CO2-Responsive Pickering Emulsions Stabilized by a Bio-based Rigid Surfactant with Nanosilica

Article abstract of DOI:10.1021/acs.jafc.8b03458

A novel CO₂-responsive surfactant, maleopimaric acid glycidyl methacrylate ester 3-(dimethylamino)propylamine imide (MPAGN), based on sustainable resource of rosin was synthesized and used to prepare a kind of CO₂-responsive Pickering emulsions with nanosilica. MPAGN can be reversibly responsive to CO₂ and N₂ between active cationic (MPAGNH⁺) and inactive nonionic (MPAGN), leading to adsorb on or desorb from the surface of nanosilica, then stabilize or break emulsion. CO₂-responsive behavior of MPAGN was verified by cycle change of pH and conductivity with bubbling CO₂ and N₂ alternately. The type of adsorption of MPAGNH⁺ at the particle-water interface was explained according to the adsorption isotherms. The mechanisms of stabilization, destabilization, and restabilization of Pickering emulsion were analyzed according to zeta potentials and droplet size. This Pickering emulsion can be reversible between stable and unstable by bubbling CO₂ and N₂ alternately. Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil. Therefore, it not only has economic benefits but also has an environmentally friendly property.

Full text of DOI:10.1021/acs.jafc.8b03458

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Biofuels and Biobased Materials
CO2-Responsive Pickering Emulsions Stabilized
by a Bio-based Rigid Surfactant with Nano-silica
Xinyan Yan, Zhaolan Zhai, Ji Xu, Zhanqian Song, Shibin Shang, and Xiaoping Rao
J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.8b03458 • Publication Date (Web): 26 Sep 2018
Downloaded from http://pubs.acs.org on September 30, 2018
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CO2ꢀResponsive Pickering Emulsions
Stabilized by a Bioꢀbased Rigid
Surfactant with Nanoꢀsilica
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Xinyan Yan †, Zhaolan Zhai †, Ji Xu†, Zhanqian Song †，
*
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Shibin Shang †,§, Xiaoping Rao †,‡,§
†
Institute of Chemical Industry of Forest Products, CAF; National Engineering Lab. for Biomass Chemical
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Utilization；Key and Open Lab. Of Forest Chemical Engineering, SFA; Key Lab. of Biomass Energy and Material.
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‡
Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, Nanjing Forestry University.
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§ Research Institute of Forestry New Technology, CAF.
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Mailing address:
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†No.16 Suojinbei Road, Xuanwu District, Nanjing, Jiangsu Province, 210000, China.
‡ NO.159 Longpan Road, Xuanwu District, Nanjing, Jiangsu Province, 210000, China.
§
NO.1 Xiangshan Road, Haidian District, Beijing, 100091, China.
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AUTHOR INFORMATION
*Corresponding author: Xiaoping Rao
Email: rxping2001@163.com
Fax: + 86 25ꢀ85413445;
Tel: + 86ꢀ25ꢀ85482452;
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Abstract
A novel CO2ꢀ responsive surfactant, maleopimaric acid glycidyl methacrylate ester
3ꢀdimethylaminopropylamine imide (MPAGN), based on sustainable resource of rosin was
synthesized and used to prepare a kind of CO2ꢀresponsive Pickering emulsions with Nanoꢀsilica.
MPAGN can be reversibly responsive to CO2 and N2 between active cationic (MPAGNH+) and
inactive nonionic (MPAGN), leading to adsorb on or desorb from the surface of Nanoꢀsilica, then
stabilize or break emulsion. CO2ꢀresponsive behavior of MPAGN was verified by cycle change of
pH and conductivity with bubbling CO2 and N2 alternately. The type of adsorption of MPAGNH+
at particleꢀwater interface was explained according to the adsorption isotherms. The mechanisms
of stabilization, destabilization and reꢀstabilization of Pickering emulsion were analyzed according
to Zeta potentials and droplets size. This Pickering emulsion can be reversible between stable and
unstable by bubbling CO2 and N2 alternately. Moreover, this emulsifier can be recycled when new
oil was added after removing the initial oil. Therefore, it not only has economic benefits, but also
has environmental friendly property.
Keywords: rosin; rigid; CO2- responsive; Pickering emulsions, recycled
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Introduction
Conventional emulsions are stabilized by amphiphilic polymers or surfactants1, and they are
thermodynamically instable systems resulting in coalescence, sedimentation, creaming, or phase
inversion2. A far higher concentration than critical micelle concentration (CMC) is usually needed
to form a stable emulsion.
Pickering emulsion can be very stable only required a little amount of surfactant because it is
stabilized by surfaceꢀactive colloid particles, which provide a mechanical barrier to cream3.
However, it is difficult to be demulsified when it is used in some areas such as drug delivery4,
emulsion polymerization5, oil transport and fuel production6. Therefore, it has become a crucial
focus that how to balance between stability and instability of a Pickering emulsion.
Responsive surfactants can be switched reversibly between inactive and active under the
control of trigger. Responsive surfactants can be classified into two broad categories: one is
responded to the electrochemical stimuli such as pH7ꢀ13, CO214ꢀ20, redox21, 22, etc., the other is
responded to the physical stimuli, such as light23ꢀ26, temperature27, and magnetic field28ꢀ30, etc..
