE
pubs.acs.org/Langmuir
New Pickering Emulsions Stabilized by Bacterial
Cellulose Nanocrystals
Irina Kalashnikova, Herv
e Bizot, Bernard Cathala, and Isabelle Capron*
UR 1268 Biopolym
eres, Interactions et Assemblages, INRA, F-44316 Nantes, France
b
S Supporting Information
1. INTRODUCTION
An emulsion is a system of dispersed droplets of one im-
miscible liquid in another stabilized by emulsifiers. Since the
pioneering work of Ramsden1
and Pickering,2
solid colloidal
particles have been shown to adsorb at fluid interfaces to form
so-called “Pickering emulsions”. The properties of these systems
are due in part to the irreversible nature of such particle
adsorption.35
Interest in Pickering emulsions has been renewed
over the past 10 years, especially for health and cosmetics
applications where the use of surfactants is undesirable. Indeed,
they not only present good mechanical properties, but much
fewer particles are required to produce good stability, thereby
leading to a reduction in the use of hazardous surfactants and
their environmental consequences.6,7
Stabilization of solid particles at the interface is generally
considered to be directed by particle wettability, which is itself
directed by the more or less hydrophilic/hydrophobic nature of
the surface. The formation of an oil in water (o/w) emulsion will
occur for hydrophilic surfaces. On the opposite, a stable water-in-
oil (w/o) emulsion will be produced for hydrophobic surfaces.8
If the particles are completely wetted by water or oil, they may
remain dispersed in either phase and no stable emulsion will
form.9
Most of the literature dealing with Pickering stabilization
concerns various types of inorganic, commercially available
particles such as silica,4,1012
montmorillonite,13
Laponite,14
calcium carbonate,3,15
carbon graphite,3
or various latexes.4,16
However the raw, non-surface-treated nanoparticles are usually
not surface active due to their lack of hydrophilicity/hydropho-
bicity, or great tendency to aggregate. Methods involving surface
modifications6
or the addition of cosurfacting compounds17,18
have been developed to produce symmetrical or asymmetrical
(Janus) particles, but such methods increase the use of chemicals
and make the process complicated for large-scale production.
Research efforts are being focused on the development of
environmentally friendly, biobased nanocomposites, but few
studies, up to now, have described stabilization by particles
derived from renewable resources.1921
Cellulose is one of the
most widespread biopolymers and is a good candidate because of
its sustainability, biodegradability, and nontoxicity.
Cellulose is a linear homopolymer of β(1-4)-D-glucose resi-
dues linked together by glycosidic oxygen bridges. It exists in
nature as highly resistant fibers. The high cohesive energy is
induced by van der Waals forces and by extensive hydrogen bond
networks within and between the chains via the hydroxyl groups
in equatorial position.22
These fibers are formed by the stacking
of microfibrils, and it is now recognized that the ordered crystal-
line regions are interrupted by amorphous or less ordered regions
along the elementary fibril where the microfibrils are distorted by
internal strain.23,24
A few investigations have used cellulose derivatives such as
microfibrillated cellulose (MFC), which are moderately de-
graded microfibrils (510 μm long), to stabilize oil in water
emulsions,19,25
but their long length compared to the size of
the droplets produced networks rather than individual drops.
In order to control the emulsion characteristics, the particle size
must be reduced. Concerning interfacial stabilization, most of
the studies have been carried out on hydrophobically modified
cellulosic material26
leading to water in oil emulsions. They
Received: March 15, 2011
Revised: May 9, 2011
ABSTRACT: We studied oil in water Pickering emulsions stabilized
by cellulose nanocrystals obtained by hydrochloric acid hydrolysis of
bacterial cellulose. The resulting solid particles, called bacterial cellu-
lose nanocrystals (BCNs), present an elongated shape and low surface
charge density, forming a colloidal suspension in water. The BCNs
produced proved to stabilize the hexadecane/water interface, promot-
ing monodispersed oil in water droplets around 4 μm in diameter
stable for several months. We characterized the emulsion and visua-
lized the particles at the surface of the droplets by scanning electron microscopy (SEM) and calculated the droplet coverage by
varying the BCN concentration in the aqueous phase. A 60% coverage limit has been defined, above which very stable, deformable
droplets are obtained. The high stability of the more covered droplets was attributed to the particle irreversible adsorption associated
with the formation of a 2D network. Due to the sustainability and low environmental impact of cellulose, the BCN based emulsions
open opportunities for the development of environmentally friendly new materials.

