Surfactant-Free High Internal Phase Emulsions Stabilized by

Jan 4, 2013 - Surfactant-Free High Internal Phase Emulsions Stabilized by Cellulose Nanocrystals ... We propose that this two-step process to create C...
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Surfactant-Free High Internal Phase Emulsions Stabilized by Cellulose Nanocrystals Isabelle Capron* and Bernard Cathala INRA, UR1268 Biopolymeres Interactions Assemblages, 44316 Nantes, France ABSTRACT: Cellulose nanocrystals (CNCs) are rod-like colloidal particles that irreversibly adsorb at the oil−water interface to produce ultrastable emulsions. When the internal phase fraction is increased, these CNCs can produce gel-like oil-in-water high internal phase emulsions (HIPEs) in which more than 90% of the hydrophobic phase is stabilized by less than 0.1% wt. of CNCs. However, a one-step preparation of HIPEs is not possible, and incorporation of the high internal phase fraction requires the prior preparation of Pickering emulsions. We propose that this two-step process to create CNC HIPEs relies on a swelling process of the droplets that does not desorb the CNCs from the interface, decreasing the coverage ratio of the droplets and leading to coalescence. As a result, this process leads to a drops deformation and a new interfacial networking organization as revealed by confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) images.

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INTRODUCTION In the past few years, high internal phase emulsions (HIPEs), also known as emulsion gels, have been the object of intense research due to their wide range of applications in many different domains such as food, cosmetics, pharmaceuticals, and paints and coatings.1−4 HIPEs are emulsified systems with an internal phase volume fraction greater than 0.74, which is the maximum packing density of monodispersed hard spheres. HIPEs are generally stabilized against coalescence using a relatively large quantity of surfactants that are solubilized in the continuous phase.5 This large quantity of surfactants constitutes a potential environmental drawback. In addition, these surfactants must be chosen with care in order to prevent any emulsion inversion.6 An alternative is the use of solid colloidal particles that are irreversibly adsorbed at the interface leading to PickeringHIPEs. Only a few Pickering-HIPEs have been reported. They are usually stabilized by synthetic particles including modified silica particles,7−9 titania particles,10 and synthetic organic polymers.11 Nevertheless, even if the amount of stabilizer required is less than that necessary for surfactant-based emulsions, the release of such nondegradable colloidal particles can also present toxicity risks. In this context, renewable polysaccharides such as cellulose appear to be an ideal source for large-scale applications of new materials. Indeed, whereas water-in-oil emulsions based on hydrophobized cellulose have rarely been reported,12−14 no oil-in-water HIPEs with cellulose nanocrystals (CNCs) have been investigated as of this time. The recent increasing interest in materials based on CNCs15−19 therefore prompted us to report our recent work that demonstrates, for the first time, the ability of CNCs devoid of any surface chemical modification to produce long-term gellike HIPEs. © XXXX American Chemical Society

We recently reported the stabilization of Pickering emulsions with unmodified CNCs, also known as cellulose whiskers.20 CNCs arise from preferential hydrolysis of the amorphous regions of cellulose fibers, which leads to highly crystalline solid rod-like particles. CNCs can be obtained from several biological sources. In the case of CNCs from cotton, crystalline rods of approximately 200 nm in length and 6−10 nm in thickness are obtained.21,22 CNCs are usually thought to be hydrophilic due to the presence of surface hydroxyl and sulfate groups. However, we have recently shown that unmodified CNCs were able to adsorb at the oil/water interface, suggesting an amphiphilic character. This character has been attributed to a surface heterogeneity induced by the crystalline organization.17 Nevertheless, CNCs are mainly hydrophilic since they can be dispersed in water but not in organic solvents. They thus lead to ultrastable and monodispersed Pickering oil-in-water emulsions. The three main characteristics of CNCs that provide a number of benefits for HIPE manufacturing are (i) irreversible adsorption at the interface, resulting in highly stable structures; (ii) a rod-like shape that gives rise to a percolating network, thus increasing cohesion and stability; and (iii) a nontoxic, abundant and renewable source.23 We found that such HIPE structures could be produced by first preparing a Pickering emulsion and subsequently adding oil with mechanical stirring resulting in HIPEs with more than 90% of internal phase. The local interfacial reorganization from individual drops to networking self-assembly is discussed based on the results of confocal laser scanning microscopy (CLSM) of the native HIPE, and scanning electron microscopy (SEM) Received: September 20, 2012 Revised: December 29, 2012

