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Phase and Steady Shear Behavior of Dilute Carbon Black Suspensions and Carbon Black Stabilized Emulsions Michael P Godfrin, Ayush Tiwari, Arijit Bose, and Anubhav Tripathi Langmuir, Just Accepted Manuscript • DOI: 10.1021/la504151z • Publication Date (Web): 03 Dec 2014 Downloaded from http://pubs.acs.org on December 6, 2014

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Phase and Steady Shear Behavior of Dilute Carbon Black Suspensions and Carbon Black Stabilized Emulsions Michael P. Godfrin†, Ayush Tiwari‡, Arijit Bose§, Anubhav Tripathi†* †

Center for Biomedical Engineering, School of Engineering, Brown University,

Providence, RI ‡

Department of Civil Engineering, Thapar University, Patiala, Punjab, India

§

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

KEYWORDS. Transient colloidal networks, attractive colloidal gels, carbon black emulsion Abstract We use para-amino benzoic acid terminated carbon black (CB) as a model particulate material to study the effect of salt-modulated attractive interactions on phase behavior and steady shear stresses in suspensions and particle-stabilized emulsions. Surprisingly, the suspension displayed a yield stress at a CB volume fraction of ϕCB = 0.008. The yield stress scaled with CB concentration with power law behavior; the power law exponent changed abruptly at a critical CB concentration, suggesting a substantial change in network structure. Crogenic scanning

*

Corresponding email: [email protected]

ACS Paragon Plus Environment

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electron microscopy revealed structural differences between the networks found in each scaling regime. Randomly oriented pores with thick CB boundaries were observed in the scaling region above the critical particle concentration, suggesting a strong gel network, and long, oriented pores were found in the scaling region below the critical particle concentration, suggesting a weak network, influenced by an induced shear stress. These findings correlate with the existence of gels and transient networks. The yield stresses of CB- gels containing oil emulsion droplets were found to scale with carbon black concentration as the CB-gels without oil. These results offer insight into salt-induced attractive colloidal networks and the difference in structure and yield-stress behavior between transient networks and gels. Furthermore, CB offers the ability to stabilize an oil phase in discrete droplets and contain them within a rigid network structure. 1. Introduction

Particle suspensions begin to display non-Newtonian behavior at high concentrations (ϕ ≥ 0.5) when interparticle interaction is driven only by volume exclusion[1]. This phenomenon can change as particles in a suspension exhibit attractive interactions. As attractive potential between particles increases, network formation can occur at dilute particle concentrations[2-6]. As attractive potential substantially exceeds thermal energy, aggregation of fractal clusters occurs under diffusion limitation until the cluster spans the entire sample, forming a gel[7]. A phase diagram can be compiled for attractive colloidal systems, where the phase is dependent on particle concentration, magnitude of interparticle attractive interaction and applied stress[8]. Gel phases emerge at finite particle concentration, ϕ, and interparticle attractive potential, U.

Gels can be classified through yield stresses, where at applied stresses less than the yield stress elastic behavior is observed. Yield stresses emerge in network systems, where a critical


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finite stress leads to microstructural breaking of any inter-particle network and macroscopic flow of the sample. Yield stresses in attractive colloidal gels are a function of particle concentration and the number and nature of inter-particle interactions necessary to overcome for sample yielding[9]. Yield stress is usually found to scale with power law behavior with reference to the critical gelation concentration, where the magnitude of the exponent is a function of interparticle interactions[7]. Pickering[10] and Ramsden[11] first reported that oil phases can be emulsified using solid particles[12]. The particles are partially wettable in both the oil and water phase,[12] and therefore prefer to locate at the interface. The position of particles at the oil-water interface and their ability to stabilize emulsions is a function of particle size[13] and shape,[14] wettability in the two liquid phases,[12] inter-particle interactions[15] and for charged particles, the salt concentration in the aqueous phase[16]. Particles at the oil-water interface can stabilize the emulsion by sterically inhibiting droplet coalescence[17], imparting charges at the interface leading to electrostatic repulsion between droplets[18], or by increasing the interfacial viscosity which leads to reduced thinning of the intermediate liquid layer between approaching droplets[19]. Many descriptions of emulsion rheology have focused on concentrated emulsions where particle stabilized droplets (ϕoil ≥ 0.4) interact due to volume exclusion and elastic and yield stress behavior results.[20-23].

