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Solvent effects on the formation of surface nanodroplets by solvent exchange Ziyang Lu, Haolan Xu, Hongbo Zeng, and Xue Hua Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b03303 • Publication Date (Web): 21 Oct 2015 Downloaded from http://pubs.acs.org on October 29, 2015
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Solvent effects on the formation of surface nanodroplets by solvent exchange Ziyang Lu,† Haolan Xu,‡ Hongbo Zeng,¶ and Xuehua Zhang∗,† School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, VIC 3001, Australia, Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, SA 5095, Australia, and Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 2V4, Alberta, Canada E-mail:
[email protected] Abstract Solvent exchange is a simple process to form oil nanodroplets at solid-liquid interfaces with well-defined location and morphology. In this process, a good solvent of the oil is displaced by a poor solvent, leading to the nucleation and growth of oil droplets from a transient oversaturation at the mixing front. Our recent work has shown that the final volume of the droplets is related to the flow conditions. In this work, we investigate the effects of the type and the composition of solvents on the droplet formation under the same flow conditions. Water nanodroplets were produced by ethanol/cyclohexane (solution A)and cyclohexane (solution B) on a hydrophilic substrate. We found that the droplet size increases first and then decreases with an increase of the initial ethanol ∗
To whom correspondence should be addressed School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, VIC 3001, Australia ‡ Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, SA 5095, Australia ¶ Department of Chemical and Materials Engineering, University of Alberta, Edmonton, T6G 2V4, Alberta, Canada †
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concentration in solution A. This is attributed to the phase separation of ethanolcyclohexane-water, in particular, the composition of solution A on the phase boundary above the Ouzo region. The same reason also contributes to the lower efficiency in droplet formation for a longer alkane. The important implication from this work is that the maximal droplet volume is limited by the phase separation of the solvents used in the solvent exchange.
Introduction Surface nanodroplets with the height less than 1 µm are of high relevance to many fundamental and practical processes, such as wetting, condensation, lubrication, and surface corrosion. 1–9 These nanodroplets are immobilised on a substrate, providing a confined microenvironment for chemical or biological reactions, 10–12 or acting as adjustable microlenses in high resolution near-field imaging techniques. 13–15 Solvent exchange is a general and simple approach for producing a large amount of surface nanodroplets on various substrates. The principle of this process is that when a good solvent of a solute is displaced by a poor solvent of that solute, 16–21 an oversaturation pulse of the solute is created at the mixing front, leading to the heterogeneous nucleation and growth of droplets at the solid-liquid interface. 22 The duration and intensity of the oversaturation mediate the number and final size of the solute droplets. Moreover, the location of droplets can be well defined on chemically patterned substrates. 23 To realize the full potential of solvent exchange, it is necessary to better understand the roles of different parameters in the nucleation and growth of the droplets. The solvent exchange has usually been performed in the regime of laminar flow. Our recent work has addressed the significant effect of the flow conditions on the diffusive growth of the droplets while the solution conditions were constant. It is equally crucial to know the effect of the solution composition on the droplet formation under constant flow conditions. Intuitively, the droplet size should increase with the solubility gap between the two solvents. However, so far 2
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no relationship has been established between the droplet size and the solution conditions. In this work, we examine how the size of surface nanodroplets varies with the solution concentration along the entire saturation boundary line in the three phase diagram of solute (water), good solvent (ethanol) and poor solvent (cyclohexane). Furthermore, to clearly reveal the effects from the solvent type, we produce nanodroplets of water on a hydrophilic substrate with a wide range of organic solvent pairs. This is in contrast to previous work with oil droplets or gas bubbles on hydrophobic substrates, where the poor solvent is restricted to an aqueous solution. 16,24 Our results show that the droplet size is dominated by the phase separation behavior of the tertiary system. In addition, our study demonstrates the application of the solvent exchange can be greatly expanded to the formation of surface water nanodroplets.
Results and discussion Glass
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Figure 1: (a) Schematic illustration of the formation of water nanodroplets by solvent exchange. During the solvent exchange, solution A (the water saturated mixture of ethanol and cyclohexane) is displaced by solution B (water saturated cyclohexane). (b) Optical and (c) confocal microscopical image of water nanodroplets. (d) A three-phase diagram of water, cyclohexane and ethanol. The ethanol/cyclohexane usedexchange. for solution A are labeled Figure 1 Schematic drawing ofratios a flowofcell during the solvent on the phase boundary.
