Eco-Friendly Pickering Emulsion Stabilized by Silica Nanoparticles

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Novel eco-friendly Pickering emulsion stabilized by silica nanoparticles dispersed with high-molecular-weight amphiphilic alginate derivatives XinYu Zhao, Gaobo Yu, Jiacheng Li, Yuhong Feng, Lei Zhang, Yang Peng, Yiyuan Tang, and Longzheng Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04508 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Sustainable Chemistry & Engineering

Novel eco-friendly Pickering emulsion stabilized by silica nanoparticles dispersed with high-molecular-weight amphiphilic alginate derivatives Xinyu Zhao, Gaobo Yu, Jiacheng Li*, Yuhong Feng**, Lei Zhang, Yang Peng, Yiyuan Tang, Longzheng Wang Key Laboratory of Advanced Materials of Tropical Island Resources, Ministry of Education, College of Materials and Chemical Engineering, Hainan University, 58 Renmin Road, Haikou, 570228, Hainan Province, China Corresponding Author *E-mail: [email protected]; [email protected] (J. L); **E-mail: [email protected] (Y. F). ABSTRACT O/W Pickering emulsions formed with silica nanoparticles adsorbed with amphiphilic alginate derivatives produced a longer stability. Ugi-Alg was obtained by a mild and efficient Ugi four-component condensation reaction, and its structure of Ugi-Alg was confirmed by 1H NMR and FT-IR spectroscopies. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) measurements were then performed to characterize the aggregation behavior of SiO2/Ugi-Alg mixture in aqueous solution. The positioning of the silica particles at the oil–water interface was obtained through confocal laser scanning microscopy (CLSM). The long-term stability and rheological property of Pickering emulsions stabilized by Ugi-Alg and SiO2 nanoparticles were respectively investigated by Turbiscan Lab® Expert stability analyzer and rotational rheometer. The rheological behaviors of the emulsions were studied and results showed that Ugi-Alg with high molecular weight led to more rigid structures. This trend can be justified by the formation of silica particle strong network attributed to the entanglement and bridging of Ugi-Alg. The results indicated that adjusting the amount of silica nanoparticles and Ugi-Alg molecular-weight could be a suitable means for manipulating stability of polymer-nanoparticles-based 1

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Pickering emulsions. This process would be of great application prospect for the preparation of Pickering emulsions stabilized by sustainable polymer surfactant and nanoparticles. KEY WORDS: Silica nanoparticles, Ugi-Alg, Pickering emulsion, long-term stability, rheological property, stability mechanism INTRODUCTION Emulsions are applied in various fields such as foods, paints, cosmetics, pesticides and coal treatment.1-2 In industrial applications, the long-term stability of emulsions required is generally achieved by a proper use of organic materials, such as surfactant-like molecules, surface active polymers or low molecular weight surfactants.1,

3

Emulsions stabilized by solid particles in place of surfactants are

known as Pickering emulsions. Compared to traditional emulsions, Pickering emulsion exhibits higher physical/chemical stabilities.4 Because they retain the basic properties of classical emulsions, so that Pickering emulsions can be substituted for classical emulsions in most applications.5 Pickering emulsions are stabilized by the adsorption of solid particles, such as silica nanoparticles,6-7 CaCO3 nanoparticles,8 clay,9-12 zein,4 magnetic particles,13 chitosan-coated alginate,14 oppositely charged latex particles,15 and biopolymer-based particles.16 Silica, an inorganic particle, is considered safe, nontoxic, and environmentally benign. A clear synergy can be found between silica and each surfactant.17-18 Moreover, phase inversion can be prevented by nanoparticles that are well–dispersed in a continuous phase, but it is difficult to form homogeneous Pickering emulsions due to they are not adsorb to the interface easily.19 Particles that are hydrophilic or hydrophobic cannot act as emulsion stabilizers.1,6 Several studies reported various methods for particle surface modification through the adsorption of organic surfactants or amphiphiles, such as propionic acid for alumina, chitosan for polystyrene,20 CTAB for silica,21 or tannic for zein.4 Pickering emulsions stabilized by particles can also form particles film on oil droplet interface and network between droplets. Pickering emulsions stabilized by particles can also provide an excellent steric stabilization to their droplets and provide a barrier against coalescence. 3, 17, 22 2


