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Aqueous Microemulsions of a Fluorinated Surfactant and Oil Studied by PFG-NMR: Transformation from Threadlike to Spherical Micelles Espen Johannessen,*,† Harald Walderhaug,‡ and Balin Balinov§ Graduate School of Materials Research, Department of Physical Chemistry, Åbo Akademi University, Porthansgatan 3-5, FI-20500 Åbo, Finland, Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, NO-0315 Oslo, Norway, and Amersham Health AS, Research Oslo, P.O. Box 4220 Nydalen, NO-0401 Oslo, Norway Received July 31, 2003. In Final Form: November 6, 2003 The aqueous microemulsion system consisting of the fluorinated surfactant tetraethylammonium perfluorooctylsulfonate (TEAFOS) and the fluorinated oil 1H-perfluorohexane (PFH) has been investigated using the pulsed field gradient NMR self-diffusion method on both 1H and 19F. Neat TEAFOS(aq) builds threadlike micelles from rather low surfactant concentrations up to ca. 80 mmol kg-1. The addition of PFH to TEAFOS(aq) solutions induces a transition from threadlike micelles to spherical micelles solubilizing the oil. In this paper, information from the self-diffusion coefficients of oil (PFH), surfactant counterion (TEA+), surfactant ion (FOS-), and water (HDO) during the transition is presented.
Introduction Microemulsions are thermodynamically stable mixtures of water, oil, and surfactant, resulting in highly reproducible formulations with long-term shelf stability. In addition, the properties of microemulsion systems may be tailored to support a variety of technical applications.1 Fluorinated microemulsions are expected to combine the superior properties of microemulsions with the excellent surface activity of fluorinated surfactants2 and their immiscibility with hydrocarbons and lipids. Dissolved in water, this type of surfactant and corresponding microemulsions behave qualitatively as ordinary hydrocarbon surfactants.3 Above the critical micelle concentration (cmc), they form micelles, and they are able to solubilize fluorinated oils. Microemulsion solution phases may exist over extended concentration regimes of both surfactant and oil. Emulsions may be formed in two-phase regions. Various lyotropic liquid crystalline mesophases also may exist for such systems. Aqueous fluorinated surfactant systems and related microemulsions have accordingly attracted a lot of academic interest and have been studied by experimental techniques including cryogenic transmission electron microscopy,4 electric birefringence, neutron scattering, rheometry,3,5,6 and NMR methods.7-12 For some anionic surfactants with an organic counterion, it has been found that the micelles grow into long threadlike structures in the L1 phase even at rather low surfactant concentrations.13 Bossev et al. reported selfdiffusion coefficients of micellar FOS- with Li+ or TEA+ as surfactant counterions in neat systems8 and mixed systems.7 A self-diffusion coefficient of 8.4 × 10-12 m2 s-1 * To whom correspondence should be addressed. E-mail: espen.
[email protected]. † A ° bo Akademi University. ‡ University of Oslo. § Amersham Health AS. (1) Solans, C.; Kunieda, H. Industrial applications of microemulsions; Marcel Dekker: New York, 1997; Vol. 66. (2) Kissa, E. Fluorinated surfactants: synthesis, properties, applications; Marcel Dekker: New York, 1994. (3) Hoffmann, H.; Wu¨rtz, J. J. Mol. Liq. 1997, 72, 191-230. (4) Matsumoto, M.; McNamee, C.; Bossev, D.; Nakahara, M.; Ogawa, T. Colloid Polym. Sci. 2000, 278, 619-628. (5) Watanabe, H.; Osaki, K.; Matsumoto, M.; Bossev, D.; McNamee, C.; Nakahara, M.; Yao, M. Rheologica Acta 1998, 37, 470-485. (6) Watanabe, H.; Sato, T.; Osaki, K.; Matsumoto, M.; Bossev, D.; McNamee, C.; Nakahara, M. Rheol. Acta 2000, 39, 110-121.
