Formation of Monodisperse Charged Vesicles in Mixtures of Cationic

pubs.acs.org/Langmuir © 2010 American Chemical Society

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Formation of Monodisperse Charged Vesicles in Mixtures of Cationic Gemini Surfactants and Anionic SDS Sylvain Pr
evost,*,†,‡ Laurent Wattebled,§, Andr
e Laschewsky,§ and Michael Gradzielski† †

)

Stranski-Laboratorium f€ ur Physikalische und Theoretische Chemie, Institut f€ ur Chemie, Technische Universit€ at Berlin, Strasse des 17. Juni 124, Sekr. TC7, D-10623 Berlin, Germany, ‡Helmholtz-Zentrum-Berlin, Soft Matter and Functional Materials, Hahn-Meitner-Platz 1, Lise-Meitner-Campus, Berlin D-14109, Germany, and § Universit€ at Potsdam, Institut f€ ur Chemie, Karl-Liebknecht-Strasse 24-25, D-14476 Golm/Potsdam, Germany, and Fraunhofer Institut f€ ur Angewandte Polymerforschung IAP, P.O. Box 600651, D-14406 Potsdam, Germany. Current address: Evonik Stockhausen GmbH, B€ okerpfad 25, D-47805, Germany Received October 3, 2010. Revised Manuscript Received November 21, 2010 The aggregation behavior of catanionics formed by the mixture of cationic geminis derived from dodecyltrimethylammonium chloride (DTAC) and anionic sodium dodecylsulfate (SDS) was studied by means of phase studies and comprehensive small-angle neutron scattering (SANS) experiments at 25
C and 50 mM overall concentration. The results are compared to those for the previously studied SDS þ DTAC system. Various gemini spacers of different natures and geometries were used, but all of them had similar lengths: an ethoxy bridge, a double bond, and an aromatic ring binding the two DTACs in three different substitutions (ortho, meta, and para). SANS and SAXS data analysis indicates that the spacer has no large effect on the spheroidal micelles of pure surfactants formed at low concentration in water; however, specific effects appear with the addition of electrolytes. Microstructures formed in the catanionic mixtures are rather strongly dependent on the nature of the spacer. The most important finding is that for the hydrophilic, flexible ethoxy bridge, monodisperse vesicles with a fixed anionic/cationic charge ratio (depending only on the surfactant in excess) are formed. Furthermore, the composition of these vesicles shows that strongly charged aggregates are formed. This study therefore provides new opportunities for developing tailor-made gemini surfactants that allow for the fine tuning of catanionic structures.

Introduction A typical goal of physicochemists is the understanding and optimization of self-assembly from basic molecular building blocks. More than 30 years ago, the introduction by Israelachvili of the packing parameter1 as a simple conceptual approach to tuning the curvature of microstructures formed upon the aggregation of surfactants helped in the analysis and the design of self-assembled objects by modification of the building block geometry. Nonionic surfactants with a PEO moiety constitute a classical case, whereby upon choosing the length of the apolar chain and the surface area of the molecule (by varying the length of the hydrophilic ethoxy chain) one can fine tune their aggregation behavior. Similarly, by employing double-chain surfactants (and thereby doubling the apolar volume) one changes the packing parameter such that bilayers or even reverse structures can be formed easily. Dimeric surfactants, or geminis,2,3 where a spacer covalently binds two identical surfactants, represent an elegant option for tailor-made designs of self-aggregating systems. The presence of the spacer affects the distribution of charges in the surfactant layers, its positioning at the interface will further modify the packing parameter, its length and flexibility will add a steric constraint, forcing the distance and the angle between adjacent blocks, and its nature will change the interactions with water, the solubility of other species, and internal interactions in the surfactant layer. In general, geminis also possess lower solubility and *To whom correspondence should be addressed. E-mail: prevost.sylvain@ gmail.com. (1) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525. (2) Laschewsky, A. Tenside, Surfactants, Deterg. 2003, 40, 246–249. (3) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 205–253.

582 DOI: 10.1021/la103976p

cmc’s because of the fact that they contain a much larger total number of hydrophobic carbon atoms than does the equivalent monoalkyl surfactant and gain less entropy by being dissolved because of the fact that they are dimeric.3 The most intensively studied geminis are cationics derived from alkyl trimethylammonium bromide (CmTAB),4-7 usually referred to as alkanediylR,ω-bis(alkyldimethylammonium bromide) with the notation m-s-m, where m is the number of carbon atoms in the main alkyl chain and s is the number of carbons in the aliphatic spacer. For short aliphatic spacers of two to three carbons, threadlike micelles are formed; for spacers smaller than the aliphatic tail, essentially spherical micelles appear; for long spacers, vesicles are prominent. The spacer nature effects have been less often examined.8 Gemini surfactants have been studied in particular for their synergistic effects in mixtures with other surfactants, being nonionic,9,10 zwitterionic,11 of similar charge12,13 or of opposite charge,14-16 and (4) Zana, R.; Talmon, Y. Nature 1993, 362, 228–230. (5) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448–1456. (6) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392–4396. (7) Zana, R. J. Colloid Interface Sci. 2002, 248, 203–20. (8) Laschewsky, A.; Wattebled, L.; Arotcarena, M.; Habib-Jiwan, J. L.; Rakotoaly, R. H. Langmuir 2005, 21, 7170–7179. (9) Alargova, R. G.; Kochijashky, I. I.; Sierra, M. L.; Kwetkat, K.; Zana, R. J. Colloid Interface Sci. 2001, 235, 119–129. (10) Chakraborty, T.; Ghosh, S. Colloid Polym. Sci. 2007, 285, 1665–1673. (11) Yaroslavov, A. A.; Udalykh, O. Y.; Melik-Nubarov, N. S.; Kabanov, V. A.; Ermakov, Y. A.; Azov, V. A.; Menger, F. M. Chem.;Eur. J. 2001, 7, 4835– 4843. (12) Tsubone, K. J. Colloid Interface Sci. 2003, 261, 524–528. (13) Bakshi, M. S.; Singh, J.; Singh, K.; Kaur, G. Colloids Surf., A 2004, 234, 77–84. (14) Liu, L.; Rosen, M. J. J. Colloid Interface Sci. 1996, 179, 454–459. (15) Bhattacharya, S.; De, S. Langmuir 1999, 15, 3400–3410. (16) Brito, R. O.; Marques, E. F.; Gomes, P.; Falcao, S.; Soderman, O. J. Phys. Chem. B 2006, 110, 18158–18165.

