Pickering Emulsion Formation of Paraffin Wax in an EthanolâWater

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Pickering Emulsion Formation of Paraffin Wax in Ethanol-Water Mixture Stabilized by Primary Polymer Particles and Wax Microspheres thereof Zuoxu Xiao, Hongyan Cao, Xubao Jiang, and Xiang Zheng Kong Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03802 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

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Pickering Emulsion Formation of Paraffin Wax in Ethanol-Water Mixture Stabilized by Primary Polymer Particles and Wax Microspheres thereof Zuoxu Xiao, Hongyan Cao, Xubao Jiang*, Xiang Zheng Kong* College of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, China KEYWORDS: Dodecene, trihydroxymethyl propane triacrylate, precipitation polymerization, Pickering particles, interface energy, wax microspheres.

ABSTRACT: Stable dispersions of paraffin wax droplets and their nano- and microspheres have broad applications. Despite intensive efforts, the production of uniform wax spheres remains a challenge. For their preparation, abundant surfactants and other additives are commonly used to stabilize the dispersions. These additives are hardly removable and entrain often adverse consequence in many applications, particularly in biological and medical applications, where microspheres with absolutely clean surface are preferred. We report here a novel process to prepare stable dispersion of wax droplets in water-ethanol mixture with narrow size distribution by simply shaking without any surfactants. The process is featured by using primary polymer particles (PPs) of poly(dodecene-trihydroxymethyl propane triacrylate) as Pickering stabilizer. PPs were prepared by precipitation polymerization without any surfactant and stabilizer. By rapidly cooling down the wax emulsion, solid wax spheres with good uniformity were obtained.

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Their size, between 50 and 480 µm, was easily adjustable by changing the shaking rate, amount of PPs and particularly the size of PPs. Morphology of the wax spheres were examined by SEM, which showed that they were covered by a layer of PPs. Formation mechanism of the microspheres was also discussed based on adsorption energy of PPs on wax spheres, estimated from the corresponding contact angle of the solvent towards the PPs and the wax. This paper presents a novel pathway to the preparation of wax microspheres with only polymer particles without need for any other additives.

1. INTRODUCTION Paraffin waxes refer to a class of white or colorless soft solids derived from petroleum or coal. They usually consist of a mixture of hydrocarbon molecules containing between twenty and forty carbon atoms. Owing to their specific characteristics, wax droplets and spheres have been widely used in leather, paper, textile, automobile and cosmetic industries.1-12 For a great part of these applications, they are often used under the form of emulsions or dispersions, particularly in house construction, paper and agriculture industries,1-5 and relevant studies have been extensively reported.2-11 Wax spheres of large size with several hundreds of microns in diameter are also useful in tissue engineering and in preparation of functional Janus particles.12-14 Wax emulsification refers to the processes, by which the wax, usually under liquid state, is dispersed into a solvent as tiny droplets with varied size from nanoscale6,9 to several microns or larger.1,3,7,10,15 In most of the cases, water is the continuous phase (the solvent). The dispersion is achieved in general by mechanical stirring or homogenization under high speed.1,2,10 To keep the dispersion stable for a desired time, surfactants or stabilizers with co-surfactants and other additives are usually necessary. The amount of the surfactants and associated additives is usually


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quite high, about 10 wt% relative to the wax is common when ionic surfactants are used, and it reaches 40 wt% when non-ionic surfactants or polymeric dispersants are used.1,2 The abundant use of the surfactants/dispersants and associated additive increases not only the prime cost of the product, it is also unpleasant in many applications,16 particularly in biological and medical applications. In addition, specific equipment and infrastructure are often required to achieve high speed agitation or homogenization.9,10,17 It is to point out that finely divided solid particles of nano- or micro-scales can also function in similar ways as surfactants and stabilizers to form stable emulsion or dispersions,5,18-25 namely Pickering emulsion. These stabilizers of solid particles are usually refereed as Pickering stabilizer. The most common solid particles used for this purpose are silica and titania.5,18-20 Other inorganic particles have been also reported, including for examples, calcium carbonate,21 magnetic oxide22 and other composite particles of different nature.23-25 Binks has done a review to describe the similarities and the differences between classical surfactants and the solid particles with regard to their uses as stabilizers in emulsion and dispersion.26 Compared to inorganic particles, very fewer works have been reported on polymer particles as Pickering stabilizers.27,28 This is also seen in recent reviews on Pickering emulsions from different particles, their applications and the effect of particle shape and nature on the resulting Pickering emulsions.29,30 In this paper, primary polymer particles were prepared by precipitation polymerization of dodecene and trihydroxymethyl propane triacrylate without any additive except the initiator, and were used as Pickering stabilizer to fabricate paraffin wax emulsion in a binary mixed solvent of ethanol-water. By cooling down to room temperature, solid wax spheres were obtained. The size of the wax spheres was easily adjustable, from about 50 to 480 µm, by


