Article pubs.acs.org/Langmuir

Synergistic Formation and Stabilization of Oil-in-Water Emulsions by a Weakly Interacting Mixture of Zwitterionic Surfactant and Silica Nanoparticles Andrew J. Worthen,† Lynn M. Foster,†,‡ Jiannan Dong,† Jonathan A. Bollinger,† Adam H. Peterman,† Lucinda E. Pastora,† Steven L. Bryant,§ Thomas M. Truskett,† Christopher W. Bielawski,‡ and Keith P. Johnston*,† †

McKetta Department of Chemical Engineering, ‡Department of Chemistry and Biochemistry, and §Department of Petroleum and Geosystems Engineering, University of Texas at Austin, Austin, Texas 78712-0231, United States S Supporting Information *

ABSTRACT: Oil-in-water emulsions were formed and stabilized at low amphiphile concentrations by combining hydrophilic nanoparticles (NPs) (i.e., bare colloidal silica) with a weakly interacting zwitterionic surfactant, caprylamidopropyl betaine, to generate a high hydrophilic−lipophilic balance. The weak interaction of the NPs with surfactant was quantified with contact angle measurements. Emulsions were characterized by static light scattering to determine the droplet size distributions, optical photography to quantify phase separation due to creaming, and both optical and electron microscopy to determine emulsion microstructure. The NPs and surfactant acted synergistically to produce finer emulsions with a greater stability to coalescence relative to the behavior with either NPs or surfactant alone. As a consequence of the weak adsorption of the highly hydrophilic surfactant on the anionic NPs along with the high critical micelle concentration, an unusually large surfactant concentration was available to adsorb at the oil−water interface and lower the interfacial tension. The synergy for emulsion formation and stabilization for the two amphiphiles was even greater in the case of a high-salinity synthetic seawater aqueous phase. Here, higher NP adsorption at the oil−water interface was caused by electrostatic screening of interactions between (1) NPs and the anionic oil−water interface and (2) between the NPs. This greater adsorption as well as partial flocculation of the NPs provided a more efficient barrier to droplet coalescence.

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INTRODUCTION Surfactants may be combined with solid particles to stabilize oil and water emulsions of interest in numerous applications, including foodstuffs,1 cosmetics,2 oil recovery,3 and oil-spill remediation.4 The surfactant layer lowers the interfacial tension to favor droplet formation and also slows emulsion destabilization through various mechanisms including drainage of lamellae, coalescence, and Ostwald ripening.5 Adsorbed nanoparticles (NPs) may be added to further improve emulsion stability by reducing coalescence,6−8 even without lowering the interfacial tension.9 Coalescence of oil droplets in water may be inhibited by monolayers or bilayers of particles at the interface10,11 and by partial flocculation of the particles to raise the viscosity of thin films of water between approaching droplets.11 Interactions between the NPs and surfactant molecules influence competition for adsorption of surfactant on the NPs versus the oil/water interface. For example, adsorption of a charged surfactant on a NP with the opposite charge may be used to lower the hydrophilic/lipophilic balance (HLB).12−15 Loss of surfactant to the NP surface typically lowers the amount of surfactant that may freely adsorb with preferred © 2014 American Chemical Society

orientations at the oil−water interface, which may raise the interfacial tension.12,15−19 For the case of strong repulsion between like-charged NPs and surfactants, the interfacial tension may decrease as surfactant is driven to the interface via depletion forces.20,21 Here, the NPs may be driven out of thinning films between oil droplets as a consequence of electrostatic repulsion, which may reduce emulsion stability.22 Finally, nonionic surfactants can adsorb strongly on hydrophilic anionic NPs to raise their hydrophobicity and adsorption at the oil−water interface.18 Often the competing phenomena of (i) adsorption of surfactant on NP surfaces to modify the particle hydrophilicity/hydrophobicity and (ii) reduction of interfacial tension by free surfactant induce a free-energy minimum in the adsorption energy of NPs at the oil−water interface. At high surfactant concentrations, the surfactant may suppress adsorption of NPs at the interface.12,17,18,23 Emulsion stability may be enhanced by weak flocculation of NP stabilizers. Here, flocculated NP networks in the thin films Received: October 25, 2013 Revised: January 7, 2014 Published: January 10, 2014 984

