Yongqi Zhang,Yongjian Xu,Chun Liu,2,Ling Yang,Jianmin Hu,Ruixia Zhang,Xiuqiong Guan

1.College of Bioengineering,Sichuan University of Science and Engineering,Yibin,Sichuan Province,644000,China

2.College of Bioresources Chemical and Materials Engineering,Shaanxi University of Science&Technology,Xi'an,Shaanxi Province,710021,China

3.Voith Paper(China)Co.,Ltd.,Suzhou,Jiangsu Province,215300,China

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

Abstract:Generally,hydrogen bonds are formed between cellulose nanocrystals(CNCs)during their water removal and drying,leading to the irreversible aggregation of CNCs,and thus a poor water-redispersibility.The present study demonstrated a novel approach that involved using hydrolyzed sugars generated from the corresponding CNC production as redispersing agents to enhance the redispersibility of CNCs.Experimental data indicated that hydrolyzed sugars can be adsorbed onto CNCs through ethanol precipitation.The oven-dried CNCs onto which hydrolyzed sugars were adsorbed via ethanol precipitation were homogeneously redispersed in water.The redispersed CNCs showed the particle size distribution,Zeta potential,and thermal decomposition properties similar to those of the CNCs without drying.This method may improve the use of hydrolyzed sugars obtained in the hydrolysate from the corresponding CNC production,as well as facilitate the transportation and storage of CNCs.

Keywords:water redispersion;cellulose nanocrystals(CNCs);hydrolyzed sugars;ethanol precipitation

1 Introduction

As one of the most promising nano-sized cellulose materials,cellulose nanocrystals(CNCs)have received tremendous attention in recent years[1-2].CNCs not only have the inherent characteristics of natural cellulose,but also exhibit appealing properties such as chirality, biocompatibility, biodegradability,nontoxicity,and ease of surface modification due to their abundant surface functional groups[3-4].Because of these unique features,CNCs can be used in a variety of potential applications,such as papermaking,water purification, paints, coatings, food, cosmetics,biomedicine, and optical/electronic devices[5-6].However,a significant challenge in the handling of CNCs arises when they must be dried from a suspension of well-dispersed individual nanoparticles.The unique properties of CNCs depend on their sizes,and the presence of individual nanoparticles is essential for the full expression of the CNC properties.Unfortunately,irreversible aggregation often occurs when CNCs are dried,leading to loss of their properties and functionality.Therefore,fabrication of dried CNCs that can retain their original properties after redispersing in water is a challenge.

Several redispersing agents,such as sodium chloride[7],vinyl alcohol[8],maltodextrin[9],and carboxymethyl cellulose(CMC)[10]have been used to improve the redispersibility of dried nanocellulose materials including CNCs.However,the above methods may be inadequate or toxic for food and pharmaceutical-related applications,restricting their use in some of the most promising sectors for the development of CNCs and their applications.

Previously,we discovered that CNCs can be prepared by an extremely low acid (ELA,acid concentration≤0.1 wt%)hydrolysis[11].The main degraded products in the hydrolysate are hydrolyzed sugars,such as oligosaccharides,glucose,and xylose.Direct disposal of hydrolyzed sugars is a great waste of resources. Low-molecular-weight sugars such as oligosaccharides,glucose,and xylose are recognized by the Food and Drug Administration of the United States as safe materials[12].The low-molecular-weight sugars can be adsorbed onto the surface of CNCs as their solubility decreases[13].In addition,some researchers have also reported that the use of sugars as bulking agents can prevent partial nano-crystal aggregation during spray or freeze-drying[14].They investigated the effect of sugars on nano-crystal(indomethacin crystalline andγ-polymorph)aggregation during drying.Thus,it may be expected that the adsorption of hydrolyzed sugars from the corresponding CNC production onto the surface of the CNCs prior to drying can enhance their redispersibility in water.

To prepare water-redispersible CNCs,hydrolyzed sugars present in the hydrolysate liquor generated in the corresponding CNC fabrication with ELA hydrolysis were selected as redispersing aids.In this work,the water redispersibility of dried CNCs on which hydrolyzed sugars were adsorbed via ethanol precipitation was studied.Analyses were conducted using scanning electron microscopy(SEM),transmission electron microscopy (TEM),selected area electron diffraction (SAED),thermo gravimetric analysis (TGA),and Zeta potential analysis,etc.

2 Experimental

2.1 Materials

Bleached kraft bamboo pulp(BKBP)was obtained from TAISON Group(Shanghai,China).The chemical compositions of the pulp was 82.8%α-cellulose,17.1%hemicelluloses,0.07%ash,and no detectable lignin,according to the analysis performed using the Technical Association of Pulp and Paper Industry(TAPPI)test methods.Hydrochloric acid(HCl,37%,w/w)and ethanol(CH3CH2OH,95%,w/w)were purchased from Sinopharm Chemical Reagent Co.,Ltd.(China).All chemicals were of analytical grade and were used without any further purification.Deionized water was used for all experiments conducted in this study.