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More recently, a growing interest on this type of Pickering emulsions, such as oil-in-water (O/W) Pickering emulsions, stabilized by particles‒polymer interaction has emerged. O/W Pickering emulsions were obtained by using polystyrene and chitosan composite particles.20 O/W Pickering emulsions stabilized by the complex of polystyrene (PS) particles and chitosan (CS) were systematically investigated, and the mechanism underlying of the emulsifying ability of the PS/CS composite particle synergistic-stabilized Pickering emulsions were studied. Meanwhile, Pickering emulsion gels with high oil volume fraction were successfully fabricated using a novel zein/tannic acid complex colloidal particles (ZTP).4 The authors explained that stable Pickering emulsion gels formed due to oil droplets closed to each other that further triggered cross-linking to form a continuous network of oil droplets and protein particles. In addition, crude oil-in-seawater emulsions with long-term stability and small droplet sizes were produced from silica nanoparticles modified in situ with rhamnolipid.3 Synergistic stabilization upon rhamnolipid addition led to particle flocculation, which increased the stability of the emulsions. Furthermore, O/W emulsions can be stabilized by Ulva fasciata polysaccharide.23 In addition, Turbiscan Lab® Expert stability analyzer can effectively determine the long-term stability of emulsions.24-26 In the present study, we sought to identify polymers that are safe, nontoxic, and environmentally friendly and found that sodium alginate is a good candidate. Sodium alginate is a natural anionic unbranched polysaccharide obtained from brown seaweed and various bacteria that is widely used in the food industry, environmental engineering,

and

biomedical

applications

because

it

is

nontoxic

and

biocompatible.27-29 However, pure sodium alginate has inherent disadvantages, such 28, 29

as poor mechanical strength and high water-solubility.

Thus, some studies on

chemical modification of alginate via esterification and amidation have been reported.30-32 Ugi-Alg, a sodium alginate derivative, is synthesized by the Ugi four component condensation reaction that an amine, an aldehyde or ketone, an isocyanide and a carboxylic acid. Meanwhile, it is the most effective and mild method which has an inherent high atom economy and chemical yield.

28, 33, 34

3


Our research group


studied the self-assembly behavior of Ugi-Alg in aqueous solution and showed that Ugi-Alg is imparts good amphiphilic functionality.28 Furthermore, stable Pickering emulsions can be prepared using modified kaolin particles in cooperation with Ugi-Alg. Thus, in this work, amphiphilic alginate derivatives were prepared via the Ugi reaction. Therefore, this paper focuses on the use of silica nanoparticles and a sustainable macromolecule surfactant, Ugi-Alg, to synergistically stabilize O/W emulsions. Sodium alginate is a well-known biopolymer heteropolysaccharide that is extracted from marine algae. However, despite its wide application, little is known about the synergistic stabilization of Pickering emulsions stabilized by a mixture of silica nanoparticles and Ugi-Alg. In this paper, we exploited the sustainable surfactant Ugi-Alg to modify the surface characteristics of hydrophilic silica nanoparticles to form stable O/W Pickering emulsions. We also investigated the influence of the particle concentration and Ugi-Alg molecular weight on emulsion stability properties, namely, stability, size, and rheological behavior; we also investigated the possible stability mechanism of these Pickering emulsions. EXPERIMENTAL SECTION Materials. Sodium alginate (NaAlg, Mw = 710,974 and Mn = 495,565), formaldehyde, octylamine, hydrochloric acid, ethanol, hydrophilic SiO2 nanoparticles (20‒40nm), H2O2 (AR), FeSO4 7H2O (AR), and Rhodamine B (>95% HPLC grade) were purchased from Aladdin Chemical Reagent Co. Ltd. (Shanghai, China). Sodium alginate (NaAlg, Mw = 290,956 and Mn = 186,653) and cyclohexyl isocyanide were purchased from J&K Chemical Reagent Technology Co. Ltd. (Beijing, China). All sodium alginates were assessed by gel permeation chromatography (GPC). Paraffin liquid (CP) was supplied by Xilong Scientific Co. Ltd (Guangzhou, China) and used as the oil phase. Synthesis of amphiphilic alginate derivatives. Ugi-Alg was prepared according to previous work by our research group.28 Ugi reaction is a four-component condensation reaction and carried out at a fixed pH of 3.6. A 2.5 wt% sodium alginate solution was adjusted to 3.6 by the addition of 0.5 mol/L of HCl. Then, 0.462 mL of 4