3. 7472 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
involved acetylated nanofibers,27,20
sililated microfibrils,28
or carboxymethylated cellulose,29
but none reported the
production of stable oil in water droplets. It has been known
for many years that stable nanoparticle suspensions can be
prepared by treating native cellulose to harsh acid
hydrolysis.30,31
Acid hydrolysis is a heterogeneous process
which involves first the diffusion of acid into the fibers followed
by the cleavage of the glycosidic bonds, resulting in a turbid
colloidal suspension. Sulfuric acid is commonly used in introdu-
cing sulfated charges at the surface. The absence of charge
generally leads to the production of crystalline aggregates.32
In
the literature, these crystallites are referred to as whiskers or
nanocrystalline cellulose.
In this work, cellulose nanoparticles were prepared via hydro-
chloric acid hydrolysis of bacterial cellulose. The bacterial
cellulose was chosen on its availability since it can be easily
produced from nata de coco, in which cellulose is present at
rather high concentration. Moreover, it is possible to get it wet,
whereas most of the other sources available provide dry material
and present limited contaminants, that is, without any hemi-
cellulose or various byproduct generally surrounding cellulose.
It is then possible to hydrolyze it with milder conditions and,
notably, to avoid the use of sulfuric acid which promotes highly
charged surfaces. Finally, characteristic features of bacterial
cellulose nanocrystals (BCNs) are their large aspect ratios and
flat, ribbonlike cross sections. As a result, pure low charged BCNs
were used, without further desulfation, modification, or addition
of any cosurfacting product, to stabilize the interface of an oil
in water emulsion. These BCNs were characterized, and the
stability of the emulsion, drop size, and coverage characteristics
were investigated.
2. MATERIALS AND METHODS
2.1. Materials. Nata de coco was a gift from Mrs. Joann Dar, who
bought it in San Jose (Central Luzon, Philippines) from a traditional
producer. All other products were purchased at 99% or greater purity.
n-Hexadecane (Sigma) was cleaned thoroughly with water to remove
any remaining surfactants. Styrene (Fluka) was purified: tert-butylcate-
chol inhibitor was removed by percolation through a cartridge filled with
activated alumina (EcoChrom N super1) regenerated by treatment at
400 °C. Polymerization was initiated with V65 (2,20
-azobis(2,4-dimethyl
valeronitrile)) which was a gift from Qingdao Runxing Photoelectric
Material Co. (LTD, Qingdao, China). For CLSM BODIPY (4,4-
difluoro-4-bora-3a,4a-diaza-s-indacene) (Molecular Probes Invitrogen,
Eugene, OR) and calcofluor (Sigma) were used without purification.
2.2. BCN Preparation. BCNs were obtained from nata de coco
cubes ground in a Waring blender at maximum speed. The resulting
slurry was filtered and resuspended in 0.5 N NaOH and then stirred in a
lidded flask for 2 h at 70 °C. Alkali was removed by rinsing with distilled
water to neutrality. After that, bleaching treatment was performed twice
by mixing the suspension with a 8.5 g/L NaClO2 solution in sodium
acetate buffer (pH 4.5) at 70 °C for 2 h. The bleached cellulose was
rinsed with distilled water until the pH remained stable.
The purified bacterial cellulose was then hydrolyzed with 2.5 M HCl
at 70 °C for 2 h as described by Gilkes et al.33
Acid was removed by
successive centrifugations during 10 min at 10 000g and redispersion of
the solid materials in ultrapure water (18.2 MΩ) up to pH 5. The
suspension was further homogenized by ultrasound treatment for about
1 min (ultrasonic processor XL 2020, Misonix, Farmingdale, NY). The
resulting 1% suspension was then extensively dialyzed against water
to remove any remaining contaminants and deionized by mixing with
mixed bed ion-exchange resin. For storage, one drop of CHCl3 per
250 mL was added to the suspension and kept at 4 °C.
2.3. Transmission Electron Microscopy (TEM). A total of
20 μL of a BCN suspension in water (0.1% w/v) was deposited on a
freshly glow-discharged carbon-coated electron microscope grid (200
mesh copper, Delta Microscopies, France) and the excess removed.
Negative staining was performed by adding 20 μL of uranyl acetate
solution (2% w/v). The excess solution was removed and the grids were
dried in an oven at 40 °C just before observation. The grid was observed
under standard conditions with a JEOL JEM-1230 transmission electron
microscope operating at 80 kV.
