Insight into the dispersive mechanism of Carboxylated Nanofibrilllated cellulose for individual montmorillonite in water

Journal Pre-proof Insight into the dispersive mechanism of Carboxylated Nanofibrilllated cellulose for individual montmorillonite in water Chuan Sun, Zhiqiang Fang, Famei Qin, Kaihuang Chen, Jingyu Wang, Zixian Ding, Xueqing Qiu PII:

S1359-8368(19)33238-X

DOI:

https://doi.org/10.1016/j.compositesb.2019.107399

Reference:

JCOMB 107399

To appear in:

Composites Part B

Received Date: 8 July 2019 Revised Date:

26 August 2019

Accepted Date: 27 August 2019

Please cite this article as: Sun C, Fang Z, Qin F, Chen K, Wang J, Ding Z, Qiu X, Insight into the dispersive mechanism of Carboxylated Nanofibrilllated cellulose for individual montmorillonite in water, Composites Part B (2019), doi: https://doi.org/10.1016/j.compositesb.2019.107399. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

SYNOPSIS: Illuminate the disperse mechanism of Carboxylated Nanofibrilllated as a green dispersant dispersing individual Montmorillonite at different dosage by AFM.

Insight into the Dispersive Mechanism of Carboxylated Nanofibrilllated Cellulose for Individual Montmorillonite in Water Chuan Suna, Zhiqiang Fangb,c,*, Famei Qina, Kaihuang Chena, Jingyu Wanga, Zixian Dinga, Xueqing Qiua,b,* a

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China

b

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China c Laboratory of Pulp and Paper Science & Technology of Ministry of Education/Shandong Province, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China

Abstract: Carboxylated nanofibrillated cellulose (CNFC) has emerged as a promising green dispersant to prepare stable aqueous individual montmorillonite (MMT) suspensions. Nevertheless, its underlying dispersive mechanism remains elusive. Herein, we attempt to unveil the dispersive mechanism of CNFC for individual MMTs in water by characterizing the interfacial interactions between the two components using a quartz crystal microbalance with dissipation monitoring (QCM-D) and an atomic force microscopy (AFM). Both electrostatic repulsion and steric hindrance contribute to the excellent stability of homogeneous individual MMT suspensions, and their individual contributions are dependent upon the dosage of CNFC dispersant. The electrostatic repulsive forces dominate over the van der Waals forces that trigger the aggregation of aqueous individual MMTs when the dosage of CNFCs is 2 wt% (based on individual MMTs). With increasing dosage of CNFC dispersant, the electrostatic repulsive forces between individual MMTs tend to be level off while the steric hindrance gradually becomes a dominant factor that influences the dispersion stability of aqueous individual MMT suspensions. Finally, the effect of the CNFC dispersant’s dosage on the optical and mechanical properties of nanocomposite film made with CNFCs and CNFC-dispersed individual MMTs is investigated. Understanding the dispersing principle of CNFCs for individual MMTs

1

in water could pave the way to extend the applications of MMT in numerous value-added fields such as high-performance nanocomposites and flexible electronics.

Keywords: Carboxylated nanofibrillated cellulose; Individual montmorillonite; Dispersive mechanism; Interfacial interactions; Green dispersant

2

1

1. Introduction

2

Montmorillonite (MMT) has considered an environmentally friendly natural

3

material for numerous high-tech applications (e.g., high-performance nanocomposites,

4

catalysis, water treatment, electronic devices, and biomedicine) due to its earth

5

abundance, geometric platelet shape, ionic exchange character, and biocompatibility

6

[1-3]. Exfoliating layered MMTs into their fundamental units (individual MMTs) with

7

an average thickness of 1 nm is a viable approach to completely unlock their potential

8

capability such as catalytic and adsorption efficiency, and superior mechanical

9

properties [4-6]. However, individual MMTs are prone to aggregate in aqueous

10

solutions because of inherent van der Waals, and/or ionic bond interactions etc.,[7]

11

which significantly limits the utilization of their unique physical properties.

