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CO2-switchable dispersion of a natural chitosan and its application as a responsive pickering emulsifier
Accepted Manuscript Title: CO2 -Switchable Dispersion of a Natural Chitosan and its Application as a Responsive Pickering Emulsifier Authors: Dongyin Ren, Shengming Xu, Dejun Sun, Qibao Wang, Zhenghe Xu PII: DOI: Reference:
S0927-7757(18)30583-1 https://doi.org/10.1016/j.colsurfa.2018.06.068 COLSUA 22637
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
5-5-2018 22-6-2018 25-6-2018
Please cite this article as: Ren D, Xu S, Sun D, Wang Q, Xu Z, CO2 -Switchable Dispersion of a Natural Chitosan and its Application as a Responsive Pickering Emulsifier, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2018), https://doi.org/10.1016/j.colsurfa.2018.06.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CO2-Switchable Dispersion of a Natural Chitosan and its Application as a Responsive Pickering Emulsifier
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Dongyin Rena, Shengming Xub, Dejun Sunc, Qibao Wanga, Zhenghe Xub, d*
School of Chemical and Environmental Engineering, China University of Mining and Technology,
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Beijing 100083, P. R. China
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, P. R.
Key Laboratory of Colloid and Interface Chemistry, Shandong University of Ministry of
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China
Department of Materials Science and Engineering, Southern University of Science and
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d
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Education, Jinan 250100, P. R. China
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Technology, Shenzhen 518055, P. R. China
Corresponding author: Zhenghe Xu (Tel: +86 755 8801 8968); Shengming Xu (Tel: +86 10
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Email: [email protected]; [email protected]
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Graphical abstract
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ABSTRACT
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The natural chitosan aggregates obtained by increasing pH are found to be responsive to CO2/N2 switching. These chitosan aggregates can be dispersed in water upon CO2 bubbling and
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precipitated out with N2 bubbling at room temperature. Liquid paraffin-in-oil Pickering emulsions are prepared using these aggregates as the stabilizer. These emulsions can be demulsified
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completely with CO2 bubbling, and the separated two phases can be re-emulsified with N2 bubbling under mechanical mixing. This CO2/N2-switched demulsification/re-emulsification cycle can be repeated several times. The CO2/N2-switched dispersion/aggregation of the chitosan
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aggregates leads to desorption/re-absorption of the aggregates at the oil/water interface and hence the disappearance/reconstruction of the three-dimensional network structure of the aggregates, which is responsible for the demulsification/re-emulsification. Experimental evidence including the measurement of interfacial tension, interfacial dilatational elastic modulus, and rheological properties and micrographs of formed emulsions is given to support the proposed stabilization mechanism.
KEY WORDS Chitosan; CO2-switchable aggregation; emulsification; demulsification; Pickering emulsion
INTRODUCTION
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Since the first synthesis by Jessop in 2006 [1], the CO2-switchable emulsifiers have become the focus of research in recent years [1-19]. The emulsifying activity of these CO2-switchable
emulsifiers can be changed by the addition or removal of CO2 as such that the emulsions can be
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readily demulsified and re-emulsified. This demulsification/re-stabilization cycle can be repeated several times without the formation of any by-products [1-19], indicating the potential of these
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CO2-switchable emulsifiers for broad range of applications.
However, most of CO2-switchable emulsifiers reported by previous studies were obtained by
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organic synthesis. Expensive raw materials, complex synthetic methods, long synthetic routes [1-3,
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6, 8, 9, 12, 17, 19] may limit the application of these switchable emulsifiers. Therefore, there is a
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clear need to search for green and low cost CO2-switchable emulsifiers. Natural chitosan appears to be an ideal candidate. As a product of natural material chitin, chitosan (CTS) is readily available at
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affordable cost [20, 21]. Previous studies have demonstrated the emulsifying activity of CTS [20-25]. CTS is hypothesized to be responsive to CO2 because of the amino group in each structure
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unit as shown in scheme 1 [23, 24, 26]. As a natural macromolecule with non-cytotoxicity, good biocompatibility and degradability [20, 22], CTS is expected to be widely used in commercial
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applications.
OH
HO H2N
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O
OH
HO O
O
H2N
O O
HO NH2
n
O
OH
Chitosan
Scheme 1. Molecular structure of chitosan Recently, Liu et al [23, 24] used a natural CTS as a Pickering emulsifier for preparing pH-responsive emulsions. After adjusting pH to ca. 6.0 by NaOH, the CTS nanoparticles were formed in situ. These nanoparticles could adsorb at the oil-water interface to stabilize Pickering
emulsions. After adjusting pH lower than 6.0 by HCl, the CTS nanoparticles dissolved in water that led to demulsification of original stable emulsions [23, 24]. It is worth noticing that the pKaH of CTS is about 6.5 [27], which is higher than the pKaH of carbonic acid (6.38) [14]. This chemical characteristics of CTS indicates that in theory the precipitation and dissolution of CTS can be switched by CO2. These prior studies motivated us to explore natural CTS as a CO2 switchable
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Pickering emulsifier. In this work，the switching characteristics of CTS between aggregation-dispersion by CO2 was first confirmed, followed by investigating the demulsification/emulsification of oil-in-water
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emulsions by bubbling of CO2 and N2, respectively. The mechanism of emulsification by CTS aggregates was found to be different from that by CTS nanoparticles. The mechanism of
demulsifying CO2-switchable emulsions stabilized by CTS aggregates was further investigated by
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determining oil-water interfacial properties of CTS containing systems.
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2. EXPERIMENTAL
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Materials
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CTS (Mw ≈ 2×104, deacetylation degree ≥ 97%) was obtained from Jinan Haidebei Marine Bioengineering Co. Ltd., China. NaOH (analytically pure), acetic acid (analytically pure), liquid o
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paraffin (chemical grade isoalkane with the viscosity and density at 20 C being 12.0 mPa∙s and 0.830 g·cm-3, respectively) and toluene (analytically pure) were obtained from Sinopharm Chemical
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Reagent Co. (China). CO2 (99.99%) and N2 (99.99%) were obtained from France Air Liquide Investment Co. (China). Deionized water was used throughout this study.