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octylamine, 0.292 mL of formaldehyde, and 0.447 mL of cyclohexyl isocyanide were successively added to the solution, and the mixture was stirred for 24 h. The product was dialyzed against distilled water for 3 days and lyophilized to obtain the pure Ugi-Alg product. High molecular weight alginate (H-Alg, Mw = 710,974) was degraded by hydrogen peroxide (30 wt%) and FeSO4 7H2O (0.5 mmol/L), and the sample (L-Alg) was dialyzed against pure water and lyophilized. Low‒molecular‒ weight modified alginate (L-Ugi-Alg) was obtained by Ugi reaction. Preparation of Pickering emulsion. A series of stock dispersions of hydrophilic SiO2 (0.1 wt%, 0.6 wt%, 1 wt%, 1.5 wt%, 2 wt %) and Ugi-Alg aqueous solution was obtained by mixing various mass fraction of hydrophilic SiO2 with Ugi-Alg aqueous solution (1 g/L) under ultrasonic condition. The critical aggregation concentrations (CAC) of Ugi-Alg were 0.196 g/L, 0.246 g/L, and 0.279 g/L (see the Supporting Information, FigureS1). Pickering emulsions were prepared by mixture of the dispersion and paraffin liquid (volume ratio=1:1) at 22,000 rpm for 10 min using a high shear dispersion homogenizer (FA25, FLUKOF, Germany) at room temperature.. During the preparation, hydrophilic SiO2 particles were used as stabilizer and paraffin liquid as the oil phase. The O/W emulsion was prepared in the laboratory at room temperature (25 °C). The type of Pickering emulsion was determined simply by observing the result of adding a drop of emulsion to a volume of either pure oil or pure water (drop test).20, 35 Fourier transform infrared spectroscopy and 1H nuclear magnetic resonance spectroscopy. The structure of Ugi-Alg was confirmed by FT-IR and 1H NMR. The FT-IR spectra of the samples were recorded on a Tensor27 (Bruker, Switzerland) Fourier transform infrared spectrometer. The samples were scanned from 400 cm‒1 to 4000 cm‒1 at a resolution of 4 cm‒1. The samples were mixed with KBr and compressed to semitransparent disks for spectroscopic analysis to determine the composition of Ugi-Alg functional groups. 1H NMR was performed on a DMX500 (Bruker, Switzerland) nuclear magnetic resonance spectrometer at 75 °C using 5 mm NMR tube. The samples were dissolved in D2O (99%) to a concentration of ~10 mg/mL. 5



Transmission

electron microscopy

(TEM).

The

morphology

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of

silica

nanoparticles with or without Ugi-Alg was observed using a JEOL transmission electron microscopy (JEM-2100, Japan). TEM images of the Ugi-Alg/SiO2 dispersion were obtained by placing fresh drops of dispersion on a carbon-coated grid, which was then dried with an infrared lamp before observation. Optical microscopy. The microstructure of the Ugi-Alg/SiO2 synergistic-stabilized Pickering emulsions were observed by optical microscopy (Nikon Eclipse E200, Nikon, Japan) equipped with an SBI Imaging camera. The average droplet size of freshly prepared emulsions was estimated by Nano Measurer 1.2 software. Confocal laser scanning microscopy (CLSM). The microstructure of emulsions was observed by a Leica TCC-SP8 confocal laser scanning microscope (Leica Microsystems Inc., Germany). The emulsions were dyed by Rhodamine B, which was added to the aqueous phase prior to emulsification to label fluorescently the silica particles. Multiple light scattering. The formed emulsions were placed into cylindrical glass tubes. The destabilization of Pickering emulsions was measured by Turbiscan Lab® Expert stability analyzer (Formulaction, France) at 25 °C. The technique allows transmitted light (T) and back-scattering light (BS) to be obtained as a function of time. Turbiscan Stability Index (TSI) is a parameter, which can be used for estimation of Pickering emulsions stability. Due to the specialized computer program connected with Turbiscan, calculating the value of TSI that is very useful in the evaluation of emulsion was possible. The following equation [Eq. (1)] was applied: =

∑

(1)

where, is average backscattering for each minute of measurement, is the mean of , and n is the number of scans.26, 36-38 Rheological measurement. Shear viscosity and oscillatory sweep measurements were conducted in a DHR rotational rheometer (TA, USA) equipped with stainless-steel parallel-plate geometry (60 mm diameter and 500 µm gap) and a Paltier system for control temperature. All rheological measurements were carried out at 6


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25 °C. Oscillation amplitude tests were performed in a range of 0.01%–1000% at 1 Hz frequency. The strain amplitude values were checked to ensure that all measurements were performed in the linear viscoelastic region (LVR) of the emulsion.27 In order to ensure that the test does not provoke structure damage in the sample, test should be performed at a stress and strain within the linear viscoelastic region. Previous oscillatory torque sweep tests at a fixed frequency were performed to estimate the maximum amplitude value of the sinusoidal shear stress function, which guarantees linear viscoelastic behavior.39 Oscillation frequency sweep tests were conducted from 0.01 Hz to 100 Hz at strain amplitude of 1% within the linear range, and the frequency dependent curves of storage modulus G′ and loss modulus G″ were recorded. Steady shear flow tests were performed in the controlled rate mode by increasing the shear rate from 0.01 s‒1 to1000 s‒1 and apparent viscosity η were recorded. RESULTS AND DISCUSSION Characterization of Ugi-Alg. Sodium alginate was hydrophobically modified through the the Ugi four-component reaction. The chemical modification of NaAlg is irreversible and drives the whole reaction without the need of catalysis.28 Figure 1 shows the proton peaks of NaAlg and Ugi-Alg marked in the 1H NMR spectrum in. The proton peaks from 3.5 ppm to 5.5 ppm are due to the H of the native NaAlg chain. Some additional proton peaks ranged from 0.5 ppm to 2.0 ppm, which were assigned to the protons of new functional groups grafted to sodium alginate, demonstrating that Ugi-Alg was successfully synthesized. The FT-IR spectra showed in Figure S1 also indicated that Ugi-Alg was successfully synthesized. The FT-IR spectra showed in Figure S2 also indicated that Ugi-Alg was successfully synthesized (see the Supporting Information, FigureS1). Different molecular weights of Alg and Ugi-Alg were assessed by gel permeation chromatography, as shown in Table 1.

