Stress-Driven Separation of Surfactant Stabilized Emulsions and Gel

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Stress-Driven Separation of Surfactant Stabilized Emulsions and Gel-Emulsions by Superhydrophobic/Superoleophilic Meshes Vickramjeet Singh, Thao Phuong Nguyen, Yu-Jane Sheng, and Heng-Kwong Tsao J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b07449 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 13, 2018

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The Journal of Physical Chemistry

Stress-Driven Separation of Surfactant Stabilized Emulsions and GelEmulsions by Superhydrophobic/Superoleophilic Meshes Vickramjeet Singha,b, Thao Phuong Nguyena, Yu-Jane Shengc*, Heng-Kwong Tsaoa* a

Department of Chemical and Materials Engineering, National Central University, Jhongli 320,

Taiwan. E-mail: [email protected] b

Department of Chemistry, Dr. B R Ambedkar National Institute of Technology,

Jalandhar144011, Punjab, India. c

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan. E-mail:

[email protected]

ABSTRACT Oil-in-water emulsions and gel-emulsions which are stabilized by surfactants for more than five months have been successfully separated by a simple gravity-driven agitation-assisted device.

The

device

contains

a

rotating

magnetic

stir-bar

and

a

flexible

superhydrophobic/superoleophilic Cu mesh which has been folded into the desired threedimensional shape. The emulsions were prepared by mixing various fractions of alkane and water (99 % to 1 %) stabilized by a water-soluble surfactant. For low water fractions which contained higher concentrations of surfactant, solid-like gel-emulsions were obtained. The stressdriven process was found to effectively separate stable emulsions with a separation efficiency ≥ 98 % and gel-emulsions with separation efficiency as high as 96 %. The mechanism for oil recovery from stable emulsions by our stress-driven device is based on momentary breakage of the water barrier layer (superhydrophobicity) and enhancement of the oil-mesh contact leading to oil permeation (superoleophilicity). For gel-emulsions, the additional function of the stress is to induce a temporary transformation from the solid-like gel into a liquid-like state. Our methodology can be employed for efficient separation of highly stable oil-in-water emulsions and gel-emulsions that are often produced by industrial processes. 1

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1. Introduction Oil exhibits a strong reluctance to become miscible with water; however, a small amount of oil can be dispersed in water to form an oil-in-water emulsion, which is more difficult to separate into two phases than the oil-water mixture with comparable ratio obtained by agitation.13

The emulsion is a heterogeneous system containing one immiscible phase dispersed in another

phase. Because of the thermodynamic driving force to reduce interfacial area, emulsions lack long-term stability. However, their stability can be greatly enhanced by the inclusion of emulsifiers such as surfactants, polymers, and finely divided particles having amphiphilic properties.2-4 Meta-stable emulsions are ubiquitous in daily life and industrial applications, and sometimes they are undesirable. For example, the disposal of oil-water mixtures into the water stream or ocean from industrial processes such as oil-extraction and metallurgy causes long-term detrimental influence on the environment.2-6 The oil-water separation has fascinated the worldwide attention, owing to an increase in oily wastewater and frequent oil-spillage accidents.5-7 Evidently, the separation of the emulsified oil-water mixture becomes highly challenging as compared to conventional oil-water mixtures, due to enhanced stability of emulsions.1,2,8 The conventional separation methods include sedimentation, centrifugation, flotation, and oil skimmers, which can separate the typical oil-water mixture but fail for emulsified mixtures, specifically for surfactant stabilized emulsions.5,8-10 A well-known example is surfactant flooding employed for the production of crude oil from its reservoir. Surfactant reduces the wettability of solids inside the reservoir and lowers the interfacial tension between the oil and water phases. In this process, the water-in-oil (W/O) emulsion is formed initially, and it is then separated via numerous steps. Eventually, the produced fluid is inverted to form the oil-in-water (O/W) emulsion.11-13 Thus, petrochemical industries have to overcome the tedious task of oil recovery from emulsions (O/W or W/O) by separation or demulsification.11-14 Demulsification is the process of breaking the emulsions into individual components, and it can be achieved by physical, chemical, or biological techniques.15-16 However, several drawbacks are often encountered including the use of a large amount of expensive chemicals, high energy consumption, low breaking efficiency, high operation cost, contamination, and long settling time.6,11,13,16,17 2