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Preparation and Characteristics of CO2-switchable CTS CTS (0.403g) and 100 mL deionized water were added into a 250-mL flask. Added slowly
into this flask was 1.00 mL acetic acid while the dispersion was continuously stirred with a magnetic stir for 24 hours, resulting in a transparent, light yellow CTS aqueous dispersion. The dispersion was filtered to remove the particulate impurities. The pH of the dispersion was then adjusted to 9.0 with 0.1 M NaOH, which led to aggregation of CTS and formation of an opaque
suspension. The CTS aggregates were dialyzed with a dialysis bag (size: 14,000 Da) for 48 hours in deionized water, resulting in a neutral, pure CTS aggregates. The transmittance (721C, Shanghai INESA Scientific Instrument Co. Ltd., China), pH (FE20, Shanghai Mettler Toledo international trade Co. Ltd., China) and conductivity (DDSJ-308A, Shanghai Rex Xinjing Instrument Co. Ltd., China) of a 0.1 wt% CTS aggregate suspension were
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measured while bubbling CO2 at 200 mL·min-1 flow rate through the CTS aggregate suspension until the conductivity value became constant where the suspension became clear dispersion. The
measurements were continued by switching to bubbling of the dispersion with N2 (200 mL·min-1)
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until the conductivity value became constant. The CO2 and N2 alternative bubbling was repeated for four cycles.
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Emulsification and Demulsification
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CTS aggregate suspension of different concentrations were mixed with liquid paraffin to a
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total volume of 50 mL at different volume fractions. Pickering emulsions stabilized by CTS
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aggregates were prepared by a mechanical mixer of digital display (JJ1A, Jiangsu Jintan Xicheng Xinrui Instrument Factory, China), stirring at 1000 r·min-1 for two minutes. The emulsions were
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then poured into a 50 mL cylinder and stored for 24 h. A digital camera was used to take pictures of emulsions under an optical microscope (XPV-990E, Shanghai Changfang Optical Instrument Co.
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Ltd., China). All the droplets in the field of view were analyzed using commercial image analysis software to determine the drop size distribution.
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The CO2-switchable emulsification/demulsification experiments were conducted as follows. A mixture of 33 mL liquid paraffin and 67 mL CTS aggregate suspension (0.1 wt%) were placed into a 250-mL round bottom flask. After stirring at 1000 r·min-1 for two minutes, the Pickering
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emulsion stabilized by CTS aggregates was formed. This emulsion was evenly divided into two 50-mL cylinders. One of the emulsions was stored for 24 hours and the other was stored for two weeks. After such storage, photos of these two emulsions were taken to determine the drop size distributions. CO2 was then bubbled at 200 mL·min-1 flow rate through the emulsions for 10 minutes to demulsify these two Pickering emulsions. Photos of these two systems were taken again for comparison and assessing the efficiency of demulsification. After demulsification, N2
was bubbled at 200 mL·min-1 through the mixture for 20 minutes. The mixture were then stirred at 1000 r·min-1 for two minutes for re-emulsification with the photos taken after the storage of the emulsion for a specified period of time. The above processes were repeated for four cycles, the droplet size of the emulsions after one cycle and four cycles were observed with optical microscope to determine drop size distribution,
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while the chemical compositions of the oil layers separated from the system after one cycle and four cycles were determined using an elemental analyzer (CE-440, Perkin Elmer, America).
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Characterization of Emulsions Stabilized by CTS aggregates
Rheological measurements of the emulsions stabilized by CTS aggregates were performed using a cone and plate geometry (cone diameter 50 mm, angle 1°) on an Anton Paar rheometer
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(MCR, Anton Paar, Austria). Strain within the linear viscoelastic region was selected for dynamic
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frequency scan in dynamic oscillatory shear mode, with the scan frequency ranging between 0.1 ~
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100 rad·s-1, to obtain corresponding storage modulus (G') and loss modulus (G") of emulsions.
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The emulsions prepared at 0.1 wt% CTS concentration in aqueous phase and an oil-to-water volume ratio of 1/2 were chosen for the following measurements. In the Laser-induced Confocal
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Scanning Microscopy (LCSM) measurement, CTS aggregates were dyed by Rhodamine B before the preparation of the emulsions. After the emulsion was prepared, a LCSM (LSM780, Beijing
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Precise Instrument Co. Ltd., China) was used to probe the adsorption of CTS aggregates at the surface of emulsion droplets. Toluene was used as the oil phase in the Scanning electron
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microscopy (KYKY-EM3900M, Thermo Fisher, America) measurement. The emulsion was o
dropped on a copper sample plate, froze at -40 C and dried for 48 h before the SEM measurement.
Interfacial Tension and Dilatational Elasticity Measurement of CTS Aggregate
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Suspensions
Dynamic oil−water interfacial tension (IFT) by pendant drop method and dilatational
elasticity modulus by oscillating drop method in the controlled surface area mode were measured o
at 25 C using a commercial tensiometer (Interface Technology Co. Ltd., Tracker, TECLIS (France). An oil droplet (10.0μL) was formed and maintained vertically at the end of an inverted needle (U
= 0.63 mm, l = 42 mm) in a transparent cuvette containing 0.1 wt% CTS aggregates before or after CO2 addition. The interfacial tensions (IFT) were taken from 0 min to 30 min to study the effect of aging. Dilatational elasticity modulus was measured at the frequency of 0.1 Hz after the equilibrium IFT was reached.
CO2-Switchable Behavior of CTS Aggregate Suspension
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3. RESULTS AND DISCUSSION
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Without any treatment, the initial CTS aggregate suspensions are opaque in appearance with a transmittance of about 35%. After bubbling of CO2, the opaque aggregate suspension becomes transparent pale yellow dispersion, with the transmittance increases to over 95%. After bubbling of
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N2, the transparent aqueous dispersion becomes opaque again with the transmittance deceases to
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about 35%, indicating the formation of CTS aggregates after the removal of CO2 from the aqueous
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dispersion. This dispersion/aggregation progress of CTS aggregates can be switched by alternately
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bubbling of CO2 and N2 (Figure 1).
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80
+CO2
－CO2
60
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Transmittance(%)
100
40
20
0
120 240 360 480 600 720 840 960 Time(s)
Figure 1. Change of transmittance of CTS aggregate suspension during repeated cycles of bubbling or removal of CO2. Insets are digital photographs of CTS aggregate suspension under CO2 bubbling (upper) and CTS dispersion under N2 bubbling (lower).
The conductivity and pH of CTS aggregate suspension are measured under alternate bubbling of CO2 and N2 for five cycles. As shown in Figure 2, the conductivity (black line) increases to the highest value of 155 µs·cm-1 with the bubbling of CO2, accompanied by a decrease in pH (red line) to 5.8. Upon bubbling of N2 to remove CO2, the conductivity decreases to the lowest value of 50 µs·cm-1, accompanied by an increase in pH to 8.5.
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These results indicate the protonation of amino groups on CTS by bubbling of CO2, making the CTS molecules more hydrated and hence dispersed in deionized water. Bubbling of N2 through the system makes the protonated amino group back to its neutral state and less hydrated as such
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that the system returned to the initial aggregation state. The proposed mechanism of the
CO2-responsive switching (similar to pH-switching) between aggregation and dispersion of CTS
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has been confirmed by molecular dynamics simulation [26].
