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Antagonistic effect in pickering emulsion stabilized by mixtures of hydroxyapatite nanoparticles and sodium oleate
Accepted Manuscript Title: Antagonistic Effect in Pickering Emulsion Stabilized by Mixtures of Hydroxyapatite Nanoparticles and Sodium Oleate Author: Bing Hu Chunhua Zhao Xiaoying Jin Hao Wang Jian Xiong Junjun Tan PII: DOI: Reference:
S0927-7757(15)30153-9 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.08.009 COLSUA 20100
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
5-6-2015 3-8-2015 6-8-2015
Please cite this article as: Bing Hu, Chunhua Zhao, Xiaoying Jin, Hao Wang, Jian Xiong, Junjun Tan, Antagonistic Effect in Pickering Emulsion Stabilized by Mixtures of Hydroxyapatite Nanoparticles and Sodium Oleate, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.08.009 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.
Antagonistic Effect in Pickering Emulsion Stabilized by Mixtures of Hydroxyapatite Nanoparticles and Sodium Oleate Bing Hua*, Chunhua Zhaob, Xiaoying Jina, Hao Wanga, Jian Xionga, Junjun Tana*
a. School of Chemistry and Chemical Engineering, Hubei University of Technology, Wuhan, 430068, Hubei, P. R. China; b. Oil Field Chemistry Research Institute, Division of Oil Field Chemistry, China Oil Field Services Limited, Yanjiao, Hebei 065201, China
Corresponding author (*): Junjun Tan; Bing Hu; Tel: 0086 27 59750460 Fax: 0086 27 59750482 E-mail address: [email protected] [email protected]
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Graphical abstracts
Highlights
Antagonistic effect was discovered in Pickering emulsion stabilized by the mixture of hydroxyapatite (HA) nanoparticles and sodium oleate. Both particles and surfactants lose their ability to stabilize oil/water interface at surfactant concentration of antagonistic effect. The origin is attributed to the adsorption of nearly all surfactant molecules on particle surfaces and the consequent transfer of sufficient hydrophobic particles from water phase into oil phase. the oil/water interface facilitates particles' transferring into oil phase through peeling off the second layer of surfactant because of the competition adsorption between oil/water interface and the saturated surface of HA nanoparticles.
Abstract In this paper, a systematic investigation on the behavior of emulsions stabilized by the mixture of hydroxyapatite (HA) nanoparticles and sodium oleate was conducted. Surprisingly, in addition to the observation of double inversion of emulsion, antagonistic effect was first discovered in surfactant range of forming W/O emulsion, in which the emulsion breaks rapidly and completely as both particles and surfactants lose their ability to stabilize oil/water interface. The origin is attributed to the adsorption of nearly all surfactant molecules on particle surfaces and the consequent transfer of sufficient hydrophobic particles from water phase into oil phase, and the surfactant molecules left are too few to stabilize oil/water interface. In this process, the oil/water interface facilitates particles’ transferring into oil phase through peeling off the second layer of surfactant because of the competition adsorption between oil/water interface and the saturated surface of HA nanoparticles. The deduction above is well supported by the results of TEM, XRD, contact angle, surface tension, TG and FTIR. This novel finding not only offers deeper recognition about roles of both HA 2
particles and surfactant at the oil/water interfaces but also clarifies specific requirements of antagonistic effect of emulsion, which help avoiding its negative impact on the fabrication of targeting materials of HA for specific applications or enhancing the phase transfer of HA nanoparticles from water to oil for stable dispersion in oil phase.
Keywords: Hydroxyapatite nanoparticles; Sodium oleate; Antagonistic effect; Phase transfer.
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1. Introduction The behavior of colloidal particles on the interface of liquid-liquid system (Pickering emulsion for example) is always a fascinating issue.1 Because of the strong steric repulsion of particle layers at the interface, solid-stabilized emulsions (Pickering emulsion) which are widely used in a range of industrial sectors, such as pharmaceutical, agrochemical, food science, home and personal care products etc. are well-known for their excellent stability of resisting coalescence; moreover, Pickering emulsions are used as powerful tool in fabricating advanced materials, such as polymer microspheres2, 3, porous materials4, core shell materials5, nanocomposites6, 7, and colloidosomes 8-10 and designing smart emulsion systems11, 12. In recent years, a thorough understanding of emulsion stabilized by solid particles alone has been achieved13-15. To a certain Pickering emulsion system, the wettability of particles dominates not only emulsion stability through varying particle detachment energy at the interface but also emulsion type by changing the preferential interfacial position of particles at a curved oil/water interface. Generally, the tailoring of particle behavior at the interface and the controlling of resulting emulsion stability and emulsion type are indispensable to meet the requirements of scientific research and practical applications. In practice, for example, it is not feasible to use unmodified inorganic particles directly to tailor the emulsion properties owing to the inherent strong hydrophilicity of most inorganic particles. So usually modification of particles is necessary. Generally, in situ surface modification with surfactant is used as a preferable alternative considering its simplicity and low cost 12
.Thus, the understanding of the behavior of emulsion stabilized by mixture of
inorganic particles and surfactant is crucial. Generally, single phase inversion of emulsion would be easily conducted by adding particles into a surfactant system or vice versa, as the wettability of particles is adjusted by the surfactant. Schulman and Leja found that water in oil (W/O) emulsions stabilized by oleic acid invert to oil in water (O/W) by adding barium sulfate particles16. In contrast, the inversion of the emulsion from o/w to w/o was observed by adding barium sulfate particles to emulsions stabilized by sodium dodecyl sulfate (SDS). Tambe and Sharma demonstrated inversion from O/W to W/O emulsions stabilized by calcium carbonate particles by increasing the addition of stearic acid surfactant17. For reports mentioned above, the authors attributed the phase inversion to the great change 4
of particle wettability caused by the adsorption of surfactants. Recently, a plausible finding, named double phase inversion of Pickering emulsion was first reported by Binks and co-authors, which challenged the previous recognition there is only single emulsion inversion stabilized by particles and surfactant18. Typically, the emulsions containing silica particles can invert initially from O/W to W/O and subsequently back to O/W by increasing the concentration of surfactant didecyldimethylammonium bromide. The origin of double inversion is attributed to double layer adsorption on particle surface. To be specific, at relatively low surfactant concentration, the stability of O/W emulsion would be improved but when the surfactant concentration overpasses a certain point, the emulsion would invert into W/O under a synergistic effect between the surfactants and the particles because of the increase of particle hydrophobicity. Further increase the surfactant concentration to a critical point, emulsion inversion occurs again from W/O to O/W type because of competitive effects between the particles and the surfactants. The wettability of particles tends to be hydrophilic and surfactant molecules dominate the oil-water interface when surfactant is double-layer adsorbed on particle surface. This finding not only provides an easy and effective way to control emulsion inversion but also is significant to the deep understanding of the behavior of particles and surfactant at the interface; sequent researches further extend its universality to other surfactant-particle systems19-24, such as SDS/Layered Double hydroxide particles (LDH)19, 19
22
24
,
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SDBS/LDH , SDS/CaCO3 , sodium carboxylate/CaCO3 , sorbitan oleate/Laponite particles23 etc. These examples about double inversion above reveal two features in common: the obtained emulsion keeps stable with increase of surfactant and particles cannot across the oil/water interface to get into oil phase; however, when we reconsider the variation of wettability of particle, it may not conclude the whole story. With increase of surfactant concentration, the wettability of particles would alter from being hydrophilic to hydrophobic and then return to hydrophilic, which corresponds to at least four stages: unsaturated monolayer adsorption, saturated monolayer adsorption, unsaturated bilayer adsorption and saturated bilayer adsorption. Owing to the continuous changes of wettability, there should be a maximum for hydrophobicity at saturated monolayer adsorption stage. In that case, hydrophobicity of the particles would reach the highest point and particles may transfer from water phase into oil phase; thus the interface would not be stabilized by particles and surfactants as the latter would be nearly all adsorbed onto particle surface. In that case, emulsion would break
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rapidly; this effect is completely different from previously reported synergistic effect2528
and competing effect between the particles and the surfactants29, 30. Here we defined
it as antagonistic effect. To verify this presumption and recognize the behavior of nanoparticles at the interface, we here reported the behavior of emulsion stabilized by the mixture of HA nanoparticles (hydroxyapatite, Ca10(PO4)6(OH)2) and surfactant (sodium oleate). The origin and requirements of antagonistic effect of Pickering emulsion were also discussed.