2.4. Emulsion Preparation and Characterization. Emulsion
Preparation. The oil in water (o/w) emulsions were prepared using
hexadecane and a BCN aqueous suspension at the required concentra-
tion without further dilution. All the emulsions were prepared using an
oil/aqueous phase ratio of 30/70. Practically, 0.3 mL of hexadecane was
added to 0.7 mL of aqueous suspension in a plastic vial and sonicated
with an ultrasonic device with a dipping titanium probe close to the
surface (power level at 1.5 dial value corresponds to 2 W/mL applied
power determined by heat balance), alternating 3 s sonication with a 3 s
standby for 20 s.
Stability Test. Stability of the generated emulsions was checked by
centrifugation for 2 min at 4000g. The emulsion volume and drop
size measured for various centrifugation times kept stable from 30 s to
40 min. Photographs of the vials containing the emulsions were taken
with a P1 digital camera (Olympus), and the thickness of the creaming
layer was measured with a digital caliper.
Light Microscopy. The emulsions were all visualized by light micro-
scopy. A total of 15 μL of the resulting Pickering emulsion was added to
1 mL of distilled water and stirred by vortex, then a single drop was
poured onto a slide and observed via microscope (BX51 Olympus).
Droplet diameter was measured by image analysis using the “ImageJ”
software and compared to the drop size distribution determined by laser
light diffraction using a Malvern 2000 granulometer apparatus equipped
with a HeNe laser (Malvern Instruments, U.K.) with Fraunhofer
diffraction. The measurements were systematically carried out in tripli-
cate. The diameter was expressed as surface mean diameter D(3,2) (the
Sauter diameter), and eventual aggregation was checked by comparing
these values with those where sodium dodecyl sulfate (SDS) was added
just before measurement. Similar results were obtained via granulometer
and ImageJ analysis.
Sugar Analysis. In order to evaluate the amount of cellulose involved
in the droplet stabilization, quantification of the cellulose released in the
aqueous subphase after centrifugation was carried out by colorimetric
titration after sulfuric acid degradation. Subphases were lyophilized,
dispersed in 26 N sulfuric acid for 1 h at 25 °C, and afterward hydrolyzed
in 2 M sulfuric acid for 6 h at 100 °C before colorimetric titration with
orcinol sulfuric acid method with a Skalar autoanalyser. Glucose was
used as a standard.
2.5. Confocal Laser Scanning Microscopy (CLSM). For
CLSM visualization, the emulsions were prepared with double staining.
Hexadecane and cellulose were stained with BODIPY (564/570)
fluorophore andcalcofluor (347/450) respectively. Images were acquired
using a Zeiss LSM 410 confocal microscope (Zeiss, Gottingen, Germany)
using a x40 water-immersion lens with an optical section thickness
around 2 μm.
2.6. Scanning Electron Microscopy (SEM). Because of hex-
adecane volatility, the emulsions formed could not be introduced in
the SEM chamber. The BCNs proved to stabilize the styrene/water
interface and form a stable emulsion. This ability to form solid particles
was then used to visualize the distribution of BCNs at the drop inter-
face by SEM. Therefore, aqueous suspensions containing 0.5, 1, and
5 g/L BCNs were sonicated for 20 s and degassed with nitrogen for
10 min. Styrene was mixed with V65, an oil-soluble initiator, to obtain a

4. 7473 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
styrene/V-65 ratio of 120/1 (w/w).14
A total of 86 μL of this mixture
was added to 1.0 mL of the BCN suspension, and an emulsion was
prepared by sonication for 20 s (3 s pulse on, alternating with 3 s pulse
off). Then 500 μL of water was added to dilute the already formed
Pickering droplets. After vortex mixing, the emulsions were again
degassed with nitrogen gas for 10 min before polymerization at 63 °C,
without stirring, for 24 h. The dried beads were metallized with platinum
and visualized with a JEOL 6400F instrument.
2.7. Coverage Determination. Coverage was determined from
the amount of particles involved in the emulsion and the oil effectively
trapped within the droplets. The absence of BCN in the nonemulsified
aqueous phase after centrifugation was confirmed by sugar analysis
after sulfuric hydrolysis. The surface coverage C is then given by the ratio
of the theoretical maximum surface susceptible to be covered by the
particles Sp and the total surface displayed by the oil droplets Sd:
C ¼
Sp
Sd
ð1Þ
where
Sp ¼ NpLl ¼
mp
hFp
ð2Þ
and
Sd ¼ 4πR2 3Voil
4πR3
¼
3Voil
R
ð3Þ
where Np is the number of BCNs, L, l. and h are, respectively, the length,
width, and thickness of BCN, mp is the mass of BCNs, Fp is the BCN
density, R is the average drop radius, and Voil the volume of oil included
in the emulsion after centrifugation.