12

An effective dispersion strategy to suppress the strong self-aggregation tendency

13

of individual MMTs in water involves the use of low cost and easy processing

14

dispersants (e.g., sodium hexametaphosphate [8], sodium polyacrylate [9] and sodium

15

polyphosphate [10]). Nevertheless, there are several issues raised by those dispersants:

16

(1) some dispersants derived from petroleum-based chemicals are hazardous to

17

environment and human’s healthcare; (2) the introduction of dispersants may

18

deteriorate the interfacial interactions between MMTs and other substances in some

19

cases [8-10].

20

Thanks to amphiphilic characteristics and tunable surface chemistry and fibril

21

morphology [11], carboxylated nanofibirllated cellulose (CNFC) derived from 2, 2, 6,

22

6-tetramethylpiperidine-1-oxyl radical (TEMPO)-mediated oxidization has recently

23

emerged as an effective and green dispersant to disperse inorganic nanomaterial (e. g.

24

carbon nanomaterials [12-14], clay nanoplatelets [15], hexagonal boron nitride (BN) 3

25

[16,17] and molybdenum disulfide (MoS2))[16] in water. As a consequence,

26

homogeneous dispersions with superior colloidal stability were prepared to use in the

27

design of advanced functional materials for electronics and energy storage devices.

28

Our group previously demonstrated a homogeneous and stable individual MMT

29

suspension in an aqueous solution by using CNFC as a dispersant, assisted by

30

ultrasonic treatment. Such stable colloidal suspension enabled the formation of highly

31

ordered structure of CNFC-individual MMT films that presented a total light

32

transmittance of 90% at 600 nm even when the loading weight of MMTs reached 50

33

wt %.

34

Although CNFC confers aqueous individual MMT suspension with excellent

35

colloidal stability, seldom endeavor has been devoted to exploring its dispersive

36

mechanism for individual MMT suspensions. In this work, CNFC derived from

37

TEMPO-oxidized system was used as an effective and green dispersant to promote

38

the colloidal stability of aqueous individual MMT suspension and its underlying

39

dispersive mechanism was comprehensively explored by characterizing the interfacial

40

interactions between CNFC and MMTs using a quartz crystal microbalance with

41

dissipation monitoring (QCM-D) and an atomic force microscopy (AFM).

42

2. Materials and methods

43

2.1. Materials

44

Hardwood pulp (Hongta paper Co,.Ltd, Zhuhai) was used as raw material for the

45

preparation of carboxylated nanofibrillated cellulose (CNFC) by TEMPO oxidization

46

system. MMT powder was purchased from Nanocor Co,. Ltd, USA. TEMPO (98%,

47

AR) was purchased from Macklin Inc., Sodium hypochlorite solution (active chlorine

48

≥7.5%，AR) and sodium bromide were purchased from GuangZhou chemical reagent 4

49

co. Ltd, China, and DaMao (Tianjin) chemical reagent co. Ltd, China, respectively.

50

2.2. Preparation of CNFC and aqueous individual MMT suspensions

51

CNFC was prepared according to our previous publication [18]. Aqueous

52

individual MMT suspensions were prepared as follows: 1g MMT powder was

53

blended with CNFC at different dosage using 100 mL deionized water as solvent.

54

After that, the MMT suspension was stirred at 1000 rpm for 10 min, followed by

55

ultrasonification using an ultrasonic Cell Crusher (Ningbo Scientz Biotechnology Co,.

56

Ltd) for 10 min. finally, the treated MMT suspension was centrifuged at 5000 rpm for

57

20 min immediately to obtain aqueous individual MMT suspensions.