The flow cell consists of a glass top window for microscope observation. The Hydrophilic silicon wafer is placed inside the cell, facing up to the transparent glass window. 3
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The solvent exchange was performed under constant laminar flow conditions inside a fluid cell with a hydrophilic silicon or glass substrate placed on the bottom wall and a glass window at the top for microscope observations (Fig. 1a). Water and cyclohexane are miscible with ethanol but immiscible with each other. Therefore, water droplets are formed when a saturated good solvent of water (ethanol and cyclohexane, solution A) is replaced by a saturated poor solvent of water (cyclohexane, solution B). The solubility gap between solvents A and B can be adjusted by the changing the ratio ethanol/cyclohexane in solution A. After the droplet formation by the solvent exchange, the size of water nanodroplet was obtained from images recorded with an optical microscope equipped with a long working distance lens (Fig. 1b). Furthermore, Fig. 1c shows an example of confocal microscopy with a fluorescent dye added to the droplets. a
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Figure 2: Influence of ethanol/cyclohexane molar ratio on the droplet size. (a)-(j) Optical images of water droplets. The scale bar applies to all images. (k) Plot of the probability density function (PDF) of droplets diameter with the ratio from 0.22 to 2.72. (l) Plot of PDF with the ratio from 4.32 to 13.9. (m) Plot of the droplet size as a function of the ratio. We examined the droplet size as we varied the molar ratio of ethanol/cyclohexane in solution A along the phase boundary in the three-phase diagram in Fig. 1d, while the com4
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position in solution B was kept constant. Along the left side of the phase boundary, the molar ratio first was increased from 0.04 to 2.72, reaching the peak at 4.32, and then further increased up to 40 along the right side of the diagram. At very low molar ratios of 0.04 and 0.1, no water droplets were observed within the resolution limit (≈ 0.42 µm) of the optical microscope. As the ethanol/cyclohexane molar ratio was increased to 0.22, a large amount of water droplets were produced on the surface (Fig. 2). With the further increase of the ratio, the size of droplets increased and the size distribution broadened (Fig. 2k ). By increasing the ratio from 0.22 to 2.72, the mean droplet diameter increased from 1 µm to 12 µm. However, there is a turning point at the molar ratio of 2.72 from which the droplet size decreased and the size distribution narrowed with an increase of the ethanol content (Fig. 2l). Specifically, the mean droplet diameter decreased from 11.1 µm to 3 µm with an increase in the molar ratio from 4.32 to 13.9. We propose the following mechanism for the dependence of the droplet size on the solution composition. In the parabolic mixing front between solution A and B, as sketched in Fig. 3, the saturation level is 1 on two ends in either solution A or solution B, between which solution A is essentially mixed with and diluted by solution B. During the solvent exchange, the ratio between ethanol and water was constant, predetermined by their concentrations in solution A. The solution composition follows the dilution curve as indicated in the phase diagram, and can remain metastable in the Ouzo region as indicated by the shaded area. In the Ouzo region, the one-phase solution is in a metastable state where the Gibbs free energy is not minimized, but the kinetic barrier to the phase separation is large. 25 The oversaturation level for a given composition is the ratio of the actual water concentration to the concentration of water on the binodal curve, that is, the projection of the line between the actual composition and the binodal curve on the water axis. The overall water supply from the solvent exchange is represented by the integrated area between the binodal curve and the dilution curve, although we can not yet get the exact correlation between the shaded area size and the sum of the oversaturation without the exact form of the phase boundary as 5
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Figure 3: Three phase diagram of cyclohexane, ethanol and water. (a) The Ouzo region, initial conditions of the solution A, and tie lines in the diagram. (b) (c) Sketch showing the composition during the solvent exchange. In (b), the initial solution starts the solvent exchange directly in the Ouzo region. The dilution curve (red thick line) is the solution path with the ratio of ethanol to water same as in solution A. The temporary supersaturation of a solution is the ratio of the actual concentration of water to the concentration of water at the binodal (i.e. the projection of the thin red lines on the water axis). The sum of the overall water supply is reflected by the size of red shaded area. In (c), the initial solution is out of the Ouzo region. The one-phase solution separates to two phases along the dotted tie lines.
