Oil Interface in Surfactant

Feb 6, 2014 - The present study prepared a size-controllable, uniform, and surfactant-free emulsification to investigate the ζ potential of the solve...
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Solvent Effect on ζ Potential at an Aqueous/Oil Interface in Surfactant-Free Emulsion Yong Wu,†,‡ Qiang Li,§ Fuli Deng,† Xiangfeng Liang,*,† and Huizhou Liu*,† †

Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China ‡ China Shenhua Coal to Liquid and Chemical Beijing Engineering Company, Beijing 100011, China § National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: The present study prepared a size-controllable, uniform, and surfactant-free emulsification to investigate the ζ potential of the solvent effect. The results showed that the ratio of electrophoretic mobility changed with droplet diameter, and the correct factor of the ζ potential was determined. The effect of functional groups on the ζ potential was further studied in the presence of an organic hydrophilic solvent. The study characterized the effects of pH, ionic strength, and ionic type on the ζ potential and indicated that the solvents were able to modulate the local electrochemical environment, thus leading to the redistribution of interface charges.

1. INTRODUCTION Stability and surface morphology are two significant properties of emulsions. The ζ potential is an important parameter for emulsion stability prediction. Recently, the interface studies of ζ potential have attracted growing attention in many fields,1,2 including biotechnology,3−5 colloid science,6−9 and nanotechnology.10−13 These studies showed that the ζ potential was able to be controlled by a variety of environmental parameters, such as pH,14,15 ionic strength,16 surfactants,17 particle size,12 and geometry.6,8 In most of these interface studies, many surfactants were used to stabilize the surface of the metastable droplets17−19 because these supplementary amphiphilic surfactants with multiple functional groups were able to change the ζ potential.20−23 However, a series of methods were developed to study the ζ potential of emulsions without a surfactant. Beattie and Djerdjev employed the electroacoustic method to measure the charges of hydrocarbon droplets by mechanical membrane emulsification.8 The finding addressed how the origination of the ζ potential was obtained by overcoming the disturbance resulting from the additional surfactants.8 However, the presence of other impuritiessuch as ethanol, acetone, and acetonitrilecreated a number of challenges, including the © 2014 American Chemical Society

need to adjust two-phase stability through the maintenance of the optimal interface properties. In particular, the impurities altered the ζ potential of the emulsions and affected the electrophile/nucleophile interaction to some extent.24−27 For instance, an electron-rich solvent at the interface was attracted to the center with a positive charge and was capable of bonding with the electron-deficient species by donating electrons.28 The long-chain groups influenced the electrophoretic mobility of nanoparticles due to their hydrophilicity and hydrodynamic permeability.29 The organic solvent created the complex interface properties of the emulsions. However, studies have seldom focused on the key effect of the ζ potential, especially in hydrophilic solvents.26 Therefore, to fill this gap, it was necessary to perform systematic research on the solvent effect of the ζ potential. This study used a surfactant-free method to prepare droplets with controllable sizes and used the emulsion droplets to study the solvent effect on the ζ potential. Moreover, the calculation of the ζ potential was also simplified. The study systematically Received: October 8, 2013 Revised: February 3, 2014 Published: February 6, 2014 1926

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mobility velocity is proportional to the electric field. Moreover, cations and anions are not able to penetrate the droplets. On the basis of the assumptions mentioned above, the Navier−Stokes equations combined with the Poisson equation produces29

examines the effect of the ζ potential of a hydrophilic solvent on pH, ionic strength, and ionic type. This work will enhance our understanding of the behaviors of droplets at the oil/water (o/w) interface and contribute to this field by quantifying the interface charges affected by a hydrophilic organic solvent.