was reported for the micellar peak of tetraethylammonium perfluorooctylsulfonate (TEAFOS) at 30 °C. For the same set of solutions, a viscoelastic behavior typical for elongated micelles was described for TEAFOS(aq) by Watanabe et al.5,6 The latter investigations comparing Li+ and TEA+ as counterions showed that LiFOS constitutes spherical micelles while TEAFOS constitutes elongated threadlike micelles. Thus the importance of the organic counterion TEA+ for the formation of elongated threadlike micelles became clear. Microemulsions of TEAFOS with small amounts of perfluoro-1-methyldecalin were studied by Matsumoto et al.4 A perfluorinated alcohol was used as a cosurfactant. Pictures from cryogenic transmission electron microscopy gave further indications of elongated micelles existing like beads on a string. Salted out with LiNO3, the micelles became spherical with the same diameter as the beads constituting the threadlike micelles. In the present study, we focus our interest on the solution structure of the aqueous system consisting of TEAFOS and the fluorinated oil 1H-perfluorohexane. This system has previously been investigated in some detail using rheometry, electric birefringence, and neutron scattering, and the results are reviewed by Hoffmann and Wu¨rtz.3 With pulsed field gradient NMR (PFG-NMR; see below), we have determined the self-diffusion coefficients of the various components in a 1H-perfluorohexane-based microemulsion system that has already been investigated using other techniques. The constituents in the system, oil, surfactant ion, and counterion, may be bound to surfactant aggregates or diffuse freely. In such a twostate model for self-diffusion, the molecules are believed to exist in two environments, either in water, diffusing as free molecules with a self-diffusion coefficient Df, or bound to the interior or exterior of the micelles, moving with the (7) Bossev, D.; Matsumoto, M.; Sato, T.; Watanabe, H.; Nakahara, M. J. Phys. Chem. B 1999, 103, 8259-8266. (8) Bossev, D.; Matsumoto, M.; Nakahara, M. J. Phys. Chem. B 1999, 103, 8251-8258. (9) Gente, G.; LaMesa, C. J. Solution Chem. 2000, 29, 1159-1172. (10) Monduzzi, M.; Chittofrati, A.; Visca, M. Langmuir 1992, 8, 12781284. (11) Mathis, G.; Leempoel, P.; Ravey, J.-C.; Selve, C.; Delpuech, J.-J. J. Am. Chem. Soc. 1984, 106, 6162-6171. (12) Oliveros, E.; Maurette, M.-T. Helv. Chim. Acta 1983, 66, 11831188. (13) Hoffmann, H. Viscoelastic Surfactant Solutions. In Structure and Flow in Surfactant Solutions; ACS Symposium Series, Vol. 578; American Chemical Society: Washington, DC, 1994.
10.1021/la035399u CCC: $27.50 © 2004 American Chemical Society Published on Web 12/19/2003
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Figure 2. The pulsed field gradient spin-echo pulse sequence.
Figure 1. Composition of the samples shown as points in the water-rich corner of the ternary phase diagram for the system of HDO, TEAFOS, and PFH.