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a function of the solution composition. Qualitatively, at very low ethanol/cyclohexane ratios, the solubility of water in solution A is simply too low to provide sufficient amount of water for the droplet formation. As the ratio of ethanol increases in solution A, more water is supplied by the solvent exchange, leading to the formation of droplets and furthermore larger droplet size. The ratio of the shaded area was 1: 3.6:19 for the ethanol/cyclohexane ratio of 0.22, 0.48 and 1.23, respectively. The droplet diameters from these three ethanol/cyclohexane ratios are 1.5 µm, 3.3 µm, and 11 µm, in agreement with the trend of the shaded area. As the ethanol/cyclohexane ratio in solution A is raised above the stable Ouzo region, the system is unstable upon the dilution by solution B, and undergoes a local phase separation. Consequently, a water-rich phase and an cyclohexane-rich phase form. 26,27 The water-rich phase is simply washed off by solution B, because of immiscibility with solution B (Unless some droplets with very small volume that can overcome hydrodynamics and adsorb onto the surface). Therefore, the contribution from the water-rich phase to the formation of the surface droplets may be negligible. On the other hand, the cyclohexane-rich phase can be mixed and displaced by solution B to produce surface water nanodroplets just as in a process of a normal solvent exchange. Therefore, the droplet size is primarily determined by the composition in the cyclohexane-rich phase after the phase separation. This proposed mechanism is consistent with the experimental results shown in Fig. 2. With the ethanol ratio of 4.32 in solution A, the droplet size lies between those produced by solution A with the ratio of 0.4 and 1.23, exactly the location of the ethanol ratio in the cyclohexane-rich phase after phase separation from the ratio of 4.32. This is the same for solution A with a ratio of 7.6 and 13.9, where the droplet size is also consistent with their cyclohexane-rich phase composition. After the phase separation from solution A with the ethanol ratio of 40, the cyclohexane-rich phase contains too little water to produce the droplets. Therefore, the droplet size increases first and then decreases with an increase of the ratio of ethanol in cyclohexane. From the above mechanism it is clear that the efficacy of producing water droplets de7
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pends on the phase separation behavior of the solvents. To explore the effect of this behavior, we conducted the solvent exchange with various solvents, replacing cyclohexane with either hexane, heptane, octane or decane. We compared the threshold of the ethanol/alkane ratio in solution A for the formation of the water nanodroplets. A solvent is considered more effective when larger drops are produced for a given ratio of ethanol/alkane. The droplet size is expected to increase with the ratio ethanol/alkane. However, the droplet size of ethanol/alkane solutions was found to decrease with alkane chain length even with a higher ratio ethanol/alkane. This suggests that the longer chain length alkanes are less effective solvents. Hexane produced the largest droplets, while decane did not produce any droplets at all. In the three-phase diagram, indeed the three phase boundary increases most sharply for the decane-ethanol-water system and slowest for the hexane-ethanol-water system. In previous work on solvent exchange, the size of oil nanodroplets was found to increase with the flow rate for a given solvent pair. 22 To compare the solvent exchange with water nanodroplets with these results, we examine the effect of flow rate for a given solvent pair. We used ethanol/cyclohexane with 20 vol% ethanol for solution A and cyclohexane for solution B. When the flow rate of solution B was 200 µL/min, many small water nanodroplets were formed (Fig. 5). The size of the interfacial water droplets was small (4.9 µm), distributed in a narrow range of 0.8 µm. The increase of the flow rate resulted in larger droplets with a wider size distribution. These results are consistent with previous solvent exchange experiments with oil nanodroplets.
Conclusions We have demonstrated the formation of water nanodroplets by solvent exchange and established the correlation between the droplet size and the solution compositions for the first time. By using ethanol and cyclohexane as solvents, the size of water nanodroplets increases first and then decreases with an increase of the ethanol/cyclohexane ratio or the
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Figure 4: Effects of the solvent type on the formation of water droplets. (a-d) Optical images of the substrates after the exchange by different organic solvents. Water droplets formed in (a-c), but not in (d). (e) Water fraction in solution A consisting of 20 vol% of ethanol in alkane. The dilution curves during the solvent exchange are indicated by dotted lines. (f) The lateral diameter of droplets formed by using different organic solvents.