The responsive surfactant can be transferred to charged particles by electrostatic interaction and
the combination can be used to prepare responsive Pickering emulsion, which can be existed for a
long time while it can be demulsified immediately exposing to a stimulus. As a biocompatible,
nontoxic, inexpensive and easily remove trigger, CO2 has been attracted more attention from
researchers. There are two methods commonly used to prepare CO2 – responsive Pickering
emulsion. One was to use CO2 – responsive particles, which are polymeric particles usually.
However, these particles generally require complex synthetic procedures31. The other was to use a
CO2 ꢀresponsive compound, which were adsorbed on the surface of nanoparticles by electrostatic
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interaction. The latter was better than the former as the performance of the particles could be
controlled by changing the concentration or structure of the compound. Liu and coꢀworkers32 first
reported in 2006 that longꢀchain alkyl amidine could be switched by bubbling CO2 and N2, and
can be used as switchable emulsifier. Jiang33 reported a responsive Pickering emulsion stabilized
by existing responsive surfactant (N’ꢀdodecylꢀN,Nꢀdimethylacetamidine) with Nanoꢀsilica particle.
Zhang and coꢀworkers31 fabricated a CO2ꢀ responsive Pickering emulsion based on long
hydrophobic tertiary amine with Nanoꢀsilica particle. They also reported a CO2 –responsive
Pickering emulsion stabilized by amine oxideꢀbased surfactant (C14PAO) with Nanoꢀsilica
particle34. In these reports, the materials of CO2 responsive surfactants were often restricted to
existing longꢀchain alkane surfactants, which were mainly derived from petrochemical resources.
Studies on sustainable chemistry have drawn attention widely because of the urgency for
resolving environmental problems35. Rosin is a kind of unique sustainable biomass resource,
obtained from pine trees. It has a unique tricyclic rigid structure36, and its derivatives are abundant,
renewable, sustainable, economical, and nontoxic37. It can be used to prepare surfactants thanks to
the excellent hydrophobicity of its tricyclic rigid backbone38. We previously synthesized some
surfactants based on rosin and studied their properties such as aggregation behavior39, emulsifying
properties 38 and dispersing properties40.
In this study, a novel CO2 ꢀ responsive surfactant based on natural tricyclic rosin was
synthesized, which have a lower critical micelle concentration (CMC) than that of longꢀchain
alkane CTAB, thanks to its hydrophobic tricyclic hydrophenanthrene rigid skeleton. This
surfactant can be switched reversibly between active cationic (water soluble) and inactive
nonionic (water insoluble) by bubbling CO2 and N2. A CO2 ꢀ responsive Pickering emulsion was
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prepared, which was stabilized by this CO2 ꢀ responsive surfactants in combine with 0.5wt%
Nanoꢀsilica. This Pickering emulsion can be switched many times between stable and unstable by
bubbling CO2 and N2. Even more, after demulsification by bubbling N2, the emulsifier can be
recycled so it embodies the ideal of green and sustainable development in the field of marketing.
Experimental Section
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Materials and Equipment
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Rosin was received from Westech chemical co. LTD (China). Nanoꢀsilica (purity>99.5%,
primary particle diameter ≈30nm；BET surface area ≈115 m2/g.) and Glycidyl methacrylate (GMA)
were obtained from Aldrich (China). 3ꢀdimethylaminopropylamine was obtained from Energy
Chemical (China). Hydroquinol, triethylamine, maleic acid, glacial acetic acid, absolute ethyl
alcohol, absolute methyl alcohol and dichloromethane were purchased from Sinopharm Chemical
Reagent co. LTD (China). Methylbenzene was obtained from Nanjing chemical reagent co. LTD.
(China). They were all analytical pure and used as received. Distilled water was produced by
Hitech laboratory purification system (with a pH of 6.1 and electrical resistivity of 18.2Mꢁ.cm).
FTꢀIR spectra were recorded on a Thermo Scientific Nicolet IS10. 1H NMR was measured by
an Advance III 400 MHz spectrometer (Bruker, Germany). Mass spectrum was performed on an
Agilentꢀ5973 spectrometer (Agilent Technologies, USA).
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Synthesis of MPAGN
Synthesis of maleopimaric acid (MPA):400g rosin and 140g maleic acid were dissolved in
160g glacial acetic acid. The mixtures were stirred at 140℃ for 4h. After cooling, the mixtures
were poured into 400g glacial acetic acid and the crystalline MPA was obtained after suction
filtering and dryer. The crystal was further purified by crystallization using acetic acid for another
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twice41 (purity: 93%).
Synthesis of maleopimaric acid glycidyl methacrylate ester (MPAG): 40g MPA and 15.6g
GMA were dissolved into 210g toluene. Triethylamine (0.5wt % with respect to MPA) was used as
the catalyst, and hydroquinone (0.2wt % with respect to GMA) was used as inhibitor. The
mixtures were stirred at 120℃ for 8h. Then, the toluene was evaporated.
Synthesis of maleopimaric acid glycidyl methacrylate ester 3ꢀdimethylaminopropylamine
imide (MPAGN): Obtained MPAG was dissolved in 200g ethyl alcohol and stirred at 85 ℃. Then,
a solution of 11.2g 3ꢀdimethylaminopropylamine dissolved in 10g ethyl alcohol was dropped in
the flask. They were stirred at 85℃ for 5h. The crude product was purified by recrystallization
using acetone and a little amount of methyl alcohol. The crystal was removed and the filtrate was
dried using a rotary vacuum dryer and purified further via column chromatography (the ratio of
methyl alcohol to dichloromethane = 1:9) to get the target product MPAGN (yield: 24%).
