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Langmuir 1999, 15, 5426-5428
Monodisperse Disk-Shaped Micelles of Perfluorooctadecanoic Acid Andreas F. Thu¨nemann* and Heimo Schnablegger Max Planck Institute of Colloids and Interfaces, Kantstrasse 55, 14513 Teltow, Germany Received January 27, 1999. In Final Form: April 21, 1999
Introduction Perfluorinated surfactants have a unique chemical and thermal stability as well as a capability to reduce the surface tension of liquids to values lower than 20 mN/m.1 Their critical micelle concentrations (cmc’s) are extremely low, typically in the order of 10-2 mol/L,1 and the shape of the micelles often deviates from the normal spherical geometry. Cylindrical and disk-shaped micelles are described for short-chain perfluorinated surfactants in water in low concentrations.2 Because of its commercial relevance, the phase behavior of short-chain perfluorinated surfactants containing six to eight CF2 groups has been investigated extensively.1-8 But little is known about the physicochemical properties of perfluorinated surfactants with more than nine CF2 groups. A knowledge of cmc’s and micelle structures of long-chain fluorinated surfactants is important for designing polyelectrolyte-fluorosurfactant complexes, which are newly developed selforganizing materials capable in forming ultralow surface energies.9-12 In this work we report on a detailed smallangle X-ray investigation of perfluorooctadecanoic acid in diluted solutions and the determination of its cmc. Experimental Section The surfactant used in this study, perfluorooctadecanoic acid, was supplied by ABCR and used in the state received. It had a melting point of 170 °C. Water was from a Milli-Q system. We performed the small-angle X-ray scattering experiments with a Kratky compact camera (Anton Paar, Austria), equipped with a stepping motor and a counting tube with an impulse-height discriminator. The light source was a conventional X-ray tube with a fixed copper target operating at 40 mA and 30 kV. No monochromator (except the built-in impulse-height discriminator of the detector) was used. Instead, the Kβ contributions were numerically calulated for in the subsequent desmearing * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Kissa, E. Fluorinated Surfactants, Surfactant Science Series; Marcel Dekker: New York, 1994; Vol. 50. (2) Hoffmann, H. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 1078. (3) Edwards, P. J. B.; Jolley, K. W.; Smith, M. H.; Thomson, S. J. Langmuir 1997, 13, 2665. (4) Easttoe, J.; Dalton, J. S.; Downer, A. Langmuir 1998, 14, 1937. (5) Boden, N.; Harding, R.; Jolley, K. W.; Thomson, S. J. Mol. Cryst. Liq. Cryst. 1997, 304, 185. (6) Dombroski, J. P.; Edwards, P. J. B.; Jolley, K. W.; Boden, N. Liq. Cryst. 1995, 18, 51. (7) Boden, N.; Harding, R.; Gelbart, W. M.; Ohara, P.; Jolley, K. W.; Heerdegen, A. P.; Parbhu, A. N. J. Chem. Phys. 1995, 103, 5712. (8) Holmes, M. C.; Leaver, M. S.; Smith, A. M. Langmuir 1995, 11, 356. (9) Antonietti, M.; Henke, S.; Thu¨nemann, A. F. Adv. Mater. 1996, 6, 41. (10) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1998, 14, 4898. (11) Thu¨nemann, A. F.; Lieske, A.; Paulke, B. R. Adv. Mater. 1999, 11, 321. (12) Thu¨nemann, A. F.; Lochhaas, K. H. Langmuir 1999, submitted for publication.
