Micellization and Adsorption Properties of Novel Zwitterionic

A series of novel zwitterionic surfactants each with two hydrophilic and two hydrophobic groups in the molecule (so-called heterogemini surfactants) h...
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Micellization and Adsorption Properties of Novel Zwitterionic Surfactants V. Seredyuk, E. Alami, M. Nyde´n, and K. Holmberg* Department of Applied Surface Chemistry, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden

A. V. Peresypkin and F. M. Menger Department of Chemistry, Emory University, Atlanta Georgia 30322 Received February 5, 2001. In Final Form: June 5, 2001 A series of novel zwitterionic surfactants each with two hydrophilic and two hydrophobic groups in the molecule (so-called heterogemini surfactants) has been synthesized. One of the hydrophilic groups is a phosphodiester anion and the other is a quaternary ammonium salt. Two methylene groups separate the two headgroups. The critical micelle concentration values of the surfactants were determined using du Nouy tensiometry and steady-state fluorescence and are of the order of 10-5 M. Very low surface areas per molecule were observed suggesting that the monolayer formed is extremely tightly packed. NMR self-diffusion measurements gave information about the micelle size distribution. A broad distribution of self-diffusion coefficients was observed and indicated that the time scale of monomer-aggregate and/or aggregate-aggregate exchange is slow compared to the NMR time scale used (100 ms). A mean aggregate size of about 55 nm is obtained for one sample. Adsorption of the gemini surfactants at hydrophilic and at hydrophobized silica was studied by reflectometry. The more symmetrical gemini surfactants gave very low values of surface area per molecule on the hydrophobic surface, indicating a very tight packing of surfactant molecules. At higher surfactant concentrations all gemini surfactant gave very high adsorbed amount, most probably due to formation of aggregates at the surface.

Introduction Surfactants are used in many industrial fields such as detergents, paints, cosmetics, and pharmaceuticals. Surfactant mixtures are commonly utilized in many surfactant formulations and practical applications, because mixtures often behave synergistically and provide more favorable or desirable properties than the individual surfactants.1-6 Zwitterionic surfactants, whose hydrophilic polar heads carry both a positive and a negative charge, are interesting in several respects. The presence of both positively and negatively charged hydrophilic groups in the same molecule leads to the headgroup hydrophilicity being an intermediate between the ionic and conventional nonionic classes.7 Depending on the nature of the polar groups, the zwitterionic surfactants may exhibit pH-dependent behavior, they often display a high foam stability, and they are known to be less irritating to the skin than many * To whom correspondence should be addressed. Chalmers University of Technology, Department of Applied Surface Chemistry, SE-412 96, Go¨teborg, Sweden. E-mail: [email protected]. (1) Holland, P. M.; Rubingh, D. N. Mixed Surfactant systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; p 1. (2) Holland, P. M.; Rubingh, D. N. Mixed Surfactant systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; p 31. (3) Rosen, M. J. Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1986; p 144. (4) Scamehorn, J. F. Phenomena in Mixed Surfactant Systems; Scamehorn, J. F., Ed.; ACS Symposium Series 311; American Chemical Society: Washington, DC, 1986; p 1. (5) Hill, R. M. Mixed Surfactant Systems; Ogino, K., Abe, M., Eds.; Surfactant Science Series 46; Marcel Dekker: New York, 1993; Chapter 11. (6) Rosen, M. J. J. Am. Oil Chem. Soc. 1986, 66, 1840. (7) Laughlin, R. G. Langmuir 1991, 7, 842.

ionic surfactants.8 Because of these useful characteristics, zwitterionic surfactants are often combined with anionic or cationic surfactants in many consumer products, such as cosmetics, health care products, and pharmaceuticals. In recent years, new classes of amphiphilic molecules have emerged and have attracted the attention of various industrial and academic research groups. One of these classes is the “gemini” surfactants, which have two hydrophilic headgroups and two hydrophobic groups per molecule. The hydrophobic groups are connected by a short linker at, or in close vicinity to, the headgroup.9,10 These surfactants allow a new way of controlling the shape of surfactant assemblies. They appear to be better in certain important properties than the corresponding, and more conventional, monomeric surfactants, which are made up of one headgroup and one hydrophobic group with one or more alkyl chains. They tend to have very low critical micelle concentration (cmc),11,12 and they can reduce the surface tension and adsorb at interfaces more efficiently than monomeric surfactants at the same molar or mass concentration.13-15 In this paper we present the micellization process in aqueous solutions, and the interfacial behavior on silica surfaces, of novel zwitterionic surfactants classified as “heterogemini” having two different hydrophilic groups, (8) Tsubone, K.; Uchida, N.; Mimura, K. J. Am. Oil Chem. Soc. 1990, 67, 455. (9) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 113, 1451. (10) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (11) Zana, R.; Benraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (12) Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir 1993, 9, 1465. (13) Zana, R. Curr. Opin. Colloid Interface Sci. 1996, 1, 566. (14) Zana, R. In Novel Surfactants; Holmberg, K., Ed.; Dekker: New York, 1998; p 241. (15) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906.

