Impact of Spontaneously Adsorbed Hydroxide Ions on Emulsification

Sep 15, 2014 - of a hexadecane−acetone−water system via the Ouzo effect (solvent shifting) ... called the Pastis or Ouzo effect.1−5 The emulsifi...
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Impact of Spontaneously Adsorbed Hydroxide Ions on Emulsification via Solvent Shifting Aijing Ma, Jie Xu, and Haolan Xu* Ian Wark Research Institute, University of South Australia, Mawson Lakes Campus, Adelaide, South Australia 5095, Australia S Supporting Information *

ABSTRACT: In this study, the influence of interfacial OH ions on spontaneous emulsification of a hexadecane−acetone−water system via the Ouzo effect (solvent shifting) was investigated. It was found that although there is no surfactant utilized in the solvent shifting method, the resulting emulsion droplets are negatively charged in a wide pH range (pH > ∼2.4) due to the spontaneous adsorption of OH ions. The OH ions remarkably affect the size, size distribution, and stability of the emulsions. The emulsion droplets obtained in the absence of OH ions are large and have a wide size distribution. Coalescence of the emulsion droplets and phase separation are prone to take place. At high OH− concentration, the resulting emulsion droplets have a small size, a narrow size distribution, and excellent colloid stability. The results suggest that the spontaneous OH− adsorption affects the nucleation process during the emulsification and the stability of the obtained emulsions during storage.

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INTRODUCTION Solvent shifting is a low-energy and spontaneous emulsification method for preparation of surfactant-free emulsions. It is also called the Pastis or Ouzo effect.1−5 The emulsification process is very simple. Only three components, a hydrophobic oil, an amphiphilic organic solvent which is fully miscible with water, and a nonsolvent (water), are involved in this method. Ethanol, acetone, or tetrahydrofuran is generally used as the amphiphilic solvent. First, oil is dissolved in the amphiphilic solvent. Then the oil solution is mixed with a large amount of water. During the mixing, interdiffusion of the oil solvent and water takes place, resulting in a supersaturation of oil. Nucleation and growth of oil droplets spontaneously take place, leading to emulsions. No external energy input and surfactants are required, making this method energy- and cost-effective. The solvent shifting method has been widely applied in drug loading and preparation of nanoemulsions, polymer nanoparticles, and nanocapsules.5−9 Recently, nonsurfactant emulsions prepared via the solvent shifting method were utilized to form interfacial nanodroplets at solid−liquid interfaces due to the relatively simple interfacial properties.10−12 The sizes, size distributions, and stability of the emulsions prepared via the solvent shifting method are crucial to their applications. The effects of experimental factors such as the volume fraction of oil, solvent ratio, temperature, and interfacial tension, etc. have been investigated.1,7,13,14 In addition, some ubiquitously existing but unperceivable factors may also affect the emulsification process by the Ouzo effect. For instance, Nobuo et al. recently studied the influence of the dissolved atmospheric gases on the emulsification and stability of the emulsion obtained by solvent shifting.15 They found that the removal of the dissolved gases from the solution resulted in relatively stable emulsions with a small droplet size and narrow © 2014 American Chemical Society

size distribution. The dissolved gases were suggested to affect the nucleation process during the spontaneous emulsification. Besides the dissolved gases, there is another species ubiquitously existing in the oil−water system, the spontaneously adsorbed OH ions (at the oil−water interface). However, the role of OH ions was little considered in the solvent shifting process. It has been well documented that OH ions spontaneously adsorb and enrich at oil−water interfaces in a wide pH range (pH > 2−3).16−20 The adsorbed OH ions render the oil−water interfaces negatively charged. For instance, the ζ potential of pristine oil droplets at pH 9 is about −100 mV.18 This negative charge causes the electrostatic repulsion between the pristine oil droplets to inhibit the coalescence. Thus, the adsorbed OH ions could solely stabilize the oil droplets, producing pristine oil-in-water emulsions in the absence of surfactants.18 Recently, Xu et al. reported that OH ions also played an important role in the formation of Janus-like Pickering emulsions.21 Both OH ions and particles are present at oil−water interfaces and act as emulsifiers to costabilize the oil droplets. These OH ion loaded oil−water interfaces remind the emulsions prepared by solvent shifting method, in which no surfactant is used to stabilize the emulsion droplets. OH ions are expected to spontaneously adsorb at oil−water interfaces. The adsorbed OH ions should be taken into account for the formation and stabilization of emulsions. In the present work, we mainly investigate the impact of OH ions on the size and stability of the emulsions prepared by the solvent shifting method. Received: July 29, 2014 Revised: September 11, 2014 Published: September 15, 2014 23175

