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Stimuli-responsive cellulose nanocrystals for surfactant-free oil harvesting Juntao Tang, Richard M. Berry, and Kam (Michael) Chiu Tam Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.6b00144 • Publication Date (Web): 11 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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Biomacromolecules

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Stimuli-responsive cellulose nanocrystals for surfactant-free oil

2

harvesting

3 Juntao Tang1, Richard M. Berry2, Kam C. Tam1*

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1

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200 University Avenue West, Waterloo, ON, N2L 3G1, Canada

Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo,

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2

CelluForce, Inc., 625 President-Kennedy Ave., Montreal, Quebec, Canada H3A 1K2

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Email address: [email protected]

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1

ACS Paragon Plus Environment

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Abstract:

14 15

Binary polymer brushes, poly(oligoethylene glycol) methacrylate (POEGMA) and

16

poly(methacrylic acid) (PMAA), grafted cellulose nanocrystals (CNC-BPB) were

17

prepared by cerium mediated polymerization in aqueous solution. The physical properties

18

of the CNC-BPB can be controlled by external triggers, such as temperature and pH,

19

which can be utilized to stabilize and destabilize oil/water emulsions. By virtue of the

20

modifications, these bi-functionalized CNCs diffused to the oil-water interface and

21

stabilized the oil droplets at high pHs. When the pH was lowered to 2, strong hydrogen

22

bonding between POEGMA and PMAA chains grafted on the CNC induced the

23

coalescence of the emulsion droplets, resulting in the phase separation of oil and water.

24

For emulsions stabilized by CNC-POEGMA and free PMAA mixtures, instantaneous

25

coalescence was not observed at low pHs. Successive stabilization-destabilization over 5

26

cycles was demonstrated by modulating the pH with the addition of acid or base without

27

any loss in efficiency. This work demonstrates that functional sustainable nanomaterials

28

can be used for small scale oil-water separations, particularly for oil droplet transportation

29

and harvesting of lipophilic compounds.

30 31

Keywords: Cellulose nanocrystals, Stimuli-responsive, Pickering emulsion, Oil

32

harvesting

33 2


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Biomacromolecules

Introduction:

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Surfactant stabilized emulsions are commonly used in the petroleum industry, where

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the destabilization of emulsions is desirable prior to the separation of oil-water mixtures1.

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Examples of chemical demulsifiers presently used are: polymeric surfactant such as

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copolymers of polyethylene-oxide and polypropylene-oxide2; alkylphenol-formaldehyde

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resins; and blends of different surface-active compounds3. These surface-active agents

40

increase the surface pressure at the oil-water interface, and induce the coalescence of the

41

emulsions. The use of demulsifiers derived from fossil fuels poses economic and

42

environmental challenges. These challenges have led to the development of switchable or

43

stimuli-responsive stabilizers from sustainable materials for controlling the stability of

44

emulsions4–8.

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Conventional surfactant based emulsion systems that are thermodynamically stable

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generally only work at surfactant concentrations exceeding the critical micelle

47

concentration (CMC) which increases cost and environmental impact. Pickering

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emulsions, which are particle-stabilized emulsions, are more attractive as they provide

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extremely stable interfaces that inhibit the coalescence at relatively low concentrations.

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Further, the stability of the interface can be reduced considerably by altering the

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amphiphilic characteristics of the particle using functional moieties that possess

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switchable stimuli-responsive properties9–11. The pioneering works of Fuji & Armes12,13 as

53

well as Saigai & Tilton14 provide the impetus and motivation for this research. A review of

54

the literature indicated that studies on the use of functional particle with responsive 3


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properties for controlling the stability of Pickering emulsions for the separation of oil and

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water mixtures is limited. Chen et al. reported an efficient oil harvesting system based on

57

poly(N-isopropylacrylamide) (PNIPAM) grafted magnetic composite particles15. The

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magnetic core offers the ease of separation using a magnetic field, and the

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temperature-responsive PNIPAM allows for thermal induced destabilization of emulsion

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droplets for the recovery of the oil phase. Wang et al. reported on a similar core-shell

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poly(2-(dimethylamino)ethyl methacrylate) grafted-hybrid magnetic nanoparticle for the

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separation of emulsified oil droplets and the recovering of oil using pH as the trigger16.

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However, the synthetic route, the choice of nanoparticle (silica on a magnetic

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nanoparticle), and the polymerization method (atom transfer radical polymerization) are

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complicated, not sustainable and are difficult to scale-up. In addition, the efficiency of

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demulsification is generally low since the emulsions are not completely coalesced on the

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application of triggers, such as pH and temperature. As a result, we propose to address

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these shortcomings using an abundant and easily-modified sustainable nanoparticle,

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cellulose nanocrystals (CNC)11.

