Sodium Dodecyl Sulfate Composite


Nov 20, 2017 - Pure LSNP dispersion could be obtained after removing free SDS by dialysis. SDS is an anionic surfactant, which has low toxicity to hum...
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Preparation of Lignin/SDS Composite Nanoparticles and Its Application in Pickering Emulsion Template based Microencapsulation Yuxia Pang, Shengwen Wang, Xueqing Qiu, Yanling Luo, Hongming Lou, and Jinhao Huang J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03784 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017

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Journal of Agricultural and Food Chemistry

Preparation of Lignin/SDS Composite Nanoparticles and Its Application in Pickering Emulsion Template based Microencapsulation Yuxia Pang,† Shengwen Wang,† Xueqing Qiu,†, ‡, * Yanling Luo,† Hongming Lou,†, ‡, *

and Jinhao Huang.†

† School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, China. ‡ State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China. * Corresponding authors: Dr. Xueqing Qiu School of Chemistry and Chemical Engineering South China University of Technology Wu Shan Road, Guangzhou 510640, China E-mail: [email protected] Dr. Hongming Lou School of Chemistry and Chemical Engineering South China University of Technology Wu Shan Road, Guangzhou 510640, China E-mail: [email protected]

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ABSTRACT

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Lignin is a vastly underutilized biomass resource. The preparation of water dispersed

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lignin nanoparticles is an effective way to realize the high-value utilization of lignin.

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However, the currently reported preparation methods of lignin nanoparticles still have

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some drawbacks, such as the requirement for toxic organic solvent or chemical

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modification, complicated operation process and poor dispersibility. Here, lignin/SDS

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composite nanoparticles (LSNPs) with outstanding water dispersibility and a size

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range of 70-200 nm were facilely prepared via acidifying the mixed basic solution of

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alkaline lignin and sodium dodecyl sulfate (SDS). No harsh chemical was needed.

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The formation mechanism was systematically studied. Results indicated that the

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LSNPs were obtained by acid precipitation of the mixed micelles formed by the

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self-assembly of lignin and SDS. In addition, based on the LSNPs-stabilized

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Pickering emulsions, lignin/polyurea composite microcapsules combining the

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excellent chemical stability of synthetic polyurea shell with the fantastic

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antiphotolysis and antioxidant properties of lignin were successfully prepared.

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Keywords: lignin, nanoparticle, Pickering emulsion, microcapsules

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INTRODUCTION

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Lignin is the most abundant aromatic natural polymer on the earth. Unfortunately, due

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to the complexity of the lignin structure with extremely broad molecular weight

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distributions and poor water solubility,1 lignin finds very limited application in certain

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areas, such as surfactant, coloring agent, dyes, adhesives, polymer additives, and other

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uses.2-5 Lignin nanoparticles dispersed in the neutral or acidic water were found to be

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a multifunctional material which have many potential applications such as adsorbing

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heavy metal ions in waste water,6 stabilizing Pickering emulsions,7-9 and

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strengthening phenolic foam.10 Thus the preparation of lignin nanoparticles is an

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effective way to realize the high value utilization of lignin and has attracted

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tremendous attention recently.7, 11, 12

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In recent years, many researchers have introduced several lignin nanoparticle

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preparation methods. Solvent-based processes are by far the most common methods

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used to prepare lignin nanoparticles, which mainly include solvent exchange

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precipitation11,

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method for the preparation of colloidally stable lignin nanoparticles through dialysis.13

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Ago et al. prepared lignin particles via aerosol flow reactor.8 Bian et al. successfully

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prepared lignin nanoparticles by simply diluting the spent lignin p-toluenesulfonic

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acid liquor.18, 19 Meanwhile, researchers also prepared lignin particles by chemical

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polymerization

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microemulsions.21 Besides, Gilca et al. obtained lignin nanoparticles by sonication.22

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Among those reported lignin nanoparticle preparation methods, toxic or harmful

13-19

and spray drying.8 For instance, Lievonen et al. proposed a

such

as

suspension

polymerization20 and cross-linking

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organic solvents, chemical modification on lignin or strict operation condition were

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needed. Meanwhile, due to the low surface charge, the lignin nanoparticles prepared

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by some reported methods were prone to aggregate after long time standing. The

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research by Stewart et al. revealed that the addition of sodium dodecyl sulfate (SDS)

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or cetyltrimethylammonium bromide (CTAB) had a dramatic effect on improving the

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stability and reducing the size of lignin microparticles.23 However, the relatively high

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concentration of surfactants in the system may limits the further application of the

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lignin microparticles.

