Influence of Dissolved Atmospheric Gases on the Spontaneous

ARTICLE pubs.acs.org/JPCC

Influence of Dissolved Atmospheric Gases on the Spontaneous Emulsification of Alkane
Ethanol
Water Systems Barbara Sowa,† Xue Hua Zhang,‡ Karen Kozielski,§ Patrick G. Hartley,† and Nobuo Maeda*,† †

CSIRO Materials Science & Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, VIC3168, Australia Department of Chemical and Biomolecular Engineering, University of Melbourne, VIC 3010, Australia § CSIRO Earth Science & Resource Engineering, Ian Wark Laboratory, Bayview Avenue, Clayton, VIC3168, Australia ‡

ABSTRACT: We previously showed that the amount of oils dispersed in an aqueous phase, and their kinetic stability, could be enhanced when the dissolved atmospheric gases were removed from the system. Here we studied the effect of the removal of dissolved atmospheric gases on the formation and kinetic stability of oil droplets dispersed by the spontaneous emulsification (the Ouzo effect) for the ternary systems of ethanol, alkane, and water. The hydrocarbons studied were decane (C10), undecane (C11), dodecane (C12), tetradecane (C14), and hexadecane (C16). Our findings show that (1) the amount of oils dispersed to the aqueous phase was greater for the degassed samples than for the control (nondegassed) counterparts, (2) the variation in the size from sample to sample of the oil droplets dispersed, measured by dynamic light scattering (DLS), was much narrower for the degassed mixtures than for the control counterparts, and (3) the size of the oil droplets dispersed decreased with the molecular weight of the oil for the control mixtures, whereas it was largely independent of the molecular weight of the oil for the degassed mixtures. Our results suggest that the nature of nucleation during the spontaneous emulsification process may have changed after the removal of dissolved atmospheric gases, becoming more homogeneous in nature than that in the control samples.

’ INTRODUCTION When water is added into Ouzo (a kind of liqueur), a milky emulsion can be formed spontaneously. This phenomenon is known as the “Ouzo effect” or spontaneous emulsification. In the literature, the Ouzo effect has been explained as follows: for three components of oil, ethanol, and water, oil is miscible with ethanol but sparingly miscible with water, whereas ethanol is highly miscible with both oil and water. When an oil-in-ethanol solution is mixed with water, ethanol diffuses through water, and the oil precipitates out.1 Thus, the Ouzo effect offers a convenient way of dispersing oil in water without the use of surfactants or vigorous stirring.1 It has been considered that there are only three components in play in the Ouzo effect. However, this is not quite true. The dissolved atmospheric gases are usually present during the spontaneous emulsification, a fact which has been largely overlooked so far. There is a good reason to believe that the presence of such ubiquitous dissolved atmospheric gases can affect the spontaneous emulsification. First, previous studies showed that the presence of dissolved atmospheric gases can affect the physical properties of colloidal systems. The dissolved gases in ethanol and water can accumulate on a solid surface and form nanobubbles,2
4 which may influence various interfacial phenomena. The dissolved atmospheric gases may also have significant influence on the stability of solid dispersions5 and emulsions6
10 and the electrical conductivity of water.11 Second, the amount of the dissolved atmospheric gases can be comparable to the amount of oil for a range of compositions of the ternary system (alkane þ ethanol þ water). The amount of dissolved atmospheric gases in water at the standard temperature r 2011 American Chemical Society

and pressure is typically 1 mM.8 This amount is higher in ethanol and in ethanol aqueous solutions. For a low concentration of oil, for example 10 μM, there would be at least 100 times more atmospheric gas molecules than the oil molecules. A mechanical vacuum pump can reduce the amount of dissolved atmospheric gases from about 1 mM to 100 nM. This reduction corresponds to, in relative terms, from about 100 times more to about 100 times less than that of the amount of oil in the above example. In the present work, we examined the effects of degassing on the oil droplets formed by the spontaneous emulsification in the system of ethanol, oil, and water. We will show that, perhaps surprisingly, there is a significant difference between the samples in the presence and absence of dissolved atmospheric gases.

