Langmuir 2000, 16, 3003-3005
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Vesicle-Forming Single-Tail Hydrocarbon Surfactants with Sulfonium Headgroup Bodo zu Putlitz, Katharina Landfester, Stephan Fo¨rster, and Markus Antonietti* Max Planck Institute for Colloids and Interfaces, Am Mu¨ hlenberg, D-14424 Potsdam/Golm, Germany Received December 2, 1999. In Final Form: February 14, 2000 It is shown by small angle neutron scattering and freeze-fracture electron microscopy that sulfonium surfactants with a single surfactant tail form bilayer vesicles of narrow size distribution. This exceptional behavior is attributed to an unusually small area requirement of the polar headgroup due to the high polarizability of the sulfonium cation.
Introduction In a recent paper, the synthesis and characterization of a new class of cationic surfactants with sulfonium headgroups were described.1 It was shown that the surfactants are highly efficient: The critical micelle concentration (cmc) values are much lower than those of other cationic surfactants, e.g., alkyltrimethylammonium surfactants, whereas the saturation value of the surface tension can go down to extremely small values, i.e., 25 mN/m. Such hydrophobization values are close to the theoretically expected values of a perfect dense packing of CH3 groups, similar to those of self-assembled monolayers on gold, but well below the surface tension of common ionic surfactants (ca. 35-40 mN m-1). Already these properties indicated a dense packing of the single surfactant tails, and the good packing was speculatively attributed to the fact that the sulfonium headgroup is more polarizable and therefore “softer” than ammonium headgroups. This would effect a weaker mutual repulsion and a coupled lower area requirement per molecule at the water/air interface. The similarity of the area requirements of the hydrophilic and the hydrophobic parts which is required for good packing at the (planar) water/air interface also means that the surfactants are expected to favor aggregates with low curvatures. In good agreement, a broad and pronounced lamellar phase region was found in the binary phase diagram of the surfactant. This paper addresses the question of what the aggregates of these special surfactants look like in dilute solution, and the aggregation will be studied by dynamic light scattering (DLS), small angle neutron scattering (SANS), and freeze-fracture transmission electron microscopy (TEM). The influence of chain length of the hydrophobic tail and the counterion on the aggregation behavior will be analyzed. Experimental Details For the synthesis of sulfonium surfactants, (2-ethylhexyl)glycidyl ether, 1,2-dodecene oxide, 1,2-octadecene oxide (all Aldrich Co.), bis(2-hydroxyethyl) sulfide, 2,2-bis(hydroxymethyl)propionic acid, acetic acid (all Aldrich Co.), and tetrahydrofuran (BASF AG) were used as received. Surfactant Synthesis. In a typical reaction, 10 g of 1,2dodecene oxide (0.0542 mol) was dissolved in 60 mL of THF. The mixture was heated to 80 °C, and the stoichiometric amount of bis(2-hydroxyethyl) sulfide (6.52 g, 0.0542 mol) was added. The reaction vessel was kept at 80 °C for 1 h. After that time, the organic acid forming the counterion, e.g., 2,2-bis(hydroxymethyl)(1) zu Putlitz, B.; Hentze, H. P.; Landfester, K.; Antonietti, M., Langmuir 2000, 16, 0000.