micelles as kinetic entities with a self-diffusion coefficient Db. If the exchange between the two states is fast on the NMR time scale (i.e., ∆; see below), the observed selfdiffusion coefficient Dobs for the molecule is given by11
Dobs ) (1 - pb)Df + pbDb
(1)
where pb denotes the fraction of molecules bound to the micelle. For very low water solubility or dissociation, we may anticipate that pb ≈ 1 and that the observed selfdiffusion coefficient thus equals the micelle self-diffusion coefficient. Using this approach, we aim to get more detailed information on the molecular organization in these solutions. Gente and La Mesa9 investigated the selfdiffusion of the various components in a related microemulsion system using trifluoroethanol as a solubilizate by PFG-NMR. Materials and Methods The investigated system is composed of three main components. As a solvent, water was used in the form of deuterium oxide (HDO, Aldrich, 99 at. % D; or Apollo, >99.92 at. % D, diluted with H2O to 99 at. % D). The surfactant was TEAFOS (Fluka, 98%). The oil intended for solubilization was 1H-perfluorohexane, C6F13H (PFH, Avocado). To prevent evaporation from the samples in NMR tubes, small amounts of poly(dimethylsiloxane) (silicon oil, Dow Corning 200 fluid, 200 CST) were used. All compounds except the water from Apollo have been used as delivered. No phase diagram has been found for the investigated system in the literature, and the systematic approach to the system then was to work with four TEAFOS concentrations and additions of PFH at given PFH/TEAFOS ratios. First, a stock TEAFOS(aq) solution at 70 mmol kg-1 was made through weighing, magnetic stirring, and cooling. Cool PFH was added to samples from the stock solution by weighing. The resulting solutions have PFH/TEAFOS mole ratios in the range 0-1 and PFH/TEAFOS weight ratios in the range 0-0.5. The sample series are shown as points in the water-rich corner of the ternary phase diagram for the system of HDO, TEAFOS, and PFH in Figure 1. After addition of oil, it sank to the bottom of the flask. The solutions were then set to magnetic stirring, and the solutions became clouded. After approximately 15 min, all the oil had solubilized and the solutions were isotropic. The samples were prepared in 25 mL Erlenmeyer flasks with glass corks and Parafilm sealing. Not much space was left for air in the flasks. After homogenizing, the samples were left a while for foam degradation. The same day, a solution sample was taken from the flask and added to a 5 mm NMR tube of the type Norell 502. The height of the test samples was 9 ((1) mm.
Figure 3. The pulsed field gradient stimulated echo pulse sequence. PFG-NMR. The PFG-NMR technique is well established and has been used to determine self-diffusion coefficients in the range 10-14-10-9 m2 s-1 for liquid systems.14,15 All experiments were carried out using 5 mm NMR tubes on Bruker DMX-200 spectrometers possessing probe inserts for 1H and 19F, respectively. The systems were capable of delivering gradient strengths in the range 0-9 T m-1 (0-900 G cm-1). The intensity of the spin-echo formed after a combination of radio frequency (rf) pulses is attenuated due to the presence of magnetic field gradient pulses (in this work two identical), between the radio frequency pulses and diffusion of the species.16 The gradient pulses are of duration δ and have magnitudes of g. In the experiments, the value of g was varied to get the signals attenuated from their maximum intensity to about e-2 times the maximum value. The number of transients was minimum 16. When the self-diffusion coefficient of water was measured, a pulsed field gradient spinecho (PGSE) pulse sequence as shown in Figure 2 was utilized. In that case, the gradient pulses are separated by a simple 180° pulse and the spins are located in the plane perpendicular to the magnetic field during the mixing time, ∆. Typically g spans from near zero to 0.40 T m-1 for determining self-diffusion coefficients of water. For the other species, the pulse sequence known as pulsed field gradient stimulated echo (PGSTE) as shown in Figure 3 was used. In this pulse sequence, two 90° pulses separate the gradient pulses so the time the spins are perpendicular to the magnetic field is minimized to the evolution times around the gradient pulses. This was necessary due to the presence of couplings between the observed nucleus and neighboring nuclei and fast spin-spin relaxation in the situations with threadlike micelles and slow dynamics. A perfect optimization of the 90° pulse would then give spin-lattice relaxation as the only relaxation mechanism during the mixing time, ∆, in the PGSTE experiment. The mixing time, ∆, is the time between the onset of the gradient pulses, and thus the time diffusion affects the resulting signal. A value of 100 ms was chosen for ∆ in all experiments. A gradient pulse length, δ, of 1.0 ms was used in all experiments. The total evolution time in the PGSTE experiment (time between the two first 90° pulses), τ1, was 2.44 ms. The attenuated signal is given by the following equation:
I(∆,g) ) I0 exp[-D(γgδ)2(∆ - δ/3)]
(2)
where I0 denotes the signal intensity without field gradients and (14) Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 1-45. (15) Soderman, O.; Stilbs, P. Prog. Nucl. Magn. Reson. Spectrosc. 1994, 26, 445-482. (16) Stejskal, E. O.; Tanner, J. E. J. Chem. Phys. 1965, 42, 288-292.