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Alkaline lignin, recovered from the pulping black liquor, has a significant solubility in

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aqueous solutions at high pH (above pH 10), however, in acidic conditions, alkaline

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lignin becomes insoluble and micron aggregations are formed. Here, in this work,

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lignin/SDS composite nanoparticles (LSNPs) were facilely prepared via acidifying the

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mixed basic solution of alkaline lignin and sodium dodecyl sulfate (SDS) under the

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absence of any toxic or harmful organic solvents which was frequently used in other

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methods. The LSNPs dispersed stably in water by the electrostatic repulsion between

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sulfates. Pure LSNP dispersion could be obtained after removing the free SDS by

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dialysis. SDS is an anionic surfactant which is low toxic to humans and is commonly

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used in household detergents, personal care products and pharmaceuticals due to their

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ability to decrease the interfacial tension.24,

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remained in the LSNPs after dialysis. Therefore, this novel LSNPs possess the

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potential of being used in some high value-added field like cosmetic and food

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industry.

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Only a small amount of SDS was

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Thanks to the special performance of antiphotolysis and antioxidant devoted by the

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polyphenolic structure of lignin,26-28 successful utilization of lignin to embed

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avermectin has been reported in literatures and shown a good antiphotolysis

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performance.29, 30 Besides, it is worth noting that based on the concept of Pickering

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emulsion template, biopolymer based microcapsules has been successfully prepared

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through ionic cross-linking recently.31, 32 The polyanionic biopolymers can be gelled

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by ionic cross-linking with divalent cations (e.g., Ca2+) at room temperature to form

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an insoluble matrix, this provides an efficient method for microencapsulation.

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However, due to the permeability of the porous hydrogel-type microstructure, it

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remains very challenging for encapsulation of some volatile small molecules by

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biopolymer-based shell materials, in particular for aldehydes, ketones, and other

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phytochemicals used as fragrances, flavors, or insect repellents.33, 34 To protect the

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liquid core, capsules with polyurea or urea formaldehyde shell materials have

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previously been shown to provide good stability and mechanical characteristics.35, 36

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As an application of the novel LSNPs in this work, the LSNPs were successfully used

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to stabilize Pickering emulsions. Besides, based on the LSNPs-stabilized Pickering

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emulsions, we prepared lignin/polyurea

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combining a strong chemical shell formed by synthetic polyurea with the excellent

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antiphotolysis and antioxidant properties of lignin. The LSNPs adsorbed at the O/W

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interface of Pickering emulsion droplets were ionically cross-linked with the

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positively charged ethylenediamine hydrochloride (EH) ions to form the lignin shell

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and the polyurea shell was formed by interfacial polymerization reaction between

composite microcapsules (LPMCs)

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isophorone diisocyanate (IPDI) and ethylenediamine (EDA). The microcapsules were

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characterized using a particle size analyzer, optical microscopy, confocal laser

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scanning microscopy (CLSM), and field emission scanning electron microscopy.

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Taking the excellent chemical stability of synthetic polyurea shell with the fantastic

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antiphotolysis and antioxidant properties of lignin into account, these LPMCs are

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expected to be a good microcontainer for some photosensitive or oxygen sensitive

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small molecules.

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In a word, stable and nontoxic LSNPs were facilely prepared via acidifying the mixed

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basic solution of alkaline lignin and SDS. Finally, oil-in-water Pickering emulsion and

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LPMCs were successfully prepared with the prepared LSNPs.

