Molecular Aggregates of Partially Fluorinated Quaternary Ammonium

Sep 8, 2007 - Keisuke Matsuoka,*,† Tomokazu Yoshimura,‡ Takashi Shikimoto,† Juri Hamada,†. Mika Yamawaki,† Chikako Honda,† and Kazutoyo En...
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Langmuir 2007, 23, 10990-10994

Molecular Aggregates of Partially Fluorinated Quaternary Ammonium Salt Gemini Surfactants Keisuke Matsuoka,*,† Tomokazu Yoshimura,‡ Takashi Shikimoto,† Juri Hamada,† Mika Yamawaki,† Chikako Honda,† and Kazutoyo Endo† Department of Physical Chemistry, Showa Pharmaceutical UniVersity, Higashi-Tamagawagakuen 3-3165, Machida, Tokyo 194-8543, Japan and Graduate School of Humanities and Sciences, Nara Women’s UniVersity, Kitauoyanishi-machi, Nara 630-8506, Japan ReceiVed May 25, 2007. In Final Form: July 12, 2007 The size and shape of novel partially fluorinated gemini surfactant 1,2-bis[dimethyl-(3-perfluoroalkyl-2hydroxypropyl)ammonium]ethane bromide (CnFC3-2-C3CnF, where n ) 4, 6, and 8) were investigated in aqueous solution by means of light scattering and transmission electron microscopy (TEM). The sizes of these molecular aggregates changed with increasing carbon number of the alkyl chain and concentration. For example, the apparent hydrodynamic radius by dynamic light scattering was 18 nm at a concentration of cmc × 5 for n ) 4, 115 nm at the cmc × 15 for n ) 6, and 62 nm at the cmc × 30 for n ) 8, at 298.2 K. The shapes of CnFC3-2-C3CnF aggregates drastically changed with the alkyl chain length; the aggregates were mainly in the form of large or irregular small aggregates (n ) 4), string-like aggregates (n ) 6), and vesicles (n ) 8). The bromide-ion activity was measured using a bromide-ion-selective electrode to determine the degree of counterion binding to the aggregates. The degree of counterion binding to aggregate was very small compared with that in the typical hydrogenated gemini surfactants. These results indicated that the small curvature of large aggregates was not influenced by an electrostatic repulsion between the cationic head groups in the case of the bulky molecular volume of fluorinated gemini surfactants.

Introduction Gemini surfactants are novel surfactants that consist of two polar head groups and two hydrocarbons connected by a spacer group.1 Gemini surfactants, in comparison to the corresponding monomeric surfactants, have a greater ability to lower the critical micelle concentration (cmc) and surface tension.1 Moreover, the size and structure of aggregates can be easily controlled by changing the length of the spacer2-4 or hydrophobic groups.5,6 Recently, a partially fluorinated gemini surfactant has been reported; the properties of its aggregates were considerably different from those of typical hydrogenated gemini surfactants.7,8 For example, C12H25N+(CH3)2-(CH2)n-N+(CH3)2C12H25Br-2 (n ) 2-10) formed small micelles (aggregation number ) 20100) in an aqueous solution regardless of the spacer chain length;2 on the other hand, the TEM micrograph of C8F17N+(CH3)2(CH2)2-N+(CH3)2C8F17 Br-2 showed unilamellar vesicles with a diameter of 15-200 nm.7 A fluorocarbon chain is stiffer than a hydrocarbon chain because of bulky fluorine atoms. Generally, the rigid structure of a fluorocarbon chain leads to formation of * To whom correspondence should be addressed. Phone: +81-42-7211566. Fax: +81-42-721-1565. E-mail: [email protected]. † Showa Pharmaceutical University. ‡ Nara Women’s University. (1) Zana, R. J. Colloid Interface Sci. 2002, 248, 203. (2) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (3) Li, Y.; Li, P.; Dong, C.; Wang, X.; Wang, Y.; Yan, H.; Thomas, R. K. Langmuir 2006, 22, 42. (4) Wang, X.; Wang, J.; Wang, Y.; Yan, H.; Li, P.; Thomas, R. K. Langmuir 2004, 20, 53. (5) Oda, R.; Huc, I.; Homo, J. C.; Heinrich, B.; Schmutz, M.; Candau, S. Langmuir 1999, 15, 2384. (6) Li, Y.; Li, P.; Wang, J.; Wang, Y.; Yan, H.; Thomas, R. K. Langmuir 2005, 21, 703. (7) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 9759. (8) Asakawa, T.; Okada, T.; Hayasaka, T.; Kuwamoto, K.; Ohta, A.; Miyagishi, S. Langmuir 2006, 22, 6053.