2.2 Methods

2.2.1 Preparation and isolation of e-CNCs(CNCs obtained through ELA hydrolysis)and hydrolysate

The e-CNCs were prepared from BKBP by ELA hydrolysis according to the method described by Zhang and coworkers[11].BKBP(0.5 g,dry basis),water,and a 0.2 wt%HCl solution were added to the reactor(50 mL stainless steel reactor operating in the batch mode)until the desired acid concentration(0.08 wt%)was reached,and then the reactor was heated to 180℃at a heating rate of 5℃/min.The reaction time and operating pressure were 60 min and 15 MPa,respectively.At the desired reaction time,the reaction was quenched by placing the reactor in cold water.Then,the nonhydrolyzed fraction of biomass was separated from the reaction mixture using a canvas with the opening of 1μm,from which a part of the filtrate(containing e-CNCs and hydrolysate)was stored in a cold room(4℃)for further testing and ethanol precipitation.Alternatively,to isolate the e-CNCs and hydrolysate,the remaining filtrate (containing e-CNCs and hydrolysate)was centrifuged using an OPTIMA XPN-10 centrifuge(Beckman Coulter,US)at 4000 r/min for 15 min.Subsequently,the supernatant was filtered using a canvas with the opening of 0.22μm to obtain hydrolysate.The collected hydrolysate was stored in a cold room (4℃) for high-performance liquid chromatography (HPLC) analysis and ethanol precipitation.The residue obtained by centrifugation was washed with water and then was centrifuged again at 4000 r/min for 15 min.The washing procedure was repeated until the pH value of suspension reached 6.At last,the washed residue was denoted as e-CNCs and stored in a cold room(4℃)for further testing and ethanol precipitation.

2.2.2 Ethanol precipitation

Ethanol precipitation is a facile method for industrial production and is often used to deposit hydrolysis products consisting of oligosaccharides,glucose,and xylose.Thus,the hydrolyzed sugars (HS)were precipitated via adding ethanol into the treating filtrate(containing e-CNCs and hydrolysate)when the volume concentration of ethanol reached 85%.The above precipitates were denoted as e-CNC/HS precipitates.

To confirm that the HS in hydrolysate can be adsorbed onto the surface of e-CNCs through ethanol precipitation,the hydrolysate generated in the previous section was treated with ethanol until the volume concentration of ethanol reached 85%.The obtained precipitate was denoted as HS precipitates.The e-CNCs isolated as described above were dispersed in water to obtain a concentration of 0.05 wt%.Then,the e-CNC suspension was treated with ethanol to achieve the same concentration of 85 vol% ethanol.The obtained precipitate was denoted as e-CNC precipitates.

2.2.3 Concentrating and drying

As shown in Fig.1,the precipitates of ethanol extraction were separated from suspensions via centrifugation(6000 r/min,10 min).Subsequently,the obtained e-CNC precipitates and HS precipitates were dried in an oven at 45℃ to a constant weight,respectively.The e-CNC/HS precipitates were also dried in an oven at 45℃to achieve the water content of 20 wt%.

Fig.1 Experimental strategy for the preparation of water-redispersible CNCs via adsorption of hydrolyzed sugars on their surface

2.2.4 Redispersion of dried e-CNCs

The e-CNC suspension was prepared via mixing of the dried e-CNC/HS precipitates and deionized water under ultrasonication(200 W,20 kHz)using ultrasonic cell crusher(JY99-IIDN,SCIENTZ,China)for 10 min.Then,the suspension was dialyzed in the deionized water for 2 d.

2.3 Characterization

The yield of isolated e-CNCs was calculated based on Eq.(1).

whereYis the yield of e-CNCs,CandVare the solid content and volume of the e-CNC suspension,andMis the oven dry weight of the raw material(BKBP).

The particle size distribution (PSD)and Zeta potential of CNCs in water at the concentration of 0.01 wt%were analyzed using a Malvern Zetasizer Nano ZS90 instrument(Malvern Panalytical,UK).All samples were ultrasonically treated (20 kHz and 200 W)for 15 min prior to the analysis.All measurements were carried out in duplicate and the average results were presented.

The morphology of CNCs was observed by SEM(S4800,Hitachi,Japan)operated at 1.3×10-3Pa and 5.0 kV.The dried samples were coated with a thin layer of gold using a sputter coater(Cressington 208HR,Cressington Scientific Instruments Ltd.,UK).

