Stabilizing Pickering Emulsions Using Octenyl Succinic Anhydride Modified with Cellulose Nanofibrils

Jialei Qu,Langman Luo,Weifeng Zhang,Shao Geng,Chunping Wang,An Wang,Yangbing Wen

Tianjin Key Laboratory of Pulp and Paper, Tianjin University of Science & Technology, Tianjin,300457,China

Abstract: In this study, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-mediated(TEMPO-mediated) oxidized cellulose nanofibrils (TOCNFs) modified with octenyl succinic anhydride (OSA) to improve their hydrophobicity and emulsification performance were investigated.OSA loadings of 9.87-14.81 mmol/g (base on oven-dry weight of TOCNFs) were explored during modification, and the obtained OSA-modified TOCNFs (TOCNF-OSA) were characterized using Fourier transform infrared spectroscopy,X-ray photoelectron spectroscopy,X-ray diffraction,and thermogravimetric analysis.Additionally,the emulsification performance of TOCNF-OSA was also investigated in the preparation of the Pickering emulsion.The results demonstrated that the molecular chain of OSA was successfully grafted onto the backbone of TOCNFs during modification.With an increase in OSA loading, the content of the carboxyl group of the modified TOCNF-OSA increased significantly while the Zeta potential decreased.Moreover, a degree of substitution of 0.95 could be achieved at an OSA loading of 14.81 mmol/g.During the preparation of Pickering emulsions, compared with original TOCNFs, the modified TOCNF-OSA significantly improved the stability and dispersibility of the Pickering emulsion.This study demonstrated a promising approach for the modification of the Pickering emulsion to produce an eco-friendly emulsifier with superior performance for use in the food industry.

Keywords:modification; TOCNF; OSA; hydrophobicity; emulsification

1 lntroduction

Pickering emulsions, which are emulsions that stabilize solid particles rather than surfactants, have attracted considerable attention[1-2].Compared with conventional emulsions stabilized by surfactants, Pickering emulsions have several advantages, such as high resistance to coalescence, low environmental impacts, and excellent storage stability[2].Moreover, the "surfactant-free"character further enables Pickering emulsions to be utilized in cosmetic, food, and pharmaceutical applications where surfactants normally cause several problems, such as chronic toxicity, poor emulsion stability, and nonbiodegradablity[3-4].When compared with surfactants, the dosage of the solid emulsion stabilizer is generally low, but the utilization of nondegradable colloidal particles, such as silica particles, titania particles, and synthetic organic polymers could also result in toxicity risks[4-6].Therefore, the development of an environmentally friendly emulsion stabilizer would further expand the application of Pickering emulsions in the production of high-value products.

Cellulose nanofibrils(CNFs)produced from renewable cellulose resources with lengths of more than a few micrometers and widths of 5-20 nm[7]could be promising raw materials to be utilized as solid stabilizers in Pickering emulsions due to several unique properties,such as low density, large surface area, biodegradability,robust mechanical properties, and high chemical tenability[8-13].The challenge of using CNFs as emulsion stabilizers is the high hydrophilicity that greatly limits their applications in the hydrophobic medium or emulsions[14-16].Therefore,conducting surface modifications that can improve the interfacial properties between the nanofibers and hydrophobic matrix would further broaden the application of CNFs.

Recently, substantial literature has been published on modifying CNFs to improve the interface hydrophobicity and expand their application in the formation of composites.The modification methods commonly used are esterification, surface adsorption, and grafting with coupling agents[17-20].Among these methods,esterification of the hydroxyl groups in cellulose is considered a promising strategy due to its easy operation, efficient reaction rate, and use of low toxic chemicals[19,21].During the esterification process, hydroxyl groups are esterified with chemicals containing acyl anhydride groups (e.g., acyl, cyclic, succinic, and N-dodecyl succinic anhydrides) to form half-esters that could introduce extra carboxyl groups into the nanofiber and increase its hydrophobicity[19,22-23].Moreover, to prepare modified CNFs to apply in the food or pharmaceutical industries, finding a safe and food-grade chemical is key.Octenyl succinic anhydride (OSA), which has been widely used in the food industry to modify starch[24],could be a promising chemical for modifying CNFs to increase their hydrophobicity.During the esterification of CNFs with OSA, the hydrophobic element is provided in the form of octenyl succinate groups[19,22].Presumably,few study on the modification of CNFs with OSA to increase their hydrophobicity and improve the stability of Pickering emulsions has been reported.

