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Design of Pickering micro and nanoemulsions based on the structural characteristics of nanocelluloses Clara Jiménez Saelices, and Isabelle Capron Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.7b01564 • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Design of Pickering micro and nanoemulsions based on the structural characteristics of nanocelluloses Clara Jiménez Saelices, Isabelle Capron* UR1268 Biopolymères Interactions Assemblages, INRA, 44316 Nantes, France

ABSTRACT

The development of biobased materials with lower environmental impact has seen an increased interest these last years. In this area, nanocelluloses have shown a particular interest in research and industries. Cellulose nanocrystals (CNC) and cellulose nanofibrils (CNF) are both known to stabilize oil-water interface forming the so-called Pickering emulsions which are surfactant-free, highly stable emulsions armored by a layer of solid particles. This work describes the emulsion’s characteristics and properties according to particle sizes, shape and surface chemistry in order to produce controlled micro- and nano-emulsions stabilized by nanocelluloses. For this purpose, four nanocelluloses which vary in source, length, width and surface charge density were used. Isolated droplets were produced by CNCs and interconnected droplets by CNFs that led to distinct drop size (micro and nano-sized), organization of nanoparticles at the surface of the droplets, stability and mechanical properties through rheological measurements. This work makes a precise description of the resulting emulsions and shows ability to produce nanosized droplets for CNC and TEMPO oxidized CNF but not for the less fibrillated CNF using HP-

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homogenizer. Individual non creaming droplets with average diameters as low as 350 nm were achieved for cotton CNCs and TEMPO oxidized CNFs.

KEYWORDS. Cellulose, Pickering emulsion, Biopolymers, nanoemulsion

INTRODUCTION The use of biobased nanoparticles converted into ordered structures has seen an increased interest these last years to develop materials with lower environmental impact as well as new or improved devices for electronic, magnetic, optical or mechanical properties. In this area, nanocelluloses have shown a particular interest.1,

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Depending on the production conditions,

nanocelluloses can be divided into two main categories: cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). CNCs have highly-crystalline structure generally obtained by acid hydrolysis, through the removal of amorphous regions. Such degradation is usually carried out with sulfuric acid that allows the grafting of anionic sulfate half ester groups (OSO3−), which induces negative charges at the surface responsible for colloidal stability in water by electrostatic repulsion.3-9 As a result, CNCs occur as rod-like nanoparticles, with a diameter of 5–30 nm and length of 100 nm up to several micrometers according to the source, well dispersed in water.10 CNFs are long and flexible nanofibers composed of both crystalline and amorphous domains and with a cross section at the nanometer scale.11 They are obtained from multiple mechanical shearing actions to delaminate the cell wall of cellulose fibers. However, the production of CNFs using mechanical disintegration requires high energy consumption. In order to reduce the production and environmental cost, chemical or enzymatic pretreatments of cellulosic fibers have been developed that facilitate delamination. These various types of nanocellulose are industrially


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produced and can be used for a wide range of applications in pharmaceutical or food industry,12, 13

or for the preparation of materials such as composites,14, 15 aerogels16, 17 or emulsions. Nanocelluloses were particularly shown to replace conventional surfactants to stabilize

oil/water interfaces. Oza first demonstrated that microcrystalline cellulose was able to form a network around oil droplets.18 Since that publication, many nanocelluloses stabilized emulsions have been reported with the various types of nanocelluloses; nanofibrillated celluloses,19,

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cellulose nanocrystals,21-24 and bacterial nanocellulose.25, 26 Nanocellulosic materials can also be hydrophobized to produce water-in-oil emulsions27, 28 and double emulsions,29, 30 or modified for functional emulsions.31-34 Such particle stabilized emulsions refer to the so-called Pickering emulsions that form surfactant-free, highly stable emulsions armored by a layer of solid particles. If the mechanism of adsorption is not fully elucidated, several points are already clearly established. The nanorods strongly adsorb to the oil phase along the longest axis, most probably via the less polar (200) crystalline plan.23,

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This reveals an interesting advantage of rod-like

particles with a high aspect ratio for which a large part of their surface cooperatively stacks to the surface without deforming it. It results an adsorption energy that is of several orders larger than surfactants.35 Such nanorods are then irreversibly adsorbed at the interface, forming a solid armor around the droplet. In such system, coalescence is prevented by steric repulsion between the droplets. The resulting emulsions can be used as excellent base for materials with high mechanical properties. The Pickering emulsions reported in the literature are microemulsions with diameters that can range from few microns up to several millimeters for emulsions produced by a low energy method. When the drop diameters are in nano range, the emulsions are referred to as nanoemulsions. They are usually produced by high shear. High-pressure homogenization is a