N

∑ ∇δni × ∇ni(0)

η∇ × ∇ × ∇ × u =

2. MATERIALS AND METHODS 2.1. Materials. Methanol (Sigma-Aldrich, ≥ 99.9%), ethanol (Sigma-Aldrich, ≥ 99.5%), 1-propanol (Sigma-Aldrich, ≥ 99.7%), acetone (Sigma-Aldrich, ≥ 99.9%), and acetonitrile (Caledon Laboratories Ltd., ≥ 99.9%) were used as the dispersers. The oil phase was dodecane (Sigma-Aldrich, ≥ 99.0%). Potassium chloride (Sigma-Aldrich, ≥ 99.0%), magnesium chloride (Sigma-Aldrich, ≥ 99.0%), and sodium chloride (Sigma-Aldrich, ≥ 99.0%) were purified by recrystallization. Sodium hydroxide (Sigma-Aldrich, ≥ 98.0%), potassium hydroxide (Sigma-Aldrich, ≥ 98.0%), and hydrochloric acid (Sigma-Aldrich, ∼37%) were used for pH adjustment. Ultrapure MilliQ water (Millipore) was used for the preparation of all samples and solutions. 2.2. Preparation of Surfactant-Free Emulsion. A controllable droplet diameter and surfactant-free emulsion was prepared via the liquid−liquid dispersive method. The hydrophilic solvent was used as not only a disturbance agent but also as a disperser. The following details the preparation procedure. The study used 10 mL of the disperser solvent (i.e., methanol, ethanol, 1-propanol, acetone, or acetonitrile) to disperse a nonpolar solvent in an aqueous solution. The ratio of nonpolar solvent to disperser solvent was 0.5 wt %. By using a syringe (0.45 mm i.d.), 1 mL of mixed solvent was rapidly injected into 50 mL of ultrapure water to form cloudy solutions. To reach a certain ionic strength, 0.2 M electrolyte solution was added into the water solution in order to prepare 0.01, 0.1, 1, 10, and 100 mM electrolyte solutions. This calculation method called for the disperser solvent to be added to the cloudy solution to reach the desired solvent concentration, that is, 2.000, 20.00, 200.0, and 2000 mM ethanol, 1.866, 18.66, 186.6, and 1866 mM acetone, and 3.640, 36.40, 364.0, and 3640 mM acetonitrile. Finally, 10 mM KOH and 10 mM HCl were used for pH adjustment. 2.3. Determination of Droplet Size and Electrophoretic Mobility. A cloudy solution (water/hydrophilic solvent/dodecane) was injected into the flow cell of the Delsa Nano C analyzer (Beckman Coulter, Inc., Japan) to measure the diameter and the electrophoretic mobility. All measurements were performed at 298.15 K. The quartz cell was filled with a 10 mL sample. The size and electrophoretic mobility of the droplets were calculated from the triplicate measurements, and they were represented as the average values. The test time was 20−30 s, including droplet size and electrophoretic mobility. 2.4. Calculation of the ζ Potential. To characterize the interface charge, the ζ potential was calculated by electrophoretic mobility.29,30 For a spherical nonconducting droplet, the ζ potential is obtained using Henry’s formula: μ = fH (κa)εrε0ζ /η

(r > a)

i=1

∇· (ni(0)u − ni(0)∇δμi ) = 0

(2) (3)

where u is the liquid velocity, μi is the electrochemical potential of the ith ionic species, η is the viscosity of the bulk phase, ni(0) is the equilibrium concentration of the ith ionic species, δμi is the deviation of μi because of the electric field. According to symmetry considerations, the equation is expressed as follows:

μ=

εr ε0a 2κ 2ζ 9η

∫a

a3 ⎞⎛ ae−κ(−a + r) aκ e−κ(−a + r) ⎞ ⎟⎟ + ⎜1 + 3 ⎟⎜⎜ 2 r ⎝ 2r ⎠⎝ r ⎠

∞⎛

⎛ 3r 2 2r 3 ⎞ ⎜1 − 2 + 3 ⎟dr ⎝ a a ⎠

(4)

As shown in eq 4, the electrophoretic mobility is related to flow viscosity, droplet radius, the electrolyte dielectric constant, and the Debye length. At the same surrounding solution, ζ1 is thought to be equal to ζ2 at different radii. At a low organic solvent concentration, the organic solvent had little influence on the dielectric constant.33 According to Henry’s formula (eq 4), the ratio of electrophoretic mobility at the different sizes forms μ2 μ1

