Modified Lignin with Anionic Surfactant and Its Application in

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Modified Lignin with Anionic Surfactant and its Application in Controlled Release of Avermectin Yuanyuan Li, Dongjie Yang, shuo Lu, sulin Lao, and Xueqing Qiu J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b00393 • Publication Date (Web): 13 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018

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Journal of Agricultural and Food Chemistry

Modified Lignin with Anionic Surfactant and its Application in Controlled Release of Avermectin Yuanyuan Li,† Dongjie Yang,*,† Shuo Lu,† Sulin Lao, † Xueqing Qiu*,†,‡ †

‡

School of Chemistry and Chemical Engineering and State Key Laboratory of Pulp

and Paper Engineering, South China University of Technology, 381 Wushan Road, Guangzhou, Guangdong 510640, People’s Republic of China 1

Abstract：：Alkali lignin (AL), an anionic polymer, is a by-product of the paper

2

industry. AL was first modified by quaternization to synthesize quaternized alkali

3

lignin (QAL). The aim of the present study is to reveal the effects of sodium dodecyl

4

benzenesulfonate (SDBS) on the microstructure of QAL. The interaction between

5

SDBS and QAL are studied by means of zeta potential, fluorescence

6

spectrophotometer and static contact angle measurement. The results indicated that

7

there are electrostatic interaction and hydrophobic interaction between QAL and

8

SDBS. SDBS/QAL complex can self-assembled into lignin-based colloidal spheres

9

(LCS) in an ethanol/water mixture, which have remarkable avermectin (AVM)

10

encapsulation efficiency and antiphotolysis performance. The cumulative release

11

amount of AVM encapsulated by LCS (LCS@AVM) after 72 h was 77% and the

12

release was still going on. The release behaviors of LCS@AVM can be controlled by

13

adjusting the ratio (w/w) of LCS to AVM. More than 85% of AVM could be preserved

14

even after 96 h of UV irradiation. LCS showed controlled release and UV-blocking

15

performance for AVM.

1 ACS Paragon Plus Environment


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16

Keywords: Quaternized alkali lignin, Sodium dodecyl benzenesulfonate, Colloidal

17

spheres, Controlled release, Anti-photolysis

18

Introduction

19

Lignin is the unique renewable aromatic polymer, as well as the second most

20

abundant component in plants.1 It is a three-dimensional amorphous polymer derived

21

from three different cinnamyl alcohol monomers：p-coumaryl alcohol, coniferyl

22

alcohol, and sinapyl alcohol.2,

23

byproduct of pulping and bio-refinery processes.4 Basic phenylpropane units and

24

other hydrophilic groups such as phenolic hydroxyl and carboxyl groups endow lignin

25

amphiphilic property. Lignin can be used as industrial dispersants,5, 6 carbon fiber,7

26

activated carbons8 and reinforcing agent.9 Essential properties of lignin are

27

biocompatibility,

28

resistance.13 Nevertheless, Lignin is mainly burnt in a recovery boiler to recover the

29

pulping chemicals and to obtain energy,14 which is a huge waste of lignin renewable

30

resources. High-value utilization of lignin is necessary and economical.

3

Commercially, lignin is usually obtained as a

biodegradability,

UV-blocking

property,10-12

and

oxidation

31

The application performance of lignin depends on its aggregation degree in bulk

32

solution. Lignin is a 3-dimensional, non-linear polymer, it is easy to form micro-sized

33

aggregates, which is seriously affected its application performance.15 Deng et al.16

34

indicated that the AL aggregation can be classified into two levels of aggregation: one

35

is the molecular aggregation of polymer chains because of van der Waals attraction,

36

and the other is π-π aggregation of the aromatic groups in lignin because of


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17

37

non-bonded orbital interaction. Li et al.

found that small amounts of straight chain

38

alcohols could promote the disaggregation of the sodium lignosulfonates aggregates.

39

The results indicate that straight chain alcohols can interact with the hydrophobic

40

region of sodium lignosulfonates aggregates, promoting the carboxyl groups, wrapped

41

inside the molecule, to be more effective. The negative potential of sodium

42

lignosulfonates molecules is increased and the aggregation degree of sodium

43

lignosulfonates in solution is reduced. Wang et al.18found that urea acted as hydrogen

44

bond breaker and was favorable to break the original inter- and intra-molecular

45

hydrogen bonds in lignin. What’s more, urea molecules could enter the inside of

46

lignin aggregates and form O-π structures with lignin, which significantly weakened

47

the π-π stackings between lignin molecules.

