Preparation of Oil-in-Seawater Emulsions Based on Environmentally

Sep 23, 2015 - One remediation technique of oil spills is the application of dispersants to oil slicks, which is essentially a process of emulsificati...
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Preparation of Oil-in-Seawater Emulsions Based on Environmentally Benign Nanoparticles and Biosurfactant for Oil Spill Remediation Guilu Pi,† Lili Mao,‡ Mutai Bao,† Yiming Li,*,† Haiyue Gong,† and Jianrui Zhang† †

Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, Ocean University of China, 238 Songling Road, Qingdao, 266100 Shandong Province, China ‡ College of Chemistry and Chemical Engineering, Ocean University of China, 238 Songling Road, Qingdao, 266100 Shandong Province, China S Supporting Information *

ABSTRACT: One remediation technique of oil spills is the application of dispersants to oil slicks, which is essentially a process of emulsification. Tetradecane and crude oil-inseawater emulsions formed with silica nanoparticles modified in situ with rhamnolipid produced a longer stability and smaller droplet size. The interactions of silica particles with rhamnolipid were characterized by contact angle, interfacial tension, TEM, and SEM measurements. The images of confocal fluorescence microscopy and SEM showed the oil droplet microstructure and the morphology of nanoparticles at the oil droplet−water interface. The average emulsion droplet size and emulsion index were investigated. These results indicated a synergistic stabilization upon rhamnolipid addition. The synergy was even more efficient in the case of seawater with a high salinity. Here, because of the strong flocculation caused by high salinity, silica nanoparticles alone were not an effective emulsifier in seawater. The modification of silica nanoparticles by rhamnolipid changed the contact angle and promoted their adsorption at the oil−seawater interface, which provided an efficient barrier to droplet coalescence. The emulsification of rhamnolipid-modified silica nanoparticles worked well in crude oil−seawater system. So, this could be a new method to deal with the issue of the marine oil spill by environmentally benign silica particles and rhamnolipid. KEYWORDS: Silica nanoparticles, Rhamnolipid, Oil spill, Emulsification, Dispersants, Seawater

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INTRODUCTION Crude oil spill accidents result in significant contamination of the ocean and shoreline environments.1 One remediation technique is the application of dispersants to oil slicks.2 The dispersion of an oil slick in water is essentially a process of emulsification. Application of dispersants can reduce the possibility of shoreline impact, decrease the severity of effects on birds and mammals, and promote the biodegradation of oil.3 The dispersants used for this purpose are usually composed of complex mixtures of surfactants, organic solvents, and additives, such as Corexit 9500A. Although the chemical surfactants are considered to be effective for crude oil dispersion, there are some reports regarding their toxicity.4 So, nontoxic and environmentally benign dispersants, although also having good dispersing ability, would be potential candidates in the treatment of oil spills. For example, soybean lecithin was studied as a dispersant for crude oil spill application recently.5 Since the original work of Ramsden and Pickering,6 considerable progress has been made in the area of Pickering emulsions.7 Pickering emulsions are emulsions stabilized by the adsorption of solid particles (e.g., clay, oxides, carbon, silica, polymer lattices alumina and fat crystals etc.).8−10 Interest in Pickering emulsions has increased over the past 10 years, especially in some application areas where the use of chemical © XXXX American Chemical Society

surfactants can cause negative consequences. The ability to form stable emulsions makes these solid particles an alternative to conventional dispersants for emulsifying crude oil following a spill. More recently, studies on particle-based dispersants that have specific application for emulsifying oil and keeping them stable in water columns in the event of an oil spill have been developed. For example, Saha et al. demonstrated the formation of crude oil-in-seawater emulsions using carbon black particles, without any supplementary acid or salt addition.11 They also illustrated a novel concept of integrating particle stabilization of emulsions together with the release of chemical surfactants from the particles for the development of an alternative, cheaper, and environmentally benign technology for oil spill remediation.12 Dodecane-in-seawater emulsions were stabilized with natural montmorillonite clay microparticles upon increasing the hydrophobicity by adsorption of surfactant bis(2-hydroxyethyl)oleylamine.13 Hydrophilic silica combined with zwitterionic surfactant, caprylamidopropyl betaine, was also found to show greater synergy for emulsion formation and stabilization in the case of a high-salinity synthetic seawater Received: June 9, 2015 Revised: September 18, 2015