Dilute emulsions generally conform to models based on

Einstein’s theory[24, 25], where inter-droplet interactions are small, however dilute Pickering emulsions stabilized by particles or molecules with attractive potential can form networks based on particle bridging[26, 27]. These systems contained intermediate droplet concentrations (0.12


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≤ ϕdroplet ≤ 0.5) and Pickering emulsions contained silica particle concentrations well outside of the very dilute regime (0.08 ≤ ϕsilica ≤ 0.2).

Carbon black (CB) is a fractal, carbon nanoparticle that has been studied as a model particulate within oil phases for rheological studies.[28-31] Within oil phases attractive potential between CB particles is based solely on van der Waals forces, and gelation, viscoelasticity and shear thickening have been reported.[30-33] Furthermore, CB-stabilized emulsions have been reported,[34, 35] the characteristics of which likely offer interesting behavior, similar to other carbon nanoparticle-stabilized emulsions.[36, 37] Here we investigate the effect of salt-induced attractive interactions between para-amino benzoic acid (PABA) terminated CB particles in the dilute regime (ϕ < 0.1) on phase behavior. Whereas many studies on network formation in attractive colloids use polymer or molecular surfactant to modulate weak attractive interactions, this investigation sheds light on the network formation in colloidal suspensions with salt-induced attractive potential. While we work with CB as a model particulate, these results should apply to any nanoscale fractal particle where attractive interparticle interactions can be induced through salt introduction. Additionally we use the model system to investigate the steady shear and structural property transitions between weak transient networks and gels. Furthermore, CB particles surface modified with para amino benzoic acid have been used to form oil-in-water emulsions, when the aqueous phase contains salt[34, 35]. Salting the surface carboxylate groups introduces hydrophobicity to otherwise hydrophilic particles, thus allowing them to locate at oil water interfaces. We investigate the effect of enveloped oil droplets within dilute CB gels, and offer insight into the structural changes that are imparted on colloidal networks when oil droplets are incorporated.


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2. Materials and Methods 2.1 Sample Preparation and Phase Diagram Para amino benzoic acid (PABA) terminated carbon black in aqueous suspension (15% w/w) was provided by Cabot Corporation. Purified de-ionized water was filtered using an Elga Purelab Classic water filtration system for use in the experiments. DI water was added to the CB stock suspension to provide CB volume fractions, ϕCB = 0.00005 -0.067, where the density of CB is ρ = 2.1 g/cm3. Sodium chloride and sodium dodecyl sulfate were purchased from Sigma Aldrich. Appropriate volumes of 5M NaCl were added to CB suspensions for final NaCl concentrations between 0.1 mM and 1 M. CB suspension with NaCl solutions were made in 1 mL centrifuge tubes and sonicated for 3 minutes at 42 kHz. These solutions were allowed to incubate for 3 days, and their physical states were classified as either a freely flowing homogeneous dispersion, a flocculated state with particles eventually settling out of suspension, or a gel. We note later that a weak network exists in some flocculated systems, and a strong network exists in the gel phase. 2.2 Rheology A TA Instruments AR-2000 shear stress-controlled rheometer was used to investigate the steady shear behavior of suspensions and emulsions. The minimum measurable torque is τ = 0.05 μN-m, with a torque resolution of τ = 1 nN-m. All yield stresses reported correspond with a measured torque of at least τ = 0.5 μN-m, one order of magnitude larger than the minimum measurable torque reported by the instrument manufacturer. Suspensions were vortexed for 2 minutes at 3000 rpm and probed within 5 minutes of mixing. This excludes the effect of any gravitational settling. 1 mL of each CB suspensions was characterized using a cone and plate geometry. Emulsions were formed by adding oil to aqueous suspensions of CB with salt and