7



O NaO C HO

OH

O HO

O O

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H4 O O

H2

H3

OH

H1

NaO C O

L-Ugi-Alg

M-Ugi-Alg

H-Ugi-Alg O C6H11

Alg

8

7

3

6

5

4

3

2

N H2 C

N H

1

(CH2)6

CH3

2

1

0

δ/ppm

Figure 1. 1HNMR spectra of NaAlg and Ugi-Alg (H- Ugi-Alg, M- Ugi-Alg, L- Ugi-Alg) Table 1. The results of molecular weight of Alg and Ugi-Alg Samples

Mn

Mw

Mp

D

H-Alg

495565

710974

763412

1.434674

H-Ugi-Alg

443056

685508

803367

1.547228

M-Alg

186653

290956

202644

1.558806

M-Ugi-Alg

176208

279842

206822

1.588136

L-Alg

83832

109194

95316

1.302531

L-Ugi-Alg

72949

92706

80405

1.270837

Mn: number-average molecular weight, Mw:weight-averaged molecular weight Mp: peak position molecular weight, D: degree of polymerization D = M w M

n

Characterization of dispersions containing silica nanoparticles and Ugi-Alg. Figure 2A shows the FT-IR spectra of unmodified and surface-adsorption silica nanoparticles. The FT-IR spectrum of SiO2 is shown in Figure 2A (a). The broad peak at 3450 cm-1 is assigned to O–H stretching vibrations. The adsorption band at 1638 cm-1 is due to moisture in the sample. The characteristic peaks appearing at 1102 cm-1 and 804 cm-1 are attributed to the asymmetric and symmetric stretching vibrations of 8


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Si–O–Si, respectively. A small peak appearing at 963 cm-1 is assigned to the Si–OH stretching vibrations.40-41 The other characteristic bands near 470 cm-1 are assigned to the out–of–plane (rocking) deformation of the Si–O–Si chain in SiO2.41 In Figure 2A(b-d), the additional peaks at 2925 cm-1 and 2845 cm-1, respectively, correspond to the C–H stretching vibrations of the saccharide structure and –CH2 bending vibrations, both of which indicate successful surface modification. Additionally, the characteristic peaks at 470 cm-1 (Figure 2A (a)) shift to 465 cm-1 (Figure 2A (b-d)). The COO– stretching vibrations at 1612 cm-1 and 1416 cm-1 are slightly shifted to 1622 cm-1 and 1405 cm-1 for the adsorbed sample. The results indicate the interaction of COO– with the SiO2 surface. The broad peak of the hydroxyl group at 3450 cm-1 is increased and red-shifted to 3429 cm-1 (H-Ugi-Alg), indicating strong interaction through hydrogen bonding between the –OH of the SiO2 surface and COO–groups of Ugi-Alg.42-43 The red-shift value of –OH is decreased with increasing Ugi-Alg molecular weight. The results suggest that hydrogen bonding in H-Ugi-Alg/SiO2 is stronger than in M-Ugi-Alg/SiO2 and L-Ugi-Alg/SiO2, and decreases in the following order: [H-Ugi-Alg/SiO2] > [M-Ugi-Alg/SiO2] > [L-Ugi-Alg/SiO2].44 Samples of silica suspensions with and without Ugi-Alg were taken for TEM analysis, as shown in Figure 3. Although some silica nanoparticles aggregated together (Figure 3a), these nanoparticles were relatively discrete in the presence of Ugi-Alg (Figure 3b-d). As the molecular weight of Ugi-Alg decreased, the degree of dispersion among silica nanoparticles decreased, resulting in aggregation. The zeta potential of silica particles increased when the measured Ugi-Alg molecular weight increased (Figure 2B). H-Ugi-Alg adsorption on the silica resulted in increased electrostatic repulsion of particles. The above results show that H-Ugi-Alg has a certain extent of dispersancy effectiveness on silica nanoparticles. The stability of suspensions is promoted through electrostatic repulsion.42

9



(A)

(B) -20

(d) L-Ugi-Alg-SiO2

-25

465

(c) M-Ugi-Alg-SiO2 3440

-30 465

1411

(b)H-Ugi-Alg-SiO2

-35

Zeta/mV

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

3434 1405

2845

3429

2925

(a) SiO2

465

1622

1638

3450

963 470

-55

1102

4000 3500 3000 2500 2000 1500 1000

-45 -50

804

1096

-40

-60 500

SiO2

Wavenumber/cm-1

L-Ugi-Alg M-Ugi-Alg H-Ugi-Alg /SiO2 /SiO2 /SiO2

Figure 2. (A) FT-IR spectra of SiO2 and Ugi-Alg-SiO2 (H-Ugi-Alg-SiO2, M-Ugi-Alg-SiO2, L-Ugi-Alg-SiO2); (B) Zeta potential of silica nanoparticles dispersed without Ugi-Alg and with different molecular weights of aqueous Ugi-Alg.