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between approaching emulsion droplets slow down droplet coalescence and creaming.11 For example, for negatively charged silica NPs combined with cationic12,14 and nonionic18,19,24 surfactants, emulsions were most stable when the nanoparticles were weakly flocculated. Binks and co-workers demonstrated that coalescence of oil-in-water emulsions was fully arrested (over 24 h) when flocculated silica NPs were used with hexadecyltrimethylammonium bromide (CTAB) near the critical micelle concentration (cmc).12 Similarly, the surfactant concentration could be reduced from 2% to 0.1% while improving emulsion stability with addition of flocculated NPs.19 In contrast with the above systems, very few studies have examined emulsion stabilities in surfactant−NP systems where the surfactant adsorbs only weakly on the NPs.20 Recently, a highly hydrophilic zwitterionic surfactant (caprylamidopropyl betaine, CAPB) was combined with hydrophilic bare colloidal silica NPs to stabilize CO2-in-water foams synergistically in the absence of strong NP−surfactant interactions.3 Further study is needed to understand how to design more efficient emulsifier systems with interfacial compositions and morphologies that more effectively lower interfacial tension with charged NPs and weakly adsorbed surfactants and retard coalescence of oil droplets with viscous films between approaching droplets. Strategies are needed to overcome kinetic barriers25−27 and thermodynamic limitations26 for adsorption of the charged NPs at the oil−water interface. The objective of this study is to form and stabilize oil-inwater emulsions at unusually low amphiphile concentrations by combining a very weakly interacting mixture of hydrophilic NPs (bare colloidal silica) and a zwitterionic surfactant (CAPB) with a high HLB. Here a high fraction of the surfactant does not adsorb on the NPs surfaces and is available to freely adsorb at the oil−water interface to lower the interfacial tension. Additionally, the high cmc of the very hydrophilic surfactant reduces surfactant loss to micelles over the range of concentrations studied. The interfacial properties and emulsion stability (creaming and coalescence) are investigated both for DI water and for synthetic seawater from 10 s to 24 h over a range of oil/water ratios and surfactant/NP concentrations. Emulsion stabilization is maintained upon reducing the surfactant concentration an order of magnitude with a NP concentration on the order of 0.1% w/v, as a result of synergistic interactions with the weakly adsorbing surfactant (on the NP surface). The synergy is shown to result from a combination of the high available concentration of free surfactant for adsorption at the oil/water interface, the compatibility of the uncharged surfactant and NPs at the interface, and the increased barrier to droplet coalescence provided by the NPs. Unlike the case of oppositely charged NPs and surfactant, the zwitterionic surfactant and negatively charged NPs in this study will not be driven out of the thinning water films by electrostatic repulsion upon approach of two charged oil droplets. In the case of SSW, the screening of the charge on the NP surfaces reduces electrostatic repulsion between the NPs as well as the repulsion of a NP with the anionic oil−water interface. These changes will be shown to raise the thermodynamic driving force for particle adsorption at the oil−water interface26 while simultaneously lowering the electrostatic kinetic barrier.25−27 Additionally, the salt is shown to produce weak NP flocculation, which increases the hydrodynamic forces for NP adsorption relative to electrostatic repulsion and also provides a more effective barrier to droplet coalescence.12,14,18,19,24

Article

EXPERIMENTAL SECTION

Materials. Bare colloidal silica NPs (NexSil 20, Nyacol Nano Technologies, NPs) were purchased as a 40% w/v aqueous dispersion and had a specific surface area of 135 m2/g, per the manufacturer. Caprylamidopropyl (octylamidopropyl) betaine (CAPB) was a gift from Rhodia (Mackam OAB, batch UP1K17X18). The chemical structure of CAPB is given in Scheme S1, Supporting Information. As per the certificate of analysis provided by the manufacturer, the surfactant solution was 37.6% w/v solids including 6.5% w/v sodium chloride, which is a byproduct of the betaine synthesis. As additional impurities are unknown, a concentration of 30% w/v CAPB was assumed for calculations in this study, based on the typical CAPB concentration per the manufacturer. Synthetic seawater (Cat. No. 8363-5, Lot 1306873, ASTM D1141, SSW) was purchased from Ricca Chemical Co. and used as received with a pH of 8.2. Dodecane (99%, Acros Organics) was purified with basic alumina prior to use. Deionized (DI) water (Nanopure II, Barnstead, Dubuque, IA) was used for all experiments. In all experiments with either DI water or SSW, the aqueous-phase pH was 8.0 ± 0.5. The pKa of the carboxylic acid group in CAPB is ca. pH 2, and thus, the cationic fraction of CAPB is negligible at pH 8.0 ± 0.5. Surface Tension and Interfacial Tension Measurements. For SSW or DI water-based systems, surface tension (γAW) and dodecane− aqueous-phase interfacial tension (γOW) were determined using axisymmetric drop shape analysis of an aqueous pendant droplet containing a known concentration of surfactant and/or NPs with methods described previously.28,29 The droplet was held for 2 min to equilibrate in air (γAW) or an excess dodecane phase (γOW). The droplet shape profile was fitted according to the Young/Laplace equation with a software package (CAM200, KSV Ltd., Finland). The mean γAW or γOW was taken as a mean of 10 measurements that were acquired 10 s apart, and the standard deviation of the measurements was typically less than 1% of the mean. When NPs were added to the surfactant solution, samples were allowed to equilibrate overnight. The density of the aqueous phase was determined by weighing 10 mL aliquots. Contact Angle Measurements. Axisymmetric drop shape analysis of a captive dodecane droplet in an excess aqueous phase was used to determine the contact angle (θOW) between dodecane/ water/silica wafer. The apparatus and techniques were adapted from a previous study.3 The apparatus consisted of a 11/16 in. i.d. stainless steel view cell30 in which a silica-coated silicon wafer was mounted. The wafer was equilibrated with surfactant solution for 20 min prior to θOW measurements. Dodecane droplets with a volume of ca. 10 μL were injected with a glass syringe and captured on the wafer. To calculate θOW, the dodecane droplet profile shape was analyzed according to the Young/Laplace equation with a software package (CAM200, KSV Ltd., Finland). The average and standard deviation of the calculated θOW for 3−4 droplets placed on different locations on the silica wafer were recorded, where 10 measurements were taken of each droplet every 5 s. To produce the silica coating on the silicon wafer (mirror-polished Si, Wafer World, Inc., USA), it was cleaned with DI water and placed in 15.8 N HNO3 solution overnight.12 The resulting silica-coated wafer was then neutralized with NaHCO3, washed with DI and ethanol, and dried prior to use. Preparation of Emulsions. Emulsions were prepared by combining DI water or SSW, NP dispersion, surfactant solution, and dodecane to a total volume of 14 mL in a 20 mL glass vial with an inside diameter of 2.5 cm and immediately homogenized with an IKA Ultra-Turrax T-25 Basic with an 8 mm head (S 25 N − 8 G Dispersing element, Ident. No. 001024200, IKA Works GmbH) operating at 13 500 rpm for 2 min at room temperature. The gap between the rotor and the stator is 0.25 mm, giving a shear rate of ca. 17 000 s−1 at 13 500 rpm. Both particles and surfactant originate in the aqueous phase, and their volume is treated as part of the aqueous-phase volume. The aqueous-phase shear viscosity in all experiments was 0.83−2.45 cP (Table S1, Supporting Information). NP and surfactant concentrations are given as mass percent per total sample volume (% w/v), and water fraction (φw) is given as the fraction of aqueous phase to the total 985