The morphology of CNCs was examined by TEM(FEI Tecnai G2 F20 S-TWIN,USA).A drop of CNC suspension was deposited on the carbon-coated grids and air-dried.The TEM image of the dried CNCs was recorded using the TEM microscope at 200 kV.SAED patterns were obtained based on the selected area in the same TEM image.

The sugar components of hydrolysate were analyzed by HPLC (Waters, 2498, Milford, US). The chromatograph was equipped with a differential refractive index detector and a SHODEX SH 1011 column(Shodex,Japan).The reaction products were eluted with sulfuric acid as the mobile phase at a flow rate of 0.5 mL/min.The column temperature was 55℃,and the differential refractive index detector temperature was 40℃.Complete HPLC analysis was carried out in 60 min.The samples were tested three times independently and the average values were reported.

The thermal decomposition behavior of CNCs were studied by TGA (STA449F3-1053-M, Netzsch,Germany)under N2atmosphere(20 mL/min),and each sample was heated at a rate of 10℃/min from 30℃to 700℃.

The Brunauer-Emmett-Teller(BET)specific surface areas of the CNC samples were determined by N2adsorption at-196℃using a surface area and porosity analyzer(Gemini VII2390,Micromeritics Instrument,China).Prior to the measurements,the samples were degassed at 105℃for 1 h.The specific surface areas were determined using the BET method from the adsorption and desorption isotherms. Pore size distributions were derived from the desorption branch of the isotherms using the Barrett-Joyner-Halenda(BJH)method.

3 Results and discussion

3.1 Preparation of water-redispersible CNCs through adsorption of the HS on their surface

During acid hydrolysis reaction,the amorphous regions in cellulose and hemicelluloses are preferentially attacked,whereas the denser crystalline regions show higher resistance to acid attack[15].As a result,the rodlike e-CNCs was obtained from BKBP through the ELA hydrolysis with a yield of 29.5%.The resulting e-CNCs had an average length of 522 nm(Fig.2(d))and a width in the range of 50-90 nm(Fig.4(b)).The different length and width compared to the results reported in the literature is because the particle size of CNCs of a specific cellulose source mostly depend on its hydrolysis conditions rather than on its source used to prepare CNCs[16-17].The acidity of the ELA hydrolysis process is much weaker than that of concentrated acid hydrolysis[18].Thus,the length and width of the CNCs in this study are larger than those previously reported in the literature.The negative Zeta potential value was measured for the resulting e-CNC suspension and showed a Zeta potential of-16 mV.Generally,cellulose hydrolysis using acids that do not generate charges on the CNC surface,such as hydrochloric acid,leads to the low absolute value of the Zeta potential of the CNC suspension[19].Based on the HPLC results,the main degradation products in hydrolysate from the corresponding e-CNC production were oligosaccharides,glucose,xylose,arabinose,furfural,formic acid,and acetic acid,in which the highest relative content was HS (oligosaccharides:4.81 g/L,glucose:1.19 g/L,xylose:0.12 g/L).

Fig.2 Redispersion of e-CNC/HS precipitates in water for(a)1 min and(b)10 min,respectively;(c)the water-redispersed e-CNC suspension after dialysis;(d)the particle size distribution and Zeta potential of the never-dried e-CNC suspension and the water-redispersed e-CNC suspension,respectively

The redispersion of the dried e-CNC/HS precipitates was carried out by ultrasonication,and the results are shown in Fig.2.No visible macroscopic particles were found in the redispersed e-CNC suspension after dispersion for 10 min(Fig.2(b)),and the redispersed suspension is transparent(Fig.2(c))after dialysis.In addition,the average size and Zeta potential of waterredispersed e-CNCs were 547 nm and-17 mV,respectively.The particle size distribution and Zeta potential of the water-redispersed e-CNCs were very similar to those of the never-dried e-CNCs(Fig.2(d)).

Fig.3 shows the TG and DTG curves of the neverdried e-CNCs and water-redispersed e-CNCs.After the removal of HS from the e-CNCs through the dialysis process,the maximum thermal degradation of the water-redispersed e-CNCs occurred at 338℃.Similarities between the thermograms of the water-redispersed e-CNCs and the never-dried e-CNCs indicate that the thermal decomposition behavior of the waterredispersed e-CNCs did not change after the removal of HS by the dialysis process.

Fig.3 TG and DTG curves of never-dried e-CNCs and water-redispersed e-CNCs

The combined results from images and light scattering provided a general description of the redispersion of the dried e-CNCs in water.HS was adsorbed on the surface of e-CNCs via ethanol precipitation,which was essential for avoiding the agglomeration of e-CNCs upon drying,enabling the easy redispersion of e-CNCs in water.

