Destabilization of Oil-in-Water Emulsions Stabilized by Non-ionic

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Destabilization of Oil-in-Water Emulsions Stabilized by Nonionic Surfactants: Effect of Particle Hydrophilicity Hari Katepalli, Arijit Bose, T. Alan Hatton, and Daniel Blankschtein Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b03289 • Publication Date (Web): 15 Sep 2016 Downloaded from http://pubs.acs.org on September 19, 2016

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Destabilization of Oil-in-Water Emulsions Stabilized by Nonionic Surfactants: Effect of Particle Hydrophilicity Hari Katepalli†, Arijit Bose‡, T. Alan Hatton†*, and Daniel Blankschtein†* †

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

‡

Department of Chemical Engineering, University of Rhode Island, Kingston, RI 02881

*

Corresponding authors: [email protected], 617-253-4594; [email protected], 617-253-4588

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Abstract We investigate the use of particle hydrophilicity as a tool for emulsion destabilization in Triton X-100 stabilized hexadecane-in-water emulsions. The hydrophilicity of the particles added to the aqueous phase was found to have a pronounced effect on the stability of the emulsion. Specifically, addition of hydrophilic fumed silica particles to the aqueous phase resulted in coarsening of the emulsion droplets, with droplet flocculation observed at higher particle concentrations. On the other hand, when partially-hydrophobic fumed silica particles were added to the aqueous phase, coarsening of the emulsion droplets was observed at low particle concentrations, and phase separation of oil and water was observed at higher particle concentrations. Surface tension and interfacial tension measurements showed significant depletion of surfactant from the aqueous phase in the presence of the partially-hydrophobic particles. The observed changes in the stability of the emulsion and the depletion of surfactant can be rationalized in terms of changes in the adsorption behavior of the surfactant molecules, from one dominated by hydrogen bonding on hydrophilic particles to one dominated by hydrophobic interactions on partially-hydrophobic particles. Our findings also provide, for the first time, an in-depth understanding of antagonistic (destabilizing) effects in mixtures of partially-hydrophobic particles and nonionic surfactant (Triton X-100) in water. Key Words: Surfactants, silica particles, emulsions, emulsion destabilization, antagonistic effects.


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Introduction Destabilization of emulsions is an essential step in various industrial applications such as enhanced-oil recovery, mineral processing, and pollution control.1 Presently, methods to destabilize emulsions include the use of chemical additives,2, separation,4,

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centrifugation, membrane

and the application of electric6 and acoustic fields.7 Unfortunately, the use of

chemical additives results in contamination of the treated water, and the use of centrifugal force is limited to small scales. Furthermore, the majority of these methods are energy intensive. Here, we report on the destabilization of oil-in-water emulsions stabilized by nonionic surfactant (Triton X-100) using colloidal particles that can be removed readily by filtration, thus overcoming these limitations. Surfactants are amphiphilic molecules with a natural affinity for the oil-water interface. They reduce the oil-water interfacial tension, thereby lowering the energy required for the formation of emulsions. Once droplets are created by agitation, surfactant molecules adsorbed at the oil-water interfaces also provide steric or electrostatic barriers against droplet coalescence.8, 9 Destabilization of emulsions involves the following three steps --- flocculation, coalescence of emulsion droplets, and separation of the two immiscible liquids (in this case, oil and water) into two bulk phases. An effective emulsion destabilizer should be able to deplete surfactant molecules from the continuous phase as well as from the oil-water interfaces to induce flocculation and coalescence of the emulsion droplets. Colloidal particles can serve as effective emulsion destabilizers due to their tunable surface properties, including specific surface area, charge, and wettability.10,

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Adsorption of surfactant molecules onto the particle surfaces can

lead to surfactant depletion from both the oil-water interfaces associated with the emulsion droplets as well as from the continuous phase. This can lead to an increase in the oil-water


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interfacial tension, and to a lowering of the steric or electrostatic barriers between the emulsion droplets. Removal of surfactant molecules from droplet interfaces also enhances the film drainage between adjacent emulsion droplets, thereby enhancing the phase separation of oil and water. Nonionic surfactants can interact with particle surfaces through hydrogen bonding or through hydrophobic interactions depending on the hydrophilicity of the particle surfaces in water.12 In particular, head groups of nonionic surfactants (long-chain fatty acids, alcohols, and polyoxyethylenated alcohols) can form hydrogen bonds with different functional groups (such as –OH, -NH, and -COOH) on hydrophilic particle surfaces (Si-OH groups on silica surfaces).8, 13 On the other hand, if the particle surfaces are partially-hydrophobic, the nonionic surfactants can adsorb on the particles through both hydrogen bonding and hydrophobic interactions. The dominant mode of adsorption has a pronounced effect on the hydrophilicity of the particles and the depletion of the surfactant from the aqueous phase. This, in turn, has significant impact on emulsion formation and stability. The synergistic effects of hydrophilic and hydrophobic silica particles with nonionic surfactants on the stabilization of oil-in-water or water-in-oil emulsions are well documented in the literature.14,