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Cellulose nanocrystals, derived from acid hydrolysis of biomass, have attracted

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increasing attention for various applications17–19, particularly for the stabilization of

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Pickering emulsions. Kalashnikova and coworkers were the first to report on the use of

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cellulose nanocrystals to stabilize oil-in-water emulsions20–22. Since then, surfactant and

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surface active polymers (such as surfactant adsorption23, periodate oxidation and

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amination24, esterification25, as well as PNIPAM26 and long alkyl chain grafting27) have 4


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been incorporated onto the CNC surface to further demonstrate its function in Pickering

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emulsions. Various strategies were used to manipulate the surface functionalities of CNC

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in controlling the physical properties of Pickering emulsions. Our research group has

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recently reported on a dual-responsive Pickering emulsion system based on

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poly[2-(dimethylamino)ethyl methacrylate] grafted cellulose nanocrystals28. However,

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when the triggers (pH and temperature) were applied, the destabilization of the emulsion

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was not sufficient for the recovery of the oil phase, thus limiting their use in industrial

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applications. To address this limitation, we adopted a new strategy by introducing a binary

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polymer brush consisting of poly(oligoethylene glycol) methacrylate (POEGMA) and

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poly(methacrylic acid) (PMAA) on the surface of CNC nanoparticles using water as the

86

reaction medium. The choice of the polymers allows for the control of the system using

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two types of triggers, i.e. POEGMA for temperature and PMAA for pH trigger. To the best

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of our knowledge, this is the first reported use of a binary brush nanoparticle with

89

tailorable interactions (hydrogen bonding) between POEGMA and PMAA chains to

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control the stability of particle-stabilized emulsions. Pickering emulsions based on the

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proposed modified CNC particles were investigated in detail in the present study. By

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taking advantage of the functional groups on the surface of cellulose nanocrystals (CNCs),

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polymer modifications can offer greater flexibility in the design and development of

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Pickering emulsifier. A reversible process of emulsification-demulsification could be

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controlled by manipulating pH. This work demonstrates that sustainable functional

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nanomaterials can be used for small scale oil-water separations, particularly in viscous oil 5


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transportation and harvesting of lipophilic compounds.

98 99 100

Experimental Section Materials

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Cellulose nanocrystals were supplied by CelluForce Inc. with an average charge

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density of 0.26 mmol/g. Di(ethylene glycol) methyl ether methacrylate (MEO2MA, 95%),

103

poly(ethylene glycol) methyl ether methacrylate (average Mn 300, OEGMA300) and

104

methacrylic acid (99%) were purchased from Sigma-Aldrich and passed through columns

105

of neutral aluminum oxide prior to use. Cerium (IV) ammonium nitrate (CAN),

106

ammonium persulfate (APS), heptane (99%), Nile red, poly(methacrylic acid, sodium salt)

107

solution (average Mw 4000~6000, 40 wt.% in water) were used as received from

108

Sigma-Aldrich. Standard solutions of hydrochloric acid and sodium hydroxide were used

109

to prepare 1 M solutions.

110 111

Preparation of CNC-POEGMA

112

For grafting POEGMA to the surface of CNC, the typical experimental procedure was

113

as follows: CNC (1.0 g) and 0.5 mL 70 wt% HNO3 were dispersed in 150 mL of deionized

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water in a 250 mL one-neck flask equipped with magnetic stirrer first. MEO2MA (9 mmol,

115

1.692 g) and OEGMA300 (3 mmol, 0.9 g) (ratio of 3:1) were mixed in 5 mL ethanol and

116

added dropwise into the reaction flask. The mixture was stirred for 15 mins and purged

117

with high purity Argon for 1 h. Then the degassed initiator solution (CAN, 200 mg in 2 6


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mL water) were added drop-wise into the reaction flask to initiate the polymerization. The

119

reaction mixture was kept at room temperature overnight and the final product was

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concentrated via centrifugation and washed 3 times with Millipore water. The product was

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purified 2 times via ultrafiltration and dialyzed against Millipore water for 3 days with

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constant changes of water. The CNC-POEGMA dispersion with a measured solid content

123

was recovered and stored in the refrigerator for future use.

124 125

Preparation of CNC-POEGMA-PMAA

126

To prepare the binary polymer brush grafted nanoparticles, 80 mL of CNC-POEGMA

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(1.0 g) aqueous dispersion was first mixed with 40 mL ethanol and dispersed using a

128

sonication bath for 15 mins. The mixture was purged with high purity Argon for 1 h under

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constant stirring. The initiator (APS, 200 mg) was charged to the reaction flask and the

130

temperature was kept at 50 oC for 10 mins. After the degassed MAA (0.7 mL) was

131

introduced, the temperature of the reaction mixture was slowly increased to 60 0C, which

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was maintained for 6 hours. By adding 1 ml of 1 M NaOH solution to the mixture, the

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modified nanoparticles were recovered by centrifugation and washed 3 times with

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deionized water. The mixture was then dialyzed against Millipore water for 3 days until

135

the conductivity remained constant. The dispersion was finally concentrated and

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freeze-dried.