procedure which is included in the program ITP,13-15 which transforms the scattering data into real space and gives radial pair-distance distribution functions (PDDF). This program uses the equation
ID(q) ∝
∞ p(r) ∫r)0
sin qr qr
dr
(1)
where p(r) is the radial PDDF to be calculated and ID(q) the desmeared scattering intensity as a function of q. The scattering vector is defined in terms of the scattering angle θ and the wavelength λ of the radiation (Cu KR ) 0.154 nm)
q ) (4π/λ) sin(θ/2)
(2)
The actual experimental data I(q) can be expressed in terms of the desmeared intensity ID(q) (see eq 7 in ref 14), and both functions, p(r) and ID(q), were computed in one step using the experimental scattering function I(q), the wavelength-distribution data, and the corresponding beam profile of the experimental equipment. The temperature of the sample was adjusted to 80 ( 1 °C using a K-HR temperature controller (Anton Paar, Austria). Surface tension measurements were carried out with a K12 Tensiometer (Kru¨ss, Germany). A 40 mL volume of aqueous surfactant solutions was heated to 80 °C and stirred for 1 h. The surface tension values were automatically obtained using the Ring Method16 with a platinum ring of 9.545 mm diameter. Results and Discussion The small-angle X-ray scattering of perfluorooctadecanoic acid in aqueous solutions were investigated at concentrations in the range of 10-3-10-2 mol/L. This is a concentration range relevant for the preparation of solid polyelectrolyte surfactant complexes.10 Because of the poor solubility of perfluorooctadecanoic acid at room temperature, the temperature was raised to 80 °C. The shape of the scattering curves did not change significantly within the concentration range investigated. From this we concluded that the scattering entities in the solution do not change significantly or cluster. As is clear from Figure 1a, two pronounced minima and a characteristic increase of the scattering curve at small scattering angles can be observed. From the shape of the curves the presence of disklike or rodlike micelles can be assumed. The two relative sharp minima are indicative for a low polydispersity of the micelles in the cross section. A quantitative evaluation of the scattering data was achieved by the indirect Fourier transform method as developed by Glatter.14,17 This method transforms the experimental scattering function into pair-distance distribution functions (PDDF), where smearing effects from the beam geometry and the wavelength distribution are simultaneously determined. The desmeared scattering intensity is shown in Figure 1 (curve b) and the PDDF in Figure 2 (solid curve). For values smaller than 20 nm, the PDDF is clearly described by a calculated distance distribution function of a micelle with a size of 33 ( 0.5 nm in diameter (13) Glatter, O. Acta Phys. Austria 1977, 47, 83. (14) Glatter, O. J. Appl. Crystallogr. 1977, 10, 415. (15) Glatter, O. J. Appl. Crystallogr. 1979, 12, 166. (16) Lecomte du Nouy, P. J. Gen. Physiol. 1919, 1, 521. (17) Glatter, O. J. Appl. Crystallogr. 1980, 13, 577.
10.1021/la990080o CCC: $18.00 © 1999 American Chemical Society Published on Web 06/17/1999
Notes
Langmuir, Vol. 15, No. 16, 1999 5427
Figure 1. Small-angle X-ray scattering curves obtained for a dilute solution (5 × 10-3 mol/L) of perfluorooctadecanoic acid at 80 °C. (a) Data points as measured with a Kratky camera (squares) and smoothed data points (solid line). (b) Desmeared scattering curve as calculated from (a) taking into account the geometry of the beam and the wavelength distribution.
Figure 2. Distance distribution function calculated from the scattering curve (solid line) and the best-fitting distance distribution function of a disklike micelle (dashed line).
and a height of about 5 nm (Figure 2, dashed curve). The deviations between the fitted curve and the experimental curve at higher distances may be due to some remaining parasitic scattering of the primary beam at very small scattering angles. Therefore, the shape of p(r) at values larger than 35 nm is not very significant. Another possible explanation for the nonvanishing values of p(r) at higher distances may be the formation of larger lamellar aggregates. These are described in the literature for shortchain fluorosurfactants, which are preliminary stages of lamellar mesophases.18 This special behavior of fluorosurfactants explains why the phase diagrams of perfluorosurfactants are generally much simpler than the phase diagrams of similar single chain hydrocarbon surfactants. In some cases the two-phase region of the lamellar phase begins right at the cmc, i.e., at a very low concentration.2 The fit of p(r) in Figure 2 is very sensitive to variations of the diameter, but this is not for the height of the micelles. Presuming that the micelle diameter is much larger than its height, the cross-section distribution function17 pt(r), i.e., that represents the direction parallel to the main axis, can be calculated by inversion of
q2I(q) ∝
∫0∞ pt(r) cos(qr) dr
(3)
The resulting pt(r) is shown in Figure 3 (solid curve). From this, the heights of the micelles can be determined with great accuracy. If a homogeneous density is present, the cross-section distribution function is simply a straight (18) Herbst, L.; Hoffmann, H.; Kalus, J.; Reizlein, K.; Schmelzer, U.; Ibel, K. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 1050.