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common mechanical emulsification system used for the preparation of nanoemulsions from preemulsified.36-39 Nanoemulsions have two main advantages over conventional emulsions with micrometer-sized droplets; a high stability against sedimentation or creaming if drops are noninteracting, and a transparent or translucent optical aspect even at high droplet volume fractions.40 The interest in nanoemulsions has experienced a continuous increase in the last years, as evidenced by the publication of reviews on the subject.41,

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This enormous attention is

triggered by the wide range of applications in the medical, pharmaceutical, cosmetic, food and chemical industries. However, their implementation requires large amounts of surfactants that are, in a large extent, petroleum-based molecules. A very limited number of nanoemulsions are stabilized by particles in the literature and they are essentially silica particles.36, 43, 44 To the best of our knowledge, there is only one example of nanoemulsions stabilized with nanocelluloses where they used chemically modified CNCs by covalent surface functionalization with hydrophobic groups.45 The recent intensification of industrially produced nanocelluloses has led to a variety of products with various characteristics. The objective of the present paper is to study the emulsion architecture and drop size limits that can be achieved according to particle sizes, shape and surface chemistry in order to produce controlled micro- and nano-emulsions stabilized by nanocelluloses. For this purpose, four nanocelluloses of various sources were used. This work makes a precise description of the resulting emulsions and compares their organization, stability and rheological properties with their respective ability to produce nanosized droplets.

EXPERIMENTAL SECTION Materials


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Cotton CNCs (c-CNCs) were obtained from Whatman filters (grade 20 Chr). Wood CNCs (wCNCs) and wood CNFs (w-CNFs) were provided by University of Maine (Orono, Maine, U.S.) in the form of aqueous suspensions at 12 and 3 wt%. W-CNCs were diluted, sonicated and intensively dialyzed for ten days before use. A 1 wt% aqueous suspension of CNFs produced from wood by TEMPO-mediated oxidation46 of spruce wood pulp (w-TCNFs) was received from Swiss Federal Laboratories for Materials Science and Technology (EMPA, Düben-dorf, Switzerland). All of the reagents used were of analytical grade (Sigma-Aldrich), and water was purified with the Milli-Q reagent system (18.2 MΩ cm Millipore Milli-Q purification system). Cotton CNC Preparation C-CNCs were prepared from cotton linters according to the method of Revol et al.,6 with minor modifications. Briefly, sulfated c-CNCs were prepared using sulfuric acid hydrolysis at 58% at 70 °C under stirring for 20 min. After hydrolysis, the suspensions were washed by centrifugation, dialyzed to neutrality against Milli-Q water for 2 weeks, and deionized using mixed bed resin (TMD-8). The final dispersion was sonicated for 10 min, filtered, and stored at 4 °C. Nanocelluloses Characterization Transmission Electron Microscopy (TEM) was used to visualize and get dimensions of all nanocelluloses. The suspensions were deposited on a freshly glow-discharged carbon-coated copper grid (Electron Microscopy Sciences, UK) for 2 min, and the excess was removed by blotting. Negative staining was immediately performed using 20 µL of phosphotungstic acid (1% w/v, adjusted to pH 6 with NaOH and filtered at 0,1 µm) for 2 min and the excess solution was blotted before drying grids at room temperature. Sample was observed under standard conditions


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using a JEOL JEM-1230 TEM at 80 kV (Figure 1). The average length and width of the nanocelluloses were analyzed from 100 images (see table in SI).

Figure 1. TEM images of the nanocelluloses

Conductometric titration was performed in order to quantify the surface charge density of nanocelluloses using a Metrohm 905 Titrando automated controlled by a computer using TIAMO software (Metrohm, Switzerland). The sulfate contents of CNC suspensions were titrated with NaOH. The total amount of sulfate groups was calculated from: ∙ / where Veq is the amount of NaOH in mL at the equivalent point, CNaOH is the concentration (mol/L) and m is the weight of titrated cellulose. The carboxylate contents of the TEMPO oxidized CNF were determined by adding a solution of HCl to the w-TCNF suspension. After 10 min of stirring, the suspensions were titrated with 0.01 M NaOH. The titration curves showed the presence of strong acid corresponding with the