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vortexing at 3000 rpm for 3 minutes. Rheological measurements were also made on emulsions formed with CB or surfactant using a 4cm diameter parallel plate assembly with a gap of 550μm. Samples were tested within 5 minutes of mixing. All samples were presheared at a shear rate of 𝛾=500𝑠−1 for 1 minute to remove the effect of any sample loading. Steady state tests were conducted using peak hold measurements. Samples were sheared at a constant torque until steady state behavior was attained, at which time shear rate, shear stress, viscosity and applied torque were recorded.

Torques corresponding with an approximate shear rate of 𝛾̇ =

500𝑠 −1 were applied first, with consequent torque measurements decreasing in magnitude. This down-sweep process removes any artifacts that may result from instrument resolution issues at low torques. 2.3 Imaging A VWR Vistavision inverted microscope was used to image emulsion droplets. Droplets were imaged with a 10x magnification objective and analyzed using ImageJ software provided by the NIH. Cryo-SEM imaging was conducted using a SIGMA VP Field Emission-SEM. Images were obtained using backscattering detection and 2 kV filament voltage. Samples were prepared using the same procedure as rheological samples and cryogenically prepared using a Gatan cryo-SEM cryotransfer system. Sublimation times of ~7 minutes were found to enhance topological features of the emulsion.

3. Results and Discussion Carbon Black Suspensions:


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Fractal CB particles had a mean size of approximately ~150nm, with a size range of 50300nm (see figure S1). CB particles are considered fractal because they are composed of ~20nm spherical carbon particles, which fuse together to form an aggregate of varying shapes with high surface area to volume ratios. A typical CB particle can be seen in figure 1a. CB particles were functionalized with para-amino benzoic acid at a ratio of 0.1-4.0 μM/m2, imparting a zeta potential of ~-60 mV and rendering them stable in aqueous suspension.[35, 38] We use NaCl to tune a reduction of electrostatic repulsive potential between particles, leading to a net attractive interaction potential. Na+ ions form a metal salt with surface carboxylate groups and reduces the hydrophilicity of the CB particles; these mechanisms result in the aggregation of CB particles, as seen in figure 1b. At CB concentrations below which networks are formed, aggregates of ~1μm in size are observed.


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Figure 1. Cryo-TEM images of (a) a typical CB particle and (b) an aggregate of CB particles induced through attractive interparticle interactions with the introduction of 0.6M NaCl. Scale bars are (a) 200nm and (b) 500nm. We use rheological measurements, cryo-scanning electron microscopy (cryo-SEM) and optical observations to investigate the phase behavior of CB particle containing suspensions. Steady shear experiments were performed using a range of CB and NaCl concentrations. Figure 2a shows measured shear stresses in response to applied shear rates for 0.00008 < ϕCB < 0.067. Suspensions below ϕCB = 0.008 exhibit Newtonian behavior. However, at a CB concentration of ϕCB = 0.008 the suspension displays a yield stress (see Figure S2 for log plot), with the stress versus shear rate behavior following a Bingham model with yield stress σy, σ(ϕCB) = σy + ηγ. Yield stress grows with increasing particle concentration, as is expected with colloidal networks


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when maintaining a constant interparticle attractive energy,[8] and scales with CB concentration, σy ~ ϕCBa. The yield stress grows with particle concentration as a larger volumetric density of possible inter-particle interactions is present in the network.

An abrupt change in scaling

behavior is observed at CB concentration of ϕCB = 0.05, where scaling changes from a = 2.4 for 0.008 ≤ ϕCB ≤ 0.05 to a = 8.3 for ϕCB > 0.05. Note that in other systems of particles with attractive interactions, such as laterite particles, the yield stresses also scale as a power law with particle concentration (a~3.2).[39] The magnitude of power law exponent indicates the strength of network interactions present in the suspension. The effect of strongly attractive inter-particle interactions leading to network formation can also be observed in zero shear viscosity measurements (inset figure 2b). At ϕCB ≥ 0.008 the viscosity diverges from values predicted by Einstein’s model for hard spheres in dilute suspension, η = ηs (1+2.5ϕCB). The abrupt change in yield stress scaling stems from a transition from a soft network to a gel at the critical CB concentration of ϕCB = 0.05. The soft network can break down due to gravitational forces over time, and therefore the yield stresses measured for the soft networks are only valid at short times after sample preparation. However this result may suggest that transient networks are observable using steady state experiments. This may offer a way of detecting soft networks, in addition to structural techniques.