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MATERIALS AND METHODS

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Materials

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Alkaline lignin was derived by first acidifying the black liquor provided by

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Xiangjiang Paper Industry Co., Ltd. (Hunan, China) and then purified by filtration

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with ultrapure water. The functional group and molecular weight data of alkaline

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lignin according to non-aqueous potential titration and GPC respectively are given in

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Table 1. Ethylenediamine (EDA), isophorone diisocyanate (IPDI), Sodium dodecyl

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sulfate (SDS) were purchased from energy chemical (China). Hydrochloric acid (HCl),

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Sodium hydroxide (NaOH), and cyclohexane were bought from Guangzhou Chemical

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Factory (China). All the reagents were analytic grade and used without further

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purification. Ultrapure water was used throughout the study.

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Preparation of lignin/SDS composite nanoparticles (LSNPs) 6

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LSNPs were successfully prepared using a method based on acid precipitation, it can

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be briefly described as follows: Firstly, 1 g alkaline lignin was added into 100 ml

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ultrapure water, and the pH of the lignin dispersion was adjusted to 11 using 4 M

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NaOH under stirring. The resulting lignin solution (1wt%, pH 11) was stirred

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overnight to ensure complete dissolution of lignin and was filtered in order to remove

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the impurities before usage. Subsequently, SDS was added in amounts that lead to

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concentrations of SDS in the final solution of 0, 4, 6, 8, 10, and 12 mM to the 100 ml

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lignin solution with a magnetic stirring for 30 min. Finally, the LSNPs were formed

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by adjusting the pH of the mixed solution to 2.5 with 1 M HCl under stirring. After

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standing overnight, in order to remove the inorganic salts like sodium chloride and the

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free SDS, the prepared LSNP dispersion was introduced into a dialysis bag

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(MWCO=10 kDa) which was then immersed in excess ultrapure water (periodically

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replaced) for 5d. The mean particle size and zeta potential of LSNPs before and after

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dialysis were determined and the sulfur content of LSNPs after dialysis were

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analyzed.

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Preparation of Lignin Particles (LPs)

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1 g alkaline lignin was added into 100 ml ultrapure water, and the pH of the lignin

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dispersion was adjusted to 11 using 4 M NaOH under stirring. The resulting lignin

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solution was stirred overnight to ensure complete dissolution of lignin and was

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filtered in order to remove the impurities before usage. LPs were then prepared by

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directly acidifying the lignin solution to a pH of 2.5 with 1 M HCl under stirring.

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Effects of SDS on LPs 7

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SDS was added in amounts that lead to concentrations of SDS in the final LP

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dispersion of 0, 4, 6, 8, 10, and 12 mM to the above LP dispersion with a magnetic

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stirring for 30 min to ensure the added SDS were completely dissolved. After standing

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overnight, the resulting dispersion were subjected to the same dialysis process as the

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LSNP dispersion. The mean particle size and zeta potential of LPs before and after

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dialysis were determined and the sulfur content of LPs after dialysis were analyzed.

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The Gelation of LSNPs

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10 ml of the dialysed LSNP dispersion prepared at an initial SDS concentration of

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12mM were transferred to a 30 ml sample bottle, and an excess of EH solution

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(prepared by adding HCl to the 1M EDA solution to a pH of 2.5.) was added to the

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LSNP dispersion with a magnetic stirring. After standing for half an hour, the particle

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size distribution of the LSNP aggregations was determined by dynamic light

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scattering measurements.

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Preparation of LSNPs-stabilized Pickering Emulsions (LSNPEs)

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The lignin concentration of the dialysed LSNP dispersion (≈0.6 wt%, pH≈3.5)

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prepared at an initial SDS concentration of 12 mM was adjusted to 0.5% by adding

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water. The resulted LSNP dispersion was used as the aqueous phase, cyclohexane

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containing 10wt% IPDI as the oil phase was added to the aqueous phase and the

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system was emulsified at 11000 rpm for 3 min with a homogenizer (IKA Ultra Turrax

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T25 basic instrument) to obtain a LSNPs stable Pickering emulsions. The volume

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fraction of cyclohexane was fixed in 10% (v/v).

