Carboxymethyl Cellulose Nanofibrils with a Treelike Matrix

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Carboxymethyl Cellulose Nanofibrils with a Treelike Matrix: Preparation and Behavior of Pickering Emulsions Stabilization Jie Wei,† Yi Zhou,† Yanyan Lv,† Jianquan Wang,† Chao Jia,§ Jianxin Liu,† Xinfang Zhang,† Jian Sun,*,‡ and Ziqiang Shao*,†

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†

Beijing Engineering Research Center of Cellulose and Its Derivatives, School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, P. R. China ‡ School of Life Sciences, Beijing Institute of Technology, Beijing 100081, P. R. China § State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, P. R. China S Supporting Information *

ABSTRACT: Carboxymethyl cellulose nanofibrils (CMCNFs) are of great importance in the fields of sustainable chemistry and energy materials but challenging in preparation, e.g., low yield, pollution, and morphology control. In this work, CMCNFs with a quantitative yield (95%) and good morphology control were successfully achieved using a simple, low-cost, and relatively ecofriendly protocol. Water-insoluble carboxymethyl cellulose (CMC) with a low degree of substitution (DS ≤ 0.35) was obtained via moderate alkalization and etherification of cellulose raw materials and then was mechanically fibrillated to prepare CMCNFs using a microfluidizer. As the DS of the CMCNFs was increased from 0.05 to 0.35, the diameter was obviously decreased from 100 to 35 nm without changing the treelike matrix that was confirmed by TEM characterization. More importantly, there is no obvious difference in the final performance of the CMCNFs derived from different cellulose raw materials (i.e., cotton and wood) through this approach. Additionally, compared with the commercial CNFs, high stabilization of Pickering emulsions stabilized by the CMCNFs (DS = 0.23) could be achieved because of its novel morphology with a network structure, implying that the CMCNFs could potentially serve as a biofunctional stabilizer. KEYWORDS: Carboxymethylation, Cellulose nanofibrils, Treelike matrix, Stabilization, Pickering emulsions

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applications in some fields where it could not be involved before.18−20 Typically, nanocellulose can be divided into two categories: cellulose nanocrystals (CNCs), also called cellulose whiskers, and cellulose nanofibrils (CNFs), also called microfibrillated cellulose.21 To the best of our knowledge, there are many approaches for the preparation of nanocellulose, which could be classified as chemical, mechanical, biological, and chemical− mechanical combination processes.22,23 CNCs displaying a rodlike morphology are commonly produced from acid hydrolysis in the presence of mineral acids, e.g., sulfuric acid, hydrochloric acid, and phosphoric acid,24−26 which require plenty of water and generate lots of waste acid.27 In addition, typically, this method suffers from a low CNCs yield of around 30%,27 and even if the yield could be optimized to 75%, it is still low for industrialization.28 On the other hand, CNFs with a linear structure and high aspect ratio are mainly produced by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) oxidation fol-

INTRODUCTION

With the advocacy of sustainable development, the attention and demand for products originating from sustainable and renewable resources are increasing rapidly. Nanobiomaterials (NBMs) have received widespread attention in recent years because of their attractive properties1−3 and are designed to achieve special functions in response to electrical, magnetic, optical, thermal, and ultrasonic signals.4,5 It has been wellbelieved that research on NBM-based biodegradable products will be a hot topic as the understanding of NBM safety and nontoxicity increases.6−10 Nanocellulose, derived from an abundantly natural polymer with renewability on Earth, is one of the most important NBMs11 because of its unique merits such as high modulus, large surface area, low thermal expansion coefficient, biodegradability, and environmental friendliness,12,13 which enables them to achieve various applications in polymer reinforcement, antimicrobial materials, shape-memory nanocomposites, biodegradable composites, etc.14−17 Moreover, abundant surface hydroxyl groups of nanocelluloses could be modified to endow them with new performance and special functionalities, thus expanding © XXXX American Chemical Society

