Heterogeneously Modified Cellulose Nanocrystals-Stabilized

Research Article pubs.acs.org/journal/ascecg

Heterogeneously Modified Cellulose Nanocrystals-Stabilized Pickering Emulsion: Preparation and Their Template Application for the Creation of PS Microspheres with Amino-Rich Surfaces Wenbo Du,†,‡ Juan Guo,*,§,∥ Huaming Li,†,‡ and Yong Gao*,†,‡ †

College of Chemistry, Xiangtan University, Xiangtan, Hunan Province 411105, China Key Laboratory of Polymeric Materials & Application Technology of Hunan Province, Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province, Xiangtan, Hunan Province 411105, China § Research Institute for Forestry New Technology, Chinese Academy of Forestry, Beijing 100091, China ∥ Department of Wood Anatomy and Utilization, Research Institute of Wood Industry, Chinese Academy of Forestry, Beijing 100091, China ‡

S Supporting Information *

ABSTRACT: A kind of manipulative oil in water Pickering emulsion stabilized by heterogeneously modified cellulose nanocrystals (CNCs) and its application as a template for the creation of functional polystyrene microspheres were presented in this study. First, 18-carbon alkyl long chains were selectively introduced at the reducing ends of CNCs through a two-step chemical process including the hydrazone reaction and the amidation reaction. The as-obtained heterogeneously modified CNCs (CNC−C18) were then used as emulsifiers for the formation of oil in water Pickering emulsions. Compared with the pristine CNCs used in this study, CNC−C18 exhibited a high emulsifying performance. Highly stable oil in water Pickering emulsions could be formed at a low content of CNC−C18 nanoparticles. Because of the pH-responsive stability of the CN bond linkages between 18-carbon alkyl chains and CNCs, the introduced 18-carbon alkyl chains could be cleaved completely at a suitable acid condition, and this endowed the formed Pickering emulsions with a pH-triggered de-emulsification character. Upon the replacement of n-hexane with a styrene/divinyl benezene mixed oil, cross-linked PS microspheres with amino-rich surfaces were created by the Pickering emulsion directed-radical copolymerization reaction, followed by acid treatment. The current work revealed CNCs could be used as a promising green and widely available resource for the fabrication of stimuli-responsive particle emulsifiers. KEYWORDS: ellulose nanocrystals (CNCs), Pickering emulsion, Reversible covalent bond, Template

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INTRODUCTION Pickering emulsions are emulsions stabilized by solid particles in place of surfactants. Compared with low molecular weight surfactant-stabilized emulsions, solid particles-stabilized Pickering emulsions have many attractive advantages, such as low toxicity, low emulsifier content, and adjustable droplet size.1,2 The coalescence among the dispersed droplets is largely suppressed due to the strong adsorption of the emulsifier particles at the oil−water interface. As a result, a Pickering emulsion is highly stable in thermodynamics. On the other hand, highly stabilized Pickering emulsions featuring a responsive de-emulsification are more desirable in some cases where emulsion stability is only needed temporarily, like oil recovery and interfacial reaction.3−6 Employing stimuli© 2017 American Chemical Society

responsive particles as emulsifiers is a prevailing method for the formation of responsive Pickering emulsions. Due to adjustable surface wettabilities in response to external stimuli, such as pH, temperature, ionic species, etc., the stimuliresponsive particle emulsifiers can be easily detached to the water phase or the oil phase from the oil−water interface without any extra energy input. A broad variety of responsive particles acting as emulsifiers for the formation of responsive Pickering emulsions have been explored over the past decades.7−19 Received: February 6, 2017 Revised: June 29, 2017 Published: July 31, 2017 7514

DOI: 10.1021/acssuschemeng.7b00375 ACS Sustainable Chem. Eng. 2017, 5, 7514−7523

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ACS Sustainable Chemistry & Engineering