58

2.3. Characterization of aqueous individual MMT suspensions

59

The yield of individual MMT suspensions was measured by ash method. Firstly,

60

the CNFC-dispersed individual MMT suspension (m0) was dried at 105

for 4 h to

61

oven dry weight (m1), which was then burned in Muffle Furnace at 800

for 2 h to

62

remove the mass of CNFC, the weight of the residual mass of MMT was denoted as

63

m2. Finally, the yield was calculated according to equation 1:

64

୫

௠

y = 100 ୫భ × ௠మ × 100% బ

(1)

భ

65

The colloidal stability of aqueous individual MMT suspensions was measured by

66

a Turbiscan Lab analyzer (Formulaction Co., L’Union, France) for 5 h, where the

67

final Turbiscan stability index (TSI) was calculated by the light passed through and

68

back scattered of the suspension.

69

The size and thickness of the individual MMT were characterized by an atomic

70

force microscope imaging (XE-100, Park Systems, Korea).

71

2.4. AFM colloidal probe technique and substrate modification

72

The modified CNFC-coating SiO2 and MMT-coating SiO2 spheres were

73

prepared by layer-by-layer self-assembly as follows: SiO2 standard spheres (23µm) 5

74

were used as base spheres, CNFCs and individual MMTs were adhered to the surface

75

of SiO2 by silane coupling agent and poly dimethyl diallyl ammonium chloride

76

（PDAC）respectively. Then, the modified spheres were attached on the tipless probe

77

at the end of cantilever by using hot-melt adhesive. (AFM tipless probe (NP-O10,

78

Bruker Inc., Germany) with nominal cantilever spring constant of 0.12 N/m was used

79

in our experiment.

80

The force measurement substrate was obtained by layer-by-layer self-assembly

81

of PDAC-MMT on SiO2 substrate, and its morphology and roughness were

82

characterized by AFM imaging. The specific AFM force measurement and calculation

83

details were shown in supporting information.

84

2.5. QCM-D measurement

85

The adsorption behavior between CNFC and individual MMTs was further

86

confirmed by using a QCM-D (Q-Sense E1 instrument, Biolin Scientific, Sweden).

87

QCM-D crystal sensor was modified by layer-by-layer self-assembly of PADC and

88

individual MMTs, and its morphology was characterized by AFM imaging. The

89

specific QCM-D measurement and calculation details were shown in supporting

90

information.

91

3. Result and discussion

92

3.1. The dispersion capability of CNFC for MMTs

93

To understand the dispersion capability of CNFC for MMTs in water, three

94

samples with/without a dispersant were prepared. Note that the dispersion capability

95

of CNFC for MMTs is evaluated in terms of the yield of individual MMTs in water

96

and its colloidal stability. Control sample means dispersant-free individual MMT

97

suspension, while CNFC or carboxymethyl cellulose (CMC) was used as a dispersant

98

in the other two samples. Fig. 1a and b show the visual appearance of three prepared 6

99

samples before and after sitting for one week. In comparison to control sample with a

100

yield of 48.5%, both CMC and CNFC have a positive effect on the yield of individual

101

MMTs, showing a higher dispersion yield of 58.1% and 56.3%, respectively. After

102

sitting for 1 week, the yield of control sample decreases from 48.5% to 40.8%.

103

However, for CNFC-dispersed suspension, only a slight change in dispersion yield is

104

observed, decreasing from 56.3% to 54.8%. There is an obvious phase separation for

105

CMC-dispersed suspension due to the aggregate of individual MMTs in water (Fig.

106

1b). CNFC is, therefore, an effective dispersant for the preparation of individual

107

MMT suspension with desired dispersion yield and colloidal stability.