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excess of HCl and weak acid corresponding to the carboxyl content. The total amount of carboxyl groups was calculated from: ∙ / where Veq is the amount of NaOH in mL at the equivalent point, CNaOH is the concentration (mol/L) and m is the weight of titrated cellulose. The starting pH of the samples was 3.15 for c-CNC, 3.02 for w-CNC, 7 for w-CNF and 6.5 for w-TCNF Emulsion Preparation and Characterization Oil-in-water (o/w) emulsions were prepared using hexadecane and the nanocelluloses aqueous suspensions at the required concentration without further dilution in 50 mM NaCl. This ionic strength was chosen to limit repulsions due to charged groups potentially present at the surface. For all experiments, 3 mL of emulsions were prepared using a 20/80 oil/aqueous phase ratio, and sonicated for 45 s with an ultrasonic device with a dipping titanium probe close to the surface (amplitude 2 corresponds to 4 W/mL applied power), using intermittent pulses. Individual average droplet diameters were measured by laser light scattering using a Horiba LA-960 particle size distribution analyzer (Kyoto, Japan). An analysis model was used with refractive index of 1.43 and 1.33 for hexadecane and water, respectively. The calibration for water as a reference was taken before each measurement. All emulsions were measured at a range of transmittance between 80 and 90%. The measurements were systematically carried out in triplicate. The diameter was expressed as surface mean diameter D(3,2) (the Sauter diameter). The emulsions were all visualized by light microscopy. A total of 15 µL of the resulting Pickering emulsion was added to 1 mL of distilled water and stirred by vortex, and then a single drop was poured onto a slide and observed with a BX51 Olympus microscope.


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Scanning electron microscopy (SEM) was used in order to visualize the nanocelluloses at the interface of polymerized styrene-in-water emulsions. Hexadecane and styrene have similar surface tension (27 mN m-1 and 32 mN m-1, respectively) and we checked by optical microscopy that they produced stable emulsion with the same average diameters. Therefore, styrene–AIBN mixtures (ratio 100:1 w/w) were mixed with nanocelluloses dispersed in 50 mM of NaCl aqueous solution. The emulsions were made via sonication and then polymerized at 65 °C without stirring for 48 hours. The resulting beads were washed by repeated centrifugation. The dried beads were metallized with platinum and visualized with a JEOL 6400F instrument. Stability of the generated emulsions was checked using a refrigerated centrifuge SIGMA 330K equipped with a swing-out rotor (model 11134). Centrifugations were carried out for 2 min at 4000g and at 20 °C. Rheological measurements were carried out using an ARES (TA Instruments) controlled strain rheometer equipped with a 40 mm plate-plate geometry equipped with a Peltier temperature control. Emulsions were poured onto the ARES plate and two different gaps were fixed; 0.15 mm for CNC stabilized emulsions and 0.2 mm for CNF stabilized emulsions. All measurements were made at 20°C, and paraffin oil was used to prevent water evaporation. For dynamic viscoelastic measurements, the linear viscoelastic range was determined with a strain sweep (0.01-100%) at a fixed frequency of 6.28 rad/s. The chosen strain for the dynamic frequency sweep measurements was 0.5% for CNC stabilized emulsions and 2% for CNF stabilized emulsions which were within the linear region of the materials. The dynamic mechanical spectra were obtained recording the storage modulus (G’), and the loss modulus (G’’) as a function of frequency. Strain sweeps measurements were carried out from 0.07 to 100% and from 100 to 0.07% at a fixed frequency of 6.28 rad/s. The evolution of storage modulus (G’) with time was


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measured for 12h after strain sweeps measurements at a fixed frequency of 6.28 rad/s and the chosen strain amplitude. In order to reduce the average drop diameters, preformed emulsions were passed through a HP-homogenizer (GEA Niro Soavi Panda Plus 2000, Italy) up to 50 times at 600 bars at room temperature. The HP-homogenizer was equipped with a high-pressure valve (HP-valve) and a low-pressure valve (LP-valve). In this study, the pressure at the LP-valve was adjusted at 100 bars and the HP valve was used to arrive at processing pressure of 600 bars which corresponds to the sum of the pressures at the both valves. A cooling device was placed just after the outlet of the HP-homogenizer to limit the increase of the emulsion temperature.

RESULTS Emulsions’ architecture, stability and rheological properties Four highly purified cellulosic nanoparticles have been selected in order to discriminate the potential impact of the particle dimension and surface accessibility on their ability to stabilize various kinds of emulsions. The various preparations are described in the material and methods part and the parameters are reported in Error! Reference source not found.. Short CNC from two different sources varying in width and surface charge density were compared to longer wood-based CNFs. The two selected CNF samples vary in the level of fiber dissociation, which is revealed by the width, and in the surface charge densities. In TEMPO oxidized CNF (wTCNF), the primary alcohol groups at the surface of cellulose are modified into anionic carboxylic groups. Accessibility might be limited by the large amount of charges but simultaneously it allows a much better dissociation into individual nanofibrils.