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changing the shaking rate or the amount and the size of the polymer particles. This report provides therefore a novel process to fabricate uniform and clean wax microspheres. 2. EXPERIMENTAL 2.1. Materials. Paraffin wax is purchased from Wako Pure Chemical Industries, Ltd (Grade I, 163-13295. MP 68 °C, Density 0.79 g/cm3). Dodecene (DC, Analytical Pure, Shanghai Aladdin Biochem Technology), trihydroxymethyl propane triacrylate (TMPTA, Chemical Pure, Tianjin Tiaojiao Chemicals Ltd), azobisisobutyronitrile (AIBN, Analytical Pure, Tianjin Damao Chemicals Ltd) and ethanol (EtOH, Tianjin Fuyu Chemicals), were all used as received. Water used was double distilled in the lab. 2.2. Preparation of Primary Polymer Particles. Preparation of uniform polymer microspheres through precipitation polymerization has been reported, mainly on the polymerization of styrene with different crosslinker monomers.31-33 Based on those reports, the primary particles were prepared by free radical polymerization of DC and TMPTA using the following process: in a glass reactor of 120 mL containing 98 mL of EtOH-H2O mixture (H2O 15 mL), a monomer mixture (2 mL, DC/TMPTA=7/3 by mass) was first charged. After hand shaking to make a homogeneous monomer solution and deoxygenation of the system by N2 under ultrasonic treatment for 5 min, the reactor was placed in a water bath shaker at 70 ºC, followed by addition of AIBN initiator of 2.0 wt% (relative to the monomer mixture) to start the polymerization under shaking at 120 osc/min. The polymerization was allowed to run for 8 h, uniform P(DC-TMPTA) particles (PPs) with the number average size of about 1.70 µm were obtained. 2.3. Preparation of Paraffin Wax Dispersion. In a typical process, 100 g of EtOH-H2O mixture was first added in a glass flask of 120 mL capacity pre-located in a water bath shaker set at 70 ºC, followed by consecutive additions of wax powder (4.0 g) and PPs (0.4 g; Except otherwise


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stated, PPs with size of 1.51 µm were used throughout). The mixture was shaken under a fixed rate at 70 ºC for 2 h. At end of the treatment, icing-water was poured in the shaker bath to quickly drop the temperature down to about 30 ºC, far below the MP (68 °C) of the wax, to solidify the wax droplets to microspheres. The wax spheres settled down to the bottom of the flask upon the flask shaking stopped, and were easily separated from the supernatant. To optimize the uniformity and to adjust the size of the wax spheres, this process was carried out under different experimental conditions, including EtOH/H2O ratio, shaking rate, time of operation as well as the amount and size of the PPs. 2.4. Characterizations and Instruments. After wax spheres separated from the system and dried, a known amount was dissolved in hexane at 65 °C under gentle stirring for 5 min, followed by filtration using a Nylon separation membrane (pore size 0.22 µm, Shanghai Mili Membrane Separation Co., Ltd.) to get rid of the wax and to collect the adsorbed PPs. This process allowed one to get the proportion of the wax under the form of spheres and that of the PPs adhered on the wax spheres. The wax and the PPs remained in the continuous phase of EtOH-H2O were obtained the same way through their separation, drying up, followed by hexane dissolution and the characterization. The size of wax spheres and that of the PPs were examined using optical microscopy (OM, Olympus BX-51, Japan) and scanning electron microscopy (SEM, Quanta FEG-250, FEI) as previously described.34-36 By counting at least 200 spheres on the pictures, their number-average size (Dn), weight-average size (Dw) and the size distribution (Dw/Dn, namely PDI or the polydispersity) were obtained. To observe the interior morphology of the wax spheres, the spheres contained in a small sack were immersed in liquid nitrogen for about 5 min, placed on a


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stainless steel panel after taken out and hit by a surface-smooth hammer to crush the microspheres, the sample was observed under SEM. 2.5. Contact Angles and Adsorption Energy. To obtain the contact angles of a liquid on the wax and those on PPs film and the adsorption energy of the PPs on the interface of wax droplet and EtOH-H2O, about 2 g of dried PPs were added in 10 mL of ethyl benzene and left swollen at 50 °C for 30 min to obtain a viscous mixture, which was taken using a pipette and placed on a glass slide. A polymer film was formed after the solvent evaporated. Contact angles of water, ethanol, their mixture and liquid wax on the surface of this P(DC-TMPTA) film and on solid wax were measured using goniometer (OCA-40, Dataphysics, Germany). The interface tension between liquid wax and EtOH-H2O was determined with an interfacial rheometer (Tracker-H, Teclis, France). Detail for the measurements is given in Supporting Information. 3. RESULTS AND DISCUSSION 3.1 Preparation of P(DC-TMPTA) Primary Particles.

Figure 1. SEM photos of PPs of different size obtained at different polymerization time (A: 1.01 µm, 0.5 h; B: 1.21 µm, 2 h; C: 1.41 µm, 4 h; D: 1.51 µm, 6 h)