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sample volume. Immediately after homogenization, the emulsion conductivity was determined using a Cole-Parmer EC conductivity meter with a Pt/Pt black electrode. The emulsion type was also confirmed using the “drop” test. Emulsion nomenclature is given as the concentration of dispersants added to the system in % w/v starting with NP followed by the CAPB (e.g., 0.5/0.1 is representative of an emulsion stabilized by 0.5% w/v NP and 0.1% w/v CAPB). Emulsion Droplet Size Determination. The volume moment mean diameter (D[4, 3]) and uniformity (U) were calculated from the droplet size distributions with a Malvern Mastersizer S static light scattering (SLS) instrument equipped with a 15 mL stirred optical cell at various times. Refractive indices of dodecane, DI water, and SSW were 1.42, 1.33, and 1.34 as measured at room temperature using a refractometer (Fisher Scientific, model 334620). Calculated droplet size distributions were determined to be insensitive to an aqueousphase refractive index variation of ±0.02, and thus, a refractive index of 1.33 was used for the aqueous phase in all experiments. D[4, 3] is given by

D[4, 3] =

Article

RESULTS Surface Tension and Interfacial Tension. Figure 1 shows surface tensions (γAW) (Figure 1a and 1b) and dodecane−

4 ∑i Nd i i 3 ∑i Nd i i

(1)

where Ni is the number of drops with diameter di. U was calculated by

U=

3 1 ∑i Nd i i |dm − di| 3 dm ∑i Nd i i

(2)

where dm is the mean drop diameter. In some cases, the emulsions were visually observed to coarsen in the first 10 min after formation, and this behavior was noted. The emulsion behavior at 24 h was visually noted regarding where fine emulsion droplets remained and where NPs were released from the emulsion. Emulsion Creaming Stability Determination. Emulsion stability to creaming was determined by monitoring the creaming front position as a function of time with a Canon PowerShot SD4000 IS Digital ELPH for unstable emulsions (20% aqueous-phase resolution time < ca. 10 min) while leaving the emulsion in the 20 mL glass vial or with a Nikon D5100 camera with Phottix TR-90 remote controller for more stable emulsions after transferring the emulsion to a capped 16 mm × 125 mm glass test tube to prevent evaporation. Digital photos were analyzed with ImageJ software (U.S. National Institutes of Health). Stability maps were constructed to show the time to resolve 20% of the aqueous phase from the emulsion as a function of CAPB and NP concentrations. The time required is color coded on a log scale in order to show resolution times quantitatively ranging from several seconds to 24 h. Shading was assigned by linear interpolation between data points to populate the maps. In cases where stability increased significantly with a small change in dispersant concentration (e.g., 0.15/0.01 to 0.2/0.01 at φw = 0.2 in Figure 6a), shading is given to delineate dispersant concentrations that gave similar stability, and the interpolated region is approximate. Emulsion Microstructure Determination. Optical microscopy and confocal fluorescence microscopy were performed with a Zeiss LSM 710 confocal microscope. For confocal fluorescence imaging, 1 × 10−5 M Auramine O was added to the aqueous phase prior to emulsification to fluorescently label the NPs. On a glass microscope slide, 30 μL of emulsion was covered with a glass cover slip for imaging. Cryogenic scanning electron microscopy (cryo-SEM) was performed with a Gatan Alto 2500 cryogenic system for SEM and Hitachi S-4800 field emission SEM. Samples were prepared by loading 5 μL of emulsion sample into a sample holder which was then plunged into slushed liquid nitrogen to rapidly freeze the sample. The sample emulsion droplet was fractured at −130 °C, sublimed for 5 min at −95 °C, and coated with Pd/Pt alloy at −130 °C. The fractured surface of the sample was observed at 3 kV and −130 °C.