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γ is the magnetogyric ratio of the observed nuclide. In each experiment, all parameters are kept constant except for g which varied according to the oil-to-surfactant ratio regarding the tracing of the micelles. When measuring the self-diffusion coefficient of PFH, it was clearly seen that it was linked to the dynamics of the micelles. For the upper half of the range of oilto-surfactant ratios, the gradient strength typically spanned from 1 to 3.5 T/m. As the oil fraction decreased, the gradient strengths had to be increased to get the required attenuation. For the lowest oil-to-surfactant ratio, the gradient strength spanned from 2.5 to 9 T/m, as also was the case when micellar FOS- ions were traced. TEA+ counterions could be traced with gradient strengths typically spanning from 0.2 to 1.4 T m-1. The self-diffusion coefficients D of constituents with differing chemical shifts and/ or self-diffusion coefficients may in principle be determined in the same or different experiments.14,15 This has been utilized in this study to determine the self-diffusion coefficients of the microemulsion’s constituents as a function of microemulsion composition (see below). The temperature was calibrated with puriss methanol during the experiments and determined to be 292 ((1) K.
Results The samples were chosen to form oil-in-water (o/w) microemulsions satisfying the requirements for selected surfactant concentrations and selected oil-to-surfactant mole ratios, as demonstrated in Figure 1. The choice of selected oil-to-surfactant mole ratios is determined from the need to understand the dilution properties of microemulsions in general. The surfactant concentration was well above the cmc of TEAFOS, determined to be 1.1 mmol/ kg.8,9 The samples were homogeneous solutions as observed under a cross polarizer. The fluorine NMR shows only one set of CHF2- spectral peaks for the PFH oil that is consistent with a one-phase region, presumably o/w microemulsion. The self-diffusion coefficients of the four species oil (PFH), water, surfactant ion, and counterion are determined. The oil diffusion reflects the center-of-mass diffusion of the micelles,14,15 due to very low water solubility of this component (see below). The self-diffusion coefficient of the counterions serves to determine the counterion binding degree (see below), while the water self-diffusion coefficient depends on the form of aggregates due to obstruction effects.17 Oil and Micelle Diffusion. The self-diffusion coefficients of PFH, as measured by the 1H nuclide, are plotted in Figure 4 as a function of PFH/TEAFOS weight ratio. Four series of different TEAFOS concentrations are plotted. The PFH/TEAFOS weight ratios are kept constant between the series. The self-diffusion coefficients of FOSat zero PFH concentration are inserted for comparison. These were measured by the 19F nuclide for neat TEAFOS solutions of given concentrations. The micellar selfdiffusion coefficient in the neat TEAFOS system was found to be 4.16 × 10-12 m2 s-1 for 50 mmol kg-1 TEAFOS at 19 °C. Bossev et al.8 reported a micellar self-diffusion coefficient of 8.4 × 10-12 m2 s-1 for 100 mmol kg-1 neat TEAFOS at 30 °C. The self-diffusion coefficient of PFH at the four TEAFOS concentrations shows the same dependency on the PFH/ TEAFOS ratio. At the first addition of PFH, the mean self-diffusion coefficient is 4.9 × 10-12 m2 s-1. This is 3 orders of magnitude lower than the self-diffusion coefficient of free PFH (3.35 × 10-9 m2 s-1), as measured from neat PFH diffusing in itself. In the next increment of PFH/ TEAFOS ratio, the diffusion has dropped to half its value compared to the lowest ratio. This is a fact for all the (17) Jo¨nsson, B.; Wennerstro¨m, H.; Nilsson, P. G.; Linse, P. Colloid Polym. Sci. 1986, 264, 77-88.