Synthesis route of MPAGN was shown in Scheme 1. This type of surfactant was polymerizable.
Therefore, it maybe has potential application value in the emulsifier free emulsion polymerization
field. Firstly, in this study, its emulsifying property was studied.
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Scheme 1 Synthesis route of MPAGN
Preparing of Pickering emulsions.
MPAGN was protonated and formed cationic tertiary amine bicarbonate by bubbling CO2. To
distinguish the cationic tertiary amine bicarbonate from nonionic form of MPAGN，the former was
written as MPAGNH+.
0.5wt% Nanoꢀsilica particles were dispersed in different concentrations of MPAGNH+ using
an ultrasonic cell disruptor (750 Watt and 20 KHz) for 1min. 7mL MPAGNH+ solution or
Nanoꢀsilica or MPAGNH+ with Nanoꢀsilica and 7mL liquid paraffin were put in a 25cm3 glass
bottle followed by homogenization at 11000rpm for 5min (IKA T18, S25Nꢀ8G head). The
concentrations of MPAGNH+ and Nanoꢀsilica reported in this paper were expressed as mmols per
liter (mM) and weight percent (wt%) referred to the aqueous phase6. The emulsions were stored
for at least 24h at 25℃ before being analyzed.
Measurements:
(a) Conductivity / pH: The conductivity / pH of MPAGN solutions with bubbling CO2 or N2
were measured by a DDSꢀ307A conductometer and Thermo Scientific Orion pH meter at 25℃
respectively. The gas flow rate of CO2 or N2 was about 100mL/min.
(b) Surface Tension: The surface tensions of different concentrations of MPAGNH+ solution
and with Nanoꢀsilica were measured by a Sigma701 at 25 °C. The CMC and surface tension (γ) of
MPAGNH+ solution were obtained when the adsorption balance was reached. They were
equilibrated at 25℃ for at least 12h before measurement.
(c) Zeta Potentials: The Zeta Potentials of 0.5% Nanoꢀsilica particles dispersed in different
concentrations of MPAGNH+ solution were measured by a Zeta sizer instrument (Nano ZS ZEN
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3600, Malvern Instruments Ltd.). They were equilibrated at 25℃ for at least 12h before being
analyzed.
(d)Adsorption of MPAGNH+ at silica -water interface: The adsorption isotherm of
MPAGNH+ at the interface of particleꢀwater at equilibrium concentration (< CMC) was obtained
according to the depletion method at 25℃, and the surface tension of MPAGNH+ solution without
Nanoꢀsilica was used as calibration. The curve of log Cꢀγ of different concentrations MPAGNH+
solution without Nanoꢀsilica was fitted by Origin 8.5 followed Szyszkowski equation.
Results and Discussion
Characterization
1
FTꢀIR spectra of MPA, MPAG and MPAGN were shown in Fig.S1. H NMR spectrum and
MS spectrum of MPAGN was shown in Fig.S2 and Fig.S3 respectively. It can be implied that the
intermediate product MPAG and target product MPAGN had been synthesized successfully.
CO2-responsive behaviour of MPAGN
It has been reported that the head group of tertiary amineꢀbased surfactant could be switched
between active states (cationic) and inactive states (nonꢀionic) with transforming the pH of
solution34. CO2 was acted as a pH trigger offering a lot of advantage such as ease of operation, low
cost, environmentꢀfriendly, and avoiding extra acid or base solution causing accumulation of salt,
contamination and any byꢀproduct in system42. Reversible transformation between cationic
MPAGNH+ and nonionic MPAGN was shown in Scheme 2. Neutral tertiary amine was protonated
and formed cationic tertiary amine bicarbonate, which was active and water soluble form as
shown in inset of Scheme2 , by bubbling CO2 to solution for 3 minutes at room temperature. The
active form was deprotonated and returned back to the inactive and water insoluble neutral form
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as shown in insets of Scheme2, by bubbling N2 to the solution for 30 minutes at 60℃. The
conductivity and pH were monitored to verify the CO2ꢀresponsive performance and
interconversion of MPAGN. When CO2 was bubbled into the solution, its conductivity was
measured to be 340 ꢂS cmꢀ1, and its pH value was about 5.1 lower than that of the water slightly
because water contained CO2 making it a weak acid. After N2 was bubbled through solution, the
conductivity of solution decreased dramatically to 177ꢂS cmꢀ1, and pH value increased to 7.7
because the CO2 in solution was forced out and the cationic was deprotonated. After bubbling CO2
through solution again, the conductivity of solution increased back to 340 ꢂS cmꢀ1 and pH value
was back to about 5.1. Their reversion can be recycled for several times without marked change in
pH and conductivity as shown in Fig.1, which indicated that the MPAGN have excellent
CO2ꢀresponsive behaviour.
O
O
CH3
N
CH3
HCO3
H2
C
H2
C
ꢀ
N
CH3
N
C
N
C
C
C
CH3
H2
H2
H2
H2O
CO2
H2
O
O
N2/air
H2
C
H2
C
H
H
COOCH2C
O
C
O
C
COOCH2C
O
C
O
C
CH2
CH2
OH
CH3
OH
CH3
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Scheme2. Reversible transformation between cationic MPAGNH+ and nonionic MPAGN;
Insets photographs are corresponding appearance of nonionic and cationic form in water.
