Oil Interfaces: Tessellated

Oct 9, 2013 - The interfacial assembly of graphene oxide (GO) at the water/oil interface and its kinetics were systematically studied. GO nanosheets w...
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Assembly of Graphene Oxide at Water/Oil Interfaces: Tessellated Nanotiles Zhiwei Sun, Tao Feng, and Thomas P. Russell* Department of Polymer Science and Engineering, University of MassachusettsAmherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: The interfacial assembly of graphene oxide (GO) at the water/oil interface and its kinetics were systematically studied. GO nanosheets were found to segregate to the water/oil interface and interact with quaternized block copolymer chains by the peripheral carboxyl groups on the GO. If the interfacial area is decreased, then GO, assembled at and confined to the interface, jams and then buckles. An analysis of the kinetics of the assembly processes leads to the conclusion that the diffusion of GO to the interface is the ratedetermining step. The morphology of the jammed GO film was investigated, and TEM images show that GO sheets form a mosaic or tile across the whole oil/water interface.

1. INTRODUCTION Amphiphilic nanoparticles (NPs) (e.g., CdSe, silica, and viruses) will be assembled at the water/oil interface as surfactants to reduce the interfacial energy. The stabilization of one fluid in another via this route is known as the formation of a Pickering emulsion.1−8 A solid film would be formed at the interface if nanoparticles were closely packed at the interface and jammed together, which provides strong protection against the coalescence of emulsion droplets, making emulsions with long-term stability. Spicer and co-workers showed a hexagonal packing of silica microspheres at the interface of arrested coalescing droplets,1 and Russell and others prepared longrange-ordered arrays of virus bionanoparticles that were 30 nm in size at a water/oil interface.9−11 Unlike these large particles, nanoparticles form a disordered array of particles, like a glass. In all cases, a monolayer of particles is formed that can be used as a membrane, for encapsulation, for functional nanodevices, and as the basis of a hierarchically ordered nanocomposite material.3,12−14 Graphene oxide is an atomically thin sheet that has a hydrophobic basal plane and some hydrophilic groups (e.g., carboxyl and hydroxyl groups) decorating the periphery.15−20 It has been argued to be a potential surfactant in stabilizing emulsions of immiscible liquids.21−23 As a giant, flexible planar sheet, GO was expected to conform to a curved interface geometry and could be a very effective barrier in blocking the direct contact of two immiscible liquids. However, the packing of GO at water/oil interfaces has not been discussed. Because GO nanosheets usually have irregular shapes, there is an open question as to how GO will effectively cover the interface and, in doing so, how the irregular shapes of the GO pack together to maximize the interfacial coverage. © 2013 American Chemical Society

Graphene oxide has been reported to have only limited adsorption to the oil/water interface, and no indications of a jammed film at the interface have been discussed.12,23 A key step in enhancing the assembly of GO at an interface is to improve its surfactancy. One route to this end is to add hydrophobic polymer ligands that interact favorably with the hydrophilic GO. These interactions can, of course, be maximized at the interface between an aqueous dispersion of GO in contact with an oil phase containing the endfunctionalized polymer. Here, we report that graphene oxide can be trapped at water/oil interfaces and jammed into a solid thin film using a block copolymer, poly(styrene-b-vinylpyridine), as a ligand. The morphology of the jammed GO film at the interface was revealed with TEM. In addition, the assembly kinetics of GO at the interface was systematically studied.

2. EXPERIMENT 2.1. Materials. Graphene oxide was obtained from the Graphene Supermarket in a concentrated aqueous solution (5 g/L) with an average size of 500 to 700 nm (AFM images shown in Figure S1, Supporting Information). Poly(styrene-b-vinylpyridine) and poly(styrene-r-vinylpyridine) with different molecular weight (as shown in Table 1) were obtained from Polymer Source and used without further purification. Anhydrous toluene from Sigma-Aldrich was used as received. 2.2. Assembly of GO at the Water/Toluene Interface. Aqueous GO solutions of various concentrations were prepared from the concentrated GO stock solution and sonicated for 5 min Received: June 27, 2013 Revised: September 12, 2013 Published: October 9, 2013 13407