15, 16, 17, 18

It is also known that partially-hydrophobic

particles are capable of forming particle-stabilized emulsions19,

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However, there are no

literature reports on the properties of aqueous mixtures of non-ionic surfactants and partiallyhydrophobic silica particles and their subsequent effects on emulsion stabilization or destabilization. Here, we investigate the effect of the controlled addition of an aqueous suspension of fumed silica particles with different hydrophilicities to Triton X-100 stabilized emulsions. In our studies, we chose not to utilize highly-hydrophobic particles for the following two reasons: (i)


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hydrophobic particles would not disperse in water, but instead, would either agglomerate, sediment, or form a space-filling immobile viscous network,22 and (ii) it is important to use conditions where the particles are mobile and not aggregated or interconnected, which will ensure that all their surfaces are available for surfactant adsorption. This, in turn, will maximize the response of the emulsions to the addition of particles that interact with the surfactant that is used to stabilize the emulsion. Fumed silica particles were chosen because of their high surface area and tunable surface chemistry. Changes to the emulsion stability upon addition of particle suspensions were monitored visually, as well as by optical and cryogenic scanning electron microscopy. We found that partially-hydrophobic fumed silica particles are very effective at destabilizing oil-in-water emulsions stabilized with nonionic surfactants. Our results provide a comprehensive understanding of the interactions between partially-hydrophobic particles and nonionic surfactants in water, including their antagonistic properties, an understanding that has not been reported previously in the literature. Materials and Methods Triton X-100 (99% purity, TX-100, critical micelle concentration (CMC) = ~0.2-0.3 mM23,

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), and hexadecane (99% purity) were purchased from Sigma-Aldrich and used as

received. Fumed silica particles Aerosil 200 (A200) and Aerosil R816 (R816) were provided by Evonik Corporation. The Aerosil 200 fumed silica particles are manufactured by the flame hydrolysis of silicon tetrachloride (SiCl4) in the presence of H2 and O2, and the presence of a large number of surface silanol groups makes the particles hydrophilic. The Aerosil R816 particles

are

prepared

via

the

surface

silanization

of

the

A200

particles

with

hexadecyltrimethoxysilane, making them partially hydrophobic.25, 26 The specific surface areas


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of the A200 and R816 particles, reported by the manufacturer, are ~200±25 m2/g and ~190±20 m2/g, respectively. Both A200 and R816 can be dispersed in water. Particle dispersions were prepared using ultra-sonication, by dispersing a known weight of fumed silica powder in the water. Deionized water (18.2 MΩ.cm) was used for the preparation of all surfactant solutions and particle suspensions. Hexadecane-in-water emulsions were prepared with 0.4 mM Triton X-100 in water as follows: 0.2 mL hexadecane was added to 1 mL of the surfactant aqueous solution, and then vortexed at 3000 rpm for 1 min to form the base emulsion. Destabilization of the emulsions was investigated by dilution of the emulsions with silica particle suspensions of volume equal to that of the aqueous phase in the base emulsion (1mL). The final surfactant concentration was 0.2 mM. The emulsions and the particle suspensions were mixed very gently to avoid foaming or creation of any new oil−water interfaces. The final silica particle concentration in the aqueous phase was varied between 0.05 and 1 wt %. To differentiate between the actual effects of the particles and of water dilution, the surfactant stabilized emulsions were also diluted with an equal volume of deionized water. The resulting emulsions were analyzed using bright-field optical microscopy. A Zeiss AxioPlan2 microscope equipped with a CCD camera was used to observe the emulsion droplets. Images of the droplets were processed with Image-J software27 to obtain average droplet sizes. Surfactant adsorption onto the particle surfaces was measured by monitoring the depletion of surfactant from the aqueous phase at different concentrations of added particles. Surface tension measurements were carried out to determine surfactant depletion as described next. Aqueous mixtures of 0.2 mM Triton X-100 and various particle concentrations (in wt %) were prepared and equilibrated for 24hrs. Subsequently, the mixtures were centrifuged to separate the particles,


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and the supernatant was used for the depletion measurements.14,

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Surfactant depletion was

estimated from a rise in the surface tension of the supernatant solution at the air-water interface. In addition, hexadecane-water interfacial tensions were measured to study the depletion of surfactant from the emulsion oil-water interfaces in the presence of added particles. Pendantdrop experiments were carried out using a KRÜSS Easy Drop goniometer to measure surface and interfacial tensions. Cryogenic Scanning Electron Microscopy (Gatan Alto 2500 Cryosystem attached to a Zeiss Sigma field emission scanning electron microscope) was used to investigate the structure of the emulsion droplets after the addition of fumed silica particles.