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Figure 2. a) Shear rate vs shear stress data for CB solutions with 0.6M NaCl. b) Yield stress of CB suspensions with 0.6M NaCl. Inset) Zero-shear viscosity vs. CB concentration Scaling of yield stress with particle concentration in gelled suspensions with attractive interactions can also be described using a critical-like function form, where the yield stress scales with particle concentration relative to a gelation point, σy = σϕ(ϕ-ϕc)μϕ,[8] where ϕc is the critical gelation concentration and σϕ is a coefficient dependent on the strength of inter-particle attraction (U). The value of the power, μϕ, has been reported to be between 2.1-3.4.[8, 40] Our data scales according to this critical form with μϕ ~ 1.7, when we consider the strong gel regime at and above the gelation concentration of ϕCB =0.05.This value is argued to correlate with inter-


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particle interactive strength where larger values indicates stronger interactions that resist can shear[7]. Hydrophobic interactions in addition to van der Waals attractive forces leads to network formation at particle concentrations orders of magnitude smaller than many previous studies [5, 6, 39]and yield stresses orders of magnitude larger than those reported at similar concentrations (σy ~ 10-6 – 10-1 Pa vs. 10-1 – 101 Pa). Furthermore, this is one of the first studies to use saltinduced attractive interactions to form colloidal networks. We observe the effect of particle concentration on salt-induced, attractive colloidal networks; it is also known that by altering the magnitude of interparticle attractive energy, U, we can change the volume fraction at which a network is formed.

We modulate the value of U by varying the NaCl concentration in

suspension with CB. Steady shear experiments were conducted with the same CB particles using 0.1 and 1 M NaCl. The results from using those concentrations can be seen in table 1. For CB suspensions with 0.1 M NaCl, yield stress scaled with CB concentration with a power law where a=2.6, similar to the scaling of CB suspensions in the transient network regime with 0.6M NaCl. For this salt concentration, there was no critical ϕCB that was reached where scaling changed abruptly. This suggests that the magnitude of interparticle attractive energy, U, induced by 0.1 M NaCl is not large enough to form a strong gel at a particle concentration of ϕCB ≤ 0.078.

Table 1. The yield stress results of CB using various NaCl concentrations. Salt Concentration (M)

Transient Network Yield Stress Scaling (a)

Gelation Concentration (ϕCB)

Yield Stress at Gelation (Pa)

Gel Yield Stress Scaling (a)


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0.1

2.6

--

--

--

0.6

2.4

0.05

1.86

8.3

1

3.4

0.044

2.93

6.5

However, the emergence of a yield stress suggests the formation of a weak, transient network, which can also be observed in cryo-SEM images of ϕCB = 0.067 (see figure 3a,b,c). Long (~15 um) aqueous pores, free of CB, of approximately ~5-8µm in width (marked by red bars) are observed oriented in the same direction (marked by red arrows), encompassed by ~2 µm wide bands of CB comprising the colloidal network. The bands are clearly comprised of concentrated CB, as seen by the rough, particulate texture.

The network may display

characteristics expected of a weak, transient network. All pores in the cryo-SEM images are strongly aligned in the same direction, suggesting that the network is not strong enough to maintain a random, porous structure, and that the network reorients under some shear stress. It is known that gravitational forces can result in the compression and collapse of weak networks of attractive colloidal particles.[41, 42]. Furthermore, many breaks in the CB network are observed, as seen in figure 3b (red, dashed circles) and figure 3c. These observations and possible explanations of network alignment further emphasize the weak, transient nature of the network at this CB and NaCl concentration.