’ MATERIALS AND METHODS Materials. The Milli-Q water (Millipore, resistivity >18.2 MΩ) was used for the preparation of all aqueous samples and cleaning of glassware. Ethanol (MERCK, g99.7%) was purified by fractional distillation. The oils used here were decane (C10, Aldrich, g99%), undecane (C11, Aldrich, g99%), dodecane (C12, BDH, g99%), tetradecane (C14, KOCH, g99%), and hexadecane (C16, BDH, g99%). Selected physical properties of these oils are summarized in Table 1. The oils were purified by passing it several times through a column packed with activated, basic aluminum oxide (Al2O3, MERCK, basic 90 active, active Received: February 7, 2011 Revised: March 8, 2011 Published: April 08, 2011 8768

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Table 1. Selected Physical Properties of the Oils Used in This Study27
32 oils density (g/mL)

decane C10 0.73

undecane C11 0.74

dodecane C12 0.75

tetradecane C14 0.76

hexadecane C16 0.77

refractive index

1.41

1.42

1.42

1.43

1.44

viscosity (Poise)

0.78 (22.3 °C)

0.94 (22.7 °C)

1.15 (23.3 °C)

2.22 (21.9 °C)

3.63 (22.2 °C)

Mw (g/mol) solubility in water (298 K)

142.3

156.3 4  10
10

170.3 4  10
10

198.4 2  10
10

226.5 0.7  10
10

(g/100 g water)

(mole fraction)

solubility in 25%

0.0804 (mol fraction

ethanol solution

(mole fraction)

(mole fraction)

of water 0.9033; T = 20 °C)

stage 1) with hexane (MERCK, g99%) as eluent and then evaporating the hexane using a rotator evaporator (BUCHI Vacuum controller V-800, BUCHI Rotavapor R-200, Switzerland). The degassing tubes were made of glass. Glassware was manufactured by our glassblower in a high-temperature furnace and cleaned using 10 wt % NaOH solution and rinsed with a large quantity of Milli-Q water immediately before use. Degassing Procedure. Dissolved gas from each degassed sample was removed by a repeated freeze
pump
thaw process.7,8 Each sample was first frozen in liquid nitrogen. The sample was immersed slowly into liquid nitrogen to prevent ice expansion from cracking the glass tube that contains the sample. Then the space in the tube above the frozen sample was evacuated using a mechanical vacuum pump (Edwards RV3, two-stage mechanical pump, Proscitech, Thuringowa, QLD, Australia), which can attain 2  10
4 Torr when connected to a dry vacuum line. Then the sample was gradually warmed to room temperature (typically 20
25 °C). Formation of a large number of bubbles in the condensed phase of the sample was observed during the thawing process, as the previously dissolved gases were pulled into the upper space. We observed that as the frozen ice gradually thawed from the outer part (in the vicinity of the wall of the sample tube) to the interior of the sample (the temperature gradient during the thawing is such that heat flows from the wall to the interior), trapped gas bubbles in the gaps (defects) of polycrystalline ice were released, expanded in volume as they were released from the defects, and buoyed upward to the surface. This freeze
pump
thaw process was repeated five times for each sample. It was observed that the number of bubbles formed and released during the gradual thawing process became less with each cycle of the freeze
pump
thaw process. Since it is important to keep the constant ratio of oil, water, and ethanol for a given pair of degassed and control counterparts, we paid close attention to potential evaporation of ethanol during the degassing procedure. Fortunately, we did not find an appreciable decrease in the volume of ethanol after each freeze
thaw cycle (so measured by marking the ethanol level on the outside wall of the sample tube), so we conclude that the vapor pressure of ethanol and/or the rate of evaporation at 77 K are negligible for our purpose. Mixing of Emulsions. All oil
ethanol
water emulsions were prepared by the following procedure: 5 mL of 0.2 vol % oil-inethanol solution was first prepared. For the control (nondegassed) samples, 15 mL of water was then added to the oil-in-ethanol solution immediately prior to the DLS measurement. For the degassed samples, 5 mL of 0.2 vol % oil-in-ethanol solution and 15 mL of water were degassed separately. These solutions were kept sealed in the glass tubes while they were gradually warmed to room temperature and then transported to the DLS instrument.

