Microemulsions with the Ionic Liquid Ethylammonium Nitrate: Phase

Jul 11, 2014 - ... oil) relates to the microstructure, spinning drop tensiometry is used to measure the interfacial tension σab and to estimate the d...
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Microemulsions with the Ionic Liquid Ethylammonium Nitrate: Phase Behavior, Composition, and Microstructure Jan C. Thater,† Violaine Gérard,†,‡ and Cosima Stubenrauch*,† †

Universität Stuttgart, Institut für Physikalische Chemie, Pfaffenwaldring 55, 70569 Stuttgart, Germany S Supporting Information *

ABSTRACT: In this study, we investigate properties of microemulsions which consist of the ionic liquid (IL) ethylammonium nitrate (EAN), the nonionic surfactant C12E3 and an n-alkane, namely n-dodecane or n-octane. The compositions of the coexisting phases are calculated from the densities and volumes of the respective phases. Since the interfacial tension between the water-rich and the oil-rich phase in traditional microemulsions (containing water and oil) relates to the microstructure, spinning drop tensiometry is used to measure the interfacial tension σab and to estimate the domain sizes. Finally, measuring the self-diffusion coefficients of all components via the Fourier Transform Pulsed Gradient Spin Echo (FTPGSE) NMR technique allowed distinguishing between continuous and discrete structures. Our results indicate that the general principles underlying water−n-alkane−CiEj microemulsions can indeed be transferred to oil-in-EAN droplet and the respective bicontinuous microemulsions, while differences are observed for EAN-in-oil droplet microemulsions. (NTf2−).16 The studies mentioned above are promising ways toward microemulsions with polar or apolar ILs. It is well-known that microemulsions consisting of water, an n-alkane and a nonionic alkyl-polyethylenglycolether surfactant (CiEj) change their microstructure as a function of temperature due to a change of the interfacial film’s curvature.17 For this reason, it is possible to predict the microstructucture of a water−n-alkane−CiEj microemulsion when the phase diagram is known. For IL−n-alkane−CiEj microemulsions, however, the relation between phase behavior and microstructure has been investigated by a very low number of studies,8−13 and numerous questions still remain to be answered. To address some of these questions, we studied microemulsions consisting of EAN, C12E3 and an n-alkane, namely n-dodecane or n-octane. Our study complements the previous work of Atkin and Warr to get a more detailed picture of these systems. For that purpose, we calculated the composition of the coexisting phases from the phase volumes and phase densities at a constant surfactant mass fraction. Additionally, we measured the interfacial tension σab between the upper and the lower phases via spinning drop tensiometry. Furthermore, we used the FTPGSE (Fourier Transform Pulsed Gradient Spin Echo) NMR technique to determine whether continuous or discrete domains are present.

1. INTRODUCTION Microemulsions are optical transparent, nanostructured, and thermodynamically stable mixtures of water, oil, and a surfactant.1,2 For about 10 years, the formulation of microemulsions has been pursued with an ionic liquid (IL) being one of the two solvents. The aim is to combine the nanostructure of a microemulsion with the unique properties of ionic liquids.3−5 As these properties may be tuned by the choice of the counterion, ionic liquids are often referred to as “designer solvents”.6 One of the properties that can be tuned in this way is the solvophobicity,7 i.e., the immiscibility with water and oil, respectively. In other words, ILs can replace either oil or water of a traditional microemulsion. Replacing water by a polar IL leads to microemulsions which lend themselves well to high temperature applications.8−10 Polar ILs that were used for the formulation of such microemulsions are mainly ethylammonium nitrate (EAN)8,11 and imidazolium salts with short alkyl chains.12 For a ternary microemulsion consisting of EAN, an nalkane, and CiEj, Atkin and Warr observed a characteristic structure peak in the SAXS curve.11 Kunz and coworkers presented evidence for domains swollen with either EAN or [bmim][BF4] in a microemulsion stabilized by a surfactant which itself is an ionic liquid.13 However, apolar ILs may be used in microemulsions as substitutes for traditional oils, which are often hazardous and volatile. For example, microemulsions were formulated with the imidazolium based IL [bmim][PF6], water, and the technical surfactant TX-100.14,15 Since the hexafluorophosphate anion (PF6−) is unstable toward hydrolysis it was exchanged by the more stable bis-triflimide anion © 2014 American Chemical Society