dx.doi.org/10.1021/jp5076109 | J. Phys. Chem. C 2014, 118, 23175−23180

The Journal of Physical Chemistry C

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EXPERIMETNAL SECTION

Materials. Hexadecane (99%) was purchased from SigmaAldrich. Acetone (99.8%), NaOH (99%), and HCl (37 wt %) were supplied by Chem-Supply (Australia). Hexadecane was purified by being passed through a column of basic alumina several times. Other chemicals were used as received. Milli-Q water with a resistance of ∼18.2 MΩ cm−1 was used for all experiments. Preparation of Hexadecane-in-Water Emulsions. The emulsions were prepared by the solvent shifting method. The typical procedure is as follows: Hexadecane was first dissolved in acetone. The volume fraction of hexadecane in acetone was set as 0.1%, 0.5%, and 10% separately. Then 1 mL of asprepared hexadecane in acetone solution was quickly injected into 20 mL of Milli-Q water at different pH values to form emulsions. The emulsions are denoted as 0.1% emulsion, 0.5% emulsion, and 10% emulsion, respectively. ζ Potential and Dynamic Light Scattering (DLS) Measurement. The ζ potentials and DLS size distributions of the resulting emulsions were measured with a Malvern Zetasizer Nano ZS instrument. All the measurements were carried out at 25 °C. The physical properties of the oil and solutions for the measurements are listed in Table S1 (Supporting Information). Transmittance and Tyndall Effect. Transmittance (T, %) spectra of the emulsion solutions were recorded with a Shimadzu UV-2600 UV−vis spectrophotometer. The incident light wavelength is 500 nm. The Tyndall effect was utilized to confirm the existence of emulsion droplets in the solutions. A 1 mW 532 nm green laser pointer was used as a light source. A visible light beam path can be observed in an emulsion due to Tyndall scattering. When phase separation takes place, the light beam path becomes weak and finally vanishes.

Figure 1. ζ potential of the emulsions as a function of the pH value. The emulsions were prepared by the solvent shifting method. The volume fraction of oil in acetone is 0.5% (0.5% emulsion).

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RESULTS The presence of OH ions at oil−water interfaces has been proved by measuring the ζ potential of pristine oil droplets dispersed in water. When there is no charged species, for instance surfactant molecules, present at oil−water interfaces, the oil droplets are still negatively charged due to the spontaneous enrichment of OH ions.17−19 The isoelectric point of pristine oil droplets was reported to be a pH of 2−3. The ζ potential decreases with an increase of the pH value, which indicates an increase of the amount of OH ions adsorbed at the surfaces of oil droplets. In this study, it is found that the emulsion droplets prepared by the solvent shifting method are also negatively charged in a wide pH range. As shown in Figure 1, one can explicitly observe the pH-dependent ζ potential of the emulsions. The emulsions have an isoelectronic point at pH ≈ 2.4 and become negatively charged at pH > 2.4. The ζ potential decreases with an increase of the pH value. This feature is similar to that of pristine emulsions stabilized solely by OH ions. Because there are no other charged species in the solvent shifting method (hexadecane was purified, the purity of acetone is 99.8%, and the impurities mainly contain water, isopropyl alcohol, methanol, and diacetone alcohol, which are all noncharged), the negative charge can be ascribed to the spontaneous OH− adsorption at oil droplet surfaces. The effect of the adsorbed OH ions on the size and polydispersity of the as-prepared emulsions was investigated. Figure 2 illustrates that both the mean size and polydispersity index (PDI) of the emulsions decrease with an increase of the