Published on Web 12/10/2010

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in some cases polymers;17,18 their solubilization properties have also been studied, in particular, in cases where the solute can be considered to be a cosurfactant or a counterion.19 In all cases, synergisms are obtained.20,21 The variety of structures obtained with catanionics22; mixtures of anionic and cationic surfactants;is an interesting choice for the study of spacer effects, adding several degrees of freedom such as the ratio between surfactant leading to charged micelles or oppositely to neutral large structures,23,24 with the choice of both headgroups effecting ion pair interactions.25 Such gemini-based catanionics combine the diversity of size and shapes for large bilayers encountered with catanionics sometimes exhibiting holes and mimicking biologic entities,26-29 with the possibility of the gentle adjustment of curvature by modifying the structure of the spacer moiety while the whole system is constituted of essentially two single surfactants as elementary units. For catanionic surfactant mixtures, one very frequently finds the spontaneous formation of unilamellar vesicles and similar observations have also been made for the case of double-chain cationic surfactants.30 Gemini-based catanionics have been most intensively studied with sodium dodecyl sulfate (SDS) as a single anionic surfactant and m-s-m gemini cationics,31-34 in parallel to most single-chain surfactants catanionics that are also based on DTAB and SDS or similar anionics.35-38 A decrease in the solubility of the mixture is obtained when compared to that of SDS/DTAB. So far, only a few studies exist on gemini-based catanionics with geminis other than m-s-m39-41 or an ion pair different from SDS.15 Overall, typical findings include the formation of unilamellar vesicles and (17) Zana, R.; Benrraou, M. J. Colloid Interface Sci. 2000, 226, 286–289. (18) Luciani, P.; Bombelli, C.; Colone, M.; Giansanti, L.; Ryhanen, S. J.; Saily, V. M. J.; Mancini, G.; Kinnunen, P. K. J. Biomacromolecules 2007, 8, 1999–2003. (19) Alehyen, S.; Bensejjay, F.; El Achouri, M.; Perez, L.; Infante, M. J. Surfactants Deterg. 2010, 13, 225–231. (20) Bergstr€om, M. Langmuir 2001, 17, 993–998. (21) ud Din, K.; Sheikh, M. S.; Dar, A. A. J. Colloid Interface Sci. 2009, 333, 605–612. (22) Kaler, E.; Murthy, A.; Rodriguez, B.; Zasadzinski, J. Science 1989, 245, 1371–1374. (23) Tondre, C.; Caillet, C. Adv. Colloid Interface Sci. 2001, 93, 115–134. (24) (a) Abecassis, B.; Testard, F.; Arleth, L.; Hansen, S.; Grillo, I.; Zemb, T. Langmuir 2006, 22, 8017–8028. (b) Abecassis, B.; Testard, F.; Arleth, L.; Hansen, S.; Grillo, I.; Zemb, T. Langmuir 2007, 23, 9983–9989. (25) Vlachy, N.; Jagoda-Cwiklik, B.; V
acha, R.; Touraud, D.; Jungwirth, P.; Kunz, W. Adv. Colloid Interface Sci. 2009, 146, 42–47. (26) Dubois, M.; Deme, B.; Gulik-Krzywicki, T.; Dedieu, J.; Vautrin, C.; Desert, S.; Perez, E.; Zemb, T. Nature 2001, 411, 672–675. (27) Zemb, T.; Dubois, M. Aust. J. Chem. 2003, 56, 971–979. (28) Glinel, K.; Dubois, M.; Verbavatz, J.; Sukhorukov, G.; Zemb, T. Langmuir 2004, 20, 8546–8551. (29) Hao, J.; Hoffmann, H. Curr. Opin. Colloid Interface Sci. 2004, 9, 279–293. (30) Marques, E.; Regev, O.; Khan, A.; Lindman, B. Adv. Colloid Interface Sci. 2003, 100, 83–104. (31) Cheon, H. Y.; Jeong, N. H.; Kim, H. U. Bull. Korean Chem. Soc. 2005, 26, 107–114. (32) (a) Wang, Y. J.; Marques, E. F. J. Phys. Chem. B 2006, 110, 1151–1157. (b) Wang, Y. J.; Bai, G. Y.; Marques, E. F.; Yan, H. K. J. Phys. Chem. B 2006, 110, 5294–5300. (c) Wang, Y. J.; Marques, E. F.; Pereira, C. M. Thin Solid Films 2008, 516, 7458–7466. (d) Wang, Y. J.; Marques, E. F. J. Mol. Liq. 2008, 142, 136–142. (33) Shang, Y.; Liu, H.; Hu, Y.; Prausnitz, J. M. Colloids Surf., A 2007, 302, 58– 66. (34) Jurasin, D.; Weber, I.; Filipovic-Vincekovic, N. J. Dispersion Sci. Technol. 2009, 30, 622–633. (35) Kondo, Y.; Uchiyama, H.; Yoshino, N.; Nishiyama, K.; Abe, M. Langmuir 1995, 11, 2380–2384. (36) S€oderman, O.; Herrington, K.; Kaler, E.; Miller, D. Langmuir 1997, 13, 5531–5538. (37) Coldren, B. A.; Warriner, H.; van Zanten, R.; Zasadzinski, J. A. Langmuir 2006, 22, 2474–2481. (38) Coldren, B. A.; Warriner, H.; van Zanten, R.; Zasadzinski, J. A. Langmuir 2006, 22, 2465–2473. (39) Zana, R.; Levy, H.; Danino, D.; Talmon, Y.; Kwetkat, K. Langmuir 1997, 13, 402–408. (40) Stathatos, E.; Lianos, P.; Rakotoaly, R. H.; Laschewsky, A.; Zana, R. J. Colloid Interface Sci. 2000, 227, 476–481. (41) Brito, R. O.; Marques, E. F.; Gomes, P.; Falcao, S.; Soderman, O. J. Phys. Chem. B 2006, 110, 18158–18165.

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fine tuning the phase diagram through the spacer length or the addition of inorganic ions, in particular, when biphasic systems are desired.33 Therefore, in this study we close a gap by comparing the effects of nature and the geometry of the different spacers based on our previous study on SDS þ DTAC.42 While retaining the spacer length as roughly constant, we varied the type of spacer in a systematic fashion. By employing a polar spacer such as EO and various aromatic bridges, we varied in a systematic way the polarity, geometrical constraints, and stacking potential of the headgroups. The aim then is to correlate the structure of the headgroup with its properties in catanionic surfactant mixtures, with a particular emphasis on the spontaneous formation of vesicles.

