Cellulose Composite: From Aqueous

Apr 6, 2016 - The poor dispersibility of Sigmacell cellulose at low pH led to attraction of the composite-covered droplets which, once aggregated, cre...
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Multiwalled Carbon Nanotube/Cellulose Composite: From Aqueous Dispersions to Pickering Emulsions Carlos Avendano,† Nicolas Brun,*,† Olivier Fontaine,*,† Martin In,‡ Ahmad Mehdi,† Antonio Stocco,‡ and André Vioux† †

Institut Charles Gerhardt de Montpellier, UMR 5253, CNRS-ENSCM-UM, Université de Montpellier, CC 1701, Place Eugène Bataillon, 34095 Montpellier, France ‡ Laboratoire Charles Coulomb, UMR 5221, CNRS-UM, Université de Montpellier, CC069, Place Eugène Bataillon, 34095 Montpellier, France S Supporting Information *

ABSTRACT: A mild and simple way to prepare stable aqueous colloidal suspensions of composite particles made of a cellulosic material (Sigmacell cellulose) and multiwalled carbon nanotubes (MWCNTs) is reported. These suspensions can be dried and redispersed in water at pH 10.5. Starting with rather crude initial materials, commercial Sigmacell cellulose and MWCNTs, a significant fraction of composite dispersed in water could be obtained. The solid composites and their colloidal suspensions were characterized by electronic microscopy, thermal analyses, FTIR and Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and light scattering. The composite particles consist of tenuous aggregates of CNTs and cellulose, several hundred nanometers large, and are composed of 55 wt % cellulose and 45 wt % CNTs. Such particles were shown to stabilize cyclohexane-in-water emulsions. The adsorption and the elasticity of the layer they form at interface were characterized by the pendant drop method. The stability of the oil-in-water emulsions was attributed to the formation of an elastic network of composite particles at interface. Cyclohexane droplet diameters could be tuned from 20 to 100 μm by adjusting the concentration of composite particles. This behavior was attributed to the limited coalescence phenomenon, just as expected for Pickering emulsions. Interestingly, cyclohexane droplets were stable over time and sustained pH modifications over a wide range, although acidic pH induced accelerated creaming. This study points out the possibility of combining crude cellulose and MWCNTs through a simple process to obtain colloidal systems of interest for the design of functional conductive materials.

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INTRODUCTION Pickering emulsions are emulsions stabilized by solid particles in place of surfactants.1 The particles have to be partially wetted by both phases for an effective stabilization. In general, hydrophilic colloids tend to stabilize the oil-in-water (o/w) emulsions, while the water-in-oil (w/o) emulsions are better stabilized by hydrophobic particles. In that case, a major advantage of the stabilization by solid particles, which adsorb irreversibly at liquid−liquid interfaces, is a considerably higher resistance to coalescence than classical emulsions stabilized by surfactants. However, the stability is lost if the particles are wetted too strongly by either water or oil, which leads to their dispersion mostly in one of these phases. The shape of the particles plays also an important role for stabilizing emulsions. Nonspherical particles at fluid interfaces are submitted to strong attractive capillary forces due to interface deformation. This leads to interfacial aggregation and increases the interfacial layer rigidity which is a key parameter for emulsion stability.2,3 The absence of surfactant, which can be detrimental to health and environment, is another competitive advantage of Pickering emulsions for applications such as food,4 cosmetics, 5 pharmaceutics,6 and oil spill remediation.7 Moreover, intrinsic © XXXX American Chemical Society

chemical and physical properties of particles can be exploited to impart specific functionalities to emulsions. This opens a wide field of specific applications, which have contributed to the renewed interest in Pickering emulsions since the end of the nineties. In this work, we were interested in stabilizing o/w emulsions by carbon nanotubes (CNTs), which represent nano-objects with high surface, mechanical, electrical and optical properties. Typically, emulsion droplets stabilized with CNTs can serve as microreactors in which reactions are promoted by catalysts supported on the CNTs. Such recoverable catalytic systems that can both stabilize w/o emulsions and catalyze reactions of relevance to biomassrefining chemistry at the liquid/liquid interface were achieved by depositing palladium onto CNT−inorganic oxide composite nanoparticles.8,9 The use of CNTs as both interfacial stabilizer and electronic conductor was shown to promote rapid electrochemically driven ion transfer between the organic and the aqueous phase, which can be applied in liquid/liquid Received: February 1, 2016 Revised: April 5, 2016