10.1021/la010182q CCC: $20.00 © 2001 American Chemical Society Published on Web 07/24/2001

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Figure 1. General route for synthesis of the zwitterionic surfactants.

one being negatively charged (a phosphodiester anion) and one positively charged (a quaternary ammonium salt). The two headgroups are separated by two methylene groups. The studies have been performed using tensiometry, fluorescence, NMR self-diffusion, and reflectometry. Materials and Methods Surfactant Synthesis. Figure 1 shows the synthesis route of the novel zwitterionic surfactants each with two hydrophilic and two hydrophobic groups in the molecule. The gemini surfactants were recrystallized from acetonitrile repeatedly. Their structure and purity were checked by 1H NMR, 13C NMR, 31P NMR, HRFAB-MS, and elemental analysis. The details of the synthesis and analysis have been published recently.16 A single 31P NMR peak was a particularly satisfying indication of purity. Cx-PO4--(CH2)2-N+(CH3)2-Cy is used as an abbreviation, where x and y are the number of carbon atoms in the hydrophobic chains. Tensiometry. Equilibrium surface tensions were measured with a SIGMA70 tensiometer (KSV) equipped with a Pt-Ir du Nouy ring. The instrument was calibrated against standard pure liquids and agreement with literature values was typically (0.1 mN m-1. Double distilled water was used for all sample preparations. All measurements were carried out with freshly made solutions at 20.0 ( 0.1 °C. Steady-State Fluorescence. Pyrene I/III ratios report changes in the environment of the probe.17 The high sensitivity of this method facilitates determination of very low critical aggregation concentration (cac) and cmc values. The equipment used for the steady-state fluorescence measurements was a SPEX Fluorolog combined with a SPEX Spectroscopy Laboratory Coordinator DM1B. The fluorescence spectra were measured between 350 and 500 nm with an excitation wavelength at 335 nm. Reflectometry. For studies of adsorption to solid surfaces, a laser giving a plane-polarized light beam was used as light source. In a typical reflectometer, monochromatic light (He-Ne laser, 633 nm) is linearly polarized and passes a 45° glass prism. This beam arrives at the interface with an angle of incidence close to the Brewster angle (θ ) arctan n2/n1) for the solvent/ substrate interface. A silicon wafer was used as substrate, water as solvent (nSi ) 3.8 and nH2O ) 1.33), and the angle of incidence light is around 71°. At this angle, any substance adsorbed at the silicon/water interface, which has a refractive index different from the two media, will change the reflectance. The adsorbed amount is calculated from the equation

Γ)Q

∆S S0

(1)

where Γ is the adsorbed amount (mg/m2), Q is the sensitivity factor (mg/m2), ∆S is the output signal due to adsorption, and S0 is the output signal at t ) 0. (16) Peresypkin, A. V.; Menger, F. M. Org. Lett. 1999, 1, 1347. (17) Kalyanasundaram, K.; Thomas, J. K., J. Am. Chem. Soc. 1977, 99, 2039.

Figure 2. Equilibrium surface tension vs concentration, C, of C14-PO4--(CH2)2-N+(CH3)2-C8 (b) and C8-PO4--(CH2)2N+(CH3)2-C14 (O) at 20 °C. The lines are guides for the eye. Polished silicon wafers, thermally oxidized to produce a SiO2 layer thickness of ∼100 nm and then cut into slides with a width of 12.5 mm, were used. The slides were cleaned in a mixture of NH4OH, H2O2, and H2O followed by cleaning in a mixture of HCl, H2O2, and H2O and then stored in ethanol until used. Just before being placed in the reflectometer cuvette, the slides were cleaned in Milli-Q water. To obtain a hydrophobic surface, the silica plate was modified by dichlorodimethylsilane (DDS). DDS reacts with the silanol groups of the surface, and the resulting -Si(CH3)2 groups are bound covalently to the silicon oxide film forming a top layer of densely packed methyl groups.18 NMR Self-Diffusion. The NMR equipment was a Varian 500 MHz spectrometer with a diffusion probe provided by DOTY Sci capable of providing 0.5T/(mA). In all experiments a stimulated echo was used since T2 was much shorter than T1. The gradient length was 4 ms in all experiments, and in order to minimize the signal loss due to T2 relaxation, the delay time between the gradient pulse and the following 90° radio frequency (rf) pulse was 1 ms. The effective diffusion time (∆) was kept constant at 100 ms, i.e., the time separation between the second and third rf pulse was ca. 95 ms. In all experiments a sine-shaped gradient pulse was used in order to minimize eddy current effects. By variation of the gradient strength in 20 linear steps, the selfdiffusion coefficient was determined by a Levenberg-Marquardt least-squares fitting routine. Surface tension, fluorescence, and reflectometry samples were prepared using double distilled water as solvent. All solutions were clear and homogeneous at the measurement temperature, which was 20 °C when not otherwise indicated. NMR samples were prepared in D2O (Isotec Inc, 99.9%).

Results and Discussion Equilibrium Surface Tensions. Four novel zwitterionic surfactants have been investigated using du Nouy tensiometry. The variation of the surface tension (γ) with the surfactant concentration (C) was determined. The results of surface tension measurements presented in a semilogarithmic representation are shown in Figure 2 for C8-PO4--(CH2)2-N+(CH3)2-C14 and C14-PO4--(CH2)2N+(CH3)2-C8 and in Figure 3 for C10-PO4--(CH2)2-N+(CH3)2-C12 and C12-PO4--(CH2)2-N+(CH3)2-C10. A break is shown at a concentration corresponding to the critical micelle concentration, cmc. The cmc values are presented in Table 1 for all surfactants. According to the Gibbs law (18) Elwing, H.; Welin, S.; Askendal, A.; Nilsson, U.; Lundstro¨m, I. J. Colloid Interface Sci. 1987, 119, 35.

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Table 1. Physicochemical Properties of the Surfactants cmc (M)

area/molecule

surfactant

tensiom

fluoresc

C8-PO4--(CH2)2-N+(CH3)2-C14 C14-PO4--(CH2)2-N+(CH3)2-C8 C12-PO4--(CH2)2-N+(CH3)2-C10 C10-PO4--(CH2)2-N+(CH3)2-C12 C10H21PO4Na2 C12H25N(CH3)3Br C10H21PO4Na2/C12H25N(CH3)3Br

6.0 × 10-6 1.5 × 10-5 2.4 × 10-5 1.5 × 10-5 3.0 × 10-4 5.0 × 10-3 3.1 × 10-4

1.0 × 10-5 1.5 × 10-5 1.0 × 10-5 1.0 × 10-5 4.0 × 10-4

Figure 3. Equilibrium surface tension vs concentration, C, of C12-PO4--(CH2)2-N+(CH3)2-C10 (b) and C10-PO4--(CH2)2N+(CH3)2-C12 (O) at 20 °C. The lines are guides for the eye.

applied to equilibrium systems, the adsorption of surfactant at the gas/liquid interface leads to the reduction in the surface tension of the solution. The surface excess concentration (Γ) and the surface area a per surfactant have been calculated using the Gibbs equation19

Γ)-

1 dγ 2.3nRT d log C

(

)

T

)

1 aNA

(2)

where R is the gas constant, T the temperature in degrees kelvin, and n a constant which depends on the number of species constituting the surfactant and which are adsorbed at the interface. The counterion-free zwitterionic surfactants are considered as neutral molecules, which correspond to n ) 1, and NA is Avogadro’s number. The surface areas per molecule have been calculated, and values of about 30 Å2 have been obtained (see Table 1). These values are much lower than one would expect suggesting that the monolayer formed is closely packed. Low values of the headgroup areas are usually obtained by mixing a cationic and an anionic surfactant. For comparison a mixed surfactant system was studied. In Figure 4, the surface tensions of two surfactants, one negatively (C10H21PO4Na2) and one positively (C12H25N(CH3)3Br) charged, and their mixture at equimolar concentration are presented. This mixture represents the best possible reference to the gemini surfactant C10-PO4--(CH2)2N+(CH3)2-C12. In accordance with literature, the surface area per molecule obtained for the mixture is lower than that of the pure surfactant, Table 1. This is due to a strong (19) Rosen, M. J. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley-Interscience: New York, 1989; pp 65-68.

γ at cmc (mN/m)

adsorbed amt (mol/m2)

at air/water interface (Å2)

at hydrophobic surface (Å2)

27 29 28 26 39 42 33

8.4 × 10-6 5.7 × 10-6 6.1 × 10-6 5.4 × 10-6 1.7 × 10-6 2.9 × 10-6 3.2 × 10-6

20 29 27 31 99 57 52

159 198 44 42

Figure 4. Equilibrium surface tension vs concentration, C, of a cationic (C12H25N(CH3)3Br) (0) and an anionic (C10H21PO4Na2) (O) surfactant and of their mixture (b) at 20 °C. The lines are guides for the eye.

electrostatic interactions between the headgroups.20,21 Note that the headgroup area for the mixture is significantly higher than the headgroup areas obtained for the zwitterionic surfactants. Most likely this is at least partly related to the fact that the two oppositely charged groups are covalently attached and therefore forced to be very close to each other. What may also be important here, as with other zwitterionic surfactants, is that neighboring charges of opposite sign reduce the strength of interaction with water, reducing the hydration sphere and permitting tighter packing. Fluorescence. The micellization of four zwitterionic surfactants was also investigated by steady-state fluorescence using the emission of pyrene (concentration 4 × 10-7 M for each micelle solution sample). Pyrene monomer fluorescence emission is useful for monitoring the selfaggregation in aqueous solution. Because the fluorescence intensities for various vibronic bands in the pyrene monomer fluorescence show strong polarity dependence, pyrene exhibits a characteristic fluorescence emission spectrum consisting of five bands. In polar media the 0-0 band for pyrene molecules is enhanced by a mechanism involving vibronic coupling similar to the Ham effect in the absorption spectra of benzene; thus, the intensity ratio of the first to the third band I/III can be taken as a measure of the polarity of the environment. When surfactant selfassembly takes place, pyrene molecules solubilized in water will be preferentially solubilized in the inner hydrophobic region of the micelles to cause an abrupt (20) Rodakiewicz-Nowak, J. J. Colloid Interface Sci. 1981, 84, 532. (21) Rodakiewicz-Nowak, J. J. Colloid Interface Sci. 1982, 85, 586.

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Figure 5. Variation of the intensity ratio I/III of the pyrene fluorescence spectrum vs concentration, C, in aqueous solutions of C12-PO4--(CH2)2-N+(CH3)2-C10 (b) and C10-PO4--(CH2)2N+(CH3)2-C12 (O) at 20 °C. The lines are guides for the eye.

Figure 7. (a) Signal amplitude decay of the protons at X ) 1.2 ppm in a sample containing 1 mM of C14-PO4--(CH2)2N+(CH3)2-C8 at 20 °C. (b) Size distribution assuming a lognormal distribution of diffusion coefficients. P(D) is the distribution of diffusion coefficients.

suring the spin-echo attenuation obtained during the influence of a pulsed field gradient. For molecules undergoing unhindered Brownian motion, and for single diffusing species, the attenuation of the signal intensities is given by Figure 6. Variation of the intensity ratio I/III of the pyrene fluorescence spectrum vs concentration, C, C14-PO4--(CH2)2N+(CH3)2-C8 (b) and C8-PO4--(CH2)2-N+(CH3)2-C14 (O) at 20 °C. The lines are guides for the eye.

change of the I/III ratio.22-24 Figures 5 and 6 illustrate the I/III ratio variation with the surfactant concentration. This value is useful for obtaining cmc values, because above the cmc the I/III ratio remains fairly constant and independent of the surfactant concentration. The cmc values taken at the inflection point are in accordance with those obtained from surface tension measurements. NMR Self-Diffusion of C14-PO4--(CH2)2N+(CH3)2-C8. The NMR PGSE (pulsed field gradient spin-echo) method was used to determine the selfdiffusion coefficients in aqueous solutions of one of the surfactants, C14-PO4--(CH2)2-N+(CH3)2-C8, at 20 °C. The PGSE method measures molecular motion by mea(22) Zana, R. In Surfactant Solutions: New Methods of Investigation; Zana, R., Ed.; Dekker: New York, 1987; Chapter 5, p 241 and references therein. (23) Thomas, J. K. Acc. Chem. Res. 1977, 10, 133; Chem. Rev. 1980, 80, 283. (24) Bohne, C.; Rednond, R. W.; Scaiano, J. C. In Photochemistry in Organized and Constrained Media; Ramammurthy, V., Ed.; VCH: New York, 1991; Chapter 3.

I exp(-kDi) I0

(3)

When sine-shaped pulsed field gradients are used, k ) (γGδ)2(4∆ - δ)/π2, where γ represents the magnetogyric ratio of the nucleus under observation (in this case 1H) and Di is the self-diffusion coefficient of species i. In mixtures of two or several diffusing species, with wellseparated self-diffusion coefficients, a double or multiexponential decay function can be applied. Figure 7a presents the NMR spin-echo decay obtained for the C14PO4--(CH2)2-N+(CH3)2-C8 sample (∼1 mM). The echo decay is characterized by a curvature when the log of the signal intensity is plotted vs k. This indicates a distribution of micelle sizes. The solid line represents a fit to a lognormal distribution of diffusion coefficients. The corresponding broad distribution of diffusion coefficients is shown in Figure 7b. Most surfactants give a very narrow distribution of diffusion coefficients (P(D)), a sign of a rapid exchange of surfactant monomers between the aggregates and the bulk solution. The fact that a broad distribution of self-diffusion coefficients is observed indicates that the time scale of monomer-aggregate and aggregate-aggregate exchange is slow compared to the NMR time scale

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Figure 8. Adsorption kinetics of C8-PO4--(CH2)2-N+(CH3)2-C14 (left) and C14-PO4--(CH2)2-N+(CH3)2-C8 (right) at hydrophilic and hydrophobized silica at 20 °C.

Figure 9. Adsorption kinetics of C10-PO4--(CH2)2-N+(CH3)2-C12 (left) and C12-PO4--(CH2)2-N+(CH3)2-C10 (right) at hydrophilic and hydrophobized silica at 20 °C.

Figure 10. Effect of surfactant concentration on the adsorption of C14-PO4--(CH2)2-N+(CH3)2-C8 at hydrophilic and hydrophobized silica at 20 °C.

used (100 ms). Assuming spherical structure, with a diffusion coefficient of about 4 × 10-12 m2/s, a mean aggregate size of about 55 nm is obtained for the sample using the Stokes-Einstein relation (assuming infinite dilution). Preliminary studies using cryo-transmission electron microscopy indicates the presence of aggregates of 40-50 nm size (results to be published separately). The morphology of the aggregates is unknown at present, however. The aggregates may well be vesicle-like rather

than micellar in character. In addition we note that there appear to be slow exchange dynamics between aggregates, an effect which is somewhat puzzling. The rationale for interpreting the results in terms of a distribution of aggregate sizes is that the functional form of the echo attenuation was seen to fit perfect to a log-normal distribution. However, it is important to note that when the exchange between monomers and aggregates and/or between aggregates occurs in an intermediate time scale,

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the two events may give the same functional form of the echo attenuation. By variation of the effective time scale in the NMR experiment, the type of exchange occurring might be revealed. However, these measurements are nontrivial due to experimental difficulties both in terms of available field gradient strength and in terms of signalto-noise ratio. Reflectometry. We have used reflectometry to study the adsorption of the gemini surfactants on two different substrates, hydrophilic and hydrophobically modified silica. All measurements were performed in equilibrium with bulk micelles above the critical micelle concentration, cmc, and at room temperature. The hydrophobic surface was composed of silica that had been made hydrophobic by reaction with dichlorodimethylsilane, DDS. For this hydrophobic DDS-silica, most of the silanol groups have been eliminated and the substrate is covered by methyl groups. It should also be noted that hydrophilic silica contains silanol groups that may interact with the surfactant headgroups by weak hydrogen bonding. A typical time dependence of adsorption of the zwitterionic surfactants is shown in Figure 8 for C14-PO4-(CH2)2-N+(CH3)2-C8 and C8-PO4--(CH2)2-N+(CH3)2C14. Indeed, since the silica surface is hydrophilic, the primary driving force for adsorption should be the interaction between the surfactant polar headgroup and the surface. As the figure shows, the adsorbed amount on hydrophilic surface is higher than that on the hydrophobic surface. Similar behavior has been obtained for C10PO4--(CH2)2-N+(CH3)2-C12 and C12-PO4--(CH2)2N+(CH3)2-C10, Figure 9. Note that the adsorbed amount for the pair C10-PO4--(CH2)2-N+(CH3)2-C12 and C12PO4--(CH2)2-N+(CH3)2-C10 is higher than that for C14PO4--(CH2)2-N+(CH3)2-C8 and C8-PO4--(CH2)2-N+(CH3)2-C14 on both surfaces, hydrophilic and hydrophobic. This might be due to the effect of the relatively larger difference between the two hydrocarbon chains of the latter surfactant pair. This unsymmetry may be unfavorable for efficient packing at planar surfaces. A higher amount of adsorbed surfactant on hydrohilic than on hydrophobic surfaces has been observed before for nonionic and zwitterionic surfactants. At hydrophilic silica, aggregates in the form of either continuous bilayer structures or globular aggregates are formed, while surfactant monolayers are formed at a hydrophobized silica surface.25,26 Figure 10 shows the effect of concentration on the adsorption of C14-PO4--(CH2)2-N+(CH3)2-C8 at hydrophilic and hydrophobic surfaces. As can be seen, there is a steep increase in adsorbed amount with increasing surfactant concentration. The same trend was obtained for the other geminis. This must be due to some kind of surface aggregation, and the issue will be further dealt with in a forthcoming publication. In Table 1 data of the apparent surface area per molecule obtained on hydrophobic surfaces at the lower concentration of surfactant (around the cmc) are summarized for the four zwitterionic surfactants. One may first observe that whereas the two more unsymmetrical gemini surfactants, C8-PO4--(CH2)2-N+(CH3)2-C14 and C14-PO4-(CH2)2-N+(CH3)2-C8, give high values of area per molecule, the more symmetrical species, C12-PO4--(CH2)2N+(CH3)2-C10 and C10-PO4--(CH2)2-N+(CH3)2-C12, give very low values, 44 and 42 Å2, respectively. The latter values reflect an extremely densely packed monolayer. (25) Tiberg, F. J. Chem. Soc., Faraday Trans. 1996, 92, 531. (26) Grant, L. M., Tiberg, F., Ducker, W. A., J. Phys. Chem. B 1998, 102, 4288-4294.

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At higher surfactant concentrations, the adsorbed amount increases drastically, giving even lower values of the area per molecule. This occurs both for the symmetrical and for the unsymmetrical geminis, and at this stage some kind of surface aggregation must occur. Conclusion The most important result from the present work is that the zwitterionic gemini surfactants show a strong tendency to self-assemble at very low concentrations and give low surface tensions. Another important result is that measurements of adsorption at a solid surface and of surface tension at varying surfactant concentration show that the geminis give extremely low values of area per molecule at the solid-liquid and air-liquid interfaces. Even if these surfactants, with two oppositely charged headgroups and two linear hydrocarbon tails, can be expected to pack efficiently at interfaces, the values obtained are lower than reasonable for a monolayer. The values obtained at the air-water interface, calculated from Gibbs adsorption equation, are particularly striking. The four geminis all show values between 20 and 31 Å2 per molecule. The mixture of anionic and cationic surfactants used as reference gave a value of 52 Å2. There are two differences between the geminis and the reference. First, whereas the geminis are counterion-free, the mixture contains counterions, Na+ and Br-. Separate tests with addition of this small amount of NaBr added to one of the geminis did not markedly change the surface tension curve, indicating that the lack of counterions are not responsible for the unusual packing values. Second, in the gemini surfactants the two halves are connected by a short bridge while in the mixture the entities are held together by electrostatic attraction. The covalent bond will of course keep the individual halves of the gemini surfactants close to each other, but this does not automatically mean an extremely close alignment of the individual gemini surfactants. The geminis, having 22 carbon atoms in their hydrophobic tails, are on the point of being water soluble. A large adsorption on solid surfaces could therefore be due to irreversible heterocoagulation instead of reversible adsorption that is governed by the law of mass action. As mentioned above, we have seen that at higher surfactant concentration the adsorbed amount of gemini surfactant increases drastically. Most probably, this is due to some kind of aggregation at the surface. However, it is more difficult to explain the low values of area per molecule obtained at the air-water interface by a surfactant aggregation. The values of area per molecule at the surface reflects behavior at very low concentration, below the cmc (which is low for the geminis). Surfactant aggregation under these conditions would be very unexpected. Nevertheless, we have no other explanation to the low values of area per surfactant recorded at this interface than that they must involve some kind of surface-induced aggregation. We plan to investigate this further by neutron reflectivity measurements. We also plan to study packing at the hydrophobic solid-water interface by ellipsometry. Acknowledgment. We thank the Swedish Institute for a grant to V.S. The Competence Center for Surfactants from Natural Products is acknowledged for support for E.A. A.V.P. and F.M.M. were supported by the National Institutes of Health. The Swedish NMR Center in Go¨teborg is acknowledged for instrument time. LA010182Q