The total coverage can be expressed as
C ¼
mpD
6hFVoil
ð4Þ
where D is the D(3,2) average radius of the droplets.
3. RESULTS AND DISCUSSION
3.1. BCN Preparation. BCNs were isolated by acid hydrolysis
to remove the amorphous part of the microfibrils and then
dispersed using a sonication device. Bacterial cellulose hydrolysis
is generally performed with sulfuric acid which leads to charged
particles,34
whereas hydrolysis with hydrochloric acid results in
slightly carboxylated surfaces.35
The nanocrystals generated by
sulfuric or hydrochloric hydrolysis have rectangular cross sec-
tions or a ribbonlike shape with dimensions of ca. 10 nm 50 nm
and range in length from 100 nm to several micrometers, with
these dimensions depending on the hydrolysis conditions.34,36
The bacterial cellulose used in this work was hydrolyzed with
hydrochloric acid. At the end of the process, the sample charge
density was measured by conductometric titration (see the
Supporting Information). This titration revealed a low level of
residual charges, involving weak charge groups such as carboxylic
groups, below 103
e/nm2
. TEM images of the suspension
showed that the crystalline particles were fairly homogeneous,
despite a tendency to aggregate due to the absence of repulsive
surface charges (Figure 1). This aggregation is not observed with
sulfated cellulosic nanoparticles such as cotton whiskers.31
Analysis of the TEM images using ImageJ software indicated
an average length of 855 nm and width of 17 nm. A thickness
of 7 nm was determined by atomic force microscopy (AFM)
(see the Supporting Information). These data are in accordance
with most other authors who reported sizes for the individual
nanocrystals from bacterial cellulose thickness ca. 510 nm and
length from 100 to 1000 nm for acid-treated samples37,38
whereas the reported width ranged from 5 to 50 nm.35,37
As a
result, the particles produced show crystalline, elongated, sub-
micrometer sized characteristics.
3.2. Emulsion Preparation. The aqueous phase, for all tested
emulsions, consisted of cellulosic nanocrystalline particles dis-
persed in water at the required concentration without further
treatment. Hexadecane was added afterward. The biphasic
system was then sonicated, resulting in a very stable oil in water
emulsion.
Respective localization of the nanocrystals at the oil interface
and ordering of the droplet’s surface was checked by CLSM and
SEM. Calcofluor was used as fluorochrome for cellulosic materi-
al, while BODIPY presenting affinity for hydrophobic liquids was
dissolved in the oil phase. This double staining allows one to
clearly distinguish the inner oily phase and the surface covered by
cellulosic particles (Figure 2a). Surface organization was exam-
ined in detail by SEM visualization of the individual droplets.
Although cryo SEM would allow direct observation of oil in water
Pickering emulsions,39
polymerizable resins offer convenient
alternatives for oil in water emulsion using the classical SEM
approach. Bon and Colver prepared latex particles using Lapo-
nite clay discs as stabilizers, with the inner part being polymerized
from a variety of hydrophobic monomers.14
BCNs showed the
same ability to stabilize styrene and hexadecane droplets, and
therefore, styrene was selected. An aqueous BCN suspension
was then mixed with styrene, emulsified using the same above-
described emulsifying protocol, and visualized (Figure 2b).
Despite this process being different from the hexadecane one,
involving styrene, the BCNs displayed analogous interfacial pro-
perties for the styrene/water emulsion and a hexadecane/water
Figure 1. BCN morphology by TEM of a negatively stained dilute suspension of BCN.

5. 7474 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
emulsion. The polymerization step served just as a fixing step
of the beads formed. We therefore assumed that the resulting
emulsion was representative of the present studied system.
Several very small noncovered white beads (diameter of the
beads was generally below 100 nm) were observed for all
the samples. These were artifactual outgrowths formed during
styrene polymerization. SEM images showed that the BCN
nanoparticles were evenly distributed at the surface and bent to
follow the curvation of each droplet. It means that the cellulosic
nanocrystals when wetted by the hydrophobic phase present
enough flexibility to bend along the surface. In other words, the
tension required for droplet formation is high enough to force
these colloidal particles to align without desorbing them. We can
assume that they will align preferentially according to the face
presenting the lower thickness which is 7 nm.