137 138 7


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Preparation of Pickering Emulsions

140

All emulsions were prepared using an UltraTurrax T25 homogenizer (IKA, Germany)

141

at 6000 rpm for 2 min at room temperature. Except for specific illustrations, the ratio of

142

oil and water was 3:7 and the aqueous phase with 0.3 wt% of nanoparticles was used in

143

this study. To distinguish the oil and water phases and to facilitate the visualization of the

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emulsion droplets, 2 mg of Nile Red was dissolved in 250 mL of toluene or heptane,

145

respectively.

146 147

Characterization Techniques

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Conductometric and potentiometric titrations, performed on a Metrohm titrator, were

149

used to quantify the surface functional groups. Typically, 40 mL of the 0.1 wt%

150

nanoparticle dispersion was charged into the vessel and the pH was adjusted to 2.5. The

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dispersion was then titrated with 0.01 M NaOH solution under continuous stirring.

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Freeze-dried samples were homogenously mixed with KBr powder to yield a transparent

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pellet for FT-IR characterization. The spectra were acquired using a Bruker Tensor 27

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FT-IR spectrometer with a resolution of 4 cm–1 and a scanning number of 32 from 400 to

155

4000 cm–1 at room temperature. The thermal responsive measurement was performed on a

156

Varian (Cary 100 Bio) UV-Vis spectrometer equipped with a temperature controller. All

157

the measurements were performed at a heating rate of 0.5 °C/min and a wavelength of 500

158

nm. Dynamic light scattering and ζ-potential experiments were acquired on a Malvern

159

Instrument Zetasizer Nanoseries and thermal analyses were performed on a TGA Q600 TA 8


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Instrument (Lukens Drive, Delaware, U.S.A.) (Temperature program: 10 °C/min until

161

100 °C, maintained at 100 °C for 5 min then 10 °C/min until 700 °C). Samples for TEM

162

characterizations (Philips CM10) was prepared by spraying a 0.5 wt% aqueous dispersion

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onto copper grids (200 mesh coated with copper) and allowed to dry. In order to clarify the

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morphology of the nanoparticles, all samples were stained using 3 drops of 1.0 M ferric

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ion solution. Elemental analysis was carried out using a Vario Micro cube, Elementar

166

Americas, Inc. Carbon, hydrogen, nitrogen, and sulphur content of the samples were

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determined by repeated measurements and the average values were reported. Surface

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tension measurements were conducted on the tensiometer Data Physics DCAT 21 system.

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Aqueous suspensions with a nanoparticle concentration of 1.0 wt% were analyzed using a

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Bohlin CS rheometer, where the steady shear viscosities versus shear rate curves were

171

determined.

172

The emulsion droplets were captured at 200x magnification by an inverted optical

173

microscope (Nikon Elipse Ti-S) equipped with a CCD camera (QImaging ReTIGA 2000R)

174

by placing the emulsion droplets on a glass slide.

175 176 177

Modification of CNC with Polymer Brushes Dual-responsive

cellulose polymer

nanocrystals

brushes,

were

synthesized

poly(oligo(ethylene

glycol)

by

grafting

178

stimuli-responsive

methyl

ether

179

methacrylates (POEGMA) and poly(methacrylic acid) (PMAA) via a two-step

180

polymerization process from the surface of CNC (Scheme 1). POEGMA is a 9


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thermo-responsive and nontoxic polymer suitable for applications in injectable hydrogels,

182

cell culture substrates as well as bio-separations by virtue of its biocompatibility and sharp

183

thermal transition chracteristics29–32. By varying the number of ether groups using

184

MEO2MA, OEGMA300, or OEGMA475 or by changing the molar proportion of different

185

types of monomers, the lower critical solution temperature (LCST) can be tuned

186

accordingly. On the other hand, PMAA is a well-known polyelectrolyte with a mild

187

hydrophobic character due to the repeating methyl groups that possesses a pH-dependent

188

behavior that is correlated to the protonation and deprotonation of the carboxyl groups33.

189

In the first step, the POEGMA copolymer was randomly grafted on the CNC surface

190

(CNC-POEGMA) via a redox initiated free radical polymerization using cerium

191

ammonium nitrate (CAN). It should be noted that the cerium ion is a strong oxidant for an

192

alcohol containing 1, 2-glycol groups. The mechanism of the reaction involves the

193

formation of a chelating complex that decomposes to generate free radicals on the

194

cellulose backbone34. The free radicals can further react with the monomers that propagate

195

until chain termination. As cerium initiated free radical polymerization can result in the

196

production of free homopolymer, centrifugation and dialysis against urea solution were

197

used to remove them from the polymer grafted CNC. In a second step, PMAA was grafted

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onto CNC-POEGMA nanoparticles via a persulfate initiator, ammonium persulfate (APS).

199

The sulfate radical anion ܱܵସି • extracts a hydrogen from a primary hydroxyl group

200

resulting in a RO• radical that serves as a macro-initiator. The detailed mechanism can be

201

found in the literatures28,35. APS has been used to synthesize carboxylated CNCs, however 10


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the concentration of APS used in the oxidation process is generally very high, of up to 1 M.