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DOI: 10.1021/acssuschemeng.5b00516 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Scheme 1. Chemical Structures of Mono-rhamnolipid (Type I) and Di-rhamnolipid (Type II)

aqueous phase.14 These studies provided an insight for the design of new dispersant systems for remediating surface and subsurface oceanic oil spills. However, the high ionic strength of seawater makes the formation of Pickering emulsions challenging. The substantial screening of electrostatic interactions may produce excess aggregation of particles.13 In contrast, salinities may also provide benefits for Pickering emulsion formation.15 For the extreme ionic strength in seawater, new concepts may be required for modifying the surface properties of solid particles to obtain stable oil-inseawater emulsions. A particularly important class of Pickering emulsifiers is silica nanoparticle, which has been used in many fields. 16 Commercially available silica nanoparticles can be used as excellent Pickering emulsion stabilizers by either ex situ coating or in situ hydrophobization to modify their surface wettability.17 The latter method, based on the adsorption of ionic surfactants on oppositely charged particle surfaces, has advantages that the wettability/surface activity of the particles can be controlled by selection of the concentration and structure of the surfactants. The wettability of particles at the oil−water interface has been shown to be crucial in optimizing Pickering emulsions stability.9 Under appropriate conditions, adsorption of surfactants turns a hydrophilic surface into hydrophobic thus allowing the partial wetting requirement of solid particles.18 Various studies have demonstrated that silica nanoparticles may be modified in situ with surfactants, small surfactant-like molecules and polymers to obtain stable o/w or w/o emulsions.19−21 The adsorption of surfactants can also lead to particle flocculation, which increases the stability of emulsion.22 Recently, we embarked on a study to identify solid particle emulsifiers that are safe, nontoxic, and environmentally benign while also having good dispersing ability against oil spills.23 So in this paper, we focused on using a biosurfactant, rhamnolipid, to modify the surface characteristics of hydrophilic silica nanoparticles to form stable o/w emulsions. Biodegradable biosurfactant would decrease the risk of toxicity already observed with a lot of common chemical surfactants. Rhamnolipid is one of the well-known biosurfactants produced by the bacterium, Pseudomonas aeruginosa. In spite of its wide applications, to our knowledge, relatively little is known about the synergistic stabilization by a mixture of silica nanoparticles and rhamnolipid, especially in solution with high ionic strength. Our results indicated that rhamnolipid played an important role in obtaining Pickering emulsions stabilized by silica nanoparticles in seawater. These studies revealed one promising dispersant candidate formed by a mixture of rhamnolipid and silica nanoparticles.