Figure 2. Changes of conductivity and pH of CTS aggregate suspension during repeated cycles of bubbling or removing of CO2
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Emulsification by CTS Aggregates and CO2-Responsive Demulsification Figure 3 shows the emulsions stabilized by 0.1 wt% CTS aggregates at different oil-to-water
volume ratios. It is worth noticing that the phenomenon of emulsification occurred in all the cases, indicating the emulsifying activity of CTS aggregates. Meanwhile, all the emulsions are similar in droplet size and size distribution (Figure S1), illustrating that the difference in oil-to-water volume
ratio has a negligible effect on the emulsification. However, there are some differences among the systems. At φO/W= 1/1, there is an excess oil layer floating on the top of the emulsion as shown in Figure 3a. At φO/W= 1/3 and 1/4, there are excess CTS aggregates precipitated in the aqueous layers of the emulsions as shown in Figures 3c and 3d. At φO/W= 1/2, neither the excess oil layer nor the excess CTS aggregates was seen in the
(a)
φO/W = 1/2
(d)
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φO/W = 1/1
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(b)
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inset picture of Figure 3b.
φO/W = 1/3
φO/W = 1/4
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Figure 3. Optical micrographs of the emulsions stabilized by CTS aggregates formed at different oil-to-water volume ratios. The concentrations of the CTS aggregates are 0.1 wt%. Insets are digital photographs of the corresponding emulsions. The influence of CTS aggregates concentration on emulsion is also studied in this work.
Similar to the results in Figure 3, the emulsification occurred in all the cases, and all the emulsions
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are of similar droplet size and size distribution (Figure S1). At cCTS= 0.05 wt%, there is an excess oil layer floating on the top of the emulsion, as shown in Figure 4a. At cCTS= 1 wt%, there are excess CTS aggregates precipitated in the aqueous layer of the emulsion as seen in Figure 4d. In At cCTS= 0.1 wt% and 0.5 wt%, neither the excess oil layer nor the excess CTS aggregates are seen in the corresponding inset pictures of Figures 4b and 4c. The results of these experiments illustrate that in the emulsions stabilized by CTS aggregates,
there is a critical proportion between the amount of CTS aggregates and the amount of oil. When the amount of CTS aggregates is too high for the oil, excess CTS aggregates will not participate in the formation of emulsion but precipitate in the aqueous phase. Binks and coworkers [28] studied the influence of particle concentration on Pickering emulsions stabilized by SiO2, and found that when SiO2 concentration was reached the limiting value, the excess SiO2 particles would stay in
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the continuous phase. The similar phenomenon was shown in the previous study on Pickering emulsions stabilized by halloysite nanotubes [29]. In summary, it is confirmed that the CTS
aggregates can be used as an effective Pickering emulsifier. It should be noted that creaming
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occurred in all the emulsions we prepared because of relatively large sizes of the droplets with a significant density difference between the dispersed phase and the continuous phase [30-32].
(b)
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(a)
(d)
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(c)
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cCTS = 0.1 wt %
cCTS = 0.05 wt %
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cCTS = 0.5 wt %
cCTS = 1 wt %
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Figure 4. Optical micrographs of the emulsions stabilized at different concentrations of CTS aggregates. The oil-to-water ratios are 1/2. Insets are digital photographs of the corresponding emulsions. The CO2/N2-triggered demulsification/re-emulsification processes of CTS aggregates
Pickering emulsions are shown in Figure 5. The presence of CTS aggregates led to the formation of stable emulsions for at least two weeks of storage, confirming the CTS aggregates to be an effective emulsifier (Figure S2). This emulsion can be demulsified and completely phase separated into two layers by bubbling of CO2, while N2 bubbling efficiently re-emulsifies the oil with the aid
of mechanical mixing. This emulsion can be readily demulsified again by bubbling of CO2. The droplet size distribution of the emulsions with increasing number of cycles is almost indistinguishable (Figure S3), indicating that the bubbling of CO2/N2 for switching has no effect on the emulsification activity of CTS aggregates. All the emulsions prepared above can be demulsified completely by CO2 bubbling (Figure S4
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and S5), illustrating that the CTS concentration and the oil-to-water volume ratio have no effect on CO2-switched demulsification. The demulsification/re-emulsification process switched by CO2
addition and removal is highly reversible and easily repeatable, implying that CTS aggregates can
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be used as a CO2-switchable emulsifier.
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+CO2
φO/W=1/2 24 hours later
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cCTS=0.1wt%
+N2, 65℃
+CO2
Srirring
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Stirring
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Figure 5. The CO2/N2-triggered demulsification/re-emulsification process of CTS aggregates Pickering emulsion. It is interesting to determine whether CTS exists in oil layers after the demulsification. In all
the emulsions discussed above, the only compound which contains N element is CTS. Therefore,
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the N element content in oil phase will help us to know whether oil layers contain CTS or not. As shown in Table 1, the N element contents of the oil layers after one or four cycles are all close to zero, which are similar to the N element content of raw liquid paraffin. The results show that the CTS almost entirely in the lower aqueous phase. Therefore, the CTS aggregates can be easily reclaimed with almost no lose and reused in the next cycle of emulsification. Furthermore, it will not introduce any new impurities to the oil phase in the processes of emulsification and
demulsification. As a kind of switchable emulsifier, CTS aggregates have clear advantages in these emulsification/demulsification processes.
Table 1. Element content in oil layer C (%)
H (%)
N (%)
Liquid paraffin
81.69
16.41
0.092
Oil (after 1 cycle)
81.72
16.11
0.114
Oil (after 4 cycles)
81.68
15.72
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Sample
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0.088
N
Rheological Properties of CTS Aggregates and Corresponding Pickering
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Emulsions
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Dynamic oscillatory frequency sweep tests were conducted in the linear viscoelastic range to determine the frequency dependence of storage modulus (G') and loss modulus (G") [33, 34]. The
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rheological properties of the emulsions could help us to understand the internal structure of the emulsions. Studying the oscillatory rheological measurements of G' and G" allows us to determine
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whether the emulsion system is strongly or weakly flocculated [35]. As shown in Figure 6, both G' and G" are hardly changed with frequency and the G' is always one order of magnitude larger than
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the G" over the frequency range scanned from 0.1 to 100 rads-1. The similar results were obtained for all other emulsions prepared (Figure S6). These results imply that the emulsions prepared by CTS aggregates exhibit gel-like behavior, predicting an elastic network structure formed in the
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emulsion. The presence of CTS aggregates caused formation of much stronger (more elastic) structure (Figure S6), indicating that the elastic network structure is formed by CTS aggregates. This network prevents the oil droplets from coalescing and makes the entire system as a gel to exhibit elastic response [33, 35-37]. The similar elastic network structure was formed in the emulsions stabilized by egg-yolk and cereal β-glucans. The research demonstrated that the formation of the gel-like structure in continuous phase was attributed to the polysaccharide. The
similar phenomenon was shown in the previous study on emulsions stabilized by carob protein [38]
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and natural silk fibroin [36].
