Letter Cite This: ACS Macro Lett. 2017, 6, 1201-1206
pubs.acs.org/macroletters
Controlling Oil-in-Oil Pickering-Type Emulsions Using 2D Materials as Surfactant Bradley Rodier, Al de Leon, Christina Hemmingsen, and Emily Pentzer* Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States S Supporting Information *
ABSTRACT: Emulsions are important in numerous fields, including cosmetics, coatings, and biomedical applications. A subset of these structures, oil-in-oil emulsions, are especially intriguing for water sensitive reactions such as polymerizations and catalysis. Widespread use and application of oil-in-oil emulsions is currently limited by the lack of facile and simple methods for preparing suitable surfactants. Herein, we report the ready preparation of oilin-oil emulsions using 2D nanomaterials as surfactants at the interface of polar and nonpolar organic solvents. Both the edges and basal plane of graphene oxide nanosheets were functionalized with primary alkyl amines and we demonstrated that the length of the alkyl chain dictates the continuous phase of the oil-in-oil emulsions (i.e., nonpolar-in-polar or polar-in-nonpolar). The prepared emulsions are stable at least 5 weeks and we demonstrate they can be used to compartmentalize reagents such that reaction occurs only upon physical agitation. The simplicity and scalability of these oil-in-oil emulsions render them ideal for applications impossible with traditional oil-in-water emulsions, and provide a new interfacial area to explore and exploit.
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chemical reactions otherwise inefficient or impossible, such as water-sensitive isocyanates, acid halides, and various catalysts, as well as application of coatings on water sensitive surfaces. For example, Müllen, Klapper, and co-workers used polyisoprene-b-poly(methyl methacrylate) to stabilize the DMF/ hexane interface for the preparation of olefin-based polymer particles catalyzed through metallocene chemistry.24,26 Likewise, Lodge and co-workers demonstrated the capability of polystyrene-b-poly(ethylene oxide) and polybutadiene-b-poly(ethylene oxide) to stabilize chloroform-based emulsions with immiscible polymers dissolved into each phase.27,28 To date, stabilization of oil−oil interfaces in Pickering-type systems has been little studied, yet is especially attractive given the increased energy required to remove solid surfactant from the interface. Recently, Tawfeek and co-workers reported the combination of clay nanosheets and a nonionic, reactive surfactant to stabilize oil-in-oil emulsions,29 while Binks and co-workers reported the stabilization of the interface between silicon oil and vegetable oil using only silica particles of tailored hydrophobicity.25,30 An ideal surfactant for stabilization of oil-in-oil emulsions would have an appropriate polarity to reside at the interface of the two phases. Moreover, if this polarity can be tuned, then the stability and structure of the emulsion could be controlled. Graphene oxide (GO) is a 2D nanomaterial that has been used to stabilize oil-in-water emulsions,31−34 but has been little
mulsions are mixtures of two immiscible liquids, in which smaller domains of one liquid are dispersed in a continuous phase of the second. Emulsions are typically thermodynamically unstable and the dispersed phase domains coalesce in order to minimize surface area. However, surfactants at the fluid−fluid interface can decrease the interfacial tension between the two immiscible liquids, providing kinetic stability and preventing aggregation or coarsening of the dispersed phase.1 Surfactants lower the surface energy between the two liquids and can be ambipolar (e.g., a polar headgroup and hydrocarbon tail) or wettable to both phases.2,3 Emulsions, and the ability to control their stability, are crucial to diverse applications including foods,4−6 cosmetics,7,8 biosensing,9−11 drug delivery,12−16 and coatings.17−19 As such, new surfactants and emulsion systems are required to expand applications of microphase separated domains and reduce cost of such systems. Typical emulsion development is centered on oil/water systems, and surfactants include small molecules, polymers, and particles (the latter of