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Rhamnolipid (purity >90%, Rha-C10C10 and Rha-Rha-C10C10 with a mixing molar ratio of 2:1), as an anionic surfactant, was purchased from Huzhou Zijin Biotech Company (China). The molecular weights of rhamnolipid are 600.05 g/mol, and its critical micelle concentration (cmc) is 20−50 mg/L. The molecular structures of two types of rhamnolipid are shown in Scheme 1. Tetradecane (purity >98%) was purchased from Aladdin (China) and used as the oil phase. The artificial seawater (ASW) consisted of (g/L): NaCl 26.726, MgCl2 2.260, MgSO4 3.248, NaHCO3 0.198, KCl 0.721, CaCl2 1.153, according to the formula of Lyma and Fleming. The pH of ASW water was measured 7.9. Crude oil was obtained from Shengli Oil field, China. The characteristics of crude oil: viscosity 72.9 Pa/s (25 °C, 3 r/ min), freezing point 23.0 °C, density 0.8552 g/cm3. Preparation of Emulsions. Both nanoparticles and rhamnolipid aqueous solution were prepared in advance, and their volume was treated as part of the aqueous-phase volume. Then, the emulsions were prepared by mixing tetradecane or crude oil with the aqueous dispersion. All the emulsions were prepared using an oil-to-water volume ratio of 1:3. After mixing, it was stirred at 11000 rpm for 2 min using an IKAT10 homogenizer instrument. Immediately after emulsification, the emulsion type was determined from the conductivity measurements, and also inferred by observing the outcome of some of emulsions added to either pure oil or pure water. Droplet Size and Emulsification Index Measurements. The emulsions were prepared as above steps and visualized by optical microscopy (Leica DM1000 LED, Leica, Germany). The average oil droplets size was estimated by Nano Measurer software. The emulsification was determined by measuring the initial emulsion droplet size and its time dependence, and the volume of the emulsion layer compared to the total volume of the mixture over a period of 24 h and expressed as an emulsification index (EI). Interfacial Tension. For DI water or ASW-based system, tetradecane-aqueous-phase interfacial tension (γ) values were determined by using drop shape analysis of an aqueous pendant droplet containing a known concentration of rhamnolipid (OCA instrument, Dataphysics ES, Germany). Tetradecane was poured into the slot first and ∼30 μL aqueous dispersant was dropped by a syringe (SNP 241/180), then the droplet shape profile was analyzed to obtain the interfacial tension. Contact Angle Measurements. To produce the silica/ rhamnolipid hybrid particles-coating on the silica wafer, the wafer was placed in a solution containing silica nanoparticles and rhamnolipid overnight. Each wafer was washed with deionized water to remove any excess silica/rhamnolipid particles and then dried using N2 gas stream. The resulting silica/rhamnolipid-coated wafer was immersed in tetradecane phase and a drop of ∼2 μL DI water or ASW was injected with a syringe and captured on the wafer surface. Then the drop shape profile was analyzed according to the Young−Laplace equation to obtain the contact angle (OCA instrument, Dataphysics ES, Germany). Each wafer was measured in triplicate and the mean contact angle was determined. Dynamic Light Scattering. The size of silica particles with different contents of rhamnolipid was measured using a Zeta Nanosizer instrument (Malvern Instruments, UK). All measurements were repeated at least three times. The measurements were performed at a fixed scattering angle of 90°. Transmission Electron Microscopy (TEM). TEM (JEM-2100, JEOL, Japan) was used to visualize the morphology of silica particles

EXPERIMENTAL SECTION

Materials. Silica nanoparticles, LUDOXCL, were purchased from Sigma-Aldrich, with the concentration of 30 wt % suspension in H2O. B

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Figure 1. (a) DI water/tetradecane interfacial tension as a function of rhamnolipid concentration with (●) and without (■) 2.0 wt % silica nanoparticle at pH = 5. (b) Adsorption isotherm of rhamnolipid on silica particles derived from data in panel a by material balance. without or with rhamnolipid. The prepared fresh aqueous dispersion was applied to a carbon-coated copper grid, and then the grid was airdried prior to imaging. Fluorescence Microscopy Analysis. Fluorescent imaging was carried out using a confocal laser scanning microscope (CLSM, Fluo View FV1000, Olympus Corporation, Japan). Rhodamine B ( 6, as silica particles were rendered hydrophobic and flocculation following pH increase, as discussed above. At lower pH values, the silica particles were too hydrophilic to be held at the oil−water interface, and oil droplet coalescence ensued readily, as observed previously with fumed silica nanoparticles.10 It was noticed that with pH > 7 the separated aqueous phase was clear and probably devoid of particles. By contrast, rhamnolipid alone can stabilize emulsions only at a concentration up to 60 mg/L at pH = 6 and no emulsification was observed at pH = 8, as illustrated in Figure S5b,c. Because of the carboxyl groups of the rhamnolipid molecule, acid condition induced an increase in hydrophobicity of rhamnolipid. However, our results indicated that 10 mg/L rhamnolipid alone did not exhibit any emulsification in the range of pH 4−8. Figure 4a indicated that a synergism existed between silica nanoparticles and rhamnolipid in enhancing emulsion stability over the range of pH 4−8 studied. The emulsion phase was stable, which showed a smaller initial droplet size and higher EI value with pH increase. A mixture of 2 wt % silica particles and 10 mg/L rhamnolipid was shown to produce remarkably smaller initial droplet sizes than systems with nanoparticles as a stabilizer alone, as compared in Figure 4b. All emulsions were oil-in-water via conductivity measurements. A substantial synergy thus existed between silica particles and rhamnolipid. When a sufficient amount of silica nanoparticles was added, only a very small amount of rhamnolipid was required to obtain evident synergism in forming stable emulsion. The initial oil droplet size was below 20 μm with pH > 7, and it was the desired oil droplet size for biodegradation by bacteria in the