Figure 1. (a) Air/DI water and air/SSW surface tensions (γAW) at various aqueous-phase concentrations of CAPB, and (b) dodecane/DI water and dodecane/SSW interfacial tensions (γOW) at various aqueous-phase concentrations of CAPB. Aqueous phase was DI equilibrated with (◊) and without (×) 5% w/v bare colloidal silica nanoparticles or SSW equilibrated with (□) and without (+) 5% w/v bare colloidal silica nanoparticles. Measured clean DI water γAW is 71.8 mN/m and SSW is 71.2 mN/m. Measured clean dodecane/DI water γOW is 52.5 mN/m and dodecane/SSW is 45.3 mN/m.

aqueous-phase interfacial tensions (γOW) (Figure 1c and 1d) determined by pendant drop experiments as a function of CAPB concentration with and without added NPs. In each case, γAW and γOW were reduced by addition of CAPB to the DI water and SSW systems up to ca. 2 % w/v CAPB, above which γAW and γOW did not decrease further. Thus, the critical micelle concentration (cmc) of CAPB was 2% w/v (3.5 M) in both DI water and SSW. γAW values for the SSW system were 1−3 mN/ m lower than for DI water, suggesting to us that the salt drove the hydrophilic surfactant toward the interface. Similarly, γOW values for the dodecane/SSW water system were 1−5 mN/m lower than the values for dodecane/DI. Introduction of 5% w/v NP to the aqueous phase did not significantly affect γAW or γOW nor the surfactant cmc. Measurements were made with NP only, and the changes in γAW or γOW were negligible. From the data shown in Figure 1a with up to 1% free surfactant (ca. 0.5 × cmc), the surfactant adsorption was below the detection limit of 0.4 μmol/m2 (0.1 mg/m2) in both DI water and SSW as discussed in detail in the Supporting Information. Contact Angle. The three-phase contact angle formed by an oil droplet on a silica surface in water is described by Young’s equation γ − γSW cos θOW = SO γOW (3) where γSO and γSW are the interfacial tensions between silica/oil and silica/water, respectively. In Figure 2, an increase in θOW 986

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(Figure 3e−h) are shown in Figure 3. Measurements were taken immediately (t = 0 h) after preparation and 1 and 24 h after preparation, provided there was sufficient emulsion volume remaining for sizing. All emulsions were oil-in-water (O/W) as observed via conductivity measurements (not shown) and “drop” tests. NPs and/or surfactants were introduced from the aqueous phase and considered part of the total aqueous volume. Calculated initial D[4, 3] for the droplet size distributions of emulsions generated using 0.05% w/v NP with 0.01, 0.1, and 1% w/v CAPB in both DI water and SSW are given in Figure 4. Calculated D[4, 3] and uniformity

Figure 2. Measured static contact angles (θOW) of captive dodecane droplets in surfactant solution on hydrophilic silicon wafers versus initial concentration of surfactant in DI water (◊) and SSW (□). (Inset) Schematic illustrating the oil/aqueous-phase/silica contact angle, where γOW, γSO, and γSW represent the oil/water, silica/oil, and silica/aqueous-phase interfacial tensions, respectively. Without surfactant, θOW was 53° ± 3° for DI water and 53° ± 1° for SSW. Error bars show ±1 standard deviation for 3−4 measurements.

(decrease in cos θOW) with added surfactant indicates better wetting of oil on the silica surface. In the DI water system, added CAPB from 0.002 to 2% w/v (cmc) did not significantly affect θOW as the mean values fell between 52° and 62° despite the large change in concentration. In contrast, addition of CAPB caused a reduction in θOW in the SSW system from 53° without surfactant to 14° with 2% w/v CAPB. As θOW was less than 90° for CAPB with DI water or SSW, the silica surface behaved primarily hydrophilic over the range of conditions shown. Emulsion Morphology Characterization. Select emulsion droplet size distributions obtained by static light scattering for dodecane-in-DI water (Figure 3a−d) and dodecane-in-SSW

Figure 4. Calculated initial volume moment mean diameter (D[4, 3]) with 0.05% w/v NP and 0.01, 0.1, and 1% w/v CAPB, prepared with DI water (blue) or SSW (red). Emulsions were prepared at a water fraction (φw) of 0.5.

(U) are given in Tables S2 and S3, Supporting Information, for exemplary samples at all time points where droplet size distributions were measurable.

Figure 3. Drop size distributions determined from static light scattering of emulsions made with (a−d) dodecane-in-DI water or (e−h) dodecane-inSSW with varying NP and surfactant concentrations. Emulsions were prepared at a water fraction (φw) of 0.5. Blue circles are data taken initially after homogenization, red triangles show data collected 1 h after emulsion formation, and green squares represent data collected at 24 h after homogenization. Panels are labeled by the concentration of dispersants added to the system in % w/v in the convention of NP/surfactant. 987

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Figure 5. Images of the emulsions by (a) digital photography, (b) optical microscopy, (c) confocal fluorescence microscopy, and (d) Cryo-SEM techniques. Emulsions in a, from left to right, are dodecane-in-DI water with 0.05/1 (labeled as in Figure 3) and 0.5/0.1 and dodecane-in-SSW with 0.05/1 and 0.5/0.1 at a water fraction (φw) of 0.5 imaged 1 h after preparation (vial outer diameter = 1.56 cm). Optical microscope image in b is of a dodecane-in-SSW emulsion with 0.5/0.1 and in c is the corresponding confocal fluorescence image. Cryo-SEM image in c is of an emulsion with 0.5/ 1 at a water fraction of 0.5 taken 24 h after preparation.