48

Zhou et al.19 found that cationic surfactants can change the aggregation

49

behavior of sodium lignosulphonate (SL) and improve its surface active. Mao and

50

Wu20 found that anionic surfactants have synergistic effects with lignin-based

51

surfactant in aqueous solution. Thus, oppositely charged surfactants have a great

52

influence on the solution behavior and aggregation behavior of lignin. Interactions

53

between the polymeric anionic/cationic surfactant and oppositely charged surfactants

54

have attracted a great deal of interest in the last decades, due to their importance both

55

in fundamental polymer physics/biophysic and in biological and industrial

56

application.21-24 Lignin is an anionic polymeric surfactant, which it contains many

57

functional groups such as aromatic ring, carboxyl group and phenolic hydroxyl group.



58

Li et al.25 modify sodium lignosulfonate with CTAB to prepare lignin-based

59

hydrophobic material SL-CTAB. SL-CTAB could encapsulate photosensitive

60

pesticide avermectin to prepare AVM@SL-CTAB microsphere, which has a high level

61

of drug loading and encapsulation efficiency. However, the mechanism for small

62

molecule surfactant to affect lignin-based polymer molecule structures has not been

63

investigated in detail.

64

Avermectin (AVM) is an effective insecticide and acaricide used extensively in

65

agriculture and animal husbandry.26 However, 16-membered ring macrolide structure

66

of AVM is sensitive to the irradiation of ultraviolet light, which results in its short

67

half-life and easily photo-oxidized.27 Overuse of AVM not only increases costs, but

68

also harm to plants and environment. To overcome this challenge, controlled release

69

AVM formulations have been a concern, various materials are used as carriers to

70

stabilize drug and confer them with controlled-release properties.28-30 It has been

71

proven that lignin is a good shell material for microcapsules due to their UV-blocking

72

property, high biocompatibility and biodegradable.25, 31

73

In the present work, we selected quaternary ammonium lignin (QAL) as

74

polymeric cationic surfactant and SDBS as anionic surfactant to investigate the

75

interaction between QAL and SDBS. On this basis, a series of lignin-based colloidal

76

spheres (LCS) were prepared by changing the mass ratio of SDBS/QAL. And the

77

effects of SDBS on the microstructure of QAL were further observed by means of

78

TEM, zeta potentiometer and particle size analyzer. LCS was applied as controlled


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release carriers and avermectin was used as a model pesticide to investigate the

80

controlled release and UV-blocking performance of LCS@AVM.

81

Materials and Methods

82

Materials. The pine alkali lignin was recovered from pulping black liquor

83

produced by Shuntai Technology Development Co., Ltd. (Hunan province, China),

84

and

85

3-chloro-2-hydroxypropyltrimethy lammonium chloride aqueous solution and sodium

86

dodecyl benzenesulfonate was purchased from Aladdin Corp. (Shanghai, China). The

87

original avermectin powder (AVM, 92%w/w) was kindly supplied by Noposion Co.,

88

Ltd. (Shenzhen, China). All other chemicals were of analytical grade including

89

ethanol (Guangdong Guanghua Sci-Tech Co. Ltd., China), sodium hydroxide

90

(Guangdong Guanghua Sci-Tech Co., Ltd., China) and sulfuric acid (Guangzhou

91

Chemical Reagent Factory, China).

was

separated

by

acidification

and

filtration

treatments.

60

wt%

92

Synthesis of QAL. The QAL was prepared according to our previous work.32

93

Briefly, 100mL of 20 wt% AL (pH=12) was heated to 85 oC with a stirring speed of

94

350 rpm, 16.67g 3-chloro-2-hydroxypropyltrimethylammonium chloride (60 wt%)

95

was added dropwise. Meanwhile, a certain amount of 20 wt% NaOH was added to

96

keep the solution above pH 12. And then, the reaction was conducted at 85 oC for 4 h

97

to obtained QAL solution. QAL solution was purified by using dialysis bag with a cut

98

off molecular weight of 1000 Da, and then dried in an ALPHA1-2 LD plus freeze

99

dryer (Christ Corp., Germany) after vacuum rotary evaporation. The content of



100

quaternary ammonium group was calculated from the contents of N (element), the

101

calculate method was described by Wartelle.33 The N content of alkali lignin is 0.19

102

wt%, after quaternization with 3-chloro-2-hydroxypropyltrimethy ammonium

103

chloride,

104

group in QAL is 1.35mmol/g.

the N content of QAL is 2.08 wt%, the content of quaternary ammonium

105

Preparation of AVM-loaded Lignin-based Colloidal Spheres. 60 mL of 1

106

mg/mL SDBS aqueous solution and 100 mL of 1 mg/mL QAL aqueous solution was

107

mixed and aged at 40 oC for 1 h until a precipitate was obtained. The precipitate was

108

isolated by centrifugation and dried to obtain SDBS/QAL complexes. 0.1 g AVM was

109

dissolved in 10 ml of ethanol solution. Then a certain amount of SDBS/QAL

110

complexes were added and placed in an ultrasonic bath for 10 min. 70ml of water was

111

dropwise added into the mixture via the peristaltic pump at a speed of 20 rpm to

112

obtain AVM-loaded colloidal spheres. The ethanol was recycled by rotary evaporation,

113

and the LCS@AVM were collected by centrifugation and freeze-dried. There

114

LCS@AVM samples were prepared based on the weight ratio of QAL/SDBS

115

complexes to AVM and named as LCS@AVM-1 (0.5:1), LCS@AVM-2 (1:1) and

116

LCS@AVM-3 (2:1), respectively.