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Preparation of lignin/polyurea composite microcapsules (LPMCs) 8

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The above prepared emulsion was transferred into a two-necked flask, heated in a 35 ℃

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water bath and slowly stirred at 200 rpm with a stir bar, 15 ml of 1.5 M EDA solution

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was added to the system drop by drop by using a peristaltic pump in 1 h. The reaction

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was conducted for 6 h, and the LPMCs were efficiently formed.

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Characterizations

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The mean particle size of the LSNPs and LPs and the particle size distribution of the

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LSNP aggregations were determined by dynamic light scattering measurements at 25 ℃

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using a particle size analyzer (Zetasizer Nano-ZS, Malvern Instruments, UK), whereas

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the zeta potential of the LSNPs and LPs were measured at 25 ℃ using the same

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instrument based on the laser Doppler microelectrophoresis technique. The pH of all

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samples were adjusted to 2.5 by adding HCl before determination.

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LSNPs were observed by transmission electron microscopy (TEM, JEM-100CX.

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Japan). The sample was prepared by dropping the LSNP dispersion to a carbon-coated

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copper grid followed by dried at room temperature.

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The sulfur on the pure alkaline lignin, LSNPs and LPs after dialysis were analyzed by

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a high frequency infrared carbon−sulfur analyzer (QL-HW2000B, QILIN, China.).

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All the samples were freeze-dried before test.

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The size distributions of LSNPEs and LPMCs were studied using a particle size

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analyzer (Mastersizer 2000, Malvern Instruments. UK) equipped with a Hydro EV

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wet dispersion unit.

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Optical microscopy images of the LSNPEs and microcapsules were recorded by a

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Polarizing Microscope (BM-5000P, China.) equipped with a camera. 9

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The morphological characteristics of microcapsules were observed by a confocal laser

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scanning microscopy (Leica TCS-SP5) and field emission scanning electron

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microscopy (SEM, LEO1530VP).

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RESULTS AND DISCUSSION

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Preparation and Formation Mechanism of lignin/SDS composite nanoparticles

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(LSNPs)

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This work has developed a simple technology for the preparation of lignin/SDS

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composite nanoparticle dispersion dispersion. To illustrate the formation mechanism

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of LSNPs, LSNPs and LPs were prepared separately, and the effects of SDS and

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dialysis process on their particle size and zeta potential were compared. LSNPs were

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successfully prepared by adjusting the pH of the lignin/SDS mixed solution from 11

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to a value of 2.5. Figure 1a showed the phenomenon of preparing LSNPs from

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lignin/SDS mixed basic solution with a SDS concentration of 12 mM, the dialysed

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LSNP dispersion after a long-time standing was still similar to the homogeneous

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lignin/SDS mixed basic solution. Similarly, LPs were successfully prepared by

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adjusting the pH of the lignin solution from 11 to 2.5. Figure 1b showed that the LPs

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aggregated and precipitated after 10 minutes of standing. Subsequently, the addition

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of SDS with the amount equal to that in the Figure 1a leading particles to become

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dispersed to some extent. However, after dialysis process, LPs aggregated and

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precipitated again.

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The effect of the concentration of SDS added before acid precipitation on the mean

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particle size and zeta potential of LSNPs are shown in Figure 2a and Figure 2b, 10

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respectively. For the pre-dialysis series, the sample with no SDS had a relatively large

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mean particle size of 1.8 µm, the particle size was significantly decreased with the

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increasing SDS concentration, and the minimum size was about 120 nm at a SDS

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concentration of 12 mM. The absolute value of zeta potential of the dispersion at each

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SDS concentration is significantly higher than that of the sample without SDS and

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decreases with the increasing SDS concentration. Meanwhile, for the dialysed series,

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the particle size of LSNPs at each SDS concentration was smaller than that of the

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pre-dialysis samples and decreased with the increasing SDS concentration, the

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minimum particle size of the dialysed LSNPs was about 70 nm at a SDS

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concentration of 12 mM. The absolute value of zeta potential of the dialysed

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dispersion at each SDS concentration was also higher than that of the pre-dialysis

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samples and increases with the increasing SDS concentration.