The seals of these glass tubes were broken, and the content was mixed in air immediately prior to the DLS measurement. We note that, although the thermal history of the degassed and the control samples was different (it is impossible to cool a control sample to 77 K without freezing, which in turn causes degassing), the temperature of both samples was well equilibrated to room temperature prior to the commencement of the mixing and the DLS measurement. Because we kept the volumes of each component of the mixture constant, we assume that any heating effect arising from the mixing of water and the oil
ethanol solution must be similar for both the degassed and the nondegassed samples. Dynamic Light Scattering (DLS). The size of the oil droplets was measured using a commercial instrument (Malvern Zetasizer Nano Instrument, model: ZEN 3600, Enigma Business Park, Malvern). The instrument used a 488 nm laser light source and measured scattered light at 90° of the incident light beam. The sample cell was temperature controlled, and the DLS measurements were carried out at 25 °C. Turbidity of the samples was also monitored by recording the count rate of the instrument. Unfortunately, our instrument employed a rectangular-shaped sample tube, which made it challenging for degassing. The rectangular shape caused distribution of nonuniform thermal stress within the sample cell during the repeated freeze
pump
thaw process and resulted in cracking of the sample cell. In the end, we decided to break the vacuum seal immediately prior to the commencement of the DLS measurements. We note that our prior study showed that it takes an extended period for air to diffuse back into a degassed sample.7 So we expect we could effectively measure the “degassed” samples up to the first 24 h and beyond after exposure to air. Measurement of Viscosity. The hydrodynamic diameter of droplets calculated from the measurements by the DLS depends on the viscosity of the continuous phase, in our case the aqueous phase. Thus, there is a concern that if the viscosity of the aqueous phase changes significantly after degassing it may affect the size of the droplets obtained by the DLS measurements. To this end, we carried out precautionary experiments to measure the viscosity of water after degassing. The viscosity of water is not very high, and the expected change, if any, would not be vey great. We used a standard Ostwald capillary viscometer which is supposed to possess sufficient sensitivity for our purpose. As it was difficult to use a capillary viscometer in vacuum, we broke the vacuum seal of the degassing tubes immediately prior to the viscosity measurement, i.e., the same treatment as we applied to our Ouzo samples prior to the DLS measurements. The measurements of the viscosity of the degassed and the control water were carried out at 18 °C. We found that there was no significant difference in the viscosity of degassed and nondegassed water. This result allows 8769

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Figure 1. Light scattering intensity of dodecane droplets in water with time after the initial mixing of dodecane and water (binary system), for the control (filled symbols) and the degassed (open symbols) states. Different shaped symbols refer to different samples. For the degassed samples, the dodecane and water were degassed separately, and then the vacuum seal was broken immediately prior to the mixing at time = 0.

us to conclude that the difference in the droplet size detected by the DLS measurements, presented below, did not arise from a change in the viscosity of the continuous phase but rather from the genuine change in the size of the oil droplets.

’ RESULTS Degassing Effect on the Dispersion of Oil-in-Water Binary Systems. Before we get into our main results of oil
ethanol