Received: March 19, 2014 Revised: June 23, 2014 Published: July 11, 2014 8283

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gradient probe DIFF/30. The STE (Stimulated Echo) pulse sequence was chosen for the Pulse Field Gradient measurements.20 A thin capillary (Ø 1 mm) was filled with the sample (filling height: 20−25 mm). Parafilm was wrapped around the top of the thin capillary before inserting it into an NMR tube (Ø 5 mm). This procedure helped to avoid thermal convection in the sample. The NMR tube was placed into the device and thermostated about 20 min before each measurement. The 1H NMR spectrum of the middle phase microemulsion in the n-dodecane system (Figure 1) is shown as an example for the obtained spectra.

2. EXPERIMENTAL SECTION 2.1. Materials. All experiments were carried out with ethylammonium nitrate (EAN) from Iolitec (Heilbronn, Germany). According to the information provided by Iolitec the purity of EAN is >97%. In addition to the data provided by Iolitec, we measured an NMR spectrum of our EAN batch and could not detect any impurities (see Supporting Information, SI). Since the water content was not provided in the data sheet of EAN we determined it by Karl Fischer titration and found a water content of 1.9 wt %. We tested the influence of the water content on the phase behavior and found that it has an effect on the temperature range of the three-phase region but that it hardly affects the efficiency (see SI Figure S3 and Table S1). Since the effect on the temperature for a water content between 0.5 and 1.9 wt % is not very pronounced we worked with the EAN “as received”. For the triethylene glycolmonododecylether (C12E3) from TCI (Zwijndrecht, Belgium) a purity of >95% is written in the data sheet. We asked for additional spectroscopic data for the batch C12E3 used in this work and were told by TCI that the used C12E3 batch has a purity of 99.2%. The oils n-dodecane (purity ≥99%) and n-octane (purity ≥99%) were purchased from Sigma-Aldrich (Steinheim, Germany). 2.2. Sample Preparation. For the determination of phase diagrams, samples were prepared by weighting in a mass mC of surfactant (C), a mass mB of oil (B) and a mass mA of EAN (A) in a glass tube (capacity 5 mL, NS 10/19) containing a magnetic stirring bar. The components were weighted directly into the tube using a balance with a precision of 0.0001 g such that the total mass was about 0.8 g. For the determination of volumes, samples of a total mass between 4 and 5 g were prepared in graduated test tubes. The relative volumes of the phases were determined optically with an accuracy of 3%. In this case, no magnetic stirring bar was added. For further experiments with the techniques listed below, samples were prepared in the same way as for the determination of volumes but in modified flasks instead of in tubes. The flask containing the sample was mixed applying a vibrational stirrer and then put in a thermostated basin at a certain temperature until the phases in the sample were completely separated and clear. Equilibration sometimes took several days or weeks. Then each phase was removed, using a syringe and put in to a separate thermostated tube. Samples for the determination of densities, interfacial tensions and self-diffusion coefficients were prepared with an oil mass fraction of α = 0.50 at a fixed surfactant mass fraction γ. The mass fractions are defined as follows: mB α= mA + mB (1)

Figure 1. 1H NMR spectrum of the system EAN−n-dodecane−C12E3 for α = 0.50 and γ = 0.10 in the middle phase (microemulsion) of the three-phase region. Peaks a−e are assigned to the groups as listed in Table 1. For each temperature, at least two different diffusion times Δ (between 30 and 200 ms) were used between the gradient pulses of magnitude g, which was chosen between 50 G cm−1 and 1150 G cm−1 such that the intensity of the signal decayed to sufficiently low values. The decrease of the peak intensity A is correlated to the self-diffusion coefficient D of the component according to the Stejskal-Tanner equation:21

⎛ δ⎞ ln(A) = ln(A 0) − γg2δ 2g 2D⎜Δ − ⎟ ⎝ 3⎠

and

γ=

mC mA + mB + mC

(3)

where A0 is the echo amplitude without applied gradients, γg the gyromagnetic ratio, δ the length of the gradient pulse, g the magnitude of the gradient pulse, and Δ the time interval between the gradient pulses, i.e., the “diffusion time”. A specific peak was used to determine the self-diffusion coefficient of each component with Matlab. The peaks used for the experimental determination of the respective Dvalues and the general peak assignment of the 1H-spectra are listed in Table 1. The peak at 1.1 ppm (peak e) was used to determine the selfdiffusion coefficients of the n-alkanes. In this case, we observed a biexponential decay curve because the signal decays due to the selfdiffusion of both n-alkane and C12E3. Since the self-diffusion coefficient of the surfactant is accessible from the peak at 3.8 ppm (peak b) one of the two self-diffusion coefficients is known and used as constant value in the biexponential fit.

(2)

The samples were thermostated at temperatures between 15 and 55 °C. The total mass was chosen from 3−6 g according to the requirements of the experiment. (The phases were used for other experiments if they were not fully used for the experiment they were prepared for.) 2.3. Spinning-Drop Tensiometry and Densities. The interfacial tensions were determined with a spinning drop tensiometer Spinning Drop Video Tensiometer SVT 15 from Dataphysics. The capillary was fully filled with a syringe with approximately 1.6 mL of the lower phase avoiding the encapsulation of air bubbles. The capillary was closed and placed in the thermostated cell of the spinning drop tensiometer. Then a droplet (between 20−100 μL) of the upper phase was injected in the capillary using a syringe. The software calculated the interfacial tension according to the Vonnegut equation18 with the data of the system, namely the radius and length of the droplet, the density of phases, the velocity of the capillary and the refractive index nD of the lower phase (≈ nD (EAN) = 1.4524 at 25 °C).19 Densities were measured with an Anton Paar DMA 5000 M density meter. 2.4. FTPGSE-NMR-Spectroscopy. Self-diffusion coefficients were measured with a Bruker AVANCE III 400 NMR spectrometer with a

3. RESULTS AND DISCUSSION 3.1. Phase Behavior. The phase behavior of microemulsions of the composition EAN−n-alkane−CiEj has been reported in literature.11 Due to the appearance of the three phase body in the T-γ-phase diagram of the system EAN−ndodecane−C12E3 at temperatures between 29 and 45 °C, which allows for a detailed investigation of the system’s structure at convenient temperatures, we chose this system for our 8284

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Table 2. Coordinates of the Point X̃ , Minima of the Interfacial Tension σab,min and Maxima of the Domain Size ξ Calculated with (4) for Different Microemulsionsa

Table 1. Assignment of the Peaks for the 1H-NMR Spectrum of the System EAN−n-Alkane−C12E3a peak

peak position

a

7.7 (± 0.1) ppm

type

b

3.8 (± 0.1) ppm

broad singlet multiplet

c d

3.3 (± 0.1) ppm 1.5 (± 0.1) ppm

quartet multiplet

e

1.1 (± 0.1) ppm

triplet

assignment

γ̃

NH3+ ↔ NH2 of EAN H2OC8C8E325,26 H2OC8C8E425,26 H2OC8C12E425,26 H2OC12C8E425

OH and CH3OCH3− of C12E3 CH2 of EAN CH2 of n-dodecane / n-octane and C12E3, CH3 of EAN CH3 of n-dodecane / n-octane and C12E3

H2OC12C12E3 H2OC8C12E3 EANC12C12E3 EANC8C12E3

a

Peaks c and e were used for the determination of the self-diffusion coefficients of EAN and the alkane, respectively. The self-diffusion coefficient of C12E3 was determined from peak b.

0.19 0.25 0.03 0.35 0.05 0.01 0.20 0.17

T̃ (°C) 16 42 13 57 5 −5 40 23

σab,min (m Nm−1)

ξmax (nm)

−2

1.5 × 10 2.8 × 10−2 9.0 × 10−4

12.4 20.9

1.5 × 10−2 1.3 × 10−2

17.0 17.9

a

The values for the systems printed in bold are estimates based on the known trends.25

complementary study. Since the use of n-alkanes with shorter chain lengths is known to improve the efficiency of aqueous microemulsions with nonionic surfactants22 we also investigated the system EAN−n-octane−C12E3 for the sake of comparison with the aqueous counterparts. We determined the phase diagrams of both systems (EAN−n-dodecane−C12E3 and EAN−n-octane−C12E3) at a constant n-alkane / (EAN + nalkane) mass fraction of α = 0.50 (corresponding to a volume fraction of φ = 0.62 for n-dodecane and φ = 0.63 for n-octane) as a function of the surfactant mass fraction γ and the temperature T (Figure 2). Note that the phase diagram of the

found at surfactant mass fractions of 0.05 ≲ γ ≲ 0.20, while it is 0.05 ≲ γ ≲ 0.17 for the system with n-octane due to the enhanced efficiency. At the point X̃ (γ̃, T̃ ) the one-phase region (1) meets the three-phase region (3). We observed temperatures of T̃ ≈ 40 °C for n-dodecane and T̃ ≈ 23 °C for n-octane as the respective phase inversion temperatures. In the twophase region 2 an oil-in-EAN microemulsion coexists with an oil excess phase; in the three-phase region a surfactant-rich microemulsion coexists with an oil and an EAN excess phase; in the two-phase region 2̅ an EAN-in-oil microemulsion coexists with an excess EAN phase and in the one-phase region the whole sample is a microemulsion. A quantitative comparison between the efficiency of EAN microemulsions and the corresponding water-containing microemulsions cannot be made since the phase diagrams of H2O−ndodecane−C12E3 and of H2O−n-dodecane−C12E3 are not reported in literature. However, a qualitative comparison is possible. Kahlweit et al. compared the efficiencies γ̃ of the systems water−n-alkane−C8E4 for different alkanes.22 Replacing n-octane by n-dodecane one observes an increase of γ̃ and T̃ (see Table 2). Moreover, increasing the alkyl chain Ci and decreasing the headgroup size Ej in the systems water−noctane−CiEj, one observes a decrease of γ̃ and T̃ . For example, the replacement of C8E4 by C12E4 and by C8E3 respectively, in the system water−n-octane−CiEj decreases γ̃ and T̃ .25 Thus, the system water−n-octane−C12E3 would have a higher efficiency γ and a lower temperature T̃ compared to water−n-octane− C12E4. The same holds true for the respective n-dodecane system. For the sake of comparison rough estimates for γ̃ and T̃ can be found in Table 2. While the decrease of T̃ in the system with EAN from T̃ ≈ 40 °C for n-dodecane to T̃ ≈ 23 °C for noctane is in the expected range compared to the aqueous systems, only a small influence on the efficiency, i.e., on the γ̃value, was observed if one replaces n-dodecane by n-octane in the microemulsions studied here. 3.2. Densities and Composition. Knowing the densities and composition of the coexisting phases one can much better understand the phase behavior and the microstructure.27,28 Thus, the densities of the coexisting phases of the two ternary systems EAN−n-dodecane−C12E3 and EAN−n-octane−C12E3 at an EAN-to-oil ratio of α = 0.50 and a fixed mass fraction of surfactant γ were determined as a function of the temperature T (Figure 3). At low temperatures (two-phase region 2) the density of the upper phase is slightly higher than the density of the pure oil. This is due to monomeric surfactant molecules in the oil excess phase. However, the density of the lower microemulsion phase

Figure 2. T−γ diagrams of the system EAN−n-alkane−C12E3 for an oil / (EAN + oil) mass fraction of α = 0.50. The numbers correspond to the different phase regions: 2 = oil-in-EAN microemulsion coexisting with oil excess phase, 3 = surfactant-rich microemulsion coexisting with oil and EAN excess phases, 2̅ = EAN-in-oil microemulsion coexisting with EAN excess phase, and 1 = one-phase region.

system EAN−n-dodecane−C12E3 determined in this study differs from that reported in literature.11 The phase diagram of the study at hand has a much smaller three-phase region which is accompanied by a much higher efficiency (γ̃ ≈ 0.20 instead of γ̃ ≈ 0.38). Checking and testing all reasonable causes for this discrepancy we have to conclude that the data reported by Atkin and Warr11 cannot be correct (see SI for details). The shape of the phase diagram is similar to that reported for “traditional” microemulsions where the polar phase is not an IL but water (Table 2).2,23,24 The three-phase body appears at a surfactant mass fraction of γ0 ≈ 0.05 for both ternary systems. For the system with n-dodecane, the three-phase region (3) is 8285

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Figure 3. Densities of the pure components and of each phase for the systems EAN−n-dodecane−C12E3 at γ = 0.10 (left) and EAN−n-octane− C12E3 for γ = 0.08 (right) as a function of temperature at α = 0.50. The vertical dotted lines correspond to the phase boundaries between the phase regions 2, 3, and 2̅.

Figure 4. Relative volumes V (± 3%) of components (EAN in white, oil in gray, surfactant in dark gray) in each phase of the system EAN−ndodecane−C12E3 for γ = 0.10 (left) and EAN−n-octane−C12E3 for γ = 0.08 (right) as a function of temperature for α = 0.50. The hatched areas correspond to the microemulsion (a = EAN-rich phase, b = oil-rich phase, c = surfactant-rich phase). The black curves correspond to the interface between the phases. The vertical dotted lines correspond to the phase boundaries between the phase regions 2, 3, and 2̅.

densities of the binary systems n-alkane−surfactant and EAN− surfactant were measured as a function of the surfactant mass fraction and temperature (see SI). Comparing the density of the excess phase at the respective temperature with the densities of the corresponding binary system one can calculate the composition of the excess phase. Having done so all other components must be in the coexisting microemulsion. The remaining components were attributed to the microemulsion phase. Figure 4 shows the relative volumes of the three components in each phase as well as the volumes of the coexisting phases as a function of T for fixed α and γ. At low temperatures (two-phase region 2) we see that the EAN-rich phase (a) is composed of EAN, oil, and surfactant. All oil in phase (a) must be dispersed in droplets because the oils are immiscible with EAN under the experimental conditions. The surfactant is dissolved in both the oil excess and the EAN-rich phases but mostly in the EAN-rich microemulsion. At average temperatures (three-phase region), a large amount of surfactant is needed to form the microemulsion. The remaining amount of surfactant is mainly dissolved in the oil, while nearly no surfactant is present in the EAN excess phase. At high temperatures (two-phase region 2̅),

is lower than the density of pure EAN. This effect is stronger for n-dodecane, which suggests that a larger amount of oil and surfactant is dispersed in the microemulsion than in the system with n-octane. In the three-phase region the densities of the EAN excess phase coincide with the density of pure EAN, i.e., only a small amount of surfactant is present in the EAN excess phase. However, the density of the oil excess phase is always slightly higher than that of the pure n-alkane indicating the presence of surfactant. At higher temperatures (two-phase region 2̅), the density of the EAN excess phase again coincides with the density of pure EAN, while the density of the microemulsion phase is higher than the density of the pure nalkane. Obviously, as was the case for the three-phase region, only a very small amount of surfactant can be present in the EAN excess phase. In addition to the densities, we determined the volumes of the coexisting phases of the ternary systems at α = 0.50 and a fixed γ as a function of the temperature T (black lines in Figure 4). Combining densities and volumes one can calculate the compositions of the coexisting phases as explained in the following. We assumed that the oil excess phase only consists of oil and monomeric surfactant, while the EAN excess phase only consists of EAN and monomeric surfactant. The 8286

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Figure 5. Interfacial tension σab between EAN-rich and oil-rich phases and estimated domain sizes calculated with eq 4 for the system EAN−ndodecane−C12E3 as a function of temperature for α = 0.50 and γ = 0.10 (left). Interfacial tension σab between EAN-rich and oil-rich phases and estimated domain sizes calculated with eq 4 for the system EAN−n-octane−C12E3 as a function of temperature for α = 0.50 and γ = 0.08 (right). The vertical dotted lines correspond to the phase boundaries between the phase regions 2, 3, and 2̅.

mean curvature H of the amphiphilic film from positive to negative.17 From the present results we infer that H is positive (oil-in-EAN droplets) at low temperatures and negative (EANin-oil droplets) at high temperatures. In the three-phase region, the minimum of σab corresponds to H = 0. Comparing with water-containing systems,26 one sees that the curve is less symmetric which could be explained by the composition of the system (Figure 4). At high temperatures, the microemulsion phase (oil-rich phase) contains nearly no EAN droplets which may be the reason for the high interfacial tension, while at low temperatures the microemulsion phase (EAN-rich phase) contains a great number of oil droplets as we extracted from the volume and density data. The value of the minimum interfacial tension (σmin ≈ 1.5 × 10−2 mN m−1 for n-dodecane) is in line with the values of water-containing systems which have a comparable efficiency and phase inversion temperature (see Table 2).22,23 The interfacial tension σab provides additional information about the microstructure. According to the following:

one observes that the oil-rich microemulsion contains almost all surfactant but only a small amount of EAN which decreases with increasing temperature. This is astonishing since the large amount of surfactant in the oil-rich phase should help dissolving EAN in the n-alkane. The results suggest that the surfactant is much more soluble in oil than in EAN at high temperatures. Speculative as it may be, we suggest an explanation for this observation. According to literature, EAN has a sponge-like nanostructure with a domain size of ∼1 nm.29 The confinement of EAN in droplets of a couple of nanometers hinders the formation of the long-range nanostructure. This confinement would thus cost energy which is why only a small amount (if any at all) of EAN is dispersed in the droplets, while in the EAN excess phase the sponge-like nanostructure can form without constraints. 3.3. Interfacial Tension. Interfacial tensions provide information on the microstructure of microemulsions. Thus, we measured the interfacial tension σab between EAN-rich and oil-rich phases of the ternary systems EAN−n-dodecane−C12E3 and EAN−n-octane−C12E3 at the chosen EAN-to-oil ratio of α = 0.50 at a fixed mass fraction of surfactant γ = 0.10 for ndodecane and γ = 0.08 for n-octane as a function of the temperature T (see Figure 5). As is seen in Figure 5, the interfacial tension exhibits a minimum at the center of the three-phase region corresponding to the phase inversion temperature. The results are in line with the trend of water-containing systems.17,26 By increasing the temperature, the interfacial tension decreases until a minimum is reached at the phase inversion temperature,17,19,26 and then increases again. This behavior is associated with a change of the

ξ≈

kBT σ

(4)

where kB is the Boltzmann constant, the length scale ξ of the microemulsion domains can be estimated.22 The size of the microemulsion domains in the n-dodecane system varies from oil droplets with a radius of 9.6 nm, over a maximum domain size of 17.0 nm in the range of the phase inversion temperature back again to EAN droplets with a radius of 5.2 nm. A similar trend can be observed for the n-octane system. The estimated 8287

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Figure 6. Relative self-diffusion coefficients Drel = D/D0 of the alkane (filled symbols) and EAN (empty symbols) as a function of temperature in the system EAN−n-dodecane−C12E3 at a surfactant mass fraction γ = 0.10 (left) and EAN−n-octane−C12E3 (right, γ = 0.10) for an oil/(EAN + oil) mass fraction of α = 0.50. Note that for temperatures in the two-phase region 2̅ no self-diffusion coefficients for EAN could be measured. The gray dotted line shows the assumed progression of the value for this region.

whether the phase transitions 2−3−2̅ are accompanied by the same changes regarding the composition of the coexisting phases, the interfacial tensions, and the microstructure. To answer this question, we first calculated the composition of the phases from the densities and the volumes of the coexisting phases. In the two-phase region 2 of the phase diagram, an oil-in-EAN microemulsion exists similar to the corresponding oil-in-water microemulsions of the aqueous counterparts. In the three-phase region of the phase diagram the amount of EAN in the microemulsion phase decreases as a function of temperature until there is only a very small amount left in the microemulsion phase of the two-phase region 2̅. This suggests that at high temperatures, either very small or even no EAN-in-oil microemulsion droplets are formed. What is also remarkable is the observation that there are only traces of monomerically solubilized surfactant present in the EAN excess phase, which means that the EAN excess phase in the twophase region 2̅ can be considered as pure EAN. Second, we showed that the interfacial tension between the upper and the lower phases σab at a constant sample composition reaches a minimum in the range of the phase inversion temperature for both systems EAN−n-dodecane− C12E3 and EAN−n-octane−C12E3, respectively. This observation is in line with the trend known from aqueous microemulsions stabilized by nonionic surfactants,26 both qualitatively and quantitatively. The change of the interfacial tension σab is accompanied by a change of the composition of the phases, as was discussed above. Since the change of the interfacial tension σab is also accompanied by a change of the microstructure in traditional water−n-alkane−CiEj systems we first estimated the domain size from the interfacial tension data. Subsequently, we carried out self-diffusion NMR measurements to learn more about the connectivity of the domains. The latter study revealed that (a) oil-in-EAN droplet microemulsions are formed at low temperatures, (b) a bicontinuous structure is formed around the phase inversion temperature, (c) and EAN-in-oil droplet microemulsions are formed at high temperatures. These results are in line with the behavior known from H2O−n-alkane−CiEj microemulsions. However, only a very small amount of EAN was present in the EAN-in-oil microemulsion phase which opens up a new possibility, namely that there are no EAN-in-oil