After filtration, the solid sample was redispersed in distilled water and then dialyzed against pure water to a constant pH of 7. Preparation of 18-Carbon Alkyl Chains-Functionalized CNCs (CNC−C18). Preparation of Amino-Functionalized CNC (CNC−NH2). Here, 300 mL of calcium acetate solution (0.02 mol/L) was added to 35 mL of CNC aqueous dispersion (17.9 mg/mL). After 16 h of stirring at room temperature, CNCs solid samples were obtained by centrifugation. The solid samples were redispersed in 100 mL of borate buffer with the assistance of ultrasonification. Subsequently, HH (3.12 g, 49.9 mmol) was added. The flask was transferred to a water bath at 35 °C. After 72 h of stirring, the mixture was transferred to a dialysis bag and dialyzed against calcium acetate solution (0.02 mol/L). The solid CNC−NH 2 samples were collected by centrifugation, which were further washed with calcium acetate solution and THF sequentially. Preparation of 18-Carbon Alkyl Chains-Functionalized CNCs (CNC−C18). In a typical reaction, 306 mg of stearic acid (SA, 1.08 mmol) was first charged into a round-bottomed flask, and then, 90 mL of fresh THF was added. After complete dissolution of SA, EDC (1.40 g, 7.3 mmol) and NHS (0.39 g, 3.4 mmol) were added, followed by 4 h of stirring at 0 °C. After this, 30 mL of the CNC−NH2 suspension in THF was dropped into the flask over 30 min, followed by the addition of 2 mL of deionized water. The mixture was stirred at 0 °C for 1 h and then for 40 h at room temperature. After the reaction, the solid product was separated by centrifugation. The collected solid sample was washed with THF to remove the nonbound SA. The purified product was dried under vacuum for 24 h at room temperature. Generation of n-Hexane in Water Pickering Emulsion. A series of n-hexane in water emulsions with a fixed oil/water volume ratio (1:1) and various CNC−C18 contents ranging from 0.3 to 0.05 wt % were prepared. For the formation of n-hexane in a water Pickering emulsion, CNC−C18 was first dispersed in water at different contents with the assistance of ultrasonification; n-hexane was then added into the CNC−C 18 aqueous dispersion, followed by homogenization for 1 min at a rate of 10,000 rpm. Solidifying Emulsion Droplets. Here, 185 μL of St and DVB (17.5/1, volume ratio) containing 1 mol % of AIBN was utilized as the oil phase to prepare emulsions, and the content of CNC−C18 emulsifiers was 0.25 wt % versus the total weight of the oil and the water. After homogenization, 2 mL of water was added to dilute the already formed Pickering emulsion. The emulsion was degassed with N2 for 40 min and then transferred into an oil bath at 65 °C. The oil phase droplets were solidified by the radical copolymerization of St with DVB at 65 °C without stirring for 24 h. The solidified emulsion droplets were separated by filtration and then washed with ethyl alcohol. The samples were dried under vacuum at room temperature. Characterization and Test. Characterization of CNCs and Modified CNCs. The morphologies of CNCs were observed with TEM (HT7700, Hitachi High Technologies, Japan) at an acceleration voltage of 200 kV and a scanning probe microscope (Multimode Nanoscope III controllor, Veeco Company, USA). CNCs for TEM observations were prepared by depositing a drop of (10 μL) the aqueous suspensions onto a carbon-coated copper grid and then stained with 3 wt % uranyl acetate aqueous solution. The average length and width of the CNCs were analyzed with Image software (National Institutes of Health, USA), and 100 CNCs were randomly selected for measurements. The height of CNCs was analyzed with Nanoscope Analysis software (Bruker Company, German). FT-IR spectra were recorded on a Perkin−Elmer Spectrum One FT-IR spectrometer by the KBr pellet method at a resolution of 2 cm−1 in the range of 4000−400 cm−1. Thermogravimetric analysis (TGA) was performed on a TA SDT Q600 instrument under a nitrogen atmosphere at a heating rate of 10 °C/min in the range from 30 to 550 °C. Elemental analysis (EA) was conducted with a Perkin−Elmer CHN 2400 analyzer. Crystalline structures of CNCs and CNC−C18 were studied on a diffractometer (D8 advance XRD, Bruker Company, Germany) using Cu Kα radiation (λ = 0.154 nm, 40 kV, and 40 mV) from 2θ = 5−40° at a scan rate of 4°/min. The average surface charge density of CNCs was calculated according to the reference reported elsewhere.26 Interfacial tension measurements were performed by the