effect on the initial mean droplet size (Figure 4). In Figure 3b and 3f, the SSW system produced smaller initial oil droplets that coarsened over 1 h to generate a distribution comparable to that in the DI water aqueous phase. In Figure 3c and 3g or 3d and 3h, the presence of salts in the aqueous phase resulted in narrower distributions that, in contrast to the DI water preparations, did not show any significant coarsening over 24 h. Dispersions containing up to 1% w/v bare colloidal silica NPs with no surfactant (1/0) in either aqueous phase produced unstable emulsions that completely resolved in less than 10 s; thus, droplet size measurements could not be performed. Macroscopic and microscopic observations of the emulsions are shown in Figure 5. In Figure 5a, macroscopic photos of 0.05/1 and 0.5/0.1 dodecane-in-DI water and dodecane-inSSW systems at 1 h after homogenization demonstrate that the emulsions are white opaque phases resting above resolving aqueous phases. In Figure 5a, it is evident that more of the aqueous phase resolved for the DI versus SSW case, consistent with the larger droplets in Figure 4 for the former. The optical microscope photo in Figure 5b shows spherical dodecane droplets in a continuous SSW phase for an emulsion with 0.5/0.1 composition at a water fraction of 0.5. This photo is representative of the morphology observed in many of the emulsions investigated, and the observed droplet sizes agree with the light scattering data (Figure 3h). Coalescence of oil droplets was relatively minor according to optical microscopy. The corresponding confocal fluorescence microscopy image for the emulsion is given in Figure 5c. The bright rings in the image show the presence of fluorescently labeled NPs around the oil droplets. Bright rings were not observed in emulsions prepared without NPs. In the cryo-SEM image presented in Figure 5d, two large, spherical flocculated oil droplets are surrounded by smaller, nonspherical NP flocs. NP flocs are visible at the oil droplet interface and in the continuous aqueous phase. In the bulk aqueous phase without any oil, NP flocculation was dependent on the choice of aqueous phase and surfactant loading. In DI water, the NPs did not flocculate due to the presence of surfactant until above the cmc (Figure S2, Supporting Information). However, in SSW the NPs flocculate without added surfactant and adding surfactant 0.1−1% w/v caused an increase in flocculation (Figure S2, Supporting Information). For example, in SSW, the D[4,3] average size increased from 3.4 μm without surfactant to 5.9 μm with 1% w/ v surfactant.

In the DI water examples (Figures 3a−d and 4), greater concentrations of surfactant and NPs produced smaller droplets, provided a threshold surfactant concentration was used (>0.01% w/v CAPB), where CAPB or NP concentrations could be increased to decrease the average droplet size. Without added NPs, DI water emulsions stabilized with 0.01 and 0.1% w/v CAPB were too unstable for size measurement at 1 h (Table S2, Supporting Information). When a NP concentration of 0.05% w/v was provided, emulsions stabilized with 0.1% w/v CAPB remained stable for 24 h (Table S2, Supporting Information). However, the same NP concentration was ineffective at stabilizing an emulsion with 0.01% CAPB, where γOW was high. For a given NP concentration, increasing the CAPB concentration reduced the initial median droplet size (i.e., produced finer emulsions) (Figure 4) and stabilized the droplet distributions over 24 h, which is demonstrated in Figure 3a−c. Interestingly, increasing NP concentration from 0.05 to 0.5% w/v while simultaneously decreasing CAPB concentration from 1 to 0.1% w/v produced droplet size distributions with very similar shape and stability, as demonstrated in Figure 3c and 3d. For given CAPB and NP concentrations, use of SSW as the aqueous phase instead of DI water produced emulsions with greater stability and narrower droplet distributions with smaller median droplet size. Without added NPs, SSW water emulsions stabilized with 0.01 and 0.1% w/v CAPB were too unstable for size measurement at 1 h (Table S3, Supporting Information). However, a NP concentration of 0.05% w/v was required to stabilize emulsions of both 0.01 and 0.1% w/v CAPB for 24 h (Table S3, Supporting Information, and Figure 3e and 3f), representing a significant increase in stabilization by the NPs compared to the corresponding emulsions with DI water (Table S2, Supporting Information, and Figure 3a and 3b). On the basis of Figure 3a and 3e, the 0.05/0.01 dodecane-in-DI water emulsion completely phase separated less than 1 h after homogenization, and thus, only initial data were collected and the drops were large (D[4, 3] = 320 μm shown in Figure 4); however, the same stabilizer concentration in SSW produced an emulsion with an initial size distribution with an order-ofmagnitude smaller average (D[4, 3] = 40 μm shown in Figure 4) that remained measurable over 24 h. Increasing the CAPB concentration at a fixed NP concentration produced more stable droplet distributions over 24 h as in the DI water examples, which is demonstrated in Figure 3e−g. However, increasing CAPB concentration from 0.01 to 1% w/v had little 988