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Figure 1. Hexadecane-in-water emulsions, from left to right: Pickering emulsions with oil/water ratios of 10/90, 50/50, 80/20, and 90/10, prepared by (a) directly mixing an aqueous CNC suspension + oil; (b) by a two-step process Pickering emulsion + addition of oil leading to emulsions at 64%, 77% and 86% of internal phase; and (c) when reversing the Pickering-HIPE tube containing 86% of internal phase. Zeiss LSM 410 confocal microscope (Zeiss, Gottingen, Germany) equipped with a 40× water-immersion lens with an optical section thickness of approximately 1 μm.

images of the dried structure. The mechanism proposed, leading to the stabilization of such large amounts of oil by unmodified CNCs alone, is based on the critical coverage stability of the droplets.

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RESULTS The CNCs used in this study are rod-like monocrystals obtained from cotton fibers by acid hydrolysis with sulfuric acid. We previously showed that CNCs were able to irreversibly adsorb at the oil/water interface and stabilize hexadecane-inwater emulsions over periods longer than a year.17,20 These emulsions are prepared by directly mixing hexadecane and an aqueous nanocrystal and sonicating for 10 s. CNCs bear sulfate groups with a surface charge density of 0.123 e/nm2. NaCl at 50 mM was then systematically added in order to prevent electrostatic repulsion. We have discussed the effect of ionic strength on such emulsions elsewhere in detail.17 Without ionic strength, repulsions prevented emulsion stabilization. Ionic strength (NaCl) was then added to screen electrostatic interactions since it led to the same stability as that observed for neutral CNCs. Under such conditions, it was observed that regardless of the oil/water volume ratio and the CNC concentration, only about 64% of internal phase could be reached. The fraction of internal phase refers to the volume fraction of hexadecane stabilized in the droplets.5,7,11 Moreover, regardless of the volume of oil introduced, no catastrophic phase inversion was ever observed, and only drop-shaped emulsions could be obtained. The drop formation follows the limited coalescence principle.26,27 As long as the particle adsorption at the interface is irreversible, the area of the interface and, therefore, the drop size, is controlled by the amount of stabilizing particles introduced. Consequently, when varying the CNC concentration for a fixed volume of oil, the drop size varies, but the percentage of internal phase cannot exceed the physical limit of compacting spheres after relaxation. Trying to incorporate larger amounts of oil, regardless of the shearing conditions (sonication, homogenizer), results only in the formation of phase-separated systems (see Figure 1a). It did not appear possible to stabilize such a high level of oil with CNCs by a one-way process. Another process to increase the internal volume fraction could be to concentrate the initial Pickering emulsion. The droplets proved highly stable unless ultracentrifugation was used, leading to a complete destabilization of the emulsion. If no coalescence occurred, the centrifugation process induced the exclusion of the excess water from the interdroplet volume. As a result, it was possible to prepare an emulsion up to 74−75% of internal phase, the theoretical value of compacting monodisperse spheres. However, when the Pickering emulsion was formed, i.e., when the CNCs were irreversibly adsorbed at the interface, the