Figure 2. Contact angle of DI water and ASW drop in tetradecane on silica wafer coated with rhamnolipid/silica particles prepared at various rhamnolipid concentrations.

higher and w/o emulsions are preferred.25 The contact angle of silica particles was 43° in the absence of rhamnolipid in DI water, and it was 41° in ASW. In DI water, the contact angle increased with rhamnolipid concentration, and reached to 85° with only 10 mg/L rhamnolipid present. When θ is close to 90°, that is, the particles at the oil−water interface are balanced and the adsorption is the strongest. This illustrated that the hydrophobicity of silica particles was increased with rhamnolipid concentration in DI water. The increased hydrophobicity was attributed to the rhamnolipid hydrophobic tails extending into the oil phase. However, the contact angle increased and reached to 82° at 10 mg/L rhamnolipid in ASW, then it decreased instead. In the case of ASW, the high salinity caused a significant charge screening of silica nanoparticles, which resulted in a weak positive charge and enhanced hydrophobicity of silica nanoparticles themselves. When rhamnolipid concentration was low, a small amount of rhamnolipid molecules can interact with silica nanoparticles via electrostatic attractions, leading to the hydrophobic groups of rhamnolipid exposed on the particles surface. So, the three-phase contact angle was enhanced. In this situation, the hydrophobic forces also existed between silica nanoparticles and rhamnolipid. However, the hydrophobic forces were relatively weak comparing with electrostatic attraction forces. With rhamnolipid concentration increased further, they interacted with the almost uncharged silica particles via hydrophobic forces. This led to hydrophilic groups of rhamnolipid exposed on the particle surface. So, the hydrophobicity of silica particles decreased instead. Other studies have reported similar variance in contact angle with added surfactant molecules in DI water and ASW.14 But they

Figure 3. TEM images of 2.0 wt % silica nanoparticle dispersion without (a) and with (b) rhamnolipid (10 mg/L) in DI water, pH = 5. (c) Intensity against the size of silica particles (2.0 wt %) with different rhamnolipid concentrations. D

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Figure 4. (a) Variance of initial droplet size and EI for emulsions of tetradecane-in-DI water stabilized by silica particles (2.0 wt %) and rhamnolipid (10 mg/L) mixture with pH increase. (b) Optical microscopy images of tetradecane-in-DI water emulsions stabilized by 2.0 wt % silica particles without (upper) and with (bottom) 10 mg/L rhamnolipid at different pH values. All scale bars are 200 μm.

Figure 5. (a) Droplet size distributions of tetradecane-in-DI water and tetradecane-in-ASW emulsions, respectively. Photograph and optical microscopy images of vessels containing emulsions of tetradecane-in-DI water at pH = 8 (b) and tetradecane-in-ASW (c). Emulsions were prepared with silica particles at concentrations of 2.0 wt %. All scale bars are 200 μm.