In this study, OSA was used to modify (2,2,6,6-tetramethylpiperidin-1-yl)oxyl-mediated (TEMPOmediated)oxidized CNFs(TOCNFs)for the production of the TOCNF-OSA composites.The obtained TOCNFOSA product was characterized using Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), degree of substitution (DS) analysis,and surface charge and Zeta potential determination.Additionally, the obtained TOCNF-OSA product was also emulsified with linoleic acid to determine the impact of surface modification on the stability of Pickering emulsions.The results demonstrate that the OSAmodified TOCNFs were successfully produced, and the produced TOCNF-OSA showed great potential as a solid stabilizer for the preparation of Pickering emulsions.

2 Experimental

2.1 Materials

Air-dried TOCNFs were provided by Tianjin Woodelf Biotechnology Co.,Ltd.(Tianjin,China).The TOCNFs were produced from Northern bleached softwood kraft(NBSK) pulp following the TEMPO-mediated oxidation process described by Saito et al[25].The carboxylate content of the TOCNFs was 1.07 mmol/g,and they had an average diameter and length of 10-20 nm and 1-5 μm,respectively.Analytical grade N-methyl pyrrolidone(NMP), OSA, 4-dimethylaminopyridine (DMAP), linoleic acid, and sodium carbonate (Na2CO3) were purchased from Sigma-Aldrich Chemical Co., Ltd.(Tianjin,China).All experiments in this study were performed at least in triplicates.

2.2 Modification of TOCNFs with OSA

The reaction between TOCNFs and OSA using DMAP as catalyst was performed using a 1000 mL three-necked round-bottom flask immersed in a water bath at 70℃ at 500 r/min for 90 min.A series of modification experiments were conducted with OSA loadings of 9.87-14.81 mmol/g (base on oven-dry weight of TOCNFs).For each experimental run,50 g of TOCNFs,250 g of NMP, 8.6 g of Na2CO3, and the calculated amount of OSA were mixed and placed in a reactor for modifications.After completion, the resultant TOCNFOSA was collected through filtration and subsequently washed with 95% ethanol (~0.25 L) and deionized water (~2.0 L).The collected TOCNF-OSA was air dried for 48 h and stored at 4℃ for further use in experimentation.

2.3 Analytical and characterization methods

The moisture content of the original and modified TOCNF samples was measured by drying at (105 ±2)℃ to a constant weight.FT-IR spectra of the original and modified TOCNF samples were recorded using an FT-IR-650 spectrometer(Tianjin Gangdong Sci.&Tech.Co., Ltd., Tianjin, China) with a wavenumber range of 400-4000 cm-1using the potassium bromide (KBr)method.Briefly, 0.03 g of the TOCNF sample and 0.3 g of KBr were mixed and pressed into a transparent pellet for FT-IR analysis.

The carboxyl group content and theDSwere determined using an acid-base titration system at a TOCNF concentration of 0.1 wt%.Briefly, 20 mL of the original or modified TOCNF suspension (fibril consistency of 1 wt%) was placed in a 500 mL beaker.Thereafter, 5 mL of NaOH solution (0.1 mol/L) was added to the mixture and stirred at 200 r/min for 30 min at room temperature.The excess NaOH was titrated with HCl solution(0.1 mol/L)using phenolphthalein as an indicator.The carboxyl group content was calculated according to Equation(1):

whereC1andV1are the concentration (mol/L) and volume (L) of the NaOH solution, respectively;C2andV2are the concentration (mol/L) and volume (L) of the HCl solution, respectively;Mis the dry weight (g) of the TOCNF sample.