Materials and Methods Materials. Sodium dodecylsulfate (SDS, BioUltra, g99%) and dodecyltrimethylammonium chloride (DTAC, puriss., g99%) were obtained from Fluka. Monoalkyl sulfates are known to hydrolyze into alcohol and HSO4-, leading to the autocatalysis of the reaction. The presence of dodecanol (thus formed from SDS) is known to influence the phase diagram of SDS significantly, causing discrepancies between results from various authors.43 Therefore, we stored our SDS at low temperature (4
C) and used only freshly prepared aqueous solutions. Sodium chloride (NaCl) was purchased from Roth, and sodium methylsulfate (SMS) was purchased from Aldrich. All materials were used as received. Deuterium oxide (D2O, 99.9%) came from Eurisotop and was used as a solvent for all SANS experiments to enhance the contrast between hydrogenated aggregates and the medium and to lower the incoherent background. For other measurements, Millipore water was used. Synthesis and characterization of the gemini surfactants are detailed elsewhere.8,44-47 The five geminis used in this study are named EO-2, t-B-2, o-X-2, m-X-2, and p-X-2 and correspond to DTAC-based surfactants with the following spacers: CH2OCH2, CHdCH, and the C6H4 aryl group in ortho, meta, and para configurations. The spacer links the two polar headgroups: C 12 H 25 N(CH 3 )2 CH 2 -spacerCH 2 (CH 3 )NC 12 H 25 (Table 1). Sample Preparation. Stock solutions of pure materials (either surfactants or salts) were prepared in volumetric flasks by dissolving a weighed amount of the compound and adjusting the quantity of solvent (the mass of which was measured). The concentration of these stock solutions was adjusted to prepare catanionic mixtures of the required concentration by simply mixing the stock solutions using micropipets. The cationic solution was always added to the SDS solution. The chargerelated molar fraction X of SDS in the surfactant mixture (X = [SDS]/{[SDS] þ n[cationic]}, where n is the valency of the surfactant) is used to characterize the sample compositions. Solutions were vigorously homogenized by vortex mixing and typically prepared within a day before the SANS spectrum acquisition, staying at rest at room temperature (23.0 ( 1.5
C). Concentrations were calculated from masses and densities. Methods. Small-Angle Neutron Scattering. SANS spectra were accumulated on several occasions on V4 of the BER reactor at the Helmholtz-Zentrum Berlin (HZB), Germany. Neutrons were recorded on a 2D gas detector with 128 pixels  128 pixels of (42) Pr
evost, S.; Gradzielski, M. J. Colloid Interface Sci. 2009, 337, 472–84. (43) (a) Kurz, J. L. J. Phys. Chem. 1962, 66, 2239–2246. (b) Motsavage, V. A.; Kostenbauder, H. B. J. Colloid Sci. 1963, 18, 603–615. (44) Laschewsky, A.; Lunkenheimer, K.; Rakotoaly, R. H.; Wattebled, L. Colloid Polym. Sci. 2005, 283, 469–479. (45) Wattebled, L.; Laschewsky, A.; Moussa, A.; Habib-Jiwan, J. L. Langmuir 2006, 22, 2551–2557. (46) Wattebled, L.; Laschewsky, A. Langmuir 2007, 23, 10044–10052. (47) Wattebled, L.; Note, C.; Laschewsky, A. Tenside, Surfactants, Deterg. 2007, 44, 25–33.

DOI: 10.1021/la103976p

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Pr evost et al. Table 1. Compilation of Data on the Pure Geminis8,44, a

compound

spacer

vm (A˚ ) 3

cmc (mM)

σcmc (A˚2)

Γ

intra ˚ dN/N (A)

inter ˚ dN/N (A)

Nfluo ag

σcmc Nag

SDS 410.0 8.2 59 64 93 DTAC 482.0 (1.00) 18.3 88 (1.00) 45.0 9.4 (1.0) 9.4 (1.0) 34 (1.00) 39 949.1 (0.93) 2.2 140 (0.80) 44.9 7.5 (0.8) 8.7 (0.9) 16 (0.92) 37 EO-2 CH2OCH2 t-B-2 CHdCH 919.2 (0.79) 2.0 95 (0.54) 41.4 5.3 (0.6) 7.5 (0.8) 16 (0.90) 112 1002.3 (1.18) 1.2 95 (0.54) 37.0 5.3 (0.6) 7.5 (0.8) 13 (0.74) 133 o-X-2 C6H4 (ortho) 1002.4 (1.18) 1.5 125 (0.71) 42.8 7.0 (0.7) 8.2 (0.9) 11 (0.66) 58 m-X-2 C6H4 (meta) 1002.3 (1.18) 2.1 150 (0.85) 45.0 7.8 (0.8) 9.0 (1.0) 11 (0.61) 34 p-X-2 C6H4 (para) a The spacer is introduced between two DTAC/C12H25N(CH3)2CH2-spacer-CH2N(CH3)2C12H25 molecules. Values in parentheses are relative differences (based on single alkyl chains) compared to DTAC. vm is the molecular apparent aqueous volume obtained from densitometry measurements (the relative difference being calculated only on dissociated headgroups after the subtraction of vm(C 12 H25 ) = 26.99  12 þ 27.21 A˚3 and vm (Cl-) = 21.9 A˚3). cmc is the critical micelle concentration, σcmc is the area per headgroup at the cmc using Gibbs’ law when intra is the distance between ammonium atoms considering each gemini to be two individual molecules, Γ is the surface tension after the cmc, and dN/N estimated on 3D models after energy minimization of the molecule in vacuum with Ghemical software.49 For DTAC, the value corresponds to the inter side of a square of area σcmc(DTAC); dN/N is the distance between ammonium atoms belonging to different molecules, assuming a rectanfluo gular placement and using σcmc and dintra N/N . N ag is the aggregation number as determined by fluorescence. The ratio 3v m /σ cmc gives an expected value for the radius of a spherical micelle on the basis of σ cmc, and Nσagcmc is the corresponding aggregation number. All experimental values are obtained in H 2O.