Cellulose nanocrystals (CNCs) are highly crystalline nanorods with widths varying from 5 to 20 nm and lengths from 100 nm to 1−2 μm.20 The dimensions of CNCs are mainly dependent on preparation conditions and cellulose resources. Due to their availability, renewability, and biodegradability, CNCs have been widely used as building blocks for the design of hierarchical functional nanomaterials.21 Recently, CNCs have also been intensively studied as emulsifiers for Pickering emulsions owing to the rod-like shape and the high aspect ratio. CNCs originated from the strong acid hydrolysis of the biomass are highly hydrophilic because of the existing negative charges and hydroxyl groups,22 and appropriate hydrophobic modification was necessary for their application as emulsifiers.22−35 Recent studies showed that pristine CNCs (normally bacterial CNCs) could be directly applied as emulsifiers for the formation of Pickering emulsions.22−27 The hydroxyl groups on the surface provide versatile strategies for the functionalization of CNCs. Hydrophobic alkyl chains and polymer chains could be easily introduced on the surface of CNCs through hydroxyl groups-derived reactions.28−35 However, such a modification strategy usually leads to the hydrophobization of the whole surface rather than local hydrophobization in a limited region of the CNCs. Local hydrophobization might be much more intriguing for the application of CNCs as emulsifiers. Herein, the synthesis of the heterogeneously modified CNCs and their application as emulsifiers for Pickering emulsions were presented in this study. In order to create local hydrophobization on the surface of CNCs, 18-C alkyl chains were selectively introduced at the reducing ends of CNCs via the reversible CN covalent linkages. Because of the pHdependent stability of the CN covalent bond, the introduced 18-C alkyl chains at the reducing ends of CNCs could be cleaved completely under a suitable acidic condition. Using the resulting heterogeneously functionalized CNCs containing C N linkages as the particle emulsifiers, n-hexane in water Pickering emulsions featuring pH-triggered de-emulsification were generated. To the best of our knowledge, this is the first report on the fabrication of Pickering emulsions with responsive de-emulsification using heterogeneously modified CNCs as emulsifiers. In addition, PS microspheres with functional amino group-rich surfaces could be created employing radical copolymerization of styrene with divinylbenzene directed by this manipulative Pickering emulsion, followed by acid treatment.

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EXPERIMENTAL SECTION

Materials. Cellulose filters (softwood sulfite pulp) were provided by the Xinhua Paper Mill (Hangzhou, China); N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC) and N-hydroxysuccinimide (NHS) were bought from Aladdin Industrial Corporation (China). Styrene (St) and divinylbenzene (DVB) were purchased from Aladdin Industrial Corporation (China), which were first passed by the basic Al2O3 column and then distilled under reduced pressure prior to usage. Calcium acetate, boric acid, sodium tetraborate, and hydrazinium hydroxide (HH) were all obtained from commercial suppliers, which were used without further purification. Preparation of Cellulose Nanocrystals (CNCs). CNCs were prepared according to the procedures described elsewhere.36 Here, 15 g of cellulose was immersed in 150 mL of sulfuric acid solution (64 wt %) in a round-bottomed flask. The hydrolysis reaction was carried out at 45 °C for 90 min in an ultrasonic cleaner at a frequency of 40 kHz (DL-480B, Shanghai Zhixin Instrument Co. Ltd., China). The hydrolysis process was terminated by addition of deionized water. 7515


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ACS Sustainable Chemistry & Engineering Scheme 1. Synthesis Route for CNC−C18

Figure 1. Photographs of (A) the pristine CNCs and (B) CNC−C18 and (C) acid-treated CNC−C18 aqueous dispersion. (D) TEM image and (E) AFM image as well as (F) the height profile of the cross section of the pristine CNCs. sample was measured three times, and the average value was calculated. Characterization of Pickering Emulsions. Optical micrographs were collected with an optical microscope (Leica, DM 4500P). The average diameter of the droplets was analyzed statistically, and more than 50 droplets were randomly selected for each measurement. The emulsion was prepared by homogenization with a high speed dispersing instrument (XHF-D) at a speed of 10,000 rpm for 1 min. Scanning electron microscope (SEM) images of PS microspheres were visualized with JSM-7600F and JSM-6610LV instruments at an acceleration voltage of 30 kV.

Wilhelmy plate method, and the experiments were conducted with a Kruss tensiometer K20 equipped with a Wilhelmy slide. Water contact angle measurements of the pristine CNCs and CNC−C18 were carried out with distilled water using a contact angle meter (JC2000D, Shanghai Zhongchen Powerach Company, China) at room temperature. The films with smooth surfaces were obtained by compressing the powder under 20 bar of pressure using an IR press. The conductometric titration and potentiometric titration were performed on T50 (Mettler Toledo, Switzerland). The reduction amination of CNC−NH2 samples were carried out according to the procedure reported elsewhere using sodium triacetoxyborohydride as the reduction agent.37 The reduced samples were dispersed in distilled water at a concentration of 1.19 mg/mL. NaOH aqueous solution with a concentration of 10 mM was adopted to adjust pH of the dispersion to around 7. Then, HCl aqueous solution (1 mol/L) was added to the dispersion dropwise under stirring at room temperature. The drop volume was fixed at 1 μL during measurements by T50. The zeta potential measurement was performed on a MALVERN Zetasizer Nano ZS instrument (Malvern Instruments Ltd., Malvern, UK). Each

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RESULTS AND DISCUSSION Preparation of CNC−C18 with Cleavable 18-C Alkyl Chains at Reducing Ends. During biosynthesis and deposition, the unidirectional parallel orientation of cellulose chains within the elementary fibrils induces the formation of crystals having the nonreducing end with pendant hydroxyl groups and the reducing end bearing hemiacetal function7516