Figure 3. Cross-section distance distribution function calculated from the scattering curve in Figure 1 (solid line) and the best-fitting cross-section distribution function for a large homogeneous disk with a height of 4.8 nm (dashed line).
line. The value pt(r) ) 0 is identical with the height of the micelles; this was determined, from a linear fit, to be 4.8 ( 0.2 nm (Figure 3, dashed line). This gives a diameterto-height ratio of 6.9, i.e., the shapes of the micelles are that of relatively flat disks. The small deviations of pt(r) from a straight line may be due to deviations from a homogeneous density profile of the micelles, possibly with a lower density near the micelle surface. An idea of the internal micelle structure is gained from simple geometric considerations: For the length of perfluorooctadecanoic acid with a fully extended chain 2.45 nm was calculated in the same way as for perfluorinated acids.19 Such an all-trans conformation seems to be reasonable because of the stiffness of the fluorocarbon chain; this is in contrast to the flexibility of its hydrocarbon analogue. This value for the lengths of the molecules is half of the diameters of the micelles, and therefore, an interdigitated arrangement of the alkyl chains can be ruled out. A tail-to-tail geometry of the internal structure of the micelles is very probable. Obviously, the cmc of perfluorooctadecanoic acid is lower than 10-3-10-2 mol/L, which is the concentration range of the SAXS investigations. In the literature, the critical micelle concentrations of perfluorinated acids are given for chain lengths up to 10. Zisman et al. found a decreasing cmc with increasing chain length down to 7.4 × 10-1 mol/L (perfluorobutyric acid), 9.1 × 10-3 mol/L (perfluorooctanoic acid), and 7.8 × 10-4 mol/L (perfluorodecanoic acid), while for perfluorododecanoic acid the solubility at 25 °C was too low to reach the minimum surface tension.20 The surface tension concentration dependence of perfluorooctadecanoic acid at 80 °C is shown in Figure 4. It can be seen that a constant surface tension of 21 mN/m is already reached at a concentration of 10-4 mol/L and the cmc is about 8 × 10-5 mol/L. This value is about 2 orders of magnitude lower than that typically found for other fluorinated surfactants having one ionic hydrophile.1 But because of the high fluorine content of perfluorooctadecanoic acid, this extreme value was not unexpected. If we compare the cmc’s of the four acids in mass per volume, the values are 158.4, 3.77, 0.40, and 0.073 g/L for chain lengths of 4, 8, 10, and 18, respectively. From an economic point of view it is more useful to compare latter values. Furthermore, it is interesting to note that the longer the carbon chains are, the steeper is the decrease of the surface tension. In contrast to this, the lowest possible surface tension has a minimum at a chain length of eight.20 For example, the surface tension of perfluorooctanoic acid is 15.3 mN/m, while it is 20.5 mN/m for perfluorodecanoic (19) Burkitt, S. J.; Ottewill, R. H.; Hayter, J. B.; Ingram, B. T. Colloid Polym. Sci. 1987, 265, 619. (20) Mernett, M. K.; Zisman, W. A. J. Phys. Chem. 1959, 63, 1911.
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Notes
perfluorooctadecanoic acid. Similarly low cmc’s as that found for perfluorooctadecanoic acid have been reported for nonionic fluorinated surfactants, which are regularly below 10-3 mol/L;1 e.g., the cmc of F(CF2)8CH2CON[(C2H4O)3CH3]2 is 1.2 × 10-5 mol/L.21 Finally it was shown by small-angle X-ray scattering that perfluorooctadecanoic acid forms flat, disklike micelles at very low concentrations of 10-3-10-4 mol/L. The arrangement of the fluorinated chains within the micelles is thought to be tail-to-tail with an all-trans chain conformation. The critical surface tension was found to be extremely low (8 × 10-5 mol/L), while the lowest observed surface tension is much higher (21 mN/m) than that of its short-chain homologues (15 mN/m). Figure 4. Surface tension of perfluorooctadecanoic acid in aqueous solution at 80 °C.
acid at the cmc (25 °C). Here it is found that the lowest possible surface tension for perfluorooctadecanoic acid is even higher (21.0 mN/m at 80 °C). From this we conclude that an arrangement of CF3 groups at the air/water interface is denser for perfluorooctanoic acid than for
Acknowledgment. We thank C. Burger for a critical discussion of the scattering results, and the Max Planck Society for financial support. LA990080O (21) Selve, C.; Ravey, J. C.; Moudjahid, C.; Moumni, E. M.; Delpuech, J. J. Tetrahedron 1991, 47, 411.