Figure 2. (a) Droplet size and (b) PDI of the emulsions prepared via the solvent shifting method at different pH values. The volume fraction of oil in acetone is 0.5%.

pH, i.e., the concentration of OH−. At the isoelectric point (pH 2.4), the droplet size is measured to be 446 ± 44 nm (Figure 2a). The PDI of the emulsion is 0.43 (Figure 2b), indicating a wide size distribution. At pH 3.2, the ζ potential of the emulsion droplets is −14 ± 4 mV. The droplet size is 409 ± 176 nm (Figure 2a), while the PDI is 0.4. With an increase of the pH, the OH− concentration on the emulsion droplet surfaces increases. The ζ potential of the emulsions is decreased to −34 ± 6 mV when the pH is increased to 4.4 (Figure 1). Correspondingly, the mean size of the emulsion droplets is 23176

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Figure 3. (a) Droplet size, (b) transmittance (T, %), and (c) photographs of the emulsions prepared by the solvent shifting method as a function of the storage time. The volume fraction of oil in acetone is 0.5%.

decreased to 208 ± 2 nm (Figure 2a), and the PDI is decreased to 0.16 (Figure 2b), indicating a relatively narrow size distribution of the emulsion droplets. A further increase of the pH value imparts a higher OH− concentration at oil−water interfaces, manifested by the lower ζ potential (Figure 1). As a result, the sizes of the emulsion are gradually decreased to 156−178 nm (Figure 2a). The PDI is lowered to 0.09 when the pH is 10.8 (Figure 2b). These results indicate that the OH ions play an import role in the nucleation and growth of emulsion droplets during the solvent shifting process. When the OH− concentration is low (pH < 3.2), the obtained emulsion droplets have large sizes and a wide size distribution. A high OH− concentration could lead to emulsions with small sizes and a narrow size distributions. It is hypothesized that, once the emulsion nuclei formed during solvent shifting, OH ions spontaneously adsorbed onto the surfaces of the nuclei. The adsorbed OH ions served as stabilizers to stabilize the emulsion nuclei by introducing electrostatic repulsions. Thus, the aggregation of the nuclei, which is one of the key factors inducing a large particle size and polydispersibility was suppressed, leading to small emulsion droplets with a narrow size distribution. On the other hand, at higher pH, more OH− ions are available to stabilize oil−water interfaces, which is favorable for the formation of small emulsion droplets. After the formation of emulsion droplets, the adsorbed OH ions cause electrostatic repulsions between the emulsion droplets and thus affect their colloidal stability. We monitored the changes in droplet size and the light transmitted through the emulsion solution during the emulsion storage at different pH values to investigate the impact of OH ions on the stability of the obtained emulsions. The light transmittance (T, %) has been used to study the coalescence of bubbles in various salt

solutions.22 An increase of T implies a decrease of the emulsion concentration via phase separation or droplet coalescence and consumption. In the absence of OH ions on the surfaces of emulsion droplets (at the isoelectric point, pH 2.4), the size of the emulsion droplets dramatically increases from 446 ± 44 to 1193 ± 16 nm in 48 h (Figure 3a). T of the emulsion solution increases from 35.3% to 44.4% in 2 h and further increases to 95.6% and 96.4% after 24 and 48 h of storage (Figure 3b). The dramatic change in droplet size and light transmittance of the emulsion solution indicates a significant decrease of the emulsion concentration. The digital photographs (Figure 3c) of the emulsion clearly demonstrate the gradual transition of the emulsion solutions from cloudy to transparent with the storage time. Correspondingly, the Tyndall effect of a laser beam passing through the emulsion solution became weak during storage (Figure 3c). These results indicate that, without the adsorbed OH ions, the emulsion prepared by the solvent shift method is unstable. The droplet sizes increase quickly, followed by phase separation. This leads to a dramatic decrease of the concentration of emulsion droplets. At pH 3.2 (the ζ potential is −14 ± 4 mV), it is noticed that there is no obvious change in droplet size (Figure 3a), while T of the emulsion solution increased from 43% to 49.2% in 2 h, to 65.1% in 24 h, and eventually to 73.6% in 48 h (Figure 3b). Distinguishable variation in the solution opaqueness and Tyndall effect can be observed from the digital photographs (Figure 3c). It is reasonable to suggest that, in the presence of a small amount of OH ions, partial emulsion droplets could be stabilized and remain in the solution, while the others, especially the large droplets, coalesced and separated from the solution. With an increase of the OH− concentration, T of the emulsion solution gradually decreases (Figure 3b), indicating improvement of the 23177