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Suspensions at 1 to 6 g/L were prepared in aqueous solutions with 0.05M NaCl in order to prevent repulsions that can destabilize emulsions.21 They were mixed to hexadecane at a 20/80 O/W ratio without the use of any additives. Hexadecane-in-water emulsions were prepared with various amounts of nanocelluloses given as the weight of nanocelluloses per milliliter of hexadecane introduced. Indeed, the volume fraction of aqueous phase was shown to have no influence on the final emulsion characteristics but on the oil volume.26 For each nanoparticle, ten emulsions have been prepared at concentrations of 0.8 to 24 mg of nanocellulose per mL of hexadecane. The D[3,2] average droplet diameter has been measured for all the concentrations. For the two CNCs, the emulsions show exactly the same concentration dependence on the diameter (Figure 2a). At low concentrations, the amount of nanocrystals is too low to stabilize small droplets so they merge creating fewer larger droplets, thereby reducing the total interface area of the system. This limited coalescence process, characterized by a sharp decrease in the drop diameter, has been previously reported for other Pickering emulsions.26, 34, 47-49

At higher concentration, the D[3,2] stops decreasing and stabilizes around 5 µm which

agrees with that of emulsions previously described.24, 50 In this domain, the droplets diameter is stable whereas the quantity of CNC increases without being released in water, as previously demonstrated by acid hydrolysis followed by colorimetric titration of the subphase.23, 26 This was attributed to the ability of rods to cooperative orientation. CNCs align inducing a densification of the CNC layer at the interface of individual droplets without diameter variation.24 Differently, in the case of CNF stabilized emulsions, two different behaviors were observed. For w-CNF, the average drop size is much larger and decreases gradually with the concentration. For w-TCNF the limited coalescence domain is very short and then the average drop diameter increases slowly with concentration. The droplet size distributions were monodisperse for the two CNCs and for


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the w-CNF. However, the w-TCNF stabilized emulsions showed two very distinct populations of oil droplets; one around 1 µm representing the individual droplets and another around 50 µm representing agglomerates of drops (Figure 2b).

Figure 2. (a) Average diameters of droplet as a function of nanocelluloses content for all kinds of emulsions. (b) Droplet size distributions of emulsions stabilized with 24 mg of nanocelluloses per mL of hexadecane.

Optical microscopy was used to visualize the droplets with two concentrations of nanocelluloses (Figure 3). As expected from the granulometry measurement, for the CNC stabilized emulsion, droplets are fairly isolated in the continuous aqueous phase. The drop size is monodisperse and decreases from 10-30 µm down to 4-6 µm with concentration. w-CNF stabilized emulsions showed much larger individual droplets at low concentrations. At higher concentrations, drops with diameters around 30 µm coexist with smaller drops and they form aggregates due to fibers that established connections between the droplets (Figure 2b). In the case of w-TCNF, at low concentration only isolated droplets are obtained, but for high concentrations only cluster-like aggregates of interconnected small droplets were observed. This


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behavior has previously been reported in emulsions stabilized by unmodified CNF29 or by longer cellulose nanocrystals isolated from cladophora.23 This is due to the long length of CNF (few microns) compared to the drop diameter. Thus, they form clusters that increase in size with concentration, which explains the results obtained by granulometry. However, Figure 3h shows that the diameter of the individual droplets decreases with the concentration down to about 1 µm, which is significantly lower than the droplets of the CNC emulsions. The lateral size of CNCs is the same as the w-TCNF and they are supposed to have the same crystalline domains; the difference between them is the presence of charges that better fibrillate and of amorphous domains that link the crystalline domains and confer higher global flexibility to w-TCNF.


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Figure 3. Photomicrograph of emulsions stabilized by c-CNC (a-b), w-CNC (c-d), w-CNF (e-f) and w-TCNF (g-h) prepared at 2 and 8 mg of nanocellulose / mL of hexadecane. The scale bar is 50µm.

In order to compare the organization at the interface of the different nanocelluloses, detailed surfaces were visualized with scanning electron microscopy (SEM) (Figure 4). Since liquid hexadecane–water emulsions cannot be depicted by SEM, styrene–water emulsions were performed and polymerized into solid beads.23,

26, 34, 50, 51

SEM images of the resulting

polystyrene beads are shown in Figure 4. They were the replica of the monomer droplets and


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they were homogenously dispersed in water. In the case of CNC stabilized emulsions (Figure 4ad), an identical organization at the surface of the individual droplets was observed for both kinds of CNCs. As already observed in previous studies,21 CNCs are never pointing out such as nest, even on the smallest droplets, but the wetting forces curve them to interact along the surface and this is at the basis of their very high adsorption energy. As a result, they align to create a dense armored layer. SEM images revealed that this layer is much less dense for emulsions stabilized by w-CNF. In this case, the fibers formed a heterogeneous and scattered layer at the surface of the drop (Figure 4f). Furthermore, the droplets are interconnected by w-CNFs which undergo lateral agglomeration upon concentration forming thick fibers (Figure 4e). It results in large droplets that are lightly covered but connected by a strong interconnected network. Considering w-TCNF, an entangled system of much smaller droplets, in the nanometer range, was observed interconnected into agglomerates (Figure 4g,h). In this case, the fibers did not undergo lateral agglomeration as evidenced for w-CNF. This is due to the electrostatic repulsion induced by the carboxylates that lead to entangled systems rather than dense fibers.