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Figure 6. Double-logarithmic plot of storage modulus G' and loss modulus G" versus frequency of a CTS aggregates Pickering emulsion.
Observations of the CTS Aggregates Pickering Emulsion
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To determine the morphology of CTS aggregates in the Pickering emulsions, optical microscope, laser-induced confocal scanning microscopy (LCSM) and scanning electron
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microscope (SEM) were applied to the CTS aggregates Pickering emulsions. As seen in the optical microscope image (Figure 7a), CTS aggregates surround the droplets like “clouds”, which entrap
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small oil droplets. In the LCSM experiment, the rhodamine B-labeled CTS aggregates can be seen on the surfaces of the droplets. The network established by CTS aggregates is also visible in this
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image (Figure 7b).
(a)
(b) (b)
(c)
Figure 7. The optical microscope (a), LCSM (b) and SEM (c) images of CTS aggregates Pickering emulsion. The arrows in (a) and (b) indicate the CTS aggregates. In the SEM image which required evaporation of oil and water from the emulsions, the distinct reticular structures formed by CTS aggregates are observed clearly. Comparison of the average cavity size with droplet diameter of the Pickering emulsions shows a close correlation
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between the two, suggesting that the cavities result from the loss of the oil drops. This finding indicates that the oil droplets are fixed in the spaces of the distinct network structures (Figure 7c).
Unlike the Pickering emulsions stabilized by CTS nanoparticles [24, 25], this study showed a
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three-dimensional network formed by CTS aggregates to stabilize Pickering emulsions. CTS
aggregates adsorbed at the oil-water interface to prevent the droplets from coalescing. Meanwhile, a
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of the emulsion, which made the emulsions more stable.
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three-dimensional network structure was formed to entrap the droplets and give a gel-like property
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Oil-Water Interfacial Properties
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Figure 8 shows the dynamic interfacial tension (IFT) between liquid paraffin and CTS aggregate suspension before and after the stimulation of CO2. The IFT decreases with time for the
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two cases, suggesting a gradual adsorption of the CTS aggregate at the oil-water interface and CTS dispersion into the aqueous phase. As the adsorption reaches equilibrium, the IFT remains
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constant. After the stimulation of CO2, the equilibrium value of the IFT is 34.1 mN·m-1, which is much higher than the value of 23 mN·m-1 before the stimulation of CO2. The increased IFT
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indicates that the CTS aggregates show a stronger interfacial activity than the dispersed CTS. The results illustrate that the CTS aggregates are an effective switchable emulsifier due to their significant change in interfacial activity by CO2 stimulation.
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Figure 9 shows the dilatational elasticity modulus of liquid paraffin and CTS aggregate
suspension before and after stimulation of CO2. In contrast to the interfacial tension, the addition of CO2 results in a significant decrease in the dilatational elasticity. Previous studies illustrated that the emulsifier with better elasticity at the oil-water interface could prevent the coalescence of droplets, leading to higher stability of the emulsion [39, 40]. The decreased dilatational elasticity indicates that the CTS aggregates show a stronger ability than the CTS dispersion to stabilize
Pickering O/W emulsions. The results again illustrate that the CTS aggregates are an effective
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switchable emulsifier due to significant change in emulsifying power by CO2 stimulation.
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Figure 8. Dynamic interfacial tension of liquid paraffin and CTS aggregation before and after stimulation of CO2
Figure 9. Dilatational elasticity modulus of liquid paraffin and CTS aggregate suspension before and after stimulation of CO2 The oil-water interfacial property measurements illustrate that the stimulation of CO2
transformed the interfacial properties of CTS aggregates. After CO2 bubbling, the CTS showed a lower oil-water interfacial activity and lower emulsifying stability than its counterpart. These transformations in interfacial properties are considered to be the primary cause of the CO2-switchable demulsification of the emulsions stabilized by CTS aggregates.
CO2-Switchable Mechanism of Emulsions Stabilized by CTS Aggregates
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On the basis of the results described above, the mechanism of the CO2-switchable emulsions stabilized by CTS aggregates is inferred. In these Pickering emulsions, CTS aggregates adsorb at
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the oil-water interface to form a three-dimensional network structure that separates the oil droplets. When the emulsion was bubbled with CO2, the neutral CTS aggregates are switched to the protonation/cationic form, resulting in an increase in their hydration, causing the CTS aggregates
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to disperse into the water phase. As a result, the three-dimensional network structure disappeared,
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resulting in the coalescence of the oil droplets and finally the demulsification. When the separated
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two phases was bubbled with N2, the dispersed CTS molecules are switched back to the deprotonation/neutral form, resulting in a decrease in their hydration. In this case; CTS
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precipitated out from water as aggregates at the oil-water interface to form a three-dimensional
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network again that stabilizes Pickering emulsions (Figure 10).
+CO2
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+N2; Stirring
+CO2
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-CO2
HCO3
HCO3 OH
HO H2N
O O
OH
HO H2N
O
+ CO2
O O
HO NH2
n
OH
OH
HO H3N
O
O O
- CO2
HO H3N
O
O O
HO NH3
OH
n
O
OH
HCO3
Figure 10. Proposed emulsification and demulsification mechanism of emulsion stabilized by
CTS aggregates with the existence and absence of CO2.
CONCLUSIONS The present work confirmed the CO2/N2-switchable property of natural chitosan aggregates by increasing pH to 9.0. The chitosan aggregates could be dispersed in water upon CO2 bubbling and
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precipitated out at oil-water interface by N2 bubbling at room temperature. These CTS aggregates were powerful in the preparation of stable Pickering emulsions. The emulsions stabilized by CTS aggregates could be demulsified and re-emulsified by CO2 and N2 bubbling. This reversible
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demulsification/re-emulsification cycle was due to the CO2/N2-switched dispersion/aggregation of the CTS. The aggregates absorb at the oil-water interface to form a three-dimensional network structure that stabilizes Pickering emulsions. When the emulsion was bubbled with CO2, the
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aggregates were dispersed into the water phase, causing the three-dimensional network structure to
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disappear and the demulsification. When the system was bubbled with N2, the CTS aggregates
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precipitated out from water and re-absorbed at the oil-water interface, forming the
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three-dimensional network structure again to stabilize the Pickering emulsion. As a kind of CO2-switchable emulsifier with low cost, good biocompatibility, degradability,
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non-cytotoxicity, low-energy intensity of emulsification and high demulsification rate, CTS aggregates have clear advantages over the existing switchable emulsifiers reported in literature.
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With such advantages, CTS demulsifiers are expected to be widely used in many processing
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industries such as coating, soil remediation, extraction of petroleum and oil transportation.
ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (grants 21333005) for financially
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supporting of this work. The financial support from Zhujiang Talent Program of Guangdong Province (K17253301) is also greatly appreciated.
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S0927-7757(18)30583-1 https://doi.org/10.1016/j.colsurfa.2018.06.068 COLSUA 22637
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
5-5-2018 22-6-2018 25-6-2018
Please cite this article as: Ren D, Xu S, Sun D, Wang Q, Xu Z, CO2 -Switchable Dispersion of a Natural Chitosan and its Application as a Responsive Pickering Emulsifier, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2018), https://doi.org/10.1016/j.colsurfa.2018.06.068 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
CO2-Switchable Dispersion of a Natural Chitosan and its Application as a Responsive Pickering Emulsifier
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Dongyin Rena, Shengming Xub, Dejun Sunc, Qibao Wanga, Zhenghe Xub, d*
School of Chemical and Environmental Engineering, China University of Mining and Technology,
b
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Beijing 100083, P. R. China
Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, P. R.
Key Laboratory of Colloid and Interface Chemistry, Shandong University of Ministry of
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c
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China
Department of Materials Science and Engineering, Southern University of Science and
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d
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Education, Jinan 250100, P. R. China
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Technology, Shenzhen 518055, P. R. China
Corresponding author: Zhenghe Xu (Tel: +86 755 8801 8968); Shengming Xu (Tel: +86 10
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Email: [email protected]; [email protected]
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Graphical abstract
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ABSTRACT
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The natural chitosan aggregates obtained by increasing pH are found to be responsive to CO2/N2 switching. These chitosan aggregates can be dispersed in water upon CO2 bubbling and
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precipitated out with N2 bubbling at room temperature. Liquid paraffin-in-oil Pickering emulsions are prepared using these aggregates as the stabilizer. These emulsions can be demulsified
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completely with CO2 bubbling, and the separated two phases can be re-emulsified with N2 bubbling under mechanical mixing. This CO2/N2-switched demulsification/re-emulsification cycle can be repeated several times. The CO2/N2-switched dispersion/aggregation of the chitosan
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aggregates leads to desorption/re-absorption of the aggregates at the oil/water interface and hence the disappearance/reconstruction of the three-dimensional network structure of the aggregates, which is responsible for the demulsification/re-emulsification. Experimental evidence including the measurement of interfacial tension, interfacial dilatational elastic modulus, and rheological properties and micrographs of formed emulsions is given to support the proposed stabilization mechanism.
KEY WORDS Chitosan; CO2-switchable aggregation; emulsification; demulsification; Pickering emulsion
INTRODUCTION
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Since the first synthesis by Jessop in 2006 [1], the CO2-switchable emulsifiers have become the focus of research in recent years [1-19]. The emulsifying activity of these CO2-switchable
emulsifiers can be changed by the addition or removal of CO2 as such that the emulsions can be
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readily demulsified and re-emulsified. This demulsification/re-stabilization cycle can be repeated several times without the formation of any by-products [1-19], indicating the potential of these
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CO2-switchable emulsifiers for broad range of applications.
However, most of CO2-switchable emulsifiers reported by previous studies were obtained by
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organic synthesis. Expensive raw materials, complex synthetic methods, long synthetic routes [1-3,
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6, 8, 9, 12, 17, 19] may limit the application of these switchable emulsifiers. Therefore, there is a
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clear need to search for green and low cost CO2-switchable emulsifiers. Natural chitosan appears to be an ideal candidate. As a product of natural material chitin, chitosan (CTS) is readily available at
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affordable cost [20, 21]. Previous studies have demonstrated the emulsifying activity of CTS [20-25]. CTS is hypothesized to be responsive to CO2 because of the amino group in each structure
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unit as shown in scheme 1 [23, 24, 26]. As a natural macromolecule with non-cytotoxicity, good biocompatibility and degradability [20, 22], CTS is expected to be widely used in commercial
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applications.
OH
HO H2N
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O
OH
HO O
O
H2N
O O
HO NH2
n
O
OH
Chitosan
Scheme 1. Molecular structure of chitosan Recently, Liu et al [23, 24] used a natural CTS as a Pickering emulsifier for preparing pH-responsive emulsions. After adjusting pH to ca. 6.0 by NaOH, the CTS nanoparticles were formed in situ. These nanoparticles could adsorb at the oil-water interface to stabilize Pickering
emulsions. After adjusting pH lower than 6.0 by HCl, the CTS nanoparticles dissolved in water that led to demulsification of original stable emulsions [23, 24]. It is worth noticing that the pKaH of CTS is about 6.5 [27], which is higher than the pKaH of carbonic acid (6.38) [14]. This chemical characteristics of CTS indicates that in theory the precipitation and dissolution of CTS can be switched by CO2. These prior studies motivated us to explore natural CTS as a CO2 switchable
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Pickering emulsifier. In this work，the switching characteristics of CTS between aggregation-dispersion by CO2 was first confirmed, followed by investigating the demulsification/emulsification of oil-in-water
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emulsions by bubbling of CO2 and N2, respectively. The mechanism of emulsification by CTS aggregates was found to be different from that by CTS nanoparticles. The mechanism of
demulsifying CO2-switchable emulsions stabilized by CTS aggregates was further investigated by
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determining oil-water interfacial properties of CTS containing systems.
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2. EXPERIMENTAL
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Materials
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CTS (Mw ≈ 2×104, deacetylation degree ≥ 97%) was obtained from Jinan Haidebei Marine Bioengineering Co. Ltd., China. NaOH (analytically pure), acetic acid (analytically pure), liquid o
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paraffin (chemical grade isoalkane with the viscosity and density at 20 C being 12.0 mPa∙s and 0.830 g·cm-3, respectively) and toluene (analytically pure) were obtained from Sinopharm Chemical
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Reagent Co. (China). CO2 (99.99%) and N2 (99.99%) were obtained from France Air Liquide Investment Co. (China). Deionized water was used throughout this study.
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Preparation and Characteristics of CO2-switchable CTS CTS (0.403g) and 100 mL deionized water were added into a 250-mL flask. Added slowly
into this flask was 1.00 mL acetic acid while the dispersion was continuously stirred with a magnetic stir for 24 hours, resulting in a transparent, light yellow CTS aqueous dispersion. The dispersion was filtered to remove the particulate impurities. The pH of the dispersion was then adjusted to 9.0 with 0.1 M NaOH, which led to aggregation of CTS and formation of an opaque
suspension. The CTS aggregates were dialyzed with a dialysis bag (size: 14,000 Da) for 48 hours in deionized water, resulting in a neutral, pure CTS aggregates. The transmittance (721C, Shanghai INESA Scientific Instrument Co. Ltd., China), pH (FE20, Shanghai Mettler Toledo international trade Co. Ltd., China) and conductivity (DDSJ-308A, Shanghai Rex Xinjing Instrument Co. Ltd., China) of a 0.1 wt% CTS aggregate suspension were
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measured while bubbling CO2 at 200 mL·min-1 flow rate through the CTS aggregate suspension until the conductivity value became constant where the suspension became clear dispersion. The
measurements were continued by switching to bubbling of the dispersion with N2 (200 mL·min-1)
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until the conductivity value became constant. The CO2 and N2 alternative bubbling was repeated for four cycles.