=

fH (κ2a 2)εrε0ζ2/η fH (κ1a1)εrε0ζ1/η

(5)

with

fH (κa) =

a 2κ 2 9

∫a

a3 ⎞⎛ ae−κ(−a + r) aκ e−κ(−a + r) ⎞ ⎟⎟ + ⎜1 + 3 ⎟⎜⎜ 2 r ⎝ 2r ⎠⎝ r ⎠

∞⎛

⎛ 3r 2 2r 3 ⎞ ⎜1 − 2 + 3 ⎟dr ⎝ a a ⎠

(6)

Subscript 1 indicates that κa is much more than 1 and that f H(κ1a1) is close to 1. Subscript 2 represents a droplet with a much smaller radius than the radii of the droplets of subscript 1. Equation 5 is simplified as eq 7: μ2 = fH (κ2a 2) μ1 (7) After eq 7 is introduced into eq 1, eq 1 may be simplified as eq 8. Therefore, the ζ potential calculation is simplified as μμ1η ζ= μ2 εrε0 (8)

(1)

where μ1 indicates that κa is much greater than 1. Subscript 2 represents the radius of a droplet much smaller than the droplet with the radius of subscript 1. In addition, it must be emphasized that μ1 and μ2 are the experimental values for the verification of the equation. In eq 8, they are constants. However, μ is the experimental data for the calculation of the ζ potential and belongs to the independent variable.

where μ is the electrophoretic mobility, εrε0 is the electrolyte dielectric constant, η is the electrolyte viscosity, a is regarded as the droplet radius, and κ−1 is the Debye length. To calculate the ζ potential, the values of Henry’s function need to be determined. There are two ways of calculating Henry’s function. First, the large droplets were prepared. The large droplets enlarged the flow resistance force, which was linearly related to the cross-sectional area of the droplets.31 The ratio of the other forces to the flow resistance force became low, and the other forces gained a slight influence on electrophoretic mobility. For small-size droplets, the Debye length depended on ionic strength.32 With an increase in ionic strength, the Debye length decreased significantly, and the κa value increased. The interface morphology is viewed as a hard sphere, which is assumed to be incompressible. The inertial terms are able to be neglected in the Navier−Stokes equations due to the small value of the Reynolds numbers of the fluid outside the droplets. The droplet

3. RESULTS AND DISCUSSION 3.1. Size and Polydispersity Index of the Emulsion Droplets. Emulsion droplets were prepared with different sizes and different polydispersity indices. As shown in Figure 1a, the droplet diameter became larger with an increase in the dodecane content. Using 1-propanol as a disperser produced the smallest droplet diameter. Meanwhile, the variation of droplet diameter reached 225 nm with a molar ratio of the 1927

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Figure 3. Effect of the electrophoretic mobility of dodecane droplets on size at pH 6.0 ± 0.1 in a methanol aqueous solution: (□) 0.1 mM KCl, experimental value = 0.715 (κ−1 ≈ 46.3 nm), theoretical value = 0.755 (κ−1 ≈ 30.4 nm); (○) 1 mM KCl, 0.727 (κ−1 ≈ 37.8 nm), 0.841 (κ−1 ≈ 9.61 nm); (△) 10 mM KCl, 0.936 (κ−1 ≈ 2.71 nm), 0.930 (κ−1 ≈ 3.04 nm); (◇) 100 mM KCl, 0.991 (κ−1 ≈ 0.891 nm), 0.976 (κ−1 ≈ 0.961 nm). Figure 1. (a) Droplet diameter and (b) distribution as a function of the molar ratio (dodecane/disperser), (c) stability of the droplets, and (d) variation of the droplet diameter using 0.1 mol % dodecane in the dispersers in 50 mL at pH 5.8: (□) methanol, (○) ethanol, (△) 1propanol, (▽) acetone, and (◇) acetonitrile.