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interfacially active, and no stable emulsion was obtained even with intense sonication. A typical surfactant should reduce the interface tension continuously before reaching an equilibrium value because surfactant molecules tend to migrate to the interface. However, the water/toluene interfacial tension remained constant at ca. 28 mN/m (Figure 1a, line (ii)) with the addition of GO to the water phase, indicating that the asreceived graphene oxide has limited surfactancy to the water/ toluene system. The loss of amphiphilicity may be a result of the pH, concentration, or ratio of hydrophilic groups on the periphery to the size of the graphene sheet (i.e., the hydrophobic area). A substantial interfacial assembly requires an appropriate affinity of GO for the toluene phase, so S2VP was added to the toluene. The poly(2-vinylpyridine) block (P2VP) in the BCP was expected to interact with carboxyl groups on graphene oxide. It was reported elsewhere that there are two types of interactions between poly(styrene-b-vinylpyridine) and graphene oxide. One is the hydrogen bonding between the poly(vinylpyridine) block and carboxyl groups on graphene oxide, and the other is the electrostatic attractions between quaternized pyridine groups and carboxylate when exposed to an acid environment. Some hierarchical structure of graphene oxide and poly(styrene-b-vinylpyridine) block copolymer has already been reported elsewhere.24,25 Effective segregation of GO to the interface was obtained when S2VP19.5k was added to the toluene phase. As shown in Figure 1b, a pendant water droplet containing GO was placed in the S2VP19.5k toluene solution with a syringe connected to a blunt needle. During equilibration of the droplet in toluene for 10 to 20 min, the interfacial tension was measured. If at equilibrium the volume of the drop was decreased by withdrawing fluid with the syringe, then the interfacial area decreased and a wrinkling and buckling of the assembled GO occurred. It should be noted that the buckling was observed only when both GO and S2VP19.5k were added to the water and toluene phases, respectively. Buckling would not be observed if either GO or S2VP19.5k was absent. The buckling indicated the formation of an elastic film at the interface

Table 1. Molecular Weight and PDI of All Polymers Used

a

materials

Mn

PDI

abbreviation

poly(styrene-b-2-vinylpyridine) poly(styrene-b-2-vinylpyridine) poly(styrene-b-4-vinylpyridine) poly(styrene-b-2-vinylpyridine) poly(styrene-r-2-vinylpyridine)a

8.2k-b-8.3k 16k-b-3.5k 24k-b-1.9k 13.8k-b-47k 71k

1.10 1.05 1.10 1.11 1.60

S2VP16.5k S2VP19.5k S4VP25.9k S2VP60.8k P(S-r-2VP)

The mole ratio of the 2-vinylpyridine monomer is 56%.

before use. An interface was made by injecting a pendant drop of the aqueous GO solution into a toluene solution containing poly(styreneb-2-vinylpyridine) (S2VP) block copolymer (BCP). GO diffused to the interface and, upon interacting with the BCP by hydrogen bonding/electrostatic interactions, was trapped at the interface. This is shown in Scheme 1. The assembly process was recorded by monitoring the interfacial tension change with time. Large-area GO films were assembled at the water/toluene interface as follows: 5 mL of a 0.10 g·L−1 aqueous GO solution was placed in a 20 mL vial with a diameter of 2.5 cm, and 2 mL of a 0.10 g·L−1 S2VP60.8k toluene solution was added on top of the GO solution. The two-phase system was then purged with N2 and gently bubbled from the water phase at the bottom for 5 min, and a wrinkled film formed at the interface. Then, the S2VP60.8k/toluene solution on top of the water phase was replaced with pure toluene several times to remove excess S2VP60.8k. The film was retrieved with a copper grid or a silicon wafer for EM characterization. 2.3. Characterization. The interfacial tension between water and the organic solvent was measured with a tensiometer (Dataphysics OCA 15plus) by the pendant drop method. The volume of the droplets was controlled to be 10 μL. The pH of the aqueous solution was measured with an Accumet model 20 pH/conductivity meter. Transmission electron microscopy (TEM) images were collected on a JEOL 2000FX at an acceleration voltage of 200 kV. Atomic force microscopy (AFM) images were collected on a Digital Instruments Dimension 3100 AFM. X-ray photoelectron spectroscopy (XPS) was obtained on Physical Electronics Quantum 2000 scanning ESCA microprobe.

