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Solubilization of pyrene in aqueous micellar solutions of gemini surfactants C12-s-C12⋅2Br
Journal of Colloid and Interface Science 300 (2006) 749–754 www.elsevier.com/locate/jcis
Solubilization of pyrene in aqueous micellar solutions of gemini surfactants C12-s-C12·2Br Ou Zheng, Jian-Xi Zhao ∗ Department of Applied Chemistry, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, People’s Republic of China Received 13 January 2006; accepted 11 April 2006 Available online 11 May 2006
Abstract Solubilization of pyrene in aqueous micellar solutions of quaternary ammonium gemini surfactants C12 -s-C12 ·2Br has been examined by UV spectra and steady fluorescence spectra at 30 ◦ C. The results showed that pyrene molecules were incorporated in the palisade layers of the micelles and interacted with the quarterary ammonium head groups through cation–π interaction, resulting in red shift of the UV absorption spectrum. C12 -s-C12 ·2Br (s = 3, 4, 6) micelles have stronger ability for solubilization of pyrene than conventional surfactant C12 TABr micelles. With increasing spacer length of C12 -s-C12 ·2Br, the micelles become more adaptable to solubilizing pyrene, which even forms the dimers as in the case of s = 3, 4, 6, while pyrene solubilizes in the micelles (s = 2) mainly in single-molecule form due to the more compact structure of the micelle in comparison with that at s > 2. © 2006 Elsevier Inc. All rights reserved. Keywords: Gemini surfactants; Pyrene; Micellar solubilization; Interaction
1. Introduction The solubility of organic substances increases with their incorporation into micelles of surfactant aqueous solutions. The phenomenon is called micellar solubilization. Solubilization is very important industrially as well as biologically [1] and very interesting chemically. For example, solubilized polycyclic aromatic molecules have often been used as photosensitive probes in photochemistry [2,3]. A number of studies have been devoted to the solubilization of aromatic compounds by aqueous micellar solutions of conventional surfactants made up of one hydrophilic head group and one hydrophobic chain. In particular, Moroi and his colleagues examined the solubilization of a series of aromatic compounds by micellar solutions of dodecylsulfonic acid [4–6], dodecylammonium perfluoroacetate [7], dodecylammonium trifluoroacetate [8], lithium 1-perfluoroundecanoate [9], and dodecyltrimethylammonium perfluorocarboxylate [10]. They treated experimental data with the stepwise association equilibria model and obtained some * Corresponding author.
E-mail address: [email protected] (J.-X. Zhao). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.04.033
important characteristic parameters such as the first stepwise association constant between solubilizate monomer and vacant micelle and the average number of aromatic molecules solubilizating in a micelle. Differently from conventional surfactants, gemini surfactants consist of two hydrophilic head groups, two hydrophobic chains, and a spacer linked at or near the head groups [11, 12]. Beyond the critical micelle concentration (cmc), gemini surfactants tend to form larger micelles than those constructed by conventional surfactants [13]. As a result, the micelles of gemini surfactants should have solubilizing capacity higher than that of the conventional surfactants. Indeed, Choi and his colleagues have observed greater solubilizing amounts of some disperse dyes in the aqueous micellar solutions of cationic gemini surfactants, such as propanediyl-α,ωbis(dimethyldodecylammonium bromide) or hexanediyl-α,ωbis(dimethyldodecylammonium bromide), than those in the conventional surfactant micelles [14]. Dam et al. also found that toluene or n-hexane can be better incorporated into micelles of alkanediyl-α,ω-bis(dimethylalkylammonium bromide) than into micelles of conventional surfactants [15]. These gemini micelles also show enhanced selectivity for aromatic compounds over paraffinic compounds [15]. Up to now, however, only a
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few reports have dealt with the solubilization of gemini micelles compared with that of conventional micelles [14–16]. Pyrene is an important photochemical probe for characterizing micellar structure and properties [2]. The aim of this work is to study the solubilization of pyrene in aqueous micellar solutions of a series of quaternary ammonium gemini surfactants, alkanediyl-α,ω-bis-(dimethyldodecylammonium bromide). Of course, the effect of the spacer length of the gemini molecule on the solubilization is an interesting subject since the length of the spacer is generally a key factor in determining the micellar shape and size [13,17].