presence of 10 mM of ionic strength. A similar result was also observed with the electrophoretic mobility of much more than 250 nm when the ionic strength reached 100 mM. However, the ratio of electrophoretic mobility was 0.715 and 0.727 in the presence of 0.1 and 1 mM KCl, respectively. As illustrated in eqs 5 and 6, the Debye length was 37.8 nm, but the theoretical value was 9.61 nm when the ratio was 0.727. Figure 3 illustrates the detailed Debye length at different salt concentrations. The presence of hydrophilic solvent changed the Debye length and increased the electric double layer thickness. The experimental Henry’s values were 0.936 and 0.991, respectively, when KCl contents were 10 and 100 mM. In a 100 mM KCl solution, the hydrophilic solvent had no obvious influence on the Debye length, which ranged from 0.891 to 0.961 nm. The data of the electrophoretic mobility of dodecane droplets on size at pH 6.0 ± 0.1 in a methanol aqueous solution can also be seen in Tables S1−S4 (Supporting Information). 3.3. Effect of pH on the ζ Potential Using Different Solvents. At an aqueous/alkane interface, the ζ potential was relevant to the hydroxide ions from the aqueous solution,7−9 but the effect of pH on the ζ potential with a hydrophilic solvent is still unknown. A standard electrophoretic mobility experiment using 200 mM hydrophilic solvent, including methanol, ethanol, 1-propanol, acetone, and acetonitrile, is shown in Figure 4. When the pH of the solution increased, the

dodecane/disperser of 7.3 × 10−2 at pH 5.8. In Figure 1b, the polydispersity index of the droplet diameter was less than 0.2 when the molar ratio between dodecane and the disperser was less than 0.004. As a result, the droplet diameter distribution was narrow. Figure 1c shows the relationship between the polydispersity index and time, where the droplets were more uniform. The polydispersity index of the droplets ranged from 0.04 to 0.14. Finally, Figure 1d shows the increase in droplet diameter, where the variation range of the droplet diameter was about 50 nm within 120 min. In Figure 2, there was no observed change in the droplet diameter in the KCl solution or in the salt-free solution when

Figure 2. Comparison of droplet diameter and polydispersity in (△) KCl solution and (□) salt-free solution. Gray represents salt-free, and light gray represents the KCl solution.

the titer was between 0 and 1 mM. However, there was a dramatic growth in the droplet diameter due to coalescence of the droplets when the concentration of the KCl solution was more than 10 mM. Moreover, the variation of the polydispersity index of the droplet diameter was between 0.1 and 0.14 with an increase in salt concentration. 3.2. Ratio of Electrophoretic Mobility at Different Droplet Diameters. According to the previous study, the Debye length decreased with an increase in the ionic strength.30 Due to the presence of a critical concentration of salt, the ratio of electrophoretic mobility at the droplet diameter had an obvious change within the range 200−250 nm, as shown in Figure 3. The ratio of the electrophoretic mobility of much less than 200 nm and much more than 250 nm was close to 1 in the

Figure 4. Effect of pH on ζ potential in a 1 mM KCl solution with 200 mM solvents: (□) methanol, (○) ethanol, (△) 1-propanol, (▽) acetone, and (◇) acetonitrile.

ζ potential decreased. The lowest value of the ζ potential in the methanol solution reached approximately −75 mV. The main reason was that the bonding between the hydroxide ion and the water molecule was a strong hydrogen bond9 and the interfacial water molecules were preferentially oriented with the oxygen atoms toward the hydrophobic phase at the boundary between 1928

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water and an apolar fluid.34 Therefore, hydroxide ions are able to alter the ζ potential even though there are hydrophilic solvents in the aqueous solutions. In addition, at the same ion strength, the ranking of the absolute values of the ζ potential from the largest to the smallest is methanol > ethanol >1propanol > acetone > acetonitrile, which indicates that the hydroxide ion is not the only origin of the ζ potential. In particular, the ζ potential was +10.5 mV at pH 4.0 in an acetonitrile solvent system. 3.4. Effect of Electrolytes on ζ Potential with Solvent. The ζ potential depended not only on the properties of the dispersed droplets but also on the surrounding droplets. As shown in Figure 5, the ζ potential moved toward positive with