117

Characterizations. The zeta potential of the SDBS/QAL mixing solution was

118

measured by zeta potential and particle size analyzer (type ZetaPlus, Brookhaven

119

Instruments Corp., USA). Ten parallel measurements were performed, and the mean

120

value was adopted.


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121

The surface tension was measured using a Wilhelmy plate with DCAT21

122

tensiometer (Dataphysics Instrument Co., Germany). Experimental errors inherent in

123

the measurement were ± 0.03 mN/m. The surface tension was determined as an

124

average value measured three times at 25 oC.

125

The contact angle of distilled water on QAL and SDBS/QAL complex disk was

126

measured by Power Each JC2000C1 static contact angle measurement instrument

127

(Shanghai zhongchen digital technic apparatus Co., Ltd., China). All the samples were

128

successively pressed at 10MPa for 1min to prepare disks before measurement.

129

Transmission electron microscope (TEM) images were obtained by using a

130

HITACHI H-7650 electron microscope with an accelerating voltage of 120 kV. The

131

TEM samples were prepared by dropping LCS dispersions onto the copper grids

132

coated with a thin carbon film and then dried under room temperature. No staining

133

treatment was performed for these measurements.

134

Dynamic light scattering (DLS) experiments were performed with a ZetaPALS

135

instrument (Brookhaven Instruments Co., America). All of the experiments were

136

performed at 25 oC.

137

Determination of the Loading Ability and Encapsulation Efficiency of

138

Lignin-based Colloidal Spheres. This analysis was based on the method described

139

by Deng 31. 10mg of LCS@AVM was dissolved in of 50 mL of methanol solution and

140

placed in an ultrasonic bath for 20 min. The resulting AVM extracts were filtered

141

organic membrane (0.45µm) and the concentration of AVM was determined by high



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142

performance liquid chromatography (HPLC, Agilent ZORBAX, Agilent, USA) with

143

isocratic elution of methanol–water (88/12, v/v) as the mobile phase. 20 µL of the

144

analyte was injected into the HPLC system and separated at 40 oC, using a flow rate

145

of 1mL·min-1 under the detection wavelength of 245 nm, where the maximum in the

146

UV spectra of avermectin is located. Three replicates were carried out for samples.

147

The AVM loading and encapsulation efficiency were respectively calculated

148

according to the following equations:

149

AVM loading (%) = (

150

Encapsulation efficiency (%) = (

@

) × 100

(1) ) × 100

(2)

151

Controlled Release of AVM. For this, samples containing the same amount of

152

AVM was suspended in 30 mL of water and then placed in a dialysis bag (Mw cutoff =

153

3000 Da). The bag was submerged in 120 mL of methanol/water (1/1, v/v) and then

154

placed in a shaking incubator at a stirring speed of 200 rpm at a constant 25 oC.

155

Periodically, 1 mL of the sample was collected, and the same volume of fresh

156

methanol-water solution was added to maintain the total volume. The concentrations

157

of AVM in the different samples were determined by HPLC.

158

Photodegradation studies. First, 1.6 mg of free AVM was dissolved in 10 mL

159

ethanol, and the AVM-load colloidal spheres containing the same amount of AVM

160

were dispersed in ultrapure water. Then poured them into 7.0 cm diameter Petri dishes

161

and dried into a thin film in dark conditions, respectively. The films were exposed to

162

UV light (emitted by a 30 W 310 nm lamp) at a distance of 30 cm. Samples were


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163

removed out at different intervals, the remaining AVM in the film was extracted by 50

164

mL methanol and detected by HPLC. Each sample was performed three times.

165

Results and Discussion

166

Effect on Zeta Potential and Surface Tension of QAL. QAL is a zwitterionic

167

surfactant and the isoelectric point is 7.5.34 When pH＜7.5，quaternary ammonium

168

groups imparted a positive charge to QAL, which behavior as a polymeric cationic

169

surfactant. In general, there exists a tendency that oppositely charged surfactants and

170

polymeric surfactant in aqueous solutions will interact with each other and formed

171

mixed aggregates.35 At pH 3, the zeta potential and phase behavior of the mixtures

172

will depend on the mixing ratio. The zeta potential of the SDBS/QAL mixing system

173

at different SDBS/QAL ratios (w/w) is determined, and the result is presented in

174

Figure 1a. With the increase of m(SDBS)/m(QAL), the zeta potential of SDBS/QAL

175

complex system gradually changes from positive to negative. When m (SDBS)/m

176

(QAL) reaches 0.6, the positive charge of QAL is almost neutralized by SDBS and the

177

zeta potential is zero. When m (SDBS)/m (QAL) is less than 0.6, QAL is excessive

178

and the solution is positively charged. For m(SDBS)/m(QAL) is greater than 0.6,

179

there is an excess of the SDBS and the charge of SDBS/QAL complexes are

180

dominated by the SDBS.