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Figure 4. SEM images of droplets issued from emulsions stabilized by c-CNC (a-b), w-CNC (cd), w-CNF (e-f) and w-TCNF (g-h) at 15 mg per mL of hexadecane at two magnifications.

Stability is commonly addressed by analysis of physical changes of the emulsion over a practical length of time. It is generally induced by flocculation or the creaming / sedimentation process that lead to coalescence. As hexadecane has a lower density than water, a creaming process was observed for most concentrations. However since no variation in the size and size distribution is noticed, the emulsion can be considered stable despite a creaming process. The


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emulsions were kept unchanged for months. To investigate stability in stressed conditions, the emulsions were centrifuged for 2 min at 4000g.

Figure 5. Emulsions stability before centrifugation (upper images) and after centrifugation (lower images) for 2 min at 4000g for increasing concentration at 2, 4, 8 and 24 mg of nanocelluloses per ml of hexadecane

Before centrifugation, for fixed oil/water ratio and oil content, the emulsion volume increased regularly with the amount of nanocelluloses introduced (Figure 5). This is explained by two phenomena; increasing concentration allows (i) the stabilization of a larger number of smaller drops that fills larger volumes and (ii) since more interfaces with nanocellulose is present, it leads to more hydration layers immobilizing more water between the droplets. As a result, it is easy to prepare non-creaming emulsions, choosing the oil/water ratio as well as the nanoparticle concentration. After centrifugation at 4000g, the emulsions stabilized by CNCs with concentrations below 8 mg of nanocelluloses per ml of hexadecane were disrupted leading to two separated phases. This is fully consistent with previous results.24, 26 For concentrations with mp above 8 mg/mL, dense white emulsions were obtained that resisted to the stress induced by centrifugation. Emulsions stabilized with w-CNFs showed the same stability as CNC emulsions.


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However, in the case of w-TCNF, all emulsions resisted to centrifugation, even with mp below 4 mg/mL. In addition to the emulsion volume, the size of the droplets was measured in order to verify their mechanical resistance. No variation in droplet size was observed after centrifugation as well as after keeping the samples for 6 months at ambient temperature (Figure S1). Thus, all the emulsions proved to displayed excellent mechanical resistance toward deformation and coalescence above 8 mg/mL and w-TCNF produce even highly stable emulsions even at very low concentration. Comparing visually emulsions prepared with 24 mg of nanocelluloses per mL of hexadecane showed that CNC stabilized emulsions were flowing whereas CNF stabilized emulsions formed gels, especially w-TCNF (Figure 6). Rheological properties were investigated on these emulsions. The dynamic mechanical spectra of all emulsions show G’ > G’’ with no crossover throughout the tested frequency range (Figure 6) revealing a typical gel-like behavior.52, 53 The two CNCs issued from cotton and wood gave the same viscoelastic properties. They result in soft gels with a tan δ of 0.6 at 1 rad/s that is expected for non-compact emulsions that means that droplets are not in close contact with each other. Stable G′ values of 2 to 3 Pa over the frequency range are measured whereas G’’ shows a minimum at a frequency of 1 rad/s. This minimum is attributed to the droplet caging effect. It reveals that the droplets organization is modified upon shearing and then slowly relax. The magnitudes of G’ and G’’ increase for emulsions stabilized by CNFs, indicating an increase in the strength of emulsion microstructure. w-CNF emulsions show stable G′ values of around 200 Pa over the frequency domain whereas w-TCNF emulsions reach G’ values up to 500 Pa (Figure 6). This increased viscoelastic property of both CNF is due to the interconnections that strongly strengthen the system.


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Figure 6. Frequency sweep of emulsions at fixed concentration of 24 mg of nanocelluloses per ml of hexadecane and emulsions pictures.

In addition to these conventional oscillatory shear studies conducted at small amplitudes, the non-linear regime can be characterized using large amplitude oscillatory shear (LAOS) flow. LAOS explores the nonlinear domain and studies the properties variation in relation to droplet size and size distribution and, thus, provides insight into the emulsion organization mechanical properties and stability. Strain sweep measurements were carried out on the four samples up to 100% deformation, which is above a critical strain where a decrease of moduli is observed (Figure 7). At low deformation, G′ is constant; the mechanical response is a reversible elastic deformation. Beyond the critical deformation, irreversible visco-plastic flow occurs. This is well evidenced by the lower moduli obtained when deformation decreases back to 0.1%. Here again, both CNCs show a very similar profile with a critical deformation, defined by the deviation of G’, at 11% deformation. CNFs present higher respective moduli values as shown in Figure 6, but also shorter linear domains revealing lower resistance to deformation. Interactions between drops