3.2 Morphologies of the HS,e-CNC,and e-CNC/HS precipitates

The TEM image of the HS precipitates is presented in Fig.4(a).It is observed that the HS precipitates were oval-shaped and their SAED pattern shown in the inset exhibited no diffuse rings,implying that the HS precipitates had a typical amorphous structure[20].The TEM image of the e-CNC precipitates shows that the precipitates are rod-like as shown in Fig.4(b).Moreover,the SAED pattern of the e-CNC precipitates shown in the inset exhibited strong and broad diffuse rings,indicating a strong crystalline structure[21].The TEM image the of e-CNC/HS precipitates is shown in Fig.4(c).It appears that the HS was adsorbed on the surface of the rod-like e-CNCs.To confirm that the HS was adsorbed on the surface of e-CNCs,SAED was used to analyze the diffraction pattern of the rod-like e-CNCs(zone 2)and the non-rod structure(zone 1)in Fig.4(c).It was found that the non-rod structure exhibited no diffuse rings,while the rod-like e-CNCs displayed clear diffuse rings.Thus,the above results showed that the HS was adsorbed onto the surface of e-CNCs by ethanol precipitation.

The pore volume and BET specific surface area of the dried e-CNCs and e-CNC/HS precipitates were determined to estimate the effect of HS adsorption on the agglomeration structure of the dried e-CNCs,with the results shown in Table 1.The pore volume and BET specific surface area of the dried e-CNCs were 0.09 cm3/g and 0.16 m2/g,respectively.Greater agglomeration of the e-CNCs during the dehydration was responsible for the smaller pore volume and surface area of the e-CNCs.By contrast,the pore volume and surface area of the dried e-CNC/HS precipitates were 1.29 cm3/g and 15.22 m2/g,respectively.These higher values can be attributed to the adsorption of HS onto e-CNCs during drying,which may prevent the formation of bonds.The larger pore volume and surface area of the e-CNC/HS precipitates can also be supported by the SEM images.In the absence of the adsorption of HS onto e-CNCs,the dried e-CNCs precipitated in a self-organized manner into a compact sheet structure with the pores largely lost(Fig.5(a)).After the adsorption of HS onto e-CNCs,the dried e-CNC/HS precipitates did not show a noticeable micro-scaled agglomeration and exhibited a porous network structure(Fig.5(b)).Therefore,the above-described changes such as larger pore volume,surface area,porous network structure,and absence of micro-scale agglomeration of the dried e-CNC/HS precipitates were beneficial for their redispersion in water.

Table 1 The pore volume and BET specific surface area of the dried e-CNCs and e-CNC/HS precipitates

Considering all of the experimental and analyticalresults described above,the possible mechanism of obtaining water-redispersible CNCs via adsorption of HS on their surface is as follows:the hydrogen bonds in the HS solution are destroyed upon the addition of ethanol[22],leading to the formation of oval-shaped HS precipitates with amorphous structure(Fig.4(a)).Peri et al[23]reported that after low-molecular-weight sugars come into contact with a CNC surface,they will be adsorbed onto the CNC surface and become immobile.Thus,the HS precipitates with amorphous structure are adsorbed onto e-CNCs(Fig.4(c)),preventing the formation of hydrogen bonds between the e-CNCs during dehydration.The actual agglomeration structure of the dried e-CNC/HS precipitates did not show a noticeable micro-scale agglomeration and exhibited a porous network structure(Fig.5(b)),which was beneficial for the redispersion of dried e-CNCs in water.When the dried e-CNC/HS precipitates were redispersed in water,the HS with amorphous structure were removed by the dialysis process.Thus,waterredispersed e-CNCs with nanoscale characteristics similar to those of the control without drying were obtained.

Fig.4 TEM images of(a)HS precipitates(inset:SAED pattern of HS precipitates);(b)e-CNC precipitates(inset:SAED pattern of e-CNC precipitates);and(c)e-CNC/HS precipitates(inset:zone 1 belongs to SAED pattern of HS precipitates;zone 2 belongs to SAED pattern of e-CNC precipitates)

Fig.5 SEM images of(a)e-CNC precipitates and(b)e-CNC/HS precipitates

4 Conclusions

This study illustrated a simple approach for the preparation of water-redispersible cellulose nanocrystals(CNCs)through adsorption of hydrolyzed sugars on their surface. Using this approach,amorphous hydrolyzed sugars were adsorbed on the surface of CNCs,inhibiting the formation of hydrogen bonds between the CNCs.Thus,dried CNCs were redispersed well in water.In addition,the size,Zeta potential,and thermal properties of CNCs were conserved after drying and water redispersion.Thus,this approach may enhance the use of hydrolyzed sugars in the hydrolysate from the corresponding CNC production,as well as facilitate the transportation and storage of CNCs.

Acknowledgments

This work was supported by the Foundation(No.202105)of Tianjin Key Laboratory of Pulp&Paper(Tianjin University of Science&Technology)and the Shaanxi University of Science and Technology Academic Leader Training Program(2013XSD25).