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Table 1. EA and Zeta Potential Results for Pristine CNCs, CNC−NH2, CNC−C18, and Acid-Treated CNC−C18 Content of element (wt %) Sample

C

H

O

N

S

Zeta potential (mV)

CNCs CNC−NH2 CNC−C18 Acid-treated CNC−C18

42.35 41.83 42.96 41.68

5.57 5.91 6.21 5.63

51.44 50.26 49.14 52.42

0 1.46 1.33 0

0.64 0.54 0.36 0.27

−19.8 −18.3 −0.9 −11.0

alities.38 The reactive aldehydes at the reducing ends have been used a tool for the fabrication of the heterogeneously functionalized CNCs.37,39−41 In the present work, we followed this strategy to synthesize the heterogeneously modified CNCs. Briefly, amino groups were first introduced at the reducing ends of CNCs via the hydrazone reaction between HH and the aldehyde groups at the reducing ends of CNCs. The introduced amino groups were then reacted with SA in the presence of an EDC/NHS combination, leading to the formation of the 18-C alkyl chains-functionalized CNCs (CNC−C18). The overall reaction route is illustrated in Scheme 1. The reducing terminals of the CNCs exist in an equilibrium between their ring form and open form.42 Acid is helpful for the shift of the equilibrium in the direction of the open ring.43 Considering the pH-dependent stability of the CN bond, the hydrazone reaction was conducted in borate buffer (pH 9). CNCs were pretreated by Ca2+ to improve resistance to the peeling-off reaction occurring under the basic condition.44 Figure 1A is the aqueous dispersion of CNCs. According to the zeta potential measurement, the surface potential of CNCs was about −20 mV, and the electrostatic repulsions made the pristine CNCs nanoparticles stably dispersed in water. TEM and AFM were utilized to characterize the morphologies of CNCs nanoparticles. A typical TEM image of CNCs is demonstrated in Figure 1D. CNCs presented a needle-like structure with an average length of 84 ± 23 nm and average width of 4.9 ± 0.6 nm, and the thickness of CNCs was about 5.4 ± 0.9 nm (Figure 1E and F). After the introduction of 18-C alkyl chains, the dispersity of the nanoparticles was remarkably decreased. In contrast to the transparent and stable aqueous dispersion of pristine CNCs, CNC−C18 dispersion in water was opaque (Figure 1B), suggesting the formation of the aggregation with a larger size. The aggregated CNC−C18 was precipitated in water after several hours standing. This poor dispersity of CNC−C18 in water was the result of the increased hydrophobic forces that were originated from both the reduction of the electrostatic repulsions and the introduction of 18-C alkyl chains at the reduction end. Zeta potential analysis showed that CNC−C18 nanoparticles were almost electrically neutral; however, this was not consistent with only about 40% of S loss in comparison with pristine CNCs (Table 1). The misleading zeta potential value might be due to the change in the effective mass average particle size caused by the aggregation of CNC−C18 nanoparticles in water.45 The crystalline structures of pristine CNCs and CNC−C18 samples were studied by X-ray diffraction. The XRD pattern of pristine CNCs is illustrated in Figure 2. Five groups of diffraction peaks at 2θ around 15.1°, 16.6°, 20.8°, 23.0°, and 34.8°, corresponding to (1−10), (110), (102), (200), and (004), respectively, appeared in the XRD pattern of pristine CNCs.46 These peaks did not display any shift after the introduction of long alkyl chains, indicating intact lattice constants of CNCs during chemical reactions.

Figure 2. XRD patterns of pristine CNCs and CNC−C18.

FT-IR characterization also supported the successful incorporation of 18-C alkyl chains. Figure 3 shows FT-IR

Figure 3. FT-IR spectra of (A) pristine CNCs, (B) CNC−C18, and (C) acid-treated CNC−C18.

spectra of pristine CNCs and CNC−C18. In the FT-IR spectrum of pristine CNCs (Figure 3A), a broad absorption peak at 3369 cm−1 was assigned to the stretching of −OH groups. The C−H stretching vibration peak was detected at 2910 cm−1. The absorption peaks at 1061 and 1030 cm−1 were allocated to C−O−C stretching.47,48 After the incorporation of 18-C alkyl chains, a typical absorption peak at 1560 cm−1, corresponding to the stretching vibration of CN, emerged in the FT-IR spectrum of CNC−C18. An obvious absorption peak at 1730 cm−1, corresponding to the characteristic stretching 7517