aggregates with lesser curvature or elongation, i.e., vesicular, lamellar, and threadlike micelles.9,10 Owing to the difficulties in the synthesis of fluorinated gemini surfactants, reports on the relationship between the alkyl chain length and micellization in them are rare in comparison to those on hydrocarbon surfactants.9,10 In this study, three cationic homologues of a novel partially fluorinated quaternary ammonium salt gemini surfactant 1,2-bis[dimethyl-(3-perfluoroalkyl-2hydroxypropyl)ammonium]ethane bromide (CnFC3-2-C3CnF, where n represents fluorocarbon chain lengths of 4, 6, and 8) were used.11 The molecular structure of CnFC3-2-C3CnF is shown in Figure 1. The synthesis, surface tension, and cmc of these surfactants have been reported in previous papers.11 The aim of the present study is to estimate the aggregates of size and shape from light scattering and TEM by changing the alkyl chain length of the surfactants. In addition, the concentration of free bromide counterion was measured using an ion-selective electrode to estimate the degree of counterion binding to the aggregates. Experimental Section Materials. The reagents 1,2-bis[dimethyl-(3-perfluorobutyl-2hydroxypropyl)ammonium]ethane bromide (C4FC3-2-C3C4F), 1,2bis[dimethyl-(3-perfluorohexyl-2-hydroxypropyl)ammonium]ethane bromide (C6FC3-2-C3C6F), and 1,2-bis[dimethyl-(3perfluoroocthy-2-hydroxypropyl)ammonium]ethane bromide (C8FC32-C3C8F) were the same as previously reported.11 The purities of these amphiphiles were tested by elemental analysis and 1H- and 19F-NMR measurements; no impurities were detected in them. Sodium bromide (99.5%), ammonium nitrate (99%), and agar powder from Wako Pure Chemical Industries (Osaka, Japan) were used without further purification. Preparation of Solutions. Ion-exchanged water was distilled once and then used for preparation of all the aqueous solutions. The (9) Kissa, E. In Fluorinated Surfactants and Repellents, 2nd ed.; Marcel Dekker: New York, 2001; Chapters 2 and 7. (10) Matsuoka, K.; Moroi, Y. Curr. Opin. Colloid Interface Sci. 2003, 8, 227. (11) Yoshimura, T.; Ohno, A.; Esumi, K. Langmuir 2006, 22, 4643.

10.1021/la701525c CCC: $37.00 © 2007 American Chemical Society Published on Web 09/08/2007

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Figure 1. Chemical structure of 1,2-bis[dimethyl-(3-perfluoroalkyl2-hydroxypropyl)ammonium]ethane bromides; CnFC3-2-C3CnF (n ) 4, 6, and 8). solutions were kept under agitation using a magnetic stirrer and incubated at 333 K for more than 1 day. This process can reduce the time required for attaining the aggregation equilibrium. The hydrodynamic radii of these aggregates remained unchanged for 0.5 years. Light Scattering. The dynamic light scattering (DLS) measurements were performed using a laser light scattering photometer (ALV5000, Germany) at 90°. The light source was a 200 mW Nd:YAG semiconductor laser with a wavelength of 532 nm. All sample solutions were filtrated through a membrane filter with a pore size of 0.8 µm (Millipore, MILLEX-AA). The filtered solution was then poured into a cylindrical cell of 15 mm diameter; the cell was in contact with circulating water thermostated at 298.2 ( 0.3 K. The sample solutions of more than 3-5 duplications were repeatedly measured 3-10 times within 1 week after preparation of the solutions. The average of the apparent hydrodynamic radii (aprh) was obtained by analyzing the normalized second-order exponential autocorrelation functions according to the method of cumulants. The accumulations were performed over 50 times. The radius of gyration (rg) was determined by static light scattering using the one concentration method.12 The scattered light intensity between 45° and 135° from solutions was analyzed by the below equation