The DS was calculated according to Equation(2):

where 162 g/mol is the molar mass of an anhydroglucose unit (AGU), 210 g/mol is the increase in the molecular weight of AGU after substituting one —OH group,mis the dry weight of the sample,NTOCNF-OSAis the amount of carboxyl group content calculated from the obtained value of the equivalent volume of known molarity of the NaOH and HCl titrated, andNTOCNFis the amount of carboxyl group in TOCNFs after titration[26-27].

The Zeta potential of the TOCNF-OSA suspension was measured using a Zeta PLAS 190 Plus analyzer(Brookhaven, USA) at 25℃ and a TOCNF-OSA concentration of 0.1 wt%.XRD patterns of the original and modified TOCNF samples were obtained using a diffractometer (D8 Advance, Bruker AXS, Germany)fitted with Cu-Kαradiation (λ= 0.154 nm) at an accelerating voltage of 36 kV and a current of 20 mA[28].The patterns in the 2θ=5°-40°range were recorded at a speed of 0.2°/min.The crystallinity index (CrI) was calculated with Equation(3):

whereI200is the diffraction intensity of the crystalline segments at 2θ=22.5°andIamis the minimum intensity at the valley between the plane(200)and(101).

XPS experiments were performed using an ESCALAB 250Xi XPS microprobe (ThermoFisher Scientific, USA)with radiation (0-5000 eV).Binding energies were calibrated by the containment carbon(C 1s=284.6 eV);an XPS Peak 4.1 was employed to deconvolve the C 1s peaks with the Shirley-type baseline using an iterative least-squared optimization algorithm[29-30].

The thermal stability of the original TOCNFs and modified TOCNF-OSA samples were determined using a TGA Q500 (TA Instruments, USA) instrument at temperatures of 100℃-600℃.For each measurement,(10 ± 0.1) mg of air-dried original TOCNFs or modified TOCNF-OSA were placed in an open platinum pan and heated at a rate of 10℃/min.The experiments were carried out in nitrogen at a flow rate of 100 mL/min.

2.4 Preparation of Pickering emulsion

Air-dried original TOCNFs and modified TOCNF-OSA were diluted to a pulp consistency of 1.05 wt% with deionized water and subsequently subjected to highpressure homogenization with a homogenizer(AH100D,ATS Engineering Inc., Canada) at a neutral pH value and 80 MPa for 6 passes to prepare TOCNF and TOCNF-OSA hydrogels before using them to prepare Pickering emulsions.For each homogenization process,10 g of original TOCNF or modified TOCNF-OSA sample was used.

The Pickering emulsion was prepared in a 200 mL beaker.Linoleic acid dosages of 1, 3, and 5 (wt%,based on the oven-dry weight of TOCNFs) were investigated.For each experimental run, 10 g of TOCNF or TOCNF-OSA hydrogel (consistency of 1.05 wt%) and the calculated amount of linoleic acid were mixed and homogenized with a rotor-stator homogenizer (Ultra Turrax T25,IKA,Germany)at 20000 r/min for 10 min.The obtained Pickering emulsions were stored at 4℃ for further use in experimentation.

2.5 Characterization of the Pickering emulsio n

The particle size distribution of the obtained Pickering emulsions was analyzed using a laser particle size analyzer(Beckman Coulter,USA)at a solid concentration of 0.1 wt%.During the determination process,ultrasound was applied for 2 min to remove bubbles from the suspension, and the morphology of the emulsions was observed using an optical microscope (Olympus BX-51,Japan).Diluting the Pickering emulsions(1.0 wt%)100 times with deionized water and applying an ultrasound for 1 min removed bubbles from the solution.Single drops of diluted emulsions were placed on glass slides and observed under the optical microscope.The droplet size and size distribution of the different emulsions were analyzed from the corresponding micrographs.

