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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Reversible Switching of the Amphiphilicity of OrganicInorganic Hybrids by Adsorption-Desorption Manipulation Chenhui Han, Eric R. Waclawik, Xiaofei Yang, Peng Meng, Hengquan Yang, Ziqi Sun, and Jingsan Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b07040 • Publication Date (Web): 07 Aug 2019 Downloaded from pubs.acs.org on August 15, 2019

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The Journal of Physical Chemistry

Reversible Switching of the Amphiphilicity of Organic-Inorganic Hybrids by Adsorption-Desorption Manipulation Chenhui Han,a Eric R. Waclawik,a Xiaofei Yang,*b Peng Meng,a Hengquan Yang,*c Ziqi Suna and Jingsan Xu,*a School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4001, Australia. b. College of Science, Institute of Materials Physics and Chemistry, Nanjing Forestry University, Nanjing 210037, China. c. School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China. a.

ABSTRACT: Surfactants are of great significance due to their wide use in fundamental research, industrial production and daily lives. It remains a grand challenge to design and synthesize surfactants exhibiting reversible amphiphilicity switching. Here, we report on a “hybrid surfactant” prepared by combining an oil-soluble molecule, stearic acid, with water-dispersible Al2O3 nanofibers via chemisorption at the oil-water interface. The long carbon-chain of stearic acid functions as the hydrophobic tail of the surfactant, while the inorganic nanofibers can act as the hydrophilic head. This “hybrid surfactant” exhibits reversible switching between hydrophilic and lipophilic states, by manipulating the adsorption-desorption volume of stearic acid attached to the Al2O3 nanofibers. Therefore, the emulsions stabilized by this organic-inorganic hybrid can reversibly transform between oil-in-water (o/w) and water-in-oil (w/o) type. Unlike conventional approaches, no other external stimulus is needed to set the amphiphilic properties of the “hybrid surfactant”. As a bonus, organic-inorganic 3D solid foams can be readily prepared based on the emulsion system, which demonstrates potential applications for remediation of oil-spills in the environment.

INTRODUCTION Surfactants have been extensively studied in fundamental research and are commonly used in industrial and household products. They are usually designed to stabilize emulsions for the purposes of cleaning and wetting, dispersing, foaming and anti-foaming. The design and synthesis of surfactants that can reversibly switch between different states (e.g., hydrophilic and lipophilic states, active and inactive states) is of great interest in many circumstances, such as in emulsion polymerization and viscous oil transportation.1 However, relying on external stimuli that involve the use of chemical reactants (acids/bases, oxidants/reductants) may result in product contamination and environmental and economic costs. To avoid these drawbacks, surfactants that are responsive to benign gases have been developed, based on the structural change upon gasing and degassing.1-3 Other useable external stimuli include temperature, light and magnetism.4 Such stimuli-responsive surfactants can only transform between active and inactive states, whereas the ability to synthesize surfactants with adjustable amphiphilic properties and thereby responsive emulsion phases, remains very limited. In recent years, the use of colloidal particles to stabilize emulsions has been of interest owing to advantages of ease of recycling, high stability and low toxicity.5-11 In many commercial products, solid particles and molecular surfactants are combined to attain an optimal emulsifying capability. The stability and type of these emulsions are largely determined by the wettability of the particles: a good emulsifier should possess intermediate wettability, where more hydrophilic

particles stabilize oil-in-water (o/w) emulsions and more hydrophobic/lipophilic particles stabilize water-in-oil (w/o) emulsions.12-14 Inorganic nanoparticles in combination with surfactants can generate phase-inversion emulsions by continuously increasing the concentration of surfactant or salt.15-16 However, the number of times phase transformation can be achieved in these systems is limited to only two-cycles (o/w to w/o and back-to o/w type) due to irreversible altering of the stabilizer. Herein, stearic acid (SA) dissolved in oil and Al2O3 nanofibers dispersed in water are combined to form an organic-inorganic “hybrid surfactant” via chemisorption at the oil-water interfaces, with the long carbon chain in SA performing the role as the hydrophobic tail and the Al2O3 nanofiber acting as the hydrophilic head. The amphiphilicity of the hybrid, which is determined by the content ratio of SA and Al2O3, undergoes reversible switching between more hydrophilic and more hydrophobic via managing the adsorption-desorption of SA molecules on the Al2O3 surface. As a result, emulsions stabilized by the hybrid can reversibly transform between o/w and w/o type upon adsorptiondesorption of SA, without using any other external stimulus. RESULTS AND DISCUSSION The Al2O3 nanofibers were synthesized based on a previously reported method with minor modifications.17 The transmission electron microscopy (TEM) images illustrate that the fibers have a diameter around 5 nm and a length in the range of 1-5 µm (Figure 1a,b). The X-ray diffraction (XRD) pattern suggests that the Al2O3 sample was of γ phase (Figure 1c). The