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For typical oil-water mixtures, the excellent separation efficiency has been achieved by using porous polymeric membranes.18-20 In addition, for industrial emulsions separation, the pressure-driven membrane-based technologies that enable the size-dependent selective separation due to the size-sieving effect have been employed.5,7,21 The porous membrane plays a dual function as wetting and coalescing medium.21 However, the separation is based on size exclusion, and the membrane should possess pore size smaller than or comparable to that of dispersed liquid droplets. For membranes with very small pore size, the separation process was usually accompanied by applying pressure. Thus, porous membranes suffer from high energy consumption, low flux, low separation efficiency, and pore pluggage/failure of permeation due to surfactant adsorption.5,7,21-24 Moreover, the fouling and blockage of the membrane by oil droplets occur during separation when its oleophilic property is ruined and turns hydrophilic. As a result, there is an urgent need to design and fabricate advanced filtration systems to replace polymeric membranes.7,24 Recently, the metallic meshes with extreme wettability (superhydrophobicity and superhydrophilicity) which can selectively repel either oil or water have been employed for efficient oil-water separation. The advantages of those mesh filters are low energy consumption and high flux rate.10,22,25-31 Among these filters, the superhydrophilic meshes (oil resistance) are mostly preferred, because of their better separation efficiency as compared to superhydrophobic surfaces (water resistance).32-34 The superhydrophilic metallic meshes have been fabricated by decorating mesh structure by hydrophilic materials (metal oxide, mineral, hydrogels, etc.).31,32,3436

However, mesh coated with hydrophilic inorganics or organic hydrogels lacks mechanical

strength, which arises due to weak adhesion force between the mesh and coating.35,37 Superhydrophobic metallic meshes have been fabricated by a combination of low energy materials with hierarchical micro/nano-structures. Such coating can be easily ruined under harsh conditions (e.g., exposure to hot water).10,38-39 Moreover, during oil/water separation, a barrier layer of water may form on the superhydrophobic mesh surface which will hinder the oil permeation.34,35 Nevertheless, superhydrophobic or superhydrophilic meshes can separate the oilwater mixtures based on phase separation mechanism.10,26-28,30-31 Only a few reports are available for separation of emulsions into individual components by using metallic meshes.22,28,40-42 However, the emulsions used for separation were obtained without the addition of surfactants or separated by employing pressure. Therefore, the high throughput separation of more complex 3



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oil-water mixtures such as surfactant stabilized emulsions or gel-emulsions by robust metallic meshes without the exertion of pressures is still challenging.28,31,43 The oil-water separation is based on using either “water-removing” or “oil-removing” mesh materials44-48 and special sponges49-50 which can selectively absorb one of the liquid component (oil or water). For instance, the oil-water separation is achieved owing to underwater superoleophobicity of the mesh coated palygorskite having water-absorbing and water-retaining abilities.44 In addition, the mesh coated with SiO2 nanoparticles that shows the total wetting property for water, is able to exhibit selective water permeation and oil rejection.46-48 In the above mentioned works, the superhydrophobic and superhydrophilic meshes were fabricated via complex preparation steps by using expensive chemicals.27,28,51,52 In this work, a highly efficient and effective separation device based on a combination of a stir-bar driven agitation and superhydrophobic/superoleophilic

Cu

mesh

has

been

desinged.

The

superhydrophobic/superoleophilic Cu mesh with a large pore size (75 µm) was facilely fabricated. The mesh demonstrates easy permeation of oil phase while repelling water phase. The efficient separation of surfactant-stabilized emulsions and gel-emulsions at ambient pressure has been illustrated using our device. The separation device consisted of a magnetic stir-bar on a 3dimensional (3-D) mesh placed atop of the beaker. The emulsions were prepared by using various fractions (1 to 99 %) of water and alkanes (e.g., heptane, decane, dodecane, and hexadecane). The water-soluble surfactants were used as an emulsifier, and the emulsions can be stable for at least five months. The agitation-assisted separation process was performed under the influence of gravity, and the device can be reused. The oil recovery from stable oil-in-water (O/W) emulsions and gel-emulsions was successfully and effectively achieved. To the best of our knowledge, the direct separation of stable solid-like gel-emulsions was accomplished for the first time. This approach can be utilized to separate highly stable surfactant containing emulsions and solid-like gel-emulsions. Such stable oil-in-water emulsions are often discarded as oily waste-water by industrial processes. Finally, the mechanism for our agitation-assisted device has been proposed.

2. EXPERIMENTAL SECTION 2.1. Materials 4


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Sodium dodecyl sulfate (SDS, 99 %) was purchased from J. T. Baker Co. (Japan), cetyltrimethylammonium bromide (CTAB, > 99 %) from MERCK (Germany), n-heptane (99 %) and Tween 60 from Sigma-Aldrich Co. (USA), n-decane (> 99 %) from Tokyo Chemical Industry Co., Ltd. (Japan), triton X-100 from Acros Organics (USA), n-dodecane and nhexadecane (99 %) from Alfa-Aesar (England). The magnetic stir-bars with (10 mm × 3 mm) and (14 mm × 4 mm) were procured from Kwo-Yi Co., Taiwan. The copper mesh (200) was the product of May-Chun Company Ltd. (Taiwan). Millipore quality pure water was used for the preparation of oil-water mixtures and stable emulsions/gel-emulsions. 2.2. Fabrication of superhydrophobic/superoleophilic Cu meshes The pristine Cu mesh was firstly oxidized and then partially reduced to obtain the superhydrophobic/superoleophilic mesh.10,53 The Cu mesh cut into the desired size was thoroughly washed with deionized water. The mesh was then immersed in 1 M aqueous solution of HCl and heated at 50 oC for 30 minutes for the removal of impurities and native oxides. The mesh was then washed with deionized water again. For partial oxidation, the clean mesh was soaked in 30 % H2O2 solution (w/w) at 50 oC for another 30 minutes. This oxidized Cu mesh was washed by using anhydrous ethanol. The mesh immersed in anhydrous ethanol was annealed at 100 oC for about 3 hours under low pressure in an oven. The partial oxidation step gave the copper oxides on the mesh surface, and the step of low pressure reduction yielded Cu2O eventually. That is, our fabrication method changes the pristine Cu mesh into that covered with hydrophobic