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Figure 3. Cryo-SEM images of CB at ϕCB = 0.067 with 0.1 M NaCl. Scale bars are (a) 10 µm and (b)(c) 2 µm. CB suspensions with 1M NaCl were investigated and an abrupt change in yield stress scaling was again observed. At this NaCl concentration, larger CB concentrations (ϕCB > 0.03) followed a Herschel-Bulkley model, σ(ϕCB) = σy + ηγn. Yield stress scaled with CB concentration where a = 3.4 for 0.008 ≤ ϕCB ≤ 0.044 and a = 6.5 for ϕCB > 0.044. This suggests that the gelation concentration for CB in 1M NaCl is approximately ϕCB = 0.044.

The formation of a gel network is also observed in cryo-SEM images, as seen in figure 4. The random orientation of CB free, aqueous pores (~5um wide, ~6-8 um long) and 2 µm wide CB bands comprising the network suggest a strong, gel-like network, that is not broken down and reoriented under shear.


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Figure 4. Cryo-SEM images of CB at ϕCB = 0.067 with 1 M NaCl. Scale bars are (a)(b) 2 µm and (c) 1 µm. Cryo-SEM imaging along with yield stress scaling from rheology suggest differences in the networks present with ϕCB = 0.067 at varying salt concentrations (1, 0.6 and 0.1M NaCl). Using DLVO theory to investigate interparticle interactions, the net energy between particles is equal to U = Uvdw + Uele, where Uvdw is van der Waals energy and Uele is electrostatic energy. The electrostatic repulsion between two particles is altered by salt through the Debye length, 1/κ = [(ε0εrkT)/(∑i(zie)2cio*)]1/2,[43] where with 1M NaCl in suspension the Debye length is approximately 0.3nm, for 0.6M it is 0.39nm and for 0.1M it is 0.96nm. These values are much smaller than the average interparticle distances at the most concentrated systems explored, r = 1/n3, where r is average interparticle distance and n is the number density of particles( ϕCB = 0.044, r ≈ 150nm). This leads to net attractive interparticle interactions at NaCl concentrations of 50 mM and larger (figure S3), correlating well with the compiled phase diagram where particles aggregate and settle out of suspension at that NaCl concentration and larger (figure 5). It appears that inter-particle potential is significantly more attractive with 1M NaCl than with 0.1M NaCl, supporting a significant difference in network structures observed at those concentrations.


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The rheological investigation and cryo-SEM imaging were used along with optical observations to build a phase diagram for CB with NaCl, seen in figure 5. With no salt induced attractive interactions, CB was observed to form a homogeneous, freely-flowing stable suspension at all concentrations up to ϕCB = 0.078. At concentrations of less than 50 mM NaCl and at all CB concentrations, the suspension remained homogeneous. Aggregate clusters likely comprise the stable suspension, because of salt induced attractive interactions. At nearly all CB concentrations and at concentrations of NaCl higher than 50 mM, the particles aggregate into clusters large enough (~1 µm, figure 1b) to drop out of suspension. At CB volume fractions of ϕCB = 0.044 – 0.078 and at salt concentrations of 0.1M and larger, a gel phase was observed (Figure 1b). There also exists the transient network phase between the gel and flocculated phases, where weak networks form, but succumb to gravitational influences. We summarize our understanding of possible microstructures existing in dilute CB suspensions in Figure 1c.