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Emulsion Creaming Stability. Contour maps of the emulsion stability to creaming (herein referred to as “stability maps”) for dodecane-in-SSW preparations at aqueous-phase fractions φw = 0.2, 0.5, and 0.8 are presented in Figure 6, where the warmth of the color indicates the length of time required for 20% of the total aqueous-phase volume to resolve, where greater times reflect greater emulsion stabilities. (This metric is illustrated with front heights in Figure S4, Supporting Information, and times are also reported in Tables S2 and S3, Supporting Information, for selected emulsions.) Foams occasionally observed at the top of the emulsions were ignored for purposes of measuring the height of the emulsion phase. The time to resolve is color coded on a log scale as a function of CAPB and NP concentrations in order to show resolution times quantitatively ranging from several seconds to 24 h. On the maps, blue shades correlate to seconds, green-yellow shades to minutes, red shades to hours, and brown to a day. The symbols overlaid on the stability maps represent emulsion behavior at 24 h after homogenization, recorded regardless of time to resolve: diamond (no emulsions), square (NPs have been released from the emulsion), and triangle (NPs not released). Open circles indicate additional points where resolution time was observed, but the behavior at 24 h was not recorded. Solid red filled symbols indicate the least stable cases where coalescence of oil droplets was visually observed within 10 min after homogenization. With limited exception, when coalescence of oil droplets was visually observed in the first 10 min after emulsion creation, the emulsion did not remain opaque white after 24 h, indicating the majority of droplets had coarsened or resolved leaving very few emulsion droplets. The solid black lines are drawn for visual ease of discerning constant total (CAPB + NP) dispersant concentrations. Several overarching trends are apparent from the stability maps at all water fractions shown: (i) preparations containing no surfactant were not stable, with resolution times of less than 30 s (and subsequent total phase separations in Fmix), adsorption of particles can be significantly retarded.25,27,38 For turbulent flow expected in a rotor-stator mixer, the hydrodynamic force pushing particles toward the oil droplet surfaces can be estimated as38 Fmix ≈ a 2ρc ε 2/3R2/3

(8)

where a is the particle radius, ρc is the continuous phase density, and ε is the rate of energy dissipation per unit mass (∼105 J kg−1 s−1 for typical lab-scale rotor-stator mixers38). In DI water, where ρc ≈ 103 kg m3 and R ≈ 100 μm, we calculate Fmix ≈ 9 × 10−13 N, which is lower than but of the same order of magnitude as the kinetic barrier calculated above. This electrostatic barrier may have led to the relatively large oil droplets sizes for emulsions in DI water. In contrast, the greater hydrodynamic forces for the weakly flocculated NPs in SSW (Figure S2, Supporting Information) as well as the lower DLVO thermodynamic barrier contributed to formation of the smaller oil droplets with R ≈ 10 μm (Figure 4) than for DI water. Emulsion Stability to Coalescence. For the cases in Figure 3, coalescence41,42 and Ostwald ripening43,44 may potentially contribute to the evolution of the oil droplet size

Gμs R γOW

(7)

(6)

where G is the shear stress (du/dz), μs is the continuous phase shear viscosity, and R is the droplet radius.37 Thus, We describes the balance of the external (shear) stress applied to the interface and the Laplace pressure (Pc) of the droplet, where Pc = 2γOW/R. The large amount of free surfactant, given 991

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known to increase interfacial elasticity50,51 and interfacial viscosity51 which may contribute to lower coalescence. Emulsion Creaming Rates. According to Stokes’ law and the Richadson−Zaki model (eqs 4 and 5), the creaming rate was reduced by decreasing the size of the emulsion droplets, increasing the continuous phase viscosity and increasing the dispersed phase volume fraction. When droplet coalescence was arrested with addition of sufficient CAPB and/or NP, the creaming behavior was well bounded by these two models for creaming velocity (Figure 7a−f). In contrast, coalescing emulsions creamed more rapidly than predicted (Figure 7d inset). Similarly, Yan and Masliyah noted accelerated creaming of mineral oil-in-water emulsions stabilized by asphaltenemodified clay particles due to coalescence of emulsion when less than a monolayer coverage of clay was present on the droplets.52 In the present study, the aqueous-phase viscosity increased only by a factor of up to 3 (up to ca. 3 cP) with added CAPB and NPs in SSW (Table S1, Supporting Information) and thus had a minor influence on the creaming rate (Figure 7). Binks and co-workers found significant improvement in oil-inwater emulsion stability to creaming using silica NPs with CTAB12 and nonionic surfactant18 due to the increased viscosity of the aqueous phase but at much higher aqueousphase viscosities (up to nearly 2000 cP). Creaming was significantly affected by increasing the dispersed volume fraction and thus changing the morphology of the emulsion. At a dispersed phase volume fraction (Φ) of 0.2 (φw = 0.8), interdroplet hydrodynamic interactions are minimal between dilute droplets, allowing them to move freely due to buoyancy and thus cream the fastest,32 as observed in Figure 7d. Increasing Φ to 0.5 (φw = 0.5), the initial interdroplet hydrodynamic interactions increase, resulting in the slower creaming rate observed in Figure 7a. At a higher Φ such as 0.8 (φw = 0.2) shown in Figure 6a, thin aqueous lamellae separate concentrated emulsion droplets in the HIPE which drain by capillary and gravitational forces.53−55 The slow drainage of lamellae allows the emulsion droplets to become more tightly packed and may be interpreted as creaming (Figure 6a). Regardless of initial Φ, as creaming progresses, the droplets become more closely packed as the aqueous phase between the droplets drains and creaming slows. Ultimately, creaming is expected to halt because the droplets have insufficient room to move. For example, in Figure 7b, creaming of an emulsion with Φ of 0.5 at t = 0 s stopped at t = 150 s when Φ reached ca. 0.8. After creaming is complete, emulsions may be stable for months or years, if coalescence has also been arrested.55