Cu2O.

The

schematic

illustration

of

the

fabrication

process

of

superhydrophobic/superoleophilic Cu meshes is shown in Fig. S1.10 The obtained mesh becomes superhydrophobic/superoleophilic as evident from static water and oil (5 µL) drops contact angles of 153o and 0o, respectively.10 The optical microscopy (OM) images of pristine and superhydrophobic/superoleophilic Cu meshes are shown in Fig. S2 of the supporting information. This Cu mesh is flexible for the 3-D structure and robust against mechanical abrasion of a rotating stir-bar.

2.3. Emulsion preparation Oil-water mixture: The unstable oil-water mixture was prepared by mixing water and oil (dodecane) thoroughly. The mixture contained the water fraction Cw = 10 wt. %. Millipore 5



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quality pure water free from emulsifier (surfactant) was used. The mixture was vigorously stirred at room temperature by using a magnetic stir bar at a rotating speed of 1000 rpm for 30 minutes. The oil phase was dyed with Sudan I and water phase dyed with methylene blue. Unstable emulsion: The unstable emulsions were prepared by mixing water phase (Cw = 10 %) with dodecane. The water phase contained a low concentration of surfactant (3 mM). The mixture was vigorously stirred at room temperature for 30 minutes using a magnetic stir bar at a rotating speed of 1000 rpm. Phase separation took place within 10 minutes after the stirring stopped. Stable O/W emulsions: Various fractions of water (Cw = 99 % to 1 %) and oil phases were mixed and stirred vigorously at room temperature for 30 minutes using a stir-bar rotating at the rate of 1000 rpm. The aqueous phase contained a water-soluble surfactant (SDS or CTAB) at a high concentration (25 mM). The emulsion can be stable for more than five months. Stable O/W gel-emulsion: The gel-emulsions were obtained by vigorous stirring surfactantcontaining water (Cw =1 or 5 wt. %) with alkanes at 1000 rpm for about three hours at room temperature. Note that occasional shaking for about 20 s after an interval of 20 minutes was performed manually to facilitate the gel formation. The gel-emulsions with Cw = 1 and 5 wt. % were prepared by using a surfactant (SDS or CTAB) of very high concentrations (0.52 M and 0.1 M, respectively) in water. The formation of gel emulsions was slower when alkane with a longer chain or less amount of water was used. The gel-emulsion can be stable for at least five months. 2.4 Emulsion separation The superhydrophobic Cu mesh was bent into the 3-D structure so that it can be placed on top of any container such as a beaker. For the agitation-assisted process, a magnetic stir-bar was placed in the 3-D mesh structure before feeding emulsions. The stir-bar was rotated inside the feed sample (emulsion or gel-emulsion) at a specified rotating speed. Typically, the speed of stirbar was fixed at 80 rpm. The average outcome of at least 10 separation experiments has been reported. 3. RESULTS AND DISCUSSION The

metallic

meshes

with

extreme

wettability

(superhydrophobicity

or

superhydrophilicity) have previously been used to separate immiscible oil-water mixtures and 6