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Figure 5. a) Partial phase diagram of CB in aqueous solution with varying NaCl concentrations. The dashed lines at 0.1M and 0.6 M NaCl describes the salt concentration/CB concentrations used for steady shear investigations (in addition to 1M atop the phase diagram). The dashed line at CB = 0.05 marks the transition from flocculated to gel for suspensions with 0.6 M NaCl. b) Images of CB samples with salt exhibiting sol, flocculated and gel characteristics. c) Schematic representations of particle behavior in homogeneous sol, flocculated suspensions, transient networks and gel solutions. CB stabilized emulsions: Oil in water CB-stabilized emulsions were formed using NaCl concentrations of 1, 0.6, 0.1 and 0.05 M (see figure 6). Emulsion droplets formed with CB in 0.6 M NaCl were polydisperse, with the size distribution depending on CB, oil and salt concentration. Other carbon nanoparticle systems also exhibit varying droplet size distributions depending on solution characteristics, such as salt concentration.[44] An oil volume of 5% results in a narrow distribution of droplet size, with a peak at 38 µm. At oil volumes of 10% and 15%, larger droplets and a wider distribution of droplet size are observed. At all conditions


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droplets tended to flocculate, which has been observed in other carbon nanoparticle emulsion systems under conditions where the particles exhibit hydrophobic interactions.[45] Knowing that the addition of salt leads to attractive interparticle interactions of aqueous CB, and network formation in very dilute concentrations, the effect of introducing oil droplets into the network was investigated. The experimental studies were conducted with a gap of 550 µm between parallel plates.

As increasing CB concentration leads to smaller oil droplets, this gap is

significantly larger than the largest oil droplets at network forming CB concentrations. This is observed in cryo-SEM images of CB networks using ϕCB = 0.033, NaCl concentration of 0.6M and 10 v% octane, as seen in figure 7a-c.

Figure 6. Histograms of distribution of emulsion droplet size using CB for o/w emulsification. 0.015 w% CB with 0.6 M NaCl and a) 5%, b) 10% octane, c) 15% octane.

Insets are

microimages of CB stabilized octane o/w emulsions with respective solution characteristics. Scale bars are 100 µm.


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Droplets are observed within a network of CB, with the largest droplets observed at around ~ 20 µm in size (figure 7a), with droplets typically between 5-10 μm in size and distorted from a spherical geometry (figure 7b). This effect may stem from oil phase being trapped as a droplet within the randomly oriented network of CB, which offers some flexibility to distort the oil droplet phase. As seen in figures 7a-c, the droplets are completely covered by CB, as observed by the thick perimeter of CB around the edge of the droplet (figure 7c). CB particles are not observed in the oil phase, suggesting that particles continue to reside in the aqueous phase despite increased hydrophobicity.

Figure 7. (a-c) Cryo-SEM images of CB networks with ϕCB = 0.033, an NaCl concentration of 0.6M containing octane droplets with 10 v% octane. Scale bars are (a) 2μm and (b)(c) 1μm. The image in (c) displays the inset in (b) marked by the red, dashed square. d) Yield stress vs. CB concentration for o/w emulsions with 10% oil with 0.6 M NaCl. Inset) Zero-shear viscosity of CB stabilized emulsions vs. CB concentration. e) Schematics describing particle structures present in emulsion solutions: (from left) no network (ϕCB < 0.008), weak network (0.008 ≤ ϕCB < 0.05), gel containing emulsion droplets (ϕCB ≥ 0.05).


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The steady shear response of CB salt-induced networks containing stabilized oil droplets was investigated (see figure 7d). Using oil volume concentrations of 10% and 0.6M NaCl, yield stress behavior in CB stabilized emulsions emerges at the same critical concentration seen in the CB suspensions (ϕCB = 0.008) without oil. Yield stress values for suspensions and emulsions are similar at CB concentrations at and above the gelation concentration in suspensions, ϕCB = 0.05, leading to similar power law-type scaling (a ≈ 7.2). This suggests that the oil phase has little effect on the network structure of the strong CB gel phase, which dominates the yield stress scaling in this regime as suggested by its similarity to CB gels without oil. The CB salt-induced networks appear to envelop oil droplets, fully stabilize them and still maintain their yield stress values.. On contrary, the yield stress behavior of silica particle stabilized water in oil emulsions was observed to show much weaker power law behavior (a ~ 1.9) at silica particle concentration of (ϕsilica = 0.1-0.2) and droplet bridging was argued as a basis for yielding behavior .[26] This suggests that the enveloping of oil droplets within a salt-induced network of attractive particles offers a different nature than that of network structures formed by droplet bridging between emulsion droplets. At CB concentrations where transient networks are observed, the yield stress of oil droplet containing CB networks is ~ 1.15 – 4 times larger than that of the CB transient networks alone, suggesting that the presence of oil droplets leads to a strengthened network structure. We infer that CB has a strong affinity for the oil/water interface because of the oil wettability induced by salt. The yield stress of the transient networks of CB containing oil droplets grows almost linearly with the CB concentration. Again, the zero shear viscosity grows rapidly for ϕCB