In addition, rhamnolipid-induced particle flocculation was also responsible for the synergy. For turbulent flow in a rotorstator mixer for emulsification, the hydrodynamic driving force Fmix between a particle and an oil droplet can be estimated from the following expression:26

ocean.11 The time stability for emulsions stabilized by silica alone and mixture of silica and rhamnolipid was compared at pH = 8, as shown in Figure S6. It indicated that the average droplet size and EI value changed little over time within 2 months. In contrast to the emulsion stabilized by silica nanoparticles alone, more excellent time stability was obtained for silica/rhamnolipid mixture. Phase separation was even not observed when the emulsions were centrifuged for 20 min at 12000 rpm. The coalescence and creaming stability of emulsion depends on the size of oil drops, the viscosity of the continuous phase, and the dispersed phase volume fraction. Despite of the contribution of particles flocculation caused by pH increase, rhamnolipid was especially effective at producing smaller drop sizes and long-term emulsion stability, presumably because of hydrophobic modification of silica surface and enhanced barriers. Here, flocculated rhamnolipid-modified nanoparticles in the thin films between approaching emulsion droplets slowed down droplet coalescence and creaming. It is known that nanoparticles can slow film rupture by forming ordered structures to provide an effective barrier to droplet coalescence. Rhamnolipids are anionic surfactants in which one or two molecules of rhamnose are linked to one or two molecules of βhydroxydecanoic acid, abbreviated as monorhamnolipid and dirhamnolipid. Such a structure of rhamnolipid explained the effective emulsion stabilization. The steric hindrance resulting from unique molecular structure of rhamnolipid coated on silica particles resulted in increased barriers at the droplet surface further, which decreased droplet size and slowed droplet approach. Rhamnolipid also contributed to a modest increase in aqueous-phase viscosity for emulsions formed in DI water, as shown in Figure S7, which slowed both droplet approach and film drainage.

Fmix ≈ a 2ρc ε 2/3R2/3

where a is the particles radius, R is the oil droplet radius, ρc is the continuous-phase density, and ε is the rate of energy dissipation per unit mass. The increased average size of silica particles with rhamnolipid concentration (Figure 3c) led to an increased Fmix and promoted their adsorption to droplet interface. The droplet size of emulsions prepared from different concentrations of silica particles or rhamnolipid was investigated. It was found that as the particle concentration increased, an emulsion with higher EI value and smaller oil droplet size was formed. In the case of a high amount of silica nanoparticles, a larger interfacial area can be stabilized, so that smaller oil droplets should result. An obvious decrease in droplet size was also observed with rhamnolipid concentration increase in DI water, as shown in Figure 5a. This effect can be associated with the increased particle wettability with rhamnolipid and barrier on the oil droplet surface.27 In DI water, the emulsion droplet size at pH = 8 was lower than that of emulsions at pH = 6, reflecting that the flocculated particles adsorbed at drop interfaces, facilitating the emulsification. Oil droplet size, macroscopic and microscopic observations of the emulsions formed in DI water and ASW are also compared (Figure 5). With the presence of silica nanoparticles alone, stable oil-in-DI water emulsions could be achieved at pH = 8. However, silica nanoparticles alone were ineffective emulsifiers E