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As is well known in Pickering stabilization by solid particles, the size of PPs particles plays an important role in the process.29,30,37 Prior to the study, a series of highly uniform PPs were prepared through precipitation polymerization using the process described above. SEM pictures of the particles are given in Figure 1, which demonstrates that highly uniform PPs were effectively achieved with narrow PDI values (Table S1 in Supporting Information). It is seen from there that the size of the PPs was regularly increasing with polymerization time, with the PDI remained quite low in the whole process. These particles were employed as Pickering particles in the preparation of the wax emulsion and spheres. 3.2. Water Amount in the Continuous Phase in Wax Sphere Preparation. In the preliminary test with only ethanol as the disperse phase, it was found that the added wax was settled down at flask bottom, and a great portion of the wax was present as floc. Addition of a surfactant should be avoided since it entrains often adverse consequence in many applications.38 Knowing that the density of the wax is slightly higher than that of ethanol, addition of water in ethanol may drive up the density of the continuous phase closer to that of the wax, and to prevent therefore the wax droplets from sedimentation. The process was therefore conducted in EtOH-H2O mixture with varying H2O amount. Typical pictures of the resulting wax spheres are given in Figure 2, the corresponding data are given in Table 1, from which one can see that the spheres with good spherical shape and uniformity were well formed as long as water amount in the binary solvent was kept at 12 wt% or lower. In Table 1 are also given the proportion of the wax under form of spheres and that of the PPs adsorbed on the surface of the wax spheres in the samples prepared with different H2O amount in EtOH-H2O. It is seen that the size of the wax spheres was decreasing with increased H2O amount up to 16 wt%, followed by an increase with further increased H2O amount, showing the smallest


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size (249.7 µm) with 16 wt% of H2O. As to the PDI of the wax spheres, the data in Table 1 show that the lowest PDI (≈1.02) of the wax spheres was observed with H2O between 10 wt% and 12 wt%, along with the highest portions of the loaded PPs (14.25% and 11.40%, respectively) in the wax spheres. Beyond these limits, the PDI was slightly increased; aggregated and non-spherical microspheres were clearly observed as shown in Figure 2 (photos C, D).

Figure 2. OM pictures of wax spheres prepared in EtOH-H2O mixture with varying H2O amount (Conditions: 70 ºC, shaking at 200 osc/min, 4.0 g of wax, 0.4 g of PPs in 100 g of EtOH-H2O mixture with H2O amount: A, 0 g; B, 10.0 g; C, 16.0 g; D, 20 g)

It is believed that H2O addition to ethanol had a dual effect on the formation of wax spheres. While the density of ethanol was driven closer to that of the wax, attenuating the wax droplets sedimentation; the compatibility between the PPs and the continuous phase was also reduced at the same time, driving more PPs to the wax phase, either in the form of droplets or floc. This was helpful to form wax spheres with better stabilization, as observed with H2O amount of up to 16 wt%. However, with more H2O added, the compatibility of the wax molecules with the medium


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was severely reduced, leading to an increase in the interface energy of the wax droplets, which might entrain at least partial aggregation for the small wax droplets, observed as an increase in the size of the wax droplets, i.e. an increase in the size of the wax spheres after steep dropping of the temperature. This was well confirmed by the presence of non-spherical and dual or triple joint-microspheres (Figure 2, C&D). These results indicate that H2O addition was effective to reduce the size of the wax droplets with a limited extent, their uniformity was deteriorated with too much H2O (>12 wt%) in the solvent. It is to note that more than 95% of the wax was found as spheres with 12 wt% to 28 wt% of H2O in the mixed solvent, and this amount was slightly decreased with H2O amount beyond these limits. Table 1. Formation of wax spheres in EtOH-H2O mixture (100 g) with varying H2O amount H2O (g)

0

2

6

10.0

12.0

16.0

20.0

28.0

36.0

Wax sphere size (Dn, µm)

480.5 315.8 305.2 279.3 271.3 249.7 263.3 270.8 271.8

PDI (Dw/Dn)

1.061 1.056 1.048 1.019 1.021 1.037 1.071 1.078 1.083

Wax spheres (g)

1.759 2.472 3.437 3.520 3.874 3.856 3.862 3.816

Wax spheres (%)a

43.97 61.81 85.93 88.00 96.86 96.41 96.54 95.42 93.51

PPs on wax spheres (g) PPs on wax spheres (%)

3.740

0.004 0.019 0.025 0.057 0.046 0.017 0.017 0.026 0.029 b

1.00

4.75

6.25

14.25 11.40

4.20

4.25

6.60

7.25

a.

Percentage of the wax collected as microspheres;

b.

Percentage of PPs found with the wax spheres. The rest of the wax and the PPs remained in

EtOH-H2O mixture.

3.3. Shaking Rate in the Formation of Wax Spheres. In order to better understand the influence of shaking rate, wax spheres were prepared with shaking rate varied from 200 osc/min to 360 osc/min while the rest of the experimental conditions was unchanged, i.e. 4.0 g of wax and 0.4 g of PPs in 100 g of EtOH-H2O mixture with 10 wt% of H2O at 70 ºC. The results are listed in Table 2, which shows that the size of the wax spheres was regularly decreasing with