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Langmuir electroanalysis.10 In the area of advanced materials, emulsion polymerization, which results in CNTs tightly bounded to the surface of the final latex particles, offers an elegant way to high performance polymer/CNT composites.11 Alternatively, CNTs can stabilize concentrated w/o emulsions where the droplets of dispersed phase are at close contact, allowing the successful use of Pickering emulsion as templates for the synthesis of polymer foams via in situ polymerization and subsequent drying.12 The polymerization of the minority organic component as the continuous phase in which the CNTs are embedded, results in highly porous, reinforced, and electrically conducting materials, called Poly-HIPE (polymerized high internal phase ratio emulsion).13 Actually, it is very difficult to disperse CNTs homogeneously in either organic or aqueous solvents due to substantial van der Waals interactions between the tubes, which lead to their aggregation in large and difficult to process bundles. However, after partial oxidation (typically acid treatment14 or by plasma15), CNTs get both hydrophobic (graphene layer) and hydrophilic (hydroxyl and carboxylic groups) characters. This duality (amphiphilicity) may stabilize CNTs at the interfaces between the aqueous and organic phases. To the best of our knowledge, the preparation of o/w Pickering emulsions only stabilized by unmodified pristine CNTs has never been reported. Additionally, only one study reported on the use of pristine CNTs to stabilize w/o (i.e., water-in-toluene) emulsions.16 Accordingly, several o/w and w/o Pickering emulsions were prepared by using oxidized single-walled (SWCNTs)17 and multiwalled CNTs (MWCNTs).15,18,19 It is worth noting that covalent surface modifications, particularly through harsh oxidative treatments, cause defects in the nanotube structure, leading to the disturbance in the π−π electron system and therefore losses in the mechanical and electronic properties of the nanotubes.14 Surfactants,20 polymers and biomacromolecules,21,22 have been successfully used as dispersing agents that did not involve any covalent modifications of CNTs. As a matter of fact, sodium carboxymethylcellulose was claimed to be one of the best dispersants for SWCNT in terms of good isolation, stability, high concentration, versatility, and film-forming ability.23 Similarly the codispersion of native cellulose and SWCNTs by ultrasonication in water at pH between 6 and 10 has been demonstrated.24 More recently, Hamedi et al. used nanofibrillated cellulose (NFC) as aqueous dispersion agent for the preparation of NFC-SWCNTs films, cryogels, and microfibers.25 Note that cellulose-MWCNT composites can be obtained by in situ biosynthesis in the presence of MWCNTs dispersed in culture medium.26,27 Moreover, such cellulosebased electroconductive composites provide a biocompatible interface for electrical stimulated drug release devices, implantable biosensors, and other in vivo applications.28 However, despite the decisive advantage of noncovalent surface modifications, there are only a few examples of emulsions stabilized by noncovalently modified CNTs29 and, to the best of our knowledge, not one involving polysaccharides as dispersing agent. The present work was motivated by the ability of nanocellulose, such as bacterial cellulose nanocrystals30 or microfibrillated cellulose,31,32 to stabilize oil/water interfaces and to produce Pickering emulsions with outstanding stability,30−35 with the prospect of further designing templated conductive materials, in which MWCNT-cellulose composite particles would play the role of building blocks. Such emulsion

templating would be relevant for preparing macroporous materials, endowed both with the conduction and mechanical properties of CNTs and with the biocompatibility and ability of chemical modification of cellulose. Herein, a detailed study of the codispersion of a cellulosic material (Sigmacell cellulose) and multiwalled CNTs (MWCNTs) in water is reported. The noncovalent surface modification of MWCNTs by cellulose was evidenced, and the composite nanoparticles were investigated by static light scattering. Subsequently, the water-dispersed MWCNT−cellulose composites were able to stabilize cyclohexane/water droplet interfaces. The resulting emulsions were shown to be stable over time and to sustain pH modification over a wide range (1.3−10.5).

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EXPERIMENTAL SECTION

Materials. Multiwalled carbon nanotubes (MWCNTs) (6−9 nm in diameter and ∼5 μm in length with purity over 95%), cellulose (Sigmacell type-50), anhydrous cyclohexane, and Sudan III (oil soluble dye) were purchased from Sigma-Aldrich and used as received. Sodium hydroxide anhydrous pellets were purchased from Carlo Erba Reagents. Methods. Preparation of MWCNT−Sigmacell Cellulose Composite. The MWCNTs were used as received without any modification. Typically, 100 mg of MWCNTs and 1.00 g of Sigmacell cellulose were added in a 600 mL flask containing 500 mL of a NaOH aqueous solution at pH 10.5 (5.75 × 10−4 M). The above mixture was ultrasonicated (Fischer, Bioblock Scientific, VibraCell 75042, 500 W) at 21% amplitude using a 13 mm probe for 15 min. The ultrasonication led to a dark dispersion at the bottom of which large floc particles settled down. To complete the separation of the dispersed particles, the sample was centrifuged at 10 000 rpm for 10 min (Beckman Coulter, Allegra 64R). The supernatant consisted of a homogeneous dark brown dispersion of cellulose-stabilized carbon nanotubes referred to as MWCNT−Sigmacell. The cake obtained by filtration of the dispersion on 0.2 μm filter led to a self-supported solid film after drying at 60 °C. Preparation of o/w Pickering Emulsions. The MWCNT− Sigmacell composites were redispersed in 10 mL of a NaOH aqueous solution at pH 10.5, sonicated with the dipping ultrasonic probe for 15 min and then allowed to cool to room temperature. Cyclohexane (3 mL) was added to the MWCNT−Sigmacell aqueous dispersion, and shaken by hand for 30 s. Finally the mixture was emulsified by ultraturrax disperser at 16,200 rpm for 30 s (IKA, T 18 digital). In some cases, a dye (5 mg of Sudan III) was previously dissolved in cyclohexane (3 mL) and sonicated in a bath for 10 min before preparing emulsions. Sudan III allowed sharper observations. The MWCNT−Sigmacell stabilized emulsions were compared with cellulose-stabilized cyclohexane/water emulsions, without MWCNTs, under the same experimental conditions. To do so, 100 mg of Sigmacell were dispersed in 50 mL of a NaOH aqueous solution at pH 10.5, sonicated with the dipping probe for 15 min, filtered, and dried. The as-prepared powder (called NaOH-treated Sigmacell) was redispersed in 10 mL of a NaOH aqueous solution at pH 10.5 and sonicated with the dipping ultrasound probe for 10 min. Afterward, cyclohexane (3 mL) was added to 10 mL NaOH-treated Sigmacell dispersion, shaken by hand for 30 s and emulsified with the ultraturrax disperser. Characterizations. All products were characterized by several techniques. For Fourier transform infrared (FTIR) spectroscopy, dried samples (1 wt %) were dispersed with KBr and pressed at 70 GPa to get thin pellets. The spectra were recorded using a Thermo Scientific Nicolet Avatar 320. Thermogravimetric analyses (TGA) were achieved under air flow using a NETZSCH STA 409 PC and the program NETZSCH Proteus Thermal Analysis. The temperature program was as follows: a rate of 5 °C/min up to 1000 °C. For transmission electron microscopy (TEM), an aliquot of dispersed MWCNTs, cellulose, cellulose−NaOH, or MWCNT−Sigmacell composites was dropped on a copper or carbon microscopy grid and images were B