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the reducing ends with the static contact angle measurement. However, unlike other hydrophobically modified CNCs,29,53 the equilibrium contact angle of CNC−C18 could not be gained. It was found that the water was spread rapidly (within 3 s) on the surface of the compassed CNC−C18 sample film or the pristine CNCs (Supporting Information, S3). This phenomenon might be ascribed to the low content of the 18C alkyl chains. Moreover, these 18-C alkyl chains were only grafted at the reducing ends of CNCs rather than the entire surface. Relative to the large hydrophilic surface of CNCs, the hydrophobic domain formed by the introduced 18-C alkyl chains was rather small. As a result, the water contact angle was dominated by the axial surface of CNCs. Thermogravimetric Analysis (TGA). The thermal stabilities of CNCs, CNC−C18, and acid-treated CNC−C18 samples were investigated by TGA, and the corresponding TGA curves are displayed in Figure 4. As illustrated in Figure 4A, about 38

vibration of CO, was also observed in the FT-IR spectrum of CNC−C18 (Figure 3B). It is well known that the stability of the CN covalent bond is dependent on pH. It is stable in the alkaline solution and would be destroyed at a higher acidity.49 This means that the introduced 18-C alkyl chains could be cleaved by adjusting the pH to a suitable acidity. In order to confirm this, a trace of HCl solution (10 μL, 1 mol/L) was added into 2 mL of the CNC−C18 aqueous dispersion, followed by gentle stirring for 30 min at room temperature. Figure 3C indicates the FT-IR spectrum of the acid-treated sample. As expected, absorption peaks at 1560 and 1730 cm−1 shown in the FT-IR spectrum of CNC−C18 disappeared completely in Figure 3C. TEM observation of Au NPs-tagged CNCs provided direct proof for the heterogeneous distribution of the introduced 18-C alkyl chains on the surface of CNCs. For this purpose, thiol groups-functionalized CNCs (CNC− SH) were first synthesized (the detailed preparation processes for CNC−SH and Au NPs-tagged CNCs are shown in the Supporting Information, S1). As displayed in Figure S1, CNCs with Au NPs bound to only one end were clearly observed because of the strong interaction of Au NPs with thiol groups, whereas no Au NPs were bound to one end of the CNCs in the aqueous dispersion of the pristine CNCs. It was difficult to accurately determine the amount of the 18C alkyl chains introduced at the reducing end of each CNC. In this study, the degree of functionalization (DF), defined as the average mmole number of the 18-C alkyl chains per gram of samples, was utilized to roughly estimate the amounts of the 18-C alkyl chains at the reducing ends of CNCs. DF was estimated according to the content of N element from EA analysis. Table 1 displayed the EA results of pristine CNCs, CNC− NH2, and CNC−C18 as well as the acid-treated CNC−C18 samples. The pristine CNCs illustrated the carbon content, hydrogen content, and sulfur content as 42.35%, 5.57%, and 0.64%, respectively. The detected sulfur element verified the existence of sulfate half-ester groups on the surface of CNCs.50 In the case of CNC−NH2, the N element was detected as expected. Here, 1.46% of the N element content corresponded to about 0.52 mmol −NH2 per gram CNC−NH2, which was almost consistent with the content of −NH2 groups of 0.47 mmol/g samples obtained from the potentiometric titration analysis (Supporting Information, S2). For the CNC−C18 sample, the detected content of the N element was 1.33 wt %, corresponding to about 0.36 mmol of −NNHCO(CH2)16CH3 at the reducing ends of per gram of CNC−C18 sample. Therefore, about 69% of the introduced −NH2 was converted to −NNHCO(CH2)16CH3 during the amidation reaction (Supporting Information, S2). For the acid-treated CNC−C18 sample, the undetectable N element strongly backed up the complete breakage of CN. The results of EA were in accordance with those of the FT-IR analysis. The surface average potential of the acid-treated CNC−C18 sample was about −11.0 mV, and the value was less than that of pristine CNCs. From pristine CNCs to the acid-treated CNC−C18 sample, the decreased sulfur content was ascribed to the removal of the sulfate groups from the surface of CNCs during the chemical reaction,51,52 which was also in accordance with the decreased tendency of the zeta potential (Table 1). The dispersity of the acid-treated CNC−C18 sample in water was better than that of the CNC−C18 sample as a result of the electrostatic repulsion effect (Figure 1C). We also attempted to indicate the successful hydrophobicity modification of CNCs at

Figure 4. TGA curves of the pristine CNCs (A), CNC−C18 (B), and acid-treated CNC−C18 (C).