5  5 mm2. The wavelength was kept at 0.605 nm (fwhm 10.5%). Sample-to-detector distances of 1, 4, and 12 or 16 m were selected to cover a wide q range. The collimation was 4, 8, and either 12 or 16 m, respectively. Samples were kept in quartz cuvettes (Hellma) of QS quality with a path length of 1 or 2 mm and an illuminated area of typically 13 mm diameter. They were placed on a thermostatted sample changer after vigorous mixing to redisperse possibly separated material; experiments were performed at 25
C. Data reduction was performed on 2D patterns using the BerSANS software package from the HZB.48 Raw data were corrected for the scattering of the empty cell; pixel efficiency and solid angle variations were taken into account by dividing by the incoherent scattering pattern of pure water in a 1 mm cuvette; note that this method neglects the deviation in the effective thickness and transmission of samples at high angles and leads to underestimated Porod coefficients at high q. Background noise was accounted for by measurements with cadmium at the sample position. The absolute scale was determined using water as a secondary standard. Reduced data were always isotropic (except in one case for p-X-2 þ SDS, with precipitation going on during the acquisition) and were therefore azimuthally averaged, and spectra from different configurations for each sample were merged with no need for any arbitrary factor. Small-Angle X-ray Scattering. The SAXS measurements were performed on ID2 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The wavelength of the incident photon flux was 0.0995 nm (fwhm 0.02%), and the beam size on the sample was 0.15  0.35 mm2. Two-dimensional SAXS patterns were corrected for electronic noise, transmitted beam intensity, and quartz capillary thickness and were scaled according to predefined calibration measurements. Being isotropic, they were radially averaged. Finally, the scattering by the solvent was subtracted. Note that samples were analyzed in different individual capillaries, and for such weak scatterers, (pure surfactants at low concentration forming micelles) the intensity from the sample itself is far less than the intensity from the sample container at small and large angles, leading to uncertainties at low and high q beyond the statistical error. Densities. Molecular volumes were obtained from density measurements with an Anton-Paar DMA 4500 densitometer, thermostatted at 25.00
C. The densities of aqueous (H2O) solutions of pure compounds at different mass fractions were measured to obtain the partial volumes of the compounds in water by linear regression and extrapolation to 100%; the mass fraction range was chosen so as to cover the range used in this article. Hence, for surfactants it was measured to be between 0.5 and 2.5% (ca. 25 to 100 mM in alkyl chains). The determined volumes were assumed to be independent of the concentration, structural (48) Keiderling, U. Appl. Phys. A: Mater. Sci. Process. 2002, 74, s1455–s1457.

584 DOI: 10.1021/la103976p

behavior, and overall compositions. Data are available in the Supporting Information, with the corresponding scattering length densities (SLD). Model-Free Analysis of Small-Angle Scattering Data. A model-free analysis of the SANS data was systematically performed to extract structural integral parameters from the curves without assuming any geometry of the fluctuations. A Porod approximation (I(q) - Iinc = 2π(ΔSLD)2Σq-4) was performed at high q, leading to the specific area Σ in the solutions and the determination of the incoherent scattering Iinc; a Guinier approximation (I(q) - Iinc = I0 exp(-(qRg)2/3)) was performed at low q to evaluate the radius of gyration Rg of the aggregates and the forward scattering I0. Both approximations were used to extrapolate experimental data toward q = 0 and ¥, R therefore allowing n us to calculate moments of I(q): ÆI(q)qnæ = ¥ 0 (I(q) - Iinc)q dq ~th = 2π2φ(1 - φ) with n = 0, 1, or 2. The theoretical invariant Q ~exp = (ΔSLD)2 was compared to the experimental invariant Q ~exp compared to Q ~th was interÆI(q)q2æ, and a lower value of Q preted as missing material from sedimentation; this was then used to recalculate the volume fraction of dispersed material and obtain together with the Porod law coefficient the area per surfactant (Σ/C) and together with I0 = φV(ΔSLD)2 the volume V of material in the aggregates. This volume was then confirmed ~exp, and finally by calculation of the Porod volume VP ≈ 2π2I0/Q ~ the correlation length lc = πÆI(q)qæ/Q, correlation cross section 2 ~ and intersection length li = 4Q/(2π(ΔSLD) ~ Σ) Ac = 2πÆI(q)æ/Q, were determined. Radii values obtained from these integral structural parameters were consistently ordered as Rg g R(VI0) ≈ R(VP) g R(Ac) g R(lc) g R(li) as expected, given the respective weighting of the data by q, and the spread in these values was interpreted as the presence of two populations of aggregates. Details are given in the Supporting Information. For all calculations, a single-contrast ΔSLD was assumed because SANS is essentially sensitive to hydrogen moieties, yet the solvation of the polar headgroup does influence the parameters (as exemplified for ~exp in the Supporting Information). Q Analytical Fit of the Small-Angle Scattering Data. The intensity scattered by a single population of monodisperse aggregates of volume V at a volume fraction φ having a constant contrast with the solvent of ΔSLD is written as IðqÞ ¼ φVðΔSLDÞ2 PðqÞ SðqÞ þ Iinc

ð1Þ

where P is the form factor with limqf0 P(q) = 1, S is the structure factor with limqf¥ S(q) = 1, and Iinc accounts for the background essentially due to incoherent scattering. The equation is used here for nonspherical populations of different aggregates by a simple sum as an approximation. As in our study the sizes of different aggregates are of different orders of magnitude, we do not expect significant interparticle correlations. Except for pure Langmuir 2011, 27(2), 582–591

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surfactants studied by SANS and SAXS, we consider a singlecontrast object/solvent. Therefore, our final model is expressed as X IðqÞ ¼ ðΔSLDÞ2 φi Vi Pi ðqÞ Si ðqÞ þ Iinc ð2Þ i

with i typically representing triaxial micelles and unilamellar vesicles or large disks (flat bilayers); a Porod law of q-4 is added to account for fluctuations that are too large to be resolved in our q range, and this typically corresponds to meso- to macroscopic domains precipitating. To this fit we also add a Gaussian to account for the smeared Bragg peak sometimes observed at high q. The orientationally averaged form factor of monodisperse triaxial core/shell ellipsoids with a constant shell thickness t and a homogeneous core and shell is Pmic ðqÞ ¼