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unstable emulsions formed by mechanical agitation. However, the separation of emulsions stabilized by emulsifier is more challenging if it is performed simply by meshes.28,31,45 As a result, the filtration technique based on the size-sieving effect has been established, but the porous structure with nanoscale pore size leads to unwanted power consumption.30,54 In this work, a simple separation device involving agitation on superhydrophobic/superoleophilic Cu mesh has been developed to separate surfactant stabilized emulsions efficiently under ambient pressure condition. The mesh pore size is as large as 75 µm and the oil-water ratio can vary from 99 % to 1 %. The emulsion type was characterized, the separation process with/without agitation was performed, and the mechanism for highly efficient separation of stable emulsion and gelemulsion has been proposed. 3.1. Characterization of the emulsion type The heterogeneous systems containing liquid droplets dispersed uniformly in another continuous liquid phase are referred to an emulsion. The emulsion formed by mechanical agitation of two immiscible liquids is thermodynamically unstable, and coalescence occurs spontaneously to reduce the interfacial area.3,55 Its stability can be greatly enhanced by the addition of an emulsifier (e.g., surfactant).3,55 In this work, emulsions with various compositions (wt. %) of water and oil (dodecane) are formed. The composition of water ranges from 1 wt. % to 99 wt. %. The emulsions can be stabilized by adding a water-soluble surfactant, including SDS (anionic), Triton X-100, Tween 60 (non-ionic), and CTAB (cationic). Fig. 1a shows various SDS stabilized emulsions which demonstrate stability for more than five months. The total water content (Cw) increases from 10 to 90 wt. %. The surfactant concentration in the water phase is approximately 25 mM (0.07 wt. %). Note that when Cw ≥ 30 wt. %, two distinct phases are observed, emulsion (yellow color) and water phase (transparent). As Cw is increased, the amount of the emulsion phase decreases. At Cw = 90 wt. %, only a thin emulsion layer appears atop the water phase. The thickness of the emulsion layer continued to decrease as Cw is increased up to 99 wt. %.

7



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Figure 1. (a) The photographic images of stable O/W emulsions of different water fractions (Cw), (b) oil droplets dispersed in water as observed by OM, and (c) drop dilution test to identify the continuous phase of emulsions.

The emulsion morphology was observed by using optical microscopy (OM). Fig. 1 b illustrates the OM images for stable emulsions. Various droplets suspended in the continuous phase have been observed. Moreover, a large size distribution of droplets was observed, varying from 1 µm to 70 µm. The kinetic stability is due to surfactant adsorption at the oil-water interface, which prevents coalescence. To determine whether the droplet is oil or water, a simple drop dilution test was performed56 simply by placing emulsion drops into both water and oil phases. As depicted inset of Fig. 1 c, the SDS stabilized emulsion drop always sank in the dodecane phase whereas it gets dispersed in the water phase instantly. This outcome indicates that the continuous phase in the emulsion is water, regardless of the oil-water ratio (Cw). That is, all the emulsions formed are oil droplets dispersed in water (O/W) based on the drop dilution test. The nature of emulsion can be described by an empirical rule developed by Bancroft56,57. 8


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According to this rule, the continuous phase is the liquid in which an emulsifier is more soluble than it in the other phase. As the emulsifier is more soluble in the water phase, oil-in-water (O/W) emulsions are developed. Consequently, the surfactant determines the type of emulsion formed rather than the relative percentage of water or oil. Our dilution results are in agreement with the prediction of Bancroft rule.56,57 As the water content becomes less than 10 wt. % and the surfactant concentration in the aqueous phase is increased, one observed gel-emulsions which are distinct from typical emulsions. The continuous phase is still water as determined by the drop dilution test; that is, one has O/W emulsions. In general, when the volume fraction of the dispersed phase is 74 % or greater, the emulsion formed is termed as gel-emulsions or high-internal-phase-ratio emulsions.58-59 Even for Cw = 1 wt. %, the emulsion is gel-like as long as the SDS concentration is high enough (0.52 M). Fig. 2 a and b demonstrate the decane-in-water and heptane-in-water gel-emulsions formed by using 0.52 M and 0.1 M of anionic SDS surfactants for Cw = 1 wt. % and 5 wt. %, respectively. The gel-emulsions can also be prepared by using cationic surfactants such as CTAB. Our gel-emulsions exhibited stability for more than at least five months as long as the evaporation of alkane was avoided. The emulsion remains to stick onto the tube wall even after inverting the tube, indicating the gel-like behavior as shown in Fig. 2a (i) (iii) and 2b (i) (iii). To verify this gel property further, a steel ball with diameter 2.5 mm and density 7.71 g/cm3 sitting on top of the gel was observed as illustrated in Fig. 2a (ii) (iv) and 2b (ii) (iv). The OM images of the gel-emulsions are given in Fig. 2 c. Clearly, oil droplets suspended in the continuous water phase have been observed. Similar to typical O/W emulsions, the large size distribution of oil droplets in gel-emulsions has been witnessed. In comparison to Fig. 1 c, the droplet shape is distorted from the spherical shape significantly, indicating densely packed droplets. Thus, the droplet motion is hindered due to a very small amount of the continuous phase.58-59 The aforementioned stable O/W emulsions and gel-emulsions were used to perform the oil-water separation experiments.

9



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Figure 2. The photographic images of (a) decane-in-water and (b) heptane-in-water gelemulsions with CW = 1 and 5 wt. %. (c) The OM images of 5 wt. % gel-emulsions.