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Langmuir obtained using a JEOL 1200 EX2 instrument operating at 200 keV. Pickering emulsions were observed by optical microscopy with a Leitz Laborlux 12 Pol S, and images were processed using the program MGrabSequence. Raman spectroscopy was performed with a LabRAM ARAMIS IR2 using a diode blue laser (λ ∼ 473 nm). X-ray photoelectron spectroscopy (XPS) was performed with a Thermo Electron Escalab 250 using a monochromatized Al Kα (1486.6 eV) source and a 400 μm spot size. In order to take into account the charging effect on the measured binding energies, the spectra were calibrated using the C 1s line at about 286.7 eV, related to C−OH bonds of cellulose. Static light scattering measurements were carried out on a standard setup (AMTEC goniometer with a BI9400 Brookhaven correlator) with yag solid laser (Cobolt Samba 150) operating at 532 nm. Incident power light ranges from 20 to 50 mW. Angle was varied from 15° to 140°, which corresponded to scattering vector 4 × 10−4 to 3 × 10−3 Å−1. Temperature was fixed at 25 °C. For static light scattering experiments, photon counts on two detectors were averaged and normalized by time, scattering volume and transmission to yield the scatterred intensity IN. The Rayleigh ratio of the dispersion reads:

R(q) =

IN/TS − I water /Twater R toluene Itoluene/Ttoluene

(1) Figure 1. Dispersions of MWCNTs (8 mg in 40 mL of water at pH 10.5) alone (a) and in the presence of 80 mg of Sigmacell (b) after ultrasonication and centrifugation.

where IN, Iwater, and Itoluene are the scattered intensity and TN, Twater, and Ttoluene are the transmittances that have been measured as explained in the Supporting Information (Figure S1). Interfacial tensions were determined by using the pendant drop tensiometer (PAT, Sinterface, Germany). The method consists of imaging aqueous drops hanging in cyclohexane at the tip of a seringe and fitting the shape of the drop to the Young−Laplace equation. The interface between cyclohexane and aqueous dispersions of MWCNT, NaOH-treated Sigmacell cellulose and MWCNT−Sigmacell composite were first studied in static conditions to evidence adsorption of material at interface. Interface elasticity was then measured upon cycles of compression and dilation at different frequencies f from 8.0 × 10−3 to 2.5 × 10−1 Hz. A sinusoidal variation of the surface area was imposed to the drop:

ΔA = A − A 0 = A a cos(ωt )