3 Results and discussion

3.1 Preparation of TOCNF-OSA

Fig.1 shows the mechanism for the surface modification of TOCNFs with OSA to prepare TOCNF-OSA.Hydroxyl groups of the TOCNF cellulose undergo esterification with cyclic anhydrides from OSA to form half-esters and introduce extra carboxyl groups into the nanofiber[19,22].Furthermore, the hydroxyl groups at C2, C3, and C6 positions might also be esterified to form octenyl groups, increasing the carboxyl group content (Fig.1).The increase in the carboxyl group content and formation of octenyl groups due to OSAmodification could increase the hydrophobicity of the TOCNFs[31], thereby expanding the utilization of TOCNFs in emulsions or hydrophobic mediums.

Fig.1 Schematic of the preparation of TOCNF-OSA

3.2 Grafting yield(GY)of TOCNF-OSA

The detailed results of grafting OSA onto TOCNFs undergoing esterification are shown in Table 1.Observe that the GY remains almost constant when the amount of OSA feed is increased from 9.87 to 14.81 mmol/g.For example, at an OSA loading of 9.87 mmol/g, the GY is 31.71%.When the OSA loading is further increased to 13.57 mmol/g, the GY of TOCNF-OSA increases to 32.50%.

Table 1 GY of modifying TOCNFs with OSA

3.3 Effect of OSA loading on the carboxyl group content of TOCNF-OSA

During the utilization of TOCNF-OSA in emulsification,the carboxyl group content is a critical parameter since it may represent the degree of OSA loading.The OSA loading is the formation of a three-dimensional network structure between particles by hydrophobic association;therefore, the long alkyl chain groups of OSA can improve the stable dispersion of oil droplets[32].Herein,we varied the loadings of OSA to produce TOCNF-OSA with different carboxyl group contents.As expected,with increasing OSA loading,the content of the carboxyl group of TOCNF-OSA increases (Fig.2(a)).For example, at an OSA loading of 9.87 mmol/g, the carboxyl group content is 4.20 mmol/g, which is significantly higher than that of the control (1.07 mmol/g).With a further increase in the OSA loading to 14.81 mmol/g,the carboxyl group content of TOCNF-OSA increases to 5.44 mmol/g,indicating that the esterification occurred efficiently under the studied conditions.To further assess the esterification efficiency, the DS of TOCNFOSA was determined.As shown, the DS of the negatively charged cellulosic surfaces increases with increasing OSA loadings.For example, increasing the OSA loading from 9.87 to 14.81 mmol/g increases the DS of TOCNF-OSA from 0.68 to 0.95,indicating that a large number of hydroxyl groups(—OH)have been replaced and esterified with cyclic anhydride groups in OSA.These results agree with the literature on improving the DS of TOCNFs by esterification with other chemicals, such as alkenyl succinic anhydride (ASA)and dodecyl succinic anhydride(DDSA)[19,33-34].

The Zeta potential of TOCNF-OSA samples were determined to further evaluate the effect of OSA modification on the properties of the TOCNFs(Fig.2(b)).As shown, with increasing OSA loadings, the Zeta potential of the TOCNF-OSA increases.This is likely due to ring-opening esterification of TOCNFs withOSA increasing the content of free carboxyl groups.The increase of Zeta potential could provide increased electrostatic repulsion and stabilize the dispersion of the fibrils in water[19].Moreover, at the highest carboxyl group content (5.44 mmol/g), the Zeta potential of the obtained TOCNF-OSA is approximately -63.9 mV, the absolute value of which is higher than that of the original TOCNFs.These results conclusively indicate that the surface esterification of TOCNFs with OSA could be used for the production of stable TOCNFOSA dispersions with high carboxyl group contents.