values for the maximum domain size of the EAN−n-alkane− C12E3 systems is in the same range as the one of aqueous microemulsions with similar efficiencies (see Table 2). 3.4. Self-Diffusion Coefficients. We determined the selfdiffusion coefficients D of the ternary systems EAN−ndodecane−C12E3 and EAN−n-octane−C12E3 at the chosen EAN-to-oil ratio of α = 0.50 and at a fixed mass fraction of surfactant by FTPGSE 1H NMR measurements20,21 as a function of the temperature T. The self-diffusion coefficients provide information about the connectivity of the subphases in a microemulsion. We chose surfactant mass fractions of γ = 0.10 for n-dodecane as the oil and for γ = 0.08 for n-octane as the oil. The self-diffusion coefficients D of EAN and the oil in the microemulsion phase were divided by the self-diffusion coefficients of EAN and the oil at the respective temperature, to obtain relative values. The relative self-diffusion coefficients D/ D0 as a function of temperature are shown in Figure 3. For temperatures below the phase inversion temperature, the relative self-diffusion coefficients D/D0 of EAN are high, while low values for D/D0 were obtained for the oil. This indicates a continuous EAN phase with dispersed oil domains. In the range of the phase inversion temperature, the relative self-diffusion coefficients of oil and EAN are of equal values. This is consistent with a bicontinuous microemulsion, which is found around the phase inversion temperature in water−oil−nonionic surfactant systems.17 At temperatures above the phase inversion temperature, we obtained high relative self-diffusion coefficients for the respective oil, which indicates a continuous oil phase. Note that at high temperatures it was not possible to measure the self-diffusion coefficients of EAN since the signals of the NMR spectra were too small to be analyzed. This observation is in accordance with the composition of the system (Figure 4): the oil-rich microemulsion contains just a very small amount of EAN which is too low to be detectable in a FTPGSE experiment.

4. CONCLUSIONS It was shown only recently by Atkin and Warr11 that the phase behavior of microemulsions formulated with EAN, an n-alkane and a nonionic surfactant of the CiEj type is similar to that of the well-studied aqueous counterparts, i.e., to systems consisting of H2O−n-alkane−CiEj. However, it was unclear 8288

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droplet microemulsions at all. We speculate that this observation is due to the sponge-like nanostructure of EAN. Confining EAN in small droplets hinders the formation of this nanostructure and may thus be energetically less favorable. As a consequence EAN is not dispersed (or only to a small extent) in the oil phase at high temperatures but forms an excess phase once the structure of the microemulsion no longer allows the formation of a long-range nanostructure.

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ASSOCIATED CONTENT

* Supporting Information S

Densities of the binary systems and phase behavior with different water content and additional references. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +49-(0)711-685-64470; fax: +49-(0)711-685-64443; email: [email protected]. Present Address ‡

Université Lyon 1, CNRS, Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Avenue Albert Einstein 2, 69626 Villeurbanne, France. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

The research was funded by Deutsche Forschungsgemeinschaft. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the German Research Foundation (DFG) for funding. We also thank Jan H. Porada and Thomas Sottman for fruitful discussions, as well as Michael Hunger, Mireia Subinya and Diana Zauser for valuable help with the measurements. Last but not least, we thank Daniel Topgaard for providing the script for NMR data treatment and Matthias Bürger for the determination of water contents by the Karl Fischer titration.

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REFERENCES

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dx.doi.org/10.1021/la501899c | Langmuir 2014, 30, 8283−8289