cellulose. This dispersible fraction is thereafter called MWCNT−Sigmacell composite. According to the literature, sodium hydroxide is known to contribute to the disruption of intra- and intermolecular hydrogen bonds24,36−39 in cellulose (phenomenon also called mercerization), which certainly favors cellulose dispersion in water and presumably should favor the formation of new bonding interactions with MWCNTs, as it will be discussed later. However, the pH range allowing stable dispersions is significantly narrower in our case than in previous works. Actually, Gokhale et al.24 obtained homogeneous aqueous dispersions of single-walled CNTs by using Sigmacell cellulose in the pH range from 6 to 10. One may assume that depending on the supplier and the nature of the CNTs, the surface chemistry can be significantly influenced. Moreover, as far as we used raw cellulose, i.e., Sigmacell cellulose, some variations were noted from one batch to another. On the other side, very reproducible dispersions were obtained by using sodium caboxymethylcellulose in the same concentration range (see Figure S3). Interestingly, the solid (MWCNT−Sigmacell composite), recovered from the supernatant liquid phase after drying, could be redispersed in water at pH 10.5, up to about 0.25 wt %. These dispersions proved to be totally stable after 2 months. However, using higher weight percentages, saturation and accumulation of the solid on the wall of the flask was observed (Figure 2). The dried MWCNT−Sigmacell composites were characterized by X-ray photoelectron spectrometry (XPS), while thermal gravimetric analysis (TGA) permitted us to assess the mass ratio of cellulose over MWCNTs. MWCNT−cellulose bonding interactions were analyzed by FTIR and Raman spectroscopy. A thorough comparison of the morphology and structure of the composites with those of MWCNTs, Sigmacell cellulose, and NaOH-treated Sigmacell, recovered from sonicated aqueous dispersions under the same conditions, was carried out by means of scanning and transmission electron microscopy (SEM and TEM) and X-ray diffraction (XRD).

(2)

and the response in interfacial tension recorded:

Δγ = γ − γ0 = γa cos(ωt + δ)

(3)

where ω = 2πf varied from 0.05 to 1.6 rad/s. The amplitude of the area varied from 1% to 40% of A0. δ was the phase shift between the perturbation and the response. Dilational storage modulus was determined from

E′(ω) =

γa A a /A 0

cos(δ) = Ed

(4)

and the loss modulus from

E″(ω) =

■

γa A a /A 0

sin(δ) = ηdω

(5)

RESULTS AND DISCUSSION Dispersion of MWCNT-Cellulose Composite in Water. The dispersion of the MWCNTs in water by using Sigmacell cellulose as a dispersing agent under ultrasonic irradiation turned out to be strongly dependent on pH (Figure S2). In a narrow range of pH around 10.5, stable dispersions were observed, while at acidic, neutral, and more basic pH (i.e., >10.5), the solid was completely nondispersible. After centrifugation at pH 10.5, the supernatant liquid phase was separated (Figure 1), filtered, and dried, yielding 30−40 wt % of the starting MWCNTs amount, i.e., less than 4 wt % related to the starting solid amount including both MWCNTs and C

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components could be discriminated after deconvolution: sp2 carbons in aromatic rings (i.e., CC) at 284.41 eV, sp3 carbons (i.e., C−C) at 285.24 eV associated with defects in the tubular structure, and carbons singly bound to hydroxyl groups (i.e., C−OH) at 286.31 eV.40,41 With respect to cellulose, four contributions could be extracted, among which the two major ones are carbons of the glucose units singly bound to hydroxyl groups (i.e., C−OH) at 286.66 eV; and carbons at position C1 of the β-1,4-glycosidic bonds (i.e., O−C−O) at 297.94 eV.42,43 Carbons at position C4 of the β-1,4-glycosidic bonds are related to the so-called C−O−C contributions at 289.20 eV. Note that, as the average depth analysis for an XPS measurement is about 10 nm (on spots of about 400 μm diameter), while the diameter of MWCNT is 9−15 nm, the relative carbon percentages measured should give a representative idea of the cellulose content in the whole composite. Thus, regarding C 1s XPS data of pristine MWCNTs, Sigmacell cellulose and MWCNT−Sigmacell composite, it could be calculated that around 34% of carbon present in the composite came from cellulose, representing around 54 wt % of cellulose in the composite (see Table S1 for details). A more accurate determination of the cellulose content in the composites was implemented from thermogravimetric analyses (TGA) under air atmosphere. As shown in Figure 4, pristine MWCNTs underwent no weight loss below 475 °C. Interestingly, at this temperature, the NaOH-treated Sigmacell sample was completely burnt down, leaving behind just inorganic ashes. This discrepancy allowed estimating the amount of cellulose from the TGA of MWCNT−Sigmacell composites. Thereby, around 55 wt % of cellulose was found in the composite, in agreement with the semiquantitative value

Figure 2. Redispersion of MWCNT−Sigmacell composites in water at pH 10.5 using different concentrations (0.10, 0.25, and 0.50 wt %).

Finally, a direct study of the aqueous dispersions of MWCNT− Sigmacell composites was carried out by means of light scattering. XPS measurements were carried out on pristine MWCNTs, cellulose and the composite MWCNT−Sigmacell (Figure 3). An increase of the O 1s peak was observed in the composite due to the incorporation of cellulose (see Figure S4). The C 1s band of the composite was deconvoluted in order to discriminate the functional groups related to MWCNTs and cellulose (Figure 3). With respect to pristine MWCNTs, three

Figure 3. C 1s X-ray photoelectron spectroscopy deconvoluted curves obtained for MWCNTs, Sigmacell cellulose, NaOH-treated Sigmacell, and MWCNT−Sigmacell composite. D

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the intensity of the broad one associated with glucosidic bonds slightly decreased. These results confirm that NaOH treatment led to the so-called mercerization of cellulose.38,48 SEM and TEM images of MWCNTs and MWCNT− Sigmacell composites are compared in Figure 5. Figure 5a and b