Received: April 1, 2019 Revised: June 22, 2019 Published: June 24, 2019 A

DOI: 10.1021/acssuschemeng.9b01822 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering lowed by mechanical operation like fibrillation or ultrasonication, giving a CNFs yield of around 90%.29,30 However, TEMPO is expensive, toxic, and corrosive. Network bacterial cellulose with high crystallinity and high purity can be prepared with the assist of Acetobacter, but strict preparation conditions and high cost make it difficult to realize large-scale production.31 In addition to the above-mentioned common chemical methods, mechanical methods like high-pressure homogenization, microfluidization, ultrasonication, high-speed blending, grinding, and cryocrushing are also used for preparing nanocellulose, but they are limited by intensive energy consumption and uneven product particle size.24 Therefore, the effective combination of mechanical and chemical treatment has become a promising preparation method. Proper carboxylation of cellulose raw materials prior to mechanical treatment is an effective pretreatment for preparing CNFs.32 Wågberg et al. prepared CNFs by high-pressure homogenization of carboxymethylated cellulose followed by ultrasonication and proved that the carboxymethylated CNFs are a promising sensor material.19 Ström et al. prepared carboxymethylated CNFs and applied it as a food additive, proving its great potential for stabilizing emulsions and food foams.33 In addition, Eyholzer et al. obtained CNFs in a dry form with the help of carboxymethylation and mechanical disintegration and evaluated the quality of CNFs powder, indicating that carboxymethylation is an effective way to prevent hornification during the CNFs drying process.34 Integration of concentrated oxalic acid hydrolysis and subsequently mechanical fibrillation has been developed recently. Although the total yield of carboxylate nanocellulose (CNFs + CNCs) can be 95%, the yield of CNFs is only about 90%.35 In addition, due to the insufficient fiber predestruction, there is a risk of microfluidizer clogging raising a concern of production operability. This approach will continue to be of interest with improving the yield and reducing the cost on the basis of the diversity of carboxylation and mechanization hybrid methods that will bring endless possibilities. As our continuous work in nanocellulose and biomass utilization,36−39 herein, we optimized the method of producing low-substituted and water-insoluble carboxymethyl cellulose (CMC) from cellulose raw materials (cotton and wood), and the treated materials were mechanically fibrillated by microfluidization to obtain carboxymethyl cellulose nanofibrils (CMCNFs) with a novel treelike matrix and high yield (about 95%). Also, we attempted to use CMCNFs with the degree of substitution (DS) of 0.23 to stabilize sunflower-oilbased Pickering emulsions to test the quality of nanocellulose. Characterizations on the CMC and CMCNFs were conducted by fourier transformed infrared spectra (FTIR), DS test, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and thermogravimetric analyses (TGA), while the stability of Pickering emulsions was characterized by ζ potential and size distribution. The preparation method is simple and easy to operate, and the solvent can be recycled, which provides a favorable way for nanocellulose industrialization.

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Ltd. Isopropyl alcohol (IPA), sodium hydroxide (NaOH), chloroacetic acid (MCA), acetic acid (HAc), and ethanol were purchased from National Pharmaceutical Group Chemical Reagent Co., Ltd. (Beijing, China). CNFs (TEMPO oxidation) were purchased from Tianjin Woodelfbio Cellulose Co., Ltd. Sunflower oil (91.9 g of fat per 100 mL) was purchased from Beijing Carrefour Supermarket. All the reagents are analytical grade and used without further purification. Methods. Preparation of CMCNFs. Intermediate product CMC with DS about 0.05, 0.15, 0.25, and 0.35 were synthesized from refined cotton and wood pulp, respectively, according to the methods used in our lab.37 In a typical procedure, 680 mL of IPA, 25 wt % NaOH aqueous solution (33.76, 42.88, 48.00, and 51.80 g), and 40 g of refined cotton or wood pulp were added to a 1 L flask, and the mixture was stirred for 90 min with a speed of 400 rpm. The temperature of the alkali treatment was controlled at 20 ± 2 °C to prevent over alkalization. Thereafter, a 15 mL MCA/IPA mixture was slowly dropped into the mixture (cellulose raw material, NaOH, water, and IPA) with an amount of 9.45, 12.00, 13.42, and 14.50 g of MCA, respectively. The stirring speed was raised to 500 rpm and mixed for 10 min to react homogeneously between the etherifying agent and the cellulose raw material. Next, the temperature was maintained at about 50 °C for 80 min and at 75 °C for 30 min, respectively. The etherification process was carried out under a stirring speed of 400 rpm. After that, the mixture was cooled down and was adjusted to pH 7 by acetic acid. The product CMC and waste solution (IPA, salts, and impurities) were separated and collected after vacuum filtration. CMC was then washed by 75% ethanol (v/v) three times after stirring for 5 min to remove salt and impurities (e.g., the residual IPA). The filtrate was also collected. After that, IPA and ethanol were recovered through distillation. The recovery rate of IPA was about 80%, while the rate of ethanol could reach 90%. The as-prepared CMC was dispersed into deionized water giving a concentration of about 2.0% (w/w). Then, the dispersion was mechanically fibrillated using a microfluidizer (Nano DeBEE, BEE International, MA) at 30k psi three times to produce CMCNFs.40,41 A portion of each CMCNFs was freeze-dried for further test. The final product CMCNFs were obtained and stored at 4 °C. The comparison of samples derived from different cellulose raw materials is listed in Table 1.