3. RESULTS AND DISCUSSION 3.1. Assembly of GO at the Water/Toluene Interface. The as-received graphene oxide in this study was not

Scheme 1. Assembly of Graphene Oxide and Block Copolymer at the Water/Toluene Interface

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Figure 2. Equilibrium interfacial tension of toluene/water at different concentrations of S2VP19.5k in toluene and GO in water.

with increasing GO concentration. The higher saturation value of the interfacial tension is a direct consequence of the assembly of GO at the interface because the interfacial area occupied by the GO sheet at the interface is much greater than that of individual S2VP19.5k and GO prevents the substantial assembly of S2VP19.5k at the interface. Although the carboxyl groups at the edge of GO interact with S2VP19.5k at the interface, the large carbon basal plane interacts unfavorably with S2VP19.5k. More GO at the interface means less S2VP19.5k and leads to a higher equilibrium interface at a fixed concentration of S2VP19.5k. Ravera and co-workers27 also reported a monotonic increase in interfacial tension between water and hexane with an increasing quantity of silica nanoparticles at the interface because nanoparticles will deplete the interface of small-molecule surfactant. Both PS-b-P2VP and PS-b-P4VP with different molecular weights and volume fractions of PS (S2VP19.5k, S2VP60.8k, S4VP25.9k, and S2VP16.5k) were studied. These showed good activity toward the interfacial assembly of graphene oxide (Figures S4−S6 and Video S2, Supporting Information). The driving force for the interfacial segregation of GO is the hydrogen bonding/electrostatic interactions between carboxyl groups on GO and pyridine groups on block copolymer because GO will not segregate at the interface at pH > 7 (Figure S3, Supporting Information). In an alkali environment, all carboxyl groups on graphene oxide become carboxylate ions and no excess protons are available for the quaternization of P2VP blocks. Therefore, neither hydrogen bonding nor electrostatic interaction exists between GO and S2VP block copolymer at pH >7. 3.2. Assembly Kinetics. The assembly kinetics of graphene oxide at the interface was studied with the buckling phenomenon. The coverage of graphene oxide on the droplet surface increased with time during assembly. As shown in Figure 3a, the droplets in the first column are 0.12 g·L−1 GO/ water solutions immersed in 0.10 g·L−1 S2VP19.5k/toluene for different times (15−300 s) before reducing the volume of the droplet. The second column shows the onset of buckling of the droplets when the volume is reduced. It is clear that the droplet surface area at the onset of buckling increases with increasing assembly time.

Figure 1. (a) Dynamic interfacial tension of the toluene/water interface at different concentrations of graphene oxide and S2VP19.5k block copolymer: (i) water against toluene, (ii) 0.12 g·L−1 GO/water against toluene, (iii) 0.25 g·L−1 GO/water against 0.10 g·L−1 S2VP19.5k/toluene, (iv) 0.12 g·L−1 GO/water against 0.10 g·L−1 S2VP19.5k/toluene, (v) 0.04 g·L−1 GO/water against 0.10 g·L−1 S2VP19.5k/toluene, and (vi) water against 0.10 g·L−1 S2VP19.5k/ toluene. (b) Buckling of the droplet surface assembled with graphene oxide, corresponding to line iv.

between the two fluids, which occurred when GO was closely packed and jammed, and this phenomenon was reported elsewhere with spherical particles (e.g., silica nanoparticles1,26). The GO film that formed at the interface between the two fluids was so robust that it could withstand massive shrinkage of the droplet. In fact, the GO film remained intact, even when the entire droplet was extracted back into the syringe and this process was repeated for numerous cycles (Video S1, Supporting Information). The surface tension, as shown in Figure 1a, also indicated the interfacial assembly of GO. First, it should be mentioned that S2VP19.5k acts as a surfactant for water and toluene, as evidenced by the decrease in the interfacial tension to a value as low as 11 mN/m when 0.10 g/L S2VP19.5k was added to the toluene phase (vi in Figure 1a). The surface tension will decrease even more with the addition of acid to the water phase, and it can be as low as 6.5 mN/m if the pH of water is adjusted to 3.0 because the poly(vinylpyridine) block at the interface would be partially quaternized by protons in water, making it more hydrophilic (Figure S2, Supporting Information). However, if GO was present in the aqueous phase (pH 4), then when S2VP19.5k was added to the toluene the interfacial tension decreased, but more slowly and leveled off at a value of 13 to 26 mN/m, which is much higher than that seen with only S2VP19.5k (iii−v in Figure 1a). The equilibrium interfacial tension was determined as a function of the GO and S2VP19.5k concentrations. As shown in Figure 2, the interfacial tension decreases with increasing concentration of S2VP19.5k at a fixed GO concentration whereas it monotonically increases 13409