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increased shaking rate, while the corresponding size distribution was moderately broadening. The wax spheres were significantly polydisperse at the highest shaking rate of 360 osc/min (See Figure S1, OM photos of the wax spheres). The granular properties of the wax spheres and the partition of the PPs between the wax spheres and the continuous phase are listed in Table 2, along with other relevant data, including the wax amount under form of spheres (%, line 1, Table 2) relative to the total wax loaded, the size of the wax spheres and size distribution (Dw/Dn), as well as the wax amount recovered. It is to note that, for all the runs, all the loaded wax was practically recovered through the method used, indicating the reliability of the process. Among the wax recovered, a predominant amount was under form of spheres (88% to 97%, line 1, Table 2). It is also seen that, with increased shaking rate, the amount of the wax spheres was in regular increase, which meant that the dissolved wax and wax flocs were in regular decrease at the same time. In Table 2 are also listed the relative amounts of the PPs vis-à-vis the wax spheres (PPs/Wax in wt%, bottom line, Table 2). With increased shaking rate, more PPs were contained in wax spheres, indicating a constant decrease in the PPs remained in the solvent. While at least 95% of the PPs recollected for all runs by summing up the amounts in the wax spheres and in the solvent (PPs recovered, %), only 14% of the PPs was found in the wax spheres prepared with shaking rate at 200 osc/min (PPs on wax spheres, %), and this value was in significant increase with shaking rate, reaching 84% with shaking rate at 320 osc/min. It is easy to understand that smaller wax droplets were formed under higher shaking (shearing), making larger interface between the wax and the solvent, requiring more PPs to maintain the stability of the wax droplets. It seemed that the wax droplets were not highly stabilized in such a system, their aggregation and tearing apart were occurring in permanence in the process, in particular at higher


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shaking, which may explain the higher PDI of the wax spheres at high shaking rate of 320 osc/min and above. Table 2. Wax spheres formation and partition of PPs particle under different shaking rate in EtOH-H2O (4 g of wax and 0.4 g of PPs) Shaking (osc/min)

200

240

280

320

360

Wax spheres (%)

88.00

89.78

92.08

96.64

96.46


279.3

186.8

136.42

103.60

101.40

PDI (Dw/Dn)

1.019

1.021

1.019

1.027

1.037

Wax recovered (%)

98.60

99.15

99.28

99.20

99.55

PPs on wax spheres (%)

14.25

30.74

51.99

84.05

85.58

PPs recovered (%)

96.35

94.58

97.85

97.18

97.80

PPs/Wax (wt%)

1.62

3.27

5.57

8.52

8.71

3.4. Operation Time and Amount of PPs versus the Formation of the Wax Spheres. In order to see how the size of the wax spheres and the PPs partition were evolved with time in the process, a set of experiments was conducted with the same formulation at shaking rate of 320 osc/min but halted at different time. The corresponding results are summarized in Table 3, which demonstrates clearly the following points: throughout the process, the wax amount that formed spheres was quite constant, slightly varying between 96% and 98%; so was the size of the wax spheres, varying between 103.6 µm and 106.4 µm. The PDI of the wax spheres remained relatively large at this high shaking, with relatively narrowed size distribution observed from 1 h to 2 h. However, the amount of PPs on wax spheres relative to total PPs charged was in constant increase up to 2 h, reached a maximum of 84% of all PPs charged being with the wax spheres, and remained quite constant afterwards.


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Table 3. Wax spheres formation and PPs partition at different time in EtOH-H2O containing 4 g of wax and 0.4 g of PPs (shaking at 320 osc/min) Shaking time (h)

0.25

0.5

1.0

2.0

4.0

6.0

Wax spheres (%)

97.98

96.58

97.14

96.64

96.87

96.12


105.7

104.8

104.3

103.6

106.4

106.3

PDI (Dw/Dn)

1.046

1.053

1.026

1.027

1.068

1.066

Wax recovered (%)

99.71

99.28

99.12

99.20

97.77

97.54


70.29

75.18

76.73

84.05

84.28

82.11

PPs recovered (%)

96.93

95.23

94.53

97.17

95.17

96.53

PPs/Wax (wt%)

6.99

7.48

7.53

8.52

8.47

8.46

Based on the mechanism of Pickering emulsion formation, the amount of PPs in this work must be crucial for the size and uniformity of the wax spheres. For this purpose, a set of samples were prepared with varied amount of PPs while the other experimental conditions kept unchanged. The results are given in Table 4. As expected, with increased amount of PPs, the size of the resulting wax spheres was regularly decreasing and their PDI was obviously broadened once the amount of PPs reached 0.8 g in the system (see Figure S2, OM photos of the wax spheres). From Table 4, it is seen that, with increased PPs amount, the wax amount under form of spheres was in slight but constant decrease; the size of the wax spheres was also decreasing significantly. As to PPs, their amount found with wax spheres, relative to their total mass, was in regular decrease. For instance, with 0.2 g of PPs added, 91.39% of them were found in wax spheres; this value was reduced to 67.89% with 0.8 g of PPs added. In addition, with exception of the lowest PPs use (0.2 g), PDI of the wax spheres was also in constant increase with PPs


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amount used, particularly when the PPs amount was augmented to 1.0 g or higher. From the results, it was concluded that wax spheres with the lowest PDI were prepared with PPs amount controlled between 0.4 g and 0.8 g in the batch given here.