Figure 4. TG curves of (a) pristine MWCNTs, (b) MWCNT− Sigmacell composite, (c) as received Sigmacell cellulose, and (d) NaOH-treated Sigmacell cellulose.

extracted from XPS data. The graphs of the derivative of the TGA curve (DTG in Figure S5) point out that the decomposition process of the composite actually occurred in two stages, the first centered at 310 °C (oxidative decomposition of cellulose) and the second centered at 560 °C (oxidative decomposition of MWCNTs). Slight shifts observed on moving from pristine Sigmacell to composites can be attributed to the changes in cellulose structure related to the action of NaOH and the hybridization with MWCNTs. Note that the residue left behind after oxidative decomposition of NaOH-treated Sigmacell cellulose under air atmosphere could be attributed to NaOH as shown by FTIR (Figure S6). Raman spectra of pristine MWCNTs and MWCNT− Sigmacell exhibited two characteristic peaks at 1324 and 1590 cm−1, denominated D and G bands, respectively (Figure S8). The D band corresponds to the vibration induced by defects, while the G band is an intrinsic feature of graphene sheets.44 The intensity ratio of these bands (ID/G) allows determining the degree of structural disorder in sp2 carbon materials. Here, the ID/G ratio was found unchanged (0.84 for pristine MWCNT vs 0.83 for MWCNT−Sigmacell composite), which confirmed that MWCNTs underwent no covalent modification. Further insights into the non covalent bonding interactions between Sigmacell cellulose and MWCNTs were given by FTIR spectroscopy (Figures S7, S9, and S10). At first impression, the presence of cellulose within the composites was clearly noticeable. For each cellulose-based sample, five main absorption bands were observed at 1032−1165 cm−1 (glucosidic bonds or ethers C−O−C stretching vibrational mode), 3410−3435 cm−1 (O−H stretching vibrational mode), 2896−1 (C−H stretching vibrational mode), 1638−1 (adsorbed H2O), and 1460−1464−1 (HCH and OCH in-plane bending vibrations).45,46 Besides the main band at 3410 cm−1, a broad shoulder centered at 3200 cm−1 could be noticed for the Sigmacell sample. This shoulder vanished after NaOH treatment. According to Oh et al.,47 this band could be assigned to O−H−O intra and/or intermolecular stretching vibrational modes. This feature supports the expected disruption of intra- and intermolecular hydrogen bonds related to NaOH treatment of Sigmacell cellulose. Moreover, clear variations in the intensity of absorption bands can be noticed. Particularly, the band centered at 1640 cm−1, and related to adsorbed water, was more intense after NaOH treatment, while

Figure 5. SEM (a) and TEM (b,c) micrographs of MWCNT− Sigmacell composite. (d) TEM micrograph of pristine-MWCNTs. Arrows point out cellulosic domains.

(and at a higher magnification Figure 5c) reveals the presence of darker domains (indicated by white arrows) within the tangle of MWCNTs. These domains can be attributed to the presence of cellulose, since they are not observed for pristine MWCNT tangles (Figure 5d). Such domains are not observed in the voids left in the tenuous network of MWCNTs, but instead always appear at contact with MWCNTs. These results confirm the actual formation of MWCNT−Sigmacell composites arising from the separation and adsorption of some cellulose matter at pH 10.5 on the external surface of MWCNT bundles. Besides, the affinity between cellulose and carbon nanotubes was also revealed in a few rare areas (Figure S11b), where bigger cellulose domains were decorated by MWCNTs. Such bigger cellulosic domains were very rarely observed in the dispersed composite and might actually constitute the major part of the flocculated part of the mixture (Figure S12). Likewise, X-ray diffraction confirmed the incorporation of cellulose in the composite (Figure 6). Three characteristic peaks of cellulose crystals were observed at 2θ = 15.86°, 22.81°, and 34.72°, while the peak around 2θ = 25.90° was assigned to the (002)-plane of graphite corresponding to lamellar arrangements of MWCNT.49−51 By using Scherrer’s formula, the average sizes of the cellulose primary crystallites for MWCNT− Sigmacell composite, NaOH-treated Sigmacell and untreated Sigmacell cellulose (Table 1) were estimated to be 42, 45, and 43 Å, respectively, from the highest intensity (2θ = 22.81°), related to (002)-plane. Besides, the crystallinity indexes (CrI) were calculated, using the formula:47 ⎛ I ⎞ CrI = ⎜1 − am ⎟ × 100 I002 ⎠ ⎝

(6)

where I002 is the intensity of the crystalline (002)-plane (2θ = 22.81) and Iam is the intensity of the amorphous reflection (2θ = 15.00). The crystallinity indexes of composite, NaOH-treated E