water ternary systems, we carried out measurements of oil-inwater binary systems for some of the oils. Compared to our previous study,7 the difference in this work is that this time the vacuum seal was broken before the mixing, and also the oils were purified by activated basic alumina. We kept the volume of an oil to 10 μL and that of water to 15 mL. The oil and water were degassed separately by the freeze
pump
thaw method. We broke the vacuum immediately before mixing the oil and water in the same manner as we would do for the ethanol
oil
water ternary systems. We found that the aqueous phase of the degassed samples became visibly much cloudier than the control counterparts of the same volumes of oil and water. Moreover, we found that the amount of the excess oil floating on top of the aqueous phase of the control samples was visibly and clearly much larger than for the degassed ones. For the amount of oil and water we used here (10 μL of oil in 15 mL of water), we could see an oil lens at the center of the top surface of the aqueous phase of a control sample immediately after the mixing. In contrast, no such lens initially appeared for a degassed mixture. Because we used a fixed amount of oil and water for both control and degassed samples, it is reasonable to conclude that the more oil collected at the top the less amount of it was dispersed in the aqueous phase. The turbidity (light scattering intensity) of decane droplets in water with time after the initial mixing of decane and water is shown in Figure 1, for the control (filled symbols) and the degassed (open symbols) samples. It can be seen that the degassed mixtures scattered light more strongly than the control mixtures. These observations point to one conclusion, that the amount of oil dispersed to the aqueous phase was much greater for the degassed samples than for the control (gassed) counterparts, even when the oil and water were degassed separately and then mixed together, in agreement with our earlier results.7

Figure 2. Photon counts, the droplet diameter (Z-average), and the polydispersity index of the degassed (white) and the nondegassed (gray) emulsions, immediately after the formation (time = 0).

Oil Droplets in the Ternary System Immediately after the Formation. Immediately after the initial mixing of water with the

oil-in-ethanol solution (which we define nominally as time = 0, practically within the first few minutes of the initial mixing), both the degassed and the control samples became visibly cloudy for all the oils studied. The light scattering measurements show that the intensity was several hundred kilocounts per second (kcps) for both types of samples. However, the degassed samples scattered more light than the control counterparts, as shown in Figure 2. Among the degassed samples, the scattering intensity was generally higher for short chain oils (decane > undecane and dodecane > tetradecane and hexadecane). The average size and the polydispersity index of the initial (time = 0) droplets of different oils are also shown in Figure 2. We note that the error bars presented in Figure 2 (and Figure 4 shown later) are referring to the variation arising from different samples, not from the polydispersity of a given sample. There are three notable features: (1) the oil droplets in the degassed samples were significantly smaller than those in the control counterparts, except for hexadecane droplets which were similar in size for the degassed and the control samples, (2) for all the oils studied, the droplet size varied in a larger range for the control samples. For a given oil, the polydiserpsity index of the control samples was much greater than that of the degassed samples, and (3) the size of the oil droplets was smaller the longer the hydrocarbon chain length (higher the molecular weight) of the oil for the control samples. In contrast, the size of the degassed oil droplets apparently remained largely independent 8770

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Figure 4. Photon counts, the droplet diameter (Z-average), and the polydispersity index of the degassed (white) and the nondegassed (gray) emulsions, two hours after the formation (time = 2 h).

Figure 3. Size (hydrodynamic diameter) of the dispersed oil droplets in 25 vol % ethanol/water solution, so measured by the DLS, in the control (filled symbols) and degassed (open symbols) states. Different shaped symbols refer to different sets of experiments. Only the data for the first 5 h from the initial mixing are shown.

of the hydrocarbon chain length. In short, we found that degassing had a significant influence on the initial size and the size distribution of the droplets formed by the spontaneous emulsification process. Evolution of the Droplets with Time. Figure 3 shows the variation in the droplet size for the first 5 h after the initial mixing. It can be seen that the size of the oil droplets for the control samples decreased with the molecular weight of the oil, whereas that of the degassed samples apparently remained largely independent of the molecular weight of the oil. Consequently, the

degassed droplets were smaller than the control counterparts for low molecular weight oils, whereas they were similar for hexadecane, the oil with the highest molecular weight studied. Figure 4 shows that after 2 h of the initial mixing the light scattering intensity already started decreasing gradually with time for both the degassed and the nondegassed control samples. The polydispersity index of the droplets of the nondegassed samples also became smaller compared to that at time = 0. These results suggest that phase separation was taking place. The decrease in the number of droplets from the experimental window could occur through the Ostawld ripening between the droplets and then the buoyancy separation due to the density mismatch between the oil and the ethanol solution. We note that the density of 25 vol % ethanol solution (0.97 g/mL at 20 °C) is higher than any of the oils studied here, and hence it is reasonable to expect that all oil droplets will float with time in the oil
ethanol
water systems (as was the case for the oil-in-water systems). From Figure 2 we see that the droplets in the control samples were much more polydisperse than those in the degassed samples. The Ostwald ripening is expected to be faster in a system with a larger size distribution of the droplets (for a given solubility of the oil). Thus, we would expect the Ostwald ripening to be faster in the nondegassed control systems than in the degassed systems. The rather slow change of hexadecane droplets is thus consistent not only with the smaller solubility of hexadecane in ethanol solution than the other oils but also with the more monodisperse initial size distribution of the droplets at time = 0. 8771