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ACS Sustainable Chemistry & Engineering in the case of ASW, as shown in Figure 5c. The sedimentation of silica particles was observed in the aqueous phase at the bottom of the bottle. The high salinity caused screening of electrostatic repulsion between the nanoparticles, as well as the repulsion of a nanoparticle with the anionic oil-seawater interface. These changes will be shown to raise the thermodynamic driving forces for particle adsorption at the oil−water interface, while simultaneously lowering the electrostatic kinetic barrier between oil droplets. It had been found that the electrostatic barrier disappears above an ionic strength of 500 mM (NaCl) for 200 nm diameter latex nanoparticles approaching a tetradecane−water interface and was expected to be negligible in high-salinity ASW for silica particles.28 Therefore, effective emulsification was not obtained by silica particles alone in ASW, as their greatly strong flocculation and the disappearance of electrostatic repulsive barrier between droplets. The sedimentation of silica nanoparticles in aqueous phase was observed below 40 mg/L rhamnolipid in Figure 5c. However, with higher rhamnolipid concentration, the sedimentation in ASW disappeared gradually. Remarkably, stable emulsion was formed with only 10 mg/L rhamnolipid present, though the EI value was lower. The emulsion with 20 μm initial average droplet size was observed in the presence of 40 mg/L rhamnolipid, despite the relatively high interfacial tension, as shown in Figure S4a. Compared to the rhamnolipid alone, the relatively high interfacial tension indicated that most rhamnolipid molecules were strongly adsorbed on silica particles. Thus, when the emulsions were prepared in ASW, the key role of rhamnolipid was not lowering interfacial tension obviously but to modify the wettability of particles and prevent growth of droplets during shear. The lack of silica nanoparticles in the resolved aqueous phase for emulsions prepared in ASW with 60 mg/L rhamnolipid indicated the adsorption of flocculated particle at the oil−ASW interface. The emulsification was improved with the aid of rhamnolipid. The compatibility of the silica particles with rhamnolipid allowed more nanoparticles to remain at the droplet interfaces, and thus, particle-free aqueous phase drained from the emulsion. These results showed the crucial role of rhamnolipid in obtaining stable emulsions in ASW. The rhamnolipid concentration used here was far below the values commonly used in other studies.29,30 And this result was different from bis(2-hydroxyethyl) oleylamine modified montmorillonite clay microparticles for stable oil-in-seawater emulsions.13 Their results showed that high salinity facilitated initial formation of small droplets during emulsification process. However, in our studies, high salinity in ASW led to more rhamnolipid was required to obtain emulsion with similar initial size and stability. We used the results above to guide the work on the emulsification of crude oil in ASW, a specific application studied here. The emulsion production steps were the same as above, just changing the tetradecane to crude oil. With rhamnolipid increase, the images of crude oil-in-ASW emulsion and initial droplet size are shown in Figure 6. Silica nanoparticles alone were not an effective emulsifier for crude oil in ASW system. Similarly, rhamnolipid alone cannot stabilize emulsions in the range of concentrations studied. However, silica/rhamnolipid mixture produced an emulsion with a longer stability and smaller droplet size. The graphs in Figure 6 showed white flocs in the bottom phase obviously for the system with lower rhamnolipid concentration, which implied that most silica particles were not acting as an effective emulsifier at this condition. With increase in rhamnolipid, the

Figure 6. Photograph of crude oil-in-ASW emulsion stabilized by 2 wt % silica nanoparticles/rhamnolipid and initial droplet size variance with rhamnolipid concentration.

separated aqueous phase below the cream was clear and devoid of particles, even in the presence of only 10 mg/L rhamnolipid. So, emulsification was enhanced greatly for the crude oil-inASW system due to the presence of rhamnolipid. With 60 mg/ L rhamnolipid present, the initial average droplet size obtained was 15 μm. The crude oil-in-ASW emulsion was observed for 2 months in our lab, during which it was found to be very stable. In summary, when silica nanoparticles were combined with rhamnolipid, as low as 60 mg/L of rhamnolipid corresponding to only a small fraction of the mass of silica, was required to achieve the desired emulsification efficiency for crude oil in ASW. Emulsion Characterization. CLSM offers an opportunity to observe nanoparticles distribution in a 3D system. The corresponding images for tetradecane-in-DI emulsion stabilized by silica alone and silica/rhamnolipid mixture are illustrated in Figure 7. The bright rings in the image showed the presence of

Figure 7. Images of the emulsion by confocal fluorescence microscopy. Emulsions were prepared by silica (2.0 wt %) without (a) or with (b) rhamnolipid (10 mg/L) in DI water, pH = 8. All scale bars are 50 μm.

fluorescently labeled silica nanoparticle around the oil droplets. In the presence of rhamnolipid, a thicker round-shaped bright fluorescence of silica nanoparticles was observed at the perimeter of the emulsion, which indicated that more nanoparticles were adsorbed at the interface. The addition of rhamnolipid resulted in smaller oil droplets according to CLSM results, which was similar to the results observed using optical microscopy in Figure 4. Although cryo-SEM would allow direct observation of oil in water Pickering emulsion, polymerizable resins offer convenient alternative for oil in water emulsion using the classical SEM approach. Silica nanoparticles showed the same ability to stabilize styrene droplets, and therefore, styrene was selected. Despite this process being different from tetradecane, the silica F