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> 0.008 (inset figure 7d). This suggests that CB can be used to emulsify oil and form an enveloping structure to contain oil droplets, even at very low ϕCB.

We compare the steady shear behavior of CB suspensions with ϕCB = 0.008 and ϕCB = 0.034 and the behavior of the emulsions stabilized by those CB concentrations in figure 8. Although the yield stresses are similar for suspensions and emulsions containing CB at ϕCB = 0.034, the steady shear viscosities are much different. This contribution of oil droplets to flow resistance is expected from Einstein’s formula. The same can be inferred for ϕCB = 0.008; yield stress is larger for the emulsion, where oil contributes significantly to the strength of oil droplet/CB networks, as discussed previously, and viscosity is also larger due to the presence of droplet. Although systems of dilute emulsions (ϕoil ~ 0.1) have been described as forming gels,[46, 47] yield stresses were not described for the surfactant-stabilized emulsions.


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Figure 8. a) Shear rate vs shear stress of CB stabilized emulsions vs CB suspensions. (b) Yield stresses of CB suspensions vs CB stabilized emulsions We also explored the steady shear behavior of emulsions stabilized by sodium dodecyl sulfate (SDS). As seen in figure 9, no yield stresses are observed in SDS-stabilized emulsions, suggesting that the emulsion droplets do not form a network at the experimental temperature of 25 °C, although it is known that surfactant stabilized emulsions can form emulsion networks at lower temperatures.[47]

Through the introduction of a chosen CB concentration into an

emulsion system, droplets can be enveloped within a particle network and will move in unison, as opposed to SDS stabilized emulsions, where droplets have little/no tendency to be linked at


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typical ambient temperatures. This, for instance, is useful for oil spill situations, where strong networks of droplets lead to their localization, after which they can be skimmed from the water surface or be burned.

Figure 9. Shear stress vs. shear rate of SDS stabilized emulsions with oil concentration of 10v%.

4. Conclusions The behavior of PABA-terminated CB in aqueous solution with varying salt concentrations has been explored. The addition of NaCl to the CB suspension leads to the tuned reduction of electrostatic repulsion, and thusly an increase in net attractive potential, and the formation of metal salts between carboxylate groups on the CB surface and Na+ ions leads to decreased hydrophilicity. concentrations.

These interactions result in network formation at very low CB

Increasing CB particle concentration and/or attractive potential between

particles through salt concentration lead to an abrupt change in yield stress scaling at a critical concentration, which along with electron microscope images suggest a distinctive boundary between a transient network and a gel. This offers new insight into salt-induced networks in


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attractive colloids, which can be applied to other systems containing charge stabilized particles. Introduced hydrophobicity of particles allows for the emulsification of oil with CB. CB with salt-induced hydrophobicity can stabilize discrete oil droplets and contain them within a rigid particulate network, which is more robust and versatile than network structures possible with molecular surfactants. These findings shed light onto colloidal networks containing discrete, stabilized oil droplets and may have potential for oil spill applications where droplet-containing CB networks lead to oil localization for mechanical removal or burning. ASSOCIATED CONTENT Supporting Information. Description of CB size distribution, log plots of shear rate vs. shear stress of CB suspensions with NaCl and analysis of DLVO interactions between particles. This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was made possible by a grant from BP/The Gulf of Mexico Research Initiative ACKNOWLEDGMENT We thank A. Saha and H. Katepalli for insightful discussions. ABBREVIATIONS


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