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decreases as q−1, as expected for cylinders (see dotted line in Figure 7). Going toward smaller q, the scattered intensity increases further, but faster than expected for cylinders. This suggests that, at larger scale, the cylinders get curved or branched. To extract the characteristic lengths of the composite, the whole scattering patterns were compared with the one of branched cylindrical structures. I(q) was calculated using general fractal model considering two levels of structuration, which reads:52,53 ⎛ −Q 2R 2 ⎞ g0 ⎟ I(Q ) = G0 exp⎜⎜ ⎟ 3 ⎝ ⎠ ⎛ −Q 2R 2 ⎞ ⎡ ⎛ QR g0 ⎞⎤3d0 g1 ⎟ 1 ⎥ ⎢ exp⎜⎜ + C0 erf⎜ ⎟ ⎟ d0 3 ⎣ ⎝ 6 ⎠⎦ ⎝ ⎠Q ⎛ −Q 2R 2 ⎞ ⎡ ⎛ QR g1 ⎞⎤3d1 1 g1 ⎟ + C1⎢erf⎜ + G1 exp⎜⎜ ⎟⎥ ⎟ 3 ⎣ ⎝ 6 ⎠⎦ Q d1 ⎝ ⎠

Figure 6. XRD patterns of (a) MWCNTs, (b) MWCNT−Sigmacell composite, (c) Sigmacell cellulose, and (d) NaOH-treated Sigmacell cellulose.

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Table 1. Crystallinity Indexes and Estimated Crystallite Sizes (by Scherrer’s Formula) of Each Sample

CrI (1 − I1/I2) crystal size (Å)

Sigmacell

NaOH-treated Sigmacell

MWCNT−Sigmacell composite

72

71

76

43

45

42

where Ci =

⎤ ⎛d ⎞ Gidi ⎡ 6di 2 ⎥Γ⎜ i ⎟ ⎢ di R gi ⎣ (2 + di)(2 + 2di) ⎦ ⎝ 2 ⎠

(8)

Each level is described by four parameters Rgi, di, Gi, and Ci among which only three are independent. First terms (subscript 0) describe features of the composite at large scale. Calculated scattering patterns I(q) were compatible with the experimental data for 350 nm < Rg0 < 450 nm and fractal dimension of 1.3 < d0 < 2.1. This fractal dimension is related to the branching density and the flexibility of the cylinders. Last terms of the equation (subscript 1) describe small scale features of the composite with d1 = 1, that we did not vary, 120 ≤ Rg1 ≤ 160 nm, and 5 < G0/G1 < 9. Analysis of the data with Beaucage model allows extrapolating the scattered intensity to q = 0 and yields R(0) which is related to the mass and the second virial coefficient according to

Sigmacell and untreated Sigmacell cellulose (Table 1) were estimated to be 76%, 71%, and 72%, respectively. These values are consistent with those reported in the literature.39 Moreover, crystallinity indexes exhibit similar values after the NaOH treatment and the hybridization with MWCNTs. These results demonstrate that no change in crystallinity of the cellulose nanocrystallites occurred. Finally, the aqueous dispersion of MWCNT−Sigmacell composite was directly analyzed by light scattering. The Rayleigh ratio divided by the concentration C is presented in Figure 7 versus the scattering vector q. At large q (from 3 × 10−3 Å−1 down to 1.8 × 10−3 Å−1), the scattered intensity

⎛1 ⎞ KC = ⎜ + 2A 2 C ⎟ ⎝ ⎠ R(0) M

where K is a constant depending on the refractive index increment, M is the molar mass of the particle, and A2 is the second virial coefficient. We do not know the refractive index increment of the composite, so we cannot determine the molar mass and the value of the second virial coefficient. It is however possible to infer from the sign of the virial coefficient, which is observed to be positive (Figure 8), that the particles repeal each other, which is consistent with the high stability of the dispersions. Note that, when studying the supernatant of MWCNT in water, the intensity is the same as the one of water, meaning that nothing in the colloidal size range remains dispersed without addition of cellulose. A similar feature was observed for the supernatant of NaOH-treated Sigmacell cellulose after few days aging, supporting a synergistic effect between the two components. Application of MWCNT/Sigmacell Composite as Stabilizing Particles in Oil-in-Water Pickering Emulsions. Several emulsions were prepared with various composite

Figure 7. Scattered light intensity normalized by the concentration versus scattering vector. Thick line is the calculated scattering pattern for branched cylindrical structures using the Beaucage model (see eq 7 in the text, with Rg0 = 440 nm, d0 = 1.6, Rg1 = 160 nm and G0/G1 = 6). Dotted line is proportional to q−1. F