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Various factors contributed to the observation that silica/ rhamnolipid mixture was an efficient emulsifier in ASW: (1) the flocculated nanoparticles partition to the aqueous side of the oil/ASW interface caused by rhamnolipid adsorption (the reduced contact angle in ASW with rhamnolipid increase shown in Figure 2 and the changed flocculation morphology shown in Figure S3d), (2) some free rhamnolipid molecules may freely adsorb to the oil-ASW interface and reduce the interfacial tension (Figure S4), though it was not remarkable, (3) the greater steric barrier to droplet coalescence provided by rhamnolipid-modified silica particles, (4) the increase in aqueous-phase viscosity and enhanced elastic modulus G′ in ASW is expected to further slow droplet approach and provide a more robust physical barrier to droplet coalescence, as shown in Figure S8.

nanoparticles showed analogous interfacial properties for the styrene-in-water emulsion. The polymerization step served just as a fixing step of the beads formed. SEM images of the oil-inwater emulsion showed the morphology of the oil droplet− water interface. Figure 8 illustrates that the polystyrene droplets

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CONCLUSIONS In this paper, we have demonstrated that stable tetradecane-inwater (or in ASW) and crude oil-in-ASW emulsions were formed and stabilized using positively charged hydrophilic silica nanoparticles modified in situ by anionic biosurfactant at usually low concentrations. The silica nanoparticles and rhamnolipid acted synergistically to produce emulsions that had smaller oil droplets and greater stability to coalescence than emulsions stabilized by either nanoparticles or rhamnolipid alone. Strong flocculation caused by high salinity in ASW led to silica nanoparticle alone being ineffective in stabilizing emulsions. However, the synergy between silica particles and rhamnolipid was greater with ASW as the aqueous phase. The interaction between rhamnolipid and the flocculated nanoparticles decreased the contact angle of particles and promoted their partition to the oil/ASW interface. Once adsorbed, the flocculated rhamnolipid-modified particles provided a greater steric barrier to droplet coalescence. The slightly decreased interfacial tension in ASW also contributed to the synergism. Additionally, highly stable crude oil-in-ASW emulsions were achieved with addition of 2 wt % silica and only 60 mg/L rhamnolipid. So, it could be an excellent method to deal with the issue of the ocean oil spill by environmentally benign silica particles and rhamnolipid.

Figure 8. SEM images of emulsions of styrene−DI water (1:3) stabilized by silica particles at concentration of 2.0 wt % without (a, b) or with (c, d) rhamnolipid at concentrations of 10 mg/L in DI water, pH = 8.

were covered by flocculated silica nanoparticles in the absence and presence of rhamnolipid, respectively. It appeared that the interfaces of drops were covered with a dense layer of flocculated silica particles, which resulted in rough droplet interfaces. A more solid-like film at the droplet interfaces was observed. For the emulsions stabilized by a silica/rhamnolipid mixture, the droplet possessed an interface where silica particles were present as more compact flocs. This is more easily observed in Figure S3. Such flocs were harder to displace from oil−water interfaces than individual silica particles because the displacement energy scales with the square of the particles size, and provide additional stability for the emulsions.11 SEM observations also revealed that the drop interfaces were not fully covered with particle flocs. Indeed, there were several reports showing microscopy pictures of stable Pickering emulsion droplets under incomplete coverage of the droplets by solid particles.31,32 Synergistic Emulsification Mechanism. Over the range of pH 4−8 studied in DI water, the interactions between silica and rhamnolipid showed synergism in the formation and stabilization of oil-in-DI water emulsions. In DI water with pH > 7, even silica particle alone was a good emulsifier. With sufficient nanoparticles and rhamnolipid, the binary mixture was shown to produce smaller initial droplet sizes than systems with either particles or rhamnolipid alone. The synergy in DI water was shown to result from a combination of the weak flocculated particles, enhanced wettability, reduced oil−water interfacial tension, and the increased barrier to droplet coalescence provided by rhamnolipid molecules. In the case of ASW, high salinity was shown to produce great particle flocculation (this is different from the flocculation caused by pH increase as shown in Figure S3), which led to an ineffective emulsification at forming an emulsion just with silica particles alone. Addition of rhamnolipid was especially effective in producing stable emulsions and smaller droplet size in ASW.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b00516. Rheological experiments; the photograph of the dispersions contained silica nanoparticles or rhamnolipid, as well as their mixture, in DI water and ASW, respectively; SEM images of dried sedimentation; ASW/tetradecane interfacial tension; time dependence of EI value and oil droplet size for tetradecane-in-DI water emulsions; steady shear viscosity and viscoelastic modulus measurement (PDF).