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The Journal of Physical Chemistry C Visual Inspection of the Degassed and the Control Samples and Their Qualitative Differences. For 1
2 hours after

the initial mixing, it was clearly visible that the excess oil floating on top of the aqueous phase was greater for the nondegassed control samples than for the degassed ones. This result is similar to the binary system of oil and water presented above. One day after the mixing, a white ring appeared on the surface of the aqueous phase and around the inner wall of the sample cell for the degassed samples but not for the control counterparts. The white ring turned out to be very fine oil droplets, which gradually disappeared with time, eventually joining the excess oil phase floating at the center of the top surface of the aqueous phase. This observation is similar to that reported by Burnett et al.,12 who reported the significantly extended lifetime of their degassed oil droplets near the surface from creaming. Two to three days after the mixing, both the degassed and the nondegassed control samples started clearing up, gradually from the bottom to the top (i.e., near the bottom of the sample cell became clearer first, and then the “clear front” gradually moved upward). This process of gradual “clearing up” of the aqueous phase continued for a number of days, and the time scale depended on the molecular weight of the oil in that the longer-chain hydrocarbons took longer time to clear up. The oil dispersed through the continuous phase eventually phase separated for both the degassed and the nondegassed systems. The time scale of the phase separation apparently increased with the molecular weight of the oil. We did not quantify the details of the correlation among such parameters as the density mismatch, the molecular weight, the solubility of the oil in the ethanol aqueous solution, and the time scale toward the phase separation. We merely note that for the case of decane, undecane, and dodecane (C10
12) the time scale was of the order of 11
12 days and that for hexadecane (C16) it took weeks for the phase separation to complete, with the other intermediate oils somewhere in between.

’ DISCUSSION Controversy Concerning Oil-in-Water Binary Systems. For binary systems, our results show that removal of ubiquitous atmospheric gases is responsible for the increased dispersion of oil in water. Burnett et al. claimed12 that degassing the oil and water separately and mixing them together in vacuum did not result in any “visible opacity, even with gentle mixing”. Their claim is in direct contrast to our current (and previous7) findings. Nevertheless, their other claims, such as freeze
thaw cycles (without pumping) had a similar effect as freeze
pump
thaw cycles of degassing, which are not surprising. None of the major gaseous components in air, nor the alkanes we studied, are known to fit into the lattice space of ice at ambient pressure or less (we note, however, that some of these components are known to form clathrate hydrates at much higher pressures13). This means that freezing of water alone will inevitably exclude dissolved gases (and oils if present) from the ice lattice and effectively phase separates them (i.e., it is impossible to freeze water without effectively degassing). In fact, Burnett et al.’s results12 that the turbidity significantly increased with the number of freeze
thaw cycles are consistent with our idea of increasing extent of (local) degassing with the number of freeze
thaw cycles. Degassing Effect on the Nucleation of Oil Droplets in Oil
Ethanol
Water Systems. Our results on the initial (time = 0) dispersions show that the oil droplets in the degassed samples

