N2-Switchable Zwitterionic Surfactant for Pickering

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Highly CO2/N2 Switchable Zwitterionic Surfactant for Pickering Emulsions at Ambient Temperature Pingwei Liu, Weiqiang Lu, Wen-Jun Wang, Bo-Geng Li, and Shiping Zhu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la502749x • Publication Date (Web): 08 Aug 2014 Downloaded from http://pubs.acs.org on August 12, 2014

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Highly CO2/N2 Switchable Zwitterionic Surfactant for Pickering Emulsions at Ambient Temperature

Pingwei Liu,1,# Weiqiang Lu,1,# Wen-Jun Wang,1,2,* Bo-Geng Li,1 Shiping Zhu,3,*

1

State Key Lab of Chemical Engineering, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, China, 310027

2

Key Lab of Biomass Chemical Engineering of Ministry of Education, Department of Chemical and Biological Engineering, Zhejiang University, Hangzhou, China, 310027

3

Department of Chemical Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7

#

*

These two authors contribute equally to this paper Corresponding authors.

[email protected] (W.-J. Wang); [email protected] (S.

Zhu)

ACS Paragon Plus Environment

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2

Abstract

Crosslinked polymer particles were prepared via surfactant-free emulsion copolymerization of 2-(diethylamino)ethyl

methacrylate

(DEAEMA)

and

N,N'-methylenebisacrylamide (MBA) as a crosslinker.

sodium

methacrylate

(SMA)

using

Generated particles are zwitterionic

possessing unique isoelectric points in the pH range of 7.5-8.0, which is readily tunable through CO2/N2 bubbling.

The particles were found to be highly responsive to CO2/N2 switching,

dissolving in water with CO2 bubbling and precipitating with N2 bubbling at room temperature. Pickering emulsions of n-dodecane were prepared using these particles as the sole emulsifier. These emulsions can be rapidly demulsified with CO2 bubbling, resulting in complete oil/water phase separations. homogenization.

Nitrogen bubbling efficiently re-emulsifies the oil with the aid of The rapid emulsification/demulsification using CO2/N2 bubbling at room

temperature provides these crosslinked zwitterionic particles with distinct advantages as functional Pickering surfactants.


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3

Introduction

Switchable surfactants are compounds possessing tunable capability to reduce surface tensions in liquid-liquid and liquid-solid systems in response to changes in environmental condition.

By using

these species, stability of (micro)emulsions can be switched with specific external stimuli.1

Such

control is highly desired in many production processes, including emulsion polymerization, oil extraction, cleaning, organic catalysis, etc.

Their use facilitates product separation, transportation

and storage, saving energy and material costs.2-7 switchable surfactant structures reported in literature.

There have been a significant number of The external stimuli utilized has included

radiation, redox reagents, acids/bases, magnetism, temperature1 and, more recently, gases such as CO2.8

CO2-responsive surfactants have received particular attention due to the advantages of using

CO2 as trigger.

CO2 is cheap and abundant.

It is also nontoxic, noncorrosive, and

non-chlorinating, and its facile addition allows for a large-scale operation.

Most importantly, it can

be readily removed by N2 or air.9

Gas processing, such CO2/N2 bubbling, has clear advantages in industrial operations. particularly true in dealing with large volume production.

This is

Triggers such as radiation and

temperature are limited to small sample sizes. (Radiation has a limited depth of penetration into process streams and temperature changes are time consuming and energy intensive.)

For acid/base

switching, operations need to take into consideration the corrosion of equipment and the possible accumulation of salts, which requires further purification.

Other triggers like redox agents involve


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chemical reactions, which can generate dangerous or toxic by-products.

4

In comparison, gas triggers,

especially involving the use of CO2, has few limitations and drawbacks making it a good candidate for commercialization.

However, CO2-responsive surfactants and materials do have several distinct

shortcomings, which need to be resolved.4-6, 10, 11, 15

In switchable surfactants based on a CO2 trigger, the CO2 is reversibly bound and disassociated to increase hydrophilicity and hydrophobicity. amidine functional groups.2, 8, 10

A number of CO2-responsive surfactants are based on

Amidine compounds demonstrate excellent CO2 switchability, but

tend to be are unstable and prone to hydrolysis in aqueous solution and thus their switching capabilities are reduced in circulation.4

Also temperatures in excess of 65 °C11 and/or the addition

of a strong base like NaOH 4 are often required to reversely remove CO2 from these species.