108

Additionally, the effect of the CNFC’s dosages on the colloidal stability of

109

aqueous individual MMT suspensions is quantified by a turbiscan lab analyzer. As the

110

dosage of CNFCs increases from 0 wt % to 50 wt %, the TSI (turbiscan stability index)

111

of aqueous individual MMT suspensions presents a declining tendency, which

112

indicates an increase in colloidal stability (Fig. 1c). Specifically, the CNFC has a

113

negligible effect on the stability of individual MMT suspension when its dosage is 2

114

wt %. An enhanced stability is achieved for aqueous suspensions with CNFC dosages

115

of 6 wt%, 10 wt%, and 15 wt%, but their corresponding TSI-time curves are almost

116

overlapping. When the CNFCs reach 50 wt%, the individual MMT suspension

117

exhibits the best colloidal stability. Interestingly, the growth trend in yields and

118

colloidal stability of aqueous individual MMT suspensions is quite similar with

119

increasing amount of CNFCs (Fig. 1d).

7

120 121

Fig. 1. Visual appearance of aqueous individual MMT dispersions with/without a

122

dispersant (CMC or CNFC) before (a) and after (b) sitting for 1 week. Note that the

123

dosage of CNFC or CMC is approximately 10 wt% (based on raw MMTs), the

124

number in pictures indicates the yield of individual MMT suspensions.

125

CNFC-dispersed individual MMT suspension exhibits better colloidal stability over

126

sample with CMC dispersant and control sample. (c) TSI-time curves and (d) yields

127

of individual MMT suspensions at different dosages (based on raw MMTs) of CNFC

128

dispersant.

129 130

In addition to improving the dispersion yield and colloidal stability, the addition

131

of CNFCs has the ability to protect individual MMT from dramatic fragmentation

132

during ultrasonic treatment. As we can see from the atomic force microscopy (AFM)

133

images in Fig. 2, CNFC-dispersed individual MMTs show a larger size than

134

dispersant-free control sample. The average size of individual MMTs without CNFCs

135

was approximately 85.4 nm measured by particle diameter analysis (Fig. 2a).

136

However, the individual MMTs with 6 wt% and 15 wt% CNFC dispersant exhibit an

137

average size of 181.3 nm (Fig. 2b) and 228.3 nm (Fig. 2c), respectively. The 8

138

improved average size of CNFC-dispersed individual MMTs is primarily ascribed to

139

the effective absorption of ultrasonication power by CNFCs.

140 141

Fig. 2. AFM height images of individual MMTs prepared with different dosage of

142

CNFC. (a) 0 wt%, (b) 6 wt%, and (c) 15 wt%.

143 144

3.2. Interfacial interaction between MMTs and CNFCs

145

To understand the underlying mechanism for the excellent dispersing capacity of

146

CNFCs for individual MMTs, the interfacial interactions between the CNFCs and the

147

individual MMTs were characterized comprehensively by using a QCM-D and an

148

AFM.

149

We initially investigated the affinity between the CNFCs and the individual

150

MMTs. Fig. 3a displays the AFM image of CNFC-dispersed individual MMTs. As we

151

can see from the AFM image that a small part of CNFCs could adsorb on the surface

152

of individual MMTs (yellow circles), and their affinity is further determined by a

153

QCM-D. Fig. 3b shows the adsorption behaviors of CNFC suspensions with different

154

concentrations on MMT film. As the concentration of CNFC dispersion increases 9

155

from 0.1 g/L to 0.75 g/L, an increase in adsorption capacity of CNFC on MMT (from

156

3.33 mg/cm2 to 4.20 mg/cm2) is observed. There is a strong adsorption between the

157

two components and increasing with the concentration of CNFC, indicating that Van

158

der Waals force and hydrogen bonds are stronger than repulsion interaction.

159

The adsorption and desorption behaviors of individual MMTs on CNFC film

160

were also performed by QCM-D analysis to further characterize the affinity between

161

the two components. We can see from Fig. 3c that △F2 (the change in frequency

162

after desorption) is almost equal to 1/2 △ F1 (the change in frequency after

163

adsorption). However, the change in frequency of MMTs adsorbed on CMC is equal

164

to its desorption change (Fig. S1). The difference between the △F1 and the △F2

165

suggests that there are stronger interfacial interactions between the individual MMTs

166

and the CNFCs in water.