Table 1. List of Product Codesa Cellulose raw materials

CMC

CMCNFs

refined cotton

CMC-C1 CMC-C2 CMC-C3 CMC-C4 CMC-W1 CMC-W2 CMC-W3 CMC-W4

CMCNFs-C1 CMCNFs-C2 CMCMFs-C3 CMCNFs-C4 CMCNFs-W1 CMCNFs-W2 CMCMFs-W3 CMCNFs-W4

wood pulp

a Note: C is short for refined cotton and W for wood pulp, and 1, 2, 3, and 4 represent DS of 0.05, 0.15, 0.25, and 0.35, respectively.

Preparation of Pickering Emulsions. Before preparation, CMCNFs and CNFs suspensions were sonicated using an ultrasonic cell pulverizer (JY 98-IIIN, Ningbo, China) for 5 min. Then, oil-inwater emulsions (40 g) were prepared with a rotor-stator homogenizer (IKA T25 Basic, Staufen, Germany) at 12 000 rpm for 3 min, which consisted of sunflower oil (10% w/w) and aqueous phase (90% w/w) containing 0.01% (w/w) CMCNFs or CNFs.42 Emulsions were then prepared by passing through a microfluidizer (Nano DeBEE, BEE International, MA) at 30k psi, and the temperature was controlled at 20 ± 10 °C, avoiding overheating. Characterization. Fourier Transform Infrared (FTIR) Spectroscopy Analysis. The aim of FTIR characterization was to confirm the existence of carboxymethyl groups. Cellulose raw materials, CMC,

EXPERIMENTAL SECTION

Materials. Refined cotton (M60, the degree of polymerization was 1200, and α-cellulose content is about 98%) and wood pulp with 90% α-cellulose after removing noncellulose components were provided by North Century Cellulose Technology Research & Development Co., B


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Figure 1. (a) FTIR spectra of cellulosic materials, CMC, and CMCNFs. “RC” represents raw material cotton. “RW” represents raw material wood. “C” is short for cotton. “W” is short for wood. (b) DS of CMC and CMCNFs made from cotton and wood pulp.

Figure 2. Schematic diagram of CMCNFs preparation. Morphology. Scanning electron microscopy (S-4800, Hitachi) was used to observe the morphology of cellulose raw materials and CMC. The samples were sputter-coated with gold to ensure sufficient conductivity. Morphologies of CMCNFs samples were observed by a TEM system (JEM 1200EX, JEOL) at an accelerating voltage of 120 kV.25 A drop of diluted CMCNFs suspension (∼0.01 wt % CMCNF in water) was deposited on a carbon-coated TEM grid. All the samples were dried before imaging and tested three times. X-ray Diffraction (XRD) Analysis. X-ray diffraction patterns of cellulose raw materials, CMC, and CMCNFs samples were tested using an X-ray diffractometer (Bruker D8 ADVANCE, Karlsruhe, Germany) in the 2θ range 10−50° with a step of 0.02°. Then, the crystallinity index (CrI) of samples was calculated according to the empirical equation below:

and CMCNFs were ground to powder and then were compressed into tablets with potassium bromide. Their spectra were then collected in the range 4500−500 cm−1 with a resolution of 4 cm−1 by an FTIR spectrometer (Nicolet IS10, Madison, WI). All the samples were dried before testing. DS of CMC and CMCNFs. The DS was tested according to the protocol for food additive-sodium carboxymethyl cellulose (GB 19042005). CMC and CMCNFs were added into an evaporating dish and then moved into a high-temperature furnace with 300 °C for 30 min. The temperature was raised to 700 ± 25 °C and held for 15 min. After that, the samples were cooled down to 200 °C, then deionized water (DI-water) and H2SO4 standard titration solution were added. The solutions were then slowly heated up to boiling for 10 min, and 2−3 drops of methyl red indicator solution was added. Then, the solutions were cooled down. The solutions were titrated with standard NaOH titration until the red color was faded. The determinations were performed in duplicate. The DS was calculated by eqs 1 and 2:

CB =

V1c1 − V2c 2 m

(1)

x DS =

0.162c B 1 − 0.080c B

(2)

CrI =

I200 − Iam × 100% I200

(3)

where I200 is the maximum peak intensity at (200) lattice diffraction, and Iam refers to the minimum diffraction intensity between planar reflections (110) and (200).43 Thermogravimetric Analysis (TGA). The TGA and thermogravimetric (DTG) analysis of the samples were measured using a thermal analysis instrument (NETZSCH STA 449 F3, Sable, Germany) in the temperature range 50−600 °C at a heating rate of 10 °C/min under nitrogen atmosphere.35 Characterization of Pickering Emulsions. The particle size distribution and ζ potential of emulsions were determined by laser diffraction using a Mastersizer 2000 (Malvern Instruments Ltd.,

where CB is the mole number of carboxymethyl groups per gram of sample (10−3 mol/g), V1 is the volume of standard H2SO4 standard titrant (mL), c1 = 0.1 mol/L, V2 is the volume of standard NaOH titration (mL), c2 = 0.1 mol/L, and m is the mass of sample (g). C


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Figure 3. SEM images of (a) cotton and wood, (b) CMC from cotton and wood, and (c) surface detailed of CMC from cotton and wood. “RC” represents raw material cotton. “RW” represents raw materials wood. “C” is short for cotton. “W” is short for wood. Malvern, UK), and the rotational velocity was 2100 rpm.42 Samples were subjected to ultrasound for 2 min before the test. All the tests were repeated three times.

As shown in Figure 2, the present protocol for CMCNFs preparation includes two steps: carboxymethylated modification of cellulose raw materials by alkalization and etherification, and subsequent microfluidization to generate the product. From Figure 3a, it can be observed that the raw materials exhibit relatively smooth surfaces, and their length is several hundred microns. Studies have shown that fibers with a large aspect ratio tend to be unevenly distributed in water, while those with a small aspect ratio tend to be homogeneously distributed.46,47 Fibers of refined cotton and wood pulp are entangled in water and cannot be dispersed due to the large aspect ratio, which leads to clogging of the microfluidizer. The SEM images of CMC powder from refined cotton and wood pulp after alkalization and etherification are displayed in Figure 3b. Some changes after the carboxymethylation of the raw materials, such as the length reduction and rough surfaces with some gaps and cracks, can be seen clearly from Figure 3c.48−50 The low-substituted CMC has a better hydrophilic property due to carboxymethyl groups, which can be welldispersed in water without dissolving, and provide a good condition for microfluidization because of an appropriate length and surface roughness. There is no obvious difference in the CMC prepared from refined cotton and wood pulp. CMCNFs were prepared from three cycles of microfluidization of CMC aqueous dispersion, and the morphology