wt % mass loss at temperatures ranging from ∼180 to ∼325 °C was attributed to the dehydration of some glucose units in the main chain of CNCs and the breakage of the molecular backbone as well as the breakage of other C−O and C−C bonds.54 The stage with about 22 wt % mass loss at temperatures ranging from ∼325 to ∼420 °C was the result of the formation of carbon. The final residue was about 31 wt %. In the case of CNC−C18, TGA curve exhibited a two-stage degradation process. It could be observed that about 80% of the polymer was decomposed at temperatures ranging from ∼240 to ∼360 °C (Figure 4B). Compared with the relative residue of the pristine CNCs of about 31 wt %, the final remaining was about 16%. For the acid-treated CNC−C18 sample, the initial decomposition temperature and the final residue were ∼200 and ∼23 wt %, respectively, as shown in Figure 4C. Relative to the pristine CNCs, the modified samples indicated an improved thermal stability. This might be ascribed to the different content of sulfate groups on the surface of the samples. A part of the sulfate groups have been removed during the chemical reaction, as evidenced by the decreased sulfur content from EA analysis. The decreased sulfate groups caused an improvement in the thermal degradation stability.52 n-Hexane in Water Pickering Emulsions Stabilized by CNC−C18. Recently, Pickering emulsions stabilized by pristine 7518


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Figure 5. Photographs of n-hexane/water biphase after homogenization in the presence of various contents of pristine CNCs (A: 0.05 wt %; B: 0.5 wt %) and acid-treated CNCs (C: 0.2 wt %). Photographs (insets) and optical micrographs of n-hexane in water Pickering emulsion droplets stabilized by CNC−C18 with various contents versus the total mass of the oil and water for emulsion formation: (D) 0.05 wt %, (E) 0.15 wt %, (F) 0.2 wt %, (G) 0.25 wt %, and (H) 0.3 wt %. Photographs were taken after 24 h of standing at room temperature. Scale bar: 50 μm.

Figure 6. (A) Photograph of n-hexane in water Pickering emulsion stabilized by 0.15 wt % of CNC−C18. (B) Photograph of broken Pickering emulsion. Photographs were taken after 24 h of standing at room temperature.

water volume ratio (1:1) could be formed at various CNC−C18 contents ranging from 0.05 to 0.3 wt %. The photographs of the formed Pickering emulsions are displayed in Figure 5D−H (insets). The thicknesses of the emulsion layers stabilized by different CNC−C18 contents were almost the same, whereas the emulsion viscosity increased with the increase in CNC−C18 content. Optical micrographs of Pickering emulsion droplets stabilized by different CNC−C18 contents are also shown in Figure 5. The average size of the droplets gradually decreased with the increase in CNC−C18 content. For example, the emulsion droplets stabilized by CNC−C18 at the content of 0.05 wt % were so large that they could be observed by naked eyes. The average diameter of the droplets was about 206 ± 44 μm (Figure 5D), whereas the average diameter of the droplets decreased to 11 ± 2 μm (Figure 5H) when the content of CNC−C18 was up to 0.3 wt %. These results backed up the conclusion that a higher concentration of CNC−C18 favored the formation of Pickering emulsions with a smaller average droplet size. The more emulsifier particles that were available at the oil−water interface, the more surface area of the oil droplets could be stabilized. As a result, the emulsion droplets gradually decreased to meet the increase in the total surface area. All Pickering emulsions exhibited a high stability. The average droplet size remained approximately constant, and no obvious oil was separated from the emulsion at room temperature within a time period of more than 4 months (Supporting Information, S4). The good emulsifying performance of CNC− C18 nanoparticles was due to their capabilities of being simultaneously wetted by both the oil and the water, leading to a strong adsorption of CNC−C18 particles at the oil−water interface. This opinion was also supported by the results of the

CNCs have been intensively investigated by Capron and her co-workers.22−24,26,55 According to their findings, the emulsifying performances of CNCs were related to several important parameters including the source, allomorph, morphology, and surface charge density as well as the surface chemistry.26 Among these factors, surface charge density was the most crucial factor. A stable emulsion could not be produced if the surface charge density of CNCs was above 0.03 charge/nm2, regardless of the source (cotton or bacterial cellulose).26 Pristine bacterial cellulose nanocrystals (BCNs) obtained from hydrolysis with hydrochloric acid are recognized to be a kind of excellent emulsifier for Pickering emulsions as a result of their large aspect ratios and flat, ribbon-like cross sections as well as a lower charge density.55 For CNCs used in the present study, it was found that Pickering emulsion could not be formed at all when they were used as emulsifiers to stabilize the n-hexane oil at a lower content, such as 0.05 wt % (Figure 5A). Only a very thin emulsion layer turned up between the oil and water phase even at a higher CNCs content of 0.5 wt %, as shown in Figure 5B, indicating a poor emulsifying performance of pristine CNCs. This poor emulsifying performance might be attributed to their higher surface charge density. The charge density of the present CNCs was 0.03 ± 0.01 e/nm2. High negative charge density kept the CNCs dispersed in the continuous water phase but adsorption at the liquid−liquid interface. Similar results were observed for the acid-treated CNC−C18 sample. Pickering emulsions could not be generated for the acid-treated CNCs sample at the content of 0.2 wt % (Figure 5C). In contrast with pristine CNCs and acid-treated CNC−C18 samples, highly stable n-hexane in water Pickering emulsions with a fixed oil/ 7519