More

recently, tertiary amine-based molecules such as 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2-(diethylamino)ethyl methacrylate (DEAEMA) and their polymers were found to be CO2 responsive.12

These monomers and polymers are commercially available and are relatively

inexpensive.

Although the monomers are susceptible to hydrolysis under acidic conditions, their

polymers are actually insensitive to it, even under extreme conditions (e.g., 80 °C and pH = 1).13 These polymers have great potential for applications such as switchable surfactants and other smart materials.5-7, 14

CO2 bubbling can only induce small changes in the pH of aqueous solutions. Switching processes using CO2 are normally not as efficient or as rapid as those that utilize the addition of acids. For example, in a previous study, PDMAEMA-b-PMMA (MMA = methyl methacrylate) was used as a


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surfactant in the emulsion polymerization of MMA. HCl was required.6

5

Prior protonation of the block copolymer with

In the recovery step, it was found that not all of the CO2-bound tertiary amines

could be deprotonated at room temperature through bubbling N2 or other inert gases.

It could be

that CO2, water, and amine groups formed some stable ammonium carbonate salts or that H2CO3 was difficult to be completely removed from water by N2 or other inert gas bubbling within a short period of time, i.e. 15 min.

Some heating was required. Jessop et al. 5 used DEAEMA as commoner to

prepare N2-coagulatable and CO2-redispersable polymer latexes. latexes were heated to 40 °C. 5 Pickering emulsifiers.

In the coagulation step, the

Similar problems were found with the use of PDEAEMA latexes as

Armes et al.15 found that at 20 °C 4 h of CO2 bubbling was required to for a

complete demulsification, and N2 purging alone was not enough to recover the polymer particles.15

We believe there are ways in engineering structures and compositions of the surfactants for improving their performances.16-17 This is particularly true with polymeric surfactants.

It is

hypothesized that a slightly crosslinked and loose chain structure would increase accessibility of amine groups, enhancing the CO2/N2 responsive performance of the polymer surfactant.

It should

be pointed out that crosslinking is essential for reversibility of polymer particle morphology, but too much crosslinking would limit accessibility of amine groups by gas molecules.

Furthermore, it is

believed that the difficulty in recovering the emulsification ability of amine-based surfactants at room temperature is due to residual protonated amine moieties inhibiting the removal of CO2 via N2 bubbling.

The introduction of anionic moieties to the chain structure of polymer surfactant through

polymerizations in the presence of anionic co-monomers or their salts should balance the residual protonated amine groups and greatly enhance the stripping of CO2 from the structure.


This

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copolymerization of amine-containing monomers with anionic comonomers results in a zwitterionic polymer with isoelectric points (IEPs) in the pH range adjustable via CO2/N2 bubbling.

It has been

reported that surface tension, zeta potential, and other properties of zwitterionic copolymers vary significantly with a very small pH changes in the IEP region.18-19

Here, DEAEMA-based polymeric particle surfactants were prepared based on the hypothesis outlined above in hopes of generating species possessing strong CO2/N2 switching responses at room temperature.

The polymer particles were prepared via a surfactant-free emulsion copolymerization

of DEAEMA and sodium methacrylate (SMA) with N,N'-methylenebisacrylamide (MBA) as a crosslinker under pH = 8.8.

The SMA was chosen because it served as a stabilizer during the

emulsion polymerization process.

The CO2/N2 switchability of the particles was evaluated both in

aqueous solutions and Pickering emulsions.

Of particular interest in this work was the

room-temperature CO2/N2 switching performance and speed of the response.

Experimental Section

Materials 2-(Diethylamino)ethyl methacrylate (DEAEMA, 99%, Aladdin) was passed through a column filled with basic alumina to remove inhibitor.

N,N’-methylene bis(acrylamide) (MBA, ≥ 98%, J&K

chemical), methacrylic acid (MAA, > 99%, Aladdin), potassium persulfate (KPS, 99%, J&K chemical), and n-dodecane (98%, Aladdin) were used as received.


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Surfactant-free emulsion copolymerization of DEAEMA and SMA The following is a typical synthetic procedure with C4 of Table 1 as an example.

In a 100-mL

round-bottom three-neck flask, MAA (0.4g, 4.65mmol), deionized water, and NaOH aqueous solution (1.0 M) with an appropriate volume ratio were added. mL.

The pH of the solution was adjusted to 8.0.

The total solution volume was 38

DEAEMA (2 g, 10.81 mmol) and MBA (40 mg,

0.26 mmol) were then added and mixed using a mechanical stirrer. was about 9.0.

After a degassing with N2 for 30 min, the flask was heated to 70 °C at a stirring

speed of 370 rpm.

2-mL aqueous solution of KPS initiator (20 mg, 1.0 wt% to the monomers) was

injected to initiate the reaction. min.

The pH value of the solution

The color of the polymerization system turned to milky-white at 30

After 5 h polymerization, the flask was removed out from the oil bath to terminate the

reaction.

The as-produced latex solution was analyzed by dynamic light scattering (DLS) to get the

particle size distribution.

To purify the latex solution, a dialysis against NaOH solution with a pH

of about 8.8 for one week was adopted, and the dialysis solution was changed twice daily in the first two days and once daily later.

Raw and purified copolymer products were obtained after

lyophilization of corresponding latex solutions under vacuum.

The yield was calculated via

comparison of the product weights of freeze-dried copolymers and same volume latex solution before and after dialysis.

The compositions of purified copolymers were analyzed by 1H NMR.

Theoretical IEP values of specific copolymers were calculated from eq. (1):20-21

1 IEP = pK b + log{ × [(1 - R)/R + [((1 - R)/R) 2 + (4/R) × 10 pK a - pK b ]1/2 ]} 2

(1)

where pKa and pKb are the dissociation constants of acidic and basic groups, respectively, herein pKa of MAA is about 5.422 and the pKb of DEAEMA is 7.3,23

R is the ratio of acidic to basic residues


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based on the 1H NMR results above.

Preparation of Pickering emulsions Subsequent to dialysis, the latex solution was diluted with deionized water and/or the NaOH aqueous solution (0.01 M) to prepare a sample solution with copolymer content of 2 wt% and pH of 8.8. The sample solution (5.0 mL) and n-dodecane (2.5 mL) were added into a 20-mL glass vial equipped with a magnetic stir bar.

The two-phase solution was emulsified with an IKA Ultra-Turrax T-18

homogenizer having a 10 mm dispersing tool operated at 11000 rpm for 2 min. stirring in the process of homogenization.

The solution kept

Further characterizations were conducted after a standing

of the resulting Pickering emulsion for 30 min at 25 °C.

All the emulsion samples were stored

more than 180 days at room temperature to observe their stabilities.

CO2/N2-responsive performance of copolymer particles in aqueous solution After dilution with the NaOH aqueous solution (pH = 8.8), purified latexes (5 mL) having 0.5 wt% copolymer content were charged into a 20-mL glass vial equipped with a magnetic stir bar.

In the

CO2-bubbling step, CO2 (0.3 L/min) was purged for 15 min under stirring at room temperature. The resulting solutions were allowed to stand for 5 min, and their pH value, particle size distribution, and Zeta-potential were analyzed.

In the N2 bubbling step, the same solutions were bubbled with

N2 (0.3 L/min) for 20 min under stirring at room temperature. was also analyzed.

The pH of resulted solutions after N2

The protonation degree (PD) of DEAEMA ( [D + ]/([D + ] + [D])) was calculated

from the solution pH values using eq. (2):


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PD =

where K’a is the acid dissociation constant,15

9

1 K 'a 1+ + [H ]

(2)

[D] and [D+] are the molarities of neutral and

protonated DEAEMA units in the copolymer, and [H+] is the molarity of hydrogen ion determined by the solution pH value.

The CO2 and N2 bubbling operations at room temperature in Pickering

emulsion systems were the same as above.

Characterizations and Measurements 1

H NMR spectra of copolymers were measured with a Bruker Advance 400 spectrometer using

CD3OD as deuterated solvent.

Particle size distributions were measured using a Malvern Zetasizer

NanoZS model ZEN 3690 at room temperature.

The instrument is equipped with an argon ion laser

with wavelength of 633 nm and the detection angle is 90º.

Zeta-potential (ζ) of the copolymer

particles was determined in the same instrument using capillary cuvette.

The NaOH aqueous

solution with pH = 8.8 or methanol was used to dilute the as-produced or dialyzed latexes in preparation of the samples having about 0.2 wt% copolymer contents. adjust the sample pH value from 8.8 to 6.0.

Diluted HCl was used to

CO2-saturated deionized water was used in the

measurement of CO2-processed copolymer particles.

The pH values of sample solutions were

measured with PHSJ-4A pH meter (Leici, Shanghai).

The type of Pickering emulsions (oil/water or water/oil) was determined via drop test and measurement of the emulsion conductivity with the conductivity meter at 25 ± 0.2 °C.

The sizes

and morphologies of oil droplets in the Pickering emulsions were observed with an optical


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microscope (Nikon ECLIPSE E600W POL) at room temperature. emulsion was applied in the sample preparation.

10X dilution of the original

The mean droplet diameter (Dm) and standard

derivation (σ) of the oil droplets were estimated from the analysis of statistical samples having >300 droplets.

Transmission electron microscopy (TEM) images of the copolymer particles in original or dialyzed latexes or the oil droplet of Pickering emulsion were acquired with a JEOL JEM-1230 microscope operated at 80 kV.

A drop of the latex sample (0.1 wt%, pH = 8.8) or Pickering emulsion (original

emulsion diluted with 10X water) was transferred onto a copper grid coated with Formvar stabilized with carbon support film.

The sample was air-dried at room temperature prior to characterization.

Results and Discussion

DEAEMA and its polymers are hydrophobic in the aqueous solution at a pH of 9 and 70 °C.

The

water-soluble comonomer SMA can thus act as a stabilizer in carrying out the surfactant-free emulsion copolymerization of DEAEMA with MBA as a cross-linker. experimental conditions and polymerization results.

Table 1 summarizes the

When the molar percentages of

SMA/DEAEMA were 7.5, 21.5 and 32.2% (C1, C2 and C3), respectively, the emulsions were not stable and formed large aggregated particles.

It appears that a minimum amount of SMA is

required to make the surfactant-free emulsion polymerization stable.

When 43.0% SMA (C4) or

more (C5, 53.8%) were used, the emulsion systems were stable and well behaved.


Figure 1

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presents the particle size distributions of C2-C5 measured by the DLS.

High levels of SMA

(43.0-53.8 mol% to DEAEMA) were required for the formation of narrowly distributed unimodal polymer particles.

The copolymer particles C6 and C7 having higher crosslinker MBA contents of 6.0 and 12.0 mol% were also prepared at the same [SMA]/[DEAEMA] = 43% as in C4 (Table 1).

Higher MBA

amounts slightly decreased particle sizes and increased polydispersity indexes (PDIs).

The C4

particles have Z-average particle size (DZ) of 211.2 nm and PDI of 0.089, while C6 of 204.9 nm and 0.125, and C7 of 201.7 nm and 0.161 (corresponding size distribution curves were shown in Figure S1 of the Supporting Information).

Table 1. PDEAEMA-co-SMA copolymer particles prepared via surfactant-free emulsion polymerization a IEP

MBA b

(mol%)

(mol%)

C1 h

7.5

2.4%

─

─

─

─

1522

0.883

C2

21.5

2.4%

63

74

7.6

─

301.9

0.236

C3

32.2

2.4%

57

72

7.5

─

228.6

0.186

C4

43.0

2.4%

62

64

7.2

8.0

211.2

0.089

C5

53.8

2.4%

60

66

7.3

─

185.9

0.099

C6

43.0

6.0%

61

66

7.3

7.8

204.9

0.125

C7

43.0

12.0%

─

─

─

7.5

201.7

0.161

Run

a

DEAEMA

SMA b

Yield %

c

content

d

(mol%)

Dz g

e

Theo. Expr.

f

PDI

(nm)

g

Other conditions: DEAEMA, 2 g; KPS, 1 wt. % of DEAEMA; SMA 0.4 g; water, 40 mL; pH = 9; temperature, 70

ºC; reaction time, 5h.

b

The molar ratio of SMA and MBA to DEAEMA, respectively. d

dialysis based on weighting method. g

Polymer yield after

DEAEMA content of the copolymer determined by 1H NMR.

Theoretical isoelectric point (IEP) calculated from eq. (1). of the copolymer particles is zero.

c

f

e

Experimental IEP is the pH at which Zeta-potential (ζ)

Z-average diameter (DZ) and polydispersity index (PDI) of copolymer

particles in aqueous solution (pH = 8.8) measured by DLS at room temperature. aggregated particles were found during the polymerization.


h

C1 was not stable and large

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Figure 1. The intensity size distributions of copolymer particles C2-C5 prepared at different molar percentages (21.5%-53.8 mol%) of SMA to DEAEMA but the same ratio (2.4%) of crosslinker MBA to DEAEMA.

Figure 2 shows TEM images of the as-produced C4 at two different magnifications. have an average diameter of 230 ± 50 nm. also present.

Grey spheres

Many white nano-objects having different shapes were

The white objects could be the aggregated SMA oligomers and polymers.

Most of

them, however, could be removed via dialysis (MW = 3000 cut-off) against an alkaline aqueous solution at pH = 8.8. white objects.

Figure 2 (c) shows the TEM graph of the dialyzed C4, which is clean of the

In addition, the dialysis process did not adversely affect the particle size distribution

of C4 (Figure S2 of the Supporting Information). 61-62% after dialysis.

The C4, C5 and C6 had similar polymer yields of

The polymer yield of C7 was unreasonably high because of difficulty in

removing its water via lyophilization.

(a)

(b)


(c)

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Figure 2. TEM images of C4 before ((a) & (b)) and after ((c)) dialysis with magnification of (a) 25000X (scale bar = 1 µm), (b) 50000X (0.5 µm), and (c) 25000X (1 µm)

Subsequently, the further investigations mainly focused on three samples C4, C6 and C7 prepared with 43% SMA, which was the minimum amount of SMA for stable emulsion polymerization.

In

water, these samples had similar volume size distributions in the range from 60 to 600 nm. However, in methanol, they showed very different size distributions.

As shown in Figure 3, C4

and C6 were significantly reduced in size, mainly in the range of 30 and 60 nm. had its size dramatically increased to the range of 300-1200 nm. of crosslink density heterogeneity in the particles.

In contrast, C7

These variations could be a result

MBA was highly active in polymerizations and

consumed and statistically incorporated into copolymer chains faster than other monomers.24-25 The C4 and C6 might have a greater amount of slightly crosslinked or loosely connected chains, which could be solvated by methanol and expanded into the solution.

The C7 particles, prepared

with the highest MBA amount among the three samples, were more densely crosslinked, which could only be swollen by methanol.

The original and purified copolymers were obtained via a freeze drying of the as-produced and dialyzed latexes, respectively. were tested.

The solubilities of these polymers in the deuterated solvent CD3OD

The original samples of C4 and C6 formed suspensions with white insoluble powders,

while the purified C4 and C6 were readily dissolved in CD3OD and formed transparent solutions without any insoluble.

The white powders should be PSMA oligomers and most of them were

removed via dialysis, consistent with the TEM results. solution.

Purified C7 formed a creamy and viscous

All unreacted monomers were removed via dialysis with no double bond signal peaks


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found at 5.00-6.00 ppm in NMR measurements. DEAEMA units in the C4 and C6.

14

There were clear signals of incorporated

The C7 had little signal response because of highly crosslinked

structure. The 1H NMR spectra were included in the Supporting Information.

Figure S3 compares

the purified C4 (a), C6 (b), and C7 (c) in CD3OD and Figure S4 identifies signals of the structure units in the purified C4.

Figure 3. Volume size distributions of the C4 latexes (MBA, 2.4 mol%), C6 (6.0%), and C7 latexes (12.0%) diluted with 10 times of NaOH aqueous solution (pH = 8.8) or methanol.

As shown in Table 1, the molar percentages of incorporated DEAEMA to SMA units in the purified copolymers were 64% for C4 and 66% for C6, respectively, determined through integration of the 1H NMR spectra.

It should be pointed out that the proton signals of densely crosslinked parts in the

copolymer particles (C7) could not be acquired by 1H NMR. representative for the lightly crosslinked and loose chain structures. result, the theoretical IEPs were calculated using eq. (1).

These data might be more Based on the composition

The IEPs of C4 and C6 were 7.2 and 7.3,

respectively.


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The purified C4, C6, and C7 were used to investigate the pH- and CO2-responsive properties of the particles.

Figure 4 (a) shows the variation of DZ with pH.

An IEP region can be identified in each

of the three copolymers in which the latex was not stable and aggregated to form large particles with DZ >1 µm.18

Compared to the C4 of pH = 7.8 - 8.5 and the C6 of pH = 7.7 - 8.4, the C7 had an IEP

region of lower pH = 7.2 - 7.9.

In addition, at pH < 7.5 outside the IEP region, smaller DZs were

found in the C4 and C6 while larger DZs in the C7. As illustrated in Scheme 1, the lightly crosslinked copolymer chain structures in the C4 and C6 particles could be hydrated and expanded by water molecules under acidic conditions when DEAEMA units were protonated by H+.

In

comparison, the same protonated units in densely crosslinked particles C7 could only be swollen after hydration. This is consistent with the analysis about the change of their sizes in methanol.

It

should be pointed out that the difference between the particle size data in water shown in Figures 3 and that shown in Figure 4(a) was due to a difference in pH.

In Figure 3, the aqueous solution had

pH 8.8 and in Figure 4(a), it was < 7.5, i.e., both outside the IEP region but on opposite sides of it.

Figure 4 (b) shows the pH dependence of zeta-potential (ζ) of the C4, C6, and C7. values of the three copolymer particles were all 300 droplets gave the mean size in III of 12.96 ± 5.09 µm (II: 12.19 ± 3.82 µm and IV: 12.88 ± 4.85 µm), close to 14.51 ± 4.59 µm in I.

The sizes and morphologies of copolymer particles at the

oil droplet interface of III were studied by TEM.

As shown in Figures 6 (c) and 6(d), spherical

copolymer particles of Dm =130 ± 30 nm were clearly observed in Figure 6 (d).

It became evident

that the N2-recorvered copolymer particles acted as a stabilizer in the Pickering emulsions.

(a) Emulsion I

Dmean = 14.51 ± 4.59 um (

(b) Emulsion III

= 20 µm)

(c) Oil droplet from Emulsion III

Dmean = 12.96 ± 5.09 um (

= 20 µm)

(d) Interface of the oil droplet

Figure 6. Optical microscopy (OM) images of Pickering emulsion I (a) and III (b) stabilized by C4


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and TEM images of (c) a representative oil droplet from Emulsion III and (b) the interface of an oil droplet.

Conclusions

Zwitterionic poly(DEAEMA-co-SMA) particles were prepared using surfactant-free emulsion copolymerization with small amount of MBA as cross-linker.

The zwitterionic nature of the

copolymers renders the particles high responsiveness to small pH variation in the IEP region, which was easily regulated by CO2/N2 bubbling.

Depending on the crosslink density, the particles could

be dissolved or swollen in aqueous solutions upon CO2 bubbling.

The CO2-treated particles could

be precipitated with N2 bubbling regardless of the level of crosslink density at room temperature. This high reversibility in CO2/N2 switching under ambient conditions resulted from the combination of the zwitterionic nature and crosslinked structure of the particles. balanced the residual protonated DEAEMA units.

The inclusion of SMA units

The lightly crosslinked structure with a

heterogeneous cross-link distribution facilitated interactions of DEAEMA units with CO2 and N2 and significantly enhanced switchability. Pickering emulsions.

The particles were used as emulsifier in preparation of

The emulsions could be rapidly demulsified into separated oil and water

phases with CO2 bubbling.

Demulsified oil/water system could be easily re-emulsified by N2

bubbling with the assistance of a homogenizer.

The demulsification and re-emulsification

processes were carried out at room temperature and could be repeated for numerous cycles.

This

work demonstrates the power of particle structure design in the development of novel CO2/N2-responsive materials.

The novelty in this design was the use of zwitterion to adjust IEPs

of the polymers into the pH range achievable by CO2/N2 and the lightly crosslinked structure with


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heterogeneous crosslink distribution enhancing the response to CO2/N2 switching.

Acknowledgements We thank National Natural Science Foundation of China (Grants 21376211 and 20976153, and Key Grant 20936006) and Chinese State Key Laboratory of Chemical Engineering at Zhejiang University (Grants SKL-ChE-12T05, SKL-ChE-11D02, and SKL-ChE-14D01) for supporting this work.

Associated Content Additional figures with supporting information are available free of charge via the Internet at http://pubs.acs.org/.

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TOC Graphic

Highly CO2/N2 Switchable Zwitterionic Surfactant for Pickering Emulsions at Ambient Temperature

Authors: Pingwei Liu, Weiqiang Lu, Wen-Jun Wang, Bo-Geng Li, and Shiping Zhu


N2-Switchable Zwitterionic Surfactant for Pickering

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