3.4 Characterization of TOCNF-OSA

Fig.3 shows the FT-IR spectra of the original TOCNFs and the modified TOCNF-OSA samples.Compared with the FT-IR spectrum of the original TOCNFs, the spectra of TOCNF-OSA has more absorption peaks and antisymmetric stretchings,such as the absorption bands at 500-900, 1730, 2900, and 3400 cm-1(Fig.3(a)).As shown, increasing the OSA loading increases the adsorption band characteristic of carbonyl bonds (C=O)at 1730 cm-1[22], whereas the intensity of the bands corresponding to the hydroxyl groups at 3400 cm-1decreases; this further confirms the esterification of hydroxyl groups in cellulose to form octenyl groups.In addition,the intensity of the absorption peak at 2900 cm-1,assigned to C—H stretching in the long alkyl chain in OSA in the TOCNF-OSA samples, also increases with increasing OSA loadings (Fig.3(a)).Another significant difference in the FT-IR spectra between the TOCNFs and TOCNF-OSA samples is in the absorption range of 500-1900 cm-1, which is magnified and shown in Fig.3(b).Compared with the FT-IR spectrum of the TOCNFs, three absorption bands at 1165, 1112, and 1058 cm-1that are attributed to the stretching of C—O from OSA[19]are strengthened in the spectra of TOCNF-OSA samples,further indicating the successful grafting of ocentyl groups onto the cellulose backbone.In addition, the absence of absorption peaks at 1850 and 1780 cm-1suggests that there are no unreacted octenyl succinic anhydride in the washed TOCNF-OSA products[19].

Fig.3 FT-IR spectra of the original TOCNFs and the modified TOCNF-OSA samples: (a) wavenumber of 400-4000 cm-1 and (b)wavenumber of 500-1900 cm-1

To further confirm the occurrence of the grafting reaction between TOCNFs and OSA, the functional groups of the obtained TOCNF-OSA products were further characterized using XPS.The C 1s signals of TOCNFs are deconvoluted into two peaks, C—C located at 284.1 eV and C—O at 286.0 eV (Fig.4(a)),agreeing with the literature that investigated the functional groups of TOCNFs[29,35-36].For TOCNF-OSA-5,the C 1s signals have four peaks which are C—C at 285.0 eV and 284.4 eV,C—O at 285.9 eV,and O—C=O at 288.5 eV(Fig.4(b)).Compared with TOCNFs,the new O—C=O signal at 288.5 eV arises from the OSA esterification of TOCNFs.In addition, the increased C—C intensity(mainly derived from OSA long alkyl chain) also indicates the successful grafting of OSA onto the structure of TOCNFs[19,37].

Fig.4 Deconvoluted C 1s XPS spectra of(a)TOCNFs and(b)TOCNF-OSA-5

To gain further insights into the esterification process, the XRD patterns and correspondingCrIfor the original TOCNFs and the modified TOCNF-OSA samples are determined (Fig.5).As shown, no obvious difference is observed between the diffraction patterns of the original TOCNFs and the modified TOCNFOSA samples.The three main peaks at 2θ=16.5°,22.5°,and 34.5° in the diffraction patterns were assigned to the typical (101), (200), and (004) reflection planes of cellulose I, respectively[38-39].In addition, esterification only has a slight effect on theCrIof cellulose.For example, theCrIof the original TOCNFs is 78.45%,while the indexes are 77.85% and 77.56% for the TOCNF-OSA samples prepared at OSA loadings of 9.87 mmol/g and 13.57 mmol/g, respectively.These results suggest that the modification of TOCNFs with OSA is limited mainly to the fibril surface[37,40].