Table 1. Solvent Polarity (D) and Partition Coefficient (log P) of the Solvents hydrophilic solvent

F(D)a

methanol ethanol 1-propanol acetone acetonitrile

0.955 0.940 0.927 − −

log Pb −0.72 −0.19 0.34 −0.16 −0.45

± ± ± ± ±

0.18 0.18 0.18 0.19 0.19

P

molar ratio

0.1905 0.6457 2.1878 0.6918 0.3548

3.390 1.000 0.295 0.933 1.820

a

Solvent polarity has been characterized by F(D), the Onsager polarity function.26 bPartition coefficient (log P) is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium36 and is calculated by Free ACD/PhysChem Suite (Advanced Chemistry Development, Inc. Canada).

Figure 5. Effect of ionic concentration and type on ζ potential: (○) 200 mM methanol at pH 6.0 and KCl solution; (□) 200.0 mM methanol at pH 9.0 and NaCl solution; (△) 200 mM methanol at pH 6.0 and NaCl solution; (▽) 200 mM methanol at pH 6.0 and MgCl2 solution.

Figure 6. Effect of hydrophilicity on ζ potential at pH 7.0: (□) methanol, (○) ethanol, and (△) 1-propanol.

the molecule had one or several lone electron pairs, which had a strong tendency to donate electrons to electron-poor reaction sites.27 Meanwhile, considering the case of hard electrophiles, such as H+ and K+, the interaction took place between the nucleophile and the electrophile.27 The hydroxyl, carboxyl, and nitrile groups were chosen as models to study the functional group effect. The partition coefficient was used to confirm the actual quality of the function groups on the surface of the emulsions droplets. As shown in Table 1, ethanol, acetone, and acetonitrile in aqueous phase were 1, 0.933, and 1.820, respectively. The quality of the functional groups of the solvents was the same at the surface of the emulsion droplets according to the partition coefficient. As shown in Figure 7, the ζ potential in the aqueous bulk phase increased as the organic solvent molecules increased at the droplet interface. The carboxyl group and the nitrile group were proton acceptors, but the oxygen atom of alcohols and anions had an ion-dipole interaction.33 Due to two lone electron pairs, the pair of anion and electron donor group

an increase in the ionic strength. Electrolyte properties can affect the diffuse double layer. Increasing the concentration or valence of the counterions compresses the double layer and increases the electrical potential gradient. The variation trend of the ζ potential is similar in a NaCl solution and a KCl solution. At the same ionic strength, univalent cations have a similar role in the ζ potential and increase positive charges on a steeper slope, but bivalent cations have a stronger impact on the ζ potential than univalent cations. The slopes of the ζ potential in methanol at pHs 6.0 and 9.0 are similar. The solvent effect on the ζ potential is independent in comparison to pH, and the ζ potential moves upward in the solution of bivalent cations. 3.5. Effect of Hydrophilicity of the Solvent on the ζ Potential. Solute identity significantly altered the local solvation environment of the solute.26 Hydrophilic urea interfered with the ordering of the water molecules near the interface.9 The ζ potential of different interfacial polarities was examined by taking advantage of the hydroxyl group. As shown in Table 1, the presence of the hydroxyl group enhanced the polarity of the droplet interface, and methanol was a stronger polarity solvent than the other two solvents. The complex of potassium ion with hydroxyl was formed.35 The ζ potential moved downward with an increase in solvent polarity. In Figure 6, the absolute values of the ζ potential gradually decreased with an increase in the cation contents. The reason is that more cations accumulated at the aqueous/alkane interface as a result of the electron-donating property of the hydroxyl substituents. In addition, the ethyl and propyl groups enhanced the hydrophobicity of the solute, and the ethanol and 1-propanol were preferentially solvated in the dodecane phase. As a result, the strong polarity solvent had slightly more influence on the ζ potential than the weak polarity solvent. 3.6. Effect of Solvent Functional Group on the ζ Potential. For some neutral molecules, the functional group of