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RESULTS AND DISCUSSION Two cellulose raw materials (refined cotton and wood pulp) are studied in this work. The FTIR spectra in Figure 1a include cellulose raw materials and typical CMC and CMCNFs prepared from cotton and wood; other samples are shown in Figure S1. In the IR spectrum, the broad band appearing at 3450 cm−1 is related to the stretching vibration of the O−H bond in the carboxyl group and hydroxyl groups.44 Affected by the six-membered ring tension in CMC and CMCNFs, bands at 2928 and 615 cm−1 could be related to the C−H stretching vibration and bending vibration in the plane.45 Two visual absorption bands at 1600 and 1415 cm−1 are related to the symmetric and asymmetric vibrations of COO− (carboxylate group), while these bands are not presented in cellulose raw materials’ spectra, proving the existence of the carboxymethyl group.44 In addition, the band at 1060 cm−1 represents the stretching vibration of C−O−C.45 The DS values of CMC and CMCNFs are shown in Figure 1b. A slight decrease of DS occurs in CMCNFs compared with that in the corresponding CMC due to partial shedding of the carboxymethyl groups during microfluidization, indicating that microfluidization has some effect on the DS but very little. D


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cellulose is collapsed to an extent, and the cellulose regularity is further decreased under the chemical effect, giving rise to a smaller size of nanocellulose with increasing the DS of CMCNFs. Along with the increase of DS from 0.05 to 0.35, it is worth noting that the diameter of CMCNF, as expected, was reduced from around 100 to 35 nm. The TEM images of CMCNFs with different DS prepared from wood pulp in Figure 4b shows the same morphology and similar size change trend with CMCNFs from refined cotton. In addition, the effect of microfluidization cycles on the morphology of CMCNFs was also investigated. Figure 4c shows the morphology of wood-based CMCNFs (with the DS of 0.25) after 4 and 6 cycles of microfluidization. Almost no change happened in size and morphology compared to the results in Figure 4b, suggesting that microfluidization will not affect the size and morphology of CMCNFs after three times. On the basis of the above results and previous studies,54−56 a possible mechanism of the treelike matrix formation is proposed. The improved hydrophilicity of CMC, benefiting from the carboxymethylation, facilitates the dispersion and swelling of samples in water after surface modification. Furthermore, the mechanical fibrillation also plays a critical role on exfoliation of CMC under the assistance of high shear and impact forces. As a result, the micron-sized fibers are cut into nanoscale fibers, while the linear fibers are exfoliated to form a treelike matrix. The typical treelike matrix is uncommon from other materials via microfluidization, while, in our work, the CMC surface possesses certain exfoliated sites caused by carboxymethylation (as shown in Figure 3) for tearing and peeling in the process of nanofibrillation, which finally exhibits the characteristic morphology. XRD patterns of cellulose raw materials, CMC, and CMCNFs with different DS are displayed in Figure 5 to imply the differences of microstructure. The pattern of refined cotton shows four peaks at around 14.4°, 16.7°, 22.5°, and 34.5°, corresponding to the (100), (010), (110), and (114) typically diffractive planes of cellulose Iα crystalline structure.57,58 In the cases of wood pulp, CMC, and CMCNFs, the etherification transfers their structures to the Iβ crystalline structure with three major peaks at 2θ about 16°, 22.5°, and 34.5°, respectively, which are assigned to the typical reflection planes (110), (200), and (004).35 XRD patterns of CMCNFs have almost no change in comparison with the CMC; thus, microfluidization does not change the crystalline structure. The crystallinity of all samples was calculated using the Segal method.59 As shown in Figure 5, the crystallinity of the refined cotton is 77.1% and 66.0%, respectively. After carboxymethylation, the crystallinity of the samples is decreased due to the partial destruction of the crystalline cellulose. The crystallinity of CMCNFs has the same tendency with CMC, while the crystallinity of CMCNFs is slightly higher than that of the CMC under the same DS. Moreover, a gentle crystallinity decrease occurs in the DS range 0−0.15. When it increases to about 0.25, the crystallinity of CMCNFs is sharply reduced from 59.5% to 56.8%, which suggests that the crystal region gradually starts to be collapsed to form an amorphous region with the DS increase under the impact of chemical modification, leading finally to an obvious crystallinity decrease. Thermal stability has a close relationship with the nanocellulose as a potentially modified agent in composite materials.60 The inherent characteristics and the molecular interactions for different macromolecules are the two factors

characterization was performed by TEM. As shown in Figure 4, cellulose nanofibrils with a treelike matrix could be observed,