propionic acid (7.26 g, 0.0542 mol) and deionized water (1.66 g, 0.092 mol) were added. The mixture was stirred for another 3 h at 80 °C. According to the length of the hydrophobic tail, the surfactants are named C12 or C18 surfactants with chloride, DPA, or acetic acid as counterion. Neutron Scattering. The measurements were performed at the 20.0, 5.0, and 1.1 m detector position at the D11 small-angle instrument at ILL, Grenoble. The neutron wavelength was λ ) 0.6 nm with ∆λ/λ ) 8% (from full width at half-maximum (fwhm)). Details of the instrumentation and data reduction can be found elsewhere.2 The surfactant concentration was c ) 5 g/L in D2O, C18 which is well above the cmc (cC12 cmc ) 0.019 g/L, ccmc ) 0.010 g/L) but sufficiently low to minimize disturbances of the form factor by interparticle correlations. The scattered intensity was put on d)1mm ) 0.857 cm-1).2 absolute scale by calibration with water (IH 2O The scattering length densities of both samples are k ) 0.0064 (cm2 mol)/g2. After subtraction of the scattered intensity of the solvent (D2O), the scattering intensity for dilute solutions is given by
I(q) ) kcMPz(q) + Iinc
(1)
where k is the neutron contrast factor, c is the surfactant concentration, M the molecular weight, P(q) the form factor, and Iinc the contribution from the incoherent scattering of the solution. The scattering vector is defined as q ) (4π/λ) sin(θ/2), where λ is the neutron wavelength and θ the scattering angle. The form factor of polydisperse vesicles with outer radius Ro, inner radius Ri, and relative standard deviations of the mean vesicle radius Rm ) (Ro + Ri)/2, σR, and of the wall thickness D ) Ro - Ri, σD, is given by3
P h (q) )
∫ ∫ P(q,R ∞
∞
0
P(q,Rm,D) )
0
m,D)h(Rm)h(D)
dR
(2)
9π (J3/2(qRo) - p2J3/2(qRi))2 2(qR)3
where J3/2(x) denotes the Bessel function. If h(x) is taken to be the Schulz-Zimm distribution
h(x) )
(z + 1)z+1 z+1
xj
Γ(z + 1)
[
exp -
]
(z + 1)x xj
(3)
the integral in eq 2 can be solved analytically in terms of hypergeometric functions.4 The distribution function h(x) is characterized by an average radius or thickness xj and a relative standard deviation σ ) (z + 1)-1/2. The parameters Ro, Ri, σR, σD, (2) Lindner, P. In Modern Aspects of Small-Angle Scattering; Brumberger, H.. Ed.; NATO, Advanced Study Institutes, Ser. C., Vol. 451; Kluwer Academic: London, 1993. (3) Gradzielski, M.; Hoffmann, H.; Langevin, D. J. Phys. Chem. 1995, 99, 12612. (4) Fo¨rster, S.; Burger, C. Macromolecules 1998, 31, 87.
10.1021/la991568s CCC: $19.00 © 2000 American Chemical Society Published on Web 03/14/2000
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Iinc as well as the absolute scattered intensity I(0) ) kcM are obtained from a nonlinear least-squares fit to the measured scattering curve using the Levenberg-Marquardt algorithm. The relative errors of the fit parameters as calculated from the covariance matrix are 6%. Photon Correlation Spectroscopy. Diluted samples of the different surfactants were investigated with photon correlation spectroscopy. A 1 W argon ion laser (COHERENT Innova 300), 488 nm wavelength, was used. The intensity of scattered light at 90° was recorded in a homodyne experiment as a function of time and processed to receive the autocorrelation function of the homodyne signal. A constrained linear least-squares fit procedure was applied to the autocorrelation function to give a distribution of translational self-diffusion coefficients D. The resulting hydrodynamic radii of the particles in solution were obtained from D according to the Stokes-Einstein equation. TEM. Electron microscopy was performed with a Zeiss 912 Omega operating at 100 kV. For the freeze fracture microscopy, the surfactant solution was rapidly frozen at -120 °C in propane cooled in liquid nitrogen and etched for 60 s.
Results and Discussion The previous result1 of having a packing on the water surface close to an ideal packing of CH3 groups hints of an aggregation behavior of sulfonium surfactants which is expected to be different from that of other standard single-tail surfactants. In the following, the aggregation behavior is examplified by the octadecylbis(hydroxyethyl)sulfonium surfactant with 2,2′-bis(hydroxymethyl)propionic acid (DPA) as counterion. The structure of the surfactant under study is the following:
The C18 surfactant was dissolved in water, and the aggregate structure was examined by dynamic light scattering as a suitable method to analyze both hydrodynamic size and polydispersity. It was found that the aggregates of a 0.5 wt % aqueous solution have a size of Rh ) 69.7 nm with a low polydispersity (smaller than 20% Gaussian width). The size of these aggregates depends just weakly on concentration. The experiments already indicate that no simple micelles are found, but the existence of larger aggregates such as vesicles is very likely. For detailed evaluation, SANS was used, which reveals not only the overall diameter but also the inner structure of the vesicles themselves. Figure 1 shows the measured scattering intensity I(q) as a function the scattering vector q for the surfactant C18 (upper curve). The characteristic feature of the scattering curve is two sets of damped oscillations with a I ∼ q-2 envelope at intermediate q, and a I ∼ q-4 Porod envelope at large q. This behavior is characteristic for hollow spherical structures such as vesicles. From the positions of the two sets of oscillations, one can determine the overall diameter of the vesicle and the bilayer thickness. For a quantitative analysis, the scattering curve is fitted to the expression for polydisperse vesicles (eq 1). The fit (solid line) yields an outer radius of Ro ) 70.9 nm, an inner radius of Ri ) 65.4 nm, and relative standard deviations σR ) 0.26 and σD ) 0.18. The value of Ro ) 70.9 nm is in good agreement with a hydrodynamic radius of Rh ) 69.7 nm measured by DLS. The fitted absolute scattered intensity Ifit(0) ) 9.5 × 104 cm-1 is smaller compared to the value calculated from the
Letters
Figure 1. Neutron scattering curve and fit for the C18 and the C12 surfactant.