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can be attributed to the limited coalescence phenomenon. As the amount of composite increased, the interfacial area that can be covered by composite particles increased, resulting in a fine control of droplet diameters.54 After one month, the emulsions were kept, but an evolution was observed, with the formation of an upper layer consisting of the o/w Pickering emulsion, and a lower layer consisting of water and some black precipitate (see Figure S13). Cyclohexane droplets in the upper layer were found slightly bigger than in the initial emulsion, which suggests that coalescence phenomena, due to the sedimentation of some composite particles, were associated with the close packing of the emulsion. Additionally, Pickering emulsions arising from MWCNT− Sigmacell composites were subjected to pH change (1.3, 3.6, and 6.8; see Figure S14). Different amounts of HCl were added to a Pickering emulsion formulated at pH 10.5 with a composite concentration of 0.10 wt %, and Ultraturrax time and speed of 30 s and 16 200 rpm, respectively. As pH decreased, the droplets were forced to concentrate into an upper layer and the excess of water was then excluded into a lower layer, leading to close packing conditions. These accelerated creaming phenomena were obtained whatever the pH value reached, i.e. 1.3, 3.6, and 6.8. Optical images (Figures 10 and S14) evidenced that the o/w emulsions were preserved, demonstrating the stability under pH modification. Surprisingly, cyclohexane droplet diameters remained almost intact despite creaming, revealing no evident coalescence. Figure 10 shows the close-packing of droplets observed at pH 1.3 in the upper layer. On the one hand, these phenomena can be explained by the pH dependence of the charge density on cellulose. At pH 10.5, cellulosic material and, as a direct consequence, composites are partially charged. This can be evidenced by the fact that both Sigmacell cellulose and composites could be only dispersed at this precise pH value, hardly sustaining pH changes, as mentioned earlier. Once adsorbed at droplets interface, composite−composite interactions determined interdroplets interactions (i.e., interactions between nearby droplets). The poor dispersibility of Sigmacell cellulose at low pH led to attraction of the composite-covered droplets which, once aggregated, creamed faster, just as flocculation accelerates sedimentation when considering particles denser than water. On the other hand, getting away from pH 10.5, one may assume that, beyond attracting each other, MWCNT− Sigmacell composites were more likely willing to migrate from the continuous aqueous phase to strongly adsorb at cyclohexane−water interfaces. This feature would rather stabilize the interface, explaining the absence of evident coalescence. Furthermore, emulsions stabilized with only NaOH-treated Sigmacell, used in the same concentration as in the composite, were prepared for comparison. Thus, considering that the composite contained around 55% cellulose (as calculated by TGA), the concentrations of NaOH-treated cellulose equivalent to 0.05, 0.10, and 0.15 wt % of composite were estimated to be 0.0275, 0.055, and 0.0825 wt %, respectively. Thereby, several emulsions were prepared with NaOH-treated cellulose using constant cyclohexane-to-water ratio (1:3.3). Figure S15 reveals that o/w emulsions exhibit droplets with similar size whatever the cellulose concentration (with a mean diameter of 22 μm). The droplets diameters shown herein are close to the ones reported in the literature for cellulosic particles-stabilized Pickering emulsions.30 In our case, the emulsions exhibited an

Figure 8. Concentration dependence of C/R(0) shows that the composite particles repeal each other. C is the weight fraction.

concentrations (0.05, 0.10, 0.15, and 0.20 wt %) with constant oil to water volume ratio (1:3.3). Droplet diameters were measured by optical microscopy. As shown in Figure 9, o/w emulsions with lower composite concentration (0.05 wt %) exhibited bigger diameter and wider distribution of droplets. Gradually increasing the concentration of composites from 0.05 to 0.20 wt % led to a decrease in droplets diameters from 76.5 ± 5.5 μm down to 16.3 ± 1.6 μm (Figure 9e). These results

Figure 9. Optical images of Pickering emulsions obtained at different MWCNT−Sigmacell composite concentrations: (a) 0.05, (b) 0.10, (c) 0.15, and (d) 0.20 wt % (scale bars: 100 μm). (e) Associated distribution of droplets diameters (Ultraturrax time and speed = 30 s and 16200 rpm, respectively). Errors bars correspond to standard deviation of uncertainty. Inset: 1/D (D = droplet diameter) versus composite-dispersion concentration (wt %). G

DOI: 10.1021/acs.langmuir.6b00380 Langmuir XXXX, XXX, XXX−XXX

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Langmuir

Figure 10. Optical images and pictures of a Pickering emulsion obtained at pH 10.5 (on the left) and the associated emulsion after adding HCl to reach pH 1.3 (on the right). Scale bars: 50 μm.