3.3. Emulsion Stability. Pickering emulsions were formulated
at a 30/70 ratio (o/w), with a 5 g/L suspension of BCN in
the water phase. These conditions were used to test stability
under various temperature conditions. Very high stability was
observed, without change in droplet size, even under various
mechanical treatments of the cream layer such as vortex or rotor
stator blender at 15 000 rpm. No variation in droplet size was
also observed after centrifugation or after keeping the samples
for 1 month at 4 °C, at 40 °C or up to 2 h at 80 °C. These various
tests even after dilution were performed over months. The
absence of drop size variation in these conditions consolidates
the fact that irreversible adsorption occurs; indeed desorp-
tion might lead to phase separation, and this has never been
observed. Further tests were then carried out to specify stability
characteristics.
An emulsion is commonly considered stable if it is resistant to
physical changes over a practical length of time. It can be tested
by several methods including centrifugation, filtration, shaking
or stirring, low intensity ultrasonic vibrations, or heating. All
the listed methods used for emulsion destabilization induce
mostly creaming or sedimentation, flocculation, and eventually
coalescence. Concretely, creaming forces contact (collision and
sticking) between the droplets and allows coalescence to occur
whereby the majority of the droplets merge creating fewer larger
droplets, thereby reducing the total interface area of the system.
As hexadecane has a lower density than water (d = 0.82 g/mL at
25 °C), a creaming process was always observed. However
an emulsion can be considered stable as long as no coalescence
occurs; that is, the size and size distribution should not change.
When the emulsions were characterized over time, a stable drop
size was observed for several months although natural creaming
occurred. Centrifugation accelerates the creaming process,
forcing the droplets to concentrate. The excess water is then
excluded from the emulsion, leading to close packing conditions.
Centrifugation at 4000g was considered suitable for following the
resistance of droplets to coalescence. Two parameters were used
to characterize and evaluate the stability of the resulting emul-
sions: (i) the average drop size obtained after dispersion of a few
microliters of the cream in water and (ii) the volume of the
emulsion after centrifugation.
The ability of BCNs to stabilize the droplets was assessed by
varying the concentration of nanocrystals in the aqueous suspen-
sion from 0 to 5 g/L. Before centrifugation, the emulsion volume
increased regularly with the amount of BCN added, for a fixed oil
content (Figure 3). This indicated that trapping of the aqueous
phase was increased when a larger amount of BCNs was present.
After centrifugation at 4000g, the emulsion with concentrations
below 2 g/L of BCNs “broke up”, and the emulsion volume could
no longer be measured. For concentrations above 2 g/L of BCNs,
dense white emulsions were obtained that resisted to the stress
induced by centrifugation.
The emulsion ratios, defined as the volume of oil over the
volume of emulsion, were calculated and are shown in Figure 4.
At BCN concentrations below 2 g/L, the oil and water phases
separate and coalescence occurs by compression during the
centrifugation process. At higher BCN concentrations, the
percentage of oil trapped into the emulsion increases sharply
to reach a plateau value of around 0.7. This value is worth noting,
as it is close to the theoretical close packing condition of 0.74 for
monodispersed spheres. It is worth noting that the same limit
was observed whatever the centrifugation speed at 4000g and
10 000g, leading to the same packing conditions whatever the
BCN concentration. These droplets, that are resistant to cen-
trifugation, can also be dispersed in water and shaken without
disruption. The centrifugation deformed the droplets and ex-
cluded the water layers present at the interface without breaking
the droplets. The total volume of oil remained trapped within the
emulsion, with no modification of emulsion volume whatever the
concentration of BCN used, when centrifugation was stopped,
that is, after relaxation of the droplets. Above 2 g/L, the
emulsions displayed excellent mechanical resistance toward
deformation and coalescence.
3.4. Droplet Characterization. Droplet size variations were
monitored by measuring the surface mean diameter D(3,2) and
varying concentrations of BCN in the water phase for a fixed
amount of oil. Results are given for a ratio of 30/70 (o/w)
(Figure 5).
Droplet size showed a clear tendency to coalescence at the
lowest concentrations of BCN. Then sizes decreased to a plateau
Figure 2. Surface contribution of BCN. (a) Confocal laser scanning micrograph of droplets stabilized by BCN with double staining (hexadecane phase
stained with BODIPY564/570, BCN stained with calcofluor). (b and c) Scanning electron micrographs of styrene Pickering emulsion stabilized by BCN
and polymerized using V65 initiator.