size and molecular weight of the vesicle (I(0) ) 1.3 × 104 cm-1, see below) due to sample polydispersity and the presence of large aggregates. A simple explanation for the thickness distribution is found in the distribution of counterions, whichsto a small extendsdissociate from the vesicle and contribute to the scattering in a systematic fashion. Remaining deviations could originate from fluctuations of the vesicular shape. However, it is unlikely that the fluctuations are of large amplitude since the vesicles are comparatively small, thus creating a large overall rigidity. The fit shown does not consider bilayer thickness fluctuations and shape fluctuations, but describes the data over 6 decades in intensity and over 2 decades in q. Similarly detailed investigations of vesicular structures by SANS have been recently performed for unilamellar vesicles of egg-PC by Pedersen et al.5 and DPPC by Mason et al.6 From the difference of outer and inner radii, the bilayer thickness can be calculated to be D ) Ro - Ri ) 5.5 nm. This measured bilayer thickness can be compared to the molecular dimension of the surfactant molecules. A crude estimate on the base of the molecular structure reveals a length for the fully extended molecule of 2.93 nm without the counterion and 3.44 nm with the counterion DPA, i.e., the bilayer thickness goes well with twice the molecular length and slightly disordered C18-tails, as expected for a liquid ordered state. Knowing the size and the thickness of the vesicle, one can also calculate the average aggregation number of surfactants forming the vesicle. The molecule volume was estimated by a sum of increments as performed in the literature7 and reveals vmolecule ) 0.656 nm3 without and vmolecule ) 0.823 nm3 with the counterion. From vmolecule and the measured vesicle radii, the aggregation number N of the vesicle is calculated to be N ) 4π(R3o - R3i )/(3vmolecule) ) 4.9 × 105 (without counterion) and N ) 3.9 × 105 (with DPA as the counterion). The real aggregation number lies between the two values and depends on the degree of dissociation and some finer details of the intermolecular packing. From the aggregation number we can obtain the area per headgroup a ) 4π/N(R2o + R2i ) ) 0.239 nm2 (without counterion) or a ) (5) Pedersen, J. S.; Egelhaaf, S. U.; Schurtenberger, P. J. Phys. Chem. 1995, 99, 1299. (6) Mason, P. C.; Gaulin, B. D.; Epand, R. M.; Wignall, G. D.; Lin, J. S. Phys. Rev. E 1999, 59, 3361. (7) van Krevelen, D. W. Properties of Polymers; Elsevier: Amsterdam, 1990; p 87.
Letters
Figure 2. Schematic presentation of the values obtained by SANS measurements.
Figure 3. TEM after freeze fracture of the vesicles formed by the C18 surfactant. Scale bar left side, 200 nm; scale bar right side, 100 nm.