3.2 Ineffective separation of stable emulsions in the absence of agitation Our

separation

device

consists

of

a

rotating

stir

bar

on

a

deformed

superhydrophobic/superoleophilic Cu mesh. The superhydrophobicity of the mesh is evident from a high CA (> 150o) of a sessile water drop with 5 µL. The superoleophilic wetting behavior was observed by placing an oil drop on the mesh. One fails to determine the oil CA because the oil drop permeates the mesh readily. This outcome is similar to the failure of acquiring CA of a water drop on a superhydrophilic or total wetting surface (positive spreading coefficient)60 because of continuous spreading wetting behavior. As a result, the Cu mesh demonstrates the superoleophilic feature. The selective rejection of water by superhydrophobic meshes was employed for efficient separation of oil-water mixtures which are generally unstable.10,26,29 The separation of oil-water mixtures and unstable emulsions were performed simply by using the superhydrophobic Cu mesh (in the absence of agitation). The oil-water mixture (surfactant free) 10


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contains two immiscible components blended mechanically. The unstable emulsion was prepared by adding a very small amount of surfactant. However, the surfactant concentration is so low that phase separation occurs within a very short period. As shown in Fig. 3 a, the mesh exhibits high mechanical flexibility and can be folded to form a desired 3-D structure. Such a structure can be placed on top of a beaker or vial to conduct the gravity-driven separation process. Fig. 3 b illustrates both the oil-water mixture and unstable emulsion, prepared by mixing water (Cw = 10 wt. %) and oil (dodecane) at constant stirring. The former is free from a surfactant, and a few large drops (dyed with methylene blue) settle soon at the bottom to form the water phase due to their higher density. The latter contains 3 mM SDS in the water phase, and two distinct phases appear within 10 minutes. Obviously, those oil-water systems are thermodynamically unstable, and phase separation occurs rapidly. As demonstrated in Fig. 3 c, the separation for both cases is successful by using Cu meshes only. First, the mixture or emulsion was poured on to the Cu mesh. The oil phase permeated the Cu mesh immediately, while the water phase was retained above the mesh. The results indicate that this methodology based on superhydrophobic meshes works for unstable oil-water systems, which will separate into two phases soon.

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Figure 3. (a) The flexible superhydrophobic Cu mesh bent into a 3-dimensional structure, (b) oil-water mixture and unstable emulsion, and (c) their separation in the absence of agitation. Dodecane dyed with Sudan I was used as the oil phase, and the water phase was dyed with methylene blue.

The separation ability of the superhydrophobic mesh (without agitation) was further evaluated by considering several surfactant stabilized O/W emulsions. As demonstrated in Fig. 1 a, the presence of high surfactant concentration (≈ 25 mM) in the water phase leads to the formation of stable emulsions. Fig. 4 a depicted the separation process of the O/W emulsion with Cw = 10 wt. %. Nothing happened for a while after pouring the emulsion phase on the separating mesh. After about 10 s, the oil permeation through the mesh occurred. Note that the oil permeation took place instantly for both the oil-water mixture and unstable emulsion. The rate of oil permeation decreased with time, and approximately 50 % of the original emulsion was retained (retentate) on top of the mesh after 30 minutes. The retentate was enriched with water, and its composition is Cw = 20 wt. %. After 1 hour of standing, the retentate was about 40 % of the original emulsion. Fig. 4 b illustrated the retentate as analyzed by using OM and the drop dilution test. Similar to the original emulsion, the retentate also demonstrated oil droplets dispersed in the continuous water phase. The outcome indicated that the superhydrophobic mesh recovered only part of the oil phase and failed to separate the stable emulsion into the water and oil phases.

12


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Figure 4. (a) The separation of a stable 10 wt. % dodecane-in-water emulsion in the absence of agitation, and (b) the OM image of the retentate and photographic image of the permeate and retentate after an hour. Here dodecane was dyed with Sudan I.

The preferential wetting of the separating mesh by one of the liquid phases plays a crucial role in removing oil or water.31 The failure of the aforementioned separation experiment for stable emulsions by using superhydrophobic meshes can be attributed to the accumulation of the water phase. This assertion can be verified by first creating a thin water layer atop a planar (2-D) superhydrophobic mesh before the stable emulsion was poured. 28,61-62 As shown in Fig. S3, there is no oil permeation, and the stable O/W emulsion can stay atop the water layer over a period of time. That is, the continuous water phase was rejected by the mesh and the dispersed oil droplets stabilized by emulsifier remained unfiltered. Obviously, oil permeation was prevented by the presence of the (water) barrier layer. If the thin water layer is not present initially, it will occur eventually because the water phase with a higher density than the oil phase tends to settle on the surface of the superhydrophobic mesh.25,34,35 For our 3-D mesh structure, oil permeation which is inhibited from the bottom surface is allowed to occur through the surrounding vertical walls. Note that the barrier layer is difficult to maintain around the vertical wall due to the gravity 13