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Emulsification and Demulsification
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CTS aggregate suspension of different concentrations were mixed with liquid paraffin to a
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total volume of 50 mL at different volume fractions. Pickering emulsions stabilized by CTS
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aggregates were prepared by a mechanical mixer of digital display (JJ1A, Jiangsu Jintan Xicheng Xinrui Instrument Factory, China), stirring at 1000 r·min-1 for two minutes. The emulsions were
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then poured into a 50 mL cylinder and stored for 24 h. A digital camera was used to take pictures of emulsions under an optical microscope (XPV-990E, Shanghai Changfang Optical Instrument Co.
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Ltd., China). All the droplets in the field of view were analyzed using commercial image analysis software to determine the drop size distribution.
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The CO2-switchable emulsification/demulsification experiments were conducted as follows. A mixture of 33 mL liquid paraffin and 67 mL CTS aggregate suspension (0.1 wt%) were placed into a 250-mL round bottom flask. After stirring at 1000 r·min-1 for two minutes, the Pickering
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emulsion stabilized by CTS aggregates was formed. This emulsion was evenly divided into two 50-mL cylinders. One of the emulsions was stored for 24 hours and the other was stored for two weeks. After such storage, photos of these two emulsions were taken to determine the drop size distributions. CO2 was then bubbled at 200 mL·min-1 flow rate through the emulsions for 10 minutes to demulsify these two Pickering emulsions. Photos of these two systems were taken again for comparison and assessing the efficiency of demulsification. After demulsification, N2
was bubbled at 200 mL·min-1 through the mixture for 20 minutes. The mixture were then stirred at 1000 r·min-1 for two minutes for re-emulsification with the photos taken after the storage of the emulsion for a specified period of time. The above processes were repeated for four cycles, the droplet size of the emulsions after one cycle and four cycles were observed with optical microscope to determine drop size distribution,
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while the chemical compositions of the oil layers separated from the system after one cycle and four cycles were determined using an elemental analyzer (CE-440, Perkin Elmer, America).
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Characterization of Emulsions Stabilized by CTS aggregates
Rheological measurements of the emulsions stabilized by CTS aggregates were performed using a cone and plate geometry (cone diameter 50 mm, angle 1°) on an Anton Paar rheometer
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(MCR, Anton Paar, Austria). Strain within the linear viscoelastic region was selected for dynamic
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frequency scan in dynamic oscillatory shear mode, with the scan frequency ranging between 0.1 ~
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100 rad·s-1, to obtain corresponding storage modulus (G') and loss modulus (G") of emulsions.
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The emulsions prepared at 0.1 wt% CTS concentration in aqueous phase and an oil-to-water volume ratio of 1/2 were chosen for the following measurements. In the Laser-induced Confocal
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Scanning Microscopy (LCSM) measurement, CTS aggregates were dyed by Rhodamine B before the preparation of the emulsions. After the emulsion was prepared, a LCSM (LSM780, Beijing
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Precise Instrument Co. Ltd., China) was used to probe the adsorption of CTS aggregates at the surface of emulsion droplets. Toluene was used as the oil phase in the Scanning electron
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microscopy (KYKY-EM3900M, Thermo Fisher, America) measurement. The emulsion was o
dropped on a copper sample plate, froze at -40 C and dried for 48 h before the SEM measurement.
Interfacial Tension and Dilatational Elasticity Measurement of CTS Aggregate
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Suspensions
Dynamic oil−water interfacial tension (IFT) by pendant drop method and dilatational
elasticity modulus by oscillating drop method in the controlled surface area mode were measured o
at 25 C using a commercial tensiometer (Interface Technology Co. Ltd., Tracker, TECLIS (France). An oil droplet (10.0μL) was formed and maintained vertically at the end of an inverted needle (U
= 0.63 mm, l = 42 mm) in a transparent cuvette containing 0.1 wt% CTS aggregates before or after CO2 addition. The interfacial tensions (IFT) were taken from 0 min to 30 min to study the effect of aging. Dilatational elasticity modulus was measured at the frequency of 0.1 Hz after the equilibrium IFT was reached.
CO2-Switchable Behavior of CTS Aggregate Suspension
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3. RESULTS AND DISCUSSION
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Without any treatment, the initial CTS aggregate suspensions are opaque in appearance with a transmittance of about 35%. After bubbling of CO2, the opaque aggregate suspension becomes transparent pale yellow dispersion, with the transmittance increases to over 95%. After bubbling of
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N2, the transparent aqueous dispersion becomes opaque again with the transmittance deceases to
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about 35%, indicating the formation of CTS aggregates after the removal of CO2 from the aqueous
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dispersion. This dispersion/aggregation progress of CTS aggregates can be switched by alternately
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bubbling of CO2 and N2 (Figure 1).
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+CO2
－CO2
60
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Transmittance(%)
100
40
20
0
120 240 360 480 600 720 840 960 Time(s)
Figure 1. Change of transmittance of CTS aggregate suspension during repeated cycles of bubbling or removal of CO2. Insets are digital photographs of CTS aggregate suspension under CO2 bubbling (upper) and CTS dispersion under N2 bubbling (lower).
The conductivity and pH of CTS aggregate suspension are measured under alternate bubbling of CO2 and N2 for five cycles. As shown in Figure 2, the conductivity (black line) increases to the highest value of 155 µs·cm-1 with the bubbling of CO2, accompanied by a decrease in pH (red line) to 5.8. Upon bubbling of N2 to remove CO2, the conductivity decreases to the lowest value of 50 µs·cm-1, accompanied by an increase in pH to 8.5.
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These results indicate the protonation of amino groups on CTS by bubbling of CO2, making the CTS molecules more hydrated and hence dispersed in deionized water. Bubbling of N2 through the system makes the protonated amino group back to its neutral state and less hydrated as such
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that the system returned to the initial aggregation state. The proposed mechanism of the
CO2-responsive switching (similar to pH-switching) between aggregation and dispersion of CTS
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has been confirmed by molecular dynamics simulation [26].