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were smaller and more uniform in size and yet scattered light more strongly than those in the control samples. Because a large droplet scatters more strongly than a small one, our results suggest that the number density of the oil droplets must have been higher in the degassed samples than in the control ones. The larger number of smaller and more uniformly sized droplets in the degassed samples may suggest that the nucleation of the oil droplets in the degassed samples was more homogeneous in nature than in the nondegassed control samples. Our hypothesis is that ubiquitous atmospheric gases should be regarded as impurities, and hence removal of them by the means of degassing would remove potential heterogeneous nucleation sites from the system. The removal of heterogeneous nucleation sites is expected to allow further diffusion of the oil-in-ethanol solution into water and build-up of the supersaturation of oil in the solution. For sufficiently large supersaturation of oil, homogeneous nucleation can trigger the precipitation of the oil from the solution. In the homogeneous nucleation, nuclei form when the local supersaturation of the oil reaches a critical level from small fluctuations in the concentration of the oil molecules. The nuclei thus have a larger than average concentration of oil, and their formation causes depletion of the oil from the surroundings. Therefore, the next nearest homogeneous nucleation can occur only some distance away from the existing nuclei around which the concentration of the oil has been depleted. As such, the homogeneous nucleation would result in spatially more uniform dispersion of smaller oil droplets than the heterogeneous nucleation.1 Time Evolution of Oil Dispersions in Oil
Ethanol
Water Systems. The larger number of smaller and more uniformly sized droplets in the degassed samples than the control samples at time = 0 may also set the course for the subsequent time evolution of the oil dispersions at time > 0. The rate of the Ostwald ripening of the oil droplets depends not only on the solubility of the oil in the medium dispersed but also, for a given oil, on the initial size and the size distribution (polydispersity) of the oil droplets. The size distribution of the oil droplets in the nondegassed control samples varied in a larger range than those in the degassed samples. Therefore, the Ostwald ripening process would be faster in the nondegassed control systems. As the larger oil droplets grow at the expense of the smaller ones, buoyancy separation due to the density mismatch between the oil and the ethanol solution is expected to accelerate. These effects would lead to faster phase separation of the oil from the ethanol solution in the control samples than in the degassed ones. We depict the situation schematically in Figure 5. Because the kinetic stability of the dispersions could be largely decided by the initial conditions of the oil droplets at time = 0, we could not establish whether the dissolved gases had any significant impact on the time evolution of the oil droplets that is independent of the initial conditions. Implications and Potential Applications of Our Results. Oil-in-water emulsions, dispersed by degassing (oil
water binary system)9 or by the spontaneous emulsification (oil
ethanol
water ternary system),1 appear to show somewhat similar properties upon heating and cooling of the dispersions—in either system, the distribution in the droplet size could be narrowed by cooling the mixture from an elevated temperature.1,9 Interestingly, our results show that degassing of oil
ethanol
water ternary systems has somewhat similar effect as heating and cooling of degassed oil
water binary systems or heating and cooling of nondegassed oil
ethanol
water ternary systems. Our findings show that the removal of atmospheric gases from 8772

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Figure 5. Schematic drawing of the evolution of the oil droplets in the degassed and control samples of the ethanol, oil, and water ternary systems.

water and ethanol may be an alternative way to narrow the distribution of the droplet size, without the need of heating the systems which may be undesirable for some delicate oils. Our results on the ternary systems of hydrocarbons, water, and ethanol might also be relevant to the fuel industry where ethanol þ gasoline blends are widely used. Remaining Questions. Unfortunately, our results and the above considerations regarding a possible role of dissolved atmospheric gases in the oil
ethanol
water ternary system are unlikely to apply to oil-in-water binary systems and hence may not shed new light on the mystery surrounding what dissolved atmospheric gases do, at the molecular level, to the kinetic stability of oil-in-water binary systems. We previously suggested7 that the presence of dissolved gases may enhance the magnitude and/or the range of the hydrophobic force,14,15 either by the cavitations of nanobubbles16 or through the potential of mean force which arises from the spatial gradient in the distribution of such gas molecules.17
19 Wennerstrom suggested that degassing may facilitate breakaway of the oil phase into smaller parts due to capillary-induced effects.20 If dissolved gas molecules accumulate in the vicinity of the oil
water interface, then the presence of such gaseous molecules could cause formation of gaseous bridges between approaching oil droplets which would pull them together and facilitate coalescence of oil droplets and/or prevent breakaway of an oil droplet to smaller parts by the same capillary induced attractive forces. Once oil droplets are dispersed in water they are known to become negatively charged. It has been reported that the zeta potential of some oil droplets, prepared by homogeneous nucleation of oil from supersaturated oil-in-water solution, induced by the application of the appropriate temperature gradient, was down to
100 mV at pH > 7,21 whereas the zeta potential of oil droplets dispersed by sonication (ultrasonic radiation) appears somewhat higher (less negative), about
35 mV.22 Such negative charge is expected to cause buildup of electrostatic repulsion between the oil droplets and hence increase the kinetic stability. One leading hypothesis as to the origin of the negative zeta potential is that autolysis of water supplies a virtually unlimited amount of negatively charged hydroxyl ions that can be adsorbed to the oil
water interface.21 Such preferential adsorption of hydroxyl ions would leave the aqueous phase acidic as the mixing proceeds, due to the overall electrical neutrality, that could be detected by titration.23 Nevertheless, we note that there are