2 π

Z

π=2 0

Z

π=2

ðFc þ Fsh Þ2 sin j dj dθ

ð3Þ

0

where Fc = Vc(Fsh - Fc) f(qr(a, b, c, j, θ)), Fsh = Vmic(Fsol - Fsh) f(qr(a þ t, b þ t, c þ t, j, θ)), f(x) = 3j1(x)/x (with j1 being the spherical first-order Bessel function j1(x) = (sin x - x cos x)/x2), and r(a, b, c, j, θ) = [(a2 sin2 θ þ b2 cos2 θ) sin2 j þ c2 cos2 j]1/2. a < b e c represents the semiaxes of the ellipsoids; Fc, Fsh, and Fsol are the SLDs of the core, shell, and solvent; and Vc and Vmic are the volumes of the core and of the whole ellipsoid, respectively. When only SANS data are fitted, a single homogeneous SLD is considered and b = c. With pure surfactants, the analytical model proposed by BabaAhmed is used to account for the effect of charges  ! - 1 UðqÞ ð4Þ Scharge ðqÞ ¼ 1 - N C0 ðqÞ kB T with N being the density number of colloids, kB = 1.381  10-23 N m K-1 being the Boltzmann constant, T being the temperature in Kelvin, C being the hard-sphere repulsion, and U being the charge perturbation. The radius and volume fraction entering this model are constrained to be defined by the maximum semiaxis of the micelle and the density used in the form factor. The full model is given in the Supporting Information. Vesicles are fitted with a model of core-shell spheres with a monodisperse shell thickness and an inner radius Ri, with the density number N(R) having a log-normal size distribution  2 Z ¥ 4π ΔSLD Pves ¼ ½ðRþtÞ3 f ðqðR þ tÞÞ - R3 f ðqðRÞÞ2 dR 3 0 ð5Þ with LN(R) = [N/Rσ(2π)1/2 exp{-[ln(R) - μ]2/2σ2} where N is the number density of vesicles of radius R.

Results and Discussion Pure Surfactants. The gemini surfactants are listed in Table 1 together with their characteristics of interest for the present study. The aqueous volume of headgroups of geminis stays within 20% of the volume of two DTAC headgroups. Although the volumes decrease for EO-2 and t-B-2 (both having a small spacer), the identical increases in volume for o-, m-, and p-X-2 do not compensate for the volume of the spacer (vm(xylene) = 203 A˚3) so that in all cases the dimerization induces a significant compression. All Krafft points of cationics except for one (p-X-2) are below 0
C. The cmc is generally decreased by about 1 order of magnitude compared to that of the DTAC monomeric analogue. (49) Hassinen, T.; Perakyla, M. J. Comput. Chem. 2001, 22, 1229–1242.

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The area per surfactant at the air/water interface determined using the Gibbs equation (treating each gemini as two monovalent surfactants) is always reduced compared to that of two DTACs but to significantly different extents depending on the spacer. The trend is expected, in particular, with σ(o-X-2) < σ(mX-2) < σ(p-X-2) [with the reduction in area for p-X-2 being small (-15%)] and σ(o-X-2) = σ(t-B-2), whose geometries are very similar, leading to a strong decrease of nearly 50%. The flexible ethoxy bridge leads to a reduction of the area at the air/water interface by only 20%. Note the excellent agreement between the spacer length as estimated from 3D models and the area at the interface. With a simplified model of the rectangular placement of intra molecules at the interface, one can estimate from σ and dN/N inter (which accounts for intramolecular N/N spacing) the distance dN/N between N atoms belonging to different molecules and therefore an aspect ratio of the geminis, which is respectively 1.9, 1.7, 1.7, 1.9, and 1.9 for EO-2, t-B-2, o-X-2, m-X-2, and p-X-2 (i.e., close to but always less than 2). dN/N,inter values are displayed in Table 1 with the aspect ratio in parentheses. It is interesting that the distance between intermolecular N groups for all geminis is less than that for DTAC. Decreases in intermolecular distances are correlated with the decrease in the surface tension at the cmc. When going from flat curvature at the air/water interface to spontaneous curvature in the bulk, fluorescence quenching45 using 9,10-dimethylanthracene as a probe and 1-n-dodecylpyridinium chloride as a quencher indicates that the area per surfactant in micelles is slightly affected by the spacer. The aggregation numbers for the geminis with respect to the alkyl chains range from 21 to 32 in comparison to 34 for DTAC. It is striking that the aggregation number varies in a very different way than foreseen from the area occupied at the air/water interface, yet the later is derived using the Gibbs equation with the simplistic assumption that each gemini can be seen as two fully dissociated charged surfactants. On the basis of the packing parameter P=νm/σl, considering that the chain length l is constant and assuming that σ in the bulk is proportional to σ at the air/water interface, we would expect the micelle size to decrease for EO-2 and m-X-2 but to increase for p-X-2 and t-B-2 and increase even more for o-X-2. Note that this approximation works for DTAC: assuming that spheres are obtained, σcmc/vm = 3/R, assuming that spheres gives an aggregation number of 39, using for an alkyl chain of i carbon atoms the length lT[A˚]=0.8(1.265i þ 1.5) and the volume ν [A˚3] = 26.99i þ 27.20. The value obtained is close to the experimental value of 34. Here all geminis have smaller aggregation numbers than DTAC. The difference among aggregation numbers for DTAC, EO-2, and t-B-2 is very small, despite an important reduction of interfacial area at the air/water interface for the geminis as compared to that for DTAC. Within the isomeric series of o-, m-, and p-X-2, Nag is not even following the trend expected from σ. The decrease in Nag means that the interfacial area in the micelle systematically increases, even for short spacers that are expected to bring each DTAC block closer. Hence all spacers, even such based on an aromatic ring, must be localized at the hydrophobic/ hydrophilic interface. Surely the packing situation is much more constrained for the geminis than for the single-chain DTAC. Small-Angle Scattering Data. The fluorescence quenching method employed for the determination of the aggregation number requires the addition of a probe molecule and a quencher, which might affect the observations and the precision of the value, especially for micelles with small aggregation numbers for which the relative molar amount of the probe becomes significant.9,50,51 (50) In, M.; Bec, V.; Aguerre-Chariol, O.; Zana, R. Langmuir 2000, 16, 141–148. (51) (a) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2001, 17, 6148– 6154. (b) Zana, R. Langmuir 2002, 18, 7759–7760. (c) Mathias, J. H.; Rosen, M. J.; Davenport, L. Langmuir 2002, 18, 7761–7761.

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Figure 1. SAXS (left) and SANS (right) spectra of SDS (bottom) and DTAC and geminis (top) at 50 mM in terms of charge at 25
C. All spectra are vertically shifted incrementally by 1 order of magnitude for better clarity, with SDS and DTAC being on a nominal scale. For practical reasons, in relation to the high Krafft point of p-X-2 in D2O (close to the experimental temperature of 25
C), this surfactant was not analyzed by SAXS.