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Figure 3. Change in droplet surface coverage with increasing assembly time: (a) droplet is 0.12 g·L−1 GO/water and ambient is 0.10 g·L−1 S2VP19.5k/toluene and (b) 0.12 g/L−1 GO/water against 0.10 g/L−1 S2VP19.5k/toluene (black line) and 0.08 g·L−1 GO/water against 0.10 g·L−1 S2VP19.5k/toluene (red line).

solution diffusion coefficient of a planar sheet and a polymer coil is considered. The sheets can diffuse in the plane of the sheet (with a cross section appearing as a rod) or normal to the sheet surface. In both cases, the sheets will diffuse much more slowly than the polymer coil. In addition, there are three distinct steps in the interfacial assembly of GO. First, GO in the aqueous phase must diffuse to the interface and then interact with S2VP19.5k in the toluene phase. To maximize interactions, GO must then reorient at the interface by assuming an orientation parallel to the interface. The diffusion of GO to the interface is the slowest and therefore the ratedetermine step in the assembly. Even though the diffusion of GO to the interface is the ratedetermining step in this interfacial assembly, the diffusion of S2VP(16.5k) ligands to the interface is also crucial to successful assembly. The pyridine group in S2VP(19.5k) is the moiety that interacts with the carboxyl groups in GO. However, it was found that the P(S-r-2VP) random copolymer did not interact well with GO at the interface. As shown in Figure 5b, no surface buckling was observed when the droplet size was reduced after the aqueous GO droplet was allowed to equilibrate in a toluene solution of P(S-r-2VP), even though the volume fraction of 2-vinylpyridine was 0.56, more than 3 times that of the S2VP(19.5k) block copolymer, where the volume fraction of 2-vinylpyridine was 0.18. In comparison to S2VP(19.5k), the interfacial tension decreased much less when P(S-r-2VP) was used (Figure 5a), indicating that P(S-r-2VP) was much less interfacially active, less likely to segregate to the interface, and therefore less likely to interact with GO. 3.3. Morphology of Jammed Graphene Oxide Nanosheets at the Interface. To examine the morphology of the jammed GO sheets at the interface, a large-area GO film was prepared and characterized by TEM and XPS. A 0.10 g·L−1

The point at which buckling began provided a simple route to assess the rate at which GO assembled at the interface. If the GO sheets are uniformly distributed over the area of the interface (termed the “free” state), then as the volume of the drop is decreased, the GO sheets will try to close pack. However, because of the irregularity in the shapes of the GO, the sheets will be jammed in a non-close-packed state (termed a “jammed” state). Buckling happens when graphene oxide sheets were closely jammed together at the interface. Therefore, the coverage (C) of GO on the droplet surface in the “free” state could be estimated as follows C≈

SJ SF

where SJ and SF are surface areas for the jammed and free states, respectively. As shown in Figure 3b, the surface coverage of GO on the droplet initially increases rapidly and plateaus in 5 min. In fact, the surface coverage could be as high as 90% for 0.10 g· L−1 S2VP19.5k/toluene and 0.12 g·L−1 GO/water (Figure 3b). Further increases in coverage are inhibited by packing constraints at the interface. Further study of the assembly kinetics shows that the diffusion of GO to the interface is the rate-determine step in the interfacial assembly. Figure 4 shows the surface coverage on a droplet at a constant time of 60 s at different GO and S2VP19.5k concentrations. The surface coverage at fixed 60 s increases rapidly with increasing concentration of GO in the aqueous phase (Figure 4a), but it remains relatively constant with increasing S2VP19.5k concentration in the toluene phase (Figure 4b). The assembly rate appears to be more sensitive to the concentration of GO but almost independent of the S2VP19.5k concentration. This can be understood when the 13410

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Figure 6. Top-down view of a large-area wrinkled graphene oxide film at the water/toluene interface in a vial.