6. 7475 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
value of 4.2 μm diameter for all emulsions containing more than
2 g/L of BCNs in the water phase. The upper image in Figure 3
taken before centrifugation showed that the emulsion volume
increased regularly with the amount of BCNs. Below 2 g/L, the
increasing amount of BCN particles resulted in decreasing average
droplet size which led to a larger interfacial area and thus to a
higher emulsion volume. Similar coalescence tendency has been
previously reported for other types of Pickering emulsions.3,7,40
The method chosen to obtain our emulsions is based on a
limited coalescence41,40
process for the samples prepared with
the lowest BCN concentrations and up to 2 g/L. This occurs
when sonication produces a much larger area of the oil/water
interface than can potentially be covered by the nanocrystals.
When sonication is stopped, the partially unprotected droplets
coalesce. This coalescence results in a decrease of the oil/water
interface and stops as soon as the interface is sufficiently covered.
For the samples prepared with the highest BCN concentrations,
at and above 2 g/L, a strong emulsion of stable droplet diameter
around 4 μm was obtained, irrespective of the amount of particles
added, which was resistant to deformation. The drop size limit
has rarely been discussed in literature. One argument comparing
the drop size according to the surface chemistry of the nano-
particles was analyzed in terms of emulsion stability. It was
observed that the droplets were smaller when more stable
emulsion conditions were used.9,42
In the present study, the
sonication process involved a rather high energy input with
production of submicrometer size droplets and the stability of
BCNs at the interface was significant. The size limitation might
therefore be due to the size and flexibility of the solid particles.
Indeed, BCNs consist of crystalline particles 855 nm in length,
and the size of the particles might be correlated to the droplet
size. No conclusions can be drawn from the present results,
but different nanocrystal shapes can be produced according to
the cellulosic origin. The comparisons should be performed with
particles differing in morphology to validate this assumption.
3.5. Evaluation of Coverage. In order to check that the
particles had been irreversibly adsorbed at the o/w interface, the
particles remaining in the aqueous subphase after emulsification
and centrifugation were quantified by determining the sugar
content. No or negligible amounts of BCNs were found outside
the emulsion (data not shown) which meant that all the particles
introduced were involved in the emulsion, at the concentration
range under study.
A relationship between the mass of solid particles (mp) and the
average droplet diameter (D) has already been proposed.16
According to the equations proposed in section 2, the final
interfacial area should be controlled by mp. The average droplet
diameter (D) should then be inversely proportional to the
amount of solid particles included (mp):
1
D
¼
mp
6hVdC
ð5Þ
where C is the surface coverage, that is, the percentage of droplet
surface area covered by cellulosic particle, h is the thickness of the
BCN covering layer (7 nm for a monolayer), and Vd is the total
volume of the dispersed phase. The evolution of the inverse
diameter (1/D) for the various amounts of BCNs introduced
showed a linear increase, followed by stabilization at the higher
concentrations (Figure 6).
The first part of the plot increased linearly over the lower
concentrations, showing the same mode of coverage for all the
emulsions. This was observed up to a critical concentration
defined by the deviation to linear mode in Figure 6. The tendency
to plateau shown by the second part of the curve would indicate
changes in the coverage arrangement.
These results indicate that when a limited amount of BCNs
is introduced, compared to the oil volume present, individual
droplets are produced for which the diameter is determined by
the amount of stabilizing particles available according to the
limited coalescence process. The coalescence process was
Figure 4. Ratio of hexadecane in the emulsions (v/v) measured after
centrifugation at 4000g for increasing BCN concentrations in the water
phase (g/L). The line serves to guide the eyes.
Figure 3. Hexadecane/aqueous BCN o/w emulsion stability (a) before centrifugation and (b) after centrifugation at 4000g for increasing BCN
concentration in the water phase from 0.1 to 5 g/L.

7. 7476 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
observed up to a critical amount of 5.2 mg of BCNn added to
the water phase to stabilize 1 mL of hexadecane to produce the
droplet diameter of 4.2 μm. It corresponded to a concentration
of 2.2 g/L BCNn in the water phase as previously defined.
Above this concentration, more and more nanocrystals can be
added without changing the droplet diameter. Cellulosic ma-
terials are known to aggregate in the aqueous phase via van der
Waals interactions and hydrogen bonding. The induced over-
load of material might result in flocculation due to particles
nonadsorbed or only partially adsorbed at the interface. This
would produce interconnected particles and change BCN
organization at the surface of the droplets resulting in the slope
variation.