167

An AFM was then utilized to investigate the surface morphology and roughness

168

of crystal sensor with an active top thin Au film for QCM-D analysis before and after

169

adsorption. Fig. 3d displays the surface morphology of thin Au film on crystal sensor,

170

with a surface roughness (Rq) of approximately 0.9 nm. After grafting individual

171

MMTs to PDAC-modified Au film, a uniform thin layer of individual MMT film was

172

deposited on the thin Au film (Fig. 3e), showing a Rq of lower than 5 nm over a 5 × 5

173

µm2 scanning area. After a saturation adsorption process using CNFC dispersion as a

174

flowing phase, a dense CNFC film fully covers on the individual MMT film (Fig. 3f).

175

And the dense structure of the CNFCs adsorbed on MMT film is similar to that of the

176

neat CNFC film (Fig. S2), suggesting a thin film of CNFC film was formed on MMT

177

film due to their strong interfacial interaction.

10

178 179

Fig. 3. The characterizations of the affinity between CNFCs and MMTs by QCM-D

180

and AFM analysis. (a) AFM height image of CNFC-dispersed individual MMTs.

181

Yellow circles suggest the adsorption of CNFC on the surface of MMTs. (b)

182

Adsorption behaviors of CNFC suspensions with different concentrations on MMT

183

film. (c) Adsorption and desorption of individual MMT suspension (0.1g/L) on CNFC

184

film. AFM images of (d) bare Au surface on crystal sensor, (e) individual MMT film

185

deposited on PDAC-modified Au surface, and (f) CNFC film adsorbed on the

186

aforementioned individual MMT film.

187 188

The strong adsorption of CNFCs by the individual MMTs in water is primarily

189

due to the hydrogen bonds formed between the hydroxyl groups in CNFC and the

190

negatively charged O atoms in MMT. To verify our hypothesis, QCM-D test and AFM

191

colloidal technique were adopted to investigate the interfacial interactions between the

192

CNFCs and the MMTs in pure water or urea solution.

193

Fig. 4a is a schematic drawing of the AFM measurement instrument including a

194

colloidal sphere probe modified with RSiX3 and CNFCs and a flat SiO2 substrate

195

modified with monolayer MMT surface. Urea is a well-known hydrogen-bond

196

breaker that has the ability to shield the hydrogen bonds between the CNFCs and the 11

197

individual MMTs. Herein, urea (1 mol/L) was added in CNFC suspension to reduce

198

the hydrogen bonds between the CNFCs and the individual MMTs. According to the

199

QCM-D characterization in Fig. 4b, the adsorption capacity of CNFC by individual

200

MMTs in urea solution is much smaller than that of CNFCs by individual MMTs in

201

pure water. In addition, AFM force measurement was applied to measure the

202

hydrogen bonding interaction between the two components. The adhesive force

203

between the CNFCs and the individual MMTs in 1 mol/L urea solution declines from

204

1.21 mN/m to 0.44 mN/m (Fig. 4c).

205

In conclusion, according to QCM-D analysis and AFM force measurements, the

206

hydrogen bonding interactions significantly contribute to the strong adsorption

207

between the individual MMTs and the CNFCs. The adsorption of CNFCs on MMTs

208

increases the electrostatic repulsive forces among individual MMTs, which lays the

209

foundation for obtaining stable colloidal suspensions of individual MMTs in water.

210 211

Fig. 4. (a) Schematic drawing (not to scale) of the colloidal sphere probe covered with

212

bilayer of RSiX3 and CNFCs and the flat MMT surface used for AFM force

213

measurements. (b) Adsorption of CNFCs on MMTs with/without urea. (c) 12

214

Force-distance curves between the CNFCs and the individual MMTs in pure water

215

and 1mol/L urea solution.