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Figure 7. (A) Optical micrograph of St/DVB in water Pickering emulsion droplets stabilized by 0.25 wt % of CNC−C18. (B, C, and D) FSEM images of PS microspheres at different magnifications. (E and F) FSEM images of acid-treated PS microspheres. Scale bars: 25 μm for A, 50 μm for B, 3 μm for C and E, and 500 nm for D and F.

Solidifying Emulsion Droplets and Cross-Linked PS Spheres with Amino Groups-Rich Surfaces. Solidifying the droplets is a direct method to visualize the emulsifier particles adsorbed at the oil−water interface. For this purpose, the nhexane oil was replaced by styrene/DVB containing the initiator of AIBN. The content of CNC−C18 was 0.25%. After homogenization, the optical micrograph of the generated Pickering emulsion droplets was as indicated in Figure 7A. The average size of the emulsion droplets was ∼18 μm. Cross-linked PS microspheres were obtained after the traditional radical polymerization reaction directed by the Pickering emulsion template. The morphologies of the obtained PS microspheres were observed by FSEM. Figure 7B−D displays FSEM images of PS spheres. As indicated in Figure 7B, nearly homogeneous PS microspheres were produced. The average size of PS microspheres was ∼16 μm. With the consideration of the volume shrinkage during conversion of the monomer to polymer, the average size of the PS microspheres was almost the same as that of the droplets of a Pickering emulsion shown in Figure 7A, revealing the high stability of St/DVB in a water Pickering emulsion, and coalescence between the droplets did not occur during the polymerization reaction. Theoretically, St/ DVB oil droplets could be completely covered by the added CNC−C18 nanoparticles (calculated coverage was 1000%),55 which agreed with the SEM observation (Figure 7C). At a higher magnification, CNCs covered on the surface of the PS microsphere were observed to be curved along the surface of the microsphere and overlapped each other and formed an armored layer, and no needle-like spikes were shown, as illustrated in Figure 7D. This result was also in favor of our conclusion on the horizontal orientation of CNC−C18 at the nhexane−water interface of the Pickering emulsion. The produced PS microspheres were further treated with HCl. For the acid-treated PS microspheres, SEM observation indicated that CNCs previously covered on the surfaces of PS microspheres disappeared entirely, and many dents were left on the smooth surface of the PS microspheres, as shown in Figure 7E and F. SEM observation also supported the breakage of CN in the acidic surrounding. For St/DVB in water Pickering emulsion stabilized by CNC−C18, the introduced hydrophobic 18-C alkyl chains at the reducing ends of CNCs were located in the St/DVB oil and

interface tension measurements. The interface tension of the pure water/n-hexane was determined to be 55.5 mN/m, which was a little higher than the referred value.56,57 For n-hexane/ water containing 0.1 wt % of CNCs, the interface tension was 53.1 mN/m. Relative to the pure water/n-hexane interface, such a minor reduction in interface intension suggested weak surface activities of pristine CNCs. However, the interface tension of n-hexane/water containing 0.1 wt % of CNC−C18 was found to be 39.5 mN/m. The significant decreasing in the interface tension revealed that CNC−C18 possessed a higher affinity for the oil−water interface owing to their amphiphilic characteristics. At the oil−water interfaces, the hydrophilic portion of CNCs and the long alkyl chains were located in the water phase and the oil phase, respectively. The hydrophobic region of CNCs, like the crystalline plane, was located at the oil−water interface. CNC−C18 was horizontally orientated at the oil−water interface, as illustrated in Figure 6. According to the above discussions, the introduced 18-C alkyl chains at the reducing ends could be detached from CNCs at the acidic surrounding. Therefore, pH would have profound effects on the stability of Pickering emulsions. In order to prove this, a trace of HCl aqueous solution (1 mol/L) was added to a Pickering emulsion that was stabilized by 0.15 wt % of CNC− C18 to adjust pH from 7 to about 4. After gentle stirring, the Pickering emulsion displayed a complete de-emulsification within 1 min (Figure 6). Similar phenomena were observed for Pickering emulsions stabilized by both 0.05 and 0.3 wt % of CNC−C18 (Supporting Information, S5). The de-emulsification was the result of breakage of the CN linkage at the acidic surrounding. Accompanying the breakage of the CN linkage, CNC−C18 nanoparticles were divided into two parts: cleaved 18-C alkyl chains and regenerated CNCs. The cleaved 18-C alkyl chains were dissolved in n-hexane, and the regenerated CNCs were shifted to the water phase from the interface and precipitated in water after several hours, as shown in Figure 6B. As a consequence, the dispersed oil droplets merged into a continuous phase. The time for the thorough deemulsification was related to the CNC−C18 content. A longer time was required for Pickering emulsions stabilized by CNC− C18 nanoparticles at a higher content (Supporting Information, S5). 7520


