Langmuir 2005, 21, 7691-7695
7691
Degree of Micelle Ionization and Micellar Growth for Gemini Surfactants Detected by 6-Methoxy-N-(3-sulfopropyl)quinolinium Fluorescence Quenching Keiko Kuwamoto, Tsuyoshi Asakawa,* Akio Ohta, and Shigeyoshi Miyagishi Division of Material Sciences, Graduate School of Natural Science & Technology, Kanazawa University, Kanazawa 920-1192, Japan Received April 11, 2005. In Final Form: June 9, 2005 The degree of micelle ionization of gemini surfactants has been investigated by using halide-sensitive fluorescence probes (e.g., 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ)). The fluorescence is quenched by the free bromide ions dissociated from surfactants. The degree of micelle ionization increased with increasing spacer chain length, but it decreased with increasing surfactant concentration. The SternVolmer plot gave two inflection points (i.e., not only at the cmc but also far above the cmc). The second inflection point suggested spherocylindrical micellar growth with decreases in the degree of micelle ionization. The spherocylindrical micellar growth was depressed with increasing spacer chain length, whereas it was enhanced with increasing tail chain length. The degree of micelle ionization of spherocylindrical micelles depended on the concentration and chain length of gemini surfactants. The change in SPQ fluorescence spectra upon hydrogenation was utilized to evaluate the solubilization site in micelle solutions. The dissolved SPQ in water was instantly reduced by the addition of NaBH4, resulting in abrupt changes in fluorescence intensity and spectral shift. All of the SPQ in micelle solution was also instantly reduced by NaBH4, indicating the existence of SPQ in the water bulk phase, but its fluorescence intensity increased upon the solubilization of hydrogenated SPQ into micelles.
Introduction A conventional surfactant has a single tail chain connected to a polar headgroup, whereas a gemini surfactant has the structure that the conventional surfactant molecules are connected with the spacer chain. Much attention have been focused on the effect of the spacer length or flexibility and hydrophobicity.1 As reported previously, gemini surfactants have remarkably low cmc values compared with those of corresponding conventional surfactants. Although geminis have two hydrophobic chains, the introduction of a long spacer chain renders the geminis water-soluble. The length of the spacer chain will affect micelle aggregation as well as adsorption behavior concerning the orientation of surfactant molecules at the interface.2,3 Fluorescence probe methods have been frequently utilized to evaluate physicochemical properties such as the critical micelle concentration (cmc) and micelle aggregation for surfactants in aqueous solution.4 Values of surfactant cmc are determined from changes in fluorescence spectra or intensities upon micelle formation. Micellar characteristics such as polarity and microviscosity can be investigated with pyrene, 1,3-pyrenylpropane, and so forth.5,6 The behavior of fluorescence quenching in micelle systems is utilized to determine the aggregation number and give valuable information as to dynamic properties of the probe, quencher, and surfactant mol* To whom correspondence should be addressed. E-mail:
[email protected]. (1) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (2) Zana, R.; Benrraou, M.; Rueff, R. Langmuir 1991, 7, 1072. (3) Li, Z. X.; Dong, C. C.; Thomas, R. K. Langmuir 1999, 15, 4392. (4) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580. (5) Kalyanasundaram, K. Langmuir 1988, 4, 942. (6) Zana, R. J. Phys. Chem. B 1999, 103, 9117.
