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Preparation of octyl-grafted alginate-amide gel particle and its application in Pickering emulsion
Accepted Manuscript Title: Preparation of octyl-grafted alginate-amide gel particle and its application in Pickering emulsion Authors: Jisheng Yang, Lei Liu, Suya Han PII: DOI: Reference:
S0927-7757(17)30580-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.018 COLSUA 21705
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21-4-2017 8-6-2017 8-6-2017
Please cite this article as: Jisheng Yang, Lei Liu, Suya Han, Preparation of octyl-grafted alginate-amide gel particle and its application in Pickering emulsion, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.018 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.
Preparation of octyl-grafted alginate-amide gel particle and its application in Pickering emulsion
Jisheng Yang*, Lei Liu, Suya Han
College of Chemistry and Chemical Engineering, Yangzhou University (Yangzhou, Jiangsu Province, 225002, China)
Corresponding author: Tel.: +86 51487975568. E-mail: [email protected]
Graphical Abstract
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Highlights
The gel particles (Ca-Alg or Ca-Alg-C8) were prepared by inverse-emulsion method.
Ca-Alg-C8 with contact angle of 88.9° can be adsorbed onto the oil-water interface.
Amphiphilic Ca-Alg-C8 can stabilize the liquid paraffin emulsions well.
Stability of the emulsion was influenced by particle concentration, ΦO and NaCl.
The rheological behavior of the emulsion system is predominantly elastic.
Abstract. Sodium alginate (Na-Alg) was hydrophobic modified to provide octyl-grafted alginate-amide derivative(Alg-C8). The prepared Alg-C8 was used to develop Ca2+ induced gel particles (CaAlg-C8), as an interfacial stabilizer for Pickering emulsions. The size distribution and morphology of Ca-Alg-C8 were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Stability and rheological property of the Pickering emulsions, liquid paraffin as oil phase, stabilized by Ca-Alg-C8, were influenced by the gel particle concentration, oil fraction and ionic strength in water phase. Optical microscopy under fluorescence evidenced that the Ca-Alg-C8 particles could be adsorbed onto interface of the emulsion droplets.
Keywords: alginate; gel particle; Pickering emulsion; stability; rheology
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1. Introduction Sodium alginate (Na-Alg), a kind of natural polysaccharide, is an anionic polyelectrolyte extracted from seaweeds. Na-Alg is a linear polysaccharide consisting of (1,4) linked β-Dmannuronate (M) and α-L-guluronate (G) residues arranged in a non-regular block pattern. The stability, solubleness, viscidity and safety of Na-Alg make it an ideal candidate for pharmaceutical excipients [1]. In 1881, E. C. Stanford, a chemist from England, found the phenomenon of gelation and film formation when Na-Alg aqueous solution was concentrated. Furthermore, multivalent cations (Ca2+, Zn2+, A13+, Ti4+ etc) can crosslink with Na-Alg easily, forming alginate hydrogels with good stability [2,3]. Ca2+ crosslinking is the most common method to prepare alginate-based hydrogels because it is sample, nontoxic and controllable. Researchers prepared microgel particles by Ca2+ induced gelification of alginate [4,5]. In aqueous solution, due to obtained gel particles with low concentration were uncontrollable size, while low concentration of the Na-Alg was required to prepare microgel particles. Therefore, emulsification method was usually used to fabricate monodisperse and micro/nano gel particles. For example, Na-Alg aqueous solution, distributed in oil phase, was turned into W/O emulsion and then crosslink with Ca2+by outer- or inner-gelation. Paques et al. prepared Ca-Alg beads as small as 200 nm by outer gelation with Ca2+ [6]. In the system of prepared W/O emulsions, containing 1% Na-Alg solution as water phase and a medium chain triglyceride as oil phase, CaCl2 nanoparticles dispersed in oil migrate to the emulsion droplet interface, where they 3
dissolve into the alginate aqueous phase and cause gelation. Liu et al. fabricated monodisperse hollow and core-shell Ca-Alg microcapsules via inner gelation in microfluidic-generated double emulsion with the capillary microfluidic device [7]. The internal gelation of the aqueous middle layer of O/W/O double emulsions is induced by crosslinking alginate polymers with Ca2+ that are released from CaCO3 nanoparticles, dispersed in water phase, upon UV exposure of the photoacid generator. Pickering emulsion, a kind of emulsion stabilized by solid particles, was discovered by Pickering as early as 1907 [8]. Later on, Pieranski described the adsorption mechanism of particle emulsifiers on the interface, and inferred it was caused by the decrease of free energy in the system [9]. Binks further researched the reason that particles were steadily adsorbed onto the oil and water interface. Due to the high attachment energy of particles to interfaces, relative to thermal energy, particles once at interfaces can be thought of as effectively irreversibly adsorbed [10]. Pickering emulsion could be widely applied in many fields [11,12]. Particle emulsifiers were researched to stabilize the emulsions for more than one century and the system of which has developed from inorganic particles to surface modified particles [13,14], natural polymer based colloid particles [15, 16]. In recent years, Na-Alg based gel particles, serving as particle emulsifiers, are favoured by scientific researchers increasingly, due to large amounts of hydrophilic carboxyl and hydroxyl groups on the backbone of chains that make it easy for chemical and biological modification [17,18]. In our previous experiment, amphiphilic alginate 4
derivatives of different substitution degree and with different carbon chain length group were prepared by the esterification [19] or amidation [20]. In this paper, octyl-grafted alginate-amide derivative (Alg-C8) was used to develop Ca2+ induced gel particles (Ca-Alg-C8) by inverse-emulsion method with the aid of confined impingement jets (CIJ) mixer. The amphiphilic gel particles were applied to Pickering emulsions. Several factors which may affect the droplet size, stability and rheological properties of the prepared emulsions were analyzed in detail. This research suggested an efficient method to prepare amphipathic gel particle and its application in Pickering emulsions, which can be widely used in food, pharmaceutical and cosmetic industries.
2. Experimental 2.1. Materials ____
Sodium alginate (Na-Alg, Mη~ 430kDa, M/G=0.18), trichloromethane (purity≥99%), calcium chloride (99%), anhydrous ethanol (99%), liquid paraffin (purity≥98.5%) were purchased from Sinopharm Chemical Reagent Co. Octyl-grafted alginate-amide derivative (Alg-C8, Noctyl/Nhexuronic= 0.3) was synthesized according to our previous report [20]. Unless specially stated, all other reagents were guaranteed analytical reagents and used as received without further purification. The deionized water was used throughout the experiments.