which give Pickering emulsions).20−23 Although less studied, oil-in-oil emulsions have applications distinct from and complementary to oil−water emulsions; unfortunately, suitable surfactants for stabilization of oil−oil interfaces are less common. Oil-in-oil emulsions exclude water and make use of two immiscible liquids: one nonpolar (e.g., octane) and one polar (e.g., N,N-dimethylformamide (DMF)). The most common surfactants or interfacial stabilizers for these systems are block copolymers, though hydrophobic polymers and silicones have also been examined.24,25 Exclusion of water from such emulsions presents opportunities to explore compounds and © XXXX American Chemical Society
Received: August 24, 2017 Accepted: October 13, 2017
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DOI: 10.1021/acsmacrolett.7b00648 ACS Macro Lett. 2017, 6, 1201−1206
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ACS Macro Letters studied for other types of emulsions (i.e., oil-in-oil). The amphiphilicity of GO is attributed to the carbon framework decorated with various oxygen-containing functional groups, such as alcohols and epoxides on the basal plane, and carboxylic acids along the edges.35−37 Unfortunately, the relatively strong hydrophilic nature of GO renders it unsuitable for use as a surfactant in oil-in-oil emulsions. However, the polarity and wettability of GO can be modified using simple reactions.33,38−42 As such, modification of GO with nonpolar functionalities could be an ideal route to tune the polarity of the nanosheets and yield surfactants for oil-in-oil emulsions. Primary amines are particularly attractive to use as reagents to functionalize GO, as modification occurs both covalently (by ring opening of epoxides) and electrostatically (by acid−base reactions with carboxylic acids). Moreover, a slew of primary amines is widely available, making this approach easily accessible and modular (i.e., the polarity of the nanosheets can be varied). Herein, we report 2D nanoparticle surfactants of tunable polarity for stabilization of oil-in-oil emulsions and control of the continuous phase based on functionalization. Both the surface and edge of GO nanosheets are functionalized with five different alkyl amines and stable oil-in-oil emulsions are formed with common immiscible systems (DMF/octane, acetonitrile (ACN)/octane). Identity of the alkyl chain length dictates the continuous phase, with longer alkyl chains yielding DMF-inoctane emulsions and shorter alkyl chains giving octane-inDMF emulsions. These systems are used for the compartmentalization of reagents whose reaction is dependent on an external stimulus (physical agitation). Notably, the tailored particle surfactants can be recovered and reused. This system is ideal for fundamentally understanding how 2D materials behave at oil−oil interfaces and how the identity of the emulsion can be controlled by decreasing particle polarity, as well as providing a platform for compartmentalization of reagents. To access GO nanosheets that stabilize oil-in-oil emulsions, the polarity of the material was adjusted by reaction with primary amines of varying alkyl lengths (Figure 1). Alkyl amines modify carboxylic acids on the edges of the nanosheets by acid−base reactions to give noncovalent modification (i.e., electrostatic interactions); alkyl amines also covalently modify
the nanosheets by ring opening of epoxide moieties. Five different alkyl amines were used to modify GO: hexyl amine (C6), nonyl amine (C9), dodecyl amine (C12), hexadecyl (C16), and octadecyl (C18) amine, collectively referred to as Cx-GO and individually referred to by the number of carbon atoms (e.g., C6-GO is GO functionalized with hexyl amine). Alkyl amine was added to a suspension of GO nanosheets in DMF to prepare C6-GO, C9-GO, and C12-GO; to access C16-GO and C18-GO, the materials precipitated from DMF were further functionalized using toluene as the solvent (see SI for details). Functionalization of GO was verified using spectroscopic and thermogravimetric characterization methods (Figure 2). The
Figure 2. Characterization of GO and Cx-GO series (A) FTIR spectra; (B) TGA thermograms; (C) high resolution C 1s XPS spectra; and (D) high resolution N 1s XPS spectra.
