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

Article pubs.acs.org/Langmuir

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

Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States Department of Chemical Engineering, University of Rhode Island, Kingston, Rhode Island 02881, United States

‡

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, the 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 the 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 the 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 a nonionic surfactant (Triton X-100) in water.

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induce flocculation and coalescence of the emulsion droplets. Colloidal particles can serve as effective emulsion destabilizers as a result of their tunable surface properties, including specific surface area, charge, and wettability.10,11 Adsorption of surfactant molecules onto the particle surfaces can lead to surfactant depletion from both the oil−water interfaces associated with the emulsion droplets and the continuous phase. This can lead to an increase in the oil−water 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. Non-ionic surfactants can interact with particle surfaces through hydrogen-bonding or hydrophobic interactions depending upon the hydrophilicity of the particle surfaces in water.12 In particular, head groups of non-ionic 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

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,2,3 centrifugation, and membrane separation4,5 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 energyintensive. Here, we report on the destabilization of oil-inwater emulsions stabilized by a non-ionic surfactant (Triton X100) 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 © 2016 American Chemical Society

Received: September 6, 2016 Revised: September 14, 2016 Published: September 15, 2016 10694

DOI: 10.1021/acs.langmuir.6b03289 Langmuir 2016, 32, 10694−10698

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base emulsion (1 mL). The final surfactant concentration was 0.2 mM. The emulsions and 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 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 chargecoupled device (CCD) camera was used to observe the emulsion droplets. Images of the droplets were processed with ImageJ software27 to obtain average droplet sizes. Surfactant adsorption onto the particle surfaces was measured by monitoring the depletion of the 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 weight percent) were prepared and equilibrated for 24 h. Subsequently, the mixtures were centrifuged to separate the particles, and the supernatant was used for the depletion measurements.14,28 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 the surfactant from the emulsion oil−water interfaces in the presence of added particles. Pendant-drop experiments were carried out using a KRÜ SS Easy Drop goniometer to measure surface and interfacial tensions. Cryogenic scanning electron microscopy (cryo-SEM, Gatan Alto 2500 cryo system 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.

non-ionic 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 a significant impact on emulsion formation and stability. The synergistic effects of hydrophilic and hydrophobic silica particles with non-ionic surfactants on the stabilization of oilin-water or water-in-oil emulsions are well-documented in the literature.14−18 It is also known that partially hydrophobic particles are capable of forming particle-stabilized emulsions.19−21 However, there are no literature reports on the properties of aqueous mixtures of non-ionic surfactants and partially hydrophobic 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 use highly hydrophobic particles for the following two reasons: (i) 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 of 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 the 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 non-ionic surfactants. Our results provide a comprehensive understanding of the interactions between partially hydrophobic particles and non-ionic surfactants in water, including their antagonistic properties, an understanding that has not been reported previously in the literature.

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RESULTS AND DISCUSSION Base Emulsion. Figure 1 shows an optical micrograph of a hexadecane-in-water emulsion prepared by vortexing a mixture

MATERIALS AND METHODS

Triton X-100 [99% purity, TX-100, critical micelle concentration (cmc) of ∼0.2−0.3 mM23,24] 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 of the A200 and R816 particles, reported by the manufacturer, are ∼200 ± 25 and ∼190 ± 20 m2/g, respectively. Both A200 and R816 can be dispersed in water. Particle dispersions were prepared using ultrasonication, 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 of 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

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

of 0.2 mL of hexadecane and 1 mL of 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 the average drop diameter during the experimental time period (3 days). Dilution Experiments. Dilution with water had no effect on emulsion stability, and the average droplet diameter remained constant without a significant change. Panels a and b of Figure 2 show images of the Triton-X-100-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 the addition of the hydrophilic particles. An increase in the average droplet diameter was observed with an increase in the particle concentration in the aqueous phase (Figure 2c). Flocculation of the emulsion droplets was also observed at a particle 10695

DOI: 10.1021/acs.langmuir.6b03289 Langmuir 2016, 32, 10694−10698

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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, hydrogenbonding interactions between the fumed silica particles and the non-ionic 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 as a result of 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. Figure 2. Images of Triton-X-100-stabilized hexadecane-in-water emulsions in the presence of increasing concentrations (in weight percent) 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, with 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, with 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−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.

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.

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 upon their relative hydrophilicities. Non-ionic 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 particles are hydrophilic, and the only mode of interaction between the non-ionic surfactant molecules and the particles is through hydrogen bonding with surface silanol groups.29,30 Panels a and b of Figure 3 clearly show that the surface and interfacial tensions of the surfactant supernatant increased with an 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 a stabilizing

R816 particles are partially hydrophobic. The coarsening of oil droplets and the phase 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 (panels a and b of Figure 3), confirming a significant adsorption of surfactant molecules onto the particle surfaces. This led to a large increase in the oil−water interfacial tension and a lowering of the steric barriers created by surfactant molecules adsorbed at the droplet interfaces. Depletion of the 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 non-ionic surfactant molecules were able to stabilize emulsions. Indeed, the depletion of non-ionic

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.

Figure 5. (a) Vortexing of the phase-separated Triton-X-100-stabilized emulsion in the presence of 0.5 wt % R816. (b) Hexadecane and aqueous phases separated immediately after vortexing was halted. 10696

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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 non-ionic surfactant (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 non-ionic surfactant (Triton X-100) and a set of two different particles, our observations can be generalized to other surfactant−particle mixtures.

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 oil-inwater emulsions,19,33,34 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 non-ionic 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 non-ionic surfactant molecules adsorb onto the particle surfaces through hydrogen bonding, the surfactant tails remain exposed to water, thereby making the particle surfaces hydrophobic (Figure 6a).35 On the other hand, if the non-

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AUTHOR INFORMATION

Corresponding Authors

*Telephone: 617-253-4588. E-mail: [email protected]. *Telephone: 617-253-4594. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Evonik Corporation for providing fumed silica particles. The authors also thank P. Brown and A. G. Rajan for useful comments.

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REFERENCES

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Figure 6. Schematic representation of adsorption behavior of Triton X-100 non-ionic 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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CONCLUSION We studied the effect of particle hydrophilicity on the destabilization of Triton-X-100-stabilized emulsions. The interactions between the particles and 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-100stabilized emulsions. This effect was attributed to the change 10697

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