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are responsible for different deformation mechanisms. Decreasing the deformation back to the initial value, a very clear decrease in moduli was observed for all samples. To focus on reversibility, G’ was measured at 1Hz and low deformation (0.5% for CNC emulsions and 2% for CNF emulsions) during 12 hours after extensive deformation. For CNC emulsions, the plateau value was fully recovered after 12 hours with the same frequency dependence (Figure S2). It illustrates the impressive stability of such systems that can be highly deformed by shear but recovers identical organization. However, in the case of emulsions stabilized with CNFs, storage modulus did not reach the plateau value after twelve hours. This is attributed to the different architecture of these emulsions. As already demonstrated, the architecture of CNF stabilized emulsions is composed by aggregates of droplets formed by entanglement and interactions between fibers. These aggregates better resist to deformation but they may suffer modifications above the critical deformation. That can change the structure and, as a result, the values of G' after deformation decrease after shear. This demonstrated that the drops are differently subjected to elastic and plastic deformations. The CNCs induced droplets can deform and return to their equilibrium position afterward, whereas CNF emulsions show improved mechanical properties but cannot return to the original point after large deformations. Two main possibilities may explain this difference; the nanofibers might be disrupted by traction forces between drops or, since the drops are rotating, they might slip on the surface and partially desorb from one interface to wrap around another one and individualize it. Both scenarios result in an irreversible reduction of the elastic moduli.


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Figure 7. Strain sweeps measurements and evolution of storage moduli after 12 hours on emulsions stabilized with 24 mg of nanocelluloses per ml of hexadecane at 1 Hz.

It is clear from these results that CNCs as well as CNFs are able to produce surfactant-free oilin-water Pickering emulsions. It is also established that the presence of charges does not interfere with drop’s architecture as long as (i) ionic strength is introduced to avoid repulsions that could prevent formation of a dense layer at the interface and thereby stability and (ii) enough energy is given during the emulsification process to remove possible existing aggregates. The adsorption to the liquid interface is believed to occur when bound water molecules are released upon ultrasonication and replaced by strong binding to the oil, which gives a large increase in the entropy of the system. Such strength is enhanced by the anisotropic form of nanocrystals.


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Considering the different nanocelluloses, the same emulsions properties are found for the two CNCs investigated from the laboratory or industrial production like recently reported in a benchmarking CNC study.54 It has to be noticed that less precise results with higher diameters and lower stability were obtained using untreated industrial products. This is probably due to the presence of oligosaccharides, as reported by Bouchard et al.55 With a degree of polymerization between 7 and 20, they can significantly modify the surface of the CNCs lowering their adsorption onto the interface whereas they have the same dimensions. After ultrasonication and an extensive dialysis, the same emulsion characteristics are recovered as with the cotton laboratory made CNCs. The surface chemistry is then a major parameter for interfacial stability, and both CNCs with identical crystalline organization lead to identical emulsions. Considering longer CNFs, they behave differently at the oil/water interface according to their preparation mode and compared to CNCs. SEM images show that the wetting force bends the CNCs along the drop. For CNFs, increasing the length and thereby the surface of contact should increase mechanically the adsorption energy. This assumption is not verified. On one hand, there is no evidence that amorphous parts are able to adsorb at the interface as crystalline parts do. On the other hand, SEM images show that, similarly to CNCs, CNFs are curved along the surface, but connections occur from a drop to another as they are long compared to the drop diameter. It reduces the adsorption surface since the non-adsorbed part is not contributing to stabilization of the interface. Simultaneously, this network strongly increases mechanical properties. Rheological measurements showed increased viscoelastic properties with moduli 100 times higher for both CNFs and better resistance to deformation.

Ability to form nanoemulsions


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These systems showed variations in drop diameters, architecture (isolated or interconnected droplets), and mechanical properties. However, it doesn’t forecast their respective ability to produce nanoemulsions. In order to test the minimal diameter that may be reached, the four preformed emulsions were passed through high-pressure homogenization. Emulsions were passed through the HP-homogenizer at a pressure of 600 bars. Once inside the chamber of the homogenizer, the emulsions are exposed to intense impact and shear forces. After 25 successive passes, the three emulsions stabilized by c-CNCs, w-CNCs and w-TCNFs gave droplets with diameters between 100 and 600 nm (Figure 8). More than 25 passes did not produce a remarkable reduction of the droplets diameter (Figure S3). Differently, w-CNF emulsions only reach a minimal drop diameter of around 5 µm after 25 passes (Figure 8) and more passes didn’t produce lower drop diameters but destabilization of the emulsion and led to phase separation. It is noticeable that this droplets diameter is the same as the minimum size usually obtained for the ultrasonicated CNC emulsions. The w-CNFs are not totally fibrillated and have large lateral sizes that prevent decreasing to nano-size drop diameter.