2.2. Preparation of micro/nano gel particles
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Micro/nano gel particles were prepared after the crosslinking of Na-Alg or Alg-C8 with Ca2+ by the inverse emulsion method, in which Na-Alg or Alg-C8 aqueous solution and trichloromethane were used as water phase and oil phase respectively. The scheme of process and principle was shown in Fig. 1. To begin with, 30 mL trichloromethane and 10 mL 2g/L NaAlg or Alg-C8 aqueous solution were mixed by high-shearing-type mixer (FA25, Fluko, China) to obtain the water-in-oil raw emulsions. The emulsions, with smaller particle size and uniform distribution, were prepared via the self-produced CIJ mixer. Two syringe pumps were used to drive the raw emulsions as two opposing liquid streams at high velocity into the mixing chamber, forming micro/nano emulsions. Then these micro/nano emulsion droplets were collected into 45 mL CaCl2 aqueous solution of 0.5mol/L, with continuous magnetic stirring for 5 hours. Afterwards, trichloromethane in the mixture was evaporated via rotatory evaporator. Finally, the water phase, containing gel particles, was isolated and purified by means of dialysis with deionized water exchange until the conductivity figures of the liquid remained unchanged.
2.3.Characterization of micro/nano gel particles The apparent average size, size distribution, and polydispersity index (PDI) of the Ca-Alg or Ca-Alg-C8 were examined by dynamic laser scattering (DLS) using a laser granulometer (Malvern Nano 90, Malvern, U.K.). Measurements were taken at 25°C and at a 90° scattering angle. The prepared gel particles were diluted to an appropriate concentration with deionized water and they were pretreated with sonication (PS-20, JLT, China) at 20 kHz for 120s to 6
prevent the gel particles from agglomerating. The surface morphology of the gel was recorded with scanning electron microscope (S-4800, Hitachi, Japan) at an acceleration voltage of 15.0 kV. To acquire superb SEM images, all the gel particles samples were placed on an aluminum tray and coated with gold to make the films conductive. The morphology of gel particles was also obtained using transmission electron microscope (Tecnai 12, PHILIPS, Holland) with an acceleration voltage of 80 kV. Samples were placed onto copper grid and negative staining with an aqueous solution of sodium phosphotungstate. Then the grid was allowed to dry at room temperature and examined under TEM.
2.4.Hydrophility/hydrophobility of the gel particles Hydrophobic/hydrophilic character of the micro/nano gel particles were measured by the optical contact angle measuring device (OCA20, Dataphysics, Germany). The samples were put into a flat base glass container with groove and dried into the membrane at room temperature. The prepared membrane was set on the platform of the device, for the measurement of contact angle, with the a drop of water at 25 ℃.
2.5.Preparation and characterization of Pickering emulsions The water suspensions of the Ca-Alg or Ca-Alg-C8 were respectively mixed with liquid paraffin with a certain volume proportion. The emulsions were obtained via a process of
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homogenization by high-shearing-type mixer (FA25, Fluko, China), with the speed of 10000 rpm for 3 min at room temperature. The types of the emulsions were judged by the measurement of conductivity. The droplet size distribution in the emulsion was measured via laser granularity analyzer (BT-9300H, Bettersize Co., China) and the data were analyzed using an optical model for a fluid with real parts of complex index set to 1.333 and 1.449 for the continuous and dispersed phases, respectively. A 3D laser scanning microscope (LSM 700, Carl Zeiss Co., Germany) was also used to observe the droplet size and structure of the emulsion. Fluorescence microscopy images were acquired as follows. Approximately 10mL of emulsion was placed in a test tube, and moderate rhodamine 6G aqueous solution (1mg/mL) was added and mixed for 30min. The mixture was then dropped on a microscope slide and covered with a coverslip before observed. The rhodamine 6G fluorescent dye was excited at 354 nm. The rheological property of the emulsion was measured and analyzed with rotational rheometer (RS600, Thermo Electron Co, USA) using a parallel plate of 60 mm and setting the temperature at 25 ℃. Then the emulsions experienced shear flow. The dynamic strain sweep measurements were performed to determine a common linear viscoelastic region. Then the steady shear mode was set to analyzed shear viscosity and the oscillation shear mode to evaluate the viscoelasticity. 8
3. Results and discussions 3.1.Size and morphologyof gel particles The size distribution of gel particles was measured by DLS. The DLS measured size of ~398nm (PDI=0.396) for Ca-Alg-C8 particles is slightly smaller than the Ca-Alg value of ~456nm (PDI=0.372), which is mainly owning to the fact that the grafted hydrophobic carbon chains limited the swelling ability of the hydrogels. According to the SEM image of Ca-Alg gel particles (Fig. 2a), the obtained Ca-Alg gel particles appear sphere and the particle mean size is about 100 nm, which is smaller than that measured by DLS. That was due to the gel dehydration during the process of desiccation to prepare the sample for SEM measurement, while the gel particles in wet condition were coated with the hydrated shell. Further, according to the TEM images (Fig.2b), the gel particles exhibit a uniform shape of sphere and the diameter is approximately 100 nm, which correspond to the result from SEM. From the SEM image of Ca-Alg-C8 gel particles (Fig. 2c), the obtained CaAlg-C8 gel particles show a collapsed spherical shape with the diameter of ~70 nm, which is smaller than that in wet gel state, resulting from volume shrinkage of the gel particles during the preparation of the samples. An inference on this result is that amphipathic Alg-C8 tends to be adsorbed on the interface of oil and water in the emulsification system, which leads to a higher concentration of Alg-C8 at the interface region than that at the core of the emulsion droplets (water phase). During the gelation of Alg-C8 and Ca2+, gel particles with shell of high 9
crosslinking density and core of low crosslinking density are produced. As a result, the gel particles show collapsed structures after dehydration. Furthermore, a same phenomenon has occurred in TEM (Fig. 2d).
3.2.Wettability of Na-Alg, Ca-Alg and Ca-Alg-C8 Wettability is one of the most important properties of solid surfaces from both fundamental and practical aspects. In principle, the contact angle (θ) of a liquid drop on a solid surface is determined by the mechanical equilibrium under the action of three interfacial tensions [21]. Fig.3 is the water contact angle of membranes prepared by Na-Alg, Ca-Alg and Ca-Alg-C8, measured in air, respectively. The water contact angle of Na-Alg membrane is about 35° (Fig. 3a), while the water contact angle of Ca-alg membrane decreases to 15.5° (Fig. 3b), which reveals that the membranes of Na-Alg and Ca-Alg gel are super-hydrophilic. Because the surface of membrane, formed by the distribution of Ca-Alg particles, is rougher, the contact angle of Ca-Alg membrane is smaller than that of Na-Alg molecular membrane. The contact angle of Ca-Alg-C8 is 88.9° (Fig. 3c). Because the hydrophobic carbon chain grafted on the backbone of Na-Alg obviously improved the hydrophobicity of Ca-Alg-C8 and led to the increase of the water contact angle. When particles are used as emulsifier, the contact angle (θow) on the oil-water interface is a highly important parameter that determines the types of the emulsions (O/W or W/O). According to the study of Binks and Lumsdon, particles with contact angle of 0-20° could 10
hardly stabilize the emulsions [22]. Theoretically, when θow=90°, adsorption ability of the particles on interface reaches the maximum and the needed energy that the particles displace from the interface to the water phase is equal to that to the oil phase. In this situation, however, the critical capillary force of interfacial film is inexistent, resulting the emulsion couldn’t be stabilized by the particles. When θow is less than 90°, the particles are hydrophilic and tend to stabilize O/W emulsions. In the contrast, the particles tend to stabilize W/O emulsions. When θow decreases to a certain value, particles are dispersed in the water phase easily and when θow is much more than 90°, the result will be opposite [23]. In conclusion, to stabilize emulsions, θow should neither be closed to 0° nor 180°. It is favorable for hydrophilic particles to locate on the interface of oil and water by increasing hydrophobility of them. Because the water contact angle of the Ca-Alg-C8 particles prepared is slightly less than 90°, the gel particles are inclined to stabilize O/W emulsions.
3.3.Stability of the emulsion The effects of gel particle hydrophility/hydrophobility, gel particle concentration, phase volume ratio and the concentration of added salt on the stability of the emulsions were researched. Fig.4 shows the photographs and optical micrographs of the emulsion fabricated with water suspension of Ca-Alg (1.5wt%) and liquid paraffin, in which volume fraction of oil phase (ΦO) was 0.5. The emulsion freshly prepared (Fig. 4a) appeared oil-water separation over 30 min (Fig. 11
4c). Obviously, Ca-Alg gel particles were not able to stabilize Pickering emulsions. As shown in the Fig. 4b, which is the microscope photo of freshly prepared sample, the droplet size of the emulsion is not uniform. When it is placed for 30 min, in Fig. 4d, the size of the emulsion droplets increases obviously and the shape is irregular. The emulsion prepared based on CaAlg is not stable due to the super-hydrophility of Ca-Alg, corresponding to the result obtained in contact angle measurement (Fig. 3b). Therefore, we confirm that Ca-Alg gel particles could hardly be used as Pickering emulsifier. It can be seen from fig. 5a,b that Ca-Alg-C8 gel particles at different concentrations show different state of emulsions, in which the volume fraction of oil phase is 0.5. With the increasing concentration of Ca-Alg-C8, the volume of water phase near the bottom decreases to disappear completely. When the Ca-Alg-C8 concentration increased to 0.2%, the formed emulsions could keep stable and the emulsions existed at upper layer while excess water dropped in the bottom. When the concentration was up to 0.8%, the uniform emulsions formed without any precipitated water phase. After 30 days, the emulsion of low particle concentration (0.1wt%) was breaked with some oil phase separated, while previously uniform emulsions (particle concentration above 0.2wt%) are still stabilized without demulsification. Moreover, the droplet size of emulsion prepared by 0.8 wt% Ca-Alg-C8 (Fig. 5d) become smaller and show a narrower size distribution compared with that of 0.2wt% Ca-Alg-C8 (Fig. 5c) stabilized emulsion. Table 1 shows the mean droplet size of emulsions stabilized by Ca-Alg-C8 at different concentrations 12
for different period times, which corresponds to conclusion via microscope. The emulsion become stable and the size of the droplets decrease with the increase of Ca-Alg-C8 concentration. When the particle concentration increased from 0.8wt% to 1.0wt%, the droplets size did not vary obviously. All results mentioned above show that the increase of the particle concentration leads to the improvement of the stability of the emulsions, which will hinder the aggregation of the droplets. With the Ca-Alg-C8 concentration increase, much more of them could be absorbed onto the interface to form a dense layer of interfacial film. As a result, oil phase with smaller droplet size was coated into the water phase and the emulsions volume increased gradually and became more stable. The concentration of the particles has a great influence on the stability and the droplet size distribution of the emulsions. However, it is not a linear relationship between particle concentration and stability of the emulsions. When the concentration reaches to a certain value, droplet size and stability of the emulsions do not vary obviously with the increase of the particle concentration [24]. Volume fraction of oil phase would also affect the types and stability of the emulsions. Fig.6 is photographs of emulsions prepared by 0.8 wt % Ca-Alg-C8 with different ΦO. When ΦO was less than 0.5, turbid water phase was separated out, which revealed that particles adsorption onto the oil-water interface had reached saturation point and the remanent particles was reserved in the water phase. When ΦO increased to 0.7, the absorbed particles were not enough to stabilize the emulsions and oil phase was separated and floated in the upper layer. Accord to 13
the conductivity measurement of the emulsions, the conductivity value varies from more than 200 μScm-1 to 0.1μScm-1, demonstrating that the phase inversion occurs [25]. Binks et al. gave similar results [26]. They prepared the W/O emulsions stabilized by nano-sized hydrophobic silica particles. They found that the W/O emulsions catastrophically invert, without hysteresis, to O/W at volume fractions of water around 0.7. Research [27] has shown that the stability of the emulsions research to the highest when it is around the phase inverse point, which is affected by particle concentration, wettability, phase volume fraction. Different concentration of NaCl was added to the Ca-Alg-C8 suspensions respectively and the size of the prepared particles was analyzed. The result was showed in Fig.7a. The particle size of Ca-Alg-C8 increased along with NaCl concentration increased, which was due to the increased flocculation degree. The suspensions above-mentioned were mixed with liquid paraffin (ΦO=0.5) and the influence of NaCl concentration on the droplet size of the emulsions was shown in Fig. 7b. The droplet size of the emulsions decreased first and then increased with the increasing concentration of NaCl. When the concentration of NaCl was less than 0.08 mol/L, droplets size decreased gradually and reached the minimum at the concentration of 0.08 mol/L. Moderate addition of electrolyte made a slight flocculation of the particles, as to improve the stability of the emulsions and make the emulsion droplets size uniform. When the concentration was more than 0.08 mol/L, the degree of flocculation is increased, and the excess electrolyte made Ca-Alg-C8 gel aggregate greatly. The particle size increased and the Ca-Alg14
C8 could hardly be absorbed onto the oil-water interface to form compact particle film. As a result, the emulsions could not keep a stable state. The addition of electrolyte would lead to the decrease of the repulsion between gel particles, as a result of flocculation. According to a research by Binks et al. [28], the stability of the emulsions could be improved greatly when the particles was flocculated slightly. Therefore, appropriate degree of particles aggregation can improve the stability of Pickering emulsions due to the changed adsorption energy of the particles while high degree of particles aggregation will lead to a rapid decrease of the stability and even demulsification.
3.4.Rheologyof the emulsions The variation of the emulsions stabilized by differently concentrated Ca-Alg-C8 shear viscosity and the viscoelasticity was shown in Fig.8. The apparent viscosities of the emulsions vary with the shear rate, as shown in Fig. 8a. The emulsions show an increasing tendency of viscosities with the increase in Ca-Alg-C8 concentration, indicating that the network structure of the emulsions become much stronger. However apparent viscosities of all the emulsions stabilized by Ca-Alg-C8 decrease as the shear rate increase, indicating that the emulsions exhibit a shear-thinning behavior and a non-Newtonian fluid [29]. As shown in Fig. 8b, when Ca-AlgC8 concentration increases, emulsions exhibit increasing values of G’ and G’’ concurrently. In
other words, the emulsions stabilized by high-concentrated Ca-Alg-C8 are more resistant to flocculation than that of low-concentrated Ca-Alg-C8 emulsions. Furthermore, in all the 15
emulsions, G’ is about one order of magnitude higher than G’’, which testifies that the rheological behavior of the emulsions system is predominantly elastic [30], result from the three-dimensional net structure of Ca-Alg-C8 in the emulsions. That could be interpreted as being due to the existence of well-developed elastic networks formed by the dispersed gel particles and emulsion droplets in the emulsions [31,32]. Several researchers have used viscoelastic properties to monitor emulsion stability [33,34].
3.5.Adsorption of gel particles on the oil-water interface The mechanism of stabilizing Pickering emulsions is that amphiphilic particles adsorbed onto the oil-water interface formed a layer of compact film to prevent the droplets from collision and coagulation. Fig.9 is the fluorescence micrograph of the emulsions prepared by 0.8 wt % CaAlg-C8 water suspension with oil fractions 0.33. According the micrograph, the droplets were coated with a layer of faint yellow membrane. Because of opposite charges attracted, the fluorescent dye of rhodamine 6G with positive charges bound to the Ca-Alg-C8 with the negative charges, so that the region of gel particles appears yellow under UV light. The bright layer outside oil droplet indicates that gel particles were adsorbed onto interface of the emulsion droplets and stabilized the emulsions well.
4. Conclusion The Ca-Alg gel particles we fabricated could disperse well in water phase and presented spherical shape, and the size distribution was homogeneous. However, the super hydrophilicity 16
of Ca-Alg gel made it hard to be adsorbed onto the oil-water interface to stabilize the emulsions. Hydrophobic-modified Ca-Alg-C8 showed amphipathy and could stabilize the emulsions well. The Ca-Alg-C8 showed spherical shape in wet state and the gel particles were collapsed after dehydration. Furthermore, the contact angle of membrane prepared by Ca-Alg-C8 was 88.9°. When concentration of Ca-Alg-C8 increased, the size of the droplets decreased and the stability of the emulsions improved. The structure and stability of emulsions were also influenced by phase fraction and the concentration of the addition electrolyte. When ΦO was less than 0.5, containing 0.8 wt% Ca-Alg-C8 aqueous suspension, water phase was separated out in the bottom and the adsorption of Ca-Alg-C8 gel particles on the oil-water interface reached to saturation. When ΦO was more than 0.7, phase inversion of the emulsions occurred and the O/W emulsions turned into the W/O ones. The droplet size of the emulsions decreased with the increasing concentration of NaCl, and when NaCl concentration was 0.08 mol/L, the drop size of the emulsions reached the minimum. When NaCl concentration was more than 0.08 mol/L, the degree of flocculation has dramatically increased, and the excess electrolyte made Ca-AlgC8 gels aggregate greatly, which influenced the formation and the stability of the emulsions. According to the rheology analysis, the prepared emulsions showed a shear-thinning behavior, and the rheological behavior of emulsions system is predominantly elastic. The fluorescence microscope image of the emulsions showed that Ca-Alg-C8 gel particles were adsorbed onto interface of the emulsion droplets, preventing the droplets from agglomerating, and stabilized 17
the emulsions well. Therefore, we foresee that the Ca-Alg-C8 gel particles for Pickering emulsions have a great chance to play an important role in the application of medicine, food and cosmetics.
Acknowledgements Authors gratefully acknowledge the financial support from Natural Science Foundation of Yangzhou (yz2014066), Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the devices support from testing center of Yangzhou University.
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[30] C. Xu, J. Chen, D. Wu, Y. Chen, Q. Lv, Polylactide/acetylated nanocrystalline cellulose composites prepared by a continuous route: A phase interface-property relation study, Carbohydr. Polym.146(2016) 58-66. [31] K. Manoi, S.S.H. Rizvi, Emulsification mechanisms and characterizations of cold, gel-like emulsions produced from texturized whey protein concentrate, Food Hydrocolloid.23 (2009)1837-1847. [32] X. Qiao, L. Wang, Z. Shao, K. Sun, R. Miller, Stability and rheological behaviors of different oil/water emulsions stabilized by natural silk fibroin, Colloid. Surface. A 475 (2015)84-93. [33] C. Bengoechea, A. Romero, J. M. Aguilar, F. Cordobés, A. Guerrero, Temperature and pH as factors influencing droplet size distribution and linear viscoelasticity of O/W emulsions stabilised by soy and gluten proteins. Food Hydrocolloid.24 (2010) 783-791. [34] E. Rezvani, G. Schleining, A.R. Taherian, Assessment of physical and mechanical properties of orange oil-in-water beverage emulsions using response surface methodology, LWT- Food Sci. Technol.48 (2014)82-88.
23
Fig.1. Procedure illustration for gel particles (Ca-Alg/Ca-Alg-C8) prepared by inverseemulsion method. Fig. 2. SEM (a,c) and TEM (b,d) images of Ca-Alg (a,b) and Ca-Alg-C8 (c,d), insert picture in c is magnified image of the gel particle. Fig. 3. The contact angle measured by dropping water droplet onto the Na-Alg (a), Ca-Alg (b) and Ca-Alg-C8 (c) membrane in air environment. Fig. 4. Photographs of the emulsion for freshly prepared (a) and over 30 min(c); optical micrographs of the emulsion for freshly prepared (b) and over 30 min(d). The Pickering emulsion was prepared by mixing 1.5 wt % Ca-Algwatersuspension and liquid paraffin(1:1, v:v), storage period at room temperature. Fig. 5. Photographs of emulsionsstabilizedbyCa-Alg-C8atdifferent concentrations, freshly prepared (a) and over 30 days storage period at room temperature (b); optical micrographs of the emulsionsprepared by 0.2 wt % (c) and 0.8 wt % (d) Ca-Alg-C8 water suspension with oil fractions 0.5. Fig. 6. Photographs of emulsionspreparedby0.8 wt % Ca-Alg-C8 with different volume fraction of oil phase (ΦO). Fig. 7. Effect of NaCl concentration on the particles size of Ca-Alg-C8 in aqueous solution (a) and droplet size of emulsions (b). The Pickering emulsion was prepared by mixing 0.2 wt % Ca-Alg-C8 water suspension with different concentration of NaCl and liquid paraffin (ΦO=0.5). 24
Fig. 8. Shear rate dependence of the viscosity (a) and oscillatory frequency sweep curves (b) for the emulsions(ΦO=0.5) stabilized by Ca-Alg-C8 at 25℃. Fig.9. Fluorescence micrographs of the emulsionsprepared by 0.8 wt % Ca-Alg-C8 water suspension with oil fractions 0.33.
25
26
b
27
28
29
30
31
32
33
34
Table 1. The mean droplet size of emulsions stabilized by Ca-Alg-C8 at different concentrations for different period times. Concentration of Ca-
Alg-C8
Mean size （μm） Freshly prepared
30 days
60 days
(wt%) 0.1
63.5
87.6
110.3
0.2
49.4
55.7
64.2
0.4
26.7
29.8
34.7
0.6
20.8
22.9
26.1
0.8
15.3
16.9
20.8
1.0
14.7
16.6
19.7
35
S0927-7757(17)30580-0 http://dx.doi.org/doi:10.1016/j.colsurfa.2017.06.018 COLSUA 21705
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
21-4-2017 8-6-2017 8-6-2017
Please cite this article as: Jisheng Yang, Lei Liu, Suya Han, Preparation of octyl-grafted alginate-amide gel particle and its application in Pickering emulsion, Colloids and Surfaces A: Physicochemical and Engineering Aspectshttp://dx.doi.org/10.1016/j.colsurfa.2017.06.018 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.
Preparation of octyl-grafted alginate-amide gel particle and its application in Pickering emulsion
Jisheng Yang*, Lei Liu, Suya Han
College of Chemistry and Chemical Engineering, Yangzhou University (Yangzhou, Jiangsu Province, 225002, China)
Corresponding author: Tel.: +86 51487975568. E-mail: [email protected]
Graphical Abstract
1
Highlights
The gel particles (Ca-Alg or Ca-Alg-C8) were prepared by inverse-emulsion method.
Ca-Alg-C8 with contact angle of 88.9° can be adsorbed onto the oil-water interface.
Amphiphilic Ca-Alg-C8 can stabilize the liquid paraffin emulsions well.
Stability of the emulsion was influenced by particle concentration, ΦO and NaCl.
The rheological behavior of the emulsion system is predominantly elastic.
Abstract. Sodium alginate (Na-Alg) was hydrophobic modified to provide octyl-grafted alginate-amide derivative(Alg-C8). The prepared Alg-C8 was used to develop Ca2+ induced gel particles (CaAlg-C8), as an interfacial stabilizer for Pickering emulsions. The size distribution and morphology of Ca-Alg-C8 were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Stability and rheological property of the Pickering emulsions, liquid paraffin as oil phase, stabilized by Ca-Alg-C8, were influenced by the gel particle concentration, oil fraction and ionic strength in water phase. Optical microscopy under fluorescence evidenced that the Ca-Alg-C8 particles could be adsorbed onto interface of the emulsion droplets.
Keywords: alginate; gel particle; Pickering emulsion; stability; rheology
2
1. Introduction Sodium alginate (Na-Alg), a kind of natural polysaccharide, is an anionic polyelectrolyte extracted from seaweeds. Na-Alg is a linear polysaccharide consisting of (1,4) linked β-Dmannuronate (M) and α-L-guluronate (G) residues arranged in a non-regular block pattern. The stability, solubleness, viscidity and safety of Na-Alg make it an ideal candidate for pharmaceutical excipients [1]. In 1881, E. C. Stanford, a chemist from England, found the phenomenon of gelation and film formation when Na-Alg aqueous solution was concentrated. Furthermore, multivalent cations (Ca2+, Zn2+, A13+, Ti4+ etc) can crosslink with Na-Alg easily, forming alginate hydrogels with good stability [2,3]. Ca2+ crosslinking is the most common method to prepare alginate-based hydrogels because it is sample, nontoxic and controllable. Researchers prepared microgel particles by Ca2+ induced gelification of alginate [4,5]. In aqueous solution, due to obtained gel particles with low concentration were uncontrollable size, while low concentration of the Na-Alg was required to prepare microgel particles. Therefore, emulsification method was usually used to fabricate monodisperse and micro/nano gel particles. For example, Na-Alg aqueous solution, distributed in oil phase, was turned into W/O emulsion and then crosslink with Ca2+by outer- or inner-gelation. Paques et al. prepared Ca-Alg beads as small as 200 nm by outer gelation with Ca2+ [6]. In the system of prepared W/O emulsions, containing 1% Na-Alg solution as water phase and a medium chain triglyceride as oil phase, CaCl2 nanoparticles dispersed in oil migrate to the emulsion droplet interface, where they 3
dissolve into the alginate aqueous phase and cause gelation. Liu et al. fabricated monodisperse hollow and core-shell Ca-Alg microcapsules via inner gelation in microfluidic-generated double emulsion with the capillary microfluidic device [7]. The internal gelation of the aqueous middle layer of O/W/O double emulsions is induced by crosslinking alginate polymers with Ca2+ that are released from CaCO3 nanoparticles, dispersed in water phase, upon UV exposure of the photoacid generator. Pickering emulsion, a kind of emulsion stabilized by solid particles, was discovered by Pickering as early as 1907 [8]. Later on, Pieranski described the adsorption mechanism of particle emulsifiers on the interface, and inferred it was caused by the decrease of free energy in the system [9]. Binks further researched the reason that particles were steadily adsorbed onto the oil and water interface. Due to the high attachment energy of particles to interfaces, relative to thermal energy, particles once at interfaces can be thought of as effectively irreversibly adsorbed [10]. Pickering emulsion could be widely applied in many fields [11,12]. Particle emulsifiers were researched to stabilize the emulsions for more than one century and the system of which has developed from inorganic particles to surface modified particles [13,14], natural polymer based colloid particles [15, 16]. In recent years, Na-Alg based gel particles, serving as particle emulsifiers, are favoured by scientific researchers increasingly, due to large amounts of hydrophilic carboxyl and hydroxyl groups on the backbone of chains that make it easy for chemical and biological modification [17,18]. In our previous experiment, amphiphilic alginate 4
derivatives of different substitution degree and with different carbon chain length group were prepared by the esterification [19] or amidation [20]. In this paper, octyl-grafted alginate-amide derivative (Alg-C8) was used to develop Ca2+ induced gel particles (Ca-Alg-C8) by inverse-emulsion method with the aid of confined impingement jets (CIJ) mixer. The amphiphilic gel particles were applied to Pickering emulsions. Several factors which may affect the droplet size, stability and rheological properties of the prepared emulsions were analyzed in detail. This research suggested an efficient method to prepare amphipathic gel particle and its application in Pickering emulsions, which can be widely used in food, pharmaceutical and cosmetic industries.
2. Experimental 2.1. Materials ____
Sodium alginate (Na-Alg, Mη~ 430kDa, M/G=0.18), trichloromethane (purity≥99%), calcium chloride (99%), anhydrous ethanol (99%), liquid paraffin (purity≥98.5%) were purchased from Sinopharm Chemical Reagent Co. Octyl-grafted alginate-amide derivative (Alg-C8, Noctyl/Nhexuronic= 0.3) was synthesized according to our previous report [20]. Unless specially stated, all other reagents were guaranteed analytical reagents and used as received without further purification. The deionized water was used throughout the experiments.
2.2. Preparation of micro/nano gel particles
5
Micro/nano gel particles were prepared after the crosslinking of Na-Alg or Alg-C8 with Ca2+ by the inverse emulsion method, in which Na-Alg or Alg-C8 aqueous solution and trichloromethane were used as water phase and oil phase respectively. The scheme of process and principle was shown in Fig. 1. To begin with, 30 mL trichloromethane and 10 mL 2g/L NaAlg or Alg-C8 aqueous solution were mixed by high-shearing-type mixer (FA25, Fluko, China) to obtain the water-in-oil raw emulsions. The emulsions, with smaller particle size and uniform distribution, were prepared via the self-produced CIJ mixer. Two syringe pumps were used to drive the raw emulsions as two opposing liquid streams at high velocity into the mixing chamber, forming micro/nano emulsions. Then these micro/nano emulsion droplets were collected into 45 mL CaCl2 aqueous solution of 0.5mol/L, with continuous magnetic stirring for 5 hours. Afterwards, trichloromethane in the mixture was evaporated via rotatory evaporator. Finally, the water phase, containing gel particles, was isolated and purified by means of dialysis with deionized water exchange until the conductivity figures of the liquid remained unchanged.
2.3.Characterization of micro/nano gel particles The apparent average size, size distribution, and polydispersity index (PDI) of the Ca-Alg or Ca-Alg-C8 were examined by dynamic laser scattering (DLS) using a laser granulometer (Malvern Nano 90, Malvern, U.K.). Measurements were taken at 25°C and at a 90° scattering angle. The prepared gel particles were diluted to an appropriate concentration with deionized water and they were pretreated with sonication (PS-20, JLT, China) at 20 kHz for 120s to 6
prevent the gel particles from agglomerating. The surface morphology of the gel was recorded with scanning electron microscope (S-4800, Hitachi, Japan) at an acceleration voltage of 15.0 kV. To acquire superb SEM images, all the gel particles samples were placed on an aluminum tray and coated with gold to make the films conductive. The morphology of gel particles was also obtained using transmission electron microscope (Tecnai 12, PHILIPS, Holland) with an acceleration voltage of 80 kV. Samples were placed onto copper grid and negative staining with an aqueous solution of sodium phosphotungstate. Then the grid was allowed to dry at room temperature and examined under TEM.
2.4.Hydrophility/hydrophobility of the gel particles Hydrophobic/hydrophilic character of the micro/nano gel particles were measured by the optical contact angle measuring device (OCA20, Dataphysics, Germany). The samples were put into a flat base glass container with groove and dried into the membrane at room temperature. The prepared membrane was set on the platform of the device, for the measurement of contact angle, with the a drop of water at 25 ℃.
2.5.Preparation and characterization of Pickering emulsions The water suspensions of the Ca-Alg or Ca-Alg-C8 were respectively mixed with liquid paraffin with a certain volume proportion. The emulsions were obtained via a process of
7
homogenization by high-shearing-type mixer (FA25, Fluko, China), with the speed of 10000 rpm for 3 min at room temperature. The types of the emulsions were judged by the measurement of conductivity. The droplet size distribution in the emulsion was measured via laser granularity analyzer (BT-9300H, Bettersize Co., China) and the data were analyzed using an optical model for a fluid with real parts of complex index set to 1.333 and 1.449 for the continuous and dispersed phases, respectively. A 3D laser scanning microscope (LSM 700, Carl Zeiss Co., Germany) was also used to observe the droplet size and structure of the emulsion. Fluorescence microscopy images were acquired as follows. Approximately 10mL of emulsion was placed in a test tube, and moderate rhodamine 6G aqueous solution (1mg/mL) was added and mixed for 30min. The mixture was then dropped on a microscope slide and covered with a coverslip before observed. The rhodamine 6G fluorescent dye was excited at 354 nm. The rheological property of the emulsion was measured and analyzed with rotational rheometer (RS600, Thermo Electron Co, USA) using a parallel plate of 60 mm and setting the temperature at 25 ℃. Then the emulsions experienced shear flow. The dynamic strain sweep measurements were performed to determine a common linear viscoelastic region. Then the steady shear mode was set to analyzed shear viscosity and the oscillation shear mode to evaluate the viscoelasticity. 8
3. Results and discussions 3.1.Size and morphologyof gel particles The size distribution of gel particles was measured by DLS. The DLS measured size of ~398nm (PDI=0.396) for Ca-Alg-C8 particles is slightly smaller than the Ca-Alg value of ~456nm (PDI=0.372), which is mainly owning to the fact that the grafted hydrophobic carbon chains limited the swelling ability of the hydrogels. According to the SEM image of Ca-Alg gel particles (Fig. 2a), the obtained Ca-Alg gel particles appear sphere and the particle mean size is about 100 nm, which is smaller than that measured by DLS. That was due to the gel dehydration during the process of desiccation to prepare the sample for SEM measurement, while the gel particles in wet condition were coated with the hydrated shell. Further, according to the TEM images (Fig.2b), the gel particles exhibit a uniform shape of sphere and the diameter is approximately 100 nm, which correspond to the result from SEM. From the SEM image of Ca-Alg-C8 gel particles (Fig. 2c), the obtained CaAlg-C8 gel particles show a collapsed spherical shape with the diameter of ~70 nm, which is smaller than that in wet gel state, resulting from volume shrinkage of the gel particles during the preparation of the samples. An inference on this result is that amphipathic Alg-C8 tends to be adsorbed on the interface of oil and water in the emulsification system, which leads to a higher concentration of Alg-C8 at the interface region than that at the core of the emulsion droplets (water phase). During the gelation of Alg-C8 and Ca2+, gel particles with shell of high 9
crosslinking density and core of low crosslinking density are produced. As a result, the gel particles show collapsed structures after dehydration. Furthermore, a same phenomenon has occurred in TEM (Fig. 2d).
3.2.Wettability of Na-Alg, Ca-Alg and Ca-Alg-C8 Wettability is one of the most important properties of solid surfaces from both fundamental and practical aspects. In principle, the contact angle (θ) of a liquid drop on a solid surface is determined by the mechanical equilibrium under the action of three interfacial tensions [21]. Fig.3 is the water contact angle of membranes prepared by Na-Alg, Ca-Alg and Ca-Alg-C8, measured in air, respectively. The water contact angle of Na-Alg membrane is about 35° (Fig. 3a), while the water contact angle of Ca-alg membrane decreases to 15.5° (Fig. 3b), which reveals that the membranes of Na-Alg and Ca-Alg gel are super-hydrophilic. Because the surface of membrane, formed by the distribution of Ca-Alg particles, is rougher, the contact angle of Ca-Alg membrane is smaller than that of Na-Alg molecular membrane. The contact angle of Ca-Alg-C8 is 88.9° (Fig. 3c). Because the hydrophobic carbon chain grafted on the backbone of Na-Alg obviously improved the hydrophobicity of Ca-Alg-C8 and led to the increase of the water contact angle. When particles are used as emulsifier, the contact angle (θow) on the oil-water interface is a highly important parameter that determines the types of the emulsions (O/W or W/O). According to the study of Binks and Lumsdon, particles with contact angle of 0-20° could 10
hardly stabilize the emulsions [22]. Theoretically, when θow=90°, adsorption ability of the particles on interface reaches the maximum and the needed energy that the particles displace from the interface to the water phase is equal to that to the oil phase. In this situation, however, the critical capillary force of interfacial film is inexistent, resulting the emulsion couldn’t be stabilized by the particles. When θow is less than 90°, the particles are hydrophilic and tend to stabilize O/W emulsions. In the contrast, the particles tend to stabilize W/O emulsions. When θow decreases to a certain value, particles are dispersed in the water phase easily and when θow is much more than 90°, the result will be opposite [23]. In conclusion, to stabilize emulsions, θow should neither be closed to 0° nor 180°. It is favorable for hydrophilic particles to locate on the interface of oil and water by increasing hydrophobility of them. Because the water contact angle of the Ca-Alg-C8 particles prepared is slightly less than 90°, the gel particles are inclined to stabilize O/W emulsions.
3.3.Stability of the emulsion The effects of gel particle hydrophility/hydrophobility, gel particle concentration, phase volume ratio and the concentration of added salt on the stability of the emulsions were researched. Fig.4 shows the photographs and optical micrographs of the emulsion fabricated with water suspension of Ca-Alg (1.5wt%) and liquid paraffin, in which volume fraction of oil phase (ΦO) was 0.5. The emulsion freshly prepared (Fig. 4a) appeared oil-water separation over 30 min (Fig. 11
4c). Obviously, Ca-Alg gel particles were not able to stabilize Pickering emulsions. As shown in the Fig. 4b, which is the microscope photo of freshly prepared sample, the droplet size of the emulsion is not uniform. When it is placed for 30 min, in Fig. 4d, the size of the emulsion droplets increases obviously and the shape is irregular. The emulsion prepared based on CaAlg is not stable due to the super-hydrophility of Ca-Alg, corresponding to the result obtained in contact angle measurement (Fig. 3b). Therefore, we confirm that Ca-Alg gel particles could hardly be used as Pickering emulsifier. It can be seen from fig. 5a,b that Ca-Alg-C8 gel particles at different concentrations show different state of emulsions, in which the volume fraction of oil phase is 0.5. With the increasing concentration of Ca-Alg-C8, the volume of water phase near the bottom decreases to disappear completely. When the Ca-Alg-C8 concentration increased to 0.2%, the formed emulsions could keep stable and the emulsions existed at upper layer while excess water dropped in the bottom. When the concentration was up to 0.8%, the uniform emulsions formed without any precipitated water phase. After 30 days, the emulsion of low particle concentration (0.1wt%) was breaked with some oil phase separated, while previously uniform emulsions (particle concentration above 0.2wt%) are still stabilized without demulsification. Moreover, the droplet size of emulsion prepared by 0.8 wt% Ca-Alg-C8 (Fig. 5d) become smaller and show a narrower size distribution compared with that of 0.2wt% Ca-Alg-C8 (Fig. 5c) stabilized emulsion. Table 1 shows the mean droplet size of emulsions stabilized by Ca-Alg-C8 at different concentrations 12
for different period times, which corresponds to conclusion via microscope. The emulsion become stable and the size of the droplets decrease with the increase of Ca-Alg-C8 concentration. When the particle concentration increased from 0.8wt% to 1.0wt%, the droplets size did not vary obviously. All results mentioned above show that the increase of the particle concentration leads to the improvement of the stability of the emulsions, which will hinder the aggregation of the droplets. With the Ca-Alg-C8 concentration increase, much more of them could be absorbed onto the interface to form a dense layer of interfacial film. As a result, oil phase with smaller droplet size was coated into the water phase and the emulsions volume increased gradually and became more stable. The concentration of the particles has a great influence on the stability and the droplet size distribution of the emulsions. However, it is not a linear relationship between particle concentration and stability of the emulsions. When the concentration reaches to a certain value, droplet size and stability of the emulsions do not vary obviously with the increase of the particle concentration [24]. Volume fraction of oil phase would also affect the types and stability of the emulsions. Fig.6 is photographs of emulsions prepared by 0.8 wt % Ca-Alg-C8 with different ΦO. When ΦO was less than 0.5, turbid water phase was separated out, which revealed that particles adsorption onto the oil-water interface had reached saturation point and the remanent particles was reserved in the water phase. When ΦO increased to 0.7, the absorbed particles were not enough to stabilize the emulsions and oil phase was separated and floated in the upper layer. Accord to 13
the conductivity measurement of the emulsions, the conductivity value varies from more than 200 μScm-1 to 0.1μScm-1, demonstrating that the phase inversion occurs [25]. Binks et al. gave similar results [26]. They prepared the W/O emulsions stabilized by nano-sized hydrophobic silica particles. They found that the W/O emulsions catastrophically invert, without hysteresis, to O/W at volume fractions of water around 0.7. Research [27] has shown that the stability of the emulsions research to the highest when it is around the phase inverse point, which is affected by particle concentration, wettability, phase volume fraction. Different concentration of NaCl was added to the Ca-Alg-C8 suspensions respectively and the size of the prepared particles was analyzed. The result was showed in Fig.7a. The particle size of Ca-Alg-C8 increased along with NaCl concentration increased, which was due to the increased flocculation degree. The suspensions above-mentioned were mixed with liquid paraffin (ΦO=0.5) and the influence of NaCl concentration on the droplet size of the emulsions was shown in Fig. 7b. The droplet size of the emulsions decreased first and then increased with the increasing concentration of NaCl. When the concentration of NaCl was less than 0.08 mol/L, droplets size decreased gradually and reached the minimum at the concentration of 0.08 mol/L. Moderate addition of electrolyte made a slight flocculation of the particles, as to improve the stability of the emulsions and make the emulsion droplets size uniform. When the concentration was more than 0.08 mol/L, the degree of flocculation is increased, and the excess electrolyte made Ca-Alg-C8 gel aggregate greatly. The particle size increased and the Ca-Alg14
C8 could hardly be absorbed onto the oil-water interface to form compact particle film. As a result, the emulsions could not keep a stable state. The addition of electrolyte would lead to the decrease of the repulsion between gel particles, as a result of flocculation. According to a research by Binks et al. [28], the stability of the emulsions could be improved greatly when the particles was flocculated slightly. Therefore, appropriate degree of particles aggregation can improve the stability of Pickering emulsions due to the changed adsorption energy of the particles while high degree of particles aggregation will lead to a rapid decrease of the stability and even demulsification.
3.4.Rheologyof the emulsions The variation of the emulsions stabilized by differently concentrated Ca-Alg-C8 shear viscosity and the viscoelasticity was shown in Fig.8. The apparent viscosities of the emulsions vary with the shear rate, as shown in Fig. 8a. The emulsions show an increasing tendency of viscosities with the increase in Ca-Alg-C8 concentration, indicating that the network structure of the emulsions become much stronger. However apparent viscosities of all the emulsions stabilized by Ca-Alg-C8 decrease as the shear rate increase, indicating that the emulsions exhibit a shear-thinning behavior and a non-Newtonian fluid [29]. As shown in Fig. 8b, when Ca-AlgC8 concentration increases, emulsions exhibit increasing values of G’ and G’’ concurrently. In
other words, the emulsions stabilized by high-concentrated Ca-Alg-C8 are more resistant to flocculation than that of low-concentrated Ca-Alg-C8 emulsions. Furthermore, in all the 15
emulsions, G’ is about one order of magnitude higher than G’’, which testifies that the rheological behavior of the emulsions system is predominantly elastic [30], result from the three-dimensional net structure of Ca-Alg-C8 in the emulsions. That could be interpreted as being due to the existence of well-developed elastic networks formed by the dispersed gel particles and emulsion droplets in the emulsions [31,32]. Several researchers have used viscoelastic properties to monitor emulsion stability [33,34].
3.5.Adsorption of gel particles on the oil-water interface The mechanism of stabilizing Pickering emulsions is that amphiphilic particles adsorbed onto the oil-water interface formed a layer of compact film to prevent the droplets from collision and coagulation. Fig.9 is the fluorescence micrograph of the emulsions prepared by 0.8 wt % CaAlg-C8 water suspension with oil fractions 0.33. According the micrograph, the droplets were coated with a layer of faint yellow membrane. Because of opposite charges attracted, the fluorescent dye of rhodamine 6G with positive charges bound to the Ca-Alg-C8 with the negative charges, so that the region of gel particles appears yellow under UV light. The bright layer outside oil droplet indicates that gel particles were adsorbed onto interface of the emulsion droplets and stabilized the emulsions well.
4. Conclusion The Ca-Alg gel particles we fabricated could disperse well in water phase and presented spherical shape, and the size distribution was homogeneous. However, the super hydrophilicity 16
of Ca-Alg gel made it hard to be adsorbed onto the oil-water interface to stabilize the emulsions. Hydrophobic-modified Ca-Alg-C8 showed amphipathy and could stabilize the emulsions well. The Ca-Alg-C8 showed spherical shape in wet state and the gel particles were collapsed after dehydration. Furthermore, the contact angle of membrane prepared by Ca-Alg-C8 was 88.9°. When concentration of Ca-Alg-C8 increased, the size of the droplets decreased and the stability of the emulsions improved. The structure and stability of emulsions were also influenced by phase fraction and the concentration of the addition electrolyte. When ΦO was less than 0.5, containing 0.8 wt% Ca-Alg-C8 aqueous suspension, water phase was separated out in the bottom and the adsorption of Ca-Alg-C8 gel particles on the oil-water interface reached to saturation. When ΦO was more than 0.7, phase inversion of the emulsions occurred and the O/W emulsions turned into the W/O ones. The droplet size of the emulsions decreased with the increasing concentration of NaCl, and when NaCl concentration was 0.08 mol/L, the drop size of the emulsions reached the minimum. When NaCl concentration was more than 0.08 mol/L, the degree of flocculation has dramatically increased, and the excess electrolyte made Ca-AlgC8 gels aggregate greatly, which influenced the formation and the stability of the emulsions. According to the rheology analysis, the prepared emulsions showed a shear-thinning behavior, and the rheological behavior of emulsions system is predominantly elastic. The fluorescence microscope image of the emulsions showed that Ca-Alg-C8 gel particles were adsorbed onto interface of the emulsion droplets, preventing the droplets from agglomerating, and stabilized 17
the emulsions well. Therefore, we foresee that the Ca-Alg-C8 gel particles for Pickering emulsions have a great chance to play an important role in the application of medicine, food and cosmetics.
Acknowledgements Authors gratefully acknowledge the financial support from Natural Science Foundation of Yangzhou (yz2014066), Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the devices support from testing center of Yangzhou University.
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Fig.1. Procedure illustration for gel particles (Ca-Alg/Ca-Alg-C8) prepared by inverseemulsion method. Fig. 2. SEM (a,c) and TEM (b,d) images of Ca-Alg (a,b) and Ca-Alg-C8 (c,d), insert picture in c is magnified image of the gel particle. Fig. 3. The contact angle measured by dropping water droplet onto the Na-Alg (a), Ca-Alg (b) and Ca-Alg-C8 (c) membrane in air environment. Fig. 4. Photographs of the emulsion for freshly prepared (a) and over 30 min(c); optical micrographs of the emulsion for freshly prepared (b) and over 30 min(d). The Pickering emulsion was prepared by mixing 1.5 wt % Ca-Algwatersuspension and liquid paraffin(1:1, v:v), storage period at room temperature. Fig. 5. Photographs of emulsionsstabilizedbyCa-Alg-C8atdifferent concentrations, freshly prepared (a) and over 30 days storage period at room temperature (b); optical micrographs of the emulsionsprepared by 0.2 wt % (c) and 0.8 wt % (d) Ca-Alg-C8 water suspension with oil fractions 0.5. Fig. 6. Photographs of emulsionspreparedby0.8 wt % Ca-Alg-C8 with different volume fraction of oil phase (ΦO). Fig. 7. Effect of NaCl concentration on the particles size of Ca-Alg-C8 in aqueous solution (a) and droplet size of emulsions (b). The Pickering emulsion was prepared by mixing 0.2 wt % Ca-Alg-C8 water suspension with different concentration of NaCl and liquid paraffin (ΦO=0.5). 24
Fig. 8. Shear rate dependence of the viscosity (a) and oscillatory frequency sweep curves (b) for the emulsions(ΦO=0.5) stabilized by Ca-Alg-C8 at 25℃. Fig.9. Fluorescence micrographs of the emulsionsprepared by 0.8 wt % Ca-Alg-C8 water suspension with oil fractions 0.33.
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26
b
27
28
29
30
31
32
33
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Table 1. The mean droplet size of emulsions stabilized by Ca-Alg-C8 at different concentrations for different period times. Concentration of Ca-
Alg-C8
Mean size （μm） Freshly prepared
30 days
60 days
(wt%) 0.1
63.5
87.6
110.3
0.2
49.4
55.7
64.2
0.4
26.7
29.8
34.7
0.6
20.8
22.9
26.1
0.8
15.3
16.9
20.8
1.0
14.7
16.6
19.7
35