EXPERIMENTAL SECTION

Materials. All of the reagents used were of analytical grade (SigmaAldrich), and water was purified with the Milli-Q reagent system (Millipore). Hexadecane was purified by extensive extraction with water. For CLSM, BODIPY (4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) was purchased from Molecular Probes Invitrogen (Oregon, USA). Sulfated Cotton CNCs. CNCs were prepared according to the method described by Revol et al.24 with minor modifications.17 Briefly, Whatman filters (grade 20 Chr) were hydrolyzed in 61% wt sulfuric acid at 72 °C under stirring for 30 min. After hydrolysis, the suspension was washed by centrifugation, dialyzed to neutrality against Milli-Q water, and deionized using mixed bed resin (TMD-8). The final dispersion was sonicated for 15 min (ultrasonic processor XL 2020; Misonix, Inc., NY, USA), filtered, and stored at 4 °C. The CNCs were characterized by transmission electron microscopy (TEM) for their average length and width (189 and 13 nm, respectively), and by atomic force microscopy (AFM) for their thickness (6 nm).25 The surface charge density of 0.123 e/nm2 was measured by conductometric titration with NaOH. Emulsion Preparation. The oil-in-water (o/w) Pickering emulsions were prepared using hexadecane and an aqueous CNC suspension at the required concentration without further dilution to match an oil/water ratio of 10/90 sonicated for 20 s. CNCs carry some sulfate half esters on the surface due to the H2SO4 hydrolysis, imparting anionic surface charges that will mutually repel neighboring adsorbed particles that may lead to unstable emulsions. Therefore, 50 mM NaCl was added to the CNC suspension to prevent electrostatic repulsions.17 Sauter mean diameters (D32) of the droplets were measured by laser light diffraction using a Malvern 2000 granulometer apparatus equipped with a He−Ne laser (Malvern Instruments, UK) with Fraunhofer diffraction. 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 autoanalyzer. Glucose was used as a standard. HIPEs were obtained in a second step by adding hexadecane or cyclohexane to the former emulsion using a rotor stator in plastic tubes. The internal phase volume fraction was measured as the volume of oil included in the emulsion divided by the total volume of the emulsion. Instrumentation. SEM images of solid emulsions were taken from cyclohexane-in-water emulsions prepared by sonication in a tube. The tubes were freeze-dried and deposited on a stub. The resulting solid emulsions were metalized with platinum and visualized with a JEOL 6400F. For CLSM, hexadecane was stained with BODIPY (564/570) fluorophore at 0.3 g/L. Images of the emulsions were acquired using a B

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Figure 2. CLSM images of emulsions stabilized by cotton CNCs containing increasing amounts of hexadecane stained with BODIPY from (a) the original 10/90 oil/water Pickering emulsion; (b) 65% of internal phase; (c) 85.6% of internal phase; (d) the same as panel c using a stacking of 2D 1-μm thick optical cross section images to form a 3D reconstitution.

where Voil is the volume of oil included in the emulsion, and R is the Sauter mean radius of the droplets. On the basis of this equation, the total surface area of a 2-mL original emulsion with an average radius of 2 μm is 0.3 μm2 (Figure 2a). The addition of only 3 mL of oil to the 2 mL of original emulsion leads to 64% of internal phase with drops with a radius of approximately 5 μm (Figure 2b). The emulsion volume increases 2.5-fold, whereas the Sd, approximately 2 μm2, increases 6.6-fold. This implies a variation of the surface coverage. The theoretical maximum surface that can be covered by the particles, Sp, is defined by mp Sp = NpLl = hρ (2)

addition of hexadecane followed by mixing with a double cylinder-type homogenizer yielded a new, highly stable gel structure (Figure 1b,c). When calculating the internal phase volume ratio, values far above 74% were reached. For example, 10/90 oil/aqueous phase Pickering emulsions were prepared in which the aqueous phase was composed of cotton CNCs at 5 g/L in 50 mM NaCl. Hexadecane (5 to 20 mL) was then added to tubes containing 2 mL of the prepared Pickering emulsion and mixed using a polytron homogenizer for 1 min. Under these conditions, the internal phase volume fraction was systematically calculated to be above 74% and up to more than 90%, without any emulsion inversion. Interestingly, these values remained stable for months at room temperature. The required preparation of Pickering emulsions prior to the incorporation of additional oil strongly suggests that the formation of HIPE structures first requires the adsorption of CNCs at the interface, followed by the rearrangement of the initial interface when oil is added. It is interesting to note that for the initial Pickering emulsion at 64% of internal phase (5 mL of added oil), the content of CNCs in the total emulsion volume was 0.18% wt. This percentage of CNC decreased to 0.045% wt for 92% of internal phase (20 mL of added oil). This means that such a process makes it possible to prepare a highly stable emulsion gel with less than 0.1% wt of unmodified CNCs. To investigate the evolution of emulsion morphology and drop size during the addition of oil to native Pickering emulsions, the hydrophobic phase (hexadecane) was stained with a BODIPY (564/570) fluorophore at 0.3 g/L and visualized by CLSM. BODIPY preferentially stained the interfacial areas, revealing the evolution of the emulsion drop size while increasing the oil volume fraction. The continuous and dispersed phases were not stained and appeared as black in the images (Figure 2). In the initial Pickering emulsion, dispersed droplets with a diameter of approximately 4 μm were observed. These droplets can move freely in the continuous phase since they represent the minor phase of the emulsion. When stained oil was added and mixed using a rotor stator, an increase in the average drop size as well as polydispersity was clearly observed with the increasing internal phase ratio. The medium internal phase emulsion (MIPE) domain, above 50% of the internal phase, was reached with larger size individual droplets still visible (Figure 2b). This increasing drop diameter suggested some coalescence of the droplets. This hypothesis is supported by the increase of interface area when oil is added. Given the volume of oil and the average drop size, we can calculate the total drop surface area (Sd): Sd = 4πR2 ×