203

In order to ensure that APS initiation did not oxidize the cellulose nanocrystal in our

204

synthetic process, we performed a control experiment to verify the impact of APS on the

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surface properties of CNC. In the control experiment, 70 mg APS was used to react with

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CNC (30 mL 1 wt% dispersion) at 60 oC for 6 hours. The dispersion was dialyzed to

207

remove unreacted chemicals and potentiometric titration was used to quantify the product.

208

As shown in the supporting information, no carboxyl groups were detected in this APS (~8

209

mM) treated CNC.

210 211

Scheme 1. Schematic illustrating the synthesis of binary polymer brushes consisting of

212

poly(oligoethylene glycol) methacrylate (POEGMA) and poly(methacrylic acid) (PMAA),

213

on cellulose nanocrystals.

214 215

Results and discussion

216

Basic characterization of stimuli-responsive nanoparticles

217

Successful grafting of POEGMA and PMAA on the CNC nanoparticles was

218

confirmed by FT-IR spectroscopy (Figure 1A). In addition to the characteristic stretching 11


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vibrations of hydroxyl groups at 3400 cm-1 and 1058 cm-1 on cellulose nanocrystals, a new

220

peak corresponding to carbonyl stretching was observed at 1729 cm-1 for CNC-POEGMA.

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In addition, the obvious change of the spectra in the region near 2900 cm-1 is a

222

consequence of the methylene moieties of POEGMA chains, as reported previously30,36.

223

When the CNC-POEGMA system was further grafted with PMAA, the absorption ratio

224

between 1730 and 1640 cm-1 was increased due to the increase in the specific absorption

225

of C=O bonds on the MAA repeating units. Another apparent sharp peak around 1550

226

cm-1 was observed in the spectrum of CNC-POEGMA-PMAA, indicating the presence of

227

carboxylate groups37,38.

228

In addition, the as-prepared nanoparticles were characterized by potentiometric

229

titration,

and

the

results

for

different

nanoparticles,

CNC-POEGMA

and

230

CNC-POEGMA-PMAA, are shown in Figure S1. CNC-POEGMA displays a classic

231

profile of strong base titration into strong acid, while the CNC-POEGMA-MAA sample

232

exhibited a three-stage process, correlating to strong acid, weak acid and strong base

233

behavior, respectively28,39,40. The weak acid region is associated to the deprotonation of

234

carboxylate groups. Depending on the titration curves, the amount of MAA repeating units

235

was calculated to be approximately ~0.850 mmol per gram of CNC-POEGMA-PMAA

236

nanoparticles.

12


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Figure 1. (A) FTIR spectral of pristine cellulose nanocrystals (CNC), CNC-POEGMA and

239

CNC-POEGMA-PMAA. (B) Thermogravimetric curves of CNC (black), CNC-POEGMA

240

(blue) and CNC-POEGMA-PMAA (red) samples.

241 242

TGA measurements were used to elucidate the thermal stability of CNC after

243

modifications (Figure 1B). It was observed that the polymer grafting lowered the thermal

244

stability of the nanoparticle. Polymer grafted CNC started to decompose at temperatures

245

around 180 oC, which is substantially lower than that of pristine CNC samples (270 oC).

246

The reduction in thermal stability after the polymer grafting may also result from the

247

oxidation process as CAN initiator will oxidize the backbone of the cellulose to produce

248

aldehyde functional groups that are less thermal stable. Compared to CNC-POEGMA,

249

CNC-POEGMA-PMAA displayed a faster degradation rate between 200 oC and 300 oC as

250

well as a lower residual mass at high temperature. The TGA analyses provided an indirect

251

confirmation of successful grafting on CNC.

252

The possible changes in the morphologies of the nanoparticles were characterized by

253

TEM. As shown in Figure 2, all the nanoparticles were well-dispersed with fairly identical 13


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rod structure. With the grafting of hydrophilic polymers, the nanoparticles present higher

255

polydispersity as well as a more homogenous dispersion. The TEM images of CNC and

256

CNC-POEGMA contains several bundle–like structure due to van der Waals interactions

257

and

258

CNC-POEGMA-PMAA system. This is probably due to the strong electrostatic repulsions

259

between CNC-POEGMA-PMAA nanoparticles. We used the zeta-sizer to check the size

260

change of the nanoparticles after each modification steps, and only a small change (several

261

nanometers) was observed.

hydrogen

bonding,

rendering

it

to

somewhat

different

from

the

262 263

Figure 2. TEM images showing the morphologies of pristine CNC (A), CNC-POEGMA

264

(B) and CNC-POEGMA-PMAA (C)

265 266

Elemental analysis (Table 1) was used to further confirm the presence of polymer

267

grafting. From the knowledge of the elemental weight of both the glucose unit and the

268

monomer used, it is possible to determine the change in the amounts of each element. In

269

the second modification step, regardless of the molecular weight and grafting density, the

270

PMAA weight ratio on CNC-POEGMA-PMAA nanoparticles was determined to be 11.5 14


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wt%, which is in agreement with the result obtained from potentiometric titration. The

272

weight ratio of PMAA to mixed brush modified cellulose nanocrystals was determined

273

from the analysis of the compositional change of carbon and oxygen. The detail

274

description is included in the supporting information.