2.3. Steady fluorescence measurements The fluorescence emission spectra of pyrene were carried out by an Edinburgh FL/FS920 TCSPC fluorescence spectrophotometer (Edinburgh, UK) at an excitation wavelength of 335 nm. The intensity ratio of the first peak (I1 at 372 nm) to the third peak (I3 at 383 nm) was used as an index of the polarity of the pyrene solubilizing medium [20]. Such emission spectra were also used to determine whether excimers of pyrene were formed [21]. 3. Results and discussion
2. Experimental 2.1. Reagents
3.1. Characteristic parameters of pyrene solubilization in C12 -s-C12 ·2Br micelles
Alkanediyl-α,ω-bis-(dimethyldodecylammonium bromide), referred to as C12 -s-C12 ·2Br (s = 2, 3, 4, 6), was synthesized in our laboratory [18]. The molecule structure is shown as follows:
In terms of the Lambert–Beer law, absorbance intensity (A) at the designated wavelength is proportional to the concentration (CP ) of pyrene in C12 -s-C12 ·2Br aqueous solutions, A = εbCP ,
Dodecyltrimethylammonium bromide (C12 TABr, Sigma) was recrystallized three times in ethyl ether–acetone mixed solvent. In this paper, the solubilization of pyrene in aqueous solutions of C12 TABr is used for comparison since C12 TABr is generally regarded as the corresponding monomer of C12 -s-C12 ·2Br. The characteristic parameters for the micellization of these compounds in aqueous solution are listed in Table 1. Pyrene (Fluka) was recrystallized three times in ethanol. All solutions were prepared with Milli-Q water. 2.2. Pyrene solubilization and absorption spectrogram determination We prepared a series of C12 -s-C12 ·2Br solutions with different concentrations in test tubes (5 ml per tube) and put ca. 0.5 mg pyrene in each test tube. The test tubes were vibrated 48 h at constant 30 ± 0.1 ◦ C to make sure of the solubilizing equilibrium of pyrene in the micelles and then centrifuged to remove excess pyrene. The absorbance of each solution was measured at λmax = 276 nm by a PE Lambda900 spectrograph using a quartz cell of path length 1 cm.
where ε and b are the molar extinction coefficient and the absorption path length, respectively, and ε is considered as a constant [5,8,10]. Fig. 1 shows the variation of A with the surfactant concentration (CS ), from which one can observe a rapid increase in A after the breakpoints of the curves. The breakpoint obviously characterizes the critical micelle concentration (cmc) since the solubilization of pyrene occurs in the micelles. Assuming respectively Acmc and At as the absorbance intensity at the cmc and at arbitrary CS beyond the cmc, CPcmc and CPt as the concentrations of pyrene corresponding to those points, one obtains the following expressions by the Lambert–Beer law, At − Acmc CPt − CPcmc CPM = = V, Acmc CPcmc CP
Surfactants cmc, mmol L−1 [18] N (at CS = 2.5 mmol L−1 ) [18] K0 [18] ηrel [19]
C12 -s-C12 ·2Br
C12 TABr
s=2
s=3
s=4
s=6
0.95 25.0 0.83 3.11
0.96 21.0 0.80 2.68
1.17 18.4 0.76 1.93
1.07 16.0 0.70 1.92
14.6 55.1∗ 0.70
Note: cmc, critical micelle concentration; N , aggregation number of micelle; ∗ , N of C TABr determined at C = 37.5 mmol L−1 ; K , degree of coun12 S 0 terion association on the micelle; ηrel , the microviscosity of C12 -s-C12 ·2Br micelles relative to that of C12 ·TABr micelles.
(2)
where CPM and CPV are the concentrations of pyrene in the micelle and in water, respectively. According to the stepwise association equilibria model brought out by Moroi and coworkers [8], the relation between CPM /CPV and the micellar concentration (CS − cmc) can be yielded as CPM CPV
Table 1 The characteristic parameters of C12 -s-C12 ·2Br (s = 2, 3, 4, 6) and C12 TABr micelles at 30 ◦ C
(1)
=
K1 (CS − cmc), N
(3)
where K1 is the first stepwise association constant between the pyrene molecule and the vacant micelle and N is the aggregation number of a micelle. Fig. 2 shows the variation of CPM /CPV with (CS − cmc). All curves indeed display good linearity and K1 can be obtained from the slope (data shown in Table 2). The free energy change (G◦ ) during the process of pyrene molecule incorporation into vacant micelles can also be calculated by the equation G◦ = −RT ln K1 .