ecules.7,8 The information sensed by fluorescence probes has been related to the microenvironment around the solubilization site of the probe in the micelles. The fluorescence spectra of water-soluble quinoline derivatives exhibit high intensities with a broad band maximum at around 440 nm in the solutions, and quenching occurs by the collision between the fluorescence probe and a halide ion.9,10 The fluorescence intensity was unaffected by dissolved oxygen and solvent polarity and hardly affected by nitrate and sulfate anions. Thus, we have applied quinoline derivatives as a fluorescence probe to micelle systems and have examined the behavior of fluorescence quenching by cationic surfactants containing halide counterions. The distinct break point appears in the Stern-Volmer plots for cationic surfactants containing bromide, whereas the quenching by sodium bromide follows the Stern-Volmer relation. We have recently reported that the fluorescence quenching behavior of N-ethoxycarbonyl-6-methoxyquinoliniumbromide (MQAE) was useful in simultaneously determining the cmc of surfactants and the degree of micellar counterion dissociation (R).11 The purpose of this article is to estimate the counterion binding behavior of a cationic gemini surfactant using the halide-sensitive fluorescence probe. On the basis of the concentration-dependent fluorescence decrease, we have evaluated the concentration of ‘‘free” counterions in aqueous solutions. The effect of the spacer chain length (s) on degree of micelle ionization has been investigated (7) Almgren, M. Adv. Colloid Interface Sci. 1992, 41, 9. (8) Gehlen, M. H.; Schryver, F. C. D. Chem. Rev. 1993, 93, 199. (9) Verkman, A. S.; Seller, M. C.; Chao, A. C.; Leung, T.; Ketcham, R. Anal. Biochem. 1989, 178, 355. (10) Biwersi, J.; Tulk, B.; Verkman, A. S. Anal. Biochem. 1994, 219, 139. (11) Asakawa, T.; Kitano, H.; Ohta, A.; Miyagishi, S. J. Colloid Interface Sci. 2001, 242, 284.
10.1021/la0509706 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/12/2005
7692
Langmuir, Vol. 21, No. 17, 2005
Kuwamoto et al.
in detail for C12-s-C12. The effect of surfactant concentration has also been investigated because Cn-12-Cn can be watersoluble up to rather high concentrations. The micellar growth of gemini surfactant has been revealed by the change in the degree of micelle ionization using the SPQ fluorescence quenching method. Experimental Procedures Materials. 6-Methoxy-N-(3-sulfopropyl)quinolinium (SPQ) was purchased from Molecular Probes, Inc. and used without purification. Alkanediyl-R,ω-bis(dimethylalkylammonium bromide) (Cn-s-Cn, n ) 10, 12, and 14, s ) 2, 3, 4, and 6) was synthesized by refluxing bromoalkane with N,N,N′,N′-tetramethyl-1,2-diaminoethane, N,N,N′,N′-tetramethyl-1,3-diaminopropane, N,N,N′,N′-tetramethyl-1,4-diaminobutane, or N,N,N′,N′tetramethyl-1,6-diaminohexane in acetonitrile, respectively.2 Cn12-Cn (n ) 12, 14, 16, and 18) was synthesized by refluxing dibromododecane with n-alkyldimethylamine in acetonitrile. n-Dodecyltrimethylammonium bromide was obtained from Tokyo Kasei Kogyo Co., Ltd. All surfactants were purified by repeated recrystallization from acetone-ethanol mixtures. Measurements. The 1.0 × 10-6 M SPQ probe aqueous solutions were made up in doubly distilled water. The fluorescence intensities of surfactant solutions were measured at 443 nm by the excitation at 346 nm using a Hitachi F-3010 spectrophotometer.11 The fluorescence intensity without quencher (I0) was used as a standard. The fluorescence spectra of 1.0 × 10-7 M pyrene were measured with scanning from 350 to 450 nm by the excitation at 335 nm.4 The ratio of the first peak to the third peak has a value of approximately 1.85 in water.5 The hydrogenation of 1.0 × 10-6 M SPQ was examined by the addition of 10 µL of 1 M NaBH4 aqueous solution.12 The hydrogenation of SPQ was performed in a quartz cell within about 3 min. The fluorescence spectra were recorded by using a Perkin-Elmer LS-55 spectrophotometer. All experiments were performed at 25 °C.
Results and Discussion 6-Methoxy-N-(3-sulfopropyl)quinolinium (SPQ) is a zwitterionic inner salt with a quinolinium backbone substituted at the 6 position with a methoxy group. SPQ has great water solubility and a low octanol-water partition coefficient.9 It was confirmed by a gel filtration method that the SPQ probe tend to be partitioned in the aqueous bulk phase without trapping in cationic micelles. The SPQ fluorescence gave a single broad emission peak centered at 443 nm by the excitation at 346 nm in the solutions. The fluorescence is quenched by halide ions via a collisional mechanism. The halide sensitivity of SPQ quenching was smaller than that of 6-methoxy-N-ethylquinolinium iodide (MEQ) because the negative sulfonate group for SPQ may result in decreasing halide sensitivity. However, the MEQ fluorescence probe has an iodide counterion, which may influence the micelle properties. Therefore, we evaluated the critical micelle concentrations and degree of micelle ionization for cationic gemini surfactants by using the fluorescence behavior of the zwitterionic SPQ probe in this article. When quenching occurs by the collision between SPQ fluorescence probe and bromide ion quencher, the variation of fluorescence intensity is related to the concentration of bromide [Br] by the Stern-Volmer relation,9 I0/I ) 1 + KSV[Br], where I0 and I are the fluorescence intensities in the absence and in the presence of quencher, respectively, and KSV is the Stern-Volmer constant. The investigated gemini surfactant is bisquaternary ammonium having an alkanediyl spacer group, which has a bromide counterion quencher. It is characterized by the low cmc’s in comparison with those of conventional (12) Biwersi, J.; Verkman, A. S. Biochemistry, 1991, 30, 7879.