Fig.5 XRD patterns of the original and modified TOCNFs (at an OSA loading of 9.87 mmol/g and 13.57 mmol/g,respectively)

Thereafter, the effect of esterification on the thermal stability of TOCNFs was determined using TGA(Fig.6).As shown, compared with the original TOCNFs,modifying with OSA slightly decreases the thermal stability.Moreover, the thermal stability of TOCNFOSA decreases with an increase in the carboxyl group content.For example, the original TOCNFs have twoTmaxtemperatures ofTmax1= 264.4℃ andTmax2= 343.3℃,during which the weight loss rates are 0.3143%/℃ and 0.6844%/℃, respectively.Regarding the TOCNF-OSA obtained from the highest OSA loading (14.81 mmol/g,i.e., TOCNF-OSA-5), the twoTmaxtemperatures are decreased toTmax1= 256.8℃ andTmax2= 323.7℃ with weight loss rates of 0.4378%/℃ and 0.7603%/℃,respectively.The slight decrease inTmaxtemperatures and increase in the rate of weight loss could be due tothe presence of large amounts of free carboxyl groups on the surface of TOCNF-OSA that facilitates the decomposition of cellulose[41].The results of the TGA show significant low-temperature shifts for the surfaceesterified TOCNF-OSA sample compared with the original TOCNFs; this could be ascribed to the decomposition and vaporization of the ester groups.Moreover, the thermogravimetric curves of TOCNFOSA(esterified products)show a two-stage weight loss process: the decomposition of the ester linkages and cellulose backbone[19].Although the esterification reaction decreases the thermal stability of TOCNFs,theTmaxtemperatures of 256.8℃ and 323.7℃ for the TOCNF-OSA-5 are still high for the stabilizing of the Pickering emulsion.

Fig.6 The thermostability of the original TOCNFs and TOCNF-OSA: (a) TGA thermograms; (b) derivative thermal gravimetry(DTG)

3.5 Effect of TOCNF-OSA on the properties of Pickering emulsion

During the production of the Pickering emulsion for active compounds or flavors and control of lipid absorption[42-43],the dispersibility is a critical parameter.In this study, both the original TOCNFs and the modified TOCNF-OSA-5 were utilized to form Pickering emulsions.The effect of esterification on the properties of the generated Pickering emulsions was investigated at linoleic acid concentrations of 1 wt%, 3 wt%, and 5 wt%.Fig.7 and Fig.8 show optical micrographs of the fresh Pickering emulsions stabilized by original TOCNFs and TOCNF-OSA-5 with linoleic acid.As shown in Fig.7, under the studied linoleic acid concentration, the particle size of the Pickering emulsion prepared with original TOCNFs is remarkably larger than that of the emulsion prepared with TOCNFOSA-5, indicating that particle flocculation occurred in the emulsion prepared with original TOCNFs.This is likely due to the esterification of OSA with long alkyl chain groups in TOCNFs improving the surface hydrophobicity to prevent flocculation of droplet particles[44].The reduction of particle size is favorable for stabilizing the emulsion[42]; increasing the storagetime of the prepared emulsion increases the particle size of the emulsion prepared with original TOCNFs(Fig.8(a) and Fig.8(b)), indicating the increasing flocculation of droplet particles with time.In contrast,prolonging the storage time only slightly changes the particle size and distribution for the Pickering emulsions prepared with TOCNF-OSA at different concentrations of linoleic acid (Fig.8(c)-Fig.8(h)), indicating that TOCNF-OSA-5 notably improves the stability of the emulsion without obvious coalescence and formation of clusters for up to 3 days.This could be attributed to TOCNF-OSA-5 containing long alkyl chain groups that may facilitate the formation of three-dimensional networks between particles,thereby improving stability[42].

Fig.7 Optical micrographs of fresh Pickering emulsions stabilized by original TOCNFs and TOCNF-OSA-5 at linoleic acid concentrations of 1-5 wt%; all micrographs were obtained in a diluted state within 1 h;scale bar:10 μm

Fig.8 Optical micrographs of fresh Pickering emulsions stabilized by original TOCNFs and TOCNF-OSA-5 with linoleic acid concentrations of 1-5 wt% on different days: (a) day 1,(b) day 3, (c) day 1, (d) day 3, (e) day 1, (f) day 3, (g) day 1,and(h)day 3;scale bar:10 μm