Table 4. Preparation and property of wax spheres prepared with different amount of PPs in 100 g of EtOH-H2O containing wax (4 g) Polymer particles (g)

0.20

0.40

0.80

1.00

1.20

1.40

Wax spheres (%)

97.03

96.64

96.32

96.06

96.13

95.63


143.50

103.60

93.57

85.46

84.19

80.36

PDI (Dw/Dn)

1.048

1.027

1.025

1.029

1.031

1.034


91.39

84.05

67.89

66.71

66.22

70.27

PPs recovered (%)

97.60

97.17

96.93

97.02

97.93

98.13

PPs/Wax (wt%)

4.62

12.78

13.74

16.90

20.42

25.42

3.5. Size Effect of PPs on Formation of the Wax Spheres. The effect of the size of PPs on the formation of the wax spheres was studied, in which a same mass of PPs with their number average size varied from 1.0 µm to 1.7 µm was used while the other experimental conditions kept unchanged. The granular properties of the resulting wax spheres are given in Table 5. One can see from there that both the wax amount (under form of spheres) and the PPs on the wax spheres (relative to total PPs loaded) were relatively constant and independent of the size of the PPs used. So was the relative amount of PPs via-a-via the wax spheres (PPs/wax, wt%). Except the samples with the largest and the smallest sized PPs (1.69 µm, 1.01µm), where the PDI of the wax spheres was slightly larger, all PDI values were quite similar for the rest of the samples. The most striking effect of the PPs size may be the size variation of the wax spheres as depicted in Figure 3. The size of wax spheres was closely dependent on that of the PPs used. It is obvious that the number of PPs was largely increased when the same weight of PPs with smaller


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size were used, since the number of PPs is proportional to the reciprocal of its volume, i.e. the cube of its radius (Eq. 2 in Supporting Information). Figure 3 shows that smaller wax spheres were obtained with PPs of smaller size, indicating that PPs with small size provide better stability to the wax droplets. From these results, an important point one can conclude here is that the use of the PPs of different size is a simple and effective means to prepare the wax spheres of different size. Table 5. Wax spheres formation and partition of PPs particle of different size in 100 g of EtOHH2O with 4 g of wax Polymer particles Size (µm)

1.01

1.21

1.41

1.51

1.69

Wax spheres (%)

95.54

97.31

97.77

96.64

97.59


57.4

67.9

79.8

103.6

112.8

PDI (Dw/Dn)

1.033

1.024

1.027

1.027

1.036


82.46

85.07

87.20

84.05

83.61

PPs/Wax (wt%)

8.43

8.31

8.62

8.52

8.37

Size of wax microspheres (µm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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100

80

60

1.0

1.2

1.4

1.6

1.8

Size of polymer particles (µm)

Figure 3. Size of the wax spheres as function of the size of the polymer particles used in preparation of wax spheres through Pickering process 3.6. Morphology of the Wax Spheres. The wax spheres as prepared were subjected to SEM observation. Selected SEM pictures are displayed in Figure 4. From exterior (Figure 4, A&B), one can see that the wax spheres were indeed covered by small PPs, of which the size was in


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agreement with those given in Figure 1. It seems that the PPs were not presented as monolayer on the wax spheres, some PPs superposed on the others as seen from the enlarged photos inserted in Figure 4 (A&B). It was thought that PPs piled up on the surface of wax spheres might be caused by a weak adhesion during the step of solvent removal, and should be removed by simply washing using H2O-EtOH. This was confirmed not to be the case. The wax spheres were dispersed in H2O-EtOH by gentle agitation for 5 min, centrifuged to separate out, and observed under SEM again. They remained as is after 1 or 2 such washing; whereas a dramatic change of the morphology (Figure 4C) for the composite spheres was observed after 5 repeated washing or with extended soaking to 30 min, always under gentle agitation. From Figure 4C, it seems that the skin of the wax composite spheres was partially peeled off, while the aggregation of the PPs on the sphere surface became even more severe. This observation suggests that the wax spheres were covered by a layer of PPs, which was adhered on the wax spheres and might be peeled off by soaking in a solvent, leaving the pure wax spheres alone. This is in good agreement with the mechanism of Pickering emulsion. To show whether the PPs were only adhered onto the surface or moved also inside the wax spheres, the wax spheres were crushed after soaking in liquid nitrogen and observed under SEM to see the interior structure. From Figure 4D, it was well confirmed that the PPs were not present inside the wax spheres, and were solely concentrated on the surface of the wax spheres.


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Figure 4. SEM photos of wax spheres as prepared (A, B), after repeated washing using H2OEtOH (C), and the interior view (D) 3.7. Estimation of PPs Number on Wax Spheres. Based on the ratio of PPs/wax in the resulting wax spheres (Tables 2~5), and knowing the amount and the sizes of the wax spheres and those of the PPs, one is able to calculate the number of the PPs on a single wax sphere (Details of the calculation is given in Supporting Information), which is interesting to see how the PPs were adsorbed on the surface of the wax spheres. Dividing simply the total number of the PPs found together with the wax spheres (np, as defined by Eq. 2 in Supporting Information) by the number of the wax spheres (nw, as defined by Eq. 1 in Supporting Information) an experimental number (Nexp) of the PPs on a single wax sphere was obtained (Eq. 3 in Supporting Information). Obviously, the quotient of the surface area of a wax sphere divided by Nexp gave the experimental area occupied by a single polymer particle on the surface of a wax sphere (Sexp, Eq. 5 in Supporting Information). The same way, a theoretical number of PPs (Nmax) that a single wax sphere can take under tightly packing was also obtained (Eq. 7 in Supporting Information);