Figure 3. TEM images of silica nanoparticles dispersion without (a) and with (b) L-Ugi-Alg, (c) M-Ugi-Alg and (d) H-Ugi-Alg

Stability of Ugi-Alg and SiO2 synergistic-stabilized Pickering emulsions. The principle of Turbiscan Lab® Expert stability analyzer is based on the variation of the droplet volume fraction (migration) or average size (coalescene), thus resulting in the variation of transmission (T) and back-scattering (BS) signals.24, 47 In contrast to other optical analytical methods, Turbiscan Lab Expert stability analysis has the advantage 10


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of non-destructive and no sample dilution. TSI value may vary within 0 and 100, and the smaller value is the more stable.36 No variation in droplet size occurs when the backscattering profile is within an interval ±2%. Variations greater than 10%, whether positive or negative on the graphical scale of backscattering, are representative of an instable formulation.24 The analyzed Pickering emulsion was contained in a cylindrical glass cell. The light source is an electro-luminescent diode in the near infrared region (λair = 880 nm). Two synchronous optical sensors receive backscattered light by the sample (45° from the incident radiation, backscattering detector), and transmitted light through the sample (180° from the incident radiation, transmission detector). To rank and easily compare any number of samples, TSI can be computed. TSI is a computation that is directly based on raw data obtained from the instrument, particularly T and BS signals. TSI sums up all of the variations in the samples, to give a unique number reflecting the destabilization of a given sample. A high TSI indicates strong the destabilization of the samples. 36 The transmission signals of the Ugi-Alg/SiO2 synergistic-stabilized Pickering emulsions were nil. Thus, variations in the backscattering of samples were analyzed (Figure S4). Positive or negative variations in the backscattering profiles of the different formulations were not correlated with the destabilization processes under sample height of 2 mm and over that of 40 mm. The backscattering light signals of Pickering emulsion at SiO2 = 0.1 wt % showed a broad, strong continuous variations during the scan, with maximum negative (‒46%) peaks for the bottom of the sample. The maximum negative peaks for the bottom of the sample with an increased SiO2 mass fraction varied from ‒46 % to ‒28 %. The ∆BS profiles of the bottom of the system for all samples were down, and the middle and top of system were up at varying times, these findings were due to a time-dependent creaming phenomenon. As shown in Figure 4b, the obtained ∆BS results for the 2 wt % SiO2 stabilized emulsion is close to ~1 % for the entire samples height. These data show that a stable Pickering emulsion stabilized by Ugi-Alg/SiO2 was obtained. Pickering emulsions stabilized solely by silica particles were easily demulsified, resulting in variations of the 11



transmission signals (Figure S4a). Thus, we did not show the ∆BS values of the emulsion. Correspondingly, for the same particle concentration (2 wt %), the Ugi-Alg causes noticeable improvement in the stability of Pickering emulsion compared to the emulsion without Ugi-Alg. Interestingly, Ugi-Alg is an amphiphilic polymer that functions similarly to that of the classic surfactant.28 Emulsions stabilized solely by silica particles exhibited the highest TSI values. Under the same Ugi-Alg concentration, a higher SiO2 concentration led to a lower TSI value and higher emulsified phase volume (Figure S3), resulting in increased stability. Under the same SiO2 concentration, the stability of emulsions synergistically-stabilized by H-Ugi-Alg and SiO2 were higher than that formed with M-Ugi-Alg or L-Ugi-Alg.

14 12

-2

H-SiO2=1.5wt% H-SiO2=2wt%

-4

∆ BS

10

TSI

(b) 0

H-SiO2=0.1wt% H-SiO2=0.6wt% H-SiO2=1wt%

(a) 16

8 6 4 2

-6 -8

H-SiO2=0.1wt% H-SiO2=0.6wt%

-10

H-SiO2=1wt% H-SiO2=1.5wt% H-SiO2=2wt%

0 0

20

40

60

80 100 120 140 160 180 200

0

20

40

60

80 100 120 140 160 180 200

t/min

t/min

(d) 0

(c) 8

-1 6

-2

∆ BS

TSI

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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4

2

0 0

20

40

60

-3

SiO2=2wt% H-SiO2=2wt%

-4

M-SiO2=2wt% L-SiO2=2wt%

-5

H-SiO2=2wt% M-SiO2=2wt% L-SiO2=2wt%

80 100 120 140 160 180 200

0

20

40

t/min

60

80 100 120 140 160 180 200

t/min

Figure 4. Effect of silica contents on the stability of Pickering emulsions: (a) Turbiscan Stability Index (TSI) values and (b) ∆BS values. Effect of Ugi-Alg molecular weight on Pickering emulsions stability: (c) Turbiscan Stability Index (TSI) values and (d) ∆BS values.