216 217

3.3. Dispersive mechanism of CNFCs for individual MMTs in water

218

To the best of our knowledge, the underlying mechanism for stabilizing

219

individual MMTs in water by CNFC dispersant remains ambiguous. Herein AFM

220

force measurement with colloidal probe technique was adopted to explore the

221

interaction mechanism from the molecular perspective [19-23]. Fig. 5a shows the

222

schematic of AFM for the measurement of force/distance (F/D) curves, which

223

includes a colloidal sphere probe first decorated with a thin layer of cationic PDAC,

224

followed by depositing individual MMTs, and an individual MMT modified substrate.

225

Generally, the AFM F/D curve contains trace curve and retrace curve. Fig. 5b is

226

a typical F/D curve between a MMT decorated probe and a MMT modified substrate.

227

In trace process, the total repulsive force increases gradually and then achieves the

228

maximum value when the MMT decorated probe approaches to the MMT modified

229

substrate. Herein we measured the approaching F/D curves in CNFC dispersions.

230

To explore the dispersion mechanism of CNFCs for individual MMTs，the

231

electrostatic and steric repulsions should be analyzed separately and individually.

232

Commonly, if just electrostatic force and van der Waals force exist in the dispersion

233

system,

234

Derjaguin-Landau-Verwey-Overbeek (DLVO) theory ( F(D)ୈ୐୚୓ = F(D)୚ୈ୛ +

235

F(D)୉ୈ୐; Where D represents the distance between colloidal probe and measurement

236

substrate, F(D)DLVO denotes DLVO force, F(D)VDW is van der Waals attraction force

237

while F(D)EDL represents electrostatic double layer repulsion force. As shown in Fig.

238

5c, the F/D curve is well agreement with DLVO theory fitting as the probe is

239

approaching the substrate. Nevertheless, a notable derivation between the measured

F/D

curve

could

be

13

well

fitted

by

classic

240

data and the fitting curves appears as the distance between the probe and the substrate

241

is less than 15 nm (Fig. 5c), which is ascribed to steric repulsion that is not considered

242

in DLVO theory.

243

After considering the influence of steric repulsion, the DLVO theory is modified

244

to DLVO-s theory ( F(D)ୈ୐୚୓ିୗ = F(D)୚ୈ୛ + F(D)୉ୈ୐ + F(D)ୗ (where F(D)S

245

represents steric repulsion force), and the DLVO-S fitting curve coincides well to

246

experiment data in the whole range. The individual proportion of electrostatic and

247

steric repulsion is as a function of distance (Fig. 5d). The electrostatic repulsion

248

belongs to long-range force that gradually decreases with the increase of distance

249

between the individual MMTs, while the steric resistance is the dominant repulsion at

250

shorter range, which rapidly decreases with increasing distance.

251 252

Fig. 5. (a) Schematic drawing (not to scale) of the colloidal sphere probe covered with

253

bilayer of PDAC and MMTs and the flat MMT surface used for AFM force

254

measurements in aqueous CNFC. (b) Typical AFM F/D curve between MMT

255

colloidal probe and MMT substrate. (c) F/D curve and corresponding DLVO and

256

DLVO-s fitting curves between the MMTs in 0.3g/L CNFC. (d) Proportion and

257

magnitude of electrostatic and steric repulsions at different distances.

258 14

259

As we presented above, the yield and stability of individual MMT suspension are

260

highly dependent on the dosage of CNFC dispersant. To understand the underlying

261

mechanism, we try to study the surface interactions between the MMTs through

262

measurements of approaching F/D curves in CNFC dispersions with varying

263

concentrations. Fig. 6a indicates the approaching F/D curves measured between the

264

individual MMTs in pure water and CNFC dispersions with different solid contents.

265

In pure water, attractive forces dominate the interfacial interaction between the

266

individual MMTs, which leads to the aggregation of aqueous individual MMT

267

suspensions. With increasing solid contents of CNFC dispersion, the interfacial

268

interactions between the individual MMTs change from attractive force to repulsive

269

force, and the intensity and range of repulsion increase as well.

270

To explain the individual contribution of electrostatic repulsion and steric

271

hindrance on the repulsive force between the individual MMTs in different CNFC

272

dispersions, the approaching F/D curves are fitted by the DLVO and DLVO-s models

273

(Fig. S3). According to the fitting results, several parameters are obtained to directly

274

determine the interaction intensity and range of repulsive forces between the

275

individual MMTs. Debye length and electrostatic interaction constant are for

276

electrostatic repulsion while characteristic length and steric interaction constant are

277

for steric repulsion. The Debye and characteristic lengths indicate the interaction

278

range of electrostatic repulsion and steric hindrance, respectively, while the

279

electrostatic and steric interaction constants relate to the interaction intensity of

280

electrostatic repulsion and steric hindrance, respectively, the larger the interaction

281

constant, the stronger the interaction intensity.

282

As shown in Fig. 6b, the Debye length and interaction constant of electrostatic

283

repulsion increases gradually and finally tends to be stable. The strong adsorption of 15

284

highly negative CNFCs (Fig. S4) on the surface of individual MMTs by hydrogen

285

bonding significantly improves their charge density, thus rendering the increase of

286

both Debye length (interaction length) and electrostatic interaction constant (intensity).

287

As the adsorption capacity of the individual MMTs reaches saturation point, the

288

electrostatic repulsion between the individual MMTs will be stable.

289

However, an obvious different phenomenon is observed for steric hindrance with

290

increasing concentrations of CNFC dispersion. The characteristic length demonstrates

291

an increasing tendency while a declining trend is observed for the steric interaction

292

constant (Fig. 6c). When the concentration of CNFC dispersion is low, the interaction

293

range of repulsive force is limited to the surface of the individual MMTs，but its

294

interaction constant is large due to the dense structure of the adsorption layer. As the

295

concentration of CNFC suspension continues to rise, the thickness of adsorbed CNFC

296

layer grows and more free CNFCs are presented, which lead to larger accessible

297

distance between the individual MMTs that finally renders the increasing interaction

298

range of steric repulsion. Moreover, the dense structure of the adsorbed CNFC layer

299

on the surface of MMTs tends to be loose with the growing thickness of adsorption

300

layer, thus resulting in the decrease of interaction constant of steric repulsion. Despite

301

the decrease of the steric interaction constant with the increasing concentrations of

302

CNFC suspension, the total steric repulsion demonstrates an increasing trend and

303

eventually become a dominant repulsion interaction.

16

304 305

Fig. 6. (a) F/D curves between individual MMTs in CNFC dispersions with varying

306

solid contents. (b) Electrostatic and (c) steric parameters in various CNFC dispersions

307

system.

308 309

In sum, the colloidal stability of individual MMTs suspensions enhances with

310

increasing concentrations of CNFC suspension. The electrostatic repulsion dominates

311

the interfacial interactions between the individual MMTs at long range while the

312

steric hindrance is a predominant factor at short range. As the increase of CNFC

313

concentration, the individual MMT suspension tends to be more stable due to the

314

increase of total repulsion. More significantly, the role of electrostatic repulsion and

315

steric resistance in the colloidal stability of individual MMT suspensions is highly

316

dependent on the concentrations of CNFC suspension. When the CNFCs’

317

concentration reaches 0.1 g/L, the colloidal stability of individual MMTs suspension

318

is mainly due to electrostatic repulsive forces, but the steric hindrance gradually

319

become a predominant repulsion as the concentration increases continuously.

320 17

321

3.4. CNFC-dispersed individual MMTs for nanocomposite films with excellent

322

properties

323

Normally, the addition of individual MMTs into pure CNFC films will inevitably

324

deteriorate their excellent optical and mechanical properties due to the easy

325

aggregation of sheet-like MMTs. However, incorporating CNFC-dispersed individual

326

MMTs into CNFC dispersion could address above challenge and obtain

327

nanocomposite films with optical and mechanical properties comparable to pure

328

CNFC films.