(4)
The saturated solubility of pyrene in water, here namely CPV , was known to be 5.3 × 10−7 mol L−1 at 30 ◦ C [8]. Thus the
O. Zheng, J.-X. Zhao / Journal of Colloid and Interface Science 300 (2006) 749–754
(a)
751
(b)
Fig. 1. Absorbance intensity (A) in pyrene–surfactant aqueous solutions as a function of the surfactant concentration (CS ) at 30 ◦ C: (a) C12 -s-C12 ·2Br and (b) C12 TABr shown for comparison.
(a)
(b)
Fig. 2. Variation of the ratio of pyrene concentration CPM in the micelle to CPV in water with the micellar concentration (CS − cmc) at 30 ◦ C: (a) C12 -s-C12 ·2Br and (b) C12 TABr shown for comparison.
numbers (Rpm ) of pyrene molecules per micelle when reaching saturated solubilization and the average numbers (RC ) of pyrene molecules corresponding to each dodecyl chain in the micelle can be further calculated by the expressions Rpm = K1 CPV
(5)
and Rpm . (6) mN The m is 2 for C12 -s-C12 ·2Br and 1 for C12 TABr, respectively. All data are listed in Table 2. RC =
3.2. Discussion of solubilization The values of K1 , Rpm , and RC for C12 -s-C12 ·2Br (s = 3, 4, 6) are higher than those of C12 TABr. This indicates that gemini surfactant micelles have a stronger ability to solubilize pyrene than conventional cationic surfactant micelles.
Table 2 Thermodynamic parameters of pyrene solubilization in aqueous micellar solutions of C12 -s-C12 ·2Br at 30 ◦ C Surfactants C12 -2-C12 ·2Br C12 -3-C12 ·2Br C12 -4-C12 ·2Br C12 -6-C12 ·2Br C12 TABr
K1 × 10−5 (L mol−1 ) 9.31 17.7 17.6 16.6 12.6
G◦ (kJ mol−1 ) −34.6 −36.3 −36.2 −36.1 −35.4
Rpm
RC
ds (nm)
0.49 0.94 0.93 0.88 0.67
0.0098 0.022 0.025 0.027 0.012
0.381 0.508 0.635 0.889 −
Note. The data of C12 TABr are also listed for comparison.
Here the data for s = 2 seem to be unusual compared with those of other s; for example, Rpm is nearly half of the corresponding value for s = 3. What makes the great disparity among the solubilizing amounts? Some valuable information about the aggregation state of pyrene in the micelles can be obtained from its steady fluorescence emission spectra as shown in Fig. 3. For example, a distinct broadband appears at 480 nm
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Fig. 3. Fluorescence emission spectra of pyrene in the aqueous micellar solutions of C12 -s-C12 ·2Br at excitation wavelength of 335 nm and at 2.5 times cmc for each surfactant. C12 TABr aqueous solution is also shown for comparison.
in the cases of s 3 and the intensity increases gradually with s increasing from 3 to 6. This means that some pyrene molecules form the excimers (excited dimers) in the micelle [21] and the content increases with s. In the s = 2 case, however, one almost cannot observe the band at 480 nm. In other words, pyrene solubilized in these micelles is mainly in single-molecule form. We attribute the phenomenon to different solubilization microenvironments caused by gemini surfactants with different length of s. The stretched length of the spacer of C12 -s-C12 ·2Br, ds , can be calculated by the formula ds (nm) = 0.127(s + 1) [22] (the data listed in Table 2). These ds s are smaller than the electrostatic equilibrium distance between the two heads of C12 TABr packed in the micelle, which was estimated to be ca. 0.9 nm [22]. With s decreasing, the two alkyl chains of the C12 s-C12 ·2Br molecule are drawn close and as a result the density of alkyl chains in a molecule increases. This enhances the hydrophobic interaction among surfactant molecules and makes the alkyl chains and heads of surfactants more closely packed within the micelles. This has been characterized by increasing values in both the relative microviscosity and the counterion association degree of the micelles, as summarized in Table 1. Gadelle and his colleagues concluded that loose micelles could solubilize aromatic hydrocarbons more easily than compact micelles [23]. Thus we could comprehend the present differences in the solubilizating amount and the pyrene dimer formation for different s, which come from different compact extents of the micelles in structure. Generally, the micelles initially formed are spherelike and the micelle volume V can be approximately estimated by the length of the alkyl chain as the radius of the spherical core. Tanford [24] suggested an equation to calculate the length (lC ) of alkyl chain having fully stretched conformation, lC (nm) = 0.15 + 0.1265nC .