181

The amount of SDBS is small, the positive charge of the QAL is not completely

182

neutralized, and a part of the positive charge is exposed to the solution, so the

183

SDBS/QAL complex system still has the QAL’s charge. As the amount of the SDBS



184

increases, more SDBS molecules adsorb on the surfaces of QAL by electrostatic

185

interaction until the charges of the QAL are fully saturated by the SDBS. A further

186

increase in the amount of SDBS, the positive charge of QAL is not enough to binding

187

SDBS. The excess SDBS will form a micelle by hydrophobic interaction with SDBS

188

and bound to the surface of QAL.36 The SDBS/QAL complex system is negative

189

charged. The combined model is shown in the illustration of Figure 1(a).

190

QAL molecule contains both hydrophobic phenylpropane units and hydrophilic

191

groups, is a natural surfactant. From Figure 1(b) we can see that the surface tension

192

value of 1g/L QAL is 50.8mN/m. With the addition of SDBS (SDBS/QAL=1/10), the

193

surface tension reduces to 39.7 mN/m. By further addition of the SDBS amounts, the

194

surface tension values of the obtained SDBS/QAL tend to be constant, approximately

195

34 mN/m. These indicate that the SDBS/QAL have stronger ability reducing the

196

surface tension in air/ water interface.

197

On the one hand, SDBS can neutralize the net positive charge of QAL and

198

reduction of the total charge of QAL molecule. Therefore, the electrostatic repulsion

199

and the stretch degree of QAL molecule will be decreased. The amount of aggregates

200

adsorbed at the air/water interface is increased and the regular degree of the QAL

201

arraying at air/water interface is improved, thus cause lower surface tension. On the

202

other hand, the long hydrophobic chains of SDBS are introduced into the QAL

203

molecules, so the QAL molecules are more hydrophobic and have stronger aggregate

204

capacity at the air/water interface.


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Effect on the Wettability of QAL. When the surface charge is zero, the

206

hydrophilic groups of the QAL is fully neutralized by SDBS, it indicated that the

207

SDBS/QAL complex system has the strongest hydrophobicity. In order to prove this

208

conclusion, the static contact angles of water droplets on the disk of SDBS/QAL

209

complexes at different SDBS/QAL mass ratios was investigate, the results are shown

210

in Figure 2. The contact angle of deionized water on the QAL disk is 67° (Figure 2a).

211

With the addition of SDBS, the contact angle of deionized water on the SDBS/QAL

212

complex disk increase to 82°(Figure 2b). Keep increasing the amounts of SDBS, the

213

hydrophobicity of SDBS/QAL complex keeps increasing. When the mass ratio of

214

SDBS to QAL is 0.6, the water is hard to spread on SDBS/QAL complex disk, the

215

positive charge of QAL is fully neutralized by SDBS, so the formation of SDBS/QAL

216

complex with strongest hydrophobicity (Figure 2c). Since the SDBS and the QAL

217

have the opposite charge, there is a strong electrostatic attraction between them, so

218

that the SDBS gradually migrate to the QAL, the head group adsorbed on the QAL

219

chain, and the hydrophobic chain is excluded. Upon exceeding the charge

220

neutralization point with further increasing SDBS amounts, the hydrophobicity of

221

SDBS/QAL complex is decrease (Figure 2d). The additional SDBS then simply is

222

able to form the SDBS micelles, which has a hydrophobic core and hydrophilic shell,

223

so the hydrophobicity of SDBS/QAL complex is decrease. Table 1 shows the

224

elemental analysis data of SDBS/QAL complex at different SDBS/QAL mass ratios.

225

With the increasing amount of SDBS, the sulfur content is increased in SDBS/QAL



226

complex. These results indicated that SDBS bind to QAL tightly.

227

Effect on Fluorescence of QAL. Recent studies have shown that fluorescence

228

spectroscopy is a very sensitive method for studying aggregation and phase separation

229

behavior of polymer at the molecular level.37, 38 Our previous study has investigated

230

the aggregation behavior and self-assembly properties of lignin by fluorescence

231

spectroscopy. The results show that lignin form aggregates driven by the π−π

232

interaction of the aromatic groups in lignin.16 At pH 3, the QAL solution is positively

233

charged and the lignin has intrinsic fluorescence emission. With the increase of the

234

SDBS/QAL mass ratio, a sharp drop of the fluorescence intensity of the QAL

235

aggregates can be observed before charge neutralization point (SDBS/QAL < 0.6).