Interface organization was investigated by calculating the
surface coverage according to eq 4, for emulsions prepared
with different BCN concentrations. These values were then
plotted against mp, that is, the mass of BCN in the aqueous
phase, as shown in Figure 7, to characterize the packing density
of the BCN particles at the interface. At low concentrations,
the few droplets showed identical coverage up to 5.2 mg BCN/
mL hexadecane, the minimum required for stable droplets
having 60% coverage. After that, the percentage coverage
Figure 5. Droplet size dependence on BCN content (upper figure) for droplet diameters D(3,2) vs BCN concentration in the water phase in an
emulsion containing hexadecane with a 30/70 oil/water ratio. (lower images) Transmission optical micrographs of the same emulsions; the
corresponding concentrations are given in the images.
Figure 6. Limited coalescence process visualized by the inverse D(3,2)
diameter plotted against the amount of particles included in the water
phase per milliliter of hexadecane.
Figure 7. Evolution of coverage vs the amount of particles included in
the water phase per milliliter of hexadecane for a 30/70 (o/w) ratio.

8. 7477 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
increased to 100% for the highest BCN concentration tested,
that is, nearly 12 mg/mL hexadecane (which corresponds to a
suspension of 5 g/L BCN). This 60% coverage value is not
surprising, as values ranging from 22% to 70% have been
reported in the literature for other Pickering systems;40,43
even
a value as low as 8% was defined for concentrated latex
micrometer-sized particles stabilized by nanosized spherical
silica particles.16
This coverage is defined as the ratio of the area which could be
covered by the particles (length width amount of particles)
and the calculated total area of the oil droplets, taking into
account the D(3,2) drop diameter measured for each emulsion of
varying BCN concentration. The average BCN dimensions of
individual nanocrystals are used, measured by TEM and AFM
microscopies, without taking into account possible aggregation.
We can therefore assume that the measured available nanocrystal
surface and consequently the calculated coverage are overesti-
mates compared to the truly available area.
As the particles are irreversibly adsorbed at the interface and
the principle of Pickering stabilization results in partial wetting
adsorption, the formation of multilayers was unlikely. However,
above 5.2 mg/mL hexadecane, the percentage coverage increased
to even more than 100%. This could be attributed to the shape
of the particles, the elongated form and relative flexibility at the
surface leading to overlapping of the particles at high concentra-
tions. Thus, a simple geometric approach presents a limitation to
describe the BCN coverage properties due its intrinsic morpho-
logical and associative characteristics. This was checked by SEM
visualization of the droplet surface prepared by using BCN
concentrations of 0.5, 1, and 5 g/L. Figure 8 shows SEM images
of the polystyrene droplets covered by nanocrystals. Comparison
of the different preparations revealed that, whatever the amount
of BCN introduced, the nanocrystals bent along the beads,
overlapping one over each other and eventually forming long
fibers. The tendency to aggregate inherent to the uncharged BCN
nanostructure was observed even at the lowest concentration.
However, coverage was lower for the emulsion prepared with the
0.5 g/L suspension, and greater overlapping of particles was
observed with the 5 g/L suspension.
Several papers highlighted the outstanding stabilizing proper-
ties of rodlike particles,26,44,45
and BCNs are generally assumed
to be rigid crystals with a high aspect ratio but their thickness of
7 nm allows a certain flexibility when aligned along the droplet
interface. We therefore never observed any “nestlike” structures
as obtained for very rigid particles such as CaCO3.46
Finally, if
this flexibility is the limitation point, the plateau observed for
droplet size around 4 μm in diameter might be correlated with
this ability of the BCNs to bend. As a result, it would also be the
main explanation to the break point observed in Figure 6. Once
the maximum curvature of the particles is reached, the interface
area is fixed and adding BCNs induces a new coverage organiza-
tion. However, if the strength necessary to stay bent at the
interface becomes high, the particles might then desorb from the
interface while we observed a very high stability of these drops.
The BCNs present the particularity to self-associate, and what-
ever the concentration a more or less densely packed 2D network
forms at the surface of the droplets. This network is highly stable
both because the energy of adsorption prevents the particles
from being removed from the interface and because particle
aggregation prevents coalescence. Steric hindrance appears to be
the major mechanism responsible for droplet stability.47
The
strongadhesionofparticlestothe interfaceconfers to the emulsion
high resistance to coalescence and high stability. This has to be
attributed to the particular behavior of cellulosic derivatives, that is,
their elongated shape and the low charged surface chemistry
forming an entangled network of isolated nanocrystals.