Figure 4. TEM images of CMCNFs made from (a) refined cotton and (b) wood pulp with different DS. (c) TEM image of CMCNFs made from wood pulp by 4 (M-4) and 6 (M-6) cycles of microfluidization.

whose morphology is different from those in the previous reports.51−53 Figure 4a shows the CMCNFs obtained from refined cotton with the DS of 0.05, 0.15, 0.25, and 0.35, separately. Each sample has a central skeleton with the length of nanoscale to microscale, just like a trunk of a tree. Nanoscale filaments protrude from each skeleton, just like the branches of a tree. During the carboxymethylation, the crystalline region of E


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Figure 5. XRD patterns of cellulosic materials, CMC, and their CMCNFs. (a) XRD patterns of refined cotton and the corresponding CMC and CMCNFs. (b) XRD patterns of wood pulp and the corresponding CMC and CMCNFs. “RC” represents raw material cotton. “RW” represents raw materials wood. “C” is short for cotton. “W” is short for wood, and 1, 2, 3, and 4 represent DS of 0.05, 0.15, 0.25, and 0.35, respectively.

Figure 6. TGA and DTG curves of (a) refined cotton and CMC, (b) corresponding CMCNFs, (c) wood pulp and CMC, and (d) corresponding CMCNFs. “RC” represents raw material cotton. “RW” represents raw materials wood. “C” is short for cotton. “W” is short for wood, and 1, 2, 3, and 4 represent DS of 0.05, 0.15, 0.25, and 0.35, respectively.

determining the thermal stability of a polymer.61 Thermal stability of samples was tested by the thermogravimetric method, and Figure 6 shows the TGA and differential thermogravimetric (DTG) curves of the samples. The thermal decomposition of cellulose involves the depolymerization, dehydration, and decomposition of glycosyl units as well as the formation of a char residue.62 From Figure 6a,c, the onset temperature (T0) of thermodegradation and the maximal decomposition temperature (Tmax) for refined cotton are 282.3 and 355.1 °C, respectively, while the counterparts of wood pulp are 286.3 and 356.6 °C. The CMC and CMCNFs display a similar decomposition temperature, showing a decreased trend with the increase of the DS for T0 and Tmax. Higher crystallinity causes a higher thermal decomposition temperature, while higher surface carboxymethyl content leads to a lower thermal decomposition temperature.63 Although the thermal decomposition temper-

ature of CMCNFs is slightly lower than that of the cellulose raw materials, the thermal decomposition temperatures are all above 290 °C. The nanocellulose has a great potential in composite materials which require high thermal stability.64−66 Emulsions stabilized by solid particles in colloidal size are called Pickering emulsions. Pickering emulsions have better stability and weatherability than other emulsions, and thus, they have been widely applied in cosmetics, coating materials, fuel, and environmental protection.67,68 More prominently, natural or biomass-derived solid particles can be used in Pickering emulsions systems, which expands Pickering emulsions’ applications in pharmaceuticals, food, and sustainable chemical materials.69 Nanocellulose is regarded as a candidate stabilizer for Pickering emulsions because of its high aspect ratio, high crystallinity, structural stability, renewability, and biocompatibility.70 F


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Figure 7. Photos of (a) oil/CMCNFs mixture and (b) Pickering emulsions, and (c) oil/CNFs mixture and (d) emulsions. The table shows the ζ potential of oil/CMCNFs and oil/CNFs Pickering emulsions. (e) Particle size distribution of oil/CMCNFs and oil/CNFs. (f) Pickering emulsion stabilization mechanism.