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Al2O3 nanofibers were dispersible in water (pH 6.5) and remained stable against sedimentation owing to strong electrostatic repulsion (zeta potential +38 mV). X-ray photoelectron spectroscopy (XPS) measurement demonstrates the binding energy of Al 2p at 74.2 eV and the peak of O 1s can be deconvoluted into one at 531.1 eV and the other at 532.4 eV, which could be assigned to the lattice oxygen in Al2O3 and the surface adsorbed hydroxide group, respectively.18

Figure 1. (a,b) TEM images, (c) XRD pattern, blue bars showing the standard diffraction pattern of γ-Al2O3; and (d) XPS spectra of Al2O3 nanofibers. (e) FTIR spectra of Al2O3, SA and SA-Al2O3 hybrid. (f) Schematic illustration of the “hybrid surfactant” formed by SA in oil adsorbed on Al2O3 in water. Fourier-transform infrared (FTIR) spectra indicate strong chemical interaction between SA and Al2O3. As shown in Figure 1e, the Al2O3 sample does not show any obvious absorption bands except the one at 3450 cm-1, corresponding to the –OH stretching vibration in adsorbed water. For the SAAl2O3 hybrid, bands at 2911 and 2844 cm-1 are assigned to the antisymmetric and symmetric stretching vibration of C-H bond from SA, respectively, indicating attachment of SA onto the Al2O3 surface. Interestingly, the stretching vibration of C=O at 1694 cm-1 completely vanishes after adsorption to the Al2O3 surface, owing to the disintegration of -COOH group from SA.19 Instead, a new peak at 1560 cm-1 appears, which is assigned to the C=O stretching vibration in the as-formed carboxylate (Figure 1f). These observations indicate that chemisorption between SA molecules and Al2O3 nanofibers occurs, generating an amphiphilic hybrid at the liquid interface. The organic-inorganic chemical interaction is found to have significant effect on the emulsifying properties of the hybrid, which is discussed below. The ability to lower the interfacial tension (e.g., between water and hydrophobic solvents) reflects the surface activity of a substance, and high surface activity is favourable to the generation of emulsions. In the present case, interfacial tension measurement illustrates that Al2O3 nanofibers did not possess

surface activity (Figure S1). Also, SA alone showed weak surface activity (Figure 2a), owing to its structural characteristics of a carbon chain connected to a carboxylic acid group. By dispersing Al2O3 in water and dissolving SA in hexane together, higher surface activity was achieved, especially when the concentration of SA (denoted as n[SA]) was greater than 0.1 mM. For instance, the interfacial tension was 41.1 mN/m when solely SA was added in hexane (8.2 mM), while it decreased to 34.6 mN/m in the presence of additional Al2O3 in water (0.33 wt%). These results demonstrate the synergistic effect of SA coupled Al2O3 for enhanced surface activity, acting like an organic-inorganic “hybrid surfactant” (Figure 1f). This effect also applies to a range of solvents including alkanes and arenes (Figure 2b), indicating the versatility of the “hybrid surfactant”.