Microstructure of Ugi-Alg and SiO2 synergistic-stabilized Pickering emulsions. Figure 5 shows photographs and the size distribution of Pickering emulsions 12


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stabilized by SiO2 particles with variable SiO2 mass fraction and molecular weight of Ugi-Alg after 3 h in storage. In the presence of Ugi-Alg, the size distribution shifted towards smaller droplet size when the particle mass fraction increased. In other words, the emulsion droplet size distribution attained a narrower range at increased nanoparticle mass fraction. Under the same nanoparticle mass fraction, the size distribution shifted towards smaller droplet size at the increased Ugi-Alg molecular weight. The small droplet size exhibit a higher stability of Pickering emulsion.

4, 47-48

Ugi-Alg is actually an amphipathic polysaccharide, which can self-assemble to form micelles when their concentration is above the CAC (Figure S1). Accordingly, Ugi-Alg (1 g/L) might form micelles and thereby causes interfacial particle displacement, decreases the size of droplets, and enhances the physical stability of Ugi-Alg-SiO2-based emulsions. 4 The microstructure of Pickering emulsions was characterized by CLSM. Before emulsification, silica particles were fluorescently labeled with Rhodamine B. The obtained images are shown in Figure 6. The bright red rings around the oil droplets were rendered visible in the CLSM images, which indicated the adsorption of fluorescently labeled silica particles around the oil droplets. For the Pickering emulsions stabilized solely by silica particles, most silica particles were dispersed in a continuous phase (aqueous phase) and few silica particles were adsorbed at the oil‒ water interface. It has been demonstrated that silica nanoparticles alone are ineffective emulsifier for flocculating. In contrast, the interfacial particle film was confirmed at the oil droplet interface of the emulsions stabilized by Ugi-Alg and silica particles, demonstrating the formation of O/W Pickering emulsions. In addition, as shown in Figure 6 of H-Ugi-Alg systems, we can see that droplet size decreases with the silica particle fraction increases from 0.1% to 2%. Meanwhile, we also can figure out that small emulsion droplets could be obtained in emulsions prepared by the high-molecular-weight Ugi-Alg, which was consistent with those results observed through optical microscopy. Moreover, for systems of 0.1%-H, 1%-H, 2%-H and 2%-M, a thickening and homogeneous droplet surface layer17 was formed at the droplet surface, which can provide an effective steric barrier.6 Furthermore, compared 13



L-Ugi-Alg with M-Ugi-Alg and H-Ugi-Alg (Figure 6), it was clearly observed that incompletely interfacial particle film was only formed in the L-Ugi-Alg system since the flocculation silica nanoparticles at the interface of oil droplet.

Figure 5. Microscopic images for nanoparticle-polymer stabilized O/W emulsions:(a) 2 wt % SiO2; (b) 0.1 wt % SiO2 + H-Ugi-Alg; (c) 1 wt % SiO2 + H-Ugi-Alg; (d) 2 wt % SiO2 + H-Ugi-Alg; and molecular weight Ugi-Alg; (d) 2 wt % SiO2 + H-Ugi-Alg; (e) 2 wt % SiO2 + M-Ugi-Alg; (f) 2 wt % SiO2 + L-Ugi-Alg;

Figure 6. Images of the emulsions were prepared by silica (i.e., 0.1%, 1%, and 2%) without and with Ugi-Alg (i.e., H-Ugi-Alg, M-Ugi-Alg, and L-Ugi-Alg) by confocal fluorescence microscopy. All scale bars are 25 µm.

Rheological properties of Pickering emulsions synergistic-stabilized by 14


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Ugi-Alg /SiO2. The rheological properties of emulsions have a prominent effect on the utilization and processing of emulsions. Moreover, flow behavior data are considered to be indicators of product quality.26,

46

The bulk phase rheology of

Ugi-Alg/SiO2 synergistic-stabilized Pickering emulsions was investigated by steady shear measurements. The Pickering emulsion stabilized with SiO2 alone showed oil release on the top phase, which was not suitable for measuring the rheological properties. Thus, we did not show the rheological behavior of the corresponding emulsion. The effects of the nanoparticle concentration on viscosity of the prepared emulsions were investigated and are shown in Figure 7. These flow curves indicated that there is a relationship between viscosity and shear rate. Experiments were performed immediately after preparation of the emulsion. The above results (Figure 7a, b) showed that all emulsions exhibited a typical shear-thinning behavior, 7 which showed a significant decrease in viscosity with increasing shear rate. For regarding coalescence and creaming, the water phase needs to easily traverse oil droplets, and therefore, intimate contact between droplets is possible. 1,6,17 The creaming rate can be estimated from the following Stokes’ equation [Eq. (2)]:

=

2r 2 ρ-ρ0 g#

9η (2)

where, is the creaming rate, r is the droplet radius, ρ is the density of the

droplet, ρ$ is the density of the dispersion medium, η is the viscosity of the

continuous phase, and g is the local acceleration due to gravity. Eq. (2) shows that

creaming is inhibited by a small droplet radius and highly viscosity of the continuous phase.6 In Figure 7a, the viscosity of emulsions increases with increasing SiO2 concentration at the same shear rate. A possible reason for the increased viscosity is that flocculated silica nanoparticles restrain the motion of oil droplets with increasing silica concentration. The enhanced viscosity slows down the droplet migration rate and the number of collisions.6 In addition, as can be seen from Figure 7b, the viscosity of emulsions increases with increased Ugi-Alg molecular weight. This is because the greater chain length of H-Ugi-Alg increases the chance of attachment on the silica 15



Page 16 of 26

surface and stronger interaction with silica. However, silica adsorbed by L-Ugi-Alg easily peels off from the particle surfaces under external force due to weaker interaction with SiO2, which may have led to the bare surfaces of the solid particles. The preference for adsorption onto SiO2 particles leads to hindered preferential adsorption onto the oil‒water interface. We also believe that the adsorption of Ugi-Alg enhanced the flocculation of silica through the polymer bridging mechanisms, 17, 42

which hindered movement by forming stronger networks when adsorbed onto the

oil-water interface. Figure 4a, c further indicate the decreased TSI curves’ slope for emulsions with an increase in SiO2 mass fraction and increase in Ugi-Alg molecular weight. The result is explained by the increased hindrance of the droplets migration. Oscillatory measurements must be used in addition to steady shear tests to better understand the viscous and elastic properties of the emulsions. The magnitudes of storage and loss modulus reflect two behaviors, namely, viscous or liquid-like behavior (G′ < G″) and elastic or gel-like behavior (G′ > G″).7, 26, 47 Figure 7c, e represents the strain sweep tests for O/W Pickering emulsions stabilized by the Ugi-Alg/SiO2 system. The emulsion system showed gel-like behavior because of the dominance of G′ over G″ at the range of 20% strain amplitude. As the SiO2 mass fraction increases from 0.1% to 2%, both G′ and G″ were increased (see from Figure7c). With increasing Ugi-Alg molecular-weight, the G′ and G″ values were increased (see from Figure7e). As seen in Figure 7c, e, the LVR range was expected to be below 1% of the strain amplitude. Therefore, frequency sweep tests for Pickering emulsions were performed at 1% strain amplitude, as presented in Figure 7d, f. Figure 7d depicts the storage and loss modulus as functions of angular frequency and SiO2 concentration at fixed strain and fixed Ugi-Alg molecular weight. H-Ugi-Alg

and

SiO2

synergistically-stabilized

Pickering

emulsions

formed

weak-gel-like structures, as indicated a G′ that was higher than G″. Increasing silica concentration does not change the trend with angular frequency but increases G′ and G″, especially G′ (Figure 7d). The results indicated that increasing SiO2 concentration enhanced the strength of the three-dimensional network structure, 1, 6 and the elasticity of the interfacial film formed at the oil–water interface with silica was increased. 7 16


Page 17 of 26

This also indicates that a network of SiO2 nanoparticles can be responsible for the emulsion’s rigidity. 1 Figure 7f shows the storage and loss modulus as functions of angular frequency and Ugi-Alg molecular weight at fixed strain (1 %) and fixed SiO2 concentration (2 wt%). The results indicated that G′ was higher than the corresponding G″ for all range of angular frequencies for Pickering emulsions stabilized by H-Ugi-Alg/SiO2. However, for M-Ugi-Alg/SiO2 and L-Ugi-Alg/SiO2 stabilized emulsions, a crossover point was observed near 200 rad/s and 20 rad/s, respectively, indicating “liquid-like” behavior after crossover. The emulsion stabilized by L-Ugi-Alg/SiO2 was not strong enough to be sustained at high frequency because both G′ and G″ increase significantly with frequency. It suggests that the network structure of emulsions may be broken with an increase in angular frequency.13 The results indicated that H-Ugi-Alg/SiO2−stabilized Pickering emulsions have a stronger three-dimensional network structure, and the interfacial film formed at the oil–water interface might be more elastic than the one stabilized by M-Ugi-Alg/SiO2 and L-Ugi-Alg/SiO2. 7 (a) 10

(b) 10

H-SiO2=0.1wt% H-SiO2=0.6wt% H-SiO2=1.5wt%

Viscosity/Pa•s

Viscosity/Pa•s

H-SiO2=1wt%

1

H-SiO2=2wt%

0.1

1

0.1

H-SiO2=2wt% M-SiO2=2wt% L-SiO2=2wt%

0.01

0.01 0.01

0.1

1

10

100

0.01

1000

0.1

Shear rate/s-1

1000

(d)

100

10

1

0.1

0.01

H-SiO2=0.1wt%-G' H-SiO2=0.6wt%-G'

H-SiO2=0.1wt%-G'' H-SiO2=0.6wt%-G''

H-SiO2=1wt%-G' H-SiO2=1.5wt%-G'

H-SiO2=1wt%-G'' H-SiO2=1.5wt%-G''