Fourier transform infrared (FTIR) spectra of GO and Cx-GO are shown in Figure 2A; the spectrum of GO contains the wellestablished signature from 3800 to 2100 cm−1 indicative of carboxylic acid and hydroxyl functionalities, as well as signatures of carbonyl and carbon−carbon single bonds (C− C) at 1750 and 1600 cm−1, respectively.41,43−45 The FTIR spectra of all functionalized GO samples contain the expected aliphatic carbon−hydrogen (C−H) stretching frequency from 3000 to 2900 cm−1, as well as increased intensity of the C−C bond signal. Moreover, the relative intensity of the C−H stretching frequency scales with chain length and carboxylates and ammonium groups are observed at 1650 and 1550 cm−1, indicating noncovalent modification. Thermogravimetric analysis (TGA) was used to study the weight loss profiles of CxGO, comparing to unreacted GO (Figure 2B). The weight loss profile of GO is consistent with previous reports,35,40,43,45 with loss of labile oxygen moieties from 150 to 250 °C, followed by less dramatic weight loss up to 650 °C. In contrast, Cx-GO materials began losing weight at a higher temperature (250 °C), indicative of possible reduction of the nanosheets upon functionalization, and show additional weight loss from 350 to 450 °C, indicative of aliphatic degradation.46 The total weight loss for Cx-GO corresponds to the length of the alkyl chain present: longer alkyl chains resulted in a higher percentage of overall weight loss. From this TGA data, the relative extent of functionalization is difficult to ascertain given
Figure 1. Overview of the work discussed herein: functionalization of GO with alkyl amines by acid−base reactions of amines with carboxylic acids and ring opening of epoxides. Continuous phase of the emulsion is controlled by the alkyl chain length. 1202
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(Figure 3B), though C 9 -GO precipitated after 24 h. Dispersibility of Cx-GO in octane is shown in Figure 3C; all samples initially disperse, but precipitate after 24 h. However, C16-GO and C18-GO remained dispersed in octane longer (at least 30 min), and after sedimentation the samples could be redispersed by gentle agitation. The differences in solubility of the Cx-GO again indicate that the polarity of the nanosheets can be tailored by simply using different alkyl chain lengths. Atomic force microscopy (AFM) was also used to understand the dispersibility of the nanosheets with height profiles indicating the presence of single nanosheets or aggregates, and any morphological changes caused by functionalization or solvent change (e.g., scrolling or wrinkling). Representative AFM images and height profiles in Figure S3 show that the CxGO nanosheets have heights ∼1−4 nm and are sheet-like, consistent with that reported for single nanosheets, and supporting a lack of substantial aggregation or changes in morphology in the solvents in which they are dispersible.50,51 While shorter alkyl chains led to a homogeneous appearance of the nanosheets, the surface of C16-GO and C18-GO appear pitted and irregular (Figure S3D, S3E), which may be attributed to aggregates of alkyl groups on the surface. Tailoring the polarity of GO nanosheets by varying the alkyl amine not only impacts the dispersibility, but also dictates the identity of the continuous phase of the emulsion formed. Essentially, GO functionalized with shorter alkyl chains are surfactants for octane-in-DMF emulsions and GO functionalized with longer alkyl chains are surfactants for DMF-inoctane emulsions. Using the dispersions discussed above (C6GO and C9-GO in DMF, and C12-GO, C16-GO, and C18-GO in octane), a small amount of the complementary organic solvent was added, and then the system was mixed by vortex to prepare oil-in-oil emulsions. As can be seen in Figure 4, C6-GO yielded