upper layer consisting of the o/w Pickering emulsion and a lower layer consisting of water. After 2 weeks, those two layers were kept and cyclohexane droplets were preserved in the upper layer (Figure S15). The fact that the emulsions prepared with Sigmacell alone and MWCNT−Sigmacell composites were qualitatively different is still open to discussion, and the elements we can provide are rather conjectural. At the moment, we believe that this is due to the difference in adsorption/desorption kinetics between cellulosic fragments and composite particles dispersed in solution. Cellulosic fragments, although certainly less hydrophobic than composite particles, should adsorb faster at the oil−water interface during the emulsification process due to smaller size. At the end of the ultraturax treatment, interfaces are closer to saturation in the presence of cellulose alone than in the presence of composite. This might explain the small droplet diameters observed. The remanence of small droplet diameters in cellulose-stabilized emulsion suggests also that Ostwald ripening is more hindered with cellulose alone than with composite as stabilizer. This would be related to the fine structure of the adsorbed layer which we could not characterize herein. Thus, in the absence of MWCNTs, one may assume that Sigmacell-stabilized emulsions behaved more like classical emulsions than like particles-stabilized Pickering emulsions, supporting the absence of limited coalescence phenomenon. On the contrary, in the presence of MWCNTs, those cellulosic fragments probably adsorbed onto carbon nanotubes and the as-obtained composites were more likely to behave purely as nanoparticles. The stability of the emulsions is related to the presence of a solidlike layer of composite at the cyclohexane/water interface. This was confirmed by the characterization of the dilational properties of the water-hexane interface. Surface tension measurements at cyclohexane/dispersions interface were carried out for dispersions of MWCNT, NaOH-treated cellulose and MWCNT−Sigmacell composite. As shown in Figure 11, the surface tension γ decreases over time from 52 to 47 mN/m for MWCNT dispersion, down to 41 mN/m for NaOH-treated Sigmacell cellulose dispersion and down to 37 mN/m for composite dispersion. The interfacial tension of cyclohexane/water interface (when both solvents have been saturated with each other) is 50 mN/m. The decrease observed for MWCNT dispersion is of the same order of magnitude as the one observed for carbon black micrometric particles at alkane/water interfaces.55 On the other hand, more significant decrease of interfacial tension is observed for NaOH-treated Sigmacell cellulose dispersion and composite dispersion. This observation supports that MWCNTs and cellulose do indeed coexist at oil/water interface. The kinetics of adsorption is slow, with characteristic time of couple of hours, as expected for large particle diffusing slowly. Measurements of a significant elasticity of the interface provide a further evidence for adsorption. Interfacial dilational

Figure 11. Interfacial tension of cyclohexane/aqueous dispersions interfaces versus time. Black diamond, MWCNTs; green square, NaOH-treated Sigmacell cellulose; red circle, MWCNT−Sigmacell composite.

modulus has been measured after the experiments under static conditions, when the interfaces could reasonably be considered at equilibrium. Over the range of accessible frequencies, all three interfaces are more elastic than viscous, it means E′ > E″ (E′ and E″ refer to storage and loss modulus, respectively; Figure 12). The storage modulus of the cyclohexane/composite

Figure 12. (a) Dilational moduli of the cyclohexane/dispersion interface versus frequency. Amplitude of the deformation is 2%. Open symbols: loss modulus E″; filled symbols, storage modulus E′. Diamond, MWCNTs; square, NaOH−cellulose; circle, MWCNT− Sigmacell composite.

dispersion interface is 20 mN/m, close to the one of the cyclohexane/cellulose dispersion interface, but significantly higher than the one of cyclohexane/MWCNTs dispersion, which is 8 mN/m. The elastic modulus obtained for cyclohexane/composite and cyclohexane/cellulose dispersion interfaces is close to the one obtained with carbon black particles (20.7 mN/m).55 It is also consistent with the one obtained for hydroxypropyl cellulose at oil/water interface at H

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the same interfacial tension.56 The dilational dynamics properties of the cyclohexane/composite dispersion interface are essentially imparted by the cellulosic component. Finally increasing the amplitude of the area variation from 2% to 40%, at f = 0.025 Hz, did not induce any variation of the dilational properties of the composite interface.

CONCLUSIONS MWCNTs/Sigmacell cellulose composite particles and associated stable aqueous colloidal suspensions were prepared from a mild and simple way, taking advantage of the noncovalent coupling of MWCNTs with cellulose. Using the adsorption ability of such composite particles at cyclohexane−water interfaces, an original way of stabilizing oil-in-water Pickering emulsions was demonstrated. The stability of the o/w emulsions was attributed to the formation of an elastic network of composite particles at the interface. Cyclohexane droplet diameters could be easily tuned from 20 to 100 μm by adjusting the concentration of composite particles. This behavior was attributed to the limited coalescence phenomenon, just as expected for Pickering emulsions. The emulsions were stable over time and sustained pH modification over a wide range (from 1.3 to 10.5). Interestingly, this study highlights the versatile ability of cellulose, even a crude one, to stabilize CNTs in water and to bring it efficiently at oil−water interfaces. These results shed light on the interest of combining cellulose and MWCNTs to prepare emulsions. Recently, this approach was extended to commercial carboxymethylcellulose57 and should be easily extended to other polysaccharides and other oil/water systems. This versatility and the stability of the resulting emulsions can considerably enlarge such applications as template synthesis of polymer composites, biphase catalysis and interfacial charge transfer reactions. The design of emulsion-templated conductive materials using MWCNT− cellulose composite systems will be published by our group in future articles. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b00380. Additional transmittance measurements, X-ray photoelectron spectroscopy, derivative thermogravimetric analyses, FTIR and Raman spectroscopy, SEM and TEM micrographs, as well as optical microscopy images of emulsions (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +33 (0)4 67 14 38 52. Tel: +33 (0)4 67 14 48 24. *E-mail: [email protected]. Fax: +33 (0)4 67 14 38 52. Tel: +33 (0)4 67 14 45 28. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was conducted within the framework of ANR-10LABX-05-01 “LABEX CheMISyst” program. Especially, C. A. is grateful to “LABEX CheMISyst” for financial support and to Sabrina Zordan for her collaboration in syntheses. I

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