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effect. However, the permeation rate will decay with time because the wall area available for oil permeation decreases. Unlike the total blockage of oil in the 2-D mesh, the 3-D mesh can partially filter the oil phase but is unsuccessful in complete separation. 3.3 Agitation-assisted separation of stable O/W emulsions As mentioned, the O/W emulsions stabilized by surfactants cannot be successfully separated by 3-D superhydrophobic/superoleophilic meshes due to the water barrier layer, and only a part of the oil phase is recovered. To provide the passageway for the oil phase, an emulsion separation device was designed by combining agitation driven by the rotating magnetic stir bar with superhydrophobicity of meshes. Interestingly, this device can separate the stable emulsions into individual components successfully without resorting to any prior addition of emulsion breakers. Fig. 5 a shows the motionless stir bar placed inside the 3-D separating mesh before pouring the emulsion phase stabilized by SDS (Cw = 10 wt. %). Fig. 5 b demonstrates the initial period of the separation process with the stir bar rotating inside the emulsion at a constant speed of 80 rpm. Within a few seconds, oil permeation occurred, and a thin oil layer was observed at the bottom (Video S1). After 30 minutes of agitation, less than 15 % retentate was left on top of the mesh, as depicted in Fig. 5 c. The retentate became enriched in the continuous water phase with a small amount of oil left, i.e., Cw = 67 wt. %. Obviously, the rate of oil permeation was greatly enhanced because the agitation was introduced. Eventually, the stable emulsion was completely separated into two phases within approximately an hour.

14


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Figure 5. Top and side views of (a) the device consisting of the 3-D superhydrophobic Cu mesh and magnetic stir-bar. (b) Agitation-assisted separation of 10 wt. % dodecane-in-water emulsion after 10 s and (c) 30 minutes of agitation.

By applying the agitation-assisted process, the feed containing a stable emulsion (Cw = 10 wt. %) was successfully separated into the retentate (water) and permeate (dodecane). The retentate and permeate were separately collected and analyzed, as shown in Fig. 6. To evaluate the effectiveness of this separation process, the OM images of both retentate and permeate are also shown. The OM image of the filtrated oil phase reveals the absence of any water droplet. However, a few dispersed oil droplets are observed in the retentate. The largest diameter of the oil drops is about 5 µm. It is believed that those left oil drops were unable to make contact with the separating mesh within the separation period. The separation processes were also performed for stable emulsion phases with Cw varying from 5 to 40 wt. % and similar results were obtained. Although no water droplets were observed in permeate, the filtrated oil was analyzed for its water content by using Karl-Fisher titrator. The water content present in the filtrated oil phase is shown in Fig. 7 a for the feed emulsions with different Cw. It is found that the water content is independent of Cw and approximately 20 to 40 ppm. This result is close to the saturated water 15



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content (about 65 ppm) in pure dodecane63, indicating that there is no O/W emulsion permeated through the mesh.

Figure 6. (a) The results of agitation-assisted separation for the dodecane-in-water emulsion (Cw = 10 wt. %). The photographic and OM images are shown in (b) for the retentate, and (c) for the permeate.

16


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Figure 7. (a) The water content (ppm) present in the permeated oil phase. (b) The separation efficiency after agitation-assisted separation of various O/W emulsions with different water fractions (Cw). The error bars are also shown to illustrate the variability of the experimental results.

The performance of our 3-D separating device was also determined by estimating the separation efficiency (η) based on the relation: η = [(M/Mo) × 100 %], where M and Mo are the oil weights of permeate and feed (emulsion), respectively. Fig.

7 b shows the separation

efficiency for various stable emulsions with different Cw. It is found that η ≥ 98 % for all cases. The result indicates that a very high fraction of oil from stable O/W emulsions is recovered and the water content in the permeated oil is within the solubility of water in dodecane. That is, our 3-D separating device can remove surfactant-stabilized dispersed oil droplets from stable emulsions via the agitation assisted process on a superhydrophobic mesh. The fraction of unrecovered oil is very small (less than 2 %) and most probably stored in the retentate. A part of unrecovered oil was observed on top of the retentate (creaming)11 or captured in the mesh structure. Note that the concentration of SDS (25 mM) in water for the preparation of stable O/W emulsions is three times its CMC, revealing that a lot of micelles are present in the water phase. 17