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The effect of the concentration of SDS added after acid precipitation on the mean

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particle size and zeta potential of LPs are shown in Figure 2c and Figure 2d,

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respectively. As can be seen, SDS added to the LP dispersion also has a significant

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effect on reducing the mean size and increasing the zeta potential of the LPs, and the

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LPs before dialysis possessed a relatively small particle size. Nevertheless, after

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dialysis process, the particle size and zeta potential at each SDS concentration

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dramatically changed to a value close to that of the sample without SDS. These results

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indicate that the SDS added after acid precipitation could be removed by dialysis.

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The sulfur content of the LSNPs and LPs after dialysis are given in Table 2. With the

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increase in SDS concentration, the sulfur content of LSNPs increased from 1.42wt% 11

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to 1.61wt%. While, the sulfur content of LPs varied little and were still close to the

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pure lignin (1.13wt%, analyzed by the same carbon−sulfur analyzer.). The relatively

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higher sulfur content of LSNPs indicated that a certain amount of SDS was retained in

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the LSNPs after dialysis and the weak change in sulfur content of LPs revealed that

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the SDS added after acid precipitation has been removed during dialysis indeed.

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Based on the sulfur content of pure lignin (1.13wt%) and the theoretical sulfur content

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of SDS (11.1wt%), the composite ratio of SDS in LSNPs could be calculated by a

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system of linear equations in two unknowns. Here, we use the molar amount of SDS

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in 1 g of lignin to represent the composite ratio. The results are shown in Table 1. The

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relatively low SDS composite ratio and pretty good stability of LSNPs is conducive to

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its high value utilization.

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The above results showed that the addition order of SDS has a significant effect on the

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mean size of the particles after dialysis. The co-dissolving of SDS with lignin is a

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necessary process for the preparation of LSNPs. Besides, lignin is known to possess

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intriguing intrinsic π-stacking and aggregation properties, as well as to present various

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hydrophobic interactions.37-39 Therefore, we suggest a hypothesis as shown in Figure

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3 on the formation mechanism of LSNPs. In the lignin/SDS mixed basic solution, the

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strong electrostatic repulsion caused by the ionized phenolic hydroxyl and carboxyl

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makes the network structure of lignin macromolecules stretched and dissolved in

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water. Under the drive of hydrophobic force, through self-assembly, the hydrophobic

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long carbon chain in SDS molecule and the stretched network structure of lignin

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macromolecules interweave together and form a mixed micelle. In this situation, when 12

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reducing the pH of the mixed solution by adding HCl, the ionized phenolic hydroxyl

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and carboxyl groups are protonated which leading to a reduction in electrostatic

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repulsion among the molecular skeleton. Subsequently, the hydrophobic interactions

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and π-π interactions drive the lignin molecular skeleton to contract and clamp the

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adsorbed SDS to form LSNPs. The intramolecular and intermolecular hydrogen

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bonding further enhance the tightness of the formed LSNPs. The SDS is a strong

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electrolyte whose sulfate radicals are still completely ionized even in a low pH

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environment, thus providing the LSNPs a large number of surface charge which

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prevent the formation of micron aggregations.

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Since the electrostatic force between ions is related to the ionic strength in the

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solution, with a high ionic strength caused by a large amount of free SDS and NaCl

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produced during the acid precipitation process, the surface negative charge of the

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LSNPs in the pre-dialysis sample is partially shielded and the absolute value of zeta

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potential is lower than the dialysed sample, thus leading to the aggregation of

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particles.