(

)

16π2n˜ 02 2 2 θ K(c - cmc) 1 1+ rg sin ) Mw 2 (Rθ - R°θ) 3λ2

(1)

where K is the optical density, c is the total surfactant concentration, Rθ and Rθ0 are the reduced light intensity of micellar and monomer solution, respectively, Mw is the molecular weight of micelles, λ is polarized incident light of wavelength, and n˜ 0 is the solvent refractive index. Hence, the mean-square radius of gyration, rg2 can be evaluated from the slope of the plot K(c - cmc)/(Rθ - R°θ) against sin2(θ/2) by first-order least-squares method. TEM Observation. A droplet of the surfactant solution was placed on a carbon-coated grid for 2 min. The excess liquid was removed by touching one end of the grid with filter paper. After the grid was partially dried, a drop of a staining solution (2% uranyl acetate) was placed on the grid for 2 min. The excess liquid was removed by filter paper, and the grid was dried at room temperature. All micrographs were obtained by a JEOL JEM-2000FX operated at 200 kV. Electrode Potential Measurement. The measurements were performed for the counterion (Br-) using a voltmeter (TOA-DKK, HM-30V), where a Br--selective electrode (TOA-DKK, Br-125) and a reference electrode (TOA-DKK, HS-305DS) were indirectly connected to the sample solution by a 5% agar bridge with 10% NH4NO3 as shown in Figure 2. The electromotive force (∆E) of the cell was measured after each stepwise introduction of an aliquot of the concentrated stock solution of a surfactant into the sample solution, (12) Brown, W. Light scattering Principles and DeVelopment; Oxford University Press Inc.: New York, 1996; Chapter 7.

Figure 2. Schematic diagram of the apparatus for determining the counterion (Br-) concentration. where the temperature of the sample was kept constant at 298.2 ( 0.3 K and the reference was a 10% NH4NO3 solution. For this cell, the Nernst equation was valid over the concentration range from 10-6 to 10-1 M when NaBr was used as the solute; the activity coefficient was determined by the Debye-Hu¨ckel equation.13