Figure 2. (a) Water-hexane interfacial tension with different SA concentrations in hexane without and with 0.33 wt% Al2O3 in water. (b) Interfacial tensions between water and different organic solvents under different conditions: pure water and pure solvents, pure water and n[SA] = 0.35 mM in solvents, and m[Al2O3] = 0.33 wt% in water and n[SA] = 0.35 mM in solvents. (c) The water-hexane emulsion phases with the increase of n[SA]; m[Al2O3] = 0.33 wt% in water. Accordingly, the SA-Al2O3 hybrid can act as a superior stabilizer for the preparation of water-oil emulsions (hexane used as a typical oil phase). As a control, Al2O3 nanofibers alone were too hydrophilic to stabilize emulsions even upon strong shearing force. In the absence of Al2O3, on the other hand, no fine emulsion was generated until quite high n[SA] was used (0.35 mM), and the emulsions exhibited low stability (Figure S2). However, by combining Al2O3 with a very small amount of additional SA in oil (0.01 mM), an o/w emulsion was readily generated by shaking the water-oil mixture (Figure 2c, left). Increasing n[SA] up to 1 mM improved the emulsifying efficiency and the obtained emulsions remained stable against coalescence for at least a year (Figure S3). These results demonstrate the high effectiveness of the hybrid emulsifier, benefiting from the improved surface activity and the steric effect of the nanofibers.20 Noticeably, when n[SA] was increased high enough (35 and 70 mM), the emulsions converted to w/o type (Figure 2c, right), as the hybrid emulsifier turning more hydrophobic due to more adsorption of SA molecules onto the Al2O3. Similar results were also observed for emulsions stabilized by some other surfactantnanoparticle mixtures.15, 21-22 We observed that when n[SA] was prepared with intermediate levels, the obtained emulsions crossed a


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The Journal of Physical Chemistry transitional state (Figure 2c, middle): they can reversibly transfer between o/w and w/o phase by simple manipulation. As illustrated in Figure 3a, an o/w emulsion was initially generated by shaking the water/hexane mixture when 0.33 wt% Al2O3 and 7 mM SA were used. After 20 min standing, this emulsion unprecedentedly transformed to w/o type upon further shaking, as shown in Figure 3b. Note that the water was dyed blue and hexane was dyed red for better light microscopy picturing. Quick sonication broke-up the w/o emulsion and followed by shaking, the emulsion was changed back to o/w type, illustrating the reversibility of the phase inversion. This reversible, dynamically-controlled, emulsion phase conversion should originate from the switchable amphiphilicity of the SA-Al2O3 hybrid.

Figure 3. Reversible water-hexane (1:1 volume ratio) emulsion phase inversion between (a) o/w type and (b) w/o type by aging-shaking and sonicating-shaking processing. Water was dyed with methylene blue and hexane was dyed with Oil Red O for light microscopy imaging. (c) Waterhexane emulsion phase diagram with different n[SA] in hexane and m[Al2O3] in water. (d) Water-solvent emulsion phase diagram with different n[SA]; m[Al2O3] = 0.33 wt%. Screening experiments show that the dynamicallyswitchable emulsion can be realized in wide concentration windows of SA and Al2O3. As shown in Figure 3c, phaseswitchable emulsions can be obtained with the concentration of Al2O3 (denoted as m[Al2O3]) in the range of 0.1 to 1.3 wt%, as long as n[SA] was set an accordingly appropriate value. For example, when m[Al2O3] was 0.2 wt%, n[SA] between 3.5 and 7.1 mM was suitable for achieving the switchable emulsions. Fixed-type emulsions (either o/w or w/o type) will otherwise be formed when n[SA] was below or above the critical concentrations, respectively. In another set of experiments, different organic solvents were used to test the versatility of the switchable emulsions stabilized by the SA-Al2O3 “hybrid surfactant”. As shown in Figure 3d, in addition to hexane, alkanes including octane and decane can also been used as the oil phase for making switchable emulsions. It is worth noting that less SA was needed for the alkanes with longer carbon chains, probably because of the structural similarity between them. The range of organic solvents can extend to arenes such as toluene, p-xylene and ethylbenzene. Higher concentrations of SA were needed in these cases, which should be owing to the structural difference between the aromatic rings and the carbon chains. The wettability of a solid emulsifier plays a critical role determining the emulsion phases. To monitor the wettability of the SA-Al2O3 hybrids, water contact angle was measured

over varied contents of SA and Al2O3. As shown in Figure 4a, the pristine Al2O3 exhibits a contact angle of 21.8o, well below 90o, confirming its high hydrophility as mentioned above. After hybridizing with SA (4.2 mM), the contact angle increased to 73o, reflecting the hybrid turning less hydrophilic. The hybrid became fully hydrophobic when n[SA] was above 14 mM, with the contact angle reaching 117o and higher. It is worth mentioning that when n[SA] = 7 mM, the hybrid exhibited switchable amphiphilicity as aforementioned, and the contact angle reached accordingly a critical value (very close to 90o). Under this condition, the amphiphilicity became very sensitive and could vary upon vibration of the solution environment.