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AUTHOR INFORMATION

Corresponding Author

*Y. M. Li. E-mail: [email protected]. Notes

The authors declare no competing financial interest. G

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stability of submicron oil-in-water emulsions. Phys. Chem. Chem. Phys. 2007, 9, 6426−6434. (22) Binks, B. P.; Desforges, A. Synergistic stabilization of emulsions by a mixture of surface-active nanoparticles and surfactant. Langmuir 2007, 23, 1098−1106. (23) Gong, H. Y.; Li, Y. M.; Bao, M. T.; Lv, D.; Wang, Z. N. Petroleum hydrocarbon degrading bacteria associated with chitosan as effective particle-stabilizers for oil emulsification. RSC Adv. 2015, 5, 37640−37647. (24) Kalashnikova, I.; Bizot, H.; Cathala, B.; Capron, I. New Pickering emulsions stabilized by bacterial cellulose nanocrystals. Langmuir 2011, 27, 7471−7479. (25) Binks, B. P.; Isa, L.; Tyowua, A. T. Direct measurement of contact angles of silica particles in relation to double inversion of pickering emulsions. Langmuir 2013, 29, 4923−4927. (26) Mohan, S.; Narsimhan, G. Coalescence of protein-stabilized emulsions in a high-pressure homogenizer. J. Colloid Interface Sci. 1997, 192, 1−15. (27) Pichot, R.; Spyropoulos, F.; Norton, I. T. Mixed-emulsifier stabilised emulsions: Investigation of the effect of monoolein and hydrophilic silica particle mixtures on the stability against coalescence. J. Colloid Interface Sci. 2009, 329, 284−291. (28) Golemanov, K.; Tcholakova, S.; Kralchevsky, P. A.; Ananthapadmanabhan, K. P.; Lips, A. Latex-particle-stabilized emulsions of anti-bancroft type. Langmuir 2006, 22, 4968−4977. (29) Garcia-Junco, M.; Gomez-Lahoz, C.; Niqui-Arroyo, J. L.; Ortega-Calvo, J. J. Biosurfactant- and biodegradation-enhanced partitioning of polycyclic aromatic hydrocarbons from nonaqueousphase liquids. Environ. Sci. Technol. 2003, 37, 2988−2996. (30) Zhu, L.; Zhang, M. Effect of rhamnolipids on the uptake of PAHs by ryegrass. Environ. Pollut. 2008, 156, 46−52. (31) Horozov, T. S.; Binks, B. P. Particle-stabilized emulsions: a bilayer or a bridging monolayer? Angew. Chem., Int. Ed. 2006, 45, 773− 776. (32) Destribats, M.; Ravaine, S.; Heroguez, V.; Leal-Calderon, F.; Schmitt, V. Outstanding stability of poorly-protected Pickering emulsions. Prog. Colloid Polym. Sci. 2010, 137, 13−18.

ACKNOWLEDGMENTS This research is supported by the Natural Science Foundation of Shandong Province (ZR2014DQ026), the Applied Basic Research Programs of Qingdao in China (14-2-4-119-jch), and National Natural Science Foundation of China (41376084). This is MCTL contribution No. 82.

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DOI: 10.1021/acssuschemeng.5b00516 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX