Abstract: The stable dodecane/water interface was formed via controllable droplet size and surfactant-free emulsions. Hydroxide ions charge was able to stabilize these oil/water emulsion droplets, and hydrophilicity and functional group of solvent can adjust interface charge.
Among the various interface properties of particles or droplets, interface charge have attracted great interest in many fields such as biotechnology,1-3 colloid science,4-7 and nanotechnology.8-12 The previous studies have provided detailed pictures of how environment alter interface charge, such as pH, ionic strength and surfactant.13-16 However, surfactant stabilizes surface of the metastable droplets in many researches,17-21 and then the amphiphilic surfactant changes the interface charge owing to the functional group of the surfactant.22-25
Liquid/liquid interface properties can be adjusted by solvent. A series of approaches to studying the interface charge of colloids are adopted in the absence of surfactant. Beattie and Djerdjev used the electroacoustic technique to measure the charge of hexadecane emulsion droplets by mechanical emulsification, and demonstrated quantitatively hydroxide ions charge.6 The calculation results showed that the amount of hydroxide ion added was a linear function of the surface area created. The similar conclusion about the pH dependence of the interface charge was also obtain by Marinova and co-workers7. Although these experiments explain the origin of interface charge and overcome the disturbance of the surfactants, in reality some solvents exist inevitably and affect the electrophile/nucleophile interactions to a different extent.28-32 e.g. an electron-rich solvent at the interface gets attracted to the centre of the positive charge and forms a bonding with an electron-deficient species by donating electrons.33 Very few studies have been undertaken to determine the key effect of interface charge in complex media, especially the solvents.
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In this report, we describe electrophoresis experiments of dodecane/water droplets with hydrophilic solvents via surfactant-free emulsions and quantify the role of the solvent effect. The solvent have markedly distinct phenomena at the liquid/liquid interface. Interface charge (zeta potential, Î¶) depends on more than simply the properties of two bulk phases. The effects of droplet size, pH, ionic strength, and ionic type are systematically studied to enhance the control of oil-in-water transport. Our results quantify how hydrophilic solvent affects the electrophoretic mobility.
2. Experimental Section.
2.1 Preparation of surfactant-free emulsions.
A controllable droplet size and surfactant-free emulsions are prepared via solvophobic effect.34 The hydrophilic solvent is not only used as a disturbance agent but as a disperser. A detailed preparation procedure is as follows. A 50 mL of ultra pure water was placed in a 100 mL beaker. A certain volume of hydrophilic disperser (e.g. methanol, ethanol, 1-propanol, acetone and acetonitrile) containing different content dodecane was rapidly injected into the ultra pure water by a syringe (Inner diameter, 0.45 mm). The electrolyte was also added into the water solution. The stirring rate had little influence on the droplet size. 10 mM KOH and 10 mM HCl were used for pH adjustment.
2.2 Determination of emulsions droplet size and electrophoretic mobility.
A cloudy solution (water/hydrophilic solvent/dodecane) was formed and then the emulsion was injected into flow cell of Delsaâ„¢ Nano C analyzer for the droplet size and electrophoretic mobility. The controllable droplet size was implemented by changing the molar ratio of dodecane to disperser and the droplet size distribution was small (Fig. S1). The stability of droplet size is fit for the electrophoretic mobility measurement (Fig. S2). Due to different diffusion rate in the water,35 the solvents have a great influence on droplet size. Meanwhile, adding the electrolyte resulted in the variation of droplet size because of salting out (Fig. 3S).
3. Results and Discussion.
3.1. Determination of Henry' function value of Î¶ potential.
To facilitate a comparison of interface charge, Î¶ potential is obtained by Henry's formula.
For Îºa < 1:
Whereas for Îºa > 1:
Here, Î¼ is the electrophoretic mobility, ÎµrÎµ0 is the electrolyte dielectric constant, Î· is the electrolyte viscosity, a is the droplet size, Îºâˆ’1 is the Debye length, and fH is a dimensionless function of Îºa involving exponential integrals. According to Eq. (2) and Eq. (3), the Henry' function values are 1 and 0.667 for very large and very small values of Îºa, respectively. Based on the experimental results, the ratio of the average electrophoretic mobility of small droplet size ( < 200 nm) to the average electrophoretic mobility of large droplet size ( > 300 nm) is approximately 0.644 in 0.01 mM KCl solution (Fig. 1). The experimental Henry's function value (fHE â‰ˆ 0.644) is approximately consistent with the theoretic value (fHT â‰ˆ 0.667).38 In addition, fHE â‰ˆ 1 closely accords with fHE â‰ˆ 1 at Îºa > 100 at 10 mM and 100 mM KCl solution. These results proved that our experiments are credible.
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Fig. 1. The effect of the electrophoretic mobility of dodecane emulsions on droplet size at pH 6: £ 0.01mM KCl, fHE=0.644; Â 0.1 mM KCl; fHE=0.727; r 1 mM, fHE=0.725; s 10 mM KCl, fHEâ‰ˆ1; Â¯ 100 mM KCl, fHEâ‰ˆ1.
An interesting phenomenon is that Henry' function value had an obvious change at 2.055 < Îºa < 2.483 or at the droplet size ranging from 200 nm to 300 nm in 0.01 mM KCl solution (Fig. 1). In addition, the similar results also appear in 0.1 mM and 1 mM KCl solution. However, the electrophoretic mobility value becomes a constant only if Îºa is more than 100. Several studies involving the modified nanoparticles reported that the long chain group influenced the electrophoretic mobility of nanoparticles due to hydrophilicity and hydrodynamic permeability of the functional group. Dodecane has high hydrophobic property and alkane chain does not easily extend into the water phase. The interface morphology is approximately viewed as the hard sphere. Note that the water solubility of the dispersed oil phase affected the shear plane37 and the interaction between the droplet surface and the water skeleton was not inevitable. At the same time, the interface properties of the emulsion droplets became complex owing to the existence of organic solvent.30 Therefore, it is difficult that all effects on electrophoretic mobility are considered.
To avoid these problems, the effects on electrophoretic mobility can be also neglected in the case of Îºa Â» 1. We are able to make a change of Îºa by the variation of ionic strength and droplet size, and then electrophoretic mobility is only influenced by the interface charge. The value of Henry function is determined by the ratio of the electrophoretic mobilities of different droplet size and the droplet size avoids existing at the corresponding range. There are two ways for reducing these impacts as follows: Firstly, we prepare the larger droplet. The large droplet enlarge the flow resistance force, which is linearly related to the cross sectional-area of the droplets.40 The ratio of the other forces to flow resistance force will become low, and then the other forces have a slight effect on electrophoretic mobility. Secondly, the Debye length is a measure of the range of the effect of the charge and the Debye length depends on the ionic strength.41 With an increase of the ionic strength, the Debye length decrease significantly, and then Îºa value is also increased. Obviously, the droplet size is an ideal way because the interface charge is also related to the ion properties. Based on the analysis, the experimental Henry's function value is obtained by the electrophoretic mobility of the different droplet size. The obtained Henry's function value is used for the calculation of interface charge. Thus, the effect of hydrophilic solvents on interface charge can be observed and discussed correctly.
3.2. The role of pH on Î¶ potential with different solvents.
Fig. 2. The effect of pH on Î¶ potential in 1 mM KCl solution for 200 mM solvents.
Many researches has shown that the Î¶ potential at an aqueous/alkane interface was relevant to the hydroxide ions from aqueous solution,5-7 but the effect pH with hydrophilic solvent on Î¶ potential has rarely been reported. To access the effect of pH with hydrophilic solvent, a standard electrophoretic mobility experiment using a 200 mM hydrophilic solvent run at 120 V is show in Fig. 2. As reported previously, the interface charges increased with an increase of pH at constant ionic strength (1 mM KCl). The pH dependence shows that hydroxide ions are responsible for negative charge of dodecane droplets. Oxygen of hydroxide ion and hydrogen of alkane forms hydrogen bonding. These pH experiments demonstrate that hydroxide ions are able to alter interface charges. However, note that Î¶ potential of dodecane emulsion droplets can be "tuned" by adding the hydrophilic solvent. The variation of droplet size has been excluded as a factor that modulates dodecane emulsion droplet mobilities. As shown in Fig. 2, very little solvent (1%) has a large influence on interface charges. The order is methanol < ethanol < 1-propanol < acetone < acetonitrile at the same ion strength and pH. Particularly, the interface charges with acetonitrile changed from negative potential to positive potential at pH 4.0.
3.3. The role of hydrophilicity of the solvent on Î¶ potential.
Table 1. Solvent polarity (D) and partition coefficient (Log P) of the solvents
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a: Solvent polarity has been characterized by the Onsager function,30 F(D):
where D is a solvent dielectric constant.
b: Partition coefficient (Log P) is the ratio of concentrations of a compound in the two phases of a mixture of two immiscible solvents at equilibrium42 and is caculated by Free ACD/PhysChem Suite (Advanced Chemistry Development, Inc. Canada).
Solute identity alter significantly local solvation environment of the solute at the interface30 and solvation accompany the change in solute position43. Meanwhile, the interface charge is relevant to the two bulk phases and the phase interface. Marinova7 proved highly hydrophilic urea possibly does not interfere with the ordering of the water molecules near the interface. To further understand the effect of the phase interface, we chose to examine the interface charges of the different interfacial polarity by taking advantage of hydroxyl group's enhanced aqueous solubility, which altered the distribution of the electrolytes across the aqueous/dodecane interface. By adding the hydrophilic solvents, the trend of Î¶ potential values was similar at the range of ionic strength (0.01mM ~ 100 mM), but there is a small change in the interface charges at the same ionic strength (Fig. 3). The concentration of counterions is drastically increased close to the surface. Î¶ potential decrease becomes steeper with increasing salt concentration. Due to the electron-donating properties of the hydroxyl substituents, there are more cations adsorbed at an aqueous/alkane interface. The change in solvent polarity with an increase of hydroxyl group on the interface charges appeal to intuition. Ethyl group and propyl group enhance the hydrophobic character of the solute, which cause ethanol and 1-propanol to be preferential solvated in dodecane phase in comparison to methanol. Methanol has a more slightly influence on interface charge than other hydrophilic solvents (ethanol and 1-propanol) when the molar concentration of the solvents of the aqueous phase is the same.
< 10 mM and > 0.1 mM
Fig.3. The effect of hydrophilicity on Î¶ potential at pH 7.0
3.3. The role of functional group of the solvent on Î¶ potential
For some neutral molecules, the functional group of the molecule has one or several pair lone electrons, which have a tendency to donate electrons or react at electron-poor sites. Meanwhile, consider for example the case of hard electrophiles such as H+ and K+, the interaction between nucleophile and electrophile has to take place.31 The formation of Î¶ potential mainly belongs to weak intermolecular bond. As a result, the functional group of molecules plays a major role in Î¶ potential. To multi-phase system, some function group was To determine the impact, we choose hydroxyl, carboxyl and nitrile as model group. Meanwhile, we applied to partition coefficient for identical number of the function groups on the surface of emulsions droplets. It is also said that ethanol in dodecane is 1 at the same molar concentration of bulk aqueous phase, acetone or acetonitrile are 1.072 and 0.549, respectively (Table 1). According to the ratio, we ensure the function group of the solvents is similar on the surface of emulsions droplets due to solubility of the solvents.
Table 2. The effect of hydrophilic functional group on Î¶ potential at 1 mM KCl solution at pH 5.9.
Ethanol in water (mM)
Î¶ potential (mV)
Acetone in water (mM)
Acetonitrile in water (mM)
Î¶ potential (mV)
The dodecane/water emulsions were prepared by the solvents (ethanol, acetone, . The emulsions slowly titrated to the corresponding pH by 10 mM HCl or 10 mM KOH. The Î¶ potential of dodecane emulsions consists of two parts. Firstly, dipole or hydrogen bond of the hydroxide ions is the main factor of Î¶ potential. Theoretically, the interfacial water molecules are preferentially oriented with the oxygen atoms toward the hydrophobic phase at the boundary between water and apolar fluid.44 The adsorption of the hydroxide ions are explained with strong dipole or hydrogen bonding of the hydroxide ions with the hydrogen atoms of the interfacial water molecules, and hydrogen bonding between the hydroxide ion and water molecule belongs to strong hydrogen bonds.7 Î¶ potential shows negative values. With an increase of pH, the amount of hydroxide ions in water is much more than the hydrogen ions. The maximal Î¶ potential reached approximately âˆ’75 mV. It should also be noted that the variation of Î¶ potential of hydrophilic solvents is similar. The phenomenon indicated that hydroxide ion was not the only source of Î¶ potential.