Figure 4. (a) Water contact angle of the SA-Al2O3 hybrid with different concentration of SA; m[Al2O3] = 0.33 wt%. (b) Dynamic interfacial tension measurement with n[SA] = 0.35 mM in hexane and with m[Al2O3] = 0, 0.1 and 0.33 wt% in water. To better understand the dynamically tuneable amphiphilicity of the SA-Al2O3 hybrid, the switchable emulsion phase inversion was carefully investigated. As aforementioned, an o/w emulsion was formed at first when the ratio of n[SA]/m[Al2O3] lied in the transitional range (Figure 3c, d). To convert this o/w emulsion to w/o type, tens-ofminutes aging (Step 1, Figure 6) was required, followed by shaking for phase inversion. We noticed that longer aging time was needed when m[Al2O3] was reduced from 0.33 to 0.1 wt%. In this context, the dynamic water-hexane interfacial tension with SA in hexane (7 mM) and with or without Al2O3 in water was monitored. It can be seen the interfacial tension quickly reached equilibrium (42.6 mN/m) within 100 s when only SA was used (Figure 4b). However, the interfacial tension decreased much more slowly when additional Al2O3 (0.33 wt%) was added in water. We can see that the interfacial tension started to get stabilized (36.3 mN/m) after 10 min and it even took longer time (20 min) when m[Al2O3] was down to 0.1 wt%. This phenomenon should be assigned to the relatively slow kinetics of SA molecules adsorbing to the Al2O3 nanofibers and thereafter reaching adsorption equilibrium at the liquid interface;23 this process became slower when the concentration of Al2O3 was lower. In addition, the adsorption process between SA and Al2O3 was in situ monitored by fluorescence-labelling experiments. Coumarin 343 X carboxylic acid (denoted as CCA, Figure S4), a blue-fluorescent molecule that resembles the structure of SA, is used to trace the location and movement of SA molecules and Al2O3 nanofibers. Firstly, we detected the diffusing of CCA onto the oil-water interface. SA was dissolved in toluene and labelled by CCA, and pristine Al2O3 nanofibers were dispersed in water. Immediately after emulsification, one emulsion droplet was taken out and pictured using confocal microscope after different time intervals. As shown in Figure 5a and b, the surface fluorescence intensity of the droplet gradually increased with



aging time, indicating that more and more molecules moved to the droplet surfaces and adsorbed onto the Al2O3 nanofibers. This conclusion is also supported by the FTIR spectra of hybrids after adsorption for different time intervals, as shown in Figure S5. Secondly, the CCA-labelled-Al2O3 nanofibers (Figure S6) were used to trace the movement of Al2O3 nanofibers toward the interface through observing an o/w emulsion stabilized by the fluorescent Al2O3 and pristine SA. As shown in Figure 5c, the fluorescence intensity at the emulsion droplet surface increased with aging time, indicating increasing amount of Al2O3. The fluorescence intensity was also quantitatively measured and shown in Figure 5d, revealing that the Al2O3 nanofibers tended to accumulate at the oil-water interface and form surface-active hybrids upon adsorbing SA molecules.

Figure 5. (a) Confocal images of a single droplet stabilized by fluorescence-labelled-SA and pristine Al2O3 at different aging time and (b) corresponding quantitative fluorescence intensity. (c) 3D confocal images of a single droplet stabilized by fluorescent Al2O3 nanofibers and pristine SA after different aging time and (d) corresponding quantitative fluorescence intensity after 0 and 60 min. Based on the above measurements and analysis, we propose a mechanism for better understanding the dynamically switchable amphiphilicity of the SA-Al2O3 hybrid, thereby resulting in reversible emulsion phase transition. In the first stage, an o/w emulsion is stabilized by the SA-Al2O3 hybrid (Figure 6). Upon aging, the remaining SA molecules in the oil shift toward and adsorb to the Al2O3 nanofibers locating at the liquid interfaces, until adsorption equilibrium is reached (Step 1, Figure 6). External disturbance, such as shaking, can induce swing of the oil-water interface and hence more of the Al2O3 surface is covered by the SA molecules, changing the wettability of the hybrid from more hydrophilic to more hydrophobic. Therefore, shaking again results in phase inversion and a w/o emulsion is obtained (Step 2 and 3, Figure 6). In the next step, this w/o emulsion is broken by sonication. Meanwhile, the ultrasonic energy results in the desorption of SA from the Al2O3 surface (Figure S7 and S8).24 The detachment of SA molecules, which could be only a minor portion, can change the hybrid back to be more hydrophilic. Therefore, an o/w emulsion is formed again by subsequent shaking (Step 4, Figure 6). This controllable emulsion phase transition, which results from the dynamically tuneable hydrophobic/hydrophilic ratio of the SA-Al2O3 hybrid relies on the reversible adsorption-desorption process. It is worth