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A theoretical area (Smin) occupied by a polymer particle on the surface of the wax spheres was also estimated accordingly (Eq. 6 in Supporting Information). Taking into account the way it was obtained, this area (Smin) was the minimal value that a polymer particle occupied on the surface of the wax sphere surface, it was therefore subscripted by minimum (min). Part of the calculated values is given in Table 6. By comparing these experimental values with those obtained by calculation, particularly Nexp versus Nmax, and Sexp (Eq. 5) versus Smin (Eq. 6), one can get some information about the location and the amount of the PPs and how the wax sphere formation was dependent on the PPs. Comparing Nexp with Nmax, one can see that, at lower shaking rate (≤240 osc/min) where the wax spheres of larger size with a lower number were obtained, Nexp was lower than Nmax, indicating that PPs adsorption on the wax spheres were under saturation; and by consequence, the experimental area occupied by a single PP (Sexp) was larger than the calculated one (Smin), i. e., each single PP took a larger area than that they were allowed by tightly packing. Under such circumstance, PPs might be only adsorbed on the surface of the wax spheres; with increased shaking rate, the size of the wax spheres became smaller, their surface became larger, more PPs were required to keep the stability of the wax spheres. At shaking rate of 280 osc/min, a change in the order of the experimental (Nexp) and the calculated number of PPs (Nmax) was observed, i.e., Nexp turned larger than Nmax, and the experimental area of a PP (Sexp) became smaller than the one calculated (Smin), implying that the number of PPs was larger than their maximal number allowed by tightly packing, and that there were more PPs at high shaking than those the wax spheres were able to take by tightly packing. This demonstrates that some of the PPs were adsorbed in multi-layer or engulfed in the wax spheres for all the samples done with shaking rate of 280 osc/min or higher, where the size of the wax spheres became obviously more disperse.


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In regard to the influence of PPs amount (see Table S2), it is particularly interesting to note that, except the sample prepared with the lowest PPs amount (0.2 g), Nexp was larger than Nmax for all the samples of the PPs amount used. This observation suggested again that part of PPs was likely adsorbed in multilayer on the wax spheres, in agreement with what revealed by SEM observation in Figure 4. Table 6. Experimental (Nexp) and theoretical (Nmax) numbers of P(DC-TMPTA) particles adsorbed and the corresponding areas (Sexp) and theoretical area (Smin) of a single polymer particle occupied on a wax sphere prepared under different shaking rate Shaking (osc/min)

200

240

280

320

360

PPs/Wax (wt%)

1.62

3.27

5.57

8.52

8.71

Nexp (PPs on a wax sphere)

98047

49393

32768

21973

21080

Nmax (PPs on a wax sphere)

125422

56406

30253

17568

16841

2

2.499

2.219

1.782

1.534

1.531

2

1.953

1.943

1.932

1.918

1.917

Sexp (µm , PP area expt.) Smin (µm , PP area theo.)

3.8. Stabilization of the Wax Spheres and Interface Energy. To better understand the formation mechanism of the wax droplets and the spheres, the interface tension between EtOHH2O and liquid wax and the contact angle of the PPs on this interface were determined (see Supporting Information for the details). The adsorption energy of the PPs at the interface of wax and EtOH-H2O was obtained through Equation 1 here below:26,39 E=πr2γow(1±cosθ)2

(1)

Where, r is the radius of the PPs particles, γow is the interface tension between wax and the mixture of EtOH-H2O. For a given system, particle adsorption at the interface is most favorable when θ=90°. It is well known that the effective adsorption of solid particles at liquid-liquid interface can be judged by its adsorption energy E: The adsorption is favored and irreversible if


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E is orders of magnitude larger than the thermal energy kT (k is the Boltzmann constant, and T the absolute temperature).26 The adsorption energy of the PPs at interface of wax and EtOH-H2O was therefore estimated and listed in Table 7. Table 7. Contact angles and adsorption energy of PPs at interface of Wax-H2O-EtOH with different H2O content in H2O-EtOH mixture H2O (wt%) in H2O/EtOH Wax-H2O/EtOH-Polymer Contact Angle (º) Interf. Tension (mN·m-1) Adsorption energy (105×kT)

0

6

10

16

100

64.9

74.7

80.1

83.4

130.7

1.87

3.54

5.10

8.70

33.73

2.70

8.35

15.2

29.7

17.8

Table 7 indicates clearly that the adsorption energy E of PPs on wax spheres was significantly larger than the thermal energy (kT) for all the tested solvent mixtures with different H2O content, which meant that all the wax spheres ought to be well stabilized under these conditions. This is in good agreement with results given in Table 1 and Figure 2, which showed that stable wax droplets were effectively formed, with the lowest PDI (≈1.02) and the highest portion of the PPs adsorbed on the wax spheres when H2O amount was between 10 wt% and 16 wt% in EtOH-H2O. 4. CONCLUSIONS Highly monodisperse polymer particles (PPs) of poly(dodecene-trihydroxymethyl propane triacrylate) were prepared by precipitation polymerization in EtOH-H2O without any surfactant and stabilizer. Using the PPs as Pickering stabilizer, stable dispersions of paraffin wax droplets with narrow size distribution were achieved in EtOH-H2O mixed solvent by simply shaking at 70 °C without any surfactant. Solid wax spheres with good uniformity were then obtained by


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quickly cooling down the system to about 30 °C, far below the melting point of the wax. The size of the wax spheres was easily adjustable between 50 and 480 µm by changing the shaking rate, amount of PPs, and particularly, the size of the PPs. SEM observation of the wax spheres revealed that the surface of the wax spheres were covered by a layer of PPs, and there were practically no PPs detected inside the wax spheres. Based on the measurement of interface tension between liquid wax and EtOH-H2O and that of the contact angle of the PPs at the same interface, the adsorption energy of the PPs at the interface of wax and EtOH-H2O was obtained, which provided a theoretical support to the formation of the wax droplets and the spheres. This paper provides therefore a novel pathway to the preparation of wax spheres of clean surface without need for any chemical additives. ASSOCIATED CONTENT Supporting Information Supporting Information, which contains the detailed process on the reduction of the equations and the calculation for experimental and theoretical number of primary polymer particles as Pickering stabilizer, surface area occupied by Pickering particles, is provided (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mails: [email protected]; [email protected] Notes The authors declare no competing interest.