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Figure 4. Self-diffusion coefficients of PFH for all the samples in the study, measured by the 1H nuclide. Also shown are the self-diffusion coefficients of neat micellar FOS-, as measured by the 19F nuclide, plotted at zero PFH/TEAFOS weight ratio to represent starting points. Each series represents one TEAFOS concentration and is plotted with the corresponding PFH/TEAFOS weight ratios at the abscissa.
Figure 5. Self-diffusion coefficients of the micellar FOS- ions, measured from the 19F nuclide, and the PFH oil, measured by both the 1H nuclide and the 19F nuclide using two different spectrometers. The TEAFOS concentration is varied, while the PFH/TEAFOS weight ratio is 0.1 in all the samples.
TEAFOS concentrations. Another increment in PFH/ TEAFOS ratio gives an abrupt increase in the self-diffusion coefficient. The self-diffusion coefficient then increases by 1 order of magnitude up to an oil-to-surfactant weight ratio of 0.35. From this weight ratio, the self-diffusion coefficient stabilizes, and further addition of oil keeps the self-diffusion coefficient constant. The increase in selfdiffusion coefficient is in agreement with the decrease in zero-shear viscosity for a 50 mM TEAFOS solution with addition of PFH reported by Hoffmann and Wu¨rtz.3 In that paper, the viscosity decreases exactly to the weight ratio of 0.35, the same weight ratio for which the PFH self-diffusion coefficient reached the plateau in Figure 4. An interesting phenomenon in Figure 4 is the dip in PFH self-diffusion coefficient at 0.1 PFH/TEAFOS weight ratio. The zero-shear viscosity measurements in ref 3 did not show any comparable phenomenon. Measurements of the self-diffusion coefficients of PFH and the FOS- surfactant ions were compared at 0.1 oilto-surfactant weight ratio. The results are shown in Figure 5. The PFH self-diffusion coefficients measured with 1H PFG-NMR are the same values as in Figure 4. Measurements were also done with 19F PFG-NMR on PFH and FOS- up to 90 mmol kg-1. In the whole concentration region, the FOS- self-diffusion coefficients are at approximately 4.5 × 10-12 m2 s-1, that is, similar to the self-
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Table 1. Average Self-Diffusion Coefficients Calculated above the 0.35 PFH/TEAFOS Weight Ratioa TEAFOS concentration /mmol kg-1 average D/10-11 m2 s-1 2 × standard deviation/ 10-11 m2 s-1
40
50
60
70
3.62 0.42
3.48 0.31
3.51 0.56
3.27 0.21
a The error margins of the calculated values are taken as twice the standard deviation from the averaging.
diffusion coefficients of neat TEAFOS. Below 80 mmol kg-1 TEAFOS, the FOS- self-diffusion coefficients are higher than the PFH self-diffusion coefficients. This is in agreement with the prediction that the oil is solubilized only in micelles, giving the true micelle self-diffusion coefficient. The FOS- surfactant ions, on the other hand, are exchanging between sites in the continuous phase as monomers and sites in the micelles, and also between the micelles. The FOS- self-diffusion coefficient is thus a modulation of the micellar self-diffusion coefficient and the FOS- monomer self-diffusion coefficient. This is reflected in Figure 5. Hoffmann and Wu¨rtz3 reported that no electric birefringence signal could be detected already from 0.2 PFH/ TEAFOS weight ratio and up and thus concluded that micelles became spherical as viscosity decreased upon addition of PFH. Reference 3 also gave the radii of aggregates estimated from neutron scattering. The reported radii were 38, 30, and 24 Å at 0.2, 0.3, and 0.4 PFH/TEAFOS weight ratios, respectively. An explanation for the phenomena is that the micellar network that is believed to consist of spherical entities connected in a chain like a pearl necklace, at least in the absence of oil,6 disentangles upon PFH solubilization so the micellar length becomes shorter and the micelles diffuse faster. Finally the micelles become perfectly spherically shaped micelles that diffuse as kinetic entities and have no entanglement points with other micelles. Oil addition from that point results in increased micellar radii, and the selfdiffusion coefficient would decrease rather than increase. The oil diffusion may, as stated above, be