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Langmuir 1996, 12, 686-690
Spontaneous Formation of Monodisperse Vesicles in Dilute Aqueous Solutions of PFPE and Betaine Sandra Ristori,† Jacqueline Appell,* and Gre´goire Porte G.D.P.C. (UA 233 CNRS) case 26, University Montpellier II, F-34095 Montpellier cedex 05, France Received July 25, 1995. In Final Form: October 20, 1995X In dilute aqueous solutions of ammonium perfluoropolyethercarboxylate (PFPE) and n-dodecylbetaine, for a small range of proportion of PFPE and betaine, we find evidence of the spontaneous formation of vesicles of a well-defined radius. This formation is tentatively explained by the fact that PFPE and betaine have only a partial miscibility allowing for a strong difference in composition of the outer and inner layer of the membrane of the vesicle. This can lead to a preferred radius of curvature for the membrane as described by Safran et al. (Phys. Rev. A 1991, 43, 1071).
Introduction Mixtures of fluoro- and hydrocarbon surfactants in aqueous solution have been extensively studied in recent years. It is well established that the mixing of fluorocarbon and hydrocarbon tails is nonideal and unfavored and this is traced back to the mutual phobicity of hydrocarbon and fluorocarbon molecules.1 In mixtures of surfactants with hydrocarbon and fluorocarbon tails, their mixing properties will depend on the interactions between their headgroups which can more or less counterbalance the phobicity of their tails so that various situations can be encountered (mixed micelles, two coexisting types of mixed aggregates, elongated aggregates, phase separation, etc.) (see, e.g. refs 2-5 and references therein). We present here the results of our investigation on the structure of the aggregates formed in dilute aqueous solutions of mixtures of ammonium perfluoropolyethercarboxylate (PFPE) and n-dodecylbetaine as studied by light scattering. Evidence is given of the existence, over a narrow range of composition, of large monodisperse vesicles probably coexisting with small micellar aggregates, while small mixed micelles are formed on the betaine-rich side. The spontaneous formation of vesicles from surfactant bilayer membranes has been experimentally observed in various situations, for example, in mixtures of sodium dodecyl sulfate and octanol in brine, the so-called L4 phase, by Herve´ et al.,6 in mixtures of cationic and anionic surfactants by Kaler et al.7 in mixtures of a nonionic surfactant and a nonionic cosurfactant by Hoffmann et al.8 and in a nonionic surfactant doped by small amounts of an ionic surfactant by Thunig et al.9 and by Ligoure et * To whom correspondence should be adressed: e-mail appell@ gdpc.univ-montp2.fr. † Present address: Department of Chemistry, University of Florence, I-50121 Florence, Italy. X Abstract published in Advance ACS Abstracts, January 15, 1996. (1) Hildebrand, J. H.; Prausnitz, J. M.; Scott, R. L.Regular and Related Solutions; Van Nostrand Reinhold; New York, 1970. (2) Mukerjee, P. Colloids Surf. A 1994, 84, 1. (3) Clapperton, R. M.; Ottewill, R. H.; Ingram, B. T. Langmuir 1994, 10, 51. (4) Hoffmann, H.; Po¨ssnecker, G. Langmuir 1994, 10, 381. (5) Guo, W.; Guzman, E. K.; Heavin, S. D.; Li, Z.; Fung, B. M.; Christian, S. D. Langmuir 1992, 8, 2368. (6) Herve´, P.; Roux, D.; Bellocq, A. M.; Nallet, F.; Gulik-Krzywicki, T. J. Phys II 1993, 3, 1255. (7) Kaler, E. W.; Herrington, K. L.; Murthy, A. K.; Zasadzinski, J. A. J. Phys. Chem 1992, 96, 6698. (8) Hoffmann, H.; Thunig, C.; Munkert, U.; Meyer, H. W.; Richter, W. Langmuir 1992, 8, 2629.
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al.10 The spontaneous curvature of symmetrical bilayers is, for symmetry reasons, equal to zero and the formation of vesicles from such bilayers is generally unfavorable. It has been argued recently by Safran et al.11 that in the case of mixed bilayers, the local composition could couple to the locally preferred curvature and that this coupling could play an important role in the spontaneous formation of vesicle. We will argue that, in the present mixed system, this is probably the origin of the formation of monodisperse vesicles. Experimental Section Materials. PFPE ) CF3O[CF2CFO]3CF2COONH4 CF3
was obtained pure from Ausimont S.p.A. and used as, is (M ) 681 Da d ) 1.8 g/cm3). n-Dodecylbetaine ) C12H25N+(CH3)2CH2COO- was synthesized and purified in the laboratory.12 The samples of pure PFPE or betaine are prepared by weight in 0.1 M NH4 Cl brine. The sample of PFPE, initially not homogeneous, is first sonicated (see discussion below). The samples of mixed PFPE and betaine are obtained by mixing appropriate amounts of the two stock solutions. The pH values of all samples have been measured to be ∼6 due to PFPE, the salt of a strong acid. This pH is roughly equal to the isoelectric point of the betaine. The samples are filtered through 0.22 µm Millipore filter into the measurement cell to eliminate dust. Light Scattering Measurements. Static and quasi-elastic light scattering have been measured on the same samples using a standard setup. The light source is an argon ion laser (λ ) 4880 Å). The sample is placed in a thermostated bath (measurements are performed at 20 °C) on the rotation axis of the detection arm. The scattered light is received onto the photocathode of a photomultiplier. The resulting pulses are received by a Brookhaven real-time correlator. The angular distribution of scattered light is measured from θ ) 10 to 150°, the accessible scattering vector q
(|q| ) (4πn/λ) sin(θ/2) with the refraction index n ) 1.33 for water) thus ranges from 3 × 10-4 to 3 × 10-3 Å-1 . The intensities are (9) Thunig, C.; Platz, G.; Hoffmann, H. in Proc. Workshop “Structure and Conformation of amphiphilic membranes” Ju¨lich Germany (Springer verl) 1991, 66. (10) Ligoure, C.; Couve, C.; Oberdisse, J.; Appell, J.; Berret, J. F.; Porte, G. in Proc.XXXth Morions Meeting “ Short and long chains at interfaces” 1995. (11) Safran, S. A.; Pincus, P. A.; Andelman, D.; Macintosh, F. C. Phys. Rev. A 1991, 43, 1071. (12) Gauthier-Fournier, F. The`se, University Montpellier II, France, 1986.
© 1996 American Chemical Society
Vesicle Formation
Langmuir, Vol. 12, No. 3, 1996 687
With relation 1 we could not adjust the anisotropic angular distributions (cf. curves b and c of Figure 1) measured here. This is traced back to the fact that qRg is no longer small and that for aggregates of larger dimensions we must use the form factor appropriate to the real form of the aggregates. As PFPE was shown by Gebel et al.14 to readily autoassociate to form bilayers, we suspected that possibly the local structure of the aggregates was a bilayer. Closed aggregates could then possibly be large vesicles. We thus tried
P(q) )
Figure 1. Illustration of the three types of angular distributions of scattered light observed in solutions of PFPE + betaine in 0.1 M NH4Cl brine. The three distributions are for a total surfactant concentration of 0.0127 g/g. The molar ratio of [betaine]/([betaine] + [PFPE]) is (a) 0.825, (b) 0.806, and (c) 0. The solid line through distribution b is the best fit obtained using relation 3 as explained in the text (with the radius of vesicles R ) 1230 Å). scaled by measuring the intensity scattered by benzene under the same conditions. The time autocorrelation functions of the scattered intensity are recorded at different angles.
sin2(qR)
(2)
(qR)2
which is the form factor appropriate to vesicles of mean radius R with a membrane thickness δ under the conditions δ , R and qδ , 1. The second condition is a reasonable assumption if the membrane is one bilayer of mixed surfactants (l < δ < 2l with l in the order of the length of the surfactant molecules). With (2) we could not adjust the angular distributions but we found that the following relationship led to a very good agreement in a narrow range of the ratio betaine over PFPE
I(q) Ibenzene
)A
sin2(qR) (qR)2
+B
(3)
Results
with Rg the radius of gyration of the aggregate.
That is the angular distribution of scattered light is the weighted sum of the form factor of large unilamellar vesicles of radius R with a remarkably narrow size distribution and of an isotropic contribution due to small aggregates. In Figure 2A we plotted q2I(q) as a function of q for three solutions with the same ratio of betaine/PFPE and three concentrations of surfactants. We use this representation which amplifies the oscillations of the form factor of vesicles (cf. (3)). It must be emphasized that the mere observation of these oscillations implies a very narrow size distribution for the vesicles. In Figure 2B the same curves are plotted in the classical I(q) versus q representation. In this representation, the adjustment is seen to be better at low q’s for the more diluted sample. The poorer adjustment at low q’s for the other samples is probably due to the neglect of the dependence on q of the structure factor (see above). This dependency is likely to become significant as the interactions between large vesicles (radius on the same order of magnitude as the wavelength of light) must range over comparable distances. For the angular distributions obtained for solutions rich in PFPE the distributions were unsatisfactorily fitted by (1) and the mean radius of gyration thus obtained was large (in the order of 1000 Å) so that the condition qRg < 1 was no longer fulfilled. But no reasonable adjustment to (3) or to other form factors was found; this can be understood if the distribution of size of the aggregates is very large so that it wipes out all important features. An interesting feature of the distributions measured in this region is that they are similar at different concentrations as illustrated in Figure 3 where we plotted the intensity normalized to unit volume fraction for three different concentrations. We will discuss this fact below. 2. Quasi-Elastic Light Scattering (QELS). In QELS we monitor the relaxation of the fluctuations of
(13) Candau, S. J. In Surfactant Science Series, 22ed.; Zana, R., Ed.; M. Dekker: New York, 1987; p 147.
(14) Gebel, G.; Ristori, S.; Loppinet, B.; Martini, G. J. Phys. Chem. 1993, 97, 8664.
1. Angular Distributions of Scattered Light. Depending on the ratio of betaine over PFPE, we find three types of angular distributions. On the betaine-rich side the angular distributions are isotropic (cf. Figure 1a). Then, for a narrow range of the ratio betaine over PFPE, anisotropic distributions are recorded (cf. Figure 1b) with, in some cases, evidence of oscillations. And regularly decreasing distributions are found on the PFPE rich side (cf. Figure 1c). The scattering pattern of light scattered by a solution of particles can be written13 as:
I(q) Ibenzene
dn [dΦ ] ΦvP(q) S(q)
)K
2
where K is a constant depending on the experimental setup, dn/dΦ is the refraction index increment upon addition of the solute particles of volume v, Φ is the volume fraction of the particles, P(q) is the form factor of the particles, and S(q) is the structure factor which is generally assumed to be constant over the range of wave vectors explored in light scattering. The angular distribution of scattered light thus reflects the form factor of the aggregates in the mixed surfactants solutions. For aggregates small with respect to the wavelength of light (typical dimensions