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Figure 8. Droplet size distributions of emulsions stabilized with 20 mg of nanocelluloses per mL of hexadecane after 0 passes (P0) and 25 passes (P25) through homogenizer at a pressure of 600 bars.

The resulting emulsions after 25 passes through the homogenizer at a pressure of 600 bars visualized by optical microscopy are shown Figure 9. It produced individual nanodroplets with diameters of 350 nm for c-CNCs and w-TCNFs and 500 nm for w-CNCs that don’t cream anymore as they are not submitted to gravity (Figure 9). Furthermore, these nanoemulsions are stable at least for 8 months with no change in drop size. In the case of emulsions stabilized by wCNFs, HP-homogenizer does not produce nanoemulsions but a significant reduction in droplet size, as demonstrated by the results of granulometry. Microscopy images show a majority of individual droplets of diameter around 5µm and the presence of some aggregates of droplets (Figure 9f). In order to check the role of the homogenizer, we confirmed that such low dimensions were not accessible even after long time using ultrasonication device.


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Figure 9. Photomicrograph of emulsions stabilized by c-CNC (a-b), w-CNC (c-d), w-CNF (e-f) and w-TCNF (g-h) at 15 mg per mL of hexadecane after 0 passes (P0) and 25 passes (P25) through homogenizer at a pressure of 600 bars. The scale bar is 50µm.

HP-homogenizer produced emulsions with lower diameters than those produced by sonication but in different ways. In the case of CNCs, the HP-homogenizer succeeds to decrease the size down to nano size. Micro sized droplets are then spontaneously stabilized by coalescence whereas nano-droplets are produced by disruption upon strong shear. This micron-size limitation was possibly explain by the stiffness of the nanocrystals which anchor strongly at the interface


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and might limit the bending and thereby the radius. The present results show that this mechanical limitation is not sufficient since smaller droplets can be obtained upon high shear stress in laminar flow. The passage through the homogenizer induces a consistent and intense deformation that causes the drops disruption down to diameters of 100 - 600 nm, which is the same order as the length of the particles and similar size as those stabilized by surfactants. It reveals an impressive bending ability of CNCs. This was also pointed out in a simulation work done by Chen et al.56 The plastic deformation limit of CNCs was defined at a critical bending angle of about 60°, which makes such nano-sized diameters possible without deformation. In the case of CNFs, two different behaviors to size variation have been observed. For wTCNF emulsions, the ultrasonication directly produced stable drop sizes at 1 µm while for wCNF the ultrasonication produces drops with diameters up to 20 µm. As mentioned before, wTCNFs have negative surface charge which causes better fibrillation due to electrostatic repulsions. Therefore, these nanofibrils have the lower cross section with alternating crystalline and amorphous domains. This low cross section increases the specific surface and thereby the stabilized interface area. On the opposite, w-CNFs are less fibrillated and result in higher lateral size even at high concentration. The passage through HP-homogenizer induced two different behaviors according to the cross section of CNFs. On one hand, w-TCNF emulsions formed aggregates of interconnected droplets with diameters around 1 µm. High shear forces first individualized droplets and then reduced the size to the nano scale. In contrast, it was not possible to prepare nanoemulsions with the w-CNF. The homogenizer suppresses inter-droplet links and causes the reduction of drops diameters only down to 5µm. It is interesting to remark that such value is close to the minimum diameter measured for emulsions prepared by ultrasonication when stabilized by CNCs. This might be the limit that can be reached when


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nanocrystals are laterally associated, like in the case of CNCs after sonication as previously demonstrated with lateral size of 22 nm.24 HP-homogenizer might reduce their dimension increasing their bending capacity and allowing stabilization of nano-sized droplets like in the case of w-TCNF. This would confirm the major role of the cross section in regard to cooperative lateral association of nanoparticles.

CONCLUSION It is here demonstrated that the different types of nanocelluloses investigated are able to produce ultrastable Pickering emulsion, irrespective of source, shape and surface charge density as long as salt is introduced to prevent repulsions. However, it illustrates that a large range of various structures can be obtained according to the shape, driven by the source of nanocellulose, and the process conditions, mainly shear and concentration. Consequently, it illustrates that various emulsion characteristics might be reached according to the targeted properties. Both CNCs, obtained in the laboratory from cotton or industrially from wood, lead to well controlled emulsions with individual droplets when the surface of the CNC is net. The emulsion may cream or fill the complete emulsion volume according to the CNC concentration and oil/water ratio. These emulsions have similar mechanical properties; above a critical concentration limit, they result in soft gels with a high resistance to deformation. These Pickering emulsions could be used for health and cosmetics applications where the use of surfactants is undesirable but also for industrial fields as printing, detergency, laundry, home care or adhesives. Longer CNFs produced individual droplets at low concentration with smaller drop diameters when they are well fibrillated as after TEMPO oxidation. At higher concentration both CNFs tend to reticulate forming clusters of microdrops for w-CNF and nanodrops for w-TCNF. Both