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Therefore, some of the unrecovered oil can be attributed to the fact that a small amount of oil tends to be entrapped within the micellar hydrophobic cores, leading to swollen micelles.3 Those micelles are thermodynamically stable and can keep oil entrapped even when they are in contact with superhydrophobic meshes. In the absence of agitation, only 60 % of the oil phase from the feed emulsion is recovered. The rest of the oil phase stabilized by surfactant remains present in the retentate. However, the agitation-assisted separation recovers almost the entire oil phase. Thus, agitation enhances the separation efficiency of the 3-D separating meshes. The agitation assisted process can be performed by both batch (video S1) and continuous separation methods (Video S2). The agitation-assisted process can be used to separate stable emulsions with any oilwater compositions, and the oil phase can be heptane, decane, dodecane, or hexadecane. 3.4 Agitation-assisted separation of stable gel-emulsions The O/W gel-emulsion is supposed to form as the volume fraction of the oil phase is about 74 % or higher in the oil-water mixture.58,59 In this work, the stable gel-emulsions were acquired at Cw = 5 wt. % with dodecane. Since the gel is solid-like, it is a challenge to separate the constituents of the gel. Therefore, it is anticipated that gel-emulsions cannot be separated simply by the gravity-driven filtration process. As shown in Fig. S4, the feed gel-emulsion was placed onto the 3-D separating mesh and observed for oil permeation. After 1 hr of standing, about 20 % of oil from the feed gel-emulsion permeates through the mesh. The gel-emulsion atop of the mesh can be clearly seen from both top and side views of the photographical images. The retentate is approximately 80 % of the gel-emulsion (Cw = 5 wt. %), and it is twice the amount of the retentate from the stable O/W emulsion with Cw = 10 wt. %. This result shows that it is much more difficult to recover oil from the gel-emulsion than from the stable emulsion. Also, complete separation of the oil phase from the gel-emulsion cannot be achieved by using 3-D superhydrophobic meshes in the absence of agitation. We have shown that the oil recovery from stable O/W emulsions can be achieved by using the agitation-assisted process. It is intuitive to apply this approach for recovering oil from stable gel-emulsions. Owing to the solid-like state of gel-emulsions, the stir-bar cannot maintain its specified rotational rate and actually exhibited a much slower speed. The stress caused by the stir-bar motion still facilitated oil permeation through the mesh. Unfortunately, some of the eroded gel particles were push through the mesh eventually, leading to failure of the separation 18


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process. To avoid the penetration of gel, the water barrier layer has to be provided atop the mesh by adding a small amount of water. Fig. 8 demonstrates the separation process of a gel-emulsion (i.e. dodecane-in-water) with Cw = 5 wt. %. The gel-emulsion was placed first, followed by the addition of water. The amount of water added is about 5 wt. % of the total feed gel-emulsion. After 30 minutes of agitation, about 75 % of the oil phase was permeated down the mesh as shown in Fig. 8 b. In an hour, about 90 % of the oil phase has been recovered as permeate. The amount of oil collected by the stress-driven process is above four-fold as compared to that in the absence of agitation. It is not surprising that the time required for the separation of gel-emulsions is much longer than that of emulsions. To increase the separation efficiency of the gel-emulsion, more separation time is needed. In fact, the separation efficiency can reach as high as 96 % within 2.5 hours. The pore size of the superhydrophobic mesh affects the performance of the oil/water separation process.10 However, increasing the pore size furthermore may enhance the oil flux, but the separation ability will be significantly reduced. The water content present in the filtrated oil phase was found to be within the saturated solubility limits. That is, the gel-emulsion can be completely separated into two phases eventually by the agitation-assisted separation process on the 3-D superhydrophobic mesh. The agitation-assisted process was also applied to recover oil from gel-emulsions formed by various alkanes such as heptane, decane, and hexadecane.

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Figure 8. The agitation-assisted separation of the gel-emulsion for dodecane-in-water (Cw = 5 wt. %). (a) In the beginning, (b) after 30 minutes and 1 hr, and (c) the permeated oil phase after 30 minutes and 1 hr of separation.

During separation of stable O/W emulsions and gel-emulsions, the agitation was continuously provided on the mesh surface by a rotating stir-bar. The loss of superhydrophobicity of the mesh due to mechanical abrasion of the stir-bar rotation can lead to failure of the separation process. Consequently, the mechanical stability of agitated superhydrophobic Cu mesh was examined by continuous rotation of the stir-bar on the mesh surface at 80 rpm for 48 hours. The resultant mesh structure still repelled the water phase (containing 25 mM surfactant) and allowed easy permeation of the oil phase. In fact, this mesh can be reused at least for 30 cycles to separate surfactant stabilized O/W emulsions successfully. The separation efficiency remains essentially the same as those shown in Fig. 7 b. That is, our flexible Cu mesh is able to resist abrasion and retains its superhydrophobicity/superoleophilicity even after a long-time agitation.

3.5 Mechanism of agitation-assisted separation For the separation of the stable O/W emulsions by superhydrophobic meshes, Fig. 9 shows the three permeation mechanisms of the oil phase. As shown in Fig. 9 a, the separation process will fail if a 2-D superhydrophobic mesh is used in a column.34-35,40 Atop the planar mesh, the water barrier layer is developed to prevent further oil permeation after the initial leakage of oil. As demonstrated in Fig. 9 b, when a 3-D mesh structure is employed, the oil phase is allowed to permeate from the vertical walls as well as from the base of the mesh initially. However, the repelled water phase will accumulate at the bottom eventually, and block the passage of oil from the base. The barrier layer reduces the contact area between the oil phase and mesh, and oil permeation will be hindered. As depicted in Fig. 9 c, when the stress is constantly provided by agitation, momentary breakage of the barrier layer ensues. Consequently, the probability for dispersed oil droplets to encounter the surface of the mesh directly increases. The oil phase will permeate the 3-D mesh from both the bottom and surrounding walls continuously.