2 Materials and methods 2.1 Materials Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, AR), sodium phosphate tribasic dodecahydrate (Na3PO4·12H2O, AR), sodium oleate (C18H33NaO2, AR), sodium dodecyl sulfate (C12H25O4NaS, AR), sodium dodecylbenzenesulfonate (C18H29NaO3S, AR) and cyclohexane (C6H12, AR) were supplied by Sinopharm Chemcial Reagent Co. Ltd. All chemical were used as received without further purification and deionized water was used throughout.
2.2 Methods 2.2.1 Prepar ation and char acter ization of HA nanorods HA nanorods were prepared by a hydrothermal method. In a typical experiment, Ca(NO3)2·4H2O solution with a fixed concentration (0.01M, 20 ml) was slowly added to the Na3PO4·12H2O aqueous solution (0.006M, 20 ml) in 10 min with continuous stirring. After that, the as-obtained mixed solution was transferred to a Teflon-lined stainless steel autoclave with 50 mL capacity. The autoclaves were cooled down naturally after hydrothermal treatment at 150oC for 24 h and the resulting product was purified by a three-cycle of centrifugation-washing process (3700g, 5 min, H1850, Changsha Xiangyi Centrifuge Instrument Co., Ltd, China) with deionized water. Finally, part of purified product was redispersed in deionized water to form aqueous dispersion with a fixed concentration, and the rest was freeze-dried into powder form for characterization. The dried powder was characterized by X-ray diffraction with a Philips Anatical X-ray diffractometer (PANalytical B.V. X'Pert PRO) using a Cu Kα radiation (1.5406 A) source at 60kV and 60mA from 20 to 80° with a scan rate of 0.5°/min. The morphology of the HA nanorods was inspected using a Transmission Electron
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Microscope (TEM, FEI Tecnai G20). The zeta potentials of the dispersion were measured by Malvern Zetasizer (nano-zs 90). No additional electrolytes were added for zeta potential measurement. 2.2.2 Prepar ation and char acter ization of mixture of HA nanorods and sur factant To investigate the interaction between the particles and the surfactants, HA nanorods dispersion in water (1wt. %) was mixed with sodium oleate by ultrasonic icebath treatment with ultrasonic energy at 300 watts (W) and 6 cycles, 5-sec activation for a 5-sec duration (JY92-II, Scie0020ntz Biotechnology Co., Ltd . China). And then the suspension was centrifuged at 3700g (H1850, Changsha Xiangyi Centrifuge Instrument Co., Ltd, China) for 30 minutes, the supernatant was used for surface tension measurement and the sediment was dried and subsequently used for TG, FR-IR, contact angle measurements. For contact angle measurements, the particles were collected and compressed to a pressure of 16 MPa into circular disks with a pellet press. The thickness of all disks is 1 mm. The contact angle of particles was measured by the compressed disk method described previously24. The advancing contact angles of sessile water drops (20 L) on the solid surface were determined (Tracker, I. T. Concept, France). The average value was taken from three measurements. This procedure is similar to the one reported by Binks31. The data of contact angle was obtained using the image analysis software (Adobe Photoshop CS2 edition). Thermogravimetric analysis (TGA) was carried out using a Thermal Analysis SDT Q600 (TA Instruments, New Castle, DE, USA). Heating was performed in a nitrogen flow (100 ml min-1 ) using an alumina sample holder. Temperature was increased from room temperature to 800oC with a heating rate of 10oC /min. Fourier transform infrared spectroscopy (FT-IR) was acquired from KBr pellets with a Nicolet 8700 FTIR spectrophotometer between 400-4000 cm-1 . The equilibrium surface tension of the supernatant was measured at 25oC by Wilhelmy plate method (KRUSS K100 Tensiometer, Germany). 2.2.3 Prepar ation and Char acter ization of Emulsions For a typical preparation, Batch emulsions of 30 mL containing equal volumes of cyclohexane and aqueous suspensions with a fixed particle concentration (1 wt. %) were prepared by ultrasonic treatment in an ice-bath with ultrasonic energy at 300 watts (W) and 6 cycles, 5-sec activation for a 5-sec duration (JY92-II, Scientz Biotechnology Co., Ltd . China). Immediately after emulsification, emulsion types were determined by drop test and conductivity measurement using a DDS-307A conductivity meter. The
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emulsions were placed into a sealed glass battle at room temperature after preparation. The stability of emulsions to creaming and coalescence was assessed by monitoring the heights of resolved water and oil from the emulsion phase over time. Photographs of vessels were taken using a digital camera (Xiaomi, MiTwo, China). Emulsion droplets were imaged with an optical microscope (XP-213, Nanjing Jiangnan Novel Optics Co., China) after a 1:6 dilution in the continuous-phase liquid for optical clarity. The average size of emulsion droplets was obtained with microscopic image analysis software (Adobe Photoshop CS2 edition). To identify phase transfer of nanoparticles from water phase to oil phase, TEM was employed using a Transmission Electron Microscope (FEI Tecnai G2/F20) with accelerating voltage of 200 kV. Samples of hydrophobic HA nanoparticles were collected from oil phase in the emulsion with complete phase separation by pipette and then 1-2 droplets were placed onto the carbon side on holey carbon-coated Cu TEMgrids. The TEM-grids were placed on a glass petri-dish with filter paper to adsorb excess liquid leaving only a thin layer of particle suspension behind, which was allowed to dry in air for about 5 minutes before being transferred for individual storage in a BEEM embedding capsules.
3 Results and discussion 3.1 HA Nanoparticles Information For particle emulsifiers, shape and size are helpful to understand behaviors of emulsion. In our experiments, HA which can be easily synthesized by a hydrothermal method is a typical biomaterial with excellent biocompatibility, biological activity and strong ion exchange capacity and is widely used in a range of fields, such as bone tissue engineering32, 33, heavy metal ions adsorption34, 35, fluorescent materials36, 37, protein separation38, 39 and drug carriers.40, 41 The crystalline structures of the obtained HA nanoparticles are shown in Figure 1a. All diffraction peaks of the XRD pattern can be easily indexed to a pure hexagonal phase of HA which agrees with the reported data (PDF No.86-740) and literature42, 43. The strong and sharp peaks observed infer the resulting product was well crystallized. The morphology of the HA nanoparticles was visualized by TEM measurements. As shown in Figure 1b, the sample consists almost entirely of nanorods with a mean length 8
of 70 nm and mean diameter of 23 nm. Further, HA is hydrophilic with contact angle 28o owing to the strong polar surface group P-OH, Ca-OH. When dispersed in deionized water, HA nanoparticles are weakly and negatively charged in neutral water (pH, 7.5) about -4 mV, which the isoelectric point of HA naoparticles is about 7-7.344, 45
3.2 Emulsion stabilized by HA nanoparticles or surfactant solely To understand the behavior of emulsion, we first prepared the emulsion stabilized by particles or surfactant solely. As shown in Figure 2a, within 2 minutes, emulsions stabilized by HA nanoparticles solely are all O/W type and completely separated into two layers at all particle concentrations investigated because of strong hydrophilicity of HA nanoparticles and result in relatively low adsorption energy at interface. Even though oil phase could be broken into small drops temporarily by ultrasonic device, the oil drops would coalescence rapidly because of lacking firm particle layer protection. Similar example could be seen in LDH, laponite and other inorganic particles46-48. But for the emulsion stabilized by surfactant only, the case is different. Sodium oleate is a hydrophilic ionic surfactant with critical micelle concentration (CMC) value of 2.15 mM in water49, which is well-known for forming O/W emulsions. As shown in Figure 2b, the emulsions are O/W type and stable when the surfactant concentration is above 0.1 mM. Further increase surfactant concentration does not affect emulsion type. This would be attributed to two aspects: reducing interfacial tension and sufficient electrostatic force among drops. The results above show emulsion type would always keep O/W type either by adding surfactant or particle solely to the systems and the type of both emulsions would not invert by changing the concentration solely.