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increase of the OH− concentration. However, the sizes did not change dramatically within 48 h. At the isoelectric point (pH 2.4), the change in the droplet size is 20% (Figure 4a), which is much lower than that of the 0.5% emulsion (167% change). The change in T of the emulsion solution is 26% (Figure 4b), which is also much smaller than that of the 0.5% emulsion (173% change). These results indicate that the degree of emulsion coalescence and phase separation at the isoelectric point is much lower when the oil volume fraction is lower. The effect of OH ions on emulsion formation and stabilization appears weak. When pH > 4.4, both the droplet size and T remain little changed, indicating excellent colloidal stability of the emulsions. Note that the corresponding border for excellent stability of the 0.5% emulsion is pH 7. The critical pH for stable emulsion formation shifts to a low value when the volume fraction of oil decreases. In contrast, when the volume fraction of hexadecane in acetone is increased to 10% (denoted as 10% emulsion), the obtained emulsion droplets have relatively larger sizes, broader size distributions (Figure S1, Supporting Information), and poor colloid stability. The initial sizes of the emulsion droplets decrease with an increase of the OH− concentration (Figure 5). At the isoelectric point (pH ≈ 2.4), the sizes of the emulsion droplets increased quickly from the initial 406 ± 47 nm to 610 ± 18 nm in 2 h and to 1281 ± 222 nm in 24 h. After 48 h of storage, the droplet sizes were undetectable. The solution became transparent (Figure 5c). T of the solution increased to 98% in 48 h (Figure 5b). No Tyndall effect was observed when a green laser passed throught the solution (Figure 5c). These results confirm the complete phase separation of the emulsion solution. Similarly at a low OH− concentration (pH of 3.2, ζ potential of −14 mV), the sizes of the emulsion droplets also increased quickly followed by phase separation within 24 h (Figure 5). In comparison, no complete phase separation occurred in the 0.1% and 0.5% emulsions at pH 3.2. This implies that the presence of OH ions becomes more important to the stability of the emulsions when the oil volume fraction is high. The emulsions are stable only when the pH is higher than 8.6; no obvious changes in droplet size and T of the emulsion solution are detected (Figure 5). The critical pH values for both a stable emulsion (pH 8.6) and phase separation (pH 3.2) increase when the volume fraction of oil increases.

colloidal stability. However, when pH < 7, a remarkable change of T can still be observed (Figure 3b). In comparison, when pH > 7, T of the emulsion solution is little changed within 24 h. After 48 h of storage, the change in T is only less than 6%. Note that when pH > 7, the spontaneously adsorbed OH ions gave rise to a strong negative charge of the droplet surfaces (ζ potential < −50 mV, Figure 1). The strong electrostatic repulsion between the emulsion droplets contributed to the excellent colloid stability. It is summarized that, at the isoelectric point (no adsorbed OH ions), both the size and concentration of the emulsions are significantly changed and the emulsions suffer severe phase separation. The emulsion is unstable. When 3.2 < pH < 7, the concentration of the emulsions decreases remarkably, while the emulsion droplet sizes are slightly changed. When pH > 7, the emulsions are stable. Both the sizes and concentrations of the emulsions are little changed. It is clear that the OH ions in the solvent shifting system play an important role in both the nucleation process and emulsion droplet stabilization. We found that the effect of OH ions on the formation and stabilization of emulsions became more significant when the volume fraction of hexadecane in acetone increased and appeared weak when the volume fraction of hexadecane decreased. Figure 4 depicts the droplet size and T evolution of the emulsions prepared when the volume fraction of hexadecane in acetone decreased to 0.1% (denoted as 0.1% emulsion). Compared with those of 0.5% emulsions, both the initial sizes and PDI of the 0.1% emulsions decreased (Figure S1, Supporting Information). Similar to those of the 0.5% emulsion, the sizes and PDI of the emulsions decreased with an