236

And the fluorescence intensity decrease to the minimum at charge neutralization point

237

(SDBS/QAL=0.6). However, by further addition of the SDBS amounts, the

238

fluorescence intensity gradually increased, as shown in Figure 3.

239

Ding et al. have shown that the complex-formation by electrostatic interaction

240

will lead to the fluorescence quenching.39 When the SDBS is added to the QAL

241

aqueous solution at low surfactant concentrations, the surfactant molecules are bind to

242

the positively charged QAL molecules by the electrostatic interaction, the lignin

243

fluorescence will be quenched. Further addition of surfactant, the additional SDBS

244

stabilize it colloidally, the intensity of fluorescence gradually increases. What’s more,

245

the addition of an opposite charged surfactant not only neutralizes the ionic charge but

246

also induce the aggregation behavior to change.40 Shirai et al. indicated that


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247

fluorescence quenching will be induced by intermolecular π-π interactions of the

248

chromophore and the degree of fluorescence quenching can be used as an indicator of

249

the intermolecular π-π interaction.41 With the increase of SDBS/QAL mass ratio, the

250

fluorescence of QAL was quenched, indicating that the π-π interaction between lignin

251

molecules was weakened. In addition, it is visible from the illustration of Figure 3 that

252

the emission wavelength is blue-shifted. According to preliminary results, the reason

253

for this change is that disaggregation of J-aggregation of the aromatic groups in

254

lignin.16 Continue to add SDBS, the excess SDBS to form micelle by the hydrophobic

255

interaction and the benzene rings near the surface of the aggregates close to each other.

256

There exists a conjugated system in the SDBS molecular, it can produce fluorescence.

257

Therefore, the fluorescence intensity increased with the increase of SDBS.

258

Effect on Microstructure of QAL. In order to visually detect the effect of

259

anionic surfactant on microstructure of QAL, the morphology of QAL and

260

SDBS/QAL complex under different conditions are observed by TEM. Figure 4a

261

shows the morphology of QAL at pH 3, which is a cluster-like aggregate. When

262

SDBS/QAL=0.6 (Figure 4b), the SDBS/QAL molecules are at charge neutralization

263

point. The aggregation is severely intensified because of the disappearance of the

264

electrostatic repulsion between SDBS/QAL aggregates. Then we dissolve the

265

aggregates in ethanol and the morphology is shown in Figure 4c. When water was

266

gradually added into the SDBS/QAL/ethanol solution, QAL and SDBS molecules

267

started to form of LCS, as shown in Figure 4d.



268

At pH 3, QAL are positively charged and has good solubility, but the

269

intermolecular π-π interactions between the benzene rings in lignin leads to the

270

formation of micro-sized aggregates. With the addition of SDBS, the SDBS

271

electrostatically bind to positively charged QAL molecules and formation of

272

SDBS/QAL complexes. In addition, the hydrophilic functional group on the surface of

273

QAL was neutralized by SDBS and the hydrophobicity of SDBS/QAL complex was

274

increased. Therefore, SDBS/QAL complex has good solubility in ethanol solution,

275

there is no obvious particles can be seen on the corresponding TEM images (Figure

276

4c). QAL insoluble in ethanol, whereas the SDBS/QAL complex with high solubility

277

in the same solvent. It was further indicated that SDBS changed the configuration of

278

QAL and promoted the disaggregation of QAL aggregates. When the water was added

279

to the SDBS/QAL/ethanol solution, the solvent gradually becomes a poor solvent for

280

the SDBS/QAL molecules. Due to the hydrophobic interaction, the SDBS/QAL

281

molecules gradually associate to form LCS.

282

Effect on Zeta Potential of Colloidal Spheres. From Figure 1 we can see that

283

the zeta potential of SDBS/QAL complexes is varied by changing the mass ratio of

284

SDBS/QAL. The SDBS/QAL complex was dissolved in ethanol and the QAL

285

aggregates were disaggregated. According to the previous studies, it was found that

286

the configuration of SDBS/QAL complex was changed in the formation of LCS. In

287

order to check whether or not the surface charges of the LCS were changed, the LCS

288

were prepared based on the different SDBS/QAL mass ratio and the surface charges


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289

were also determined. The results are shown in Figure 5.