Results and Discussion Base Emulsion Figure 1 shows an optical micrograph of a hexadecane-in-water emulsion prepared by vortexing a mixture of 0.2 ml hexadecane and 1ml 0.4 mM Triton X-100 in water. The average emulsion droplet size was ~6 µm with a polydispersity of ~0.3, and the emulsion remained stable with no measurable change in average drop diameter during the experimental time period (three days).

Dilution Experiments Dilution with water had no effect on emulsion stability and the average droplet diameter remained constant without a significant change. Figures 2a and 2b show images of the Triton X100 stabilized emulsions after dilution with hydrophilic (A200) and partially-hydrophobic (R816) fumed silica particles. No visible phase separation of the oil and water phases was noted upon addition of the hydrophilic particles. An increase in the average droplet diameter was observed with an increase in particle concentration in the aqueous phase (Figure 2c).


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Flocculation of the emulsion droplets was also observed at a particle concentration of 1 wt % in the aqueous phase (Figure 2d). On the other hand, with partially-hydrophobic fumed silica particles in the aqueous phase, coarsening of the emulsion droplets occurred at concentrations below 0.5 wt % followed by visible phase separation of oil and water phases above 0.5 wt %. These changes in emulsion stability can be attributed to the change in the adsorption behavior of the surfactant molecules onto the particle surfaces depending on their relative hydrophilicities. Nonionic surfactants can interact with metal oxide surfaces by forming hydrogen bonds with OH groups on the particle surfaces, or through hydrophobic interactions if the particles are hydrophobic. A200 particle are hydrophilic and the only mode of interaction between the nonionic surfactant molecules and the particles is through hydrogen bonding with surface silanol groups.29, 30 Figures 3(a) and 3(b) clearly show that the surface and interfacial tensions of the surfactant supernatant increased with increasing particle concentration, suggesting that surfactant molecules were depleted from both the aqueous phase and the oil-water interfaces in the presence of the hydrophilic fumed silica particles. The loss of stabilizing surfactant, in turn, led to a coarsening of the emulsion droplets,31 but the surfactant depletion was not sufficient to induce oil-water phase separation. In addition, hydrogen bonding interactions between the fumed silica particles and the nonionic surfactant molecules adsorbed on the surfaces of the adjacent emulsion droplets led to the flocculation of the emulsion droplets. Aggregation of the fumed silica particles was also observed at high particle concentrations due to surfactant-induced flocculation.14, 32 The cryo-SEM image of the emulsion droplets (Figure 4) reveals the existence of particle bridges in the bulk and between the emulsion droplets at higher particle concentrations, and supports the above hypothesis. R816 particles are partially hydrophobic. The coarsening of oil droplets and the phase


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separation of oil and water phases at higher particle concentrations are consistent with the transfer of a significant number of surfactant molecules from both the oil-water interfaces and the aqueous phase to the particle surfaces. Interfacial tension measurements also confirmed that the depletion of Triton X-100 was greater in the presence of the R816 particles than in the presence of the A200 particles (Figures 3(a) and 3(b)), confirming a significant adsorption of surfactant molecules onto the particle surfaces. This led to a large increase in oil-water interfacial tension and to a lowering of the steric barriers created by surfactant molecules adsorbed at the droplet interfaces. Depletion of surfactant also enhanced the film drainage between emulsion droplets, thereby driving the phase separation of oil and water at high R816 particle concentrations. The effectiveness of the R816 particles as emulsion destabilizers was also confirmed by attempts to re-emulsify the phase-separated oil and water samples. Rapid phase separation of oil and water was observed following vortexing of the mixtures (Figure 5), clearly demonstrating that neither the particles nor the nonionic surfactant molecules were able to stabilize emulsions. Indeed, the depletion of nonionic surfactant molecules from the aqueous phase by adsorption onto the particle surfaces was responsible for this inability of the surfactants to stabilize emulsions. Moreover, while partially hydrophobic R816 particles on their own can stabilize oilin-water emulsions.19, 33 they lose their ability to do so in the presence of Triton X-100. This observation is consistent with the notion that hydrophobic interactions are dominant between the nonionic surfactant tails and the hydrophobic portions of the surfaces of the R816 particles, rendering the particles more hydrophilic, and therefore less prone to adsorption at the droplet surfaces. If the nonionic surfactant molecules adsorb onto the particle surfaces through hydrogen