275 276

Table 1. Atomic composition of different nanoparticles determined by elemental analyses

277

Sample

N%

C%

H%

S%

CNC

0.02

41.11

5.62

0.84

CNC-POEGMA

0.01

49.25

6.43

0.34

CNC-POEGMA-PMAA

0.00

50.01

6.68

0.22

278 279

Figure S2 shows the viscosity-shear rate relationship of the polymer grafted CNC

280

suspensions under steady shear flow. The viscosity profile of the 1.0 wt% CNC dispersion

281

is independent of shear rate since the concentration of the nanoparticle is in the dilute

282

solution regime, and significant particle-particle interaction is absent. However, shear

283

thinning behavior was clearly evident for CNC-POEGMA, which is attributed to the

284

network structures produced by the grafted polymer chains induced by hydrogen bonding.

285

For the CNC-POEGMA-PMAA system, the viscosity increased by 20 times due to more

286

hydrogen bonds and the ionization of carboxyl groups that induce electrostatic repulsions

287

between the nanostructures.

15


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Figure 3. Surface tension values of 0.3 wt% CNC (black), CNC-POEGMA (blue) and

290

CNC-POEGMA-PMAA (red) dispersions

291

The surface tensions of the three nanoparticle dispersions (0.3 wt%) are shown in

292

Figure 3. It should be noted that the dispersion of polymer grafted CNC displayed a lower

293

surface tension than pristine CNC due to the amphiphilic characteristics of the polymer

294

chains, which is consistent with previous reports28,41.

295

Stimuli-responses of the nanoparticles to pH and temperature

296

The impact of pH on the surface properties of the nanoparticles was determined by

297

zeta-potential measurements. As shown in Figure 4A, the zeta-potentials of

298

CNC-POEGMA samples in water were between -12 to -16mV, which is independent of

299

pH due to the absence of pH-responsive functional groups. CNC-POEGMA-PMAA

300

however possessed a pH-dependent zeta-potential due to the presence of the weak

301

polyelectrolyte (PMAA), which undergoes subtle molecular rearrangements close to the 16


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pKa of MAA. Below the pH of 4.5, all the COOH groups are protonated, and the hydrogen

303

bonding

304

CNC-POEGMA-PMAA aggregates and precipitates, causing a hazy dispersion. Above the

305

pH of 4.5, the COOH groups become deprotonated, where repulsive carboxylate anions

306

disrupt the hydrophobic association and hydrogen bonding to yield a stable dispersion42,43.

307

The zeta-potential decreased from -25 to -40mV when the pH was increased from 4.5 to

308

8.0 due to the progressive deprotonation of carboxylic acid groups. Based on previous

309

reported studies on polyethylene glycol and PMAA37,44–52, we infer that the hydrogen

310

bonding between POEGMA and PMAA contributed to the instability of the nanoparticles

311

at low pH, and this will be discussed later.

between

POEGMA

and

PMAA

may

lead

to

the

formation

of

312

Temperature-dependent UV-Vis transmittance measurements at a wavelength of 500

313

nm were performed on 0.2 wt% samples as shown in Figure 4B. Both CNC-POEGMA

314

and CNC-POEGMA-MAA dispersions were translucent at low temperature (25 oC, see

315

inserts), and the transmittance was around 90 % due to the limited scattering by

316

nanoparticles. Upon heating, the transmittance decreased dramatically to around 10% for

317

CNC-POEGMA due to the dehydration of ethylene-oxide side chains distributed along the

318

backbone, producing a compact globular structure induced by the intra- and

319

intermolecular aggregations. A transition temperature of around 37ºC was observed, which

320

is in agreement with polymers prepared using controlled polymerization techniques32. This

321

result indicates that the thermo-responsive behavior of co-polymers prepared by free

322

radical polymerization (FRP) is similar to those prepared by controlled polymerization. 17


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Page 18 of 35

323

Our study is in agreement with previous studies and supported by the molecular

324

characteristics of the polymers (only a small impact of the polymer tacticity on the

325

thermo-responsiveness was observed29,31. Since the thermal transition temperature is

326

independent of molecular weight, it would not be feasible to quantify the molecular length

327

using this approach. For CNC-POEGMA-MAA samples, the transition temperature

328

increased slightly and the transition region broadened maybe due to the change in the

329

hydrophilicity of the nanoparticles.

330 331

Figure 4. (A) Effect of pH values on Zeta-potential profile of CNC-POEGMA and

332

CNC-POEGMA-PMAA in aqueous suspension; (B) Transmittance versus temperature of

333

0.2 wt% CNC-POEGMA (blue) and CNC-POEGMA-PMAA (red) aqueous suspensions at

334

a heating rate of 0.5 °C/min and a wavelength of 500 nm.