290

From Figure 5, with the increase of the SDBS/QAL mass ratio, the positive

291

charge of LCS is weakened. It is worth noting that when the mass ratio of SDBS/QAL

292

is greater than 0.6, that is, the SDBS/QAL complex is negatively charged, while the

293

formed LCS are still positively charged. At pH 3, QAL easily forms micro-sized

294

aggregates through π-π interactions among the aromatic rings in lignin, there are a lot

295

of quaternary ammonium groups in the core of QAL aggregate. When a small amount

296

of SDBS was added to the QAL solution, SDBS was only adsorbed on the surface of

297

the QAL aggregates by electrostatic interaction. When an excess amount of SDBS

298

was added, a part of SDBS is adsorbed on the surface of QAL aggregate through

299

hydrophobic interaction and electrostatic interaction, and the other part of them start

300

to form free SDBS micelle. The free micelle was present in the supernatant of

301

SDBS/QAL complex and will be discarded by centrifugation. Combined with the

302

previous experimental results, it can be found that SDBS can promote the

303

disaggregation of QAL aggregates. After the SDBS/QAL complex is dissolved in

304

ethanol, QAL was disaggregated and quaternary ammonium group in the core of

305

aggregates was exposed. At this time, the number of SDBS is insufficient to neutralize

306

the positive charge of QAL, so the formation of the colloidal spheres is positively

307

charged.

308

Effect on the Size of Colloidal Spheres. The dynamic light scattering (DLS)

309

instrument was used to study the effects of the mass ratio of SDBS/QAL on the size



310

changes of LCS, the results are shown in Figure 6. The size of colloidal spheres

311

increases with the increasing mass ratio of SDBS/QAL. The effect of m (SDBS)/m

312

(QAL) ratio on the morphology of LCS was further observed by transmission electron

313

microscopy (TEM). From Figure 7 we can see that the size of LCS increases with the

314

increase of the mass ratio of SDBS/QAL, which is consistent with the result of DLS.

315

Small molecular surfactants can promote disaggregation of lignin, which

316

promoting the quaternary ammonium cation groups, wrapped inside the molecule, to

317

be more effective. For the m(SDBS)/m(QAL) is less than 0.6, only the SDBS

318

adsorbed on the surface of QAL aggregates by electrostatic interaction is involved in

319

the formation of LCS. At this time, the size of the LCS is smaller. When the mass

320

ratio of m (SDBS)/m (QAL) is larger than 0.6, a part of SDBS is adsorbed on the

321

surface of QAL by electrostatic interaction, and the other part is accumulated around

322

the hydrophobic core of QAL by hydrophobic action. The SDBS/QAL complex was

323

dissolved in ethanol, QAL was disaggregated into small aggregates and the quaternary

324

ammonium group was exposed. The SDBS accumulated around QAL by hydrophobic

325

interaction tends to associate with the exposed quaternary ammonium group with the

326

addition of water. The amount of SDBS participated in the formation of micelle

327

increases, resulting in an increase in the size of the LCS. Table 2 shows the elemental

328

analysis data of colloidal spheres at different SDBS/QAL ratios. The sulfur content in

329

the colloidal sphere increases with an increase of the mass ratio of SDBS/QAL, which

330

indicated that the large amount of SDBS incorporated into colloidal sphere, thus


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331

forming large colloidal spheres.

332

Combined with the foregoing conclusions, a model is proposed concerning the

333

effect of SDBS on the microstructure of QAL (Figure 8). In the first stage, QAL is

334

aggregated in aqueous solution due to the presence of π-π interaction between the

335

benzene rings in the lignin (Figure 8a). After the addition of SDBS, the SDBS is

336

adsorbed on the QAL molecules by electrostatic and hydrophobic interactions. SDBS

337

and QAL have opposite charge, there is a strong electrostatic attraction between them.

338

With the addition of SDBS, the positive polar head-group migrates toward the QAL,

339

and the hydrophobic tail chain is excluded. By further addition of the SDBS amounts,

340

the excess SDBS to form free micelle, the driving force of this stage is the

341

hydrophobic interaction between the tailings of the surfactant. In the next stage, the

342

SDBS/QAL complex was dissolved in ethanol, the QAL aggregate was disaggregated

343

and the quaternary ammonium groups inside the aggregates were exposed (Figure 8c).

344

At the same time, the excess SDBS was dissolved in the ethanol solution. When water

345

is added to the SDBS/QAL/ethanol solution, the SDBS is bound to the quaternary

346

ammonium group in lignin molecules. The more the amount of SDBS in ethanol, the

347

more SDBS bond on the QAL by electrostatic interaction. With the addition of water,

348

the SDBS/QAL molecules associate to form LCS by hydrophobic interaction (Figure

349

8d). The size of LCS increases with increasing amount of SDBS in the SDBS/QAL

350

complex.

351

Controlled Release of AVM from LCS. Since the SDBS and QAL can form



352

complex by the electrostatic interaction and hydrophobic interaction, and this

353

complex further self-assembled into LCS in an ethanol/water mixture. The colloidal

354

sphere with hydrophobic core and hydrophilic shell, its special self-assembly behavior

355

is expected to be used for encapsulate hydrophobic pesticides. Lignin has a

356

three-dimensional network structure, which can control the drug release. In addition,

357

lignin has excellent UV blocking performance, lignin-based colloidal spheres were

358

applied to encapsulate photosensitive pesticide avermectin (AVM). Figure 9 is typical

359

TEM images of LCS and LCS@AVM-2 (1:1). It can be seen that the particle size is

360

increased after encapsulating AVM. The AVM loading ability and encapsulation

361

efficiency of the LCS are shown in Table 3.