distribution. Coalescence typically results in an increase in droplet polydispersity (increase in U) versus time, while Ostwald ripening causes a decrease in droplet polydispersity.45 An increase in U was observed in all cases, except Figure 3e and 3f, suggesting coalescence was the main destabilization mechanism prior to creaming. Furthermore, the very low solubility of dodecane in water and the lack of surfactant micelles to serve as carriers of dodecane led to extremely low and negligible calculated rates of Ostwald ripening (see Figure S3, Supporting Information). Coalescence occurs in a three-step process: (1) approach of droplets through the continuous aqueous phase; (2) deformation of droplets to form a thin film between them; and (3) thinning of the film to a critical thickness, below which a thermally activated hole in the film can form, causing the droplets to coalesce.46 The NPs improved dodecane-in-DI water emulsion stability to coalescence relative to NP-free systems with the same surfactant concentration (Figure 3, Tables S2 and S3, Supporting Information) by adsorbing at the oil−water interface (Figure 5c and 5d). Analogously, the NPs reduced the surfactant concentration required to produce emulsions stable for 24 h due to the added barriers to droplet coalescence. It is known that dispersed NPs can slow film rupture by forming ordered structures to provide steric barriers to hole formation,22,47 but this is typically observed at higher NP concentrations (10+ vol %). In our case, the stabilization at lower NP concentrations is influenced by the high surfactant adsorption at the oil−water interface. Also, the NPs contributed to a modest increase in aqueous-phase viscosity, which slowed both droplet approach (discussed in detail below in terms of creaming) and film drainage.46 Various factors contributed to the observation that the NPs were more efficient at improving emulsion stability to coalescence in SSW than in DI water: (i) the increased adsorption of NPs at the oil−water interface due to the reduced kinetic barrier for adsorption (discussed above) and (ii) the greater steric barrier provided by the partially flocculated NPs.12,14,18,19,24 Interestingly, the significant increase in stability to coalescence at surfactant concentrations ≤ 0.1% w/v occurred despite the observation that aqueous-phase viscosities were only slightly higher in SSW than in the DI water cases for equivalent concentrations of NP and surfactant (Table S1, Supporting Information). The slight increase in continuous phase viscosity was unlikely the cause of the increased stability, in contrast to previous studies where large increases in continuous phase viscosity may have provided significant stabilizing effects.18 The lack of NPs in the resolved aqueous phase for many of the emulsions prepared in SSW indicates that the adsorption at the oil−water interface was improved and NPs were not driven out of the thinning films by surfactant molecules or micelles. The compatibility of the silica NPs with the zwitterionic surfactant allowed more NPs to remain in the emulsion during droplet approach (creaming), and thus, the concentration of NPs around the emulsion droplets increased over time as NP-free aqueous phase drained from the emulsion. The increase in NP concentration is expected to further increase aqueous-phase viscosity48 to slow droplet approach and provide a more robust physical barrier to droplet coalescence.11 At the longest time scales investigated (1−24 h), gelation of the NPs is expected,49 which would effectively arrest any further droplet approach or film thinning. This phenomenon was observed visually, where many emulsions behaved like viscous gels after 24 h of aging. The NPs also are

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CONCLUSIONS Dodecane-in-water (or synthetic seawater) emulsions were formed and stabilized at low amphiphile concentrations by combining a mixture of hydrophilic NPs (i.e., bare colloidal silica) with a weakly adsorbing zwitterionic surfactant (caprylamidopropyl betaine) with an unsually high HLB. The NPs and surfactant acted synergistically to produce emulsions that had smaller oil droplets and greater stability to coalescence than emulsions stabilized by either NPs or surfactant alone. With the low adsorption of the highly hydrophilic surfactant of less than 0.4 μmol/m2 (0.1 mg/m2) on the anionic NPs, representing less than 5% of a monolayer, a large fraction of surfactant was available to freely adsorb at the oil−water interface to lower the interfacial tension. Furthermore, little of the high HLB surfactant was lost to micelles given the high cmc 992