335 336

Emulsion stability

337

Evaluation of Coverage

338

Surface coverage is an important parameter to understand the organization of

339

nanoparticles at the oil-water interface. Heptane-in-water emulsions stabilized by different 18


Page 19 of 35

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340

concentrations of CNC-POEGMA-PMAA were investigated. It shows that the emulsion

341

droplet size decreased from 43.7 to 10 µm with increasing concentration from 0.08 to 0.6

342

wt % (see Figure 5A), which is several micrometers larger than previously reported

343

emulsion droplet sizes20,21. From the calculation of the surface coverage described in the

344

supporting information, the values of surface coverage determined and plotted against the

345

mass of nanoparticle in the aqueous phase. The results show that a minimum of

346

approximately 85% of covered surface is sufficient to stabilize an emulsion with

347

CNC-POEGMA-PMAA nanoparticles (see Figure 5B). This value is in accordance with

348

the data reported in the literature for Pickering emulsion system stabilized by cellulose

349

nanocrystals18, 19. Surface coverage of 84% has been previously reported by Kalashnikova

350

et al.22 In their study, they compared three cellulose nanocrystals derived from different

351

sources: cotton (CCN), bacterial cellulose (BCN) and Cladophora (ClaCN). The aspect

352

ratio ranged from 13 to 160. Results revealed that an obvious denser organization was

353

achieved with CCN, while a looser network structure was observed for longer nanorods.

354

(i.e., CCN tends to cover the entire bead surfaces, whereas BCN and ClaCN form porous

355

multilayered coverage.) The surface coverage was determined and a minimum of

356

approximately 44% of covered surface is sufficient to stabilize the emulsion with ClaCN,

357

whereas 60% and 84% is required for BCN and CCN respectively. It is expected that stiff

358

nanorods may not be able to rearrange to produce a very tight-packing and we believe the

359

difference probably results from the flexibility of the nanoparticles. For cellulose

360

nanocrystals with a low aspect ratio, a higher surface coverage is needed to stabilize the 19


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Page 20 of 35

361

emulsion.22 Future studies to gain a better understanding on the packing at the interface

362

would be extremely useful.

363 364

Thermo-responsive behavior

365

Heptane-in-water-emulsion systems prepared using a 0.3 wt% nanoparticle

366

concentration were used to study the effect of temperatures on the stability of the

367

emulsions. As shown in Figure 6, CNC-POEGMA-PMAA systems phase separated when

368

incubated at 70 oC for 1 hour. Beyond the transition temperature of between 37 and 40 oC,

369

the hydrophobicity of the polymer was increased by the disruption of hydrogen bonds

370

between water and ether groups resulting in the hydrophobic association of the polymer

371

chains and nanoparticles. As a consequence, the oil droplets became unstable as the

372

polymer grafted CNC began to associate resulting in the coalescence of oil droplets to

373

form larger droplets. (~20% coalescence based on the volume ratio of oil collected to the

374

emulsified oil before emulsification) When the temperature was increased, the droplet

375

collision frequency was enhanced, while the interfacial viscosity and film-drainage rate

376

were reduced leading to greater droplet coalescence53.

20


Page 21 of 35

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Biomacromolecules

377 378

Figure 5 (A) Heptane in water emulsion droplet size profile as a function of

379

CNC-POEGMA-PMAA concentrations; (B) Evolution of surface coverage vs the amount of

380

CNC-POEGMA-PMAA included in the water phase per ml of heptane; (C-F) Optical micrographs

381

of heptane emulsions stabilized by different concentration of CNC-POEGMA-PMAA (C: 0.1 wt%;

382

D 0.15 wt%; E: 0.3 wt%; F: 0.5 wt%), the oil to water ratio is 30:70.

383 384

Figure 6. Photographs of heptane-in-water emulsions stabilized by CNC-POEGMA-MAA

385

(0.3 wt%) and its response to being placed in a 70 oC water bath for 1 hour. 21


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386

Page 22 of 35

pH-responsive behavior

387

The stability of Pickering emulsions with nanoparticles grafted with pH-responsive

388

polyelectrolyte brushes is expected to depend on pH. Heptane was used as the organic

389

phase and a 0.3 wt% aqueous dispersion of nanoparticle was used as the aqueous phase.

390

The pH values of the aqueous dispersions were tuned to the targeted values by adding 1M

391

NaOH or HCl. As discussed earlier, CNC-POEGMA did not possess pH-dependent

392

characteristics; hence changing pH did not impact the emulsion, which remained stable

393

against coalescence for more than 6 months (Figure 7A). A slight creaming was evident

394

due

395

CNC-POEGMA-PMAA, the stability of the Pickering emulsion is dependent on pH.