362

The release behavior of AVM from LCS@AVM was investigated in 50%

363

methanol/water mixture at 25 oC. Figure 10 shows their release kinetics. The release

364

rate of pure AVM very rapidly and 94.75% of AVM was released after 72 h.

365

Comparatively, the release rate of AVM from LCS@AVM was very slow and the

366

release of AVM from LCS@AVM was still going on after 72 h. The release rate after

367

72 h would become more slowly. What’s more, the release behaviors of LCS@AVM

368

can be controlled by adjusting the mass ratios of QAL/SDBS complex and AVM.

369

LCS@AVM-1 with the lowest mass ratio of QAL/SDBS complex and AVM (1︰2),

370

which has a thinner shell and higher loading efficiency. The release rate of

371

LCS@AVM-1 is higher than that of other LCS@AVM. The cumulative release

372

amount of AVM from LCS@AVM-1 colloidal spheres within 72h was 77%, and the


Page 18 of 40

Page 19 of 40


373

release process was still continuing. With the increasing amount of QAL/SDBS

374

complex, the loadings gradually decreased, the drug release rate slowed down.

375

Photodegradation Studies. The efficiency of the AVM and LCS@AVM against

376

photodegradation was investigated by HPLC. The anti-photolysis performance is

377

characterized by the percentage of remaining AVM versus irradiation time, as

378

presented in Figure 11. Obviously, AVM was almost decomposed and only 8% was

379

retained after 96 h of UV irradiation. By contrast, the active ingredient of AVM in

380

LCS@AVM colloidal spheres was kept well. What’s more, with the change of

381

QAL/SDBS ratio, the decomposition rate of AVM loaded in LCS@AVM colloidal

382

spheres did not change significantly and more than 85% of AVM could be preserved

383

even after 96 h of UV irradiation. The results demonstrate that the lignin-based

384

colloidal spheres exhibited excellent UV-blocking properties.

385

In conclusion，The zeta potential and static contact angle test results indicate that

386

small amounts of SDBS can interact with the QAL by electrostatic interaction. For

387

excess of SDBS addition, there are electrostatic interaction and hydrophobic

388

interaction between SDBS and QAL. The fluorescence and TEM results show that the

389

SDBS facilitate the disaggregation of QAL aggregates, promoting the quaternary

390

ammonium groups, wrapped inside the molecules, to be more effective. SDBS/QAL

391

complex can form regular LCS by self-assembly in ethanol/water mixture. With

392

increasing addition amount of SDBS, the particle size of LCS was increased, and the

393

zeta potential was decreased. LCS can effectively encapsulate the hydrophobic drug



394

AVM. LCS@AVM exhibited excellent controlled-release behavior and anti-photolysis

395

ability. This system shows good application prospects for using green bioresources in

396

the controlled release of photosensitive pesticide, such as organophosphorus and

397

organochlorine pesticide.

398

Funding

399

This work was financially supported by the National Natural Science Foundation

400

of China (NSFC) (Nos. 21436004, 21576106) and Natural Science Foundation of

401

Guangdong (2017A030308012).

402

References

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Figure Captions Figure 1. (a) Zeta potential and (b) surface tension of the complex at fixed concentration (1g/L) with different SDBS/QAL mixing ratios (w/w) at pH 3. Figure 2. Wettability of the water droplet on the (a) QAL, (b) m(SDBS)/m(QAL)=0.3, (c) m(SDBS)/m(QAL)=0.6 and (d) m(SDBS)/m(QAL)=1. Figure 3. PL spectra of QAL in water solutions (100 mg/L, λex = 350 nm) with different SDBS/QAL mixing ratios at pH 3. Figure 4. TEM images of (a)QAL, (b)m(SDBS)/(QAL)=0.6 complex, (c) m(SDBS)/m(QAL) =0.6 complex solution in ethanol (d) colloidal spheres from m(SDBS)/m(QAL) =0.6 complex. Figure 5. The effect of m(SDBS)/m(QAL) on the zeta potential of colloidal spheres. Figure 6 The effect of m (SDBS)/m (QAL) on the size distribution of LCS. Figure 7 TEM images of LCS with different m (SDBS)/m (QAL) mixing ratios at fixed initial concentration (1g/L). m(SDBS)/m(QAL) mixing ratios: (a) 0.3, (b) 0.6, and (c)1.0. Figure 8. Proposed model of the effect of SDBS on the microstructures of QAL. Figure 9. Typical TEM image of (a) LCS and (b) LCS@AVM-2 (1:1). Figure 10. Release curves of AVM and LCS@AVM in 50% methanol/water mixture. Figure 11. Percentage of AVM remaining in LCS@AVM at different UV irradiation times.