3.2 Emulsions Stabilized by HA Nanoparticles/Sodium Oleate Mixtures. Figure 3. Photograph of vessels containing equal volumes of 0.5-2 wt % HA nanorods and cyclohexane as a function of sodium oleate concentration (mM) in water. The images were taken 3
Subsequently, the behaviors of emulsion stabilized by the mixture of HA 9
nanoparticles and surfactants were investigated, as shown in Figure 3. Overall, the emulsion stability improves greatly compared with that using particle solely. Take 1 wt % HA nanoparticles for example, emulsion stability improves with increase of surfactant concentration from 0 mM to 4 mM and emulsion keeps O/W type with the naked eye observation. When the surfactant ranges from 4 mM to 15 mM, emulsion inverts into W/O type. Further increase surfactant concentration up to 20 mM, emulsion inverts back into O/W type. These emulsion type changes are similar to previously reported double emulsion inversion. Changing particle concentration from 0.5 wt. % to 2 wt. % would not lead to any essential change but the two points for phase inversion change to high surfactant concentrations respectively. More importantly, a surprising phenomenon could be observed. Emulsion stability gets worse gradually when the surfactant concentration ranges from 4 mM to 8 mM, further increase the surfactant concentration from 8 mM to 20 mM, the emulsion stability gets better. Complete phase separation occurs at 8 mM. In other words, both particles and surfactant become invalidated at the surfactant concentration. A similar variation is also observed in the microscopic scale. Figure 4 shows representative optical micrographs of various emulsion, as it shows at the low and high surfactant concentration, droplets are spherical and polydisperse. But at the intermediate surfactant concentration, no stable emulsion droplets could be observed but some broken particle layers, which further infer that particle and surfactant at a certain concentration could become invalid simultaneously.
To confirm the change of emulsion type at fixed concentration of HA nanoparticles (0.5-2 wt %) in an aqueous dispersion and surfactant ranging from 0-40 mM, conductivities of emulsions were conducted as shown in Figure 5. The test was carried out immediately after preparation of each emulsion sample. When the surfactant ranges from 0-4 mM, high conductivities (>100 S/cm) were obtained, inferring continuous phase is the aqueous phase and the emulsions are O/W; then the conductivities decrease to low values (<10 S/cm) at intermediate surfactant concentrations, which indicates that W/O emulsions are obtained and the oil phase is the continuous phase. Finally, the 10
conductivities increase to high values at high surfactant concentrations as water becomes the continuous phase again. The conductivity measurements confirm the previous observation of emulsion double inversion and the antagonistic effect occurs at the range of surfactant concentration of W/O emulsion. Previous results clearly illustrate there is double inversion of emulsion stabilized by mixture of particles and surfactant. Moreover, an antagonistic effect was observed in the surfactant concentration in the range of forming W/O emulsion, which has not been reported in the previous literatures relevant to double inversion. We deduce theoretically that in the surfactant concentration of antagonistic area, nearly all surfactants are adsorbed by particles which transfer into oil phase when the hydrophobicity reaches a certain point and therefore becomes invalidated in stabilizing the interface; besides, as the surfactant molecules left in the aqueous phase are too few to stabilize the interface, demulsification occurs rapidly. To confirm the deduction, it is critical to prove that the nanoparticles do transfer into oil phase, so TEM measurement was carried out, as shown in Figure 6: for emulsion stabilized by mixture of sodium oleate and HA particles, oil and water phases separate completely when the surfactant concentration is 8 mM; as can be observed from the TEM images, rod-like nanoparticles, similar to those synthesized by hydrothermal method in appearance, do exist in the oil phase, proving that the particles went into oil phase and stay stable there; this deduction was also supported by the color change of the oil phase from being initially total transparent to being a little bit blue-white. Besides, to prove that such a phenomenon is universal, SDS (sodium dodecyl sulfate) dodecyl benzenesulfonate)
were also
and SDBS (sodium
been tested and similar effects were found
as well at the surfactant concentration of 4 mM and 3 mM respectively (see figure S1 in supporting information).
3.3 Origins of Antagonistic Effect of Emulsions Stabilized by HA Nanoparticles and Sodium Oleate Mixtures. So far, it is realized that by changing surfactant concentration in the aqueous phase, the behavior of particle at the oil/water interface is beyond the understanding about
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double inversion of emulsion. However, an interesting question would be asked: what are the origins and requirements for antagonistic effect of emulsions? What will be discussed next are some thoughts relevant to antagonistic effect derived from variation of wettability, surface tension and adsorption by changing surfactant concentration. According to previous recognition13-15, 50, there is a close relationship between particle wettability and behavior of obtained emulsions. A quantitative description relevant to such relationship has been reported by Binks13. The energy needed to detach the particles from the interface can be estimated by using eq 1:
cosow2 (1) where is the oil-water interfacial tension, is the particle radius, and ow is the contact angle. As ow determines the particle binding energy at the interface, emulsion stability strongly depends on their wettability. Figure 7 provides the variation of wettability of HA particles after being mixed with surfactant. The contact angles of particles increase from 28o to 122o rapidly as surfactant concentration increases from 0 mM to 8 mM, indicating that surfactant adsorption on particle surfaces is established from unsaturated to saturated monolayer. After that, the contact angles of particles drop down dramatically to 70o when surfactant increases to 15 mM, indicating surfactant adsorption on the particle surfaces is established from unsaturated to saturated bilayer. Further increase of surfactant concentration does not bring any essential change, indicating that bilayer adsorption is complete. The top of contact angle value is at about 8-10 mM surfactant concentration, at which antagonistic effect of emulsion is most obvious. For phase transfer of nanoparticles from water phase to oil phase, it’s well recognized that the more hydrophobic the particle is, the easier it is for particles to transfer into oil phase. Although theoretically, the hydrophobicity would reach the highest point when the particle surface is exactly covered by saturated monolayer of surfactant, the fact is a little bit different; experiments show that it would be more feasible for particles to keep colloidally stable in oil phase when the surfactant concentration is 1-3 mM after the theoretically believed point, and this would be attributed to the competition adsorption 12
of surfactant between particle surface and O/W interface. Take steps along this path, a couple of clues were found when we tested the surface tension change of supernatant which was centrifuged from the mixture of particle and surfactant. As shown in Figure 8, the surface tension of supernatant is barely changed when surfactant concentration is between 0 mM - 8 mM, indicating that almost all the surfactant molecules are adsorbed onto particle surface; in other words, the adsorption ability for surfactant on particle surface is far greater than that on the gas/water interface; however, for surfactant concentrations between 10 mM -13 mM, the surface tension of supernatant drops sharply and close to the platform value. This, combined with the data of contact angles, shows that bilayer adsorption occurs on particle surfaces and at the meantime gas/liquid interface adsorbs the surfactant molecules left in the emulsion, the adsorption ability of both above are at the same order of magnitude. During the emulsification of mixture of oil and water, a large area of oil/water interface would be created. Even if the addition amount of surfactant is between amount of saturated monolayer adsorption of surfactant on particle surfaces and amount of saturated bi-layer adsorption of surfactant on particle surfaces, the large newly formed O/W interface would consume the second surfactant layer on particle surfaces during emulsification and subsequently help HA nanoparticles phase transfer into oil phase. Such deduction could be traced from the difference between the variation of contact angles (Figure 7) and the variation of surface tension (Figure 8) in surfactant concentration range 10-15 mM. It is such a competitive adsorption that results in the sound saturated monolayer adsorption which makes it easier for particles to transfer into oil-water interface and stay stable for several months without any aggregation according to our observation. However, such a phenomenon is strictly restricted to the range of several millimoles per litre above the theoretical value of surfactant concentration required by saturated monolayer; too higher than the theoretical value, the antagonistic effect would be weakened quickly and disappear eventually because the capacity of O/W interface for surfactant adsorption is easily saturated for a certain emulsion system; in this situation, demulsification would not be observed in experiments. 13