3Voil 3

4πR

=

3Voil R

where Np is the number of CNCs, L and l are the length and width, respectively, mp is the mass of CNCs included in the emulsion (g), h is the thickness defined by AFM (6 nm),25 and ρ is the CNC density (1.6 g/cm3). The theoretical surface coverage C is then given by the ratio of the maximum surface area that can be covered by the particles, Sp, and the total surface displayed by the oil droplets, Sd:20

C=

Sp Sd

=

m pR 3hρVoil

(3)

Given that CNCs are irreversibly adsorbed, the only parameters that vary are the volume of oil and the average diameter of the droplets. Under these conditions, the initial surface coverage, 275%, decreases to about 65%. In other words, about 35% of the surface is uncovered, which might be the triggering factor of the coalescence of the droplets, yielding larger droplets with higher polydispersity, as demonstrated in Figure 2b by CLSM. When the volume of hexadecane is further increased above the close packing conditions, the droplets undergo deformation into a polygonal shape without phase separation. This is induced by a reorganization of the interface, resulting in the formation of a gel-like structure with a continuous cellulosic network (Figure 2c). By varying the observed focal plane in depth, CLSM allowed us to visualize various layers of the sample. Figure 2d is a stacking of 10 focal planes in order to form a 10-μm-thick three-dimensional (3D) image, and shows the sample in volume. It revealed that some continuous walls were shared by different cells. The drops merged, and the CNCs formed a new interface organization. To investigate this evolution of the droplet wall structure, emulsified materials with different oil contents were freeze-dried and examined by SEM. Emulsions were prepared using cyclohexane, freeze-dried, and metalized with platinum for SEM visualization. Cyclohexane showed the same properties as hexadecane (drop diameter, internal phase percentage) but had a melting point (6

(1) C

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Figure 3. SEM images of solid emulsions from freeze-dried cyclohexane-in-water emulsions stabilized by CNCs containing (a, b) a concentrated droplet of a 2-mL initial emulsion; (c, d) the same emulsion after 9 mL of cyclohexane were added (77% of internal phase measured); and (e, f) after 22 mL of cyclohexane was added (95% of internal phase).

Figure 4. Schematic illustration of the formation process of MIPE and HIPE structures from CNC-stabilized Pickering emulsions.

°C) that made it more suitable for a freeze-drying process. Images of the freeze-dried initial Pickering emulsion (Figure 3a−c) revealed droplets of approximately 4 μm in diameter. The morphology of this material closely resembled that of the native emulsions. It suggested that porosity is induced by the original drop size and that the freeze-drying procedure does not affect the structure, at least in terms of pore size. As a consequence, a cellular architecture was produced. The reorganization of the wall already observed when increasing the oil content is confirmed by the foam morphology. The first emulsion visualized in Figure 4a showed a densely covered cellular organization. When the oil phase fraction is increased, the former drop-like structure disappeared. It is conceivable that the walls of the droplets have collapsed, giving rise to an expanded film-like reorganization of the cellulose. The original drop structural integrity resulted in an open structure. We observed a thinning tendency of the wall when the oil fraction was increased, but it was not possible to precisely determine the wall thickness. As long as no desorption occurs, it seems that the CNCs may possibly diffuse along the interface. As a result, the structure produced is an ultrathin network able to maintain established drop/cellular organization either in the liquid emulsified state or in the solid form.