Figure 7. Effect of the functional group on the ζ potential at 1 mM KCl solution at pH 5.9; hydrophilic solvent in aqueous phase; C1: 2.000 mM ethanol, 1.866 mM acetone, 3.640 mM acetonitrile; C2: 20.00 mM ethanol, 18.66 mM acetone, 36.40 mM acetonitrile; C3: 200.0 mM ethanol, 186.6 mM acetone, 364.0 mM acetonitrile; C4: 2000 mM ethanol, 1866 mM acetone, 3640 mM acetonitrile. Ethanol (dark gray), acetone (gray), acetonitrile (light gray). 1929

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Notes

forms an ion−dipole force. Under the same experimental conditions, there are more electron pairs in the nitrile group than in the hydroxyl and carboxyl groups. As more cations absorb more lone electron pairs on the interface, the ζ potential increases. Although ion−dipole interaction neutralized a part of the negative charges, the ζ potential remained negative.32 Hydroxide ions interacted with the hydrogen of alkane, which belonged to the weak intermolecular force. The dissociation energy of the hydrogen bond (50−67 kJ) was much higher than that of the ion−dipole interaction (2−8 kJ).32 The solvents created positive ζ potential values. According to the dissociation energy, the functional group has a smaller influence on the ζ potential in comparison to hydrogen bonding. The experiments about the effects pH and functional group on the ζ potential also prove these explanations. In addition, molecular dynamics techniques clearly indicated that the concentration of hydroxide ions was much higher in the vicinity of the surfaces than in the bulk regions.7 The potential energy difference between the adsorbed and the bulk states in the hydroxide ions was about −12 kJ/mol, which was much greater than in the other electrolytes.7 The negative charges are partly neutralized by ion−dipole cations in the vicinity of the phase interface, and the ζ potential moves toward the positive value. Therefore, the ζ potential of the emulsion droplets can be easily tuned by adding the hydrophilic solvent.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Science Foundation of China (no. 21136009, no. 21106152, and no. 21206175). We acknowledge Dr. Xiong Xiaochao from Washington State University and Faheem Nawaz for the revision of the paper.

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4. CONCLUSIONS This work demonstrates the feasibility of obtaining stable dodecane/water emulsions and studies the solvent effect on the ζ potential. The dispersive droplet diameter is tuned by the ratio between dodecane and the disperser. The ratio of electrophoretic mobility at different droplet diameters affects an obvious change in the experimental method. In the presence of a hydrophilic solvent at pH 6.0 ± 0.1 and 1 mM KCl, the Debye length increases from 9.61 to 37.8 nm in the methanol aqueous solution. In addition, the nitrile group of acetonitrile provides a stronger capacity of nucleophilicity than the carboxyl and hydroxyl groups, and the ζ potential in acetonitrile aqueous solution is 10.5 mV at pH 4.0 and 1 mM KCl. The presence of the hydroxyl, carboxyl, and nitrile groups made the ζ potential move toward positive values. Meanwhile, the presence of electrolytes also plays an important role in controlling interface charges. Therefore, the formation of the ζ potential not only relates to the interface properties of the emulsion droplet but also depends on the environmental conditions of the emulsion droplets.

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ASSOCIATED CONTENT

S Supporting Information *

Additional details on the electrophoretic mobility of the droplets of different sizes and tables showing the diameter and mobility of the droplet at 0.1, 1, 10, and 100 mM KCl and pH 6.0 in the methanol aqueous solution. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: +86-10-82544955; fax: +86-10-62554264; e-mail: lxf@ ipe.ac.cn. *E-mail: [email protected]. 1930

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dx.doi.org/10.1021/la403900e | Langmuir 2014, 30, 1926−1931