Previous studies have proven that nanocellulose fibers (CNFs) prepared by the TEMPO method exhibit good stability against coalescence, and the interconnected system of nanocellulose offers a higher emulsion volume and stability at low concentration.71,72 However, TEMPO is expensive and corrosive. For exploring the application of the prepared CMCNFs as a stabilizer, further experiments were carried out as below. Two Pickering emulsions were prepared by mixing sunflower oil with CMCNFs and CNFs, respectively, and the results are depicted in Figure 7. It is necessary to note that the DS of CNFs made by TEMPO oxidation is 0.28. Herein, the DS of the prepared CMCNFs is around 0.23 in our work, indicating that the DS of as-prepared CMCNFs is similar to that of CNFs. By naked-eye observation, there are no differences between CNFs and CMCNFs samples (Figure 7a−d). Both CNFs and CMCNFs can keep the emulsions stable for over 6 days. Detailed analysis reveals that the magnitudes of the ζ potential of the two emulsions are all higher than 61 mV, which reveals that the prepared Pickering emulsions are stable.73 However, the magnitude of the ζ potential of oil/CMCNFs is larger than that of oil/CNFs, indicating that the electrostatic repulsion among droplets of oil/CMCNF is stronger. In fact, a slight difference in the DS does not result in a substantial change for the final results and weakens the effect of hydrophilic groups on emulsion stability at the same time, to illustrate the contribution of the morphology of CMCNFs to emulsions stability. Treelike

matrix CMCNFs can form a better three-dimensional network than CNFs, control the porosity of the interface, and improve the stability of the Pickering emulsions.67 On the basis of the above analysis, although the DS of CMCNFs in our work is slightly lower than that of CNFs modified by TEMPO, it seems that the factor has almost no influence on the behavior of the Pickering emulsions. More importantly, the typical treelike morphology for CMCNFs plays a positive role in maintaining the stability of the Pickering emulsions compared with CNFs, illustrating the significance of a three-dimensional structure as a result from the characteristic morphology. The particle size distribution is shown in Figure 7e as well. According to the literature,42 emulsions stabilized by cellulose particles performed with a bimodal or multimodal behavior. In the present work, bimodal distribution can be observed in two samples. Both of them have similar D[3,2] values and are 1.527 (oil/CMCNFs) and 1.567 (oil/CNFs) μm, respectively. Compared with the oil/CNFs system, the D[4,3] value and span are obviously decreased in the oil/CMCNFs system, exhibiting a narrower distribution and higher stability than oil/ CNFs.74 As CMCNFs are low-cost and easier to prepare than CNFs, they are capable of being utilized in the sustainability industry.

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CONCLUSION Herein, we demonstrated an environmentally benign and lowcost method based on appropriate alkalization, etherification, and mechanical fibrillation to prepare CMCNFs with a high G


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yield (about 95%) and treelike matrix. It is also found that as the DS increases from 0.05 to 0.35, the diameter of the CMCNFs is reduced from about 100 nm to below 35 nm. From the results of XRD and TGA, the detailed information about high crystallinity and thermal stability of CMCNFs is presented. Further study demonstrates that although the DS of as-prepared CMCNFs is lower than that of CNFs treated with TEMPO, the CMCNFs with a typical treelike matrix could serve as an excellent stabilizer for sunflower-oil-based Pickering emulsions, which is attributed to the confinement effect of the three-dimensional network on the droplets in emulsion. This work provides a new opportunity and application of treelike matrix nanocellulosic materials in sustainable chemistry.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01822.

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FTIR spectra (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jie Wei: 0000-0002-7226-6970 Yi Zhou: 0000-0002-7401-9156 Yanyan Lv: 0000-0001-7251-4658 Jianquan Wang: 0000-0002-2200-0031 Chao Jia: 0000-0001-7263-2080 Jianxin Liu: 0000-0001-5138-0355 Xinfang Zhang: 0000-0002-8042-5249 Jian Sun: 0000-0001-5723-6522 Ziqiang Shao: 0000-0002-8461-4979 Funding

This work was financially supported by Natural Science Foundation of Beijing Municipality (2192050). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The characterization results were supported by Beijing Zhongkebaice Technology Service Co., Ltd. We also appreciate the help of Beijing physical and chemical analysis and testing Center.

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REFERENCES

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Carboxymethyl Cellulose Nanofibrils with a Treelike Matrix

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