H-SiO2=2wt%-G'

H-SiO2=2wt%-G''

0.1

1

10

1

10

100

1000

Shear rate/s-1

1000

Storage modulus¸G'[Pa] Loss modulus¸G"[Pa]

(c)


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


100

1000

100

10

1

0.1

0.01 0.1

1

H-SiO2=0.1wt% G' H-SiO2=0.6wt% G'

H-SiO2=0.1wt% G'' H-SiO2=0.6wt% G''

H-SiO2= 1 wt% G' H-SiO2=1.5wt% G'

H-SiO2= 1 wt% G'' H-SiO2=1.5wt% G''

H-SiO2= 2 wt% G'

H-SiO2= 2 wt% G''

10

ω(rad/s)

Strain%

17


100

1000


(e) 1000


(f)

100


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

10 1

0.1

H-SiO2=2wt%-G' M-SiO2=2wt%-G'

H-SiO2=2wt%-G'' M-SiO2=2wt%-G''

L-SiO2=2wt%-G'

L-SiO2=2wt%-G''

0.01 0.1

1

10

100

1000

100

10

1

0.1

0.01

H-SiO2=2wt%-G' M-SiO2=2wt%-G'

H-SiO2=2wt%-G'' M-SiO2=2wt%-G''

L-SiO2=2wt%-G'

L-SiO2=2wt%-G''

0.1

1000

Page 18 of 26

Strain%

1

10

100

1000

ω(rad/s)

Figure 7. Flow curves of O/W emulsions at (a) different SiO2 mass fraction and (b) different molecular weight Ugi-Algs. Strain sweep measurements of O/W Pickering emulsions at 25 °C: (c) 0.1‒2 wt % SiO2+H-Ugi-Alg; (e) 2 wt% SiO2+ Ugi-Alg (H-Ugi-Alg; M-Ugi-Alg; L-Ugi-Alg). Frequency sweep measurements at strain amplitude of 1 % at 25 °C: (d) 0.1‒2 wt % SiO2+H-Ugi-Alg; (f) 2 wt% SiO2+ Ugi-Alg (H-Ugi-Alg; M-Ugi-Alg; L-Ugi-Alg). G′ (solid symbols) and G″ (open symbols).

Figure 8. Schematic illustration of stability mechanism of Pickering emulsions using silica particles with Ugi-Alg adsorbed.

CONCLUSIONS In this study, three–different–molecular–weights of modified-Alg were synthesized using the Ugi reaction. Ugi-Alg chains were adsorbed on the surface of silica nanoparticles through hydrogen bonding in the Ugi-Alg/SiO2 suspensions. The suspensions’ stability was promoted by electrostatic repulsion. We demonstrated that 18


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stable O/W Pickering emulsions were formed by hydrophilic silica nanoparticles adsorbed Ugi-Alg. Pickering emulsions synergistically stabilized by silica nanoparticles and Ugi-Alg produced small oil droplets and led to good stability. Increasing the silica concentration improved the stability of Pickering emulsions by enhancing the rigidity of three-dimensional network structure in the emulsions. Meanwhile, with the increasing molecular weight of Ugi-Alg, both the stability and viscoelastic of the Pickering emulsions were increased, and both the value of TSI and droplet size were decreased. The results indicated that emulsions stabilized by H-Ugi-Alg/SiO2 had higher stability, which was attributed to the stronger interaction between H-Ugi-Alg and SiO2 and bridging effect of Ugi-Alg. Highly stable O/W emulsions were achieved with the addition of 2 wt% SiO2 and 1 g/L Ugi-Alg. To better understand the underlying mechanism of Ugi-Alg/SiO2 synergistic-stabilized Pickering emulsions, we propose a schematic illustration to explain the formation pathway of Pickering emulsion, as shown in Figure 8. Thus, the combination of hydrophobic modified sodium alginate and hydrophilic silica can be a promising dispersant and emulsifier. ASSOCIATED CONTENT

Supporting Information The Supporting information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. Fluorescence measurement; FT-IR spectroscopy measurement; the time-varying photograph of the Pickering emulsions contained silica nanoparticles and Ugi-Alg; the Transmission and backscattering for Pickering emulsions (PDF).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] (J. L); [email protected] (Y. F). Notes 19



The authors declare no competing financial interest. ACKNOWLEDGMENTS We gratefully acknowledge the financially supported from the National Natural Science Foundation of China (21566009, 21706045 and 21366010), the Natural Science Foundation of Hainan Province (217021, 20162013 and 20162016), and the Key Laboratory of Water Environment Pollution Treatment & Resource of Hainan Province.

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Synopsis: The mixture of environment-friendly macromolecule, Ugi-Alg, and SiO2 nanoparticles may be a promising green and sustainable emulsifier candidate. Abstract graphic:

Schematic illustration of stability mechanism of Pickering emulsions using silica particles with Ugi-Alg adsorbed.

26


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Eco-Friendly Pickering Emulsion Stabilized by Silica Nanoparticles

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