(7)
Thus, the lC is 1.66 nm for an alkyl chain with nC = 12. As N is known, the occupied volume of each alkyl chain in the micelle (V12C ) can be calculated by V /2N for C12 -s-C12 ·2Br. Fig. 4 shows variation of V12C and RC with s. The fact that V12C
Fig. 4. The average numbers (RC ) of pyrene molecules corresponding to each dodecyl chain in the micelle and the volume (V12C ) of each alkyl chain occupied in the micelle as a function of the methylene number (s) in a spacer. Filled symbol 2 represents the twice actual values that is designated as 1 in the case of C12 -2-C12 ·2Br.
increases linearly with s demonstrates in another view that the micellar structure becomes gradually loose. If most of pyrene in the C12 -2-C12 ·2Br micelles also form dimers as in the cases of other s, the value of RC should be twice the actual value, here marked as 2 substituting for the original 1 as shown in Fig. 4. This point lies quite well in the RC ∼ s straight line. This again supports the view that pyrene solubilized in the micelles of C12 2-C12 ·2Br is mainly in single molecule form. 3.3. Solubilization sites of pyrene in the micelles 3.3.1. Characteristics of I1 /I3 As described in the experimental section, the I1 /I3 of pyrene is sensitive to the polarity of the medium located by the probe molecules. For example, the I1 /I3 of pyrene in water, ethanol, and n-dodecane, the last of which can be used as analogous environment of the micelle core consisted of dodecyl chains as in the present case, is 1.87, 1.18, and 0.59, respectively [20]. Yoshida and Moroi [10] determined the I1 /I3 of pyrene in the aqueous micellar solution of dodecyltrimethylammonium perfluorocarboxylate and got 1.25–1.33 of I1 /I3 during the temperature variation from 15 to 55 ◦ C. Thus they pointed out that the pyrene should be solubilized in the palisade layer of micelles. Heindl et al. [25] also confirmed the similar location of benzene or its derivatives in the hexadecyltrimethylammonium bromide micelles by NMR. The present I1 /I3 s are 1.24, 1.35, 1.36 and 1.37, for s = 2, 3, 4, and 6, respectively. This suggests that pyrene should also be located in the palisade layers of C12 s-C12 ·2Br micelles. In these cases, the I1 /I3 of pyrene in C12 -2-C12 ·2Br micellar solution is evidently lower than the others, which also supports the micelles of C12 -2-C12 ·2Br having more a compact structure than other micelles. 3.3.2. Characteristics of UV spectra Taking C12 -2-C12 ·2Br as an example, ultraviolet absorption spectra of pyrene in the aqueous micellar solution, n-dodecane,
O. Zheng, J.-X. Zhao / Journal of Colloid and Interface Science 300 (2006) 749–754
Fig. 5. The UV spectra of pyrene in (1) n-dodecane, (2) water, and (3) C12 2-C12 ·2Br micellar solution (CS = 2.5 mmol L−1 ), respectively.
and water are respectively shown in Fig. 5. It is well known that the character of electronic spectra of aromatic nuclei is connected with the polarity of the medium. As pyrene is incorporated into the palisade layer of the micelle, the maximum absorption wavelength λmax ought to lie between those of ndodecane and water. But one actually observes an obvious red shift of λmax in Fig. 5. Similar phenomenon also occurs in the s = 3, 4, 6, and C12 TABr cases. In fact, besides the medium properties, λmax of the aromatic compound is related to whether the π electronic cloud of the aromatic nucleus is disturbed. Obviously, it is a case that cations are close to aromatic. Recent investigations have revealed so-called cation–π interaction in some cationic surfactants and aromatic additives mixtures. For example, Huang et al. [26] investigated the interaction between aromatic counterions and the micelles of C12 TABr and observed an obvious red shift of the characteristic wavelength of aromatic counterions. However, similar phenomenon was not found in the aqueous micellar solutions of anionic surfactants. They suggested that aromatic counterions might incorporate into the palisade layer of the micelle and interact with the quaternary ammonium head groups through a cationic–π effect, which resulted in a red shift of the UV absorption spectrum. In the present case, the observed red shift rather than the expected blue shift means a strong cation–π interaction between pyrene and quaternary ammonium head groups. Thus, one can come to the conclusion that pyrene molecules must be close to quaternary ammonium head groups. In other words, those probe molecules are indeed located in the palisade layer of the micelle. Fig. 6 shows the variation of the shift (λ) of maximum absorption wavelength (λmax ) of pyrene in C12 -s-C12 ·2Br micellar solutions corresponding to that in water with the spacer length s. With increasing s, the charge density of the heads of C12 -s-C12 ·2Br decreases, which weakens the cation–π interaction between pyrene and the heads of C12 -s-C12 ·2Br. Therefore, decrement of λ is reasonable. 4. Conclusion The solubilization ability of C12 -s-C12 ·2Br (s = 3, 4, 6) micelles for pyrene is greater than that of C12 TABr micelles. The
753
Fig. 6. The difference (λ) between the maximum absorption wavelength (λmax ) of pyrene in C12 -s-C12 ·2Br micellar solutions and that in water as a function of the spacer length (s).