To further understand the interaction between HA nanorods and sodium oleate at different concentrations, a FT-IR spectrum was employed, as shown in Figure 9a. For case of HA sample without sodium oleate, the 565 cm-1, 605 cm-1and 1028 cm-1 bands are attributed to the presence of PO43- and the 3570 cm-1 band is attributed to the presence of OH in the spectra of HA
40, 42
. 1640 cm-1 band in the spectra of HA is
resulted from the presence of COO group, which are attributed to CO32- in HA nanocrystals which would be derived from the carbon dioxide (CO2) from solution. For case of sodium oleate sample, the 2930 cm-1 band is attributed to the presence of C-H and the 1560 cm-1 band is attributed to the presence of COO in the spectra of sodium oleate
51
. As to the sample of HA modified with sodium oleate of 2 mM, two new
adsorption peaks appear compared with the peaks in the HA sample, which are characteristic adsorption peaks of pure sodium oleate, indicating that the surfactant molecules are adsorbed on the particle surface. More importantly, the intensity of the two new adsorption peaks increases at 40 mM of the surfactant concentration, indicating that the bilayer adsorption of sodium oleate on HA nanorods appears at high surfactant concentration which is in good agreement with the results of variation of contact angle and surface tension. Furthermore, to confirm the deduction from the FTIR results, TG measurement was employed for samples modified by sodium oleate, as shown in Figure 9b. The weight loss of samples below 150oC is mainly attributed to the volatilization of trace water and water crystallization; weight loss of samples between 200oC and 800oC is resulted from the thermal decomposition of sodium oleate adsorbed on the particle surface. Comparatively for the same temperature condition, the weight loss for sample synthesized without the addition of sodium citrate is 2.06%, and for surfactant concentration of 4 mM and 40 mM, the weight losses were 5.81%, 16.10% respectively. Moreover, Single step weightlessness curve for samples with surfactant concentration of 4mM and double step weightlessness curve for samples with surfactant concentration of 40 mM show the differences of monolayer adsorption and bilayer adsorption on HA nanoparticle surfaces, which is in good agreement with previously reported literature52. 3.4 A Proposed Mechanism of Antagonistic Effect in Pickering Double Inversion 14
Based on all the experimental data and discussions above, a clear schematic illustration could be proposed in figure 10. In addition to the recognition of double inversion of emulsion, there is an antagonistic effect between particles and surfactant when the amount of surfactant is just enough to meet the needs for monolayer adsorption of particles. However, such an antagonistic effect is sensitive to the surfactant concentration, only limited to a range of several millimoles per liter. In other words, both particles and surfactant would be invalidated to stabilize the O/W interface only when particle surfaces are sufficient hydrophobic because in this case the adsorption energy is so low that it is easier for particles to transfer into oil phase and deemulsification occurs consequently. In the whole process, The amount of surfactant on the water/air interface is ignored as no obvious foam appears, under such a condition to a specific emulsion system, the occurrence of antagonistic effect in Pickering emulsion should meet two requirements at least; first, the total amount of surfactant should be above the needed one to form saturated monolayer coverage on particle surface. At the same time, the total amount of surfactant on second layer of particle surface and left in water phase should be much smaller than that needed to saturate oil/water interface created by ultrasonic treatment under emulsification process.
4. Conclusions An antagonistic effect in Pickering emulsion stabilized by mixtures of hydroxyapatite nanoparticles and sodium oleate was first observed in Pickering emulsion induced by increasing surfactant concentration. In the antagonistic range, emulsion breaks rapidly as both particles and surfactants lose their ability to stabilize oil/water interface. The origin of antagonistic effect is attributed to the adsorption of nearly all surfactant molecules on particle surfaces and the consequent transfer of sufficient hydrophobic particles from water phase into oil phase, and the surfactant molecules left are too few to stabilize oil/water interface. In this process, the oil/water interface promotes particles transferring into oil phase through peeling off the second layer of surfactant because of the competition adsorption between oil/water interface and the saturated surface of HA nanoparticles. This novel finding not only offers deeper 15
recognition about roles of both particle and surfactant in double inversion of emulsion at the oil/water interfaces but also clarifies specific requirements of antagonistic effect of emulsion which are helpful for enormous potential applications, especially for phase transfer of nanoparticles from aqueous to oil.
5 Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 21203059) and the Doctor Foundation of Hubei University of Technology (No. BSQD12043). The authors thank Ms. Yuying Lian for help in preparation of the manuscript.
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Fe3O4 nanoparticles and their application in pH-responsive Pickering emulsions. J. Colloid Interface Sci. 2007, 310, (1), 260-269.
Figure captions Fig. 1 XRD patterns (a), TEM images (b) and wettability (c) of HA nanoparticles. Fig. 2 (a) Attempt at forming an emulsion using HA nanoparticles solely with different particle concentrations. (b) Attempt at forming an emulsion using sodium oleate solely with different surfactant concentrations. The cyclohexane to aqueous phase ratio is 1:1.
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Fig. 3 Photograph of vessels containing equal volumes of 0.5-2 wt % HA nanorods and cyclohexane as a function of sodium oleate concentration (mM) in water. The images were taken 3 h after emulsification. Fig. 4 Optical microscope images of cyclohexane-water (1:1 volume) emulsion stabilized by various concentrations of sodium oleate. The initial concentrations (mM) of sodium oleate are 0.5 wt. % HA at 1, 4.5, 15 mM (a1, a2, a3), 1 wt. % HA at 1, 8, 30 mM (b1, b2, b3) and 2 wt. % HA at 1, 14, 30 mM (c1, c2, c3). Fig. 5 Effect of surfactant concentrations on the conductivity of emulsions of cyclohexane and aqueous HA nanoparticles. Fig. 6 TEM images for HA nanoparticles in oil phase induced by different surfactants. (a1, a2) sodium oleate at 8 mM, (b1, b2) SDS at 4 mM, (c1,c2) SDBS at 3 mM. The particle concentration is 1 wt. %. Fig. 7 Contact angles of aqueous drops on planar HA substrates modified with different sodium oleate concentration in air. Fig. 8 Variation of the equilibrium interfacial tensions of the air/water interface at different surfactant concentrations. Figure 9 FT-IR (a) and TG (b) of modified HA mixed with different concentrations of sodium oleate. Fig. 10 Schematic illustration of antagonistic effect in Pickering emulsions.
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Intensity (a.u.)
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Figure.1 XRD patterns (a), TEM images (b) and wettability (c) of HA nanoparticles.
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Figure 2. (a) Attempt at forming an emulsion using HA nanoparticles solely with different particle concentrations. (b) Attempt at forming an emulsion using sodium oleate solely with different surfactant concentrations. The cyclohexane to aqueous phase ratio is 1:1.
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Figure 3. Photograph of vessels containing equal volumes of 0.5-2 wt % HA nanorods and cyclohexane as a function of sodium oleate concentration (mM) in water. The images were taken 3 h after emulsification.
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Figure 4. Optical microscope images of cyclohexane-water (1:1 volume) emulsion stabilized by various concentrations of sodium oleate. The initial concentrations (mM) of sodium oleate are 0.5 wt. % HA at 1, 4.5, 15 mM (a1, a2, a3), 1 wt. % HA at 1, 8, 30 mM (b1, b2, b3) and 2 wt. % HA at 1, 14, 30 mM (c1, c2, c3).
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Figure 5. Effect of surfactant concentrations on the conductivity of emulsions of cyclohexane and aqueous HA nanoparticles.
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Figure 6. TEM images for HA nanoparticles in oil phase induced by different surfactants. (a1, a2) sodium oleate at 8 mM, (b1, b2) SDS at 4 mM, (c1,c2) SDBS at 3 mM. The particle concentration is 1 wt. %.
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Figure 7. Contact angles of aqueous drops on planar HA substrates modified with different sodium oleate concentration in air.
Figure 8. Variation of the equilibrium interfacial tensions of the air/water interface at different surfactant concentrations.
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Figure 9. FT-IR (a) and TG (b) of modified HA mixed with different concentrations of sodium oleate.