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ACKNOWLEDGMENT This work is supported by grants from National Natural Science Foundation of China (NSFC 21274054, 21304038 and 51473066) and Science & Technology Development Plans of Shandong Province (2017GGX202009), China. REFERENCES (1) Su, J.; Wang, X. Fabrication and Properties of Microencapsulated-Paraffin/Gypsum-Matrix Building Materials for Thermal Energy Storage. Energ. Convers. Manage. 2012, 5, 101–107． (2) Zhao, H.; Li, H. P.; Liao, K. J. The Preparation of Wax Emulsions Stabilized by C5 Petroleum Resin. Petrol. Sci. Technol. 2013, 231, 284–292. (3) Yu, S.; Luo, W. Preparation and Characterization of Microencapsulated Paraffin Polyurea Phase Change Materials. J. Mater. Eng. 2015, 43, 100–104. (4) Zhang, W.; Lu, P.; Qian, L.; Xiao, H. Fabrication of Superhydrophobic Paper Surface via Wax Mixture Coating. Chem. Eng. J. 2014, 250, 431–436. (5) Destribats, M.; Schmitt, V.; Backov, R. Thermostimulable Wax@SiO2 Core-Shell Particles. Langmuir 2010, 26, 1734–1742. (6) Vilasau, J.; Solans, C.; Gómez, M. J.; Dabrio, J.; Mújika-Garai, R.; Esquena, J. Phase Behavior of a Mixed Ionic/Nonionic Surfactant System Used to Prepare Stable Oil-in-Water Paraffin Emulsions. Colloids Surf. A 2011, 384, 473–481. (7) Liu, W.; Sun, D.; Li, C.; Liu, Q.; Xu, J. Formation and Stability of Paraffin Oil-in-Water Nano-Emulsions Prepared by the Emulsion Inversion Point Method. J. Colloid Interf. Sci. 2006, 303, 557–563.


21

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Page 22 of 26

(8) Giermanska, J.; Thivilliers, F.; Backov, R.; Schmitt, V.; Drelon, N.; Leal-Calderon, F. Gelling of Oil-in-Water Emulsions Comprising Crystallized Droplets. Langmuir 2007, 23, 4792– 4799. (9) Fanselow, S.; Emamjomeh, S. E.; Wirth, K.; Schmidt, J.; Peukert, W. Production of Spherical Wax and Polyolefin Microparticles by Melt Emulsification for Additive Manufacturing. Chem. Eng. Sci. 2016, 141, 282–292. (10) Li, C.; Mei, Z.; Liu, Q.; Wang, J.; Xu, J.; Sun, D. Formation and Properties of Paraffin Wax Submicron Emulsions Prepared by the Emulsion Inversion Point Method. Colloids Surf. A 2010, 356, 71–77. (11) Liu, H.; Chen, W.; Liu, Y.; Yao, E. Current Situation of the Special Wax and Preparation Methods. Appl. Chem. Ind. 2016, 5, 533–537. (12) Zhang, J.; Wu, L.; Jing, D.; Ding J. A Comparative Study of Porous Scaffolds with Cubic and Spherical Macropores. Polymer 2005, 46, 4979–4985. (13) Hong, L.; Jiang, S.; Granick S. Simple Method to Produce Janus Colloidal Particles in Large Quantity. Langmuir 2006, 22, 9495–9499. (14) Kaupp, M.; Tischer, T.; Hirschbiel, A. F.; Vogt, A. P.; Geckle, U.; Trouillet, V.; Hofe, T.; Stenzel, M. H.; Barner-Kowollik C. Photo-Sensitive RAFT-Agents for Advanced Microparticle Design. Macromolecules 2013, 46, 6858–6872. (15) Dai, D.; Li, F.; Zhao, T. Effects of Preparation Parameters on Paraffin Wax Microemulsion. China Petrol. Process. Petrochem. Technol. 2012, 14, 15–19. (16) Mestach, D. Reactive Surfactants for Commercial Polymer Dispersions. Prog. Colloid Polym. Sci. 2004, 124, 37–41.