pyrene molecules are incorporated into the palisade layer of C12 -s-C12 ·2Br micelles and interact with cationic heads, resulting in a striking red shift in the UV spectrum. With increasing the spacer length of C12 -s-C12 ·2Br, the micelles become more adaptable to solubilizing pyrene. Because of the more compact structure of C12 -2-C12 ·2Br, pyrene solubilizes in the micelles in the single-molecule form while it forms a dimer in the micelles of s = 3, 4, and 6. Acknowledgment Support from the National Sciences Fund Foundation of China (No. 20173010) is gratefully acknowledged. References [1] R.A. Kroc, R.L. Kroc, G.D. Whedon, W. Garey, Hepatology, The Chemistry of Bile in Health and Disease, vol. 4 (5), Williams & Wilkins, Baltimore, 1984. [2] F. Grieser, C.J. Drummond, J. Phys. Chem. 92 (1988) 5580. [3] P. Lianos, M.L. Viriot, R. Zana, J. Phys. Chem. 88 (1984) 1098. [4] Y. Moroi, R. Matuura, J. Colloid Interface Sci. 125 (1988) 463. [5] Y. Moroi, K. Mitsunobu, T. Morisue, Y. Kadobayashi, M. Sakai, J. Phys. Chem. 99 (1995) 2372. [6] M. Take’uchi, Y. Moroi, Langmuir 11 (1995) 4719. [7] Y. Moroi, T. Morisue, J. Phys. Chem. 97 (1993) 12668. [8] T. Morisue, Y. Moroi, O. Shibata, J. Phys. Chem. 98 (1994) 12995. [9] M. Take’uchi, Y. Moroi, J. Colloid Interface Sci. 197 (1998) 230. [10] N. Yoshida, Y. Moroi, J. Colloid Interface Sci. 232 (2000) 33. [11] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1993) 1451. [12] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1993) 10083. [13] R. Zana, Adv. Colloid Interface Sci. 97 (2002) 205. [14] T.S. Choi, Y. Shimizu, H. Shirai, K. Hamada, Dye Pigments 45 (2000) 145. [15] Th. Dam, J.B.F.N. Engberts, J. Karthauser, S. Karaborni, N.M. van Os, Colloids Surf. A 118 (1996) 41. [16] F. Devinsky, I. Lacko, T. Imam, J. Colloid Interface Sci. 143 (1991) 336. [17] R. Zana, J. Colloid Interface Sci. 248 (2002) 203. [18] Y. You, O. Zheng, Y. Qiu, Y.H. Zheng, J.X. Zhao, G.B. Han, Acta Phys. Chim. Sin. 17 (2001) 4 [in Chinese]. [19] R. Zana, M. In, H. Levy, G. Duportail, Langmuir 13 (1997) 5552. [20] D.C. Dong, M.A. Winnik, Can. J. Chem. 62 (1984) 2560. [21] B. Valeur, Molecular Fluorescence, Principles and Applications, Wiley– VCH, New York, 2001. [22] D. Danino, Y. Talmon, R. Zana, Langmuir 11 (1995) 1448.
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[23] F. Gadelle, W.J. Koros, R.S. Schechter, J. Colloid Interface Sci. 170 (1995) 57. [24] C. Tanford, J. Phys. Chem. 76 (1972) 3020.