Figure 1. Effect of spacer chain lengths of gemini surfactants on SPQ fluorescence quenching. (O) C12-2-C12, (4) C12-3-C12, (0) C12-4-C12, (b) C12-6-C12, and (2) C12-12-C12. Table 1. Stern-Volmer Constant of the Surfactant Monomer, cmc, and Degree of Counterion Dissociation by the Quenching of SPQ Fluorescence at 25 °C surfactant
KSV/M-1
cmc/mM
R
C12-2-C12 C12-3-C12 C12-4-C12 C12-6-C12 C12-12-C12
212 222 223 217 205
0.97 0.97 1.13 1.02 0.51
0.21 0.20 0.27 0.35 0.49
surfactants containing the same tail chain length. Figure 1 shows the Stern-Volmer plots of SPQ fluorescence quenching for C12-s-C12 with spacer chain lengths of s ) 2, 3, 4, 6, and 12 in the cmc regions. The slopes for the Stern-Volmer plots of C12-s-C12 below the cmc appear to be in fairly good agreement with 221 M-1 for dodecyltrimethylammonium bromide (DTAB). The distinct inflection point gave the cmc in the same manner as conductivity measurements. The variation of the slope in the Stern-Volmer plot can be ascribed to the bromide counterion binding of cationic micelles. The degree of micelle ionization, R, was taken as the ratio of the values of KSV above and below the cmc in the same manner as for the conductivity data.11 The values of KSV below their cmc’s, the cmc values, and the degree of micelle ionization are summarized in Table 1. The cmc gave a maximum at C12-4-C12 upon increasing the spacer chain length. This was interpreted by the intramolecular interaction of monomeric C12-s-C12 in aqueous solutions as pointed out by Zana et al.2 Some contacts between the two alkyl chains in the monomeric state may result in the lower contribution of hydrophobic interaction along with micelle formation and thus a larger cmc. As the spacer chain length increased further, the cmc of C12-12-C12 decreased significantly. This is due to the incorporation of a spacer chain into the micelle contributing a hydrophobic interaction when the spacer chain is long enough. The degree of micelle ionization has been quantified by the two-site model considering free ions in the aqueous bulk phase and bound ions in the micelle phase.13 The degree of micelle ionization was often evaluated by using the conductivity method.2 Assuming that micelles do not contribute significantly to the conductivity, the slope of conductivity against surfactant concentration can be used to evaluate the degree of micelle ionization. As reported previously, the fluorescence of the SPQ probe is insensitive to not only surfactant monomers but also micelles.14 The bound halide electroneutralized with micelles was insen(13) Stilbs, P.; Lindman, B. J. Phys. Chem. 1981, 85, 2587.
Degree of Micelle Ionization for Gemini Surfactants
Figure 2. Stern-Volmer plots of SPQ fluorescence quenching by bromide ions. (O) NaBr, (4) DTAB, and (0) C10-3-C10.