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On the other hand, SDS added after acid precipitation will adsorb on the surface of the

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formed lignin particles and result in more dispersive lignin particles, but these SDS

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weakly adsorbed on the surface were different from the SDS added before acid

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precipitation and will diffused to the outside of the dialysis bag during dialysis. After

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dialysis, the lignin particles aggregate together to form larger particles, and the

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absolute value of zeta potential of the dispersion decreases to a value close to that of

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the sample without SDS. 13

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The dialysed LSNPs prepared at an initial SDS concentration of 12 mM were studied

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by TEM, the result indicated a number-average particle diameter of 40 nm (shown in

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Figure 4). Nevertheless, dynamic light scattering (DLS) studies of highly dilute

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aqueous dispersion (0.025wt %) of the same LSNP dispersion indicated their

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intensity-average diameter was about 70 nm rather than 40 nm, the difference of the

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results should be ascribed to the different testing conditions between TEM and DLS,

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the nanoparticles were swollen in the aqueous media for DLS while the hydrophobic

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chain of SDS tend to collapse onto the lignin core during the drying process of the

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TEM sample. Besides, the difference can also be attributed to the nonspherical shape

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of the particles, because DLS measures hydrodynamic particle size.

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Because of the convenient operation, we purified the residual SDS and salt by dialysis

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in this work. It is worth noting that dialysis is not practical for large scale production.

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Fortunately, in industrial production, we can use ion exchange resin to remove

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residual SDS and salt, the residual SDS and salt can be completely purified after the

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LSNP dispersion was passed through the strong-acid cation exchange resin and

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strong-basic anion exchange resin in turn. The particle size of gel type ion exchange

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resin is 400-600 µm, the pore size of gel type ion exchange resin is 0.5~5 nm, and

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the particle size of LSNP before purification is about 200 nm, thus the LSNP

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dispersion can flow smoothly through the resin column. We have successfully purified

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the SDS and salt by ion exchange resin, the resulting LSNPs has the same particle size

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and sulfur content as the one purified by dialysis.

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Characterization of LSNP Gel 14

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As shown in Figure 5a, an excess ethylenediamine hydrochloride (EH) solution

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(prepared by adding HCl to the 1M EDA solution to a pH of 2.5, which would

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undergo the reaction shown in Figure 5c) was added to the LSNP dispersion, and after

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10 minutes of standing it was observed that the LSNP gel formed. The result of

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dynamic light scattering measurements (Figure 5b) showed that the size of the LSNP

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aggregations was around 5 µm which is evidently larger than that of the acid

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precipitated lignin particles. This result indicated that by exploiting the electrostatic

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attractions between the negatively charged sulfate on LSNPs and the positively

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charged EH ions, the LSNPs could be gelled by ionic cross-linking with EH ions as

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shown in Figure 5d.

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Preparation of LSNPs-stabilized Pickering Emulsions (LSNPEs)

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Unlike the traditional nonionic or ionic surfactant-based emulsions, Pickering

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emulsions are stabilized by solid particles at the interface between the dispersed phase

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and the continuous phase.40,

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successfully prepared by emulsifying the mixture system of the oil phase

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(cyclohexane containing 10wt% IPDI) and the aqueous phase (LSNP dispersion,

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0.5wt%, pH=2.5) with a homogenizer at 11000 rpm for 3 min. Figure 6b is a typical

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optical microscopy images of the emulsions. The emulsion droplets of

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cyclohexane-in-water were spherical. Neither obvious droplet size change nor any

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continuous oil layer was observed for the LSNPEs stored for more than 60 days

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(Figure 6a,c) which means that the LSNPs with the hydrophobic lignin core and the

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hydrophilic sulfate is a good emulsifier. Both the formation of a steric barrier by the

41

Here, LSNPEs at 10% (v/v) oil loading were

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hydrophobic lignin core and an electrostatic repulsion provided by the composed SDS

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contributed to the remarkable stability of the emulsion. Despite of great stability, the

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emulsions exhibited relatively fast creaming over a period ranging from about 1 h due

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to the great buoyant forces.

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Characterization of lignin/polyurea composite microcapsules (LPMCs)

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LPMCs, based on the LSNPEs template, were prepared by following the procedure

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displayed in Figure 7. Pickering emulsions were prepared by the method described

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above. The EDA added to the Pickering emulsion firstly reacted with HCl in the

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aqueous phase as the way shown in Figure 5c to form ethylenediamine hydrochloride

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(EH). And as a result, the LSNPs were cross-linked by EH ions at the O/W interface

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as the way shown in Figure 5d thus forming the lignin microcapsules (LMCs).