329

Fig. 7a and b shows the optically transparent appearance of nanocomposite films

330

containing 30 wt% individual MMTs obtained by using 0 wt% or 50 wt% CNFC

331

dispersant. Pure CNFC film presents a transmittance of 90.4% at 550 nm and a tensile

332

strength of 128 MPa. However, nanocomposite film demonstrates a decrease in the

333

optical transparency (88.0% at 550 nm) and tensile strength (118 Mpa) when

334

individual MMTs are added (Fig. 7c and d). When individual MMTs with 50 wt %

335

CNFC dispersant were blended with CNFC to prepare nanocomposite film, the

336

resulting nanocomposite film presents a similar transparency (90.3% at 550 nm) and a

337

better tensile strength (143 MPa) compared to pure CNFC films.

338

The reason behind the enhanced transparency is mainly due to the superior

339

colloidal stability of individual MMT suspension using 50 wt% CNFCs as a

340

dispersant that significantly contributes to the formation of ordered structure within

341

film during self-assembly [24-25]. In addition, with increasing CNFC dispersant’s

342

dosage, larger planar size is obtained for individual MMTs, which enables the

343

enhancement in tensile strength.

18

344 345

Fig.7. Visual appearances of nanocomposite films with 30 wt% individual MMTs

346

obtained by using (a) 0 wt% or (b) 50 wt% CNFC dispersant. (c) Total light

347

transmission and (d) tensile strength of pure CNFC film and nanocomposite films

348

with 30 wt% individual MMTs obtained by using 0 wt% or 50 wt% CNFC

349

dispersant .

350 351

4. Conclusions

352

In summary, a comprehensive understanding of the dispersive mechanism of

353

CNFCs for individual MMTs is unveiled by AFM colloidal technique and QCM-D.

354

There is a strong affinity between the two components primarily resulting from the

355

hydrogen bonding interaction, which lays the foundation for preparing stable colloidal

356

suspensions of individual MMTs. Moreover, both the long-range electrostatic

357

repulsion and short-range steric resistance contribute to the colloidal stability of

358

individual MMTs in water and their individual contributions are dependent on the

359

dosage of CNFC dispersant. When the concentration of CNFC dispersant is lower

360

than 2 wt%, the electrostatic repulsion is the dominant interaction for the colloidal

361

stability of individual MMTs. As the CNFC’s dosage continues to increase, the role of

362

steric hindrance in the interfacial interactions grows gradually and eventually

363

becomes a predominant repulsion. Finally, the addition of CNFC-dispersed individual

364

MMTs into pure CNFC film will not deteriorate their optical and mechanical 19

365

properties. When the dosage of CNFC dispersant is 50 wt%, the resulting

366

nanocomposite film with 30 wt% individual MMTs demonstrated a similar

367

transparency (90.3%) at 550 nm and a better strength (143 MPa) compared to pure

368

CNFC film. This work gives a comprehensive insight on the dispersive mechanism of

369

CNFCs for individual MMTs in water and may extend the use of MMTs in numerous

370

value-added fields, such as high-performance nanocomposites and flexible

371

electronics.

372 373

Author information

374

*Corresponding author: [email protected] (Z. Fang), [email protected] (X. Qiu),

375 376

Acknowledge

377

We appreciate the Pearl River S&T Nova Program of Guangzhou (grant no.

378

201806010141), the National Natural Science Foundation of China (grant no.

379

21978103, 31700508), the Natural Science Foundation of Guangdong Province,

380

China (grant no. 2017A030310635), Science and Technology Program of Guangdong

381

Province (grant no. 2017B090903003), and the Foundation (grant no. KF201812) of

382

Key Laboratory of Pulp and Paper Science and Technology of Ministry of

383

Education/Shandong Province of China.

384 385 386 387 388 389 390 391 392 393 394 395 396 397

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