that GO may be reduced to a different extent for each sample and that the two modes of functionalization (electrostatic and covalent) may not have similar degradation profiles. X-ray photoelectron spectroscopy (XPS) was also used to characterize the chemical functionalization of GO with alkyl amines. Survey scans of GO and Cx-GO reveal the presence of C, O, and N, as expected (Figure S1). Comparison of the high resolution C 1s scans of GO and Cx-GO (Figure 2C) show the typical C−C/CC (284 eV), C−O (286 eV), and CO (288 eV) binding energies, but at varying ratios. 47 Upon functionalization, a sharp increase in the ratio of C−C to C− O is observed, indicating functionalization with alkyl chains, with the decrease in C−O contribution commensurate with longer alkyl chains, which contribute more C−C character. The increase in C−C/CC bond contribution could also be attributed to reduction, further supported by Raman spectroscopy (Figure S2).41,48,49 The high resolution N 1s XPS spectra of Cx-GO shows the presence of nitrogen, and can be used to evaluate the nature of functionalization of GO (Figure 2D), either electrostatic or covalent.40,45 The spectra of C6-GO, C9GO, C12-GO, and C16-GO all show nitrogen present as ammonium cations and amines with binding energies of 400− 401 and 398−400 eV, respectively. Interestingly, the ratio of electrostatic (ammonium) to covalent (amine) modification increases with increasing alkyl chain length, with the spectrum of C18-GO having the highest ratio of noncovalent to covalent modification, and suggesting that functionalization of carboxylic acid groups is pivotal to modifying the polarity of GO nanosheets. After verification of functionalization, dispersibility of Cx-GO in polar and nonpolar organic aprotic solvents (i.e., oil) was evaluated. In brief, GO functionalized with shorter alkyl chains could be dispersed in the polar solvents DMF (dielectric constant of 38.25) and acetonitrile (ACN, dielectric constant of 36.64), while GO nanosheets functionalized with longer alkyl chains were dispersible in the nonpolar octane (dielectric constant of 1.95). The photographs in Figure 3A show that C6-
Figure 4. Optical images of emulsions formed using only Cx-GO as surfactant. Top: Microscopy images of as prepared emulsions. Bottom: Images of same emulsions after a designated time. All scale bars are 200 μm.
octane-in-DMF emulsions ∼50−400 μm in diameter that were stable for at least 5 weeks. In contrast, GO functionalized with substantially longer alkyl chains (C16-GO, and C18-GO) led to the formation of DMF-in-octane emulsions with diameters ∼25 to 450 μm. Octane emulsions floated on top of the continuous DMF phase, while the DMF emulsions sunk to the bottom of the continuous octane phase, as expected. Control experiments using only alkyl amine or its ammonium counterpart did not result in emulsion formation, and emulsions formed with GO functionalized with alkyl chains of intermediate length (C9-GO and C12-GO) were poorly stable and irregular. As such, by simply tuning the polarity of the 2D surfactant by altering alkyl chain length, the continuous phase of oil-in-oil emulsions is controlled, and a crossover point of alkyl length is present for
Figure 3. Photographs showing the dispersibility of C6-GO, C9-GO, C12-GO, C16-GO, and C18-GO at 0.3 mg/mL in (A) DMF, (B) ACN, and (C) octane. Images were taken immediately after preparation (top) and after 24 h (bottom). The middle image for octane is taken 0.5 h after preparation.
GO and C9-GO homogeneously dispersed in DMF, and were stable up to 24 h, while C12-GO, C16-GO, and C18-GO did not disperse well in DMF and precipitated from solution. This demonstrates that GO nanosheets functionalized with shorter alkyl chains retain adequate polarity for dispersion in polar solvents, especially compared to longer alkyl chain derivatives. Similar results were observed for C6-GO and C9-GO in ACN 1203
DOI: 10.1021/acsmacrolett.7b00648 ACS Macro Lett. 2017, 6, 1201−1206
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ACS Macro Letters formation of polar-in-nonpolar or nonpolar-in-polar emulsions using modified GO as a surfactant. Of note, these 2D surfactants can be recycled, by first isolating Cx-GO by centrifugation, then redispersing it in the appropriate solvent. This second batch of emulsions have similar stability to the initial emulsions (Figure S4), indicating these surfactants are not only easy to prepare, but also can be reused without detriment. The optical images in Figure 5 demonstrate that oil-in-oil emulsions prepared using Cx-GO are not limited to DMF/
Figure 6. Optical images under ambient light (top) and UV light (bottom, λex = 365 nm) of (A) Alizr and PBA in DMF; (B) DMF-inoctane emulsion of Alizr and emulsion of PBA combined into one before agitation; (C) Sample in B following vortex and centrifugation.