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Meanwhile, the decrease in the repulsion forces permitted the deposition of

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un-adsorbed LSNPs on the outer wall of the lignin microcapsules. Once the HCl in the

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aqueous phase was consumed, with the subsequent addition of EDA, the interfacial

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reaction between EDA and IPDI was conducted to form a layer of polyurea. Finally,

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LPMCs were generated. Since the isocyanate groups of IPDI at the oil–water interface

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not only react with EDA but can also reacted with the hydroxyl group in lignin

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molecules and be hydrolyzed by water to amine groups, also urethane linkages can be

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found in the capsule shell.

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The microcapsules were observed with optical microscope, confocal laser scanning

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microscope (CLSM) and scanning electron microscopy (SEM), the results are shown

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in Figure 8a-c. Based on the images of SEM and optical microscopy, the 16

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microcapsules were spherical in shape and possessed a double layer structure of rough

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outer layer and relatively smooth inner layer. Taking advantage of the

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autofluorescence of lignin, CLSM observation were conducted to confirm the

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existence of lignin on these microcapsules, and the obvious fluorescence indicated

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that the lignin aggregations were located at the outer surface of the microcapsules.

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The SEM images showed the lignin layer of the microcapsules was severely cracked,

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this may be caused by the drying process during the SEM sample preparation. Based

332

on the double-layer structure of the microcapsules and the fantastic antiphotolysis and

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antioxidant properties of lignin, it is expected that the properties of the microcapsules

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like

335

photodegradation and oxidative degradation will increase significantly compared to

336

the conventional polyurea microcapsules.

controlling

release

and

protecting

the

embedding

materials

against

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In addition, the result of the size distribution (see Figure 9) indicated that the

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volume-average diameter of the microcapsules was larger than the emulsion droplets

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and showed a bimodal distribution. There are several reasons contributed to this

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phenomenon. Firstly, the positively charged EH ions produced during the

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microencapsulation process may have affected the stability of the emulsion stabilized

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with the negatively charged LSNPs, causing the merging of the droplets thus forming

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a larger droplets and further forming a larger microcapsules.42,

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composite shell formed on the surface of the droplets increased the size of the droplets.

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Thirdly, in the presence of EH ions, some formed LSNP aggregations with a smaller

346

size than microcapsules leading to the bimodal size distribution. 17

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Secondly, the

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In summary, a new kind of LSNPs were facilely prepared by acidifying the mixed

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basic solution of alkaline lignin and SDS, LSNPs with different mean particle size can

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be prepared by changing the initial concentration of SDS in the mixed solution. The

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relatively smaller particle size, higher absolute value of zeta potential and sulfur

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content of LSNPs after dialysis leaded to the conclusion that the co-dissolving of SDS

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with lignin is a necessary process for the preparation of LSNPs. During the acidifying

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process, the lignin molecular skeleton in the mixed micelle of lignin and SDS

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contracted and clamped the adsorbed SDS to form LSNPs. This method can be termed

355

“the physical sulfation of alkaline lignin”. The LSNPs could be gelled by ionic

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cross-linking with EH ions. To the best of our knowledge, this is the first report about

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this novel LSNPs. As a proof of possible uses of the LSNPs, they were shown to be

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effective in stabilizing O/W Pickering emulsions, and could be gelled by ionic

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cross-linking with divalent cations. Spherical LPMCs covered with a large amount of

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lignin on the surface were successfully prepared by interfacial polymerization. Thanks

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to the excellent chemical stability of synthetic polyurea shell with the fantastic

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antiphotolysis and antioxidant properties of lignin, these LPMCs possess the potential

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of being a good microcontainer for some photosensitive or oxygen sensitive small

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molecules.

365

Acknowledgment

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The authors acknowledge the financial supports of the National Natural Science

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Foundation of China (21436004, 21676109) and Fundamental Research Funds for the

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Central Universities (2015ZZ120).