Figure 10. Schematic illustration of antagonistic effect in Pickering emulsions.
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S0927-7757(15)30153-9 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.08.009 COLSUA 20100
To appear in:
Colloids and Surfaces A: Physicochem. Eng. Aspects
Received date: Revised date: Accepted date:
5-6-2015 3-8-2015 6-8-2015
Please cite this article as: Bing Hu, Chunhua Zhao, Xiaoying Jin, Hao Wang, Jian Xiong, Junjun Tan, Antagonistic Effect in Pickering Emulsion Stabilized by Mixtures of Hydroxyapatite Nanoparticles and Sodium Oleate, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.08.009 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.
Antagonistic Effect in Pickering Emulsion Stabilized by Mixtures of Hydroxyapatite Nanoparticles and Sodium Oleate Bing Hua*, Chunhua Zhaob, Xiaoying Jina, Hao Wanga, Jian Xionga, Junjun Tana*
a. School of Chemistry and Chemical Engineering, Hubei University of Technology, Wuhan, 430068, Hubei, P. R. China; b. Oil Field Chemistry Research Institute, Division of Oil Field Chemistry, China Oil Field Services Limited, Yanjiao, Hebei 065201, China
Corresponding author (*): Junjun Tan; Bing Hu; Tel: 0086 27 59750460 Fax: 0086 27 59750482 E-mail address: [email protected] [email protected]
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Graphical abstracts
Highlights
Antagonistic effect was discovered in Pickering emulsion stabilized by the mixture of hydroxyapatite (HA) nanoparticles and sodium oleate. Both particles and surfactants lose their ability to stabilize oil/water interface at surfactant concentration of antagonistic effect. The origin is attributed to the adsorption of nearly all surfactant molecules on particle surfaces and the consequent transfer of sufficient hydrophobic particles from water phase into oil phase. the oil/water interface facilitates particles' transferring into oil phase through peeling off the second layer of surfactant because of the competition adsorption between oil/water interface and the saturated surface of HA nanoparticles.
Abstract In this paper, a systematic investigation on the behavior of emulsions stabilized by the mixture of hydroxyapatite (HA) nanoparticles and sodium oleate was conducted. Surprisingly, in addition to the observation of double inversion of emulsion, antagonistic effect was first discovered in surfactant range of forming W/O emulsion, in which the emulsion breaks rapidly and completely as both particles and surfactants lose their ability to stabilize oil/water interface. The origin is attributed to the adsorption of nearly all surfactant molecules on particle surfaces and the consequent transfer of sufficient hydrophobic particles from water phase into oil phase, and the surfactant molecules left are too few to stabilize oil/water interface. In this process, the oil/water interface facilitates particles’ transferring into oil phase through peeling off the second layer of surfactant because of the competition adsorption between oil/water interface and the saturated surface of HA nanoparticles. The deduction above is well supported by the results of TEM, XRD, contact angle, surface tension, TG and FTIR. This novel finding not only offers deeper recognition about roles of both HA 2
particles and surfactant at the oil/water interfaces but also clarifies specific requirements of antagonistic effect of emulsion, which help avoiding its negative impact on the fabrication of targeting materials of HA for specific applications or enhancing the phase transfer of HA nanoparticles from water to oil for stable dispersion in oil phase.
Keywords: Hydroxyapatite nanoparticles; Sodium oleate; Antagonistic effect; Phase transfer.
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1. Introduction The behavior of colloidal particles on the interface of liquid-liquid system (Pickering emulsion for example) is always a fascinating issue.1 Because of the strong steric repulsion of particle layers at the interface, solid-stabilized emulsions (Pickering emulsion) which are widely used in a range of industrial sectors, such as pharmaceutical, agrochemical, food science, home and personal care products etc. are well-known for their excellent stability of resisting coalescence; moreover, Pickering emulsions are used as powerful tool in fabricating advanced materials, such as polymer microspheres2, 3, porous materials4, core shell materials5, nanocomposites6, 7, and colloidosomes 8-10 and designing smart emulsion systems11, 12. In recent years, a thorough understanding of emulsion stabilized by solid particles alone has been achieved13-15. To a certain Pickering emulsion system, the wettability of particles dominates not only emulsion stability through varying particle detachment energy at the interface but also emulsion type by changing the preferential interfacial position of particles at a curved oil/water interface. Generally, the tailoring of particle behavior at the interface and the controlling of resulting emulsion stability and emulsion type are indispensable to meet the requirements of scientific research and practical applications. In practice, for example, it is not feasible to use unmodified inorganic particles directly to tailor the emulsion properties owing to the inherent strong hydrophilicity of most inorganic particles. So usually modification of particles is necessary. Generally, in situ surface modification with surfactant is used as a preferable alternative considering its simplicity and low cost 12
.Thus, the understanding of the behavior of emulsion stabilized by mixture of
inorganic particles and surfactant is crucial. Generally, single phase inversion of emulsion would be easily conducted by adding particles into a surfactant system or vice versa, as the wettability of particles is adjusted by the surfactant. Schulman and Leja found that water in oil (W/O) emulsions stabilized by oleic acid invert to oil in water (O/W) by adding barium sulfate particles16. In contrast, the inversion of the emulsion from o/w to w/o was observed by adding barium sulfate particles to emulsions stabilized by sodium dodecyl sulfate (SDS). Tambe and Sharma demonstrated inversion from O/W to W/O emulsions stabilized by calcium carbonate particles by increasing the addition of stearic acid surfactant17. For reports mentioned above, the authors attributed the phase inversion to the great change 4
of particle wettability caused by the adsorption of surfactants. Recently, a plausible finding, named double phase inversion of Pickering emulsion was first reported by Binks and co-authors, which challenged the previous recognition there is only single emulsion inversion stabilized by particles and surfactant18. Typically, the emulsions containing silica particles can invert initially from O/W to W/O and subsequently back to O/W by increasing the concentration of surfactant didecyldimethylammonium bromide. The origin of double inversion is attributed to double layer adsorption on particle surface. To be specific, at relatively low surfactant concentration, the stability of O/W emulsion would be improved but when the surfactant concentration overpasses a certain point, the emulsion would invert into W/O under a synergistic effect between the surfactants and the particles because of the increase of particle hydrophobicity. Further increase the surfactant concentration to a critical point, emulsion inversion occurs again from W/O to O/W type because of competitive effects between the particles and the surfactants. The wettability of particles tends to be hydrophilic and surfactant molecules dominate the oil-water interface when surfactant is double-layer adsorbed on particle surface. This finding not only provides an easy and effective way to control emulsion inversion but also is significant to the deep understanding of the behavior of particles and surfactant at the interface; sequent researches further extend its universality to other surfactant-particle systems19-24, such as SDS/Layered Double hydroxide particles (LDH)19, 19
22
24
,
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SDBS/LDH , SDS/CaCO3 , sodium carboxylate/CaCO3 , sorbitan oleate/Laponite particles23 etc. These examples about double inversion above reveal two features in common: the obtained emulsion keeps stable with increase of surfactant and particles cannot across the oil/water interface to get into oil phase; however, when we reconsider the variation of wettability of particle, it may not conclude the whole story. With increase of surfactant concentration, the wettability of particles would alter from being hydrophilic to hydrophobic and then return to hydrophilic, which corresponds to at least four stages: unsaturated monolayer adsorption, saturated monolayer adsorption, unsaturated bilayer adsorption and saturated bilayer adsorption. Owing to the continuous changes of wettability, there should be a maximum for hydrophobicity at saturated monolayer adsorption stage. In that case, hydrophobicity of the particles would reach the highest point and particles may transfer from water phase into oil phase; thus the interface would not be stabilized by particles and surfactants as the latter would be nearly all adsorbed onto particle surface. In that case, emulsion would break
5
rapidly; this effect is completely different from previously reported synergistic effect2528
and competing effect between the particles and the surfactants29, 30. Here we defined
it as antagonistic effect. To verify this presumption and recognize the behavior of nanoparticles at the interface, we here reported the behavior of emulsion stabilized by the mixture of HA nanoparticles (hydroxyapatite, Ca10(PO4)6(OH)2) and surfactant (sodium oleate). The origin and requirements of antagonistic effect of Pickering emulsion were also discussed.