ConsideringBCNs themselves, these quasi-neutral particles are
generally considered as hydrophilic matter, but they can stabilize
at the oil/water interface. Being regular crystalline particles, they
are organized according to various crystalline plans and some
of these plans expose a more hydrophobic surface.48
When well
oriented, this may promote the particle interfacial properties. As a
result, the BCNs prepared present both hydrophilic and hydro-
phobic characters in a good ratio. Indeed, with higher surface
charged density, they do not stabilize emulsions because their too
high affinity for the aqueous phase prevents stabilization at the
o/w interface. On the other hand, the absence of charge would
induce formation of big aggregates unable to form individual
Figure 8. SEM images for metallization of a polymerized styrene/water emulsion stabilized by BCN suspensions, from left to right, at 0.5, 1, and 5 g/L.

9. 7478 dx.doi.org/10.1021/la200971f |Langmuir 2011, 27, 7471–7479
Langmuir ARTICLE
droplets. Contact angle determination is difficult to conduct at
the liquid fluid interface; however, some methods such as the one
developed by Paunov49
might help to understand the attachment
of the particles to the interface. As a result, bearing no chemical
function and resisting to high acid hydrolysis, the present BCNs
allow droplet stabilization over a large domain of pH, time, and
temperature.
Bacterial cellulose nanocrystals present, for example, an
aspect ratio significantly larger than that of wood and cotton
nanocrystals.34
For further emulsion studies, it is known that cel-
lulosic nanoparticles varying in shape (length and aspect ratios)
can be produced according to the cellulose origin (cotton, algae,
animal) or after treatments such as recrystallization (regenerated
cellulose II). From the literature, various lengths can be obtained
from some tens of nanometers to several micrometers. Since the
coverage organization can be modified by the crystal morphol-
ogy, we can expect variations in the emulsion characteristics
particularly from the curvature adaptability and networking
aptitude. Their shape versatility and high stability added to their
biorenewable property as raw material make the present BCNs or
other cellulosic derivatives outstanding sources for study of the
Pickering interfacial stabilization process and large application
domains such as for cosmetic, medical, or food industry notably.
4. CONCLUSIONS
Bacterial cellulosic nanocrystals were produced by hydrochlo-
ric acid hydrolysis and resulted in nanoparticles of very low
charge density. After sonication, they resulted in a suspension
stable for several months. This process provided nanosized
cellulosic particles which were directly able to stabilize irrever-
sibly an oil in water Pickering emulsion. BCNs can therefore be
used to prepare a monodispersed emulsion without any further
modification or wrapping process.
The emulsions produced have been characterized and proved
to be stable over months whatever the BCN concentration as
long as no centrifugation was performed. However, above a limit
evaluated at 5.2 mg of BCN to stabilize 1 mL of hexadecane, an
emulsion able to be dispersed in water and resistant to centrifu-
gation at 4000g was obtained.
Studies varying the amounts of nanoparticles were carried out
to investigate the surface organization of the droplets stabilized by
BCNs. At the lowest concentrations, limited coalescence occurred
and the droplet size decreased up to about 4 μm diameter.
Droplets with a constant surface coverage of 60% were formed,
which represents for our system the minimum required tostabilize
the emulsion. When more than 5.2 mg of BCN/mL of hexadecane
was introduced, the BCN particles were in excess and presented a
much greater available surface area than was required to stabilize
4 μm diameter droplets. The surface coverage increased toward
100% as the amounts of added BCN were increased. However, no
BCN was found in the aqueous phase after centrifugation. We
propose that irreversible adsorption of the particles, associated
with steric hindrance of the 2D network formed, at the interface
was responsible for the very high droplet stability obtained.
’ ASSOCIATED CONTENT
b
S Supporting Information. Additionnal results about the
BCN surface charge density by conductometric titration and
thickness by AFM determination. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: capron@nantes.inra.fr. Telephone: 33(0)240675095.
’ ACKNOWLEDGMENT
The authors thank Brigitte Bouchet for assistance in TEM
experiments (BIBS platform, INRA Nantes, France), Nicolas
Stephan for assistance in SEM experiments (IMN, Nantes,
France), Patricia Bertoncini for AFM measurements, Solene
Grosbois for excellent technical assistance, and R
egion Pays de
la Loire (RMB network) for financial support.
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