396

Above pH 6, the emulsions were stable, and negligible coalescence or creaming of the

397

emulsions was evident. The sizes of emulsion droplets at different pHs were fairly uniform

398

because the emulsions are reasonably stable (average oil droplet diameter of 12.3 µm) at

399

pH 10. The fluorescence micrographs of heptane-in-water emulsions stabilized by 0.3 wt %

400

CNC-POEGMA-PMAA at pH 10 are shown in Figure 7C. When the pH was decreased to

401

4, incomplete coalescence of emulsions and formation of particulate flocs were observed

402

(Figure 7B and 7C), along with a dramatic increase in the viscosity. We infer that by

403

lowering the pH, the carboxyl groups on PMAA chains become protonated and complex

404

with –CH2-CH2-O- groups on the POEGMA chains via hydrogen bonding (Figure 8). As a

405

result, the polymer brushes undergo conformational change forming hydrophobic

406

associations and globular domains that lead to inter- or intra-nanoparticle aggregation. The

to

the

density

difference

between

water

and

heptane.

However,

for

22


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407

aggregation will result in incomplete coverage of the oil-water interface, promoting the

408

coalescence of oil droplets to compensate for the reduced surface coverage of the droplet

409

interfaces (Figure 8). Further reducing the pH to 2 resulted in the formation of two

410

distinctive layers soon after emulsification and large aggregates were evident. These

411

results reflect a strong correlation between emulsion stability and interactions between

412

modified nanoparticles, where the emulsion stability increases with the introduction of

413

electrostatic repulsions by manipulating the pH values.

414

In order to demonstrate the importance of both grafted polymer chains on controlling

415

the stability of the emulsions, other systems stabilized by CNC-PMAA or PMAA and

416

CNC-POEGMA mixtures were investigated. It should be noted that the results differ

417

significantly from the CNC-POEGMA-PMAA systems. As shown in Figure S3, the

418

emulsion droplets using CNC-PMAA as stabilizer possessed pH-dependent stability

419

characteristics, displaying an opposite trend to the CNC-POEGMA-PMAA system. The

420

emulsions readily coalesced without agitation after 24 hours at high pH conditions (pH = 8

421

or 10), while they were stable at low pH conditions (pH =2, 4 or 6) because the

422

carboxylate groups are protonated at low pH and the hydrophobic PMAA chains will

423

partition to the oil phase to cover the oil-water interface.42,43 The modified nanoparticles

424

can further stabilize the oil droplets compared to pristine CNC nanoparticles, which is in

425

agreement with our previous studies28. However, at high pH, the charged polymeric chains

426

induced strong electrostatic repulsions between nanoparticles, which decreased the affinity

427

of the nanoparticles to the oil/water interface resulting in the destabilization of the oil 23


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Page 24 of 35

428

droplets. Although the pH-responsive behaviors were achieved with the grafting of

429

polyelectrolyte chains, CNC-PMAA is still not an ideal oil harvesting stabilizer as the

430

demulsification efficiency at high pH values is not very effective.

431

Ungrafted PMAA chains are unable to stabilize emulsion droplets in its protonated or

432

deprotonated state as shown in Figure S5A. To systems stabilized by CNC-POEGMA (0.3

433

wt%), various amounts of free PMAA chains were introduced. At pH 2, a viscous

434

emulsion phase was observed in the supernatant in all the samples (see in Figure S4B and

435

S5), which suggests the presence of hydrogen bond interactions between the ether and

436

carboxylic groups on the POEGMA and PMAA respectively. We observed that neither of

437

them instantaneously destabilized the emulsions, even in excess of PMAA chains. (Note:

438

at low concentrations (Figure S5A-0.05 %, 0.25%, 0.5 %), free PMAA chains were prone

439

to interact with CNC-POEGMA resulting in a clear dispersion. At high polymer

440

concentration (2.5 %), excess free PMAA chains will adopt a collapsed structure in water,

441

producing hydrophobic domains that partition hydrophobic florescence dye molecules,

442

such as Nile Red resulting in a pink solution (Figure S5A-2.50 %)). The stability of the

443

emulsions was monitored, and Figure S6 shows the changes in the size of emulsion

444

droplets as a function of time, where the size increased dramatically due to the

445

coalescence of the emulsion droplets (Figure S6A). We hypothesize that chain flexibility

446

and the molecular weight of PMAA may play important roles in destabilizing the

447

emulsions. At pH 2, adding free PMAA polymer into the CNC-POEGMA Pickering

448

emulsion may lead to bridging of oil droplets driven by hydrogen bonding between 24


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Biomacromolecules

449

PMAA and POEGMA chains on the surface of CNC. The nanoparticles at the oil-water

450

phase may form large hydrophobic aggregates with time that reduce the total surface

451

coverage of CNC-POEGMA on the emulsion droplets. This induces the oil droplet to

452

"ripen" or coalesce to compensate for the reduced surface coverage of the emulsion

453

droplets. More studies are in progress to elucidate the observed phenomenon.