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bonding, the surfactant tails remain exposed to water, thereby making the particle surfaces hydrophobic (Figure 6a).34 On the other hand, if the nonionic surfactant molecules adsorb onto the particle surfaces through hydrophobic interactions, the surfactant heads remain exposed to water, thereby making the particles hydrophilic (Figure 6b).13 In aqueous solutions, the free energy of the system is lowered to a greater extent when surfactant tails associate through hydrophobic interactions with partially-hydrophobic particle surfaces than when the surfactant heads form hydrogen bonds with hydroxyl groups on the particle surfaces.35, 36 This makes the hydrophobic attractions between the nonionic surfactant molecules and the partially-hydrophobic particle surfaces more favorable than the corresponding hydrogen bonding interactions, and leads to a greater depletion of surfactant from the aqueous phase. Therefore, when mixed with nonionic surfactant (Triton X-100), the hydrophobic attractions between the partiallyhydrophobic particle surfaces and the nonionic surfactant tails in water leads to the previously unobserved antagonistic effects that have been emphasized in this article. Conclusions We studied the effect of particle hydrophilicity on the destabilization of Triton X-100 stabilized emulsions. The interactions between the particles and the surfactant molecules were tuned by changing the hydrophilicity of the silica particles. Complete phase separation of oil and water was achieved by the addition of partially-wettable particles to Triton X-100 stabilized emulsions. This effect was attributed to the change in the adsorption behavior of the surfactant molecules onto the particle surfaces, from hydrogen bonding on hydrophilic particles to hydrophobic interactions on partially-hydrophobic particles. The antagonistic properties of aqueous mixtures of partially-hydrophobic silica particles and Triton X-100 are highlighted. The results presented here also emphasize the qualitative differences in the interactions between a nonionic surfactant


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(Triton X-100) and colloidal particles with different hydrophilicities in water, including their subsequent effects on emulsion formation or stability. Although we examined a specific nonionic surfactant (Triton X-100) and a set of two different particles, our observations can be generalized to other surfactant–particle mixtures.

Acknowledgement We thank Evonik Corporation for providing fumed silica particles. We would also like to thank P. Brown and A. G. Rajan for useful comments.