Most importantly, it is not able to give any information regarding the shape and size of the aggregates. Therefore, SAXS and SANS measurements were performed on aqueous (D2O) solutions of individual surfactants at 50 mM in alkyl chains (meaning that we employed as stock solutions 50 mM SDS and DTAC and 25 mM gemini surfactant). SAXS probes electronic densities, which are higher for the ionic shell than for the water and the aliphatic core (shell contrast), and SANS probes the hydrogenated moieties, which essentially indicates the core (bulk contrast). As discussed previously,42 the hydration of the shell leaves the tetramethylammonium headgroup essentially transparent for neutrons at high q. SAXS and SANS spectra are displayed in Figure 1. The main difference between the SANS spectra is the weaker structure factor for DTAC, which is actually due to the higher concentration of monomerically dissolved DTAC (2/5 of the total amount), resulting in a smaller volume fraction and a much higher ionic strength that screens the electrostatic repulsion between similarly charged micelles to a much stronger extent. The signal vanishes into the incoherent background at the same q value for all surfactants, at around 2 nm-1, indicating an identical smallest half cross section of the hydrogenated micelles of ca. 2.25 nm (4.49/q). The peak due to the structure factor represents the average distance between micelles and is always around the same position (0.50.6 nm-1). Because the surfactants are at approximately the same volume fraction with the same counterion concentration, we can infer that all micelles have similar sizes. The aggregation number Nag is obtained from this peak position, assuming a cubic micelle organization (Nag=(C - cmc)10-24NA(2π/qpeak)3 with q in nm-1, 586 DOI: 10.1021/la103976p

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C being the concentration in mol dm-3, and NA being Avogadro’s constant). The assumption of a cubic approximation is generally good given that the presence of a visible peak does indicate a very significant repulsion and therefore a rather organized system; the deduced value is therefore affected only by the choice of qpeak, which can be off by 10 to 20% because of the convolution of the structure factor (showing the peak) and the form factor (generally not constant and therefore modifying the apparent position of the peak). The values displayed in Table 2 indicate that the aggregation numbers of geminis stay actually very close to Nag(DTAC). Finally, with the decrease in intensity being similar at low q for all cationics and considering their similar sizes, we can deduce that all bear approximately the same charge. This applies in particular to all of the gemini surfactants. Therefore, the spacer has a pronounced visible effect neither on ion adsorption nor on the micellar structure. Similar to the effect seen in the SANS data, the SAXS intensities vanish at the same q value for all spectra, at ca. 3 nm-1, corresponding to a half cross section of the micelles of ca. 1.5 nm. In combination with the SANS data, we can deduce a shell thickness of around 0.7 nm for all surfactants. Compared to DTAC, the bump due to excess electronic density in the shell is more visible for geminis, though not to the level of SDS. This is particularly true for surfactants containing the benzene moiety, which show a different scattering pattern to that of the other gemini surfactants. This has to do with the fact that the presence of the aromatic ring induces a more pronounced contrast in the shell region. In addition, the local roughness of the micellar interface might be less in the case of the gemini surfactants than for DTAC. Scattering data are fitted with monodisperse triaxial ellipsoids as suggested by Bergstrom and Pedersen for SDS and DTAB,52 with the improvement in the simultaneous fitting of neutron and X-ray data allowing us to obtain more precise values, in particular, for the shell.42 As previously observed by us for DTAC, fits for surfactants with the ammonium headgroup are successful only when the volume (or equivalently the scattering length density) of the headgroup is free; minute deviations from partial volumes calculated by subtracting from the entire surfactant volume the partial volumes of the alkyl chain and the chloride counterion are obtained. This problem does not occur with SDS; the difference in hydration of these two different types of headgroups, as probed by dielectric spectroscopy,53 might be related to our findings. In general, the results of the fits are only mildly satisfying for geminis but excellent for DTAC and good for SDS. This is certainly due to the oversimplified assumption of a core-shell model whereas the scattering length density profile must be more complicated, in particular, when a benzene ring is present. All fits are displayed in the Supporting Information. The obtained parameters have uncertainties that obviously depend on the quality of the fit. When the modeling is satisfying, the values should be better than 5% as both SANS and SAXS are simultaneously fitted in absolute scale. Interestingly, the dimerization does not result in significant changes in the micelles formed at low concentration in water, with the aggregation number staying approximately the same, as similarly concluded before.45 At least for the high curvatures encountered with spherical micelles, the spacer is not causing strong steric constraints. The following results can thus be interpreted mainly in terms of the spacer nature or geometry (favorable curvature). (52) Bergstr€om, M.; Pedersen, J. S. Phys. Chem. Chem. Phys. 1999, 1, 4437– 4446. (53) Buchner, R.; Baar, C.; Fernandez, P.; Schrodle, S.; Kunz, W. J. Mol. Liq. 2005, 118, 179–187.

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Table 2. Parameters Resulting from the Simultaneous Fit of SANS and SAXS Data of the Surfactants at 50 mM in Alkyl Chains in D2Oa surfactant

Nag(peak)

fit quality

Nag(fit)

σi (nm2)

σo (nm2)

a (nm)

b (nm)

c (nm)

t (nm)

h

Z/Nag

SDS 67 þ 73 0.59 1.23 1.4 1.4 2.8 0.8 0.9 0.46 DTAC 38 þþ 47 0.65 1.21 1.2 1.6 1.9 0.6 0.8 0.50 EO-2 39 þ 62 0.60 1.07 1.4 1.4 2.4 0.6 0.7 0.41 t-B-2 42 60 0.66 1.51 1.1 1.5 2.9 0.9 0.9 0.39 o-X-2 54 58 0.63 1.05 1.3 1.4 2.5 0.5 0.6 0.64 m-X-2 40 69 0.60 0.91 1.3 1.6 2.7 0.4 0.5 0.52 p-X-2 39 (SANS only) 47 0.64 1.17 1.4 1.4 1.9 0.5 0.7 0.41 a Nag(peak) is the aggregated number of alkyl chains calculated from the structure peak position assuming a cubic spatial arrangement of micelles. A qualitative appreciation of the fit quality is given. Nag(fit) is the aggregation number resulting from the fit, and σi and σo are the area per alkyl chain at the apolar/polar and polar/solvent interfaces, respectively. a, b, and c are the semiaxes of the ellipsoids, t is the shell thickness, h is the volume fraction of water in the shell, and Z/Nag is the average number of charges per surfactant.