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DISCUSSION AND CONCLUSION The impact of the spontaneous interfacial adsorption of OH ions on emulsion formation and stabilization in a solvent shifting system has been investigated. The adsorbed OH ions play important roles in emulsion nucleation and stabilization. The adsorbed OH ions could effectively inhibit the aggregation of the emulsion nuclei during emulsification and the as-formed emulsion droplets during storage. In the absence of OH ions, i.e., at the isoelectric point, the emulsion nuclei generated via solvent shifting may aggregate quickly due to the absence of electrostatic repulsion (Figure 6a1,a2). The aggregation of emulsion nuclei led to a large droplet size and wide size distribution (Figure 6a3). During storage, the obtained emulsion droplets easily aggregated and coalesced (Figure 6a4), leading to phase separation (Figure 6a5). In this scenario, the emulsion droplets are polydisperse and unstable. The sizes of the emulsion increased quickly until phase separation. Accordingly, T (%) of the solution dramatically increased, and the solution became transparent. At low OH− concentration, during the nucleation process, partial emulsion nuclei could be

Figure 4. (a) Droplet size and (b) transmittance (T, %) of the emulsions prepared by the solvent shifting method as a function of the storage time. The volume fraction of oil in acetone is 0.1%. 23178

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Figure 5. (a) Droplet size, (b) transmittance (T, %), and (c) photographs of the emulsion prepared by the solvent shifting method as a function of the storage time. The volume fraction of oil in acetone is 10%.

small emulsion droplets with a narrow size distribution (Figure 6c3). The aggregation of the obtained emulsion droplets was also suppressed (Figure 6c4,c5). The emulsions showed excellent colloid stability. There are critical pH values for phase separation and excellent stability of the emulsions. The volume fraction of hexadecane affects the critical pH values. A low oil volume fraction causes low supersaturation during solvent exchange and nucleation of the emulsion. As a result the number of nuclei decreases, requiring less OH ions to stabilize the nuclei. In contrast, an increase of the oil volume fraction leads to a larger number of nuclei. More OH ions are required to stabilize the nuclei and as-formed emulsion droplets. Therefore, the critical pH value increased. In conclusion, we have proved that the spontaneous interfacial adsorption of OH ions remarkably affects emulsification via solvent shifting. The size, size distribution, and stability of the obtained emulsions vary with the concentration of OH−. From the standpoint of emulsion applications, the findings of this research will benefit the optimization of emulsions prepared via the Ouzo effect for wide applications. With respect to the colloid and interface sciences, our results imply that the OH ions involve in the nucleation process, affect the stability of emulsion nuclei and emulsion droplets via control of the electrostatic interactions. As the OH ions ubiquitously exist at hydrophobic surfaces in contact with water, the effect of the adsorbed OH ions should not be ignored in these processes.

Figure 6. Schematic illustration of the formation and evolution of the emulsion droplets prepared via solvent shifting (a) without OH−, (b) at a low OH− concentration, and (c) at a high OH− concentration.

stabilized by OH ions, while others aggregated (Figure 6b1,b2). As a result, both large and small emulsion droplets were obtained (Figure 6b3). Due to the buoyancy and weak electrostatic repulsion, the large oil droplets creamed, coalesced, and eventually separated from the emulsion solution (Figure 6b4, b5). The small ones were stabilized by OH− and acetone and remained stable. The emulsions suffered a partial phase separation (Figure 6b5). At high OH− concentration, the high concentration of the adsorbed OH− gives rise to strong electrostatic repulsions between emulsion nuclei to effectively prevent their aggregation (Figure 6c1,c2), resulting in relatively 23179

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ASSOCIATED CONTENT

S Supporting Information *

Physical properties of the oil and solvents and initial size and PDI of emulsions prepared at different oil volume fractions and pH values. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Australia Research Council (Grant DE120100042) and University of South Australia (ResearchSA Fellowship).

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