interpreted as the micelle self-diffusion. This conclusion may be drawn from the following argument. In a two-state model for self-diffusion, the oil molecules are believed to exist in two environments, either in water, diffusing as free oil molecules with a self-diffusion coefficient Df, or solubilized in the interior of the micelles, moving with the micelle as a kinetic entity with a self-diffusion coefficient Db. For very low water solubility, we may anticipate that pb ≈ 1 and that the observed oil self-diffusion coefficient thus equals the micelle self-diffusion coefficient according to eq 1. Due to the stabilization in the self-diffusion coefficient after the 0.35 PFH/TEAFOS weight ratio, it was assumed that the micelles do not disentangle further from that point and that they have more or less constant structure, probably spherical. The average self-diffusion coefficient was calculated for all four surfactant concentration series above the 0.35 PFH/TEAFOS weight ratio as shown in Table 1. The values are shown with double standard deviations from the averaging to represent statistical uncertainty in the values. The standard deviations are quite large due to the propagation of uncertainty of only four original values. As is seen in Figure 6, the average self-diffusion coefficient is decreasing proportionally with the weight fraction of the aggregates. This is reasonable because the increased weight fraction of aggregates enhances the
Figure 6. Average self-diffusion coefficients calculated from the four highest oil-to-surfactant ratios as a function of aggregate weight fraction, i.e., the total weight fraction of PFH and TEAFOS. The line represents the equation given from linear regression performed on the values. Extrapolation to zero concentration gives a self-diffusion coefficient of 4.03 ((0.20) × 10-11 m2 s-1.
obstruction effects in the system. For solutions of finite concentrations, where the surfactant aggregates obstruct each other’s diffusion path, theoretical considerations predict a decrease in the micellar self-diffusion coefficient, for which the first term in a virial expansion gives Dsphere o ) Dsphere (1 - kΦ), where Φ is the volume fraction of the 0 is the particles, k is an empirical term, and Dsphere intrinsic self-diffusion coefficient of the micelles.18 While the aggregate weight fraction varies between the series, the PFH/TEAFOS weight ratio on the other hand is kept constant, so the shape and size of the aggregates should be constant. From these considerations, linear regression was performed on the self-diffusion coefficients averaged for each series above the 0.35 PFH/TEAFOS weight ratio. The intersection of the linear equation with the ordinate gave an intrinsic self-diffusion coefficient of 4.03 ((0.20) × 10-11 m2 s-1, which thus is the intrinsic self-diffusion coefficient of the aggregates between 0.35 and 0.5 PFH/ TEAFOS weight ratio. From the Stokes-Einstein equation,19 it is possible to estimate the aggregate size if one assumes that the aggregates are spherical;
R ) kbT/6πηD
(3)
where kb is the Boltzmann constant, T is the temperature, η is the solvent viscosity, D is the self-diffusion coefficient of the aggregates, and R is the hydrodynamic radius of the aggregate. The averaged self-diffusion coefficients in Table 1 were used to calculate the sphere radius between 0.35 and 0.5 PFH/TEAFOS weight ratio for each TEAFOS concentration series. The results are plotted in Figure 7. The Stokes-Einstein equation as presented in eq 3 only accounts for the hydrodynamic interactions between the aggregate and the solvent and takes no consideration of the obstruction effects. With consideration of the aggregate weight fraction similarly as for the self-diffusion coefficient, linear regression was performed also on the sphere radii from the four series as shown in Figure 7. When the linear equation is extrapolated to zero aggregate weight fraction, a near true hydrodynamic micellar radius should be given. The result of the calculation was 54 ((1) Å. The quantitative value of this radius cannot be estimated with (18) Ohtsuki, T.; Okano, K. J. Chem. Phys. 1982, 77, 1443-1450. (19) Doi, M.; Edwards, S. The theory of polymer dynamics; Clarendon Press: Oxford, 1986; Vol. 73.