Figure 2. Changes of conductivity and pH of CTS aggregate suspension during repeated cycles of bubbling or removing of CO2
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Emulsification by CTS Aggregates and CO2-Responsive Demulsification Figure 3 shows the emulsions stabilized by 0.1 wt% CTS aggregates at different oil-to-water
volume ratios. It is worth noticing that the phenomenon of emulsification occurred in all the cases, indicating the emulsifying activity of CTS aggregates. Meanwhile, all the emulsions are similar in droplet size and size distribution (Figure S1), illustrating that the difference in oil-to-water volume
ratio has a negligible effect on the emulsification. However, there are some differences among the systems. At φO/W= 1/1, there is an excess oil layer floating on the top of the emulsion as shown in Figure 3a. At φO/W= 1/3 and 1/4, there are excess CTS aggregates precipitated in the aqueous layers of the emulsions as shown in Figures 3c and 3d. At φO/W= 1/2, neither the excess oil layer nor the excess CTS aggregates was seen in the
(a)
φO/W = 1/2
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(c)
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φO/W = 1/1
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(b)
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inset picture of Figure 3b.
φO/W = 1/3
φO/W = 1/4
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Figure 3. Optical micrographs of the emulsions stabilized by CTS aggregates formed at different oil-to-water volume ratios. The concentrations of the CTS aggregates are 0.1 wt%. Insets are digital photographs of the corresponding emulsions. The influence of CTS aggregates concentration on emulsion is also studied in this work.
Similar to the results in Figure 3, the emulsification occurred in all the cases, and all the emulsions
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are of similar droplet size and size distribution (Figure S1). At cCTS= 0.05 wt%, there is an excess oil layer floating on the top of the emulsion, as shown in Figure 4a. At cCTS= 1 wt%, there are excess CTS aggregates precipitated in the aqueous layer of the emulsion as seen in Figure 4d. In At cCTS= 0.1 wt% and 0.5 wt%, neither the excess oil layer nor the excess CTS aggregates are seen in the corresponding inset pictures of Figures 4b and 4c. The results of these experiments illustrate that in the emulsions stabilized by CTS aggregates,
there is a critical proportion between the amount of CTS aggregates and the amount of oil. When the amount of CTS aggregates is too high for the oil, excess CTS aggregates will not participate in the formation of emulsion but precipitate in the aqueous phase. Binks and coworkers [28] studied the influence of particle concentration on Pickering emulsions stabilized by SiO2, and found that when SiO2 concentration was reached the limiting value, the excess SiO2 particles would stay in
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the continuous phase. The similar phenomenon was shown in the previous study on Pickering emulsions stabilized by halloysite nanotubes [29]. In summary, it is confirmed that the CTS
aggregates can be used as an effective Pickering emulsifier. It should be noted that creaming
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occurred in all the emulsions we prepared because of relatively large sizes of the droplets with a significant density difference between the dispersed phase and the continuous phase [30-32].
(b)
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(a)
(d)
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(c)
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cCTS = 0.1 wt %
cCTS = 0.05 wt %
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cCTS = 0.5 wt %
cCTS = 1 wt %
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Figure 4. Optical micrographs of the emulsions stabilized at different concentrations of CTS aggregates. The oil-to-water ratios are 1/2. Insets are digital photographs of the corresponding emulsions. The CO2/N2-triggered demulsification/re-emulsification processes of CTS aggregates
Pickering emulsions are shown in Figure 5. The presence of CTS aggregates led to the formation of stable emulsions for at least two weeks of storage, confirming the CTS aggregates to be an effective emulsifier (Figure S2). This emulsion can be demulsified and completely phase separated into two layers by bubbling of CO2, while N2 bubbling efficiently re-emulsifies the oil with the aid
of mechanical mixing. This emulsion can be readily demulsified again by bubbling of CO2. The droplet size distribution of the emulsions with increasing number of cycles is almost indistinguishable (Figure S3), indicating that the bubbling of CO2/N2 for switching has no effect on the emulsification activity of CTS aggregates. All the emulsions prepared above can be demulsified completely by CO2 bubbling (Figure S4
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and S5), illustrating that the CTS concentration and the oil-to-water volume ratio have no effect on CO2-switched demulsification. The demulsification/re-emulsification process switched by CO2
addition and removal is highly reversible and easily repeatable, implying that CTS aggregates can
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be used as a CO2-switchable emulsifier.
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φO/W=1/2 24 hours later
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cCTS=0.1wt%
+N2, 65℃
+CO2
Srirring
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Stirring
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Figure 5. The CO2/N2-triggered demulsification/re-emulsification process of CTS aggregates Pickering emulsion. It is interesting to determine whether CTS exists in oil layers after the demulsification. In all
the emulsions discussed above, the only compound which contains N element is CTS. Therefore,
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the N element content in oil phase will help us to know whether oil layers contain CTS or not. As shown in Table 1, the N element contents of the oil layers after one or four cycles are all close to zero, which are similar to the N element content of raw liquid paraffin. The results show that the CTS almost entirely in the lower aqueous phase. Therefore, the CTS aggregates can be easily reclaimed with almost no lose and reused in the next cycle of emulsification. Furthermore, it will not introduce any new impurities to the oil phase in the processes of emulsification and
demulsification. As a kind of switchable emulsifier, CTS aggregates have clear advantages in these emulsification/demulsification processes.
Table 1. Element content in oil layer C (%)
H (%)
N (%)
Liquid paraffin
81.69
16.41
0.092
Oil (after 1 cycle)
81.72
16.11
0.114
Oil (after 4 cycles)
81.68
15.72
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Sample
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0.088
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Rheological Properties of CTS Aggregates and Corresponding Pickering
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Emulsions
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Dynamic oscillatory frequency sweep tests were conducted in the linear viscoelastic range to determine the frequency dependence of storage modulus (G') and loss modulus (G") [33, 34]. The
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rheological properties of the emulsions could help us to understand the internal structure of the emulsions. Studying the oscillatory rheological measurements of G' and G" allows us to determine
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whether the emulsion system is strongly or weakly flocculated [35]. As shown in Figure 6, both G' and G" are hardly changed with frequency and the G' is always one order of magnitude larger than
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the G" over the frequency range scanned from 0.1 to 100 rads-1. The similar results were obtained for all other emulsions prepared (Figure S6). These results imply that the emulsions prepared by CTS aggregates exhibit gel-like behavior, predicting an elastic network structure formed in the
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emulsion. The presence of CTS aggregates caused formation of much stronger (more elastic) structure (Figure S6), indicating that the elastic network structure is formed by CTS aggregates. This network prevents the oil droplets from coalescing and makes the entire system as a gel to exhibit elastic response [33, 35-37]. The similar elastic network structure was formed in the emulsions stabilized by egg-yolk and cereal β-glucans. The research demonstrated that the formation of the gel-like structure in continuous phase was attributed to the polysaccharide. The
similar phenomenon was shown in the previous study on emulsions stabilized by carob protein [38]
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and natural silk fibroin [36].