sitive to fluorescence quenching. Thus, we can evaluate the concentration of free counterions dissociated from surfactants by the SPQ fluorescence quenching behavior. The data in Table 1 shows the significant increase in R values with increasing spacer chain length. This is due to the decrease in micelle surface charge density arising from the extended spacer chain. The R value 0.49 for C1212-C12 is somewhat smaller than 0.62 reported by Zana et al.2 The difference may come from the contribution of ionized micelles in the conductivity method. Increasing the spacer chain length will increase the area (a0) per surfactant at the micelle surface. The surfactant packing parameter, v/a0lc, was previously proposed in order to explain the behavior of micellar morphology, the sphere-rod transition, and so forth with the length (lc) and volume (v) of the tail chain of the surfactant.15 Gemini surfactants with large a0 values have smaller packing parameters and thus prefer to form spherical micelles. However, as the surfactant concentration increases, a change in the packing parameter is expected. Our attention is focused on the concentration dependence of micellar aggregations by tracing the SPQ fluorescence quenching. As shown in Figure 2, the SternVolmer plot for quenching by the bromide ion followed a straight line up to 100 mM NaBr, whereas two straight lines were observed for aqueous surfactant solutions. No shift of the emission maximum was observed by the addition of surfactants and/or salts. The KSV value of NaBr was 236 M-1 at 25 °C. The inflection point gave a cmc of 14.6 mM for the conventional surfactant, DTAB, which is more than 10 times the gemini surfactant cmc. Because the fluorescence intensity of SPQ monitors the concentration of the free bromide ion in the micellar region, we can evaluate the degree of micelle ionization, R, by using the mass balance equation.11 The straight line above the cmc suggests that the degree of micelle ionization remains constant up to 200 mM of DTAB as well as 100 mM of C10-3-C10. The cmc’s of Cn-3-Cn were determined by the SPQ fluorescence quenching method as well as the pyrene fluorescence probe method. Table 2 lists their cmc’s, R, the pyrene intensity ratio I1/I3 for micelles at 5 times the cmc, and the micelle aggregation number Nagg determined by the pyrene fluorescence quenching method using cetylpyridinium chloride. The cmc values decreased with (14) Asakawa, T.; Ishino, S.; Hansson, P.; Almgren, M.; Ohta, A.; Miyagishi, S. Langmuir 2004, 20, 6998. (15) Israelachvilli, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525.
Langmuir, Vol. 21, No. 17, 2005 7693
Figure 3. SPQ fluorescence quenching in gemini surfactant systems at high concentrations. (O) C12-3-C12 and (4) C12-12C12. Table 2. Micellar Solution Properties by the Fluorescence Probe Method SPQ
pyrene
surfactant
cmc/mM
R
cmc/mM
I1/I3
Nagg
DTAB C10-3-C10 C12-3-C12 C14-3-C14
14.6 6.31 0.97 0.15
0.29 0.25 0.20 0.09
15.6 6.20 1.05 0.19
1.44 1.59 1.56 1.54
42.3 23.4 22.3 20.6
increasing alkyl chain length in fairly good agreement with the data of the pyrene fluorescence probe method.16 The R values for geminis were smaller than that of DTAB and significantly decreased for C14-3-C14. The I1/I3 ratios for geminis were larger than that of DTAB and hardly depended on the alkyl chain length. The I1/I3 ratio will be affected by the solubilization site of pyrene. The location of pyrene has been shown to interact with quaternary ammonium headgroups.17 The observed micropolarity suggests that the location of pyrene will be closer to the micellar surface on a time-averaged basis. Because gemini surfactant molecules tend to be packed more loosely at the micellar surface owing to the existence of a spacer chain, the I1/I3 values for geminis will be larger than that for DTAB. Moreover, the I1/I3 values for geminis were almost unaffected by the increasing alkyl chain length. These results also suggest that the solubilization site of pyrene would be close to the micellar surface despite the increase in the alkyl chain length. The micellar aggregation number for geminis was about 1/2Nagg for DTAB. This behavior can be rationalized using simple geometric considerations proposed by Israelachvili et al.15 If the alkyl chain length (lc) and the area (a0) per alkyl chain at the micelle surface are not very different from those of the spherical DTAB micelle, then a small number of gemini surfactant molecules are sufficient to form the micelle owing to two dodecyl chains with geminis (i.e., the large volume (v) of the surfactant tail). We noticed that the concentration dependence of the degree of micelle ionization for C12-3-C12 at high surfactant concentrations is different from that of C10-3-C10. Figure 3 shows the Stern-Volmer plots for SPQ fluorescence quenching up to 100 mM C12-s-C12. The slope of the SternVolmer plot for C12-3-C12 remained approximately constant (16) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 205. (17) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039.
7694
Langmuir, Vol. 21, No. 17, 2005
Kuwamoto et al.