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water concentration in the first solution. The reduction of the water droplet size at high ethanol/cyclohexane ratios is attributed to the phase separation of the system in contact with the second solution. We conclude that the droplet size is determined by the cyclohexane-rich phase resulting from the phase separation. Due to the different phase separation behavior, longer alkanes showed reduced efficiency in producing water nanodroplets. The results of this work provide a guideline for control of the droplet size in the solvent exchange.
Experimental Substrate and solutions: Hexane (anhydrous, 99.5 %), heptane, octane, decane, and ethanol (99.5 %) were purchased from Sigma-Aldrich. All chemicals were used without further purification. Water was added to saturate alkane/ethanol (80 vol%: 20 vol%) solution (solution A). Solution B was alkane that was saturated with water. Silicon wafers were purchased from Mitsubishi Silicon (USA). The hydrophilic silicon substrate was cleaned in piranha solution [H2 SO4 (70 %): H2 O2 (30 %)] at 75 °C for 20 min, followed by thorough rinsing in water, and then ethanol. The substrates were blown dry with nitrogen, and further dried at 400 °C for 2 hours. The advancing and receding contact angles of water in air were 40°and 17°, respectively. Phase diagram and Ouzo region: The phase diagrams were obtained from the IUPACNIST solubility database (69. Ternary Alcohol–Hydrocarbon–Water Systems: http://www. nist.gov/srd/upload/jpcrd566.pdf). The boundary of the Ouzo region on the binodal curve was determined from our experiments, based on the amount of the added solvent into the system when Ouzo droplets formed. Although the location of the spinodal curve is not accurate, it does not influence our analysis, because the dilution curve is close to the binodal curve. Preparation, and characterisation of droplets: The experimental setup is shown in the schematic drawing in Fig. 1. A flow cell consisted of a glass plate, an O-ring and a base. The
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silicon substrate was fixed on the base. 5 mL of solution A was first injected into the flow cell, followed by the injection of 10 mL of solution B. The flow rate of solution B was kept constant by a syringe pump. The flow conditions were the same for all the experiments. After the formation of the droplets, optical images of the substrate were acquired by reflectionmode optical microscopy. At least 500 droplets were analyzed to obtain the droplet diameter under each experimental condition. The water droplets were also visualized by a confocal microscope (LSCM, N-STORM, Nikon) when the droplets were dyed by fluorescein.
Acknowledgement X.H.Z and H.X gratefully acknowledge the support from the Australian Research Council (FFT120100473, DE120100042 and DP140100805). H.Z. acknowledges support from The Natural Sciences and Engineering Research Council of Canada (NSERC). The authors thank Martin Klein Schaarsberg for suggestions and proofreading. The authors also acknowledge the use of facilities and the associated technical support at the RMIT MicroNano Reserach Facility and the Microscopy and Microanalysis Facility.
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nanodroplets under controlled flow conditions. Proc. Natl. Acad. Sci. U.S.A. 2015, 112, 9253–9257. (23) Bao, L.; Rezk, A. R.; Yeo, L. Y.; Zhang, X. Highly Ordered Arrays of Femtoliter Surface Droplets. Small 2015, (24) Zhang, X. H.; Zhang, X. D.; Lou, S. T.; Zhang, Z. X.; Sun, J. L.; Hu, J. Degassing and temperature effects on the formation of nanobubbles at the mica/water interface. Langmuir 2004, 20, 3813–3815. (25) Vitale, S.; Katz, J. Liquid droplet dispersions formed by homogeneous liquid-liquid nucleation: “The ouzo effect”. Langmuir 2003, 19, 4105–4110. (26) Antosik, M.; Gałka, M.; Malanowski, S. K. Vapor-Liquid Equilibrium in a Ternary System Cyclohexane + Ethanol + Water. J. Chem. Eng. Data 2004, 49, 7–10. (27) Moriyoshi, T.; Uosaki, Y.; keiichiro Takahashi,; Yamakawa, T. Liquid + liquid equilibria of water + ethanol + cyclohexane at the temperatures 298.15 K and 323.15 K. J. Chem. Thermodyn. 1991, 23, 37–42.
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