The samples describe hereafter, with salt or an oppositely charged surfactant, were analyzed only by SANS, first because we are interested in the overall size and shape of the aggregates formed, for which SANS is particularly suited because of the single contrast between D2O and the hydrogenated material (as a good approximation), whereas SAXS would essentially yield information about the shell thickness. Second, the analysis of the SAXS data would be even more difficult than in the case of pure surfactants because of less-precise knowledge and constraint over the partial volumes of the headgroups upon mixing, leading to large uncertainties over the scattering length density profile to be used. Effect of Salt on Pure Surfactant. Geminis were tested at 25 mM (50 mM in charges and alkyl chains) for their behavior in the presence of electrolytes, being either classical NaCl with the distinct advantage of corresponding to the counterions of the catanionics (at a maximum concentration of 25 mM for X = 0.5) or a salt mimicking the headgroup of SDS, such as sodium methylsulfate (SMS). All spectra are available in the Supporting Information, and the main points are highlighted here with the comparison of spectra for DTAC and EO-2. Results with other salts are reported elsewhere.46,47 Note that bringing together two headgroups of similar charge naturally leads to strong repulsions and tension on the spacer so that ion pairing on at least one of these headgroups should be extremely favored. For both salts, DTAC is remarkably unaffected except at very large concentrations, with only screened interactions between otherwise unchanged micelles.42 In contrast, all geminis are sensitive to salts. At concentrations of up to 50 mM, EO-2, t-B-2, and m-X-2 are only mildly affected by both salts whereas o-X-2 forms cylindrical micelles with SMS and p-X-2 precipitates. Above 100 mM, geminis with an aromatic spacer either form cylindrical micelles or a precipitate. At 250 mM, while EO-2 and t-B-2 are still forming small micelles with NaCl, cylindrical micelles are obtained with SMS in the case of EO-2, whereas t-B-2 precipitates. Therefore, the salt effect strongly depends on the nature of the ion. SMS has a more pronounced synergistic effect which can be partially explained by the propensity of MeSO4- to adsorb at the interface, leading to charge neutralization starting from low concentration. Indeed, both the sulfate and ammonium groups are considered to be soft ions, and ion pairing is hypothesized as being favored between ions of similar “hardness”.25 Hence, the interaction between all cationic surfactants investigated here, with soft di- or trimethylammonium headgroups, must be involved in stronger pairing with the soft MeSO4- as compared to the harder Cl-. The polar, flexible spacer of EO-2 is electrolyte-resistant. The very short spacer of t-B-2 helps the surfactant to stay soluble in the presence of salt but yields relatively early to flat curvature, meaning that the effect of salt is reduced only by the protection of the spacer from the solvent by the close packing of ammonium headgroups. This is also true for o-X-2. By keeping the same spacer but going to slightly longest effective distances, the salt Langmuir 2011, 27(2), 582–591

Figure 2. SANS spectra for DTAC and EO-2 at 50 mM in alkyl chains in D2O and 25
C, with increasing concentrations of NaCl and SMS. The millimolar concentrations are displayed on the plots.

effect becomes larger: the space between quaternary ammonium headgroups becomes too large to shield the spacer from the outer solvent and the residing charges.

Catanionics Phase Behavior. When geminis are admixed with SDS, a visual change is systematically observed, in contrast to the reference system SDS/DTAC, for which the SDS-rich samples (X g 70%) are monophasic and transparent, with elongated micelles as the only microstructures present.42 All samples except that with DTAC exhibit a phase separation in the form of crystals for cationic-rich samples and a white liquid for SDS-rich mixtures. In all cases, vesicles are seen only on the SDS-rich side, but for EO-2, a bluish aspect is seen for all molar ratios. SDS-rich samples with up to X = 0.5 (included) are bluish as a sign of the presence of large structures, typically vesicles, yet a whitish component is also present. After a few days, a gradient of color will appear, with the upper part of the solution being white and the lower part being bluish (i.e., slow phase separation is taking place). This stays after more than a year, but it should be noted that on such a timescale the presence of dodecanol from SDS hydrolysis is inevitable. o-X-2 is an exception, with the mixture at X=0.9 (and after some time, also X=0.8) being transparent with sedimented crystals. In gemini-rich samples, white birefringent volutes appear immediately as a sign of the quasicrystal lamellar packing of surfactants. After a few days, the samples show a transparent lower phase and a white, birefringent upper phase, and this situation remains unchanged for the full observation time (more than 1 year). One exception here is t-B-2 for which the SDS-rich aspect extends up to X = 0.4. The amount of the milky (SDS-rich) or crystalline (gemini-rich) upper phase is strongly surfactant-dependent and reaches a maximum for p-X-2. When DOI: 10.1021/la103976p

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Figure 3. SANS spectra of SDS/anionics mixtures at 25
C in D2O and the overall surfactant concentration of 50 mM in charge, from X = 1 (pure SDS, front) to X = 0 (pure anionic, back). Spectra are incrementally shifted vertically ( 1010(1 - X)) and horizontally ( 101 - X) for clarity: spectra of pure SDS are on nominal scales. The high-q data for m-X-2 with X = 0.7 are missing.

samples are prepared in D2O, the surfactant-rich phase (being either crystals or a white, milky solution) goes up (creaming). When samples are prepared in H2O, the surfactant-rich phase goes down (sedimentation). Therefore, we can conclude that the density of the catanionic mixtures is between the density of light water (1.0 g cm-3) and the density of heavy water (1.1 g cm-3).

SANS Measurements To gain a deeper understanding of the structures present in the catanionic mixtures, SANS measurements were performed over the whole mixing range of the gemini surfactants and SDS. The spectra are shown in Figure 3. All spectra share a few characteristics: the intensity at low q goes up approximately as q-4 in all cases, but for some mixtures involving o-X-2 or DTAC, the equimolar mixtures with X = 0.5 almost exclusively follow the Porod law (q-4), as for o-X-2. A Bragg peak is always present at or close to equimolarity (even if not immediately visible for o-X-2) as is typically observed with increasing amounts of catanionic salt forming stacked lamellae. The height of the peak decreases in the following order: m-X-2 > p-X-2 > t-B-2 > EO-2 > DTAC > o-X-2. The peak position is converted in real space to obtain the spacing between crystalline planes (d = 2π/q) and follows this increasing order (with distances in nm): p-X-2 (1.62) < m-X-2 (1.65)1000 (0.40) P P P SMS 50 M: 73 (0.71) M: 101 (0.61) M: 126 (0.57) M: 146 (0.61) C: 742 (0.49) P 100 M: 81 (0.69) M: 118 (0.59) C: 736 (0.50) C: 1069 (0.50) C: >1000 (0.48) P 250 M: 97 (0.67) C: 343 (0.50) P P P P 500 M: 96 (0.66) C: >1000 (0.50) P P P P 1000 M: 92 (0.66) P P P P P a M stands for micelles, C stands for cylinders, and P stands for precipitate. The length of long cylinders cannot be determined from the experimental q range (Nag > 1000). NaCl

spectra with molar ratios of SDS/EO-2 1:1 for the EO-2-rich side and 4:1 for the SDS-rich side, corresponding to molar ratios of charges of 1:2 and 2:1. Possible tessellations leading to these ratios are depicted in the table of contents graphic, where stretched hexagonal packing accommodates the larger size of the gemini as compared to that of SDS. Thus, in both cases one-third of the aliphatic chains in the vesicles are bearing one excess charge, despite the fact that catanionic bilayers are sometimes referred to as being globally neutral. This charge is apparently not neutralized by counterions because our fits require a strongly repulsive structure factor for vesicles as apparent in Figure 4, yet the complexity of the spectra and the systems cannot lead to an final answer for the question of the overall charge of the vesicles. The existence of a regular packing of headgroups also implies that the presence of the double-chained gemini would not disturb the packing of (identical) C12 aliphatic tails in the membrane whereas different apolar chains might drive the inner-layer phase separation. Our results are in line with a sometimes-suggested strongly preferential stoichiometry.54-56 However, only recent studies deal precisely with the ordering of ion pairs, typically for very large (flat) structures and in the case of true catanionics (acid/ base mixtures),57 and so far establish the existence of a shortrange liquid order between ion pairs. The formation of monodisperse small vesicles from catanionics with a fixed stoichiometry independent of the overall composition and determined only by the surfactant in excess are reported here for the first time.

Conclusions We studied the aggregation behavior of a series of cationic gemini surfactants in the presence of oppositely charged surfactant SDS by means of phase behavior investigations and SANS experiments. The geminis are based on two DTAC blocks, with the spacer between the two hydrophilic headgroups being varied systematically. The aggregation behavior of the pure surfactants is relatively similar irrespective of the spacer whereas the aggregates formed in the catanionic mixtures vary substantially for the investigated surfactant concentration, which was chosen to be 50 mM. An essential result of this study is that the overall phase diagram is little affected by the dimerization of the surfactant structure, and many characteristics observed with SDS þ DTAC are found here as well, in particular, the larger stability of (54) Dubois, M.; Lizunov, V.; Meister, A.; Gulik-Krzywicki, T.; Verbavatz, J. M.; Perez, E.; Zimmerberg, J.; Zemb, T. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 15082–15087. (55) Antunes, F. E.; Brito, R. O.; Marques, E. F.; Lindman, B.; Miguel, M. J. Phys. Chem. B 2007, 111, 116–123. (56) Gonzalez-Perez, A.; Schmutz, M.; Waton, G.; Romero, M. J.; Krafft, M. P. J. Am. Chem. Soc. 2007, 129, 756–757. (57) Carriere, D.; Belloni, L.; Deme, B.; Dubois, M.; Vautrin, C.; Meister, A.; Zemb, T. Soft Matter 2009, 5, 4983–4990.

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microstructures when the anionic SDS is present in excess but also to a lesser extent the SDS molar ratio where peculiar structures are observed, such as 0.4 and 0.6 (i.e., close to but not at equimolarity) for which compositions most of the mixtures contain vesicles. This is probably directly related to the convenient choice of the spacer lengths, matching the packing geometry of the starting block DTAC relatively well as shown with the relatively unaffected aggregation number (in alkyl chains) for pure surfactants. It is, however, striking that the constraint of bridging together two positively charged headgroups does not significantly alter the aggregation behavior of oppositely charged surfactants, as would be foreseen with a well-ordered lattice repartitioning of charges. The present study highlights the role of the spacer nature and precise geometry, with a comparison among benzene spacers with ortho, meta, and para substitutions. The ortho substitution leads to essentially stable aggregates to the point that the equimolar mixture still contains microstructures and the whole phase diagram finally resembles that of SDS/DTACs. From this, it can be concluded that π stacking does not play a significant role. On the contrary, the meta-isomer to some extent and the para-isomer more strongly form crystals very easily. Clearly, the close packing induced by the ortho geometry is a lesser constraint than the larger spacing due to the meta and para geometries. In addition, the chain packing required for crystal formation is more preferred the closer the chains are bound together by the headgroup. This is due to the exposure of aryl groups to the protic polar solvent, as also seen for the pure surfactants in the presence of salts. The same result can be applied to t-B-2 (ethylene spacer), where the small double bond does not impede the formation of relatively stable vesicles. A very interesting aspect is that for the first time we report the observation of well-defined mixed vesicles with a narrow size distribution and precise composition that is independent from the samples’ overall composition, having an important net charge, in opposition to what is commonly believed for such systems. Such well-defined structures were obtained only with an ethoxy bridge as a spacer, and this gemini furthermore exhibited an excellent resistance to electrolytes. Among five different spacers, the most hydrophilic and most flexible one is the most promising for forming stable vesicular structures in water because of the weak geometrical constraint and the ambivalent nature of the ethoxy, hydrophilic, and not extensively lipophobic. In contrast, the aromatic spacers with their low flexibility and their higher apolarity compared to the same parameters for the ethoxy spacer (favoring a conformation where the spacer is part of the apolar core) generally lead to precipitates. However, all geminis are actually forming colloidally more stable mixtures with SDS than their simple counterpart, DTAC (i.e., the gemini Langmuir 2011, 27(2), 582–591

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architecture renders similar catanionic systems more robust against precipitation). Short spacers are also favorable in general because of the shield formed by the surfactant headgroups isolating the spacer from the solvent. These results can help in the design of tailor-made geminis for precise control of the resulting systems, with short spacers respecting the distance between original blocks and containing hydrophilic groups. Our studies therefore shall be the basis for further chemical variations to add functionality to catanionic structures. Acknowledgment. We are grateful to Ingo Bressler for his contribution to the fitting program Sasfit, Jacek Kozuch for his work on the concentrated geminis, and Theyencheri Narayanan

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for his help with the SAXS experiment at the European Synchrotron Radiation Facility (ESRF, Grenoble, France). The beam time on V4 at the Helmholtz Zentrum Berlin (HZB) has been supported by the European Commission under the 6th Framework Program through the key action Strengthening the European Research Area, Research Infrastructures, contract RII3-CT-2003-505925 (NMI3). Supporting Information Available: Details of the apparent molecular volumes, scattering length densities, model-free invariant analysis, and additional SANS spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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