nents, that is until the emulsions are broken by physical agitation. Herein, we have demonstrated a facile method to fabricate 2D nanoparticle surfactants to stabilize oil-in-oil emulsions. XPS, FTIR, and TGA confirm that GO is functionalized by alkyl amines on the basal plane and edges, and that the functionalization depends on the length of the alkyl group. Modification of GO results in a change in solubility profile, with longer alkyl chains rendering the nanosheets dispersible in more nonpolar solvents, as expected. By varying the alkyl chain length of the amine, the polarity of GO nanosheets is tuned, which in-turn defines the dispersed phase of the oil-in-oil emulsion. This system can be used to compartmentalize reagents, allowing for their reaction only when physical agitation is applied, and the surfactants can be reused without detriment to emulsion formation. This system is ideal for water sensitive reactions impossible with traditional emulsions, and ongoing efforts are exploring many of these applications.
Figure 5. Optical images of octane-in-ACN emulsions prepared using C6-GO (left) and ACN-in-octane emulsions prepared using C18-GO (right). Top image is as prepared and bottom image is after 2 weeks. Scale bar is 200 μm. Magnified images of ACN-in-octane are not presented due to large size of emulsions.
octane mixtures. In the ACN/octane system, only two surfactants led to the formation of emulsions: C6-GO gave an octane-in-ACN emulsion and C18-GO gave an ACN-in-octane emulsion. Both emulsions were ∼300−600 μm in diameter and both emulsions were stable for at least 2 weeks. Many other polar and nonpolar solvents could potentially be used, but volatility of low boiling point oils (e.g., hexanes) makes imaging difficult, and viscosity of higher boiling point solvents makes vigorous agitation difficult. Moreover, the Cx-GO surfactant must be dispersible in the solvent for the continuous phase. To explore the utility of these oil-in-oil emulsions as compartmentalized reactors, two separate DMF-in-octane emulsions were formed, each containing a reagent encapsulated in the discontinuous DMF phase: alizarin red (Alizr) and phenylboronic acid (PBA). Individually, neither Alizr nor PBA are fluorescent in DMF (Figure S5); however, when combined, a fluorescent boronic ether is formed, as shown in Figure 6A (mechanism in Figure S6).52 When individually encapsulated in a DMF-in-octane emulsion, neither Alizr nor PBA are fluorescent (Figure S5), even when the two emulsions are combined into one vial (Figure 6B). However, when this emulsion containing two different types of capsules, one containing Alizr and one containing PBA, is agitated by vortex, weak fluorescence is observed (see Figure S5). Graphene oxide inhibits observation of the bright fluorescence of the boronic ether, however centrifugation of the sample makes the fluorescence more easily observed (Figure 6C). Of note, centrifuging without mixing by vortex also leads to fluorescence. These results indicate that emulsions can be combined without reaction of the compartmentalized compo-
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00648. Survey XPS spectra, Raman spectra, AFM images and height profiles, optical images of emulsions and compartmentalized reactions, and mechanism for formation of fluorescent compound (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: 216-368-3697. ORCID
Emily Pentzer: 0000-0001-6187-6135 Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest. 1204
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ACKNOWLEDGMENTS The authors would like to thank CWRU College of Arts and Sciences and NSF CAREER Award #1551943 for financial support. B.J.R. is a NASA Harriett G. Jenkins Predoctoral Fellow (Grant #NNX13AR93H). XPS measurements were performed at the Swagelok Center for Surface Analysis of Materials (SCSAM) at CWRU. C.H. would like to thank the CWRU SURES program.
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