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Figure 8. SEM images of acid-treated PS microspheres after stirring in ethanol containing terephthalaldehyde.

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were embedded in the polymer bulk after the polymerization reaction. Theoretically, many NH3+ cations would be formed on the surfaces of cross-linked PS microspheres accompanying a breakage of CN bonds at the acidic medium. EDX measurements revealed that the contents of the N element were ∼1.38 wt % for the original PS spheres and ∼1 wt % for acid-treated PS microspheres. The contents of the N element were nearly the same for the two samples within the experimental error. In order to further prove the formation of PS spheres with NH3+-rich surfaces, the acid-treated PS microspheres were first dispersed in ethanol at a concentration of 0.8 mg/mL, and ammonia was then used to adjust pH to ∼9. Terephthalaldehyde was added to the above dispersion, followed by 24 h of stirring at room temperature. Subsequently, the morphologies of PS microspheres were imaged by SEM. Cross-linking between the different PS microspheres occurred. It was found that some PS microspheres with a larger size were coupled simultaneously by several PS microspheres with a smaller size, as exhibited in Figure 8. The cross-linking between PS microspheres was ascribed to the new formation of CN bonds, which were formed via a hydrazone reaction between the amino groups on the surface of PS spheres and the added terephthalaldehyde compound with two aldehyde groups. So, functional spheres with amion functionalities-rich surfaces could be created employing the current Pickering emulsion as a template.

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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.7b00375. TEM images of Au NPs-tagged CNC−SH and Au NPstagged pristine CNCs samples. Specific analysis of the contents of the introduced −NH2 groups and the 18-C alkyl chains at the reducing ends of CNCs. Timedependencies of the contact angle of the pristine CNCs and the CNC−C18 sample. Variation of n-hexane in water emulsion droplets stabilized by CNC−C18 with different contents after different displacement time periods. HCl-induced de-emulsification rate of n-hexane in water Pickering emulsions stabilized by different contents of CNC−C18. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Juan Guo. E-mail: [email protected]. *Yong Gao. E-mail: [email protected]. ORCID

Yong Gao: 0000-0001-7962-7196 Notes

The authors declare no competing financial interest.

CONCLUSIONS

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ACKNOWLEDGMENTS

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REFERENCES

The authors appreciated the financial supports from the National Natural Science Foundation of China (21574112, 21174118), Hunan Provincial Natural Science Foundation of China (2017JJ2243), Open Project of Hunan Provincial University Innovation Platform (15K123), and a project of Research Institute for Forestry New Technology, Chinese Academy of Forestry (CAFINT2014K03), the National Nonprofit Institute Research Grant of CAFINT, China.

Heterogeneous modification of CNCs has been successfully achieved by selectively introducing the hydrophobic 18 C alkyl chains at reducing ends of CNCs via the hydrazone reaction and the amidation reaction combination. Owing to the pHresponsive CN linkage, the incorporated 18-C alkyl chains at the reducing ends of CNCs could be utterly cleaved. The obtained CNC−C18 demonstrated higher emulsifying performances by comparison with the pristine CNCs. Stabilized nhexane in water Pickering emulsions could be generated at a low CNC−C18 content of 0.05 wt % versus the total weight of oil and water. CNC−C18 presented a horizontal orientation at the oil−water interface. The formed n-hexane in water Pickering emulsions displayed a pH-triggered de-emulsification feature due to the reversible CN bonds linkage between the CNCs and the introduced 18-C alkyl chains. Using CNC−C18 stabilized (St+DVB) in a water Pickering emulsion as a template, PS microspheres with amino-rich surfaces could be facilely produced. This method could be extended to prepare other functional nano-sized or micro-sized materials with amino-rich surfaces, like SiO2 spheres. These kinds of materials with amino-rich surfaces could be utilized in many important application fields, such as adsorption and separation fields.

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