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FIGURE CAPTIONS Figure 1 (a) Illustration of the preparation of LSNPs from lignin/SDS mixed basic solution; (b) Illustration of the preparation of LPs from lignin basic solution and the effect of SDS and dialysis on LPs. Figure 2 Effect of the concentration of SDS added before acid precipitation on (a) the mean particle sizes and (b) zeta potential of LSNPs; Effect of the concentration of SDS added after acid precipitation on (c) the mean particle sizes and (d) zeta potential of LPs. Figure 3 Illustration of the formation mechanism of LSNPs. Figure 4 Typical TEM images of LSNPs prepared at an initial SDS concentration of 12mM. Figure 5 (a) Illustration of the gelation of LSNPs; (b) Size distribution of the LSNP aggregations; (c) Reaction between EDA and HCl; (d) Reaction between LSNPs and EH ions. Figure 6 (a) Digital camera photo of the LSNPEs after a 60-day storage period; (b) Optical microscopy images of the LSNPEs at day 0; (c) Size distribution of the LSNPEs at day 0 and after a storage period of 60 days. Figure 7 Illustration of the preparation of LPMCs. Figure 8 (a1-a2) Optical microscopy images of the LPMCs; (b1-b2) Confocal micrograph of the LPMCs; (c1-c2) SEM images of the LPMCs. Figure 9 Size distribution of the LPMCs and LSNPEs.

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Table 1 Functional group and molecular weight data of alkaline lignin according to non-aqueous potential titration and GPC respectively. Phenolic OH (mM/g) 2.18

Carboxylic (mM/g) 2.00

Mw (Da) 4500

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Mn (Da) 2470

Mw/Mn (PDI) 1.82

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Table 2 The sulfur content on the LSNPs and LPs after dialysis and the composite ratio of SDS on LSNPs SDS concentration (mM) 8 10 12

Sulfur content Sulfur content of LPs (wt%) of LSNPs (wt%) 1.12 1.42 1.15 1.50 1.17 1.61

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Composite ratio of LSNPs (mM/g) 0.104 0.129 0.167

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Figure 1 (a) Illustration of the preparation of LSNPs from lignin/SDS mixed basic solution; (b) Illustration of the preparation of LPs from lignin basic solution and the effect of SDS and dialysis on LPs. (This figure should be published in color.) 254x118mm (300 x 300 DPI)

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Figure 2 Effect of the concentration of SDS added before acid precipitation on (a) the mean particle sizes and (b) zeta potential of LSNPs; Effect of the concentration of SDS added after acid precipitation on (c) the mean particle sizes and (d) zeta potential of LPs. (This figure should be published in color.) 44x31mm (600 x 600 DPI)

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Figure 3 Illustration of the formation mechanism of LSNPs. (This figure should be published in color.) 254x91mm (300 x 300 DPI)

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Figure 4 Typical TEM images of LSNPs prepared at an initial SDS concentration of 12mM. 63x31mm (300 x 300 DPI)

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Figure 5 (a) Illustration of the gelation of LSNPs; (b) Size distribution of the LSNP aggregations; (c) Reaction between EDA and HCl; (d) Reaction between LSNPs and EH ions. (This figure should be published in color.) 254x220mm (300 x 300 DPI)

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Figure 6 (a) Digital camera photo of the LSNPEs after a 60-day storage period; (b) Optical microscopy images of the LSNPEs at day 0; (c) Size distribution of the LSNPEs at day 0 and after a storage period of 60 days. (This figure should be published in color.) 254x95mm (300 x 300 DPI)

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Figure 7 Illustration of the preparation of LPMCs. (This figure should be published in color.) 254x89mm (300 x 300 DPI)

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Figure 8 (a1-a2) Optical microscopy images of the LPMCs; (b1-b2) Confocal micrograph of the LPMCs; (c1c2) SEM images of the LPMCs. (This figure should be published in color.) 254x287mm (300 x 300 DPI)

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Figure 9 Size distribution of the LPMCs and LSNPEs. (This figure should be published in color.) 44x31mm (600 x 600 DPI)

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Graphic for table of contents (This figure should be published in color.) 254x129mm (300 x 300 DPI)

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