wrinkles arise from a compression of the GO film at the interface that, during N2 purging, was stretched by entrapped N2. The GO film at the interface was retrieved with a copper grid or silicon wafer after replacing the S2VP60.8k/toluene solution with pure toluene at least 10 times to remove excess S2VP60.8k ligands in the toluene phase. The TEM image showes that GO sheets jammed together at the interface and a continuous film was formed, essentially a tessellation of GO (Figure 7). It shows that GO sheets aligned parallel to the oil/ water interface, forming a mosaic or tiling across the entire interface, leaving no empty space in between GO sheets. The thickness of the GO film was not uniform. Some parts of the assembly are single layers whereas there is some overlap at the boundaries of two graphene oxide sheets in other areas, and crumpling of the GO was observed when GO was compressed during the assembly. Moreover, poly(styrene-b-2-vinylpyridine) has many vinylpyridine repeat units in a P2VP block, so it was possible that two or more graphene sheets were attached to the same poly(styrene-b-2-vinylpyridine) molecule and overlap each other. Block copolymer might act as a macromolecular cross-linker. However, it should be mentioned that the overlapping of the film was not permanent, graphene oxide sheets would be released back to the free state, and the solid

Figure 4. Coverage of graphene oxide on the droplet surface at a constant time of 60 s at different concentrations of graphene oxide and S2VP19.5k: (a) 0.10 g·L−1 and 1.0 g·L−1 S2VP19.5k/toluene against different concentrations of GO/water, respectively, and (b) 0.08 g·L−1 and 0.12 g·L−1 GO/water against different concentrations of S2VP19.5k/toluene, respectively.

S2VP60.8k toluene solution was placed on top of an aqueous 0.10 g·L−1 GO solution, and N2 was gently bubbled into the bottom water phase. A wrinkled film could be seen at the interface when the bubbling was stopped (Figure 6). The

Figure 5. Different activities of random copolymer and block copolymer toward the interfacial assembly of GO. The concentration of graphene oxide is 0.08 g·L−1 in water and 3.0 g·L−1 in toluene for both P(S-r-2VP) and S2VP19.5k. 13411

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P2VP blocks becomes more hydrophilic and may partially penetrate the water phase after protonation. The amphiphilic S2VP block copolymer is a surfactant, reducing the toluene/ water interface tension continuously as more and more molecules segregate to the interface. The GO sheets in water diffuse to the oil/water interface, are effectively “caught” by the quaternized P2VP through either electrostatic attractions or hydrogen bonding, and are held at the interface. The flexible nature of GO makes it easy for GO to orient parallel to the interface, interact with more quaternized P2VP, and be anchored to the interface. In so doing, the GO also limits the amount of P2VP that can segregate to the interface because it is only the periphery of the GO that is of importance. Two or more GO sheets can attach to the same S2VP molecules. Single layers of GO can form at the interface, and during the assembly process, the overlap of GO sheets is also possible. This has not been quantified here. Ultimately, upon compression, a closepacked GO film covering the entire interfacial area is formed at the interface that is mechanically robust. It may provide a facile way to prepare large area graphene oxide thin films.

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. TEM image of the assembled graphene film at the water/ toluene interface.

AFM image of graphene oxide nanosheets, the effect of pH on the dynamic interfacial tension, dynamic interfacial tension with different block copolymer ligands, and videos of buckling on the droplet surface. This material is available free of charge via the Internet at http://pubs.acs.org/.

film would disappear if the interfacial area was enlarged again after compression (Video S1). In addition, the XPS spectrum of the film was obtained after the retrieval of the film with a silicon wafer, as shown in Figure 8. Si 2s and Si 2p peaks come from

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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 This work was supported by the U.S. Department of Energy, Office of Basic Energy Science through contract DE-FG0204ER46126.

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REFERENCES

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Figure 8. XPS spectrum of the graphene film assembled at the water/ toluene interface.

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4. CONCLUSIONS From the observation described above, the assembly of graphene oxide at the water/oil interface with block copolymer (S2VP) can be described as follows. Upon contact with the water phase, the S2VP block copolymer molecules in toluene migrate to the water/toluene interface and the P2VP block is partially quaternized with free protons in the aqueous phase. 13412

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