Figure 5. Dependence of R and C* on tail chain length for Cn-12-Cn systems. (O) R at 2 mM Cn-12-Cn, (4) R at 100 mM Cn-12-Cn, and (b) C*.
Figure 4. Variations in the degree of micelle ionization with surfactant concentration. (O) C12-3-C12 and (4) C12-12-C12.
pirical equation based on experimental studies of micelle growth.21
ln at cmc values of up to 22.8 mM, but it changed with increasing surfactant concentration in the high concentration region. The decrease in slope corresponds to the decrease in the degree of micelle ionization, suggesting micellar growth at high concentrations.18 We could assign the second inflection point C* far above the cmc to the transition for spherocylindrical micelles. The C* values were 22.8, 38.5, 40.0, and 44.9 mM for s ) 3, 4, 6, and 12, respectively; that is, the spherocylindrical micelle growth was enhanced for the short spacer length. The significant micellar growth for the short spacer length has been investigated by the pyrene fluorescence probe method, suggesting that the micelle aggregation number increased with increasing concentration.19 Spherical micelles exist in the low-concentration range, but a transition in micelle shape was observed in the high-concentration range by cryo-TEM.20 SPQ fluorescence quenching behavior shows the concentration dependence of the degree of micelle ionization along with the change in micelle shape. This observation was also confirmed by the quenching behavior of N-sulfopropylacridinium (SPA) fluorescence. Figure 4 shows the marked decrease in R values for C12-12-C12 in the low-concentration range just above the cmc. As the concentration increases, the extended spacer chain will tend to fold into the micelle palisades layer. The successive aggregation of small C1212-C12 micelles led to counterion binding at the micelle surface. The R values at 100 mM were 0.13, 0.12, 0.15, and 0.29 for s ) 3, 4, 6, and 12, respectively; that is, the degree of micelle ionization of C12-s-C12 markedly decreased for the short spacer length, suggesting spherocylindrical micelle growth. Abrupt decreases in the degree of micelle ionization upon spherocylindrical growth were also observed for Cn12-Cn systems. The C* values corresponding to spherocylindrical growth were 44.9, 30.8, 18.7, and 10.3 mM, whereas the R values at 100 mM were 0.29, 0.14, 0.15, and 0.15 for n ) 12, 14, 16, and 18, respectively. As the tail chain length increased, the spherocylindrical growth was enhanced by the accompanying counterion binding of micelles. Missel et al. reported the following semiem(18) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90, 5435. (19) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448. (20) Bernheim-Groswasser, A.; Zana, R.; Talmon, Y. J. Phys. Chem. B 2000, 104, 4005.
K κ ) + ∆ ln Cs + Γnc - FrH + β′′ n0 T
(1)
where a thermodynamic parameter (K) governing the sphere-rod transition is given by
K ) exp
[
]
n0(µs - µc) RT
where n0 is the aggregation number in a spherical micelle, T is the temperature in Kelvin, and µs and µc are the standard chemical potentials per monomer associated with spherical and cylindrical micelle regions, respectively. Cs, nc, and rH are the salt concentration, tail chain length, and effective Stokes radius of the hydrated counterion, respectively. κ, ∆, Γ, F, and β′′ are the parameters determined experimentally. The concentration-dependent growth of micelles occurs when K(X - XB) exceeds n02. Here, X and XB are the concentrations of the surfactant and the monomer in mole fraction units, respectively. If surfactant molar concentration C* at the spherocylindrical transition point was used instead of X, then the following equation can be derived from eq 1 because XB is negligible:
ln C* ) ln n0 -
[Tκ + ∆ ln C + Γn - Fr s
c
H
+ β′′
]
(2)
This equation predicts that log C* will have a linear dependence on tail chain length nc. Figure 5 shows the dependence of R and C* on tail chain length for Cn-12-Cn systems. Because the enhancement of hydrophobic interaction contributes to micelle growth, spherocylindrical micelle growth is enhanced with increasing tail chain length. The effective cross-sectional area per surfactant at the micelle surface will increase with increasing spacer chain length. As shown in Figure 6, the effect led to the increase in C* because it might resemble the increase in the effective radius of the hydrated ion. That is, the electrostatic contribution plays an important role in determining the spherocylindrical micelle growth. The formation of vesicles has often been reported for gemini surfactants.19,22 We aim to determine the existence of vesicles or elongated micelles by using the change in SPQ fluorescence spectra upon hydrogenation. If vesicles contain the inner water core solubilizing SPQ, then the (21) Missel, P. J.; Mazer, N. A.; Carey, M. C.; Benedek, G. B. J. Phys. Chem. 1989, 93, 8354. (22) Oda, R.; Huc, I.; Danino, D.; Talmon, Y. Langmuir 2000, 16, 9759.
Degree of Micelle Ionization for Gemini Surfactants
Langmuir, Vol. 21, No. 17, 2005 7695 Table 3. Fluorescence Properties of SPQ and H-SPQ SPQ surfactant
C12-3-C12
DTAB
Figure 6. Dependence of R and C* on spacer chain length for C12-s-C12 systems. (O) R at 10 mM C12-s-C12, (4) R at 100 mM C12-s-C12, and (b) C*.
Figure 7. Spectral changes in SPQ fluorescence by the addition of NaBH4 in aqueous solutions. Solid lines (a and b) indicate SPQ and hydrogenated SPQ in water, respectively, and dotted lines (c and d) indicate SPQ and hydrogenated SPQ in 50 mM C12-3-C12 aqueous solutions, respectively.
inhibition of hydrogenation toward the entrapped SPQ can be expected. It was confirmed that the hydrogenated SPQ by NaBH4 gave a spectral shift and a marked decrease in fluorescence intensity in aqueous solutions. The change in SPQ fluorescence spectra upon hydrogenation could be utilized to evaluate the solubilization site in micelle aggregates. Figure 7 shows the fluorescence spectral changes of 10-6 M SPQ in aqueous solutions by the addition of NaBH4. The fluorescence emission peak was shifted to 465 nm accompanied by a significant decrease in intensity. It was confirmed that all of the SPQ in aqueous solution was instantly reduced by NaBH4 under these experimental conditions. For C12-3-C12 systems, the shift of the emission peak was also observed upon hydrogenation, whereas the fluorescence intensity increased. We confirmed by another experiment that the addition of C12-3-C12 to the hydrogenated SPQ aqueous solutions (peak b) gave identical
H-SPQ
C/mM
Em/nm
I
Em/nm
IH
0 10 30 50
443 443 443 441
1175 531 316 247
465 458 457 457
270 450 425 423
30 50 100
443 442 442
230 188 127
456 457 456
457 433 426
spectra to peak d of Figure 7. This means that the fluorescence of hydrogenated SPQ changed owing to the micellar solubilization of hydrogenated SPQ. The cationic C12-3-C12 micelles tend to solubilize the hydrogenated SPQ that was converted to the negatively charged one. Moreover, the hydrogenated SPQ is hardly quenched by bromide ions, in contrast to SPQ. Consequently, the fluorescence intensity for C12-3-C12 systems increased upon the hydrogenation of SPQ. Table 3 shows the surfactant concentration dependence of fluorescence spectral changes upon hydrogenation for C12-3-C12 systems in comparison with conventional DTAB. The fluorescence intensities of SPQ decreased with increasing concentration, whereas those of hydrogenated SPQ remained almost constant. All of the SPQ in the DTAB micellar solution was instantly reduced by NaBH4, indicating the existence of SPQ in the aqueous bulk phase. No significant difference between C12-3-C12 and conventional surfactants was confirmed. Thus, the usual micellar growth can be expected for C123-C12 systems even at rather high surfactant concentration. Conclusions The SPQ fluorescence quenching method can trace the decrease in the degree of micelle ionization (R) with increasing concentration up to high surfactant concentration. The R value of C12-12-C12 was larger than those of other geminis owing to the extended long spacer chain, whereas its cmc was relatively lower owing to the contribution of the hydrophobicity of the long spacer chain. The successive aggregation of C12-12-C12 for small micelles led to counterion binding at the micelle surface. The spherocylindrical micellar growth of gemini surfactants was observed as the distinct inflection point in the SternVolmer plot corresponding to the increased counterion binding of micelle. No evidence of the existence of vesicles was observed for C12-3-C12, as shown from the result that all of the SPQ in the C12-3-C12 micellar solution was instantly reduced by NaBH4. LA0509706