Page 26 of 40

Table 1 Elemental of complex at different SDBS/QAL mass ratios m(SDBS)/m(QAL) 0.3 0.6 1.0

C 61.98 62.93 63.33

Element content (wt%) H O N 7.62 24.93 1.73 7.87 23.35 1.72 7.98 22.65 1.72


S 3.75 4.13 4.32

Page 27 of 40


Table 2 Elemental of colloidal spheres at different SDBS/QAL ratios m(SDBS)/m(QAL) 0.3 0.6 1.0

C 45.78 45.46 46.48

Element content (wt%) H O N 6.20 39.90 1.44 6.15 40.09 1.42 6.49 38.39 1.45


S 6.68 6.88 7.19


Table 3 AVM loading ability and encapsulation efficiency Sample LCS@AVM-1 (0.5:1) LCS@AVM-2 (1:1) LCS@AVM-3 (2:1)

AVM loading 63.19% 51.38% 42.34%

Encapsulation efficiency 85.31% 93.84% 96.25%


Page 28 of 40

Page 29 of 40


Figure graphics -30

54

a Surface Tension(mN/m)

Lignin

Zeta potential (mV)

b

52

-20 -10 0 Lignin

Lignin

10 20

50 48 46 44 42 40 38 36 34

30

32 0.0

0.2

0.4

0.6

0.8

1.0

0.0

1.2

0.2

0.4

0.6

0.8

1.0

m(SDBS)/m(AML)

m(SDBS)/m(AML)

Figure 1. (a) Zeta potential and (b) surface tension of the complex at fixed concentration (1g/L) with different SDBS/QAL mixing ratios (w/w) at pH 3.



Figure 2. Wettability of the water droplet on the (a) QAL, (b) m(SDBS)/m(QAL)=0.3, (c) m(SDBS)/m(QAL)=0.6 and (d) m(SDBS)/m(QAL)=1.


Page 30 of 40

Intensity (a.u.)


Intensity (a.u.)

Page 31 of 40

400 450 500 550 600 650 Wavenumber (nm)

400

450

500

QAL SDBS/QAL=0.2 SDBS/QAL=0.4 SDBS/QAL=0.6 SDBS/QAL=0.8 SDBS/QAL=1.6 SDBS/QAL=3.2 SDBS/QAL=4.0

550

600

650

Wavenumber (nm)

Figure 3. PL spectra of QAL in water solutions (100 mg/L, λex = 350 nm) with different SDBS/QAL mixing ratios at pH 3



Figure 4. TEM images of (a)QAL,(b)m(SDBS)/(QAL)=0.6 complex, (c) m(SDBS)/m(QAL) =0.6 complex solution in ethanol (d) colloidal spheres from m(SDBS)/m(QAL) =0.6 complex.


Page 32 of 40


8

Zeta potential (mv)

Page 33 of 40

6

4

2

0 0.3

0.6

1.0

m(SDBS)/m(QAL)

Figure 5. The effect of m(SDBS)/m(QAL) on the zeta potential of colloidal spheres.



1.0

SDBS/AML=0.3 SDBS/AML=0.6 SDBS/AML=1.0

Intensity(a.u)

0.8 0.6 0.4 0.2 0.0 100

1000

Rh(nm)

Figure 6 The effect of m (SDBS)/m (QAL) on the size distribution of LCS.


Page 34 of 40

Page 35 of 40


Figure 7 TEM images of LCS with different m (SDBS)/m (QAL) mixing ratios at fixed initial concentration (1g/L). m(SDBS)/m(QAL) mixing ratios: (a) 0.3, (b) 0.6, and (c)1.0.



Figure 8. Proposed model of the effect of SDBS on the microstructures of QAL.


Page 36 of 40

Page 37 of 40


Figure 9. Typical TEM image of (a) LCS and (b) LCS@AVM-2 (1:1).


Cumulative Relaese of AVM (%)


Page 38 of 40

100 80 60

AVM LCS@AVM-1 (0.5:1) LCS@AVM-2 (1:1) LCS@AVM-3 (2:1)

40 20 0 0

50

100

150

200

Release Time (h)

Figure 10. Release curves of AVM and LCS@AVM in 50% methanol/water mixture.


Page 39 of 40


Avermectin Remaining (%)

100 80

LCS@AVM-3 (2:1) LCS@AVM-2 (1:1) LCS@AVM-1 (0.5:1) AVM

60 40 20 0 0

20

40

60

80

100

Irradiation Time (h)

Figure 11. Percentage of AVM remaining in LCS@AVM at different UV irradiation time.



Graphic for table of contents


Page 40 of 40

Modified Lignin with Anionic Surfactant and Its Application in

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