Another critical parameter for using the Pickering emulsion is the stability and dispersibility of oil droplets.Fig.9 shows the particle size distribution of the Pickering emulsion prepared with different linoleic acid loadings and stored for different times.As shown in Fig.9(a),original TOCNFs contain low proportions of fine particles and have particle sizes that are mainly concentrated in the range of 0.459-1.637 μm.After the emulsification of linoleic acid, the proportion of fine particles in the emulsion prepared with original TOCNFs increases(Fig.9(a)).Moreover,slight phase separation in the presence of a defined interface(light and cream layer)was observed in pictures of emulsions prepared using linoleic acid (Fig.9(a)).This is probably due to original TOCNFs being in contact with the oil phase only via their surface and not being immersed in the oil[32].Compared with the emulsion prepared with the original TOCNFs,the Pickering emulsion prepared with the modified TOCNF-OSA has a more even particle size distribution.Moreover, the particle sizes and distributions of TOCNF-OSA-5 are mainly concentrated in the range of 0.632-9.117 μm.Compared with TOCNF-OSA-5 only,theparticle size of Pickering emulsion with TOCNF-OSA-5 and linoleic acid significantly decreases (Fig.9(b)).As shown in Fig.9(b), TOCNF-OSA-5 suspensions have strong stability and no obvious flocculation and stratification.This could be due to the coated droplet particles of TOCNFs with long alkyl chain groups substantiating the dispersibility of the emulsions.

Fig.9 Particle size distribution of the Pickering emulsions prepared with(a)TOCNFs with linoleic acid concentrations of 1-5 wt%,(b)TOCNF-OSA-5 with linoleic acid concentrations of 1-5 wt%,(c)TOCNF with a linoleic acid concentration of 3 wt%stored for 1-3 days,(d)TOCNF-OSA with a linoleic acid concentration of 5 wt%stored for 1-3 days.Images shown in Fig.9(a)-Fig.9(d)are pictures of the emulsions.

Fig.9(c) and Fig.9(d) show the stability of the Pickering emulsions prepared with the original TOCNFs and modified TOCNF-OSA-5, respectively.As shown in Fig.9(c),for the emulsion prepared with the original TOCNFs,the increase in time from 1 to 3 days resulted in flocculation occurring and large particle sizes formed.Additionally, phase separation and layering effect are also observed in the emulsion (Fig.9(c)).In contrast, when storing the Pickering emulsion prepared with the modified TOCNF-OSA-5 for 3 days, the average particle size of the emulsion was stabilized at 2.401 μm with no obvious stratification phenomenon,which is much smaller than that of the original TOCNFs emulsified sample(Fig.7,Fig.8,Fig.9(c),and Fig.9(d)).This is probably due to the formation of a three-dimensional network structure between particles by hydrophobic association with the long alkyl chain groups of OSA.Moreover, the introduction of extra carboxyl groups into TOCNFs through esterification might also improve the surface hydrophobicity of TOCNFs,preventing flocculation of droplet particles[6,42].

4 Conclusions

In this study, (2,2,6,6-tetramethylpiperidin-1-yl)oxylmediated (TEMPO-mediated) oxidized cellulose nanofibrils (TOCNFs) modified with octenyl succinic anhydride (OSA) to improve their hydrophobicity were investigated.The results showed that the esterification between hydroxyl groups of TOCNFs and cyclic anhydride groups of OSA occurred effectively under the studied conditions.During the modification process,increasing OSA loadings increased the carboxyl group content, absolute Zeta potential and degree of substitution in the resultant TOCNF-OSA polymer, while the thermal stability decreased slightly.Furthermore, the stability of the Pickering emulsion prepared using the modified TOCNF-OSA improved significantly, compared with that of the original TOCNFs.In addition,the particle size of the Pickering emulsion prepared with TOCNF-OSA was also much smaller than that of the Pickering emulsion stablized by original TOCNFs.In summary, modifying TOCNFs with OSA is a promising strategy for expanding the application of TOCNFs in the preparation of Pickering emulsions.