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Figure 7. Average sphere radii calculated from the four highest oil-to-surfactant ratios. The line represents the radii calculated from linear regression on the corresponding self-diffusion coefficients. The intersection with the ordinate gives a true aggregate radius of 54 ((1) Å.
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Figure 9. TEA+ binding degree as a function of PFH/TEAFOS weight ratio for four TEAFOS concentrations. Table 2. Average Counter Ion Binding at Each TEAFOS Concentrationa TEAFOS concentration/ mmol kg-1
average counterion binding, p
2 × standard deviation
40 50 60 70
0.711 0.717 0.725 0.730
0.015 0.015 0.018 0.011
a
Figure 8. Reduced HDO self-diffusion coefficients as a function of PFH/TEAFOS weight ratio for four TEAFOS concentrations.
high accuracy though. According to Cussler,20 the StokesEinstein equation used in this manner has a relative error of 20%. When equating the radius from the self-diffusion coefficient and vice versa, the error is thus significant. HDO Water Diffusion. In Figure 8, the reduced selfdiffusion coefficients for water (HDO) are displayed for four surfactant concentrations. A weak decrease of the water self-diffusion coefficients is observed for all four data sets. The decrease in the water diffusion is most likely due to binding of water to surfactant and hydration of the counterion. This effect has not been quantified in this study. The decrease of the water self-diffusion coefficient is lower than predicted from pure obstruction effects estimated from Stokes-Einstein. The self-diffusion results for the two components water and oil taken together indicate that the aggregate structure in the investigated composition range is of the type o/w.11 TEA+ Counterion Diffusion. The self-diffusion coefficients of the tetraethylammonium (TEA+) counterion were determined in order to calculate the mobility and the degree of counterion binding to the micelles. The counterion binding degree is here defined according to eq 1 as pb, that is, the fraction of counterions kinetically bound to the aggregates.8,9,14,15 The free self-diffusion coefficient for the TEA+ ion was determined in an aqueous solution of TEA+ chloride at a concentration of 50 mmol kg-1 and was found to be 6.47 × 10-10 m2 s-1 (see above). This value (20) Cussler, E. L. Diffusion, 2nd ed.; Cambridge University Press: Cambridge, 1997.
The error margin in p is taken as twice the standard deviation.
was assumed to be the self-diffusion coefficient of free counterion. The bound self-diffusion coefficient is assumed to be equal to the micelle self-diffusion coefficient, that is, the self-diffusion coefficient for the oil at the corresponding composition (see above). The results are presented in Figure 9, and as can be seen, the counterion binding degree thus defined stays quite constant and is independent of microemulsion composition. Calculated degrees of counterion binding are presented in Table 2. Twice the standard deviations of the calculated p’s from averaging at each TEAFOS concentration are given to reveal the constancy of the binding degree. The actual uncertainty of the binding degrees will be larger though, about 9%, in consideration of the propagation of uncertainty from using eq 1, assuming 5% uncertainty in the self-diffusion coefficients. The calculated values of p are somewhat smaller than the value of 0.80 found for the corresponding binary system TEAFOS/water at a surfactant concentration of 100 mmol kg-1. However, they compare well with values in the range 0.60-0.90, depending on composition, found for the microemulsion system TEAFOS/trifluoroethanol/water.9 These values for the counterion binding degree are all calculated from PFG-NMR measurements using the same reasoning. Using conductivity measurements, values quite close to the above-cited ones are determined,21 thus lending further support to the cited values for this parameter. If the aggregates do not change very much in shape in the investigated microemulsion composition range (see above), then it is not surprising that the counterion binding degree is large, a phenomenon known as “ion condensation”.22 Discussion and Conclusion The water-rich corner of the system composed of water, TEAFOS, and PFH consists of oil-in-water microemulsion. The counterion binding degree has been shown to be (21) Hoffmann, H.; Tagesson, B. Z. Phys. Chem. Neue Folge 1978, 110, 113-134. (22) Lindman, B.; Wennerstro¨m, H. Top. Curr. Chem. 1980, 87, 1.
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constant between 0.70 and 0.75 for all investigated compositions. On the basis of self-diffusion coefficients for the oil, we were able to identify the conditions where threadlike micelles break to form spherical micelles and the dependence of this transformation upon oil solubilization. Disentangling begins at 0.1 PFH/TEAFOS weight ratio, and the micelles are fully disentangled at the PFH/ TEAFOS weight ratio of 0.35. At this point, the hydrodynamic radius of the micelles is 54 Å. What should be included in this is the solubilized PFH in the micellar core, the perfluorooctyl tail, the sulfonate headgroup, bound counterions (relatively large organic ions), and bound water out to the hydrodynamic radius. The alltrans length of perfluorooctane is about 13.6 Å. This value is small compared to the calculated length of 54 Å for the hydrodynamic radius. Hoffmann and Wu¨rtz3 presented neutron scattering data from the same system, and at 0.3 PFH/TEAFOS weight ratio an estimated radius of 30 Å is reported for the aggregate. There is thus a significant difference from the radius reported in this paper. The oil-to-surfactant weight ratio in this paper goes up to 0.5 though, and the oil should take up considerable volume. In that case, there would be relatively large microemulsion droplets, or even emulsion droplets. The threadlike micelle structure is believed to consist of spherical micelles like beads on a string. The transition to spherical micelles is thus a breakup of the threadlike micelles. It could be reasonably believed that the micelles still are clustered to some extent even above the 0.35 PFH/TEAFOS weight ratio and that this is reflected in the larger hydrodynamic aggregate radius. The authors are not sure how such a clustering affects neutron scattering data. From the counterion binding data, it is clear that the micelle surface charge remains constant throughout the transition. The electrostatic interactions are thus not the driving factor in the transition. The hydrophobic effect is the main driving factor in micellar packing.23 Hydrophobic forces are mainly of short-range nature but could also be effective in longer ranges.24 Regev et al.25 have shown that the aggregate size and shape of fluorinated ammonium carboxylate surfactants are highly dependent on the hydrophobic degree of the counterion and that organic (23) Tanford, C. The hydrophobic effect: Formation of micelles and biological membranes, 2nd ed.; John Wiley & Sons: New York, 1980. (24) Jo¨nsson, B. Surfactants and polymers in aqueous solution; John Wiley & Sons: Chichester, 1998. (25) Regev, O.; Leaver, M. S.; Zhou, R.; Puntambekar, S. Langmuir 2001, 17, 5141-5149.
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Figure 10. A schematic representation of the structures of an aqueous micelle. Two possibilities are illustrated: (a) the surfactant ions pack through chain protrusion and bending, and (b) oil solubilizes in the micelle and as the packing parameter decreases the density of hydrophobic groups in the double layer decreases.
counterions with increasing hydrophobicity lower the micelle curvature and thereby increase the packing parameter2 of the micelle. Entropic effects of the counterions are also important for intermicellar interactions. It is clear that the TEA+ counterion is necessary for the formation of threadlike micelles with FOS- surfactant ions. Possibly the transition from threadlike to spherical micelles could be explained from the fact that the packing parameter of the micelle decreases as PFH oil is added. In the binary composition of TEAFOS in water, the surfactant ions must pack in a manner that is similar to the situation in Figure 10a. Then, when oil is solubilized, the volume of the micelle core would become larger and the micelle shape would become more like that in Figure 10b. In this situation, the packing parameter is reduced, and the hydrophobic interactions near the double layer become less effective as water penetrates the micelle surface and each counterion gets less micelle surface area. As a result, the micelles break up from threadlike to spherical. Acknowledgment. The authors are grateful for the hospitality of Dr. Istvan Furo at KTH in Stockholm, Sweden. Great thanks are given to Familien Stillesen og Professor S. A. Sexes legat for financial support. Dr. Sune Backlund and Dr. Rauno Friman at the Department of Physical Chemistry, Åbo Akademi University, are thanked for valuable comments during the writing of the article. LA035399U