454 455

Figure 7. (A) Photographs of CNC-POEGMA (0.3 wt%) suspensions as well as emulsions

456

(heptane water ratio 3:7) stabilized by them under different pH values; (B) Photographs of

457

CNC-POEGMA-PMAA (0.3 wt%) suspensions as well as emulsions (heptane water ratio

458

3:7) stabilized by them under different pH values; (C) Optical micrographs of emulsions

459

stabilized by CNC-POEGMA-PMAA at different pHs; fluorescence micrographs of 25


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Page 26 of 35

460

heptane in water emulsion stabilized by CNC-POEGMA-PMAA (0.3 wt%) at pH 10, the

461

oil phase is dyed with Nile red. The scale bars shown in all pictures are 50 µm.

462 463

Reversibility and Recycling

464

It is vital to demonstrate the reversibility of the stimuli-responsive nanoparticles by

465

manipulating the external environmental conditions. Two short videos illustrating this

466

phenomenon were recorded and can be viewed in the Supporting information. The

467

demulsification occurred immediately when several drops of 1 M HCl were added to the

468

vial containing the stable Pickering emulsions (pH 6.8), resulting in the formation of two

469

separate layers (Figure 9A). When a similar amount of 1 M NaOH was added to reverse

470

the pH, stable emulsions were restored with a slight creaming effect due to the presence of

471

salts. Successive stabilization-destabilization cycles were possible by alternate addition of

472

acid or base, demonstrating the reversibility of the emulsification and oil-water separation

473

protocols. As shown in Figure 9B, a stable heptane-in-water emulsion could be stabilized

474

by 0.3 wt% CNC-POEGMA-PMAA nanoparticles. Tuning the pH to an acidic condition

475

leads to strong hydrogen bonding between the nanoparticles, resulting in the coalescence

476

and phase separation of the emulsion droplets. The oil phase could be recovered by gravity

477

separation with up to 92% capture efficiency (by volume ratio). The aggregated flocs can

478

be re-dispersed by changing the dispersion pH to 7, and the oil phase can be emulsified to

479

form stable emulsions. This process can be repeated 5 times without any loss in efficiency.

480

To further strengthen the potential application in oil transportation, a viscous silicone oil 26


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Biomacromolecules

481

(100 mPa s at 20 oC) was used as organic phase. The stabilization-destabilization process

482

by tuning the pH values was observed as shown in Figure S7.

483 484

Figure 8. Schematic illustrating the pH-responsive behavior of Pickering emulsions

485

stabilized by CNC-POEGMA-PMAA.

486

487 488

Figure 9. (A) Flowchart showing the reversible stabilization-destabilization by alternate

489

addition of acid or base and (B) a demonstration of oil harvesting by manipulating the pH.

490 27


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491

Page 28 of 35

Conclusions

492

Binary polymer brushes, poly(oligoethylene glycol) methacrylate (POEGMA) and

493

poly(methacrylic acid) (PMAA), grafted cellulose nanocrystals were synthesized using a

494

straightforward synthetic protocols. The modified nanoparticles can respond to specific

495

triggers, such as temperature (CNC-POEGMA), pH (CNC-PMAA) or temperature/pH

496

(CNC-POEGMA-PMAA). Destabilization of the emulsion was achieved by increasing the

497

temperature caused by the conformational change of POEGMA chains. By incorporating

498

PMAA chains,

499

pH-responsive stability caused by the tunable interaction between the nanoparticles.

500

Bifunctionalized CNCs diffused to the oil-water interface and stabilized the oil droplets at

501

high pHs. When the pH was reduced to 2, strong hydrogen bonding between POEGMA

502

and PMAA chains grafted on the CNC induced instant coalescence of emulsions, resulting

503

in the phase separation of oil and water. However, for emulsions stabilized by

504

CNC-POEGMA and free PMAA mixtures, instant coalescence was not observed at low

505

pHs. We hypothesized that the chain flexibility and molecular weight of PMAA may play

506

important roles in destabilizing the emulsions. Furthermore, we demonstrated a reversible

507

emulsification-demulsification process controlled by pH, where the emulsification and

508

oil-water separation can be repeated 5 times without any loss in efficiency. A new

509

approach to prepare surface-tailorable sustainable nanomaterials for oil-water separations,

510

especially for oil droplet transportation and lipophilic substances harvesting was

511

developed.

the

emulsions

stabilized

by CNC-POEGMA-PMAA displayed

28


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512

Corresponding Author

513

* Kam Chiu (Michael) Tam,

514

[email protected], to whom correspondence should be addressed.

515 516

Supporting information

517

Potentiometric titration, rheology profile, size distribution of the emulsion droplets, and

518

emulsion pictures are included. This material is available free of charge via the internet at

519

http://pubs.acs.org. Two videos describing the stabilization-destabilisation process induced

520

by pH can be viewed in the supporting information.

521 522

29


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523

Acknowledgement

524

We wish to acknowledge Celluforce Inc. for providing the cellulose nanocrystals. K.C.T.

525

wishes to acknowledge funding from CFI and NSERC. J.T. wishes to acknowledge

526

Quanquan Pang for assisting with the TGA measurement and Prof. Juewen Liu for

527

providing access to the optical microscope as well as the zeta-potential analyzer.

528 529

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