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Fig.1 Cycle change of pH and conductivity with bubbling CO2 and N2 alternately
Pickering Emulsion stabilized by MPAGNH+ combined with Nano-silica
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Now that MPAGN could be converted to cationic by bubbling CO2, it was predicted that the
cationic MPAGNH+ could adsorb on the surface of negatively charged Nanoꢀsilica in aqueous
solution by electrostatic interaction and the combination could be used to prepare Pickering
emulsion. Although MPAGNH+ alone can also stabilize liquid paraffinꢀinꢀwater emulsion, a high
concentration (10mM>>CMC) was needed as shown in Fig.2a (taken 24h after preparation) and
2b (taken 1 week after preparation). The bare Nanoꢀsilica cannot stabilize emulsion either because
of its strong hydrophilic33 as shown in Fig.2c (the farthest left). However, only a little amount of
+
MPAGNH（<<CMC）was needed to form a stable Pickering emulsion by combining with 0.5wt%
Nanoꢀsilica as shown in Fig.2c (taken 24h after preparation) and 2d (taken 1 week after
preparation). The emulsion type was oilꢀinꢀ water confirmed by drop test method31. As shown in
Fig.S4, a drop of emulsion could be dispersed into water but could not be dispersed in oil.
Although creaming still could not be avoided as shown in Fig.2c and 2d, their appearance
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was stable after 1 week without obvious change, that consisted with those emulsion stabilized by
conventional surfactant combining with Nanoꢀsilica 3, 43, 44. Besides, their stabilities also can be
proved by the distribution of average droplets size between 24h and 1 month after preparation as
shown in Fig.3. There was no obvious change, either.
The average droplet size of Pickering emulsion decreased with MPAGNH+ concentration
increased as shown in Fig.4 (AꢀC). Although the stability of emulsion was not affected by droplet
size significantly45, in general, it was regarded that the smaller droplet size presented better
stability against creaming43. However, their average droplet sizes have a larger magnitude than
that of stabilized by MPAGNH+ alone (shown in Fig.4D) because the droplets were stabilized by
MPAGNH+ꢀcoated particles, which was bigger than MPAGNH+ molecules alone. So it can be
concluded that this kind of emulsion was Pickering emulsion rather than traditional emulsion.
Besides, the mean volume diameter (d43) and mean surface diameter (d32) of emulsion stabilized
by MPAGNH+ with different concentrations measured by a laser diffraction particle size analyzer
(Mastersizer 2000) were also presented in Fig.4. The droplet size of emulsion could be compared
more intuitively according to their d43 and d32.
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Fig.2 Photographs of liquid paraffinꢀinꢀwater (7mL/7mL) emulsion stabilized by MPAGNH+ alone at different
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concentrations (mM) were taken 24h (a) and 1 week (b) after preparation; with 0.5wt% Nanoꢀsilica taken 24h(c)
and 1 week (d) after preparation
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Fig.3 Droplet diameter distributions of emulsion stabilized by 0.05mM MPAGNH+
with 0.5wt% Nanoꢀsilica measured after storage for 24h and 1month.
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Fig.4 Optical micrographs of emulsions stabilized by different concentrations of MPAGNH+ with (A, B, C )or
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without (D) 0.5wt% Nanoꢀsilica measured after storage at least 24h at 25℃;(A) 0.05mM; (B) 0.5mM; (C) 1mM;
(D)10mM taken by digital camera (scale bars =100ꢂm); (Insets) d43 and d32 of the corresponding emulsions.
Mechanism Analysis of stabilization of Pickering emulsion
The CMC of MPAGNH+ solution was about 0.4 mM as shown in Fig.5 lower than that of a
conventional cationic surfactant cetyltrimethyl ammonium bromide (CTAB, 0.9 mM)6 and lower
than that of tertiary amine with long hydrophobic tails Nꢀ(3ꢀ(dimethylamino) propyl) alkyl amide
(C14PMA, 2.35 mM)30, which indicated that cationic surfactant MPAGNH+ had excellent surface
activity. At equilibrium concentration less than CMC, the adsorption isotherm of MPAGNH+ on
the interface of silica ꢀwater was measured according to the depletion method at 25℃. At the
presence of particles, the surface tension of solution would not be affected so they need not be
separated. However, the surfactant in solution adsorbed on the surface of silica, resulting in the
equilibrium concentration of surfactant in solution reduced and the surface tension increased.
Therefore, the adsorbed amount can be calculated based on the difference between the surface
tension of MPAGNH+ solution alone and with Nanoꢀsilica.
The curves of log Cꢀsurface tension (γ) of different concentrations of MPAGNH+ solution
with and without Nanoꢀsilica were shown in Fig.5, and that without Nanoꢀsilica was fitted by
Origin 8.5 followed Szyszkowski equation (1). After fitting, as shown in Fig.S5, the parameter Γ∞
and k were obtained. They are 1.24×10ꢀ10mol/cm2, 29247 respectively, and the correlation
coefficient R2 was about 0.96. The crossꢀsectional area at the water ꢀ air interface adsorption (a∞)
was 1.34nm2 calculated according to the equation (2). The equilibrium concentration of surfactant
in dispersion was known based on the surface tension of MPAGNH+ solution with Nanoꢀsilica
according to equation (1). And then, adsorption amount of MPAGNH+ on the water ꢀ particle
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interface was obtained according to the equation (3). Then, the adsorption isotherm was drawn and
shown in Fig.5. The adsorbed amount of MPAGNH+ on the particleꢀwater interface increased with
the concentration of MPAGNH+ in solution increased. For instance, the adsorbed amount of
MPAGNH+ on the particle surface increased to 0.07 mmol/g when the equilibrium concentration
was CMC (0.4 mM). Therefore, the average molecular area of MPAGNH+ on the particle surface
(a) can be obtained by the equation (4), and it was 2.72nm2 (>a∞) implying that it was monolayer
between adsorption particle surface and MPAGNH+.
γ 0
ꢀ
γ
= nRTΓ∞ ln
(
1+ KC
)
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(1)
where γ0 was the surface tension of distilled water (72 mN/m);
γ was the surface tension of MPAGNH+ solution with Nanoꢀsilica (mN/m);
R was the gas constant (8.314×107 erg/(mol K));
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T was the Kelvin temperature (298.15 K);
Г∞ was the maximum adsorption amount of surfactant at gas liquid interface (mol/cm2);
K was constant (L/mol);
C was equilibrium concentration of surfactant (mol/L).
ꢀ
a∞ = ꢁꢂ Γ∞
(2)
where a∞ was the crossꢀsectional area at the water ꢀ air interface adsorption when the amount
of adsorbed surfactant was maximum ( mmol/g)
Γ∞ was the maximum adsorbed amount of surfactant at gas liquid interface (mol/cm2)
NA was Avogadro’s number
V (C0 − C)
Γs / w
=
w
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(3)
where Гs/w was the adsorption amount of MPAGNH+ on the water ꢀ particle
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interface( mmol/g);
V was the volume of MPAGNH+ solution (cm3)
C0 was the initial concentration of MPAGNH+ before adsorption ( mol/L);
C was the equilibrium concentration of MPAGNH+ after adsorption (mol/L);
w was the quality of Nanoꢀsilica (g).
ꢃꢄ
a = ꢁꢂ Гꢅ/ꢆ
(4)
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where a was the average area molecular on the surface of Nanoꢀsilica;
a0 was the average surface area of the Nanoꢀsilica;
NA was Avogadro’s number;
Г
s/w was the adsorption amount of MPAGNH+ on the particleꢀwater interface.
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Fig.5 Surface tension of MPAGNH+ solutions (■) with 0.5 wt% Nanoꢀsilica (●),
and adsorption isotherm of MPAGNH+ at the particleꢀwater interface (
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◆).
The 0.5wt% Nanoꢀsilica was negatively charged in pure water (ꢀ37.9mV). When it was
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dispersed in MPAGNH+ solution, the Zeta potential of dispersion increased to positive with
concentration of MPAGNH+ increased as shown in Fig.6. It can be concluded that the adsorption
between cationic MPAGNH+ and silica occurred forming a monolayer via electrostatic
interactions.
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Fig.6 Zeta potential of 0.5wt% Nanoꢀsilica dispersed in different concentrations of MPAGNH+ solution;
(Insert) graph of cycles of 0.5wt% Nanoꢀsilica dispersed in 0.4 mM MPAGN solution
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Destabilization/Re-stabilization cycling of this Pickering emulsion
CO2/N2 responsive behaviour of MPAGN has been confirmed in the above discussion. So it
was predicted that the Pickering emulsion stabilized by MPAGNꢀcoated particles may be also
CO2/N2 responsive and can be cycled between stable and unstable by bubbling CO2 and N2
alternately. To prove this assumption, a liquid paraffinꢀinꢀwater emulsion stabilized by 0.05 mM
MPAGNH+ and 0.5wt% Nanoꢀsilica was cycled by bubbling CO2 followed by homogenization
and then bubbling N2. In cycle 1, a stable emulsion was formed by bubbling CO2 at 25℃ for only
3 min followed by homogenization. However, when N2 was bubbled to replace CO2 at 60℃ for
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30 min, the stable Pickering emulsion was demulsified completely as shown in Fig.7 and it cannot
form stable emulsion even though reꢀhomogenization was performed. To rule out the effect of
temperature on the demulsification, the emulsion did not appear demulsification and remained
stable as shown in Fig.S6 when it was heated at 60℃ for 30 min without bubbling N2, which
indicated that N2 was the key factors rather than temperature in process of demulsification.
Interestingly, stable emulsion was formed once more when bubbling CO2 through solution again
under the same condition with the above described. The size distribution had little change between
initial emulsion and the 2nd emulsion after response as shown in Fig.7, while the droplet size
would present an obvious increase if pH was trigger by adding HCl/NaOH alternately because of
the accumulation of NaCl44. The emulsion would be unstable in the presence of electrolyte 45. In
conclusion, CO2 as trigger in Pickering emulsion had a lot of advantages compared to the pH
trigger, because the composition of system did not be changed after bubbling CO2 and N2
alternately.
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Fig.7 Photographs and size distribution of Pickering emulsion stabilized by 0.05mM MPAGNH+ and 0.5wt%
Nanoꢀsilica with CO2/N2 cycles
After deꢀemulsification, could the emulsifier be recovered and reused? Based on this doubt,
the oil phase of completely demulsified Pickering emulsion was moved out, and the residual water
phase was collected. 7 mL new liquid paraffin was added into the collected water phase followed
by homogenization at 11000rpm for 5min and a new Pickering emulsion formed. The operational
processes were shown in Fig.8. After this cycle, the size distribution had little change between
liquid paraffinꢀinꢀwater Pickering emulsion stabilized by 0.05mM MPAGNH+ and 0.5wt%
Nanoꢀsilica with initial oil phase and the Pickering emulsion with new oil phase as shown in Fig.9.
After emulsion worked, the emulsifier can be recycled so it not only has economic benefits, but
also has environmentally friendly properties.
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Fig.8 Photographs of Pickering emulsion stabilized by 0.1 mM MPAGNH+ and 0.5wt% Nanoꢀsilica, in which
initial oil was replaced by the new one
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Fig.9 The size distribution of Pickering emulsion stabilized by 0.05mM MPAGNH+ and 0.5wt% Nanoꢀsilica with
initial oil and new oil
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Mechanism of destabilization and re-stabilization of emulsions
In the presence of CO2, the Nanoꢀsilica was hydrophobized by the adsorption of the
MPAGNH+ molecules by electrostatic interaction between cationic surfactant and anionic particles.
However, when N2 was bubbled into the system to expel CO2, cationic surfactant was reversed
back to nonionic form and the electrostatic interaction disappeared. Therefore, the coated particle
was recovered to the original bare form again, and demulsification was happened. When CO2 was
bubbled into system again, the MPAGN was switched to the cationic form MPAGNH+ and
adsorbed on the surface of particle once again. This supposition can be proved by the
transformation of Zeta potential of the Nanoꢀsilica dispersed in MPAGNH+ solution with CO2/N2
cycles. The Zeta potential was ꢀ37.9 mV when the Nanoꢀsilica was dispersed in water but it
increased to ꢀ8.5 mV when was dispersed in 0.4 mM MPAGNH+ implying MPAGNH+ has
adsorbed on the Nanoꢀsilica by electrostatic interaction in this process. After bubbling N2, the Zeta
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potential of dispersion was back to ꢀ37.9 mV implied that the coated particle was recovered to the
original bare form and the electrostatic interaction disappeared between cationic surfactant and
anionic particles. This cycle can be repeated for several times as shown in insert graph of Fig.7
In summary, a novel rigid CO2ꢀresponsive surfactant based on natural forestry resources rosin
MPAGN was reported for the first time, and its CMC was 0.4mM lower than that of a
conventional cationic surfactant CTAB (0.9 mM) and lower than that of tertiary amine with long
hydrophobic tails C14PMA (2.35 mM). Upon bubbling CO2 into the system, the nonionic MPAGN
was reversed to active cationic form and the cationic was adsorbed on the surface of negatively
charged particles via electrostatic interactions forming a monolayer. The coated particles were
used to prepare paraffinꢀinꢀwater Pickering emulsion. This kind of Pickering emulsion can exist
stably for a long time. After bubbling N2, cationic quaternary ammonium was deprotonated and
recovered to inactive nonionic tertiary amine molecule so the electrostatic interaction between
surfactant and particles disappeared leading to desorption from the surface of Nanoꢀsilica and
demulsification occurred immediately. Interestingly, stable emulsion was formed again when CO2
was bubbled into the system again under the same condition with the last time. This Pickering
emulsion can be reversible between emulsification and demulsification with bubbling CO2 / N2.
Moreover, this emulsifier can be recycled when new oil was added after removing the initial oil.
Therefore, it not only has economic benefits, but also has environmental friendly properties.
Besides, this type of CO2ꢀresponsive surfactant not only possesses good emulsifying performance
but also was polymerizable. Therefore, it maybe has potential application in the emulsifier free
emulsion polymerization field. We will start explore further more in next work.
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ASSOCIATED CONTENT
Supporting information
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Characterization of molecules: FTꢀIR, H NMR and MS spectrum, photographs of a drop of
emulsion dispersed into water and into oil, surface tension of MPAGN solution without SiO2 as a
function of concentration of MPAGN at 25℃ and fit line, photographs of liquid paraffinꢀinꢀwater
emulsion stabilized by 0.05mM MPAGN and 0.5wt% nanoꢀsilica at 60℃ for 30 min without N2
were shown in Figures S1ꢀS6. This supporting information is available free of charge on the ACS
Publication websites at DOI:
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Author Information
Corresponding Author: Xiaoping Rao, Email: rxping2001@163.com
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Acknowledgments
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This research was financially supported by grants from National Natural Science Foundation
of China (31622017 and 31470596) and Fundamental Research Foundation of the Central
Commonwealth Institute of the Chinese Academy of Forestry (CAFYBB2018QC002).
Notes
The authors declare no competing financial interest.
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REFERENCES
1.
Liu K, Jiang J, Cui Z, Binks BP: pH-Responsive Pickering Emulsions Stabilized by Silica
Nanoparticles in Combination with a Conventional Zwitterionic Surfactant. Langmuir 2017,
33(9):2296.
2.
3.
Mcclements DJ: Food Emulsions: Principles, Practice, and Techniques: CRC Press; 1998.
Zhu Y, Jiang J, Liu K, Cui Z, Binks BP: Switchable Pickering emulsions stabilized by silica
nanoparticles hydrophobized in situ with a conventional cationic surfactant. Langmuir 2015,
31(11):3301-3307.
4.
Fernandes DA, Fernandes DD, Li Y, Wang Y, Zhang Z, Rousseau D, Gradinaru CC, Kolios MC:
Synthesis of Stable Multifunctional Perfluorocarbon Nanoemulsions for Cancer Therapy and
Imaging. Langmuir 2016, 32(42):10870-10880.
5.
6.
Schrade A, Landfester K, Ziener U: Pickering-type stabilized nanoparticles by heterophase
polymerization. Chem. Soc. Rev. 2013, 42(16):6823-6839.
Zhu Y, Jiang J, Liu K, Cui Z, Binks BP: Switchable Pickering Emulsions Stabilized by Silica
Nanoparticles Hydrophobized in Situ with a Conventional Cationic Surfactant. Langmuir 2015,
31(11):3301-3307.
7.
Liu H, Wang C, Zou S, Wei Z, Tong Z: Simple, Reversible Emulsion System Switched by pH on
the Basis of Chitosan without Any Hydrophobic Modification. Langmuir 2012,
28(30):11017-11024.
8.
9.
Fujii S, Mochizuki M, Aono K, Hamasaki S, Murakami R, Nakamura Y: pH-responsive aqueous
foams stabilized by hairy latex particles. Langmuir 2011, 27(21):12902-12909.
Binks BP, Murakami R, Armes SP, Fujii S, Schmid A: pH-responsive aqueous foams stabilized by
22
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Page 23 of 28
Journal of Agricultural and Food Chemistry
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
ionizable latex particles. Langmuir 2007, 23(17):8691-8694.
10.
11.
Shu X, Meng Y, Wan L, Li G, Yang M, Jin W: pH-Responsive Aqueous Foams of Oleic
Acid/Oleate Solution. J. Disper. Sci. Technol.2014, 35(2):293-300.
Lin Q, Liu K, Cui Z, Pei X, Jiang J, Song B: pH-Responsive Pickering Foams Stabilized by Silica
Nanoparticles in Combination with Trace Amount of Dodecyl Dimethyl Carboxyl Betaine.
Colloids Surfaces A. 2018.
12.
Fujii S, Akiyama K, Nakayama S, Hamasaki S, Yusa S, Nakamura Y: pH- and
temperature-responsive aqueous foams stabilized by hairy latex particles. Soft Matter 2014,
11(3):572-579.
13.
14.
15.
Li J, Stöver HDH: Doubly pH-Responsive Pickering Emulsion. Langmuir 2008,
24(23):13237-13240.
Liang C, Liu Q, Xu Z: Surfactant-free switchable emulsions using CO2-responsive particles. ACS
Appl. Mater. Inter. 2014, 6(9):6898.
Su X, Jessop PG, Cunningham MF: Surfactant-Free Polymerization Forming Switchable Latexes
That Can Be Aggregated and Redispersed by CO2 Removal and Then Readdition.
Macromolecules 2012, 45(2):666–670.
16.
17.
18.
Yan Q, Zhou R, Fu C, Zhang H, Yin Y, Yuan J: CO2-responsive polymeric vesicles that breathe.
Angew. Chem. 2011, 50(21):4923-4927.
Lin S, Theato P: CO2 ‐ Responsive Polymers. Macromol. Rapid Comm. 2013,
34(14):1118-1133.
Zippin JH, Levin LR, Buck J: CO2/HCO3−-responsive soluble adenylyl cyclase as a putative
metabolic sensor. Trends in Endocrin. Met. 2001, 12(8):366-370.
23
ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry
Page 24 of 28
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
19.
Zhang Q, Yu G, Wang WJ, Yuan H, Li BG, Zhu S: Preparation of N2/CO2 Triggered Reversibly
Coagulatable and Redispersible Latexes by Emulsion Polymerization of Styrene with a
Reactive Switchable Surfactant. Langmuir 2012, 28(14):5940.
20.
Zhang Q, Yu G, Wang WJ, Li BG, Zhu S: Preparation of COâ /Nâ -triggered reversibly
coagulatable and redispersible polyacrylate latexes by emulsion polymerization using a
polymeric surfactant. Macromol. Rapid Comm. 2012, 33(10):916-921.
21.
22.
23.
24.
Quesada M, Muniesa C, Botella P: Hybrid PLGA-Organosilica Nanoparticles with
Redox-Sensitive Molecular Gates. Chem. Mater. 2013, 25(13):2597-2602.
Zhang Y, Guo S, Ren X, Liu X, Fang Y: CO2 and Redox Dual Responsive Pickering Emulsion.
Langmuir 2017, 33(45):12973.
Anwar N, Willms T, Grimme B, Kuehne AJC: Light-Switchable and Monodisperse Conjugated
Polymer Particles. Acs Macro Lett. 2013, 2(9):766-769.
Asshoff SJ, Lancia F, Sukas S, Blonde JB, Yamaguchi T, Hommersom CA, Gac SL, Katsonis N:
Photo-switchable distributors of light based on self-assembled monodisperse chiral nematic
micropsheres. 2015.
25.
26.
Tan TT, Ahsan A, Reithofer MR, Tay SW, Tan SY, Hor TS, Chin JM, Chew BK, Wang X:
Photoresponsive liquid marbles and dry water. Langmuir 2014, 30(12):3448-3454.
Lei L, Xie D, Song B, Jiang J, Pei X, Cui Z: Photoresponsive Foams Generated by a Rigid
Surfactant Derived from Dehydroabietic Acid. Langmuir : the ACS journal of surfaces and
colloids 2017, 33(32):7908-7916.
27.
Thompson KL, Fielding LA, Mykhaylyk OO, Lane JA, Derry MJ, Armes SP: Vermicious
thermo-responsive Pickering emulsifiers. Chem. Sci. 2015, 6(7):4207-4214.
24
ACS Paragon Plus Environment

Page 25 of 28
Journal of Agricultural and Food Chemistry
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
28.
29.
30.
31.
32.
33.
Melle S, Lask M, Fuller GG: Pickering emulsions with controllable stability. Langmuir 2005,
21(6):2158.
Lam S, Blanco E, Smoukov SK, Velikov KP, Velev OD: Magnetically responsive pickering foams.
J. Am. Chem Soc. 2011, 133(35):13856-13859.
Xie CY, Meng SX, Xue LH, Bai RX, Yang X, Wang Y, Qiu ZP, Binks BP, Guo T, Meng T: Light and
Magnetic Dual-Responsive Pickering Emulsion Micro-Reactors. Langmuir 2017, 33(49).
Zhang Y, Guo S, Wu W, Qin Z, Liu X: CO2-Triggered Pickering Emulsion Based on Silica
Nanoparticles and Tertiary Amine with Long Hydrophobic Tails. Langmuir 2016, 32(45).
Liu Y, Jessop PG, Cunningham M, Eckert CA, Liotta CL: Switchable Surfactants. Science 2006,
313(5789):958-960.
Jiang J, Zhu Y, Cui Z, Binks BP: Switchable pickering emulsions stabilized by silica nanoparticles
hydrophobized in situ with
125(47):12599–12602.
a switchable surfactant. Angewandte Chem. 2013,
34.
Zhang Y, Ren X, Guo S, Liu X, Fang Y: CO2-Switchable Pickering Emulsion Using Functionalized
Silica Nanoparticles Decorated by Amine Oxide-Based Surfactants. ACS Sustain. Chem. Eng.
2018, 6(2):2641-2650.
35.
Yoshimura Y, Nojiri M, Arimoto M, Ishimoto K, Aso Y, Ohara H, Yamane H, Kobayashi S: Green
polymer chemistry: One-pot, metal-free synthesis of macromonomer via direct
polycondensation of lactic acid and its radical polymerization to graft and comb polymers.
Polymer 2016, 90:342-350.
36.
Feng L, Xie D, Song B, Zhang J, Pei X, Cui Z: Aggregate evolution in aqueous solutions of a
Gemini surfactant derived from dehydroabietic acid. Soft Matter 2018, 14(7):1210-1218.
25
ACS Paragon Plus Environment

Journal of Agricultural and Food Chemistry
Page 26 of 28
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
37.
36.
37.
38.
Sahu NH, Mandaogade PM, Deshmukh AM, Meghre VS, Dorle AK: Biodegradation Studies of
Rosin-Glycerol Ester Derivative. J. Bioact. Compat. Pol. 1999, 14(4):344-360.
Yan X, Zhai Z, Song Z, Shang S, Rao X: Synthesis and properties of polyester-based polymeric
surfactants from diterpenic rosin. Ind. Crop. Prod. 2017, 108:371-378.
Zhai Z, Yan X, Song Z, Shang S, Rao X: Annular and threadlike wormlike micelles formed by a
bio-based surfactant containing an extremely large hydrophobic group. Soft Matter 2017.
Yan X, Zhai Z, Song Z, Shang S, Rao X: Synthesis of comb-like polymeric surfactants with a
tricyclic rigid core and their use as dispersants in pymetrozine water suspension concentrates.
RSC Adv. 2017, 7(88):55741-55747.
39.
Wang J, Chen YP, Yao K, Wilbon PA, Zhang W, Ren L, Zhou J, Nagarkatti M, Wang C, Chu F et al:
Robust antimicrobial compounds and polymers derived from natural resin acids. Chem.
Cmmun, 2012, 48(6):916-918.
40.
41.
Liu D, Suo Y, Tan J, Lu H: CO2-Switchable microemulsion based on a pseudogemini surfactant.
Soft Matter 2017, 13(20):3783-3788.
Liu K, Jiang J, Cui Z, Binks BP: pH-Responsive Pickering Emulsions Stabilized by Silica
Nanoparticles in Combination with a Conventional Zwitterionic Surfactant. Langmuir 2017,
33(9):2296-2305.
42.
Cui ZG, Yang LL, Cui YZ, Binks BP: Effects of Surfactant Structure on the Phase Inversion of
Emulsions Stabilized by Mixtures of Silica Nanoparticles and Cationic Surfactant. Langmuir
2010, 26(7):4717-4724.
43.
Huang X, Li Q, Liu H, Shang S, Shen M, Song J: Oil-in-water emulsions stabilized by saponified
epoxidized soybean oil-grafted hydroxyethyl cellulose. J. Agr.Food Chem. 2017, 65(17).
26
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494
495
496
497
498
499
500
501
502
503
44.
Morse AJ, Armes SP, Thompson KL, Dupin D, Fielding LA, Mills P, Swart R: Novel Pickering
emulsifiers based on pH-responsive poly(2-(diethylamino)ethyl methacrylate) latexes.
Langmuir 2013, 29(18):5466-5475.
45.
Wang DQ, Li H, Liu BL, Chu JY: Soap-free emulsion polymerization of MMA in the presence of
cosolvent, electrolyte and competing solvent of monomer. J. Polym. Mater. 2004,
21(4):371-376.
504 For Table of Contents Use Only
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SYNOPSIS: A CO2ꢀresponsive Pickering emulsion stabilized by a rigid surfactant based on
sustainable resources rosin was prepared and this surfactant can be recycled.
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ABSTRACT GRAPHIC
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