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Figure 6. Double-logarithmic plot of storage modulus G' and loss modulus G" versus frequency of a CTS aggregates Pickering emulsion.
Observations of the CTS Aggregates Pickering Emulsion
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To determine the morphology of CTS aggregates in the Pickering emulsions, optical microscope, laser-induced confocal scanning microscopy (LCSM) and scanning electron
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microscope (SEM) were applied to the CTS aggregates Pickering emulsions. As seen in the optical microscope image (Figure 7a), CTS aggregates surround the droplets like “clouds”, which entrap
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small oil droplets. In the LCSM experiment, the rhodamine B-labeled CTS aggregates can be seen on the surfaces of the droplets. The network established by CTS aggregates is also visible in this
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image (Figure 7b).
(a)
(b) (b)
(c)
Figure 7. The optical microscope (a), LCSM (b) and SEM (c) images of CTS aggregates Pickering emulsion. The arrows in (a) and (b) indicate the CTS aggregates. In the SEM image which required evaporation of oil and water from the emulsions, the distinct reticular structures formed by CTS aggregates are observed clearly. Comparison of the average cavity size with droplet diameter of the Pickering emulsions shows a close correlation
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between the two, suggesting that the cavities result from the loss of the oil drops. This finding indicates that the oil droplets are fixed in the spaces of the distinct network structures (Figure 7c).
Unlike the Pickering emulsions stabilized by CTS nanoparticles [24, 25], this study showed a
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three-dimensional network formed by CTS aggregates to stabilize Pickering emulsions. CTS
aggregates adsorbed at the oil-water interface to prevent the droplets from coalescing. Meanwhile, a
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of the emulsion, which made the emulsions more stable.
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three-dimensional network structure was formed to entrap the droplets and give a gel-like property
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Oil-Water Interfacial Properties
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Figure 8 shows the dynamic interfacial tension (IFT) between liquid paraffin and CTS aggregate suspension before and after the stimulation of CO2. The IFT decreases with time for the
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two cases, suggesting a gradual adsorption of the CTS aggregate at the oil-water interface and CTS dispersion into the aqueous phase. As the adsorption reaches equilibrium, the IFT remains
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constant. After the stimulation of CO2, the equilibrium value of the IFT is 34.1 mN·m-1, which is much higher than the value of 23 mN·m-1 before the stimulation of CO2. The increased IFT
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indicates that the CTS aggregates show a stronger interfacial activity than the dispersed CTS. The results illustrate that the CTS aggregates are an effective switchable emulsifier due to their significant change in interfacial activity by CO2 stimulation.
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Figure 9 shows the dilatational elasticity modulus of liquid paraffin and CTS aggregate
suspension before and after stimulation of CO2. In contrast to the interfacial tension, the addition of CO2 results in a significant decrease in the dilatational elasticity. Previous studies illustrated that the emulsifier with better elasticity at the oil-water interface could prevent the coalescence of droplets, leading to higher stability of the emulsion [39, 40]. The decreased dilatational elasticity indicates that the CTS aggregates show a stronger ability than the CTS dispersion to stabilize
Pickering O/W emulsions. The results again illustrate that the CTS aggregates are an effective
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switchable emulsifier due to significant change in emulsifying power by CO2 stimulation.
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Figure 8. Dynamic interfacial tension of liquid paraffin and CTS aggregation before and after stimulation of CO2
Figure 9. Dilatational elasticity modulus of liquid paraffin and CTS aggregate suspension before and after stimulation of CO2 The oil-water interfacial property measurements illustrate that the stimulation of CO2
transformed the interfacial properties of CTS aggregates. After CO2 bubbling, the CTS showed a lower oil-water interfacial activity and lower emulsifying stability than its counterpart. These transformations in interfacial properties are considered to be the primary cause of the CO2-switchable demulsification of the emulsions stabilized by CTS aggregates.
CO2-Switchable Mechanism of Emulsions Stabilized by CTS Aggregates
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On the basis of the results described above, the mechanism of the CO2-switchable emulsions stabilized by CTS aggregates is inferred. In these Pickering emulsions, CTS aggregates adsorb at
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the oil-water interface to form a three-dimensional network structure that separates the oil droplets. When the emulsion was bubbled with CO2, the neutral CTS aggregates are switched to the protonation/cationic form, resulting in an increase in their hydration, causing the CTS aggregates
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to disperse into the water phase. As a result, the three-dimensional network structure disappeared,
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resulting in the coalescence of the oil droplets and finally the demulsification. When the separated
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two phases was bubbled with N2, the dispersed CTS molecules are switched back to the deprotonation/neutral form, resulting in a decrease in their hydration. In this case; CTS
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precipitated out from water as aggregates at the oil-water interface to form a three-dimensional
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network again that stabilizes Pickering emulsions (Figure 10).
+CO2
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+N2; Stirring
+CO2
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-CO2
HCO3
HCO3 OH
HO H2N
O O
OH
HO H2N
O
+ CO2
O O
HO NH2
n
OH
OH
HO H3N
O
O O
- CO2
HO H3N
O
O O
HO NH3
OH
n
O
OH
HCO3
Figure 10. Proposed emulsification and demulsification mechanism of emulsion stabilized by
CTS aggregates with the existence and absence of CO2.
CONCLUSIONS The present work confirmed the CO2/N2-switchable property of natural chitosan aggregates by increasing pH to 9.0. The chitosan aggregates could be dispersed in water upon CO2 bubbling and
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precipitated out at oil-water interface by N2 bubbling at room temperature. These CTS aggregates were powerful in the preparation of stable Pickering emulsions. The emulsions stabilized by CTS aggregates could be demulsified and re-emulsified by CO2 and N2 bubbling. This reversible
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demulsification/re-emulsification cycle was due to the CO2/N2-switched dispersion/aggregation of the CTS. The aggregates absorb at the oil-water interface to form a three-dimensional network structure that stabilizes Pickering emulsions. When the emulsion was bubbled with CO2, the
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aggregates were dispersed into the water phase, causing the three-dimensional network structure to
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disappear and the demulsification. When the system was bubbled with N2, the CTS aggregates
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precipitated out from water and re-absorbed at the oil-water interface, forming the
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three-dimensional network structure again to stabilize the Pickering emulsion. As a kind of CO2-switchable emulsifier with low cost, good biocompatibility, degradability,
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non-cytotoxicity, low-energy intensity of emulsification and high demulsification rate, CTS aggregates have clear advantages over the existing switchable emulsifiers reported in literature.
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With such advantages, CTS demulsifiers are expected to be widely used in many processing
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industries such as coating, soil remediation, extraction of petroleum and oil transportation.
ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (grants 21333005) for financially
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supporting of this work. The financial support from Zhujiang Talent Program of Guangdong Province (K17253301) is also greatly appreciated.
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