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controversies as to the nature and the origin of the charge at an interface between water and a nonpolar medium.23
25 Moreover, titration of oleic acid in aqueous media shows that the aqueous phase pH rises rapidly from acidic to basic with the addition of a minute amount of NaOH.26 The inference is that the bulk majority of oleic acid remains unexposed to the aqueous phase, even with vigorous stirring. Dispersion of negatively charged oleic acid in water would be easier than dispersion of a neutral oil of a similar alkyl chain length, because of the electrostatic repulsion, but this is apparently not so. In other words, a neutral alkane can take the trouble of picking up hydroxyl ions from the aqueous phase (against the image charge repulsion) to make itself negatively charged and then use the electrostatic repulsion thus acquired to disperse itself in water. On the other hand, oleic acid, with an intrinsic negative charge of its own, cannot use the electrostatic repulsion to disperse itself or take the same trouble of picking up hydroxyl ions from the aqueous phase to facilitate its dispersion. It remains to be seen how all these different experimental observations may fit together one day.

’ CONCLUSION We systematically examined the effect of the removal of dissolved atmospheric gases on the spontaneous emulsification or the Ouzo effect. The oils we studied are decane (C10), undecane (C11), dodecane (C12), tetradecane (C14), and hexadecane (C16). We found that the removal of dissolved gases significantly altered the nature of the nucleation of the oil droplets during spontaneous emulsification in that the nucleation process became more homogeneous. (1) The amount of oils dispersed to the aqueous phase was greater for the degassed samples than the control (non-degassed) counterparts. (2) The variation in the size (hydrodynamic diameter) from sample to sample of the oil droplets dispersed was smaller for the degassed mixtures than for the control counterparts. (3) The size of the oil droplets dispersed decreased with the molecular weight of the oil for the non-degassed mixtures, whereas it was largely independent of the molecular weight of the oil for the degassed mixtures. Because the removal of dissolved gases led to the difference in the initial emulsion, we could not establish whether the dissolved gases had any significant impact on the time evolution of the oil droplets that is independent of the initial conditions. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT N.M. acknowledges the support of an Australian Research Council Future Fellowship (project number FT0991892). X.H.Z. is the recipient of an Australian Postdoctoral Fellowship (DP0880152). ’ REFERENCES (1) Vitale, S. A.; Katz, J. L. Langmuir 2003, 19 (10), 4105–4110. (2) Lou, S.; Ouyang, Z.; Zhang, Y.; Li, X.; Hu, J.; Li, M.; Yang, F. J. Vac. Sci. Technol. B 2000, 18 (5), 2573–2575. (3) Zhang, X. H.; Maeda, N.; Craig, V. S. J. Langmuir 2006, 22 (11), 5025–5035. (4) Zhang, X. H.; Zhang, X. D.; Sun, J. L.; Zhang, Z. X.; Li, G.; Fang, H. P.; Xiao, X. D.; Zeng, X. C.; Hu, J. Langmuir 2007, 23 (4), 1778–1783. 8773

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