0.300 nm2 (with counterion). The measured area per headgroup of the C18-surfactant is found to be unusually small when compared to values for other surfactants which form lamellar phases such as egg phosphatidyl choline (a ≈ 0.717 nm2),8,9 single-chain zwitterionic surfactants such as n-alkyl betains (a ) 0.60 nm2)10 or nonionic surfactants, e.g., C16E6 (a ) 0.38 nm2).11 The found values are about the value calculated for a dense, liquid packing of the alkyl tails. It must be repeated that the low area requirement per surfactant was already deduced from the very low surface tensions of the saturated air-water interface,1 which was the stimulus of the current work. From the experimentally determined surface excess Γ∞, it is possible to calculate an area requirement of a′ ) 0.286 nm2, which is very close to the actual value determined by the scattering experiment. All structure data of the vesicles obtained by SANS are summarized in a model shown in Figure 2. The formation of vesicles can be also proven and visualized by TEM measurements. Since vesicles usually are not stable by simply drying the solution on a grid, the freeze fracture technique was used to preserve the structure of the aqueous solution. The fractured surface is coated by carbon, and this carbon replica is analyzed. The images (such as Figure 3) clearly underline the presence of vesicles. The shadowing shows that the aggregated structures were spherical. Since during the preparation the vesicles are broken randomly, the size of the structures seen in the image does not reflect the diameter or the size distribution of the vesicles in the aqueous solution. Up to now just data of vesicles for the C18 sulfonium surfactant with DPA as counterion were presented. Additional information is obtained throughout the analysis of the aggregation behavior with different counterions (8) Small, D. M. J. Lipid Res. 1967, 8, 551. (9) Reiss-Husson, F. J. Mol. Biol. 1967, 25, 363. (10) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. Biochim. Biophys. Acta 1977, 470, 185. (11) Van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physicochemical properties of selected anionic, cationic and nonionic surfactants; Elsevier: Amsterdam, 1993.
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and different chain lengths. In the case of the C12 tail surfactant with chloride as counterion, spontaneous vesicle formation was observed, too. In this case the radius of the vesicles is 90 nm. Therefore, it can be concluded that the formation of vesicles is not induced by the very special counterion DPA, since the use of chloride induces a similar aggregation pattern. It is just that the solubility and the size of the vesicles changes. In the case of the surfactant with the C12 tail with acetic acid (Ac) as counterion, again vesicles were found. The sizes obtained by DLS and SANS (SANS data shown in Figure 1, lower curve) are smaller than those obtained in the case delineated above. The outer radius was determined to be Ro ) 27.5 nm, the inner radius Ri ) 24.9 nm, and the bilayer thickness 2.6 nm. Using again the single molecule volume (the increment technique results in vmolecule ) 0.548 nm3 without and vmolecule ) 0.613 nm3 with counterion), the aggregation number and the area per headgroup were calculated to be N ) 4.1 × 104 and a ) 0.422 nm2 (lower estimate without counterion) or N ) 3.7 × 104 and a ) 0.472 (upper estimate with counterion). The area requirement is obviously significantly higher and the packing is worse than in case of the C18, which should be accompanied by a lower vesicle stability. It also becomes evident from the scattering curve (Figure 1) that the vesicles are less defined, i.e., the fine structure details are smeared out. In this sequence, vesicle formation is successfully suppressed when lowering the chain length to eight carbons. Light scattering on the C8 surfactant revealed micelles with an diameter of about 4 nm. It is concluded that formation of a bilayer relies on a minimal thickness, which is reasonable for mechanical arguments. Conclusion It was shown using DLS, SANS, and TEM measurements for the first time that vesicles can be formed spontaneously by a monotail hydrocarbon surfactant, the peculiarity of which is that it carries a sulfonium headgroup. Vesicle formation was found over quite a broad range of tail lengths (carbon number between 12 and 18) and counterions (DPA, acetate, chloride), and bilayer thicknesses between 2.6 and 5.5 nm were realized. Quantitative evaluation of the scattering data reveals a comparatively small effective headgroup area of lower than 0.3 nm2, which is of the order of the cross section of densely packed, elongated alkyl tails. This results in a surfactant ratio of about 1 and the presented unusual case that singlechain surfactants spontaneously form vesicles. The insensitivity against counterion variation excludes an alternative mechanism that also leads to vesicles as reported by Kaler et al.12 where single-chain surfactants are electrostatically coupled to catanionic dimers which possess the appropriate surfactant ratio. Opposite to that, we have to attribute the special behavior to the highly polarizable nature of the sulfonium headgroup which is connected with a lower degree of hydration and simplifies the packing of similarily charged objects. Acknowledgment. The authors thank Anna Peytcheva for help with dynamic light scattering, Brigitte Tiersch for the TEM characterization, and Peter Lindner for help with the SANS measurements. Financial support by the Max Planck Society and the Fonds der Chemischen Industrie is gratefully acknowledged. LA991568S (12) Kaler, E. W.; Murthy, A. K.; Rodriguez, B. E.; Zasadzinski, J. A. N. Science 1989, 245, 1371.