mentioning that chemisorption plays a predominant role in the phase-switching process, although there is a part of physical adsorption in the overall adsorption. As shown in Figure S9, the SA-Al2O3 hybrid was recycled from a phase-switchable emulsion and thoroughly washed with ethanol to remove physically adsorbed SA. The obtained hybrid is still able to stabilize the same type of emulsions and realize phaseswitching, confirming the dominant effect of chemisorption.

Figure 6. Proposed mechanism of the switchable emulsion phase inversion between o/w and w/o type induced by tuneable amphiphilicity of the hybrid surfactant. Due to the strong interactions between the “hard” Al2O3 nanofibers and the “soft” SA molecules at the liquid interfaces, the as-stabilized emulsions possess certain mechanical strength. Hence, the emulsions can serve as a skeleton for preparation of hydrophobic bulk foams. The hybrid foams were simply prepared by centrifuging the w/o emulsions (m[Al2O3] = 0.5 wt%, n[SA] = 35 mM) and drying the collections in a vacuum oven. The bulk foams show a 3D network with the Al2O3 nanofibers closely connected together by SA molecules (Figure S10), originating from the 3D structure of the emulsions. Compared with the pristine Al2O3, the foams were highly hydrophobic with a water contact angle up to 134o (Figure 7a, b). It had a density lower than water and showed no adsorption capacity of water (Figure S11). Benefiting from this property, the hybrid foam was able to easily remove oil film from water surface by adsorption. Figure 7c illustrates the fast adsorption of decane, with 73 mg of decane completely adsorbed by a piece of the SA-Al2O3 foam in 6 s. Such superior adsorption performance of the hybrid foam occurred with a range of organic liquids, including alkanes, p-xylene, silicone oil and peanut oil (Figure 7d), reaching up to 10-15 times its own weight. The low cost, easy processing and environmental-friendly endow the SAAl2O3 foam high potential for practical oil-water separation.

Figure 7. Water contact angle of (a) pristine Al2O3 and (b) the hybrid foam (m[Al2O3] = 0.5 wt%, n[SA] = 35 mM). (c)


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The Journal of Physical Chemistry Images of the foam adsorbing decane (dyed with Oil Red O) floating on water surface. (d) Adsorption capacity of the hybrid foam for different organic liquids.

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CONCLUSIONS In summary, we have developed a versatile organic-inorganic “hybrid surfactant” by combining SA molecules in oil and Al2O3 nanofibers in water via chemisorption at the oil-water interfaces. The long carbon chains in SA function as the hydrophobic tail while the Al2O3 nanofibers act as the hydrophilic head. The amphiphilicity of the hybrid, which is determined by the content ratio of SA and Al2O3, exhibits reversible switching between more hydrophilic and more hydrophobic by dynamically managing the adsorption amount of SA on Al2O3. Consequently, emulsions stabilized by the hybrid can reversibly transform between o/w and w/o type upon adsorption-desorption manipulation, without the use of any external stimulus, highlighting the distinct feature of this hybrid. As an added benefit, 3D solid foams can be facilely derived from the emulsions, which show high adsorption capacity and thus potential applications in remediation of oilspills in environment, owing to the low cost, easy processing and environmental-friendly. We believe this work highlights a new chemical design for the preparation of switchable surfactants that may raise both fundamental research interest and industrial application values.

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ASSOCIATED CONTENT

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Supporting Information. Material synthesis, interfacial tension measurement, emulsion stability testing, and SEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author

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* Jingsan Xu, Email: [email protected] * Xiaofei Yang, Email: [email protected] * Hengquan Yang, Email: [email protected]

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Notes The authors declare no conflicts of interest.

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ACKNOWLEDGMENT The authors are grateful to the Discovery Early Career Researcher Award (DECRA) by Australian Research Council (DE160101488) and the Start-Up Fund from Nanjing Forestry University.

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