22

Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

(17) Karbstein, H.; Schubert, H. Developments in the Continuous Mechanical Production of Oil-in-Water Macro-Emulsions. Chem. Eng. Process 1995, 34, 205–211. (18) Jiang, J.; Ma, Y.; Cui, Z.; Binks, B. P. Pickering Emulsions Responsive to CO2/N2 and Light Dual Stimuli at Ambient Temperature. Langmuir 2016, 32, 8668–8675. (19) González, E.; Bonnefond, A.; Barrado, M.; Barrasa, A. M. C.; Asua, J. M.; Leiza, J. R. Photoactive Self-cleaning Polymer Coatings by TiO2 Nanoparticle Pickering Miniemulsion Polymerization. Chem. Eng. J. 2015, 281, 209–217. (20) Bai, R.; Xue, L.; Dou, R.; Meng, S.; Xie, C.; Zhang, Q.; Guo, T.; Meng, T. LightTriggered Release from Pickering Emulsions Stabilized by TiO2 Nanoparticles with Tailored Wettability. Langmuir 2016, 32, 9254–9264. (21) Leong, J.; Tey, B.; Tan, C.; Chan, E. Nozzleless Fabrication of Oil-Core Biopolymeric Microcapsules by the Interfacial Gelation of Pickering Emulsion Templates. ACS Appl. Mater. Interfaces 2015, 7, 16169–16176. (22) Lin, Z.; Zhang, Z.; Li, Y.; Deng, Y. Magnetic Nano-Fe3O4 Stabilized Pickering Emulsion Liquid Membrane for Selective Extraction and Separation. Chem. Eng. J. 2016, 288, 305–311. (23) Li, J.; Stöver, H. D. H. Pickering Emulsion Templated Layer-by-Layer Assembly for Making Microcapsules. Langmuir 2010, 26, 15554–15560. (24) 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, 11861–11867. (25) Peng, L.; Feng, A.; Liu, S.; Huo, M.; Fang, T.; Wang, K.; Wei, Y.; Wang, X.; Yuan, J. Electrochemical Stimulated Pickering Emulsion for Recycling of Enzyme in Biocatalysis. ACS Appl. Mater. Interfaces 2016, 8, 29203−29207.


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Page 24 of 26

(26) Binks, B. P. Particles as Surfactants-Similarities and Differences. Curr. Opin. Colloid Interfaces Sci. 2002, 7, 21−41. (27) Li, Z.; Ngai, T. Macroporous Polymer from Core-Shell Particle-Stabilized Pickering Emulsions. Langmuir 2010, 26, 5088–5092. (28) Madivala, B.; Fransaer, J.; Vermant, J. Self-Assembly and Rheology of Ellipsoidal Particles at Interfaces. Langmuir 2009, 25, 2718–2728. (29) Wu, J.; Ma, G. Recent Studies of Pickering Emulsions: Particles Make the Difference. Small 2016, 12, 4633–4648 (30) Dugyala, V. R.; Daware, S. V.; Basavaraj, M. G. Shape Anisotropic Colloids: Synthesis, Packing

Behavior,

Evaporation

Driven

Assembly

and

Their

Application

in

Emulsion Stabilization. Soft Matter 2013, 9, 6711–6725. (31) Kong, X. Z.; Gu, X. L.; Zhu, X. L.; Zhang, L. N. Precipitation Polymerization in Ethanol and Ethanol/Water to Prepare Uniform Microspheres of Poly(TMPTA-Styrene). Macromol. Rapid Commun. 2009, 30(11), 909−914. (32) Zhang, Z.; Gu, X. L.; Zhu, X.; Kong X. Z. Preparation and Formation Mechanism of Uniform Polymer Microspheres in Precipitation Polymerization of Pentaerythritol Triacrylate and Styrene. Acta Phys. Chim. Sin. 2012, 28, 892−896. (33) Gu, X.; Sun, H.; Kong, X. Z.; Fu, C.; Yu, H.; Li, J.; Wang, J. A Green Protocol to Prepare Monodisperse Poly(TMPTMA-St) Microspheres by Photoinitiated Precipitation Polymerization in Low-Toxicity Solvent. Colloid Polym. Sci. 2013, 291, 1771−1779. (34) Kong, X. Z.; Jiang, W.; Jiang, X.; Zhu, X. Preparation of Core–Shell and Hollow Polyurea Microspheres via Precipitation Polymerization Using Polyamine as Crosslinker Monomer. Polym. Chem. 2013, 4, 5776–57848.


24

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Langmuir

(35) Jiang, X.; Li, X.; Zhu, X.; Kong, X. Z. Preparation of Highly Uniform Polyurea Microspheres through Precipitation Polymerization and Their Characterization. Ind. Eng. Chem. Res. 2016, 55(44), 11528−11535. (36) Zhang, F.; Jiang, X.; Zhu, X.; Chen, Z.; Kong, X. Z. Preparation of Uniform and Porous Polyurea Microspheres of Large Size through Interfacial Polymerization of Toluene Diisocyanate in Water Solution of Ethylene Diamine. Chem. Eng. J. 2016, 303, 48−55. (37) Binks, B. P.; Lumsdon, S. O. Pickering Emulsions Stabilised by Monodisperse Latex Particles: Effects of Particle Size. Langmuir 2001, 17, 4540−4547. (38) Mestach, D. Reactive Surfactants for Commercial Polymer Dispersions. Prog. Colloid Polym. Sci. 2004, 124, 37−41. (39) Binks, B. P.; Fletcher, P. D.; Holt, B. L.; Parker, J.; Beaussoubre, P.; Wong, K. Drop Sizes and Particle Coverage in Emulsions Stabilised Solely by Silica Nanoparticles of Irregular Shape. Phys. Chem. Chem. Phys. 2010, 12, 11967–11974.


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TOC


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Pickering Emulsion Formation of Paraffin Wax in an EthanolâWater

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