Results and Discussion Size and Shape of Aggregates. The cmc values of CnFC32-C3CnF (n ) 4, 6, and 8) at 298.2 K as determined from the surface tension-concentration plot are as follows: 11.3 (n ) 4), 0.172 (n ) 6), and 0.00593 mM (n ) 8).11 In these experiments, the surface tension of C8FC3-2-C3C8F was extremely small, corresponding to 13.7 mN m-1 above the cmc.11 Generally, fluorinated surfactants have a tendency to from aggregates with lesser curvature, and these aggregates are very sensitive to the surfactant concentration and alkyl chain length.14,15 Since the refractive indices of fluorocarbons are very close to that of water, a variable high laser power of 200 mW (532 nm light source) was employed to determine the apparent hydrodynamic radii (aprh) and the gyration of radii (rg). The light scattering method is useful for the large aggregates or relatively high concentration solution due to the scattering detection limit. For CnFC3-2-C3CnF (n ) 4, 6, and 8), the changes in aprh with the logarithm of concentration at 298.2 K are illustrated in Figure 3, where the respective error bars were derived from standard deviations of the measurements performed for 3-5 samples. Each gyration of radius (rg) was analyzed by plots of K(c - cmc)/(Rθ - R°θ) vs sin 2(θ/2) using eq 1. The simple linear plots were omitted here, but all correlation coefficients for linear regression were more than 0.98. The three surfactants of rg at their representative concentrations are plotted together in Figure 3. The shape of aggregate can be analyzed by comparing the dependence of rg/rh with theoretical values.14-17 The theoretical rg/rh value was estimated by the geometrical formulas, and the result was as follows: sphere (0.8), oblate ellipsoid (ca.1), and rigid rod or prolate oblate (1.2-2.5).14-17 Moreover, the authors attempted to visualize the micellar size and shape of the above amphiphiles by TEM. The TEM images for the above three surfactants are shown in Figure 4; the letters (a-g) in this figure correspond to those in Figure 3. (13) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions; Butterworth: London, 1970; Chapter 9. (14) Matsuoka, K.; Yonekawa, A.; Ishii, M.; Honda, C.; Endo, K.; Moroi, Y.; Abe, Y.; Tamura, T. Colloid Polym. Sci. 2006, 285, 323. (15) Matsuoka, K.; Ishii, M.; Yonekawa, A.; Honda, C.; Endo, K.; Moroi, Y.; Abe, Y.; Tamura, T. Bull. Chem. Soc. Jpn. 2007, 80, 1129. (16) Young, C. Y.; Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1978, 82, 1375. (17) Van de Sande, W.; Persoons, A. J. Phys. Chem. 1985, 89, 404.

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Figure 3. Change of apparent hydrodynamic radius (rh) and gyration of radius (rg) with the logarithmic concentration of CnFC3-2-C3CnF (n ) 4, 6, and 8) at 298.2 K. The arrows indicate the cmc of each surfactant. The letters (a-g) in this figure correspond to those in Figure 4.

Figure 4. TEM image of CnFC3-2-C3CnF (n ) 4, 6, and 8) aggregates: C4FC3-2-C3C4F at (a) 33.9 mM (cmc × 3) and (b) 56.5 mM (cmc × 5), C6FC3-2-C3C6F at (c) 0.86 mM (cmc × 5) and (d) 2.6 mM (cmc × 15), and C8FC3-2-C3C8F at (e) 0.09 mM (cmc × 15) and (f and g) 0.24 mM (cmc × 40). The arrows in micrograph d indicate the large aggregate.

The C4FC3-2-C3C4F solution could be measured over a wide concentration rangesfrom 22.6 (cmc × 2) to 67.8 mM (cmc × 6)sby DLS. With an increasing surfactant concentration, a rapid decrease in aprh is observed (Figure 3). The relatively large aprh of 300 nm at 22.6 mM rapidly drops to ca. 20-30 nm after passing through a minimum at 45 mM (cmc × 4). Such a behaviorsdecrease in aprh with increasing concentrationsis quite unique; it indicates a change in the structure. The abrupt change

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of rg was also observed and overlapped with rh as shown in Figure 3. The ratio rg/rh is changed from 1.1 to 1.3 with increasing concentration. The values on the basis of theoretical estimation indicated that the shape of aggregates changed from oblate ellipsoid to prolate ellipsoid. The shape of C4FC3-2-C3C4F aggregates in a relatively dilute solution (33.9 mM) is ellipsoid shape of large aggregate, as shown in the TEM micrograph in Figure 4a. On the other hand, the micrograph in Figure 4b for a relatively concentrated solution (56.5 mM) indicates the existence of irregular small aggregates that resemble distorted bulky aggregates. The aprh values obtained from DLS almost agreed with the aggregate sizes determined from TEM micrographs. The primary large aggregate perfectly changed to secondary small aggregates, depending on the concentration. The authors assume that changes of the C4FC3-2-C3C4F shapes with concentration originate from the delicate critical balance between fluorocarbon and hydrocarbon length in the hydrophobic group. In general, the aggregates of fluorinated amphiphiles have a tendency to form a structure with less surface curvature, whereas hydrogenated surfactants form a relative small condensed structure. In this case, the relatively dilute solution is preferentially under the control of fluorinated properties, whereas the concentrated solution is dominated by hydrogenated properties. However, the detailed mechanism remains unknown. C6FC3-2-C3C6F forms string-like aggregates above the cmc as shown in Figure 4c, which was obtained in a 0.86 mM solution (cmc × 5). As Figure 3 indicates, aprh of the string-like aggregates monotonously increases with concentration; its DLS value increases from 70 (cmc × 7) to 120 nm (cmc × 15). aprh is a rough estimate of the aggregate shape because the derived parameter is based on the hard-sphere model. The geometrical expectation of rg/rh at 1.5 mM showed a rigid rod shape in Figure 3, which was in good agreement with the TEM micrograph. Moreover, vesicles are also observed in Figure 4d along with the string-like aggregates at a concentration of 2.6 mM (cmc × 15). The solid white circles represent large vesicles that have diameters of less than approximately 200 nm. On the other hand, characteristic curved lines represent the string-like aggregates with ring and branched shapes. Figure 4e shows the C8FC3-2-C3C8F aggregates at a concentration of 0.09 mM (cmc × 15). These aggregates are vesicles with a water phase inside, as indicated by the black area of the circle. Figure 3 illustrates an increase in aprh with concentration for C8FC3-2-C3C8F; aprh ranged from 20 (cmc × 15) to 90 nm (cmc × 50) at 298.2 K. In addition, the ratio of rg (92 nm) to rh (82 nm) at cmc × 40 indicates that C8FC3-2-C3C8F forms an oblate ellipsoidal shape of aggregates. The sizes of aggregates in the TEM observations are consistent with the radii obtained from the DLS data, as shown in Figure 3. As Figure 4f shows, the aggregates of C8FC3-2-C3C8F remain as vesicles at relatively higher concentrations (0.24 mM, cmc × 40) at which large and small aggregates exist in a polydisperse state. Moreover, giant vesicles with a size of micrometer order are rarely observed in solution, as shown in Figure 4g. According to the report by Oda et al., an analogous molecular structure, which is one hydrocarbon methylene group longer than the present surfactant, C8FC4-2C4C8F, formed large and small unilamellar vesicles whose diameter changed from 20 to 250 nm at 4 mM.7 The polydisperse state of aggregates in their micrograph is also similar to that in our micrograph in Figure 4f. Comparing C8FC4-2-C4C8F and C8FC3-2-C3C8F, it is clear that the size and shape of aggregates remained nearly unaffected by the additional methylene group in the hydrogenated alkyl chain. On the other hand, 12-2-12 surfactants form long, threadlike, and entangled micelles even

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Figure 5. Change in electrode potential (∆E) with Br- activity for NaBr and CnFC3-2-C3CnF (n ) 4, 6, and 8) solutions at 298.2 K. The Nernst slope for NaBr solutions is 57 mV equiv-1.

at a low concentration, as revealed by TEM.18 Moreover, Danino et al. reported that hydrogenated gemini surfactants formed threadlike micelles in a 1.3% solution; further, the aggregation numbers of the homologous surfactants 12-s-12 (s ) 3-8) were less than 100 over a wide range of concentrations.2 From the above discussion, it follows that micellization of CnFC3-2C3CnF (n ) 4, 6, and 8) leads to formation of larger aggregates and that, in comparison to the corresponding hydrogenated surfactants, it exhibits a greater sensitivity to the alkyl chain length for aggregate growth. Extension of the rigid fluorocarbon chain length transforms the shapes of CnFC3-2-C3CnF aggregates to large or small aggregates, string-like ones, and vesicles. A similar result has been reported for the single fluorinated surfactants with alkyl chain lengths, namely, Cn-1F2n-1CH2N+ (CH3)3Cl- (n ) 8, 10, 12, and 14), where the aggregates were mostly disc-like or ellipsoid-shaped lamella that exhibited an exponential growth in their size from 10 (n ) 8) to 500 nm (n ) 14).14,15 In a recent study, Asakawa et al. reported the micellization of partially fluorinated gemini surfactants that contain a six-methylene spacer chain and are referred to as C4FC66-C6C4F, C6FC3-6-C3C6F, and 12-6-12 in our abbreviation rule.8 These surfactants formed small aggregates with aggregation numbers of 5.6, 6.7, and 22.2, respectively. In the case of fluorinated gemini surfactants with a long spacer chain, the structures are not easily influenced by the hydrophobic fluorinated alkyl chain. Degree of Counterion Binding to Micelle. An electromotive force using an ion-selective electrode is well suited for measuring the activity of ionic species. The relation between the electrode potential (∆E) and the activity of bromide ion (aBr-) obeys the Nernst equation

∆E ) -

RT log(aBr-) + constant F

(2)

where F is the Faraday constant and the ideal slope (RT/F) is 59.2 mV equiv-1 at 298.2 K.13 First, the linearity was checked using NaBr solution, and the Nernst slope was confirmed to hold well over the measurement concentration range (Figure 5): 57 mV equiv-1 over the detectable concentration range. Figure 5 illustrates the relationship between the electromotive force (∆E) and the activity of bromide ion for CnFC3-2-C3CnF (n ) 4, 6, and (18) Zana, R.; Talmon, Y. Nature 1993, 362, 228.

Figure 6. Change in Br- concentration (left axis) and degree of counterion binding to aggregates (m/n) (right axis) with the total CnFC3-2-C3CnF (n ) 4, 6, and 8) concentration at 298.2 K. The dashed line indicates twice the concentration of CnFC3-2-C3CnF: (a) C4FC3-2-C3C4F, (b) C6FC3-2-C3C6F, and (c) C8FC3-2-C3C8F.

8) solutions from which the Br- concentration is calculated by extrapolating the linear relationship of ∆E with the logarithm of the activity below the cmc. As a result, good linearity was observed for the present amphiphiles solutions below the cmc, and the Nernst slope was 54 mV equiv-1 for C4FC3-2-C3C4F and 59 mV equiv-1 for C6FC3-2-C3C6F and C8FC3-2-C3C8F. As shown in Figure 5, the linear relationship between ∆E and log aBr- holds good up to the cmc for the above solutions, and then the plots deviate remarkably from linearity. The linearity can be used to estimate [Br-] using a gap from linearity. The obtained [Br-] is plotted as a function of the surfactant concentration in Figure 6: (a) C4FC3-2-C3C4F, (b) C6FC3-2-C3C6F, and (c) C8FC3-2-C3C8F. As revealed by the TEM micrographs in Figure 4, the aggregates were polydisperse for CnFC3-2-C3CnF (n ) 4, 6, and 8) solutions. Therefore, the obtained electrode potential (∆E) reflects the total contribution of polydisperse aggregates, and it cannot be separated for each component of the aggregates. The

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following equation represents the equilibrium between counterion and surfactant ions

nS2+ + mBr- T M2n-m n

(3)

where m is the average number of bound counterions per micelle and n is the average aggregation number. Moreover, S2+, Br-, and Mn represent, respectively, a surfactant ion, counterion, and micelle with an aggregation number n.19 The mass balance for surfactant ion and counterion are, respectively, expressed as

2Ct ) [Br-] + m[Mn] f [Mn] ) (2Ct - [Br-])/m (4) Ct ) [S2+] + n[Mn] f [Mn] = (Ct - cmc)/n

(5)

where [S2+], [Br-], and [Mn] are the corresponding concentrations and Ct is the total surfactant concentration. Further, the monomer concentration of [S2+] is assumed to be the cmc in eq 5. The degree of counterion binding to aggregates (m/n)′ can be evaluated from eqs 4 and 5 at any Ct (total concentration)

(m/n)′ = (2Ct - [Br-])/(Ct - cmc) (0 e (m/n)′ e 2)

(6)

However, it is a convention to assume the maximum degree of counterion binding as 1. Therefore, the definition of the parameter is rearranged as follows

(m/n) ) 0.5 × (m/n)′ (0 e (m/n) e 1)

(7)

Now that both [Br-] and the cmc’s11 are available, the experimental values of m/n can be evaluated using eq 7. For C4FC3-2-C3C4F aggregates, the m/n values decrease with the total concentration up to 3 times the cmc and then remain constant at ca.0.4 (Figure 6a); this corresponds to a decrease in the size and appearance of secondary aggregates. On the other hand, the m/n values of C6FC3-2-C3C6F aggregates remain almost constant at ca. 0.25 with increasing total concentration, as shown in Figure 6b; this implies that the aggregates exist as string-like aggregates in the measurement concentration range. As mentioned above, C8FC3-2-C3C8F forms large and small vesicles, which increases with concentration. As Figure 6c illustrates, the m/n values of C8FC3-2-C3C8F aggregates increase with concentration; however, the values are less than 0.3-0.225 mM (cmc × 35). A series of common results showed that the molecular aggregates of CnFC32-C3CnF (n ) 4, 6, and 8) had relatively small m/n values that (19) Moroi, Y. Micelles: Theoretical and Applied Aspects; Plenum Press: New York, 1992; Chapter 4.

decreased with fluorinated alkyl chain length. The relatively small m/n values have also been confirmed by an electric conductivity method based on the equivalent conductivity, the m/n value roughly estimated to 0.1-0.2 as average at the cmc.11 When the present results are compared with those for an analogous hydrogenated gemini surfactant (dimethylene-1,2-bis(dodecyldimethylammonium bromide; named 12-2-12), it is found that the present fluorinated gemini surfactants have much smaller m/n values than 12-2-12. The m/n values for 12-2-12 have been reported to be ca. 0.8 at 298.2 K from electric conductivity and SANS measurements.20-22 The small counterion binding to the aggregates implies that the bromide-ion binding has a slight influence on the growth of CnFC3-2-C3CnF (n ) 4, 6, and 8) aggregates. Their electrostatic repulsion between cationic head groups of surfactant molecules which arranged regularly to large aggregates seems to be smaller than the condensed structure of small aggregates for such a gemini type of fluorinated surfactant because the tails of double fluorocarbons have bulky volume originally and their positively charged terminal head groups are separated by spacer chains.

Conclusions The shapes of CnFC3-2-C3CnF (n ) 4, 6, and 8) aggregates depend on the carbon number of the fluorinated alkyl chain; these aggregates are in the form of large and irregular small aggregates and vesicles (n ) 4), string-like aggregates (n ) 6), and vesicles (n ) 8). The aggregate sizes are estimated to be ca. 10-40 nm at cmc × 5 (n ) 4), 50-150 nm at cmc × 5 (n ) 6), and 50-100 nm at cmc × 15 (n ) 8) from TEM micrographs. The various kinds of aggregate have an extremely small degree of counterion binding (m/n) as compared with their homologous hydrogenated gemini surfactants. The former have an m/n value of less than 0.4, while the latter (for example, 12-2-12) are reported to have an m/n value of ca. 0.8. These results indicate that bromide-ion binding does not promote growth of CnFC32-C3CnF aggregates because the electrostatic repulsion between the head groups is intrinsically small due to the bulky molecular volume of the fluorocarbons and as the positively charged terminal head groups are separated by spacer chains. The experimental results presented in this study on the effect of alkyl chain length on fluorinated gemini surfactants will be useful for further study on fluorinated gemini surfactants. Acknowledgment. The TEM work was performed at the “Hanaichi Ultrastructure Research Institute” in Okazaki (Japan). LA701525C (20) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (21) Wettig, S. D.; Verrall, R. E. J. Colloid Interface Sci. 2001, 235, 310. (22) Grosmaire, L.; Chorro, M.; Chorro, C.; Partyka, S.; Zana, R. J. Colloid Interface Sci. 2002, 246, 175.