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Dispersants. Industrial & Engineering Chemistry Research 2015, 54 (16), 4243-4251. 19. Whitby, C. P.; Fornasiero, D.; Ralston, J. Effect of adding anionic surfactant on the stability of Pickering emulsions. Journal of Colloid and Interface Science 2009, 329 (1), 173181. 20. Binks, B. P.; Lumsdon, S. O. Influence of Particle Wettability on the Type and Stability of Surfactant-Free Emulsions. Langmuir 2000, 16 (23), 8622-8631. 21. Binks, B. P. Particles as surfactants—similarities and differences. Current Opinion in Colloid & Interface Science 2002, 7 (1–2), 21-41. 22. Rosen, M. J. Surfactants and Interfacial Phenomena; John Wiley & Sons, Inc: USA, 1978. 23. Tiller, G. E.; Mueller, T. J.; Dockter, M. E.; Struve, W. G. Hydrogenation of Triton X100 eliminates its fluorescence and ultraviolet light absorption while preserving its detergent properties. Analytical Biochemistry 1984, 141 (1), 262-266. 24. Zhao, J.; Brown, W. Comparative Study of the Adsorption of Nonionic Surfactants: Triton X-100 and C12E7 on Polystyrene Latex Particles Using Dynamic Light Scattering and Adsorption Isotherm Measurements. The Journal of Physical Chemistry 1996, 100 (9), 37753782. 25. Degussa Technical Bulletins: Basic Characteristics of Aerosol (No. 11); Degussa Corp.: Akron, OH2006. 26. Yan, N.; Maham, Y.; Masliyah, J. H.; Gray, M. R.; Mather, A. E. Measurement of Contact Angles for Fumed Silica Nanospheres Using Enthalpy of Immersion Data. Journal of Colloid and Interface Science 2000, 228 (1), 1-6. 27. Abràmoff, M. D.; Magalhães, P. J.; Ram, S. J. Image processing with ImageJ. Biophotonics international 2004, 11 (7), 36-42. 28. Binks, B. P.; Rodrigues, J. A.; Frith, W. J. Synergistic Interaction in Emulsions Stabilized by a Mixture of Silica Nanoparticles and Cationic Surfactant. Langmuir 2007, 23 (7), 3626-3636. 29. Tiberg, F. Physical characterization of non-ionic surfactant layers adsorbed at hydrophilic and hydrophobic solid surfaces by time-resolved ellipsometry. Journal of the Chemical Society, Faraday Transactions 1996, 92 (4), 531-538. 30. Tiberg, F.; Brinck, J.; Grant, L. Adsorption and surface-induced self-assembly of surfactants at the solid–aqueous interface. Current Opinion in Colloid & Interface Science 1999, 4 (6), 411-419. 31. Katepalli, H.; Bose, A. Response of Surfactant Stabilized Oil-in-Water Emulsions to the Addition of Particles in an Aqueous Suspension. Langmuir 2014, 30 (43), 12736-12742. 32. Alexeev, V. L.; Ilekti, P.; Persello, J.; Lambard, J.; Gulik, T.; Cabane, B. Dispersions of Silica Particles in Surfactant Phases. Langmuir 1996, 12 (10), 2392-2401. 33. Whitby, C. P.; Fischer, F. E.; Fornasiero, D.; Ralston, J. Shear-induced coalescence of oil-in-water Pickering emulsions. Journal of Colloid and Interface Science 2011, 361 (1), 170177. 34. Zdziennicka, A.; Szymczyk, K.; Jańczuk, B. Correlation between surface free energy of quartz and its wettability by aqueous solutions of nonionic, anionic and cationic surfactants. Journal of Colloid and Interface Science 2009, 340 (2), 243-248. 35. Grant, L. M.; Ederth, T.; Tiberg, F. Influence of Surface Hydrophobicity on the Layer Properties of Adsorbed Nonionic Surfactants. Langmuir 2000, 16 (5), 2285-2291. 36. Israelachvili, J. Intermolecular and Surface Forces, Third Edition; Academic Press2010.


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Figure 1. Optical microscopy image of a hexadecane-in-water emulsion stabilized with 0.4 mM Triton X-100 in water.


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Figure 2. Images of Triton X-100 stabilized hexadecane-in-water emulsions in the presence of increasing concentrations (in wt %) of added hydrophilic and partially-hydrophobic fumed silica particles in the aqueous phase: (a) A200 (i) 0, (ii) 0.05, (iii) 0.1, (iv) 0.5, and (v) 1; creaming of the emulsion droplets with no visible phase separation of oil and water. (b) R816 (i) 0, (ii) 0.05, (iii) 0.1, (iv) 0.5, and (v) 1; creaming of the emulsion droplets with no visible phase separation of oil and water below 0.5 wt % of R816 in the aqueous phase (see (i) to (iii)), and visible phase separation of oil and water above 0.5 wt % of R816 in the aqueous phase (see (iv) and (v)). (c) Average diameter of the emulsion droplets after the addition of the fumed silica particles. (d) Optical microscopy image showing the flocculated emulsion droplets in the presence of 1 wt % of the A200 particles.


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Figure 3. (a) Surface tensions of surfactant solution supernatants following centrifugation of mixtures containing 0.2 mM Triton X-100 and fumed silica particles at varying particle concentrations in water. (b) Interfacial tensions of surfactant solution supernatants at hexadecane-water interfaces at varying particle concentrations.


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Figure 4. Cryo-SEM image of Triton X-100 stabilized emulsion droplets in the presence of 1 wt % A200 shows bridges formed between the emulsion droplets and aggregation of fumed silica particles in the bulk.


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Figure 5. (a) Vortexing of the phase-separated Triton X-100 stabilized emulsion in the presence of 0.5 wt % R816, (b) the hexadecane and aqueous phases separated immediately after vortexing was halted.


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Figure 6. Schematic representation of adsorption behavior of Triton X-100 nonionic surfactant molecules on particle surfaces of different hydrophilicities. (a) hydrogen bonding between the ethoxylated head groups (red circles) of Triton X-100 molecules and OH groups on the hydrophilic fumed-silica particle surface, (b) hydrophobic interactions between the alkyl groups of the surfactant tails (black wavy lines) of Triton X-100 and the silanized fumed-silica particle surface.


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Destabilization of Oil-in-Water Emulsions Stabilized by Non-ionic

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