2 Materials and methods 2.1 Materials Calcium nitrate tetrahydrate (Ca(NO3)2·4H2O, AR), sodium phosphate tribasic dodecahydrate (Na3PO4·12H2O, AR), sodium oleate (C18H33NaO2, AR), sodium dodecyl sulfate (C12H25O4NaS, AR), sodium dodecylbenzenesulfonate (C18H29NaO3S, AR) and cyclohexane (C6H12, AR) were supplied by Sinopharm Chemcial Reagent Co. Ltd. All chemical were used as received without further purification and deionized water was used throughout.
2.2 Methods 2.2.1 Prepar ation and char acter ization of HA nanorods HA nanorods were prepared by a hydrothermal method. In a typical experiment, Ca(NO3)2·4H2O solution with a fixed concentration (0.01M, 20 ml) was slowly added to the Na3PO4·12H2O aqueous solution (0.006M, 20 ml) in 10 min with continuous stirring. After that, the as-obtained mixed solution was transferred to a Teflon-lined stainless steel autoclave with 50 mL capacity. The autoclaves were cooled down naturally after hydrothermal treatment at 150oC for 24 h and the resulting product was purified by a three-cycle of centrifugation-washing process (3700g, 5 min, H1850, Changsha Xiangyi Centrifuge Instrument Co., Ltd, China) with deionized water. Finally, part of purified product was redispersed in deionized water to form aqueous dispersion with a fixed concentration, and the rest was freeze-dried into powder form for characterization. The dried powder was characterized by X-ray diffraction with a Philips Anatical X-ray diffractometer (PANalytical B.V. X'Pert PRO) using a Cu Kα radiation (1.5406 A) source at 60kV and 60mA from 20 to 80° with a scan rate of 0.5°/min. The morphology of the HA nanorods was inspected using a Transmission Electron
6
Microscope (TEM, FEI Tecnai G20). The zeta potentials of the dispersion were measured by Malvern Zetasizer (nano-zs 90). No additional electrolytes were added for zeta potential measurement. 2.2.2 Prepar ation and char acter ization of mixture of HA nanorods and sur factant To investigate the interaction between the particles and the surfactants, HA nanorods dispersion in water (1wt. %) was mixed with sodium oleate by ultrasonic icebath treatment with ultrasonic energy at 300 watts (W) and 6 cycles, 5-sec activation for a 5-sec duration (JY92-II, Scie0020ntz Biotechnology Co., Ltd . China). And then the suspension was centrifuged at 3700g (H1850, Changsha Xiangyi Centrifuge Instrument Co., Ltd, China) for 30 minutes, the supernatant was used for surface tension measurement and the sediment was dried and subsequently used for TG, FR-IR, contact angle measurements. For contact angle measurements, the particles were collected and compressed to a pressure of 16 MPa into circular disks with a pellet press. The thickness of all disks is 1 mm. The contact angle of particles was measured by the compressed disk method described previously24. The advancing contact angles of sessile water drops (20 L) on the solid surface were determined (Tracker, I. T. Concept, France). The average value was taken from three measurements. This procedure is similar to the one reported by Binks31. The data of contact angle was obtained using the image analysis software (Adobe Photoshop CS2 edition). Thermogravimetric analysis (TGA) was carried out using a Thermal Analysis SDT Q600 (TA Instruments, New Castle, DE, USA). Heating was performed in a nitrogen flow (100 ml min-1 ) using an alumina sample holder. Temperature was increased from room temperature to 800oC with a heating rate of 10oC /min. Fourier transform infrared spectroscopy (FT-IR) was acquired from KBr pellets with a Nicolet 8700 FTIR spectrophotometer between 400-4000 cm-1 . The equilibrium surface tension of the supernatant was measured at 25oC by Wilhelmy plate method (KRUSS K100 Tensiometer, Germany). 2.2.3 Prepar ation and Char acter ization of Emulsions For a typical preparation, Batch emulsions of 30 mL containing equal volumes of cyclohexane and aqueous suspensions with a fixed particle concentration (1 wt. %) were prepared by ultrasonic treatment in an ice-bath with ultrasonic energy at 300 watts (W) and 6 cycles, 5-sec activation for a 5-sec duration (JY92-II, Scientz Biotechnology Co., Ltd . China). Immediately after emulsification, emulsion types were determined by drop test and conductivity measurement using a DDS-307A conductivity meter. The
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emulsions were placed into a sealed glass battle at room temperature after preparation. The stability of emulsions to creaming and coalescence was assessed by monitoring the heights of resolved water and oil from the emulsion phase over time. Photographs of vessels were taken using a digital camera (Xiaomi, MiTwo, China). Emulsion droplets were imaged with an optical microscope (XP-213, Nanjing Jiangnan Novel Optics Co., China) after a 1:6 dilution in the continuous-phase liquid for optical clarity. The average size of emulsion droplets was obtained with microscopic image analysis software (Adobe Photoshop CS2 edition). To identify phase transfer of nanoparticles from water phase to oil phase, TEM was employed using a Transmission Electron Microscope (FEI Tecnai G2/F20) with accelerating voltage of 200 kV. Samples of hydrophobic HA nanoparticles were collected from oil phase in the emulsion with complete phase separation by pipette and then 1-2 droplets were placed onto the carbon side on holey carbon-coated Cu TEMgrids. The TEM-grids were placed on a glass petri-dish with filter paper to adsorb excess liquid leaving only a thin layer of particle suspension behind, which was allowed to dry in air for about 5 minutes before being transferred for individual storage in a BEEM embedding capsules.
3 Results and discussion 3.1 HA Nanoparticles Information For particle emulsifiers, shape and size are helpful to understand behaviors of emulsion. In our experiments, HA which can be easily synthesized by a hydrothermal method is a typical biomaterial with excellent biocompatibility, biological activity and strong ion exchange capacity and is widely used in a range of fields, such as bone tissue engineering32, 33, heavy metal ions adsorption34, 35, fluorescent materials36, 37, protein separation38, 39 and drug carriers.40, 41 The crystalline structures of the obtained HA nanoparticles are shown in Figure 1a. All diffraction peaks of the XRD pattern can be easily indexed to a pure hexagonal phase of HA which agrees with the reported data (PDF No.86-740) and literature42, 43. The strong and sharp peaks observed infer the resulting product was well crystallized. The morphology of the HA nanoparticles was visualized by TEM measurements. As shown in Figure 1b, the sample consists almost entirely of nanorods with a mean length 8
of 70 nm and mean diameter of 23 nm. Further, HA is hydrophilic with contact angle 28o owing to the strong polar surface group P-OH, Ca-OH. When dispersed in deionized water, HA nanoparticles are weakly and negatively charged in neutral water (pH, 7.5) about -4 mV, which the isoelectric point of HA naoparticles is about 7-7.344, 45
3.2 Emulsion stabilized by HA nanoparticles or surfactant solely To understand the behavior of emulsion, we first prepared the emulsion stabilized by particles or surfactant solely. As shown in Figure 2a, within 2 minutes, emulsions stabilized by HA nanoparticles solely are all O/W type and completely separated into two layers at all particle concentrations investigated because of strong hydrophilicity of HA nanoparticles and result in relatively low adsorption energy at interface. Even though oil phase could be broken into small drops temporarily by ultrasonic device, the oil drops would coalescence rapidly because of lacking firm particle layer protection. Similar example could be seen in LDH, laponite and other inorganic particles46-48. But for the emulsion stabilized by surfactant only, the case is different. Sodium oleate is a hydrophilic ionic surfactant with critical micelle concentration (CMC) value of 2.15 mM in water49, which is well-known for forming O/W emulsions. As shown in Figure 2b, the emulsions are O/W type and stable when the surfactant concentration is above 0.1 mM. Further increase surfactant concentration does not affect emulsion type. This would be attributed to two aspects: reducing interfacial tension and sufficient electrostatic force among drops. The results above show emulsion type would always keep O/W type either by adding surfactant or particle solely to the systems and the type of both emulsions would not invert by changing the concentration solely.
3.2 Emulsions Stabilized by HA Nanoparticles/Sodium Oleate Mixtures. Figure 3. Photograph of vessels containing equal volumes of 0.5-2 wt % HA nanorods and cyclohexane as a function of sodium oleate concentration (mM) in water. The images were taken 3
Subsequently, the behaviors of emulsion stabilized by the mixture of HA 9
nanoparticles and surfactants were investigated, as shown in Figure 3. Overall, the emulsion stability improves greatly compared with that using particle solely. Take 1 wt % HA nanoparticles for example, emulsion stability improves with increase of surfactant concentration from 0 mM to 4 mM and emulsion keeps O/W type with the naked eye observation. When the surfactant ranges from 4 mM to 15 mM, emulsion inverts into W/O type. Further increase surfactant concentration up to 20 mM, emulsion inverts back into O/W type. These emulsion type changes are similar to previously reported double emulsion inversion. Changing particle concentration from 0.5 wt. % to 2 wt. % would not lead to any essential change but the two points for phase inversion change to high surfactant concentrations respectively. More importantly, a surprising phenomenon could be observed. Emulsion stability gets worse gradually when the surfactant concentration ranges from 4 mM to 8 mM, further increase the surfactant concentration from 8 mM to 20 mM, the emulsion stability gets better. Complete phase separation occurs at 8 mM. In other words, both particles and surfactant become invalidated at the surfactant concentration. A similar variation is also observed in the microscopic scale. Figure 4 shows representative optical micrographs of various emulsion, as it shows at the low and high surfactant concentration, droplets are spherical and polydisperse. But at the intermediate surfactant concentration, no stable emulsion droplets could be observed but some broken particle layers, which further infer that particle and surfactant at a certain concentration could become invalid simultaneously.
To confirm the change of emulsion type at fixed concentration of HA nanoparticles (0.5-2 wt %) in an aqueous dispersion and surfactant ranging from 0-40 mM, conductivities of emulsions were conducted as shown in Figure 5. The test was carried out immediately after preparation of each emulsion sample. When the surfactant ranges from 0-4 mM, high conductivities (>100 S/cm) were obtained, inferring continuous phase is the aqueous phase and the emulsions are O/W; then the conductivities decrease to low values (<10 S/cm) at intermediate surfactant concentrations, which indicates that W/O emulsions are obtained and the oil phase is the continuous phase. Finally, the 10
conductivities increase to high values at high surfactant concentrations as water becomes the continuous phase again. The conductivity measurements confirm the previous observation of emulsion double inversion and the antagonistic effect occurs at the range of surfactant concentration of W/O emulsion. Previous results clearly illustrate there is double inversion of emulsion stabilized by mixture of particles and surfactant. Moreover, an antagonistic effect was observed in the surfactant concentration in the range of forming W/O emulsion, which has not been reported in the previous literatures relevant to double inversion. We deduce theoretically that in the surfactant concentration of antagonistic area, nearly all surfactants are adsorbed by particles which transfer into oil phase when the hydrophobicity reaches a certain point and therefore becomes invalidated in stabilizing the interface; besides, as the surfactant molecules left in the aqueous phase are too few to stabilize the interface, demulsification occurs rapidly. To confirm the deduction, it is critical to prove that the nanoparticles do transfer into oil phase, so TEM measurement was carried out, as shown in Figure 6: for emulsion stabilized by mixture of sodium oleate and HA particles, oil and water phases separate completely when the surfactant concentration is 8 mM; as can be observed from the TEM images, rod-like nanoparticles, similar to those synthesized by hydrothermal method in appearance, do exist in the oil phase, proving that the particles went into oil phase and stay stable there; this deduction was also supported by the color change of the oil phase from being initially total transparent to being a little bit blue-white. Besides, to prove that such a phenomenon is universal, SDS (sodium dodecyl sulfate) dodecyl benzenesulfonate)
were also
and SDBS (sodium
been tested and similar effects were found
as well at the surfactant concentration of 4 mM and 3 mM respectively (see figure S1 in supporting information).
3.3 Origins of Antagonistic Effect of Emulsions Stabilized by HA Nanoparticles and Sodium Oleate Mixtures. So far, it is realized that by changing surfactant concentration in the aqueous phase, the behavior of particle at the oil/water interface is beyond the understanding about
11
double inversion of emulsion. However, an interesting question would be asked: what are the origins and requirements for antagonistic effect of emulsions? What will be discussed next are some thoughts relevant to antagonistic effect derived from variation of wettability, surface tension and adsorption by changing surfactant concentration. According to previous recognition13-15, 50, there is a close relationship between particle wettability and behavior of obtained emulsions. A quantitative description relevant to such relationship has been reported by Binks13. The energy needed to detach the particles from the interface can be estimated by using eq 1:
cosow2 (1) where is the oil-water interfacial tension, is the particle radius, and ow is the contact angle. As ow determines the particle binding energy at the interface, emulsion stability strongly depends on their wettability. Figure 7 provides the variation of wettability of HA particles after being mixed with surfactant. The contact angles of particles increase from 28o to 122o rapidly as surfactant concentration increases from 0 mM to 8 mM, indicating that surfactant adsorption on particle surfaces is established from unsaturated to saturated monolayer. After that, the contact angles of particles drop down dramatically to 70o when surfactant increases to 15 mM, indicating surfactant adsorption on the particle surfaces is established from unsaturated to saturated bilayer. Further increase of surfactant concentration does not bring any essential change, indicating that bilayer adsorption is complete. The top of contact angle value is at about 8-10 mM surfactant concentration, at which antagonistic effect of emulsion is most obvious. For phase transfer of nanoparticles from water phase to oil phase, it’s well recognized that the more hydrophobic the particle is, the easier it is for particles to transfer into oil phase. Although theoretically, the hydrophobicity would reach the highest point when the particle surface is exactly covered by saturated monolayer of surfactant, the fact is a little bit different; experiments show that it would be more feasible for particles to keep colloidally stable in oil phase when the surfactant concentration is 1-3 mM after the theoretically believed point, and this would be attributed to the competition adsorption 12
of surfactant between particle surface and O/W interface. Take steps along this path, a couple of clues were found when we tested the surface tension change of supernatant which was centrifuged from the mixture of particle and surfactant. As shown in Figure 8, the surface tension of supernatant is barely changed when surfactant concentration is between 0 mM - 8 mM, indicating that almost all the surfactant molecules are adsorbed onto particle surface; in other words, the adsorption ability for surfactant on particle surface is far greater than that on the gas/water interface; however, for surfactant concentrations between 10 mM -13 mM, the surface tension of supernatant drops sharply and close to the platform value. This, combined with the data of contact angles, shows that bilayer adsorption occurs on particle surfaces and at the meantime gas/liquid interface adsorbs the surfactant molecules left in the emulsion, the adsorption ability of both above are at the same order of magnitude. During the emulsification of mixture of oil and water, a large area of oil/water interface would be created. Even if the addition amount of surfactant is between amount of saturated monolayer adsorption of surfactant on particle surfaces and amount of saturated bi-layer adsorption of surfactant on particle surfaces, the large newly formed O/W interface would consume the second surfactant layer on particle surfaces during emulsification and subsequently help HA nanoparticles phase transfer into oil phase. Such deduction could be traced from the difference between the variation of contact angles (Figure 7) and the variation of surface tension (Figure 8) in surfactant concentration range 10-15 mM. It is such a competitive adsorption that results in the sound saturated monolayer adsorption which makes it easier for particles to transfer into oil-water interface and stay stable for several months without any aggregation according to our observation. However, such a phenomenon is strictly restricted to the range of several millimoles per litre above the theoretical value of surfactant concentration required by saturated monolayer; too higher than the theoretical value, the antagonistic effect would be weakened quickly and disappear eventually because the capacity of O/W interface for surfactant adsorption is easily saturated for a certain emulsion system; in this situation, demulsification would not be observed in experiments. 13
To further understand the interaction between HA nanorods and sodium oleate at different concentrations, a FT-IR spectrum was employed, as shown in Figure 9a. For case of HA sample without sodium oleate, the 565 cm-1, 605 cm-1and 1028 cm-1 bands are attributed to the presence of PO43- and the 3570 cm-1 band is attributed to the presence of OH in the spectra of HA
40, 42
. 1640 cm-1 band in the spectra of HA is
resulted from the presence of COO group, which are attributed to CO32- in HA nanocrystals which would be derived from the carbon dioxide (CO2) from solution. For case of sodium oleate sample, the 2930 cm-1 band is attributed to the presence of C-H and the 1560 cm-1 band is attributed to the presence of COO in the spectra of sodium oleate
51
. As to the sample of HA modified with sodium oleate of 2 mM, two new
adsorption peaks appear compared with the peaks in the HA sample, which are characteristic adsorption peaks of pure sodium oleate, indicating that the surfactant molecules are adsorbed on the particle surface. More importantly, the intensity of the two new adsorption peaks increases at 40 mM of the surfactant concentration, indicating that the bilayer adsorption of sodium oleate on HA nanorods appears at high surfactant concentration which is in good agreement with the results of variation of contact angle and surface tension. Furthermore, to confirm the deduction from the FTIR results, TG measurement was employed for samples modified by sodium oleate, as shown in Figure 9b. The weight loss of samples below 150oC is mainly attributed to the volatilization of trace water and water crystallization; weight loss of samples between 200oC and 800oC is resulted from the thermal decomposition of sodium oleate adsorbed on the particle surface. Comparatively for the same temperature condition, the weight loss for sample synthesized without the addition of sodium citrate is 2.06%, and for surfactant concentration of 4 mM and 40 mM, the weight losses were 5.81%, 16.10% respectively. Moreover, Single step weightlessness curve for samples with surfactant concentration of 4mM and double step weightlessness curve for samples with surfactant concentration of 40 mM show the differences of monolayer adsorption and bilayer adsorption on HA nanoparticle surfaces, which is in good agreement with previously reported literature52. 3.4 A Proposed Mechanism of Antagonistic Effect in Pickering Double Inversion 14
Based on all the experimental data and discussions above, a clear schematic illustration could be proposed in figure 10. In addition to the recognition of double inversion of emulsion, there is an antagonistic effect between particles and surfactant when the amount of surfactant is just enough to meet the needs for monolayer adsorption of particles. However, such an antagonistic effect is sensitive to the surfactant concentration, only limited to a range of several millimoles per liter. In other words, both particles and surfactant would be invalidated to stabilize the O/W interface only when particle surfaces are sufficient hydrophobic because in this case the adsorption energy is so low that it is easier for particles to transfer into oil phase and deemulsification occurs consequently. In the whole process, The amount of surfactant on the water/air interface is ignored as no obvious foam appears, under such a condition to a specific emulsion system, the occurrence of antagonistic effect in Pickering emulsion should meet two requirements at least; first, the total amount of surfactant should be above the needed one to form saturated monolayer coverage on particle surface. At the same time, the total amount of surfactant on second layer of particle surface and left in water phase should be much smaller than that needed to saturate oil/water interface created by ultrasonic treatment under emulsification process.
4. Conclusions An antagonistic effect in Pickering emulsion stabilized by mixtures of hydroxyapatite nanoparticles and sodium oleate was first observed in Pickering emulsion induced by increasing surfactant concentration. In the antagonistic range, emulsion breaks rapidly as both particles and surfactants lose their ability to stabilize oil/water interface. The origin of antagonistic effect is attributed to the adsorption of nearly all surfactant molecules on particle surfaces and the consequent transfer of sufficient hydrophobic particles from water phase into oil phase, and the surfactant molecules left are too few to stabilize oil/water interface. In this process, the oil/water interface promotes particles transferring into oil phase through peeling off the second layer of surfactant because of the competition adsorption between oil/water interface and the saturated surface of HA nanoparticles. This novel finding not only offers deeper 15
recognition about roles of both particle and surfactant in double inversion of emulsion at the oil/water interfaces but also clarifies specific requirements of antagonistic effect of emulsion which are helpful for enormous potential applications, especially for phase transfer of nanoparticles from aqueous to oil.
5 Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 21203059) and the Doctor Foundation of Hubei University of Technology (No. BSQD12043). The authors thank Ms. Yuying Lian for help in preparation of the manuscript.
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Figure captions Fig. 1 XRD patterns (a), TEM images (b) and wettability (c) of HA nanoparticles. Fig. 2 (a) Attempt at forming an emulsion using HA nanoparticles solely with different particle concentrations. (b) Attempt at forming an emulsion using sodium oleate solely with different surfactant concentrations. The cyclohexane to aqueous phase ratio is 1:1.
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Fig. 3 Photograph of vessels containing equal volumes of 0.5-2 wt % HA nanorods and cyclohexane as a function of sodium oleate concentration (mM) in water. The images were taken 3 h after emulsification. Fig. 4 Optical microscope images of cyclohexane-water (1:1 volume) emulsion stabilized by various concentrations of sodium oleate. The initial concentrations (mM) of sodium oleate are 0.5 wt. % HA at 1, 4.5, 15 mM (a1, a2, a3), 1 wt. % HA at 1, 8, 30 mM (b1, b2, b3) and 2 wt. % HA at 1, 14, 30 mM (c1, c2, c3). Fig. 5 Effect of surfactant concentrations on the conductivity of emulsions of cyclohexane and aqueous HA nanoparticles. Fig. 6 TEM images for HA nanoparticles in oil phase induced by different surfactants. (a1, a2) sodium oleate at 8 mM, (b1, b2) SDS at 4 mM, (c1,c2) SDBS at 3 mM. The particle concentration is 1 wt. %. Fig. 7 Contact angles of aqueous drops on planar HA substrates modified with different sodium oleate concentration in air. Fig. 8 Variation of the equilibrium interfacial tensions of the air/water interface at different surfactant concentrations. Figure 9 FT-IR (a) and TG (b) of modified HA mixed with different concentrations of sodium oleate. Fig. 10 Schematic illustration of antagonistic effect in Pickering emulsions.
20
Intensity (a.u.)
3000
40
(a)
60
80
(211) 3000
(002) (300)
2000
2000
(213) (202) (222) (310) (004)
1000
1000
0
0
20
40
60
80
2Theta (degree)
Figure.1 XRD patterns (a), TEM images (b) and wettability (c) of HA nanoparticles.
20
Figure 2. (a) Attempt at forming an emulsion using HA nanoparticles solely with different particle concentrations. (b) Attempt at forming an emulsion using sodium oleate solely with different surfactant concentrations. The cyclohexane to aqueous phase ratio is 1:1.
21
Figure 3. Photograph of vessels containing equal volumes of 0.5-2 wt % HA nanorods and cyclohexane as a function of sodium oleate concentration (mM) in water. The images were taken 3 h after emulsification.
22
Figure 4. Optical microscope images of cyclohexane-water (1:1 volume) emulsion stabilized by various concentrations of sodium oleate. The initial concentrations (mM) of sodium oleate are 0.5 wt. % HA at 1, 4.5, 15 mM (a1, a2, a3), 1 wt. % HA at 1, 8, 30 mM (b1, b2, b3) and 2 wt. % HA at 1, 14, 30 mM (c1, c2, c3).
23
Figure 5. Effect of surfactant concentrations on the conductivity of emulsions of cyclohexane and aqueous HA nanoparticles.
24
Figure 6. TEM images for HA nanoparticles in oil phase induced by different surfactants. (a1, a2) sodium oleate at 8 mM, (b1, b2) SDS at 4 mM, (c1,c2) SDBS at 3 mM. The particle concentration is 1 wt. %.
25
Figure 7. Contact angles of aqueous drops on planar HA substrates modified with different sodium oleate concentration in air.
Figure 8. Variation of the equilibrium interfacial tensions of the air/water interface at different surfactant concentrations.
26
Figure 9. FT-IR (a) and TG (b) of modified HA mixed with different concentrations of sodium oleate.
Figure 10. Schematic illustration of antagonistic effect in Pickering emulsions.
27