oil volume. The absence of CNCs in residual water was checked by sugar analysis after sulfuric hydrolysis (see Experimental Section). Since no detectable sugar was revealed, we postulated that all the particles were adsorbed at the interface, leading to a surface coverage of 275%. This value is very high compared to the minimum of 84% of surface coverage that was required to stabilize the emulsions, as determined in a previous study.25 It can be assumed that the CNCs form a multilayer at the interface. Nevertheless, this excess of CNCs provides a stock of particles that can be used for stabilizing the interface when the droplet volume increases. Indeed, since the detachment energy (kT) depends on the square of the particle radius, it is considerably increased for large particles. High energy is then required to desorb such particles from the interface, but CNCs might move laterally at the interface.28,29 Assuming that the droplets do not coalesce before the surface coverage limit of 84% and, thus, that the number of droplets remains identical from 275 to 84% coverage, oil allows only an increase of the average drop diameter. We can then estimate that the coverage limit is reached for 40% of internal phase, i.e., below the limit of MIPE formation (Step 2 of Figure 4). Beyond 40% of internal phase, the coverage is no longer sufficient to stabilize the interface and a new reorganization occurs according to the limited coalescence process, forming larger droplets that are sufficiently covered by CNCs to remain stable. The drop size of Pickering emulsions is mainly governed by the surface coverage of the interface. This point is supported by both experimental and modeling studies.26,30 However, it should be noted that the droplet size polydispersity increases, as

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DISCUSSION Considering the mechanism of the HIPE formation, since the amount of CNCs remains identical throughout the HIPE formation process, the increased drop volume leads to the decrease of the surface coverage. Under our initial experimental conditions, a large amount of CNCs was used compared to the D

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shown by the CLSM images. This is in contrast with the low polydispersity generally displayed by Pickering emulsions, and should be investigated in future experiments. The droplet size continuously increases until it exceeds the close packing limit. At this point, deformation of the drops occurs with the merging of the walls, for an optimum distribution of the stabilizing CNCs (Step 4 of Figure 4). Although some recent studies have investigated the organization of particles at the oil/water interface, they have generally focused on spherical particles.31−33 The present study used rod-like CNCs. The effect of using nonspherical particles on the structure and properties of monolayers is relevant for different reasons. Shape is an important parameter in controlling the maximum packing, and the particles may also reorganize as a function of variations in the system. As a result, in the original Pickering emulsion, limited coalescence occurs until a percolating aggregate network forms at the interface.34 Some authors have illustrated the specific interfacial properties of nonspherical particles,35,36 but to our knowledge, no one has yet used submicrometer rods for such an application. One major difference with the other systems is that CNC, once confined at the interface might associate with each other according to an aggregative process. It produces multiple cooperative contacts distributed all along the crystals, deeply reinforcing the cohesion of the network at the interface. Moreover, they are flexible enough to bend at the interface of the drop. The result is that CNCs are fixed at the interface all along their length, allowing diffusion and reorganization. The wetting-induced self-assembly arises from particles trapped at the fluid−fluid interface.36 Even at the nanoscale, deformation of the interface gives rise to high deformation-mediated capillary interactions37 that are responsible for the exceptional stability of the initial Pickering emulsion, leading to the highly stable HIPE formation. The 3D packing of such an array will induce a new distribution of the available particles, leading to a gel structure maintained by thin cellulosic films. This reveals a dual property of CNCs: (i) they can undergo dynamic displacements under flow at the interface, which homogeneously redistributes the particles to promote versatile emulsions of controllable internal phase ratios; and (ii) the adsorbed CNCs cooperatively self-associate, giving rise to steric interfacial stabilization when the flow is stopped, which is a powerful mechanism for conferring strength and thereby allowing the preservation of any structural reorganization. As far as we know, this is the first time that such pristine cellulosic architectures have been reported.

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AUTHOR INFORMATION

Corresponding Author

*E-mail address: [email protected]. Mailing address: INRA, rue de la Géraudière, 44316 Nantes, France. Tel.: +33 (0)2 40 67 50 95. Fax: +33 (0)2 40 67 51 67. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Joelle Davy for SEM imaging, Nicolas Stephan for SEM assistance (IMN, Nantes, France), Brigitte Bouchet for her assistance in CLSM experiments (BIBS platform, INRA Nantes, France), and INRA for financial support (AIC 2011-2012).

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CONCLUSION The present study focuses on oil-in-water HIPEs that occur as a gel structure stabilized by a cellulosic network of colloidal particles. We demonstrate that this HIPE is only possible when a Pickering emulsion is prepared first, suggesting that a reorganization of the CNCs at the oil/water interface occurs as the volume of the internal phase and the interfacial area increases. We explain it by a critical coverage ratio process. First an increase of the drop size decreases the coverage up to the limited coverage, the stability is further controlled via a coalescence process. As a result, these emulsions could lead to liquid and solid controlled porosity materials using very small quantities of interfacial agent (below 0.1 wt %). These qualities, combined with their environmentally friendly character, could be of great interest in many application domains such as cosmetics or paint products. E

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