Secondly, hydrophilicity and functional group of the solvents have an obvious influence on Î¶ potential. Methanol, ethanol and 1-propanol have hydroxyl group. The complex of potassium ion with alcohol is formed in the emulsions. In addition, methanol content in dodecane are approximately 1% of water content in dodecane by water titration and GC analysis for methanol, and the order of hydrophilicity is methanol > ethanol > 1-propanol.35 The amount of hydroxyl group on dodecane emulsion surface increased at low hydrophilicity. As shown in Fig. 2, at all pH, Î¶ potential moves towards the positive values with a decrease in the polarity or an increase in hydrophobicity of alcohol. Moreover, the effect of functional group of hydrophilc solvents was also investigated (Fig. 2). According to the polarity of hydrophilic solvents, acetone and acetonitrile have a similar solubility in dodecane, and 1-propanol have a high solubility in dodecane in comparison to the other solvents.35
3.4 Interaction between the solvents and the electrolytes
In order to understand mechanism of interface charges, the partial charge of the solvents was computed by software. of Hydroxyl group can weakly interact with some anions and change the interface charges of the emulsions. Interface charges with acetone and acetonitrile were slightly decreased in comparison to ethanol. Carboxyl group and nitrile group are one of the most important proton acceptor groups. Oxygen atom in alcohols and anions have a ion-dipole interaction.34 Owing to two lone electron pairs, an ion-dipole force is formed by the pairs of anions and electron donor group (carboxyl group).
We attribute the enhanced band resolution to changes in the diffuse double-layer thickness. Decreasing the ionic strength increases the Debye length, leading to lower dodecane droplets migrating toward the positive electrode.
The observation support
The water molecules. the dodecane/water emulsions are stable electrostatically by a high interface charge due to a relatively thick double layer on the order of 10 nm or more. As the ionic strength (0.01 ~ 100 mM) increased, the electrical double layer shrinks to a few nanometers. The interface charges became small, and then the emulsions become unstable.
Fig. 4. KCl solution: £ Methanol at pH 5.9 and; Â¢ Methanol at pH 9.0; Â Acetone at pH 5.9; Â˜ Acetone at pH 9.0; r Acetonitrile at pH 5.9;p Acetonitirile at pH 9.0; MgCl2 solution: s Methanol at pH 5.9; NaCl solution: Â¯ Methanol at pH 5.9.
Although ion dipole interaction neutralizes a part of negative charges, the interface charges show negative. Hydroxide ions interact with hydrogen of alkane, which belongs to intermolecular force. Hydroxide ions are the charge-generating species at the alkane/water interface due to hydrogen bond. The dissociation energy of hydrogen bond (50 ~ 67 kJ) is much higher than that of ion-dipole interaction (2 ~ 8 kJ).41 In addition, molecular dynamics techniques shows that the concentration of the hydroxide ions was much higher in the vicinity of the surfaces than in the bulk regions and the potential energy difference of hydroxide ions between the adsorbed state and the bulk state is about âˆ’12 kJ/mol, which is much more than other electrolytes.5 However, the cations interact with hydroxyl group of ethanol when the hydrophilic solvent exists at the interface. The part of the charges is compensated by K+ ions in the vicinity of the phase interface. As a result, the Î¶ potentials were observed with the electrolyte dependence as the hydroxyl group of the dodecane/water interface varied, but hydroxide ion and hydrogen bond play a major role in interface charges.
These ion show a much weaker tendency when they adsorb at the surface.
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105 mV 0.2 mM NaCl
Table 2 The comparison of organic solvent-free and organic solvent on interface charge at pH 9.0
potential decreases exponentially with increasing distance.
Ion-dipole interaction becomes stronger as the charge on the ion increases or the magnitude of the dipole of the polar molecule increases.
In summary, this work has demonstrated the feasibility to obtain Î¶ potential by a facile method to prepare stable dodecane/water emulsions. The emulsion droplet size can be tuned by the ratio between dodecane and hydrophilic solvent. In addition, hydrophilicity and functional group of solvent was able to adjust interface charge. Furthermore, the hydrophilic solvent in the absence of surfactant can stablize the dodecane/water interface via hydroxide ions. The detailed mechanistic studies are currently undergoing in our laboratory, and will be reported in due course. Thus, the solvent at two adjacent phases will play a leading role in controlling interface charge.