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(3) Worthen, A. J.; Bryant, S. L.; Huh, C.; Johnston, K. P. Carbon dioxide-in-water foams stabilized with nanoparticles and surfactant acting in synergy. AIChE J. 2013, 59 (9), 3490−3501. (4) Li, Z.; Kepkay, P.; Lee, K.; King, T.; Boufadel, M. C.; Venosa, A. D. Effects of chemical dispersants and mineral fines on crude oil dispersion in a wave tank under breaking waves. Mar. Pollut. Bull. 2007, 54 (7), 983−993. (5) Langevin, D. Influence of interfacial rheology on foam and emulsion properties. Adv. Colloid Interface Sci. 2000, 88 (1,2), 209− 222. (6) Hunter, T. N.; Pugh, R. J.; Franks, G. V.; Jameson, G. J. The role of particles in stabilising foams and emulsions. Adv. Colloid Interface Sci. 2008, 137 (2), 57−81. (7) Binks, B. P. Particles as surfactants - similarities and differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (8) Horozov, T. S. Foams and foam films stabilised by solid particles. Curr. Opin. Colloid Interface Sci. 2008, 13 (3), 134−140. (9) Vignati, E.; Piazza, R.; Lockhart, T. P. Pickering Emulsions: Interfacial Tension, Colloidal Layer Morphology, and Trapped-Particle Motion. Langmuir 2003, 19 (17), 6650−6656. (10) Horozov, T. S.; Binks, B. P. Particle-stabilized emulsions: a bilayer or a bridging monolayer? Angew. Chem., Int. Ed. 2006, 45 (5), 773−776. (11) Dickinson, E. Food emulsions and foams: Stabilization by particles. Curr. Opin. Colloid Interface Sci. 2010, 15 (1−2), 40−49. (12) 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. (13) Akartuna, I.; Studart, A. R.; Tervoort, E.; Gonzenbach, U. T.; Gauckler, L. J. Stabilization of Oil-in-Water Emulsions by Colloidal Particles Modified with Short Amphiphiles. Langmuir 2008, 24 (14), 7161−7168. (14) Lan, Q.; Yang, F.; Zhang, S.; Liu, S.; Xu, J.; Sun, D. Synergistic effect of silica nanoparticle and cetyltrimethyl ammonium bromide on the stabilization of O/W emulsions. Colloids Surf., A 2007, 302 (1−3), 126−135. (15) Gonzenbach, U. T.; Studart, A. R.; Tervoort, E.; Gauckler, L. J. Tailoring the Microstructure of Particle-Stabilized Wet Foams. Langmuir 2006, 23 (3), 1025−1032. (16) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. Effect of Nanoparticles on the Interfacial Properties of Liquid/Liquid and Liquid/Air Surface Layers. J. Phys. Chem. B 2006, 110 (39), 19543− 19551. (17) Ghouchi Eskandar, N.; Simovic, S.; Prestidge, C. A. Synergistic effect of silica nanoparticles and charged surfactants in the formation and stability of submicron oil-in-water emulsions. Phys. Chem. Chem. Phys. 2007, 9 (48), 6426−6434. (18) Binks, B. P.; Desforges, A.; Duff, D. G. Synergistic Stabilization of Emulsions by a Mixture of Surface-Active Nanoparticles and Surfactant. Langmuir 2007, 23 (3), 1098−1106. (19) Midmore, B. R. Preparation of a novel silica-stabilized oil/water emulsion. Colloids Surf., A 1998, 132 (2−3), 257−265. (20) Ma, H.; Luo, M.; Dai, L. L. Influences of surfactant and nanoparticle assembly on effective interfacial tensions. Phys. Chem. Chem. Phys. 2008, 10 (16), 2207−2213. (21) Luo, M.; Song, Y.; Dai, L. L. Heterogeneous or competitive selfassembly of surfactants and nanoparticles at liquid−liquid interfaces. Mol. Simul. 2009, 35 (10−11), 773−784. (22) Velikov, K. P.; Durst, F.; Velev, O. D. Direct Observation of the Dynamics of Latex Particles Confined inside Thinning Water−Air Films. Langmuir 1998, 14 (5), 1148−1155. (23) Katepalli, H.; John, V. T.; Bose, A. The Response of Carbon Black Stabilized Oil-in-Water Emulsions to the Addition of Surfactant Solutions. Langmuir 2013, 29 (23), 6790−6797. (24) Gosa, K.-L.; Uricanu, V. Emulsions stabilized with PEO−PPO− PEO block copolymers and silica. Colloids Surf., A 2002, 197 (1−3), 257−269.

(2% w/v or 3.5 M). The role of the NPs was primarily to provide a steric barrier to droplet coalescence, resulting in minimal coalescence at relatively low amphiphile concentrations. For cationic or anionic surfactants, the type of synergy demonstrated in this study would be less likely as a result of either: (1) strong surfactant adsorption on the NPs when the charges are opposite, or (2) insufficient adsorption of both NP and surfactant at the oil/water interface when the charges are the same. Highly stable oil-in-SSW emulsions could be achieved by reducing the surfactant concentration from 1 to 0.1% w/v with addition of 0.5% w/v NP at all water fractions tested. The synergy between the two amphiphiles was even greater with a high-salinity SSW as the aqueous phase as a result of electrostatic screening of the highly charged NPs and moderate NP flocculation. The screening reduced the unfavorable repulsion between nanoparticles in the aqueous phase and the anionic oil/water interface as well as between the adsorbed nanoparticles, which increased the nanoparticle adsorption at the oil−water interface. The small flocculated nanoparticles, as measured by SLS, are driven to adsorb more effectively by hydrodynamic forces, and once adsorbed, the flocculated NPs provide a greater steric barrier to coalescence. These factors allowed formation and stabilization of smaller droplets against coalescence and thus slower creaming rates.

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ASSOCIATED CONTENT

S Supporting Information *

Molecular structure of CAPB; procedure for calculation of specific adsorption of CAPB on bare colloidal silica NPs; continuous phase shear viscosities of NPs and CAPB in DI water and SSW; NP floc size data for bare silica NPs in DI water and SSW; predicted Ostwald ripening rates; graphical representation of the time required for 20% of the aqueousphase volume to resolve from an emulsion; mean emulsion droplet sizes and time for 20% of the aqueous-phase volume to resolve for exemplary samples for DI water and SSW; methodology for calculation of the force between a NP and an oil droplet. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. V. T. John and Dr. J. He for cryo-SEM imaging. This work was supported in part by the Gulf of Mexico Research Initiative and the DOE Center for Frontiers of Subsurface Energy Security. K.P.J. and T.M.T. also acknowledge the Robert A. Welch Foundation (F-1319 and F-1696, respectively). L.E.P. was supported by the Welch Summer Scholars Program.

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REFERENCES

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