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emulsions result in strong gels. This characteristic is useful because CNF emulsions can serve as templates or intermediaries for various kinds of materials, in addition to the interesting classical applications of Pickering emulsions. Finally, oil-in-water nanoemulsions have been successfully produced from CNCs and w-TCNF emulsions decreasing the drop size to 100 – 600 nm by passes through a HP-homogenizer whereas w-CNF could only produce microemulsions with average diameter of 5µm. These stable nanoemulsions are interesting for industrial applications of latexes including coatings, paints, adhesives, rubbers, sealants, drug release systems and others. These results may allow a selective architecture which is of high importance in the design of nanocellulose based biphasic materials and in optimization of their industrial processes. ASSOCIATED CONTENT Figure S1: Comparison of droplet size distributions of emulsions before and after centrifugation. Figure S2: Evolution of storage moduli during 12 hours after extensive deformation of CNCs emulsions. Figure S3: Evolution of droplet size distributions of emulsions after passes through the HP-homogenizer. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT This work was carried out in the framework of the French National Research Agency (ANR Project CE08 2015). We are equally grateful to Emilie Perrin and Joëlle Davy for their excellent


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technical assistance for TEM and SEM visualizations, Genevieve Llamas for granulometry and to Catherine Garnier and Camille Jonchère for rheological measurements as well as to Nicolas Stephant (Institut des Matériaux Jean Rouxel, Université de Nantes) for the access to the SEM equipment. REFERENCES 1. Habibi, Y.; Lucia, L. A.; Rojas, O. J., Cellulose Nanocrystals: Chemistry, SelfAssembly, and Applications. Chemical Reviews 2010, 110, (6), 3479-3500. 2. Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J., Cellulose nanomaterials review: structure, properties and nanocomposites. Chemical Society Reviews 2011, 40, (7), 3941-3994. 3. Rånby, B. G., Fibrous macromolecular systems. Cellulose and muscle. The colloidal properties of cellulose micelles. Discuss. Faraday Soc. 1951, 11, (0), 158-164. 4. Marchessault, R. H.; Morehead, F. F.; Walter, N. M., Liquid Crystal Systems from Fibrillar Polysaccharides. Nature 1959, 184, (4686), 632-633. 5. Marchessault, R. H.; Morehead, F. F.; Koch, M. J., Some hydrodynamic properties of neutral suspensions of cellulose crystallites as related to size and shape. Journal of Colloid Science 1961, 16, (4), 327-344. 6. Revol, J. F.; Bradford, H.; Giasson, J.; Marchessault, R. H.; Gray, D. G., Helicoidal self-ordering of cellulose microfibrils in aqueous suspension Int. J. Biol. Macromol. 1992, 14, (3), 170-172. 7. Favier, V.; Chanzy, H.; Cavaille, J. Y., Polymer Nanocomposites Reinforced by Cellulose Whiskers. Macromolecules 1995, 28, (18), 6365-6367. 8. Dong, X. M.; Revol, J.-F.; Gray, D. G., Effect of microcrystallite preparation conditions on the formation of colloid crystals of cellulose. Cellulose 1998, 5, (1), 19-32. 9. de Souza Lima, M. M.; Borsali, R., Static and Dynamic Light Scattering from Polyelectrolyte Microcrystal Cellulose. Langmuir 2002, 18, (4), 992-996. 10. Nechyporchuk, O.; Belgacem, M. N.; Bras, J., Production of cellulose nanofibrils: a review of recent advances. Industrial Crops and Products 2016, 93, 2-25. 11. Klemm, D.; Heublein, B.; Fink, H.-P.; Bohn, A., Cellulose: fascinating biopolymer and sustainable raw material. Angewandte Chemie International Edition 2005, 44, (22), 3358-3393. 12. Li, Q.; Zhou, J.; Zhang, L., Structure and properties of the nanocomposite films of chitosan reinforced with cellulose whiskers. Journal of Polymer Science Part B: Polymer Physics 2009, 47, (11), 1069-1077. 13. Ye, L. J.; Wang, H. Q.; Yao, Z., Effect of surface modification with silane coupling agent on enhancing pzc value and enzyme loading capacity of mesoporous TiO 2 whiskers. CIESC J 2013, 64, 2160-2168. 14. Capadona, J. R.; Van Den Berg, O.; Capadona, L. A.; Schroeter, M.; Rowan, S. J.; Tyler, D. J.; Weder, C., A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates. Nature Nanotechnology 2007, 2, (12), 765-769.


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