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Evidently, the permeation rate of oil and separation efficiency in the presence of agitation are much higher than those in the absence of agitation.

Figure 9. The schematics of the separation mechanism. (a) Planar (2-D) superhydrophobic mesh with the water barrier layer, (b) 3-D mesh in the absence of agitation, and (c) 3-D mesh with momentarily ruptured barrier layer due to agitation.

By using the agitation-assisted device, the separation of solid-like gel-emulsion became successful if a small amount of water was added initially. Similar to the function in separating the stable emulsion, the stress provided by agitation enhances the probability of dispersed oil droplets to encounter the mesh structure via the disruption of the water barrier layer. In addition, the stress also causes water to mix with the gel-emulsion, thus transforming the solid-like gel to the liquid-like state temporarily. As a result, the stress assists the oil phase to permeate 3-D mesh without the penetration of gel particles. It is worth emphasizing that subject to agitation in a bottle, the mixture of the gel-emulsion and a small amount of water becomes liquid-like first, but 21



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turns into gel-like eventually. Fig. S5 demonstrates this transformation process for the dodecanein-water gel-emulsion (Cw = 5 wt. %, dyed with Sudan I) in a bottle after mixing with water (dyed with methylene blue) whose weight is 5 % of the gel-emulsion. The added water tends to be dissolved into the gel-emulsion, and the gel turned into liquid after 30 minutes of agitation. As illustrated in Fig. S5 f, the dispersed oil droplets exhibit their shapes close to the spherical morphology, in comparison to the distorted oil droplets in the network structure (Fig. S5 e) based on OM characterization. However, after 2 hours of agitation, the mixture (5 wt. % water and total gel) in the bottle will form solid-like gel-emulsion again. Evidently, the agitation plays a dual role: the stress induces the temporary liquid-like state whose separation is facilitated by this stress through the breakage of the barrier layer.

4. CONCLUSIONS A simple separation device comprising of stir-bar driven agitation on the 3-dimensional Cu mesh has been developed for separating stable O/W emulsions and gel-emulsions at ambient pressure. The pristine Cu mesh was first oxidized and then partially reduced to obtain a superhydrophobic/superoleophilic mesh. This mechanically flexible mesh was folded into the desired structure that can hold emulsion as well as rotating magnetic stir-bar. The emulsions used for separation were prepared by mixing various fractions of oil and water (from 99 % to 1 %) stabilized by the addition of surfactants such as SDS and CTAB. Alkanes such as heptane, decane, dodecane, and hexadecane were used as the oil phase for the preparation of emulsions. The emulsion type was characterized by the drop dilution test. The result revealed that the emulsion consists of oil droplets dispersed in the continuous water phase. Solid-like gelemulsions were formed at low water fractions (1 or 5 wt. %), which contained a higher concentration of surfactant (0.52 M or 0.1 M, respectively). The OM images of gel-emulsions exhibited distorted spherical shapes of crowded oil droplets enclosed in the network structure of the water phase. Generally, the occurrence of the water barrier layers on the superhydrophobic mesh hinders oil permeation, leading to failure or low separation efficiency of the separation process. In this work, both O/W emulsions and gel-emulsions were successfully separated by 3-D meshes with the help of agitation. While the water phase gets repelled by the mesh 22


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(superhydrophobicity), the oil phase has the opportunity to permeate because it is forced to encounter the mesh (superoleophilicity). The agitation-assisted process was found to effectively separate stable O/W emulsions with a separation efficiency greater than 98 % and separate solidlike gel-emulsions with a separation efficiency as high as 96 %. The mechanism for oil recovery from stable emulsions by our stress-driven device is based on momentary breakage of the water barrier layer and enhancement of the collision rate between oil droplets and superhydrophobic mesh. For gel-emulsions, the additional function of the stress is to transform the solid-like gel into the temporary liquid-like state.

Supporting Information The schematic illustration of the fabrication process of superhydrophobic/superoleophilic Cu meshes, the OM images of pristine Cu mesh and superhydrophobic/superoleophilic Cu mesh, the photographic images for a water drop and an emulsion drop (10 wt % dodecane-in-water) atop planar Cu meshes, the separation of the gel-emulsion in the absence of agitation for dodecane-inwater (Cw = 5 wt %), and the transformation process for the dodecane-in-water gel-emulsion (Cw = 5 wt. %, dyed with Sudan I) in a bottle after mixing with water (dyed with methylene blue) are supplied as Supporting Information.

Acknowledgments The authors thank the Ministry of Science and Technology of Taiwan for financial support.

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Stress-Driven Separation of Surfactant Stabilized Emulsions and Gel

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