[25] A. Heindl, J. Strnad, H.H. Kohler, J. Phys. Chem. 97 (1993) 742. [26] M. Mao, J.-B. Huang, J.-X. Xiao, X. He, B. Zhu, Acta Chim. Sin. 58 (11) (2000) 1358 [in Chinese].
Solubilization of pyrene in aqueous micellar solutions of gemini surfactants C12-s-C12·2Br Ou Zheng, Jian-Xi Zhao ∗ Department of Applied Chemistry, College of Chemistry and Chemical Engineering, Fuzhou University, Fuzhou 350002, People’s Republic of China Received 13 January 2006; accepted 11 April 2006 Available online 11 May 2006
Abstract Solubilization of pyrene in aqueous micellar solutions of quaternary ammonium gemini surfactants C12 -s-C12 ·2Br has been examined by UV spectra and steady fluorescence spectra at 30 ◦ C. The results showed that pyrene molecules were incorporated in the palisade layers of the micelles and interacted with the quarterary ammonium head groups through cation–π interaction, resulting in red shift of the UV absorption spectrum. C12 -s-C12 ·2Br (s = 3, 4, 6) micelles have stronger ability for solubilization of pyrene than conventional surfactant C12 TABr micelles. With increasing spacer length of C12 -s-C12 ·2Br, the micelles become more adaptable to solubilizing pyrene, which even forms the dimers as in the case of s = 3, 4, 6, while pyrene solubilizes in the micelles (s = 2) mainly in single-molecule form due to the more compact structure of the micelle in comparison with that at s > 2. © 2006 Elsevier Inc. All rights reserved. Keywords: Gemini surfactants; Pyrene; Micellar solubilization; Interaction
1. Introduction The solubility of organic substances increases with their incorporation into micelles of surfactant aqueous solutions. The phenomenon is called micellar solubilization. Solubilization is very important industrially as well as biologically [1] and very interesting chemically. For example, solubilized polycyclic aromatic molecules have often been used as photosensitive probes in photochemistry [2,3]. A number of studies have been devoted to the solubilization of aromatic compounds by aqueous micellar solutions of conventional surfactants made up of one hydrophilic head group and one hydrophobic chain. In particular, Moroi and his colleagues examined the solubilization of a series of aromatic compounds by micellar solutions of dodecylsulfonic acid [4–6], dodecylammonium perfluoroacetate [7], dodecylammonium trifluoroacetate [8], lithium 1-perfluoroundecanoate [9], and dodecyltrimethylammonium perfluorocarboxylate [10]. They treated experimental data with the stepwise association equilibria model and obtained some * Corresponding author.
E-mail address: [email protected] (J.-X. Zhao). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.04.033
important characteristic parameters such as the first stepwise association constant between solubilizate monomer and vacant micelle and the average number of aromatic molecules solubilizating in a micelle. Differently from conventional surfactants, gemini surfactants consist of two hydrophilic head groups, two hydrophobic chains, and a spacer linked at or near the head groups [11, 12]. Beyond the critical micelle concentration (cmc), gemini surfactants tend to form larger micelles than those constructed by conventional surfactants [13]. As a result, the micelles of gemini surfactants should have solubilizing capacity higher than that of the conventional surfactants. Indeed, Choi and his colleagues have observed greater solubilizing amounts of some disperse dyes in the aqueous micellar solutions of cationic gemini surfactants, such as propanediyl-α,ωbis(dimethyldodecylammonium bromide) or hexanediyl-α,ωbis(dimethyldodecylammonium bromide), than those in the conventional surfactant micelles [14]. Dam et al. also found that toluene or n-hexane can be better incorporated into micelles of alkanediyl-α,ω-bis(dimethylalkylammonium bromide) than into micelles of conventional surfactants [15]. These gemini micelles also show enhanced selectivity for aromatic compounds over paraffinic compounds [15]. Up to now, however, only a
750
O. Zheng, J.-X. Zhao / Journal of Colloid and Interface Science 300 (2006) 749–754
few reports have dealt with the solubilization of gemini micelles compared with that of conventional micelles [14–16]. Pyrene is an important photochemical probe for characterizing micellar structure and properties [2]. The aim of this work is to study the solubilization of pyrene in aqueous micellar solutions of a series of quaternary ammonium gemini surfactants, alkanediyl-α,ω-bis-(dimethyldodecylammonium bromide). Of course, the effect of the spacer length of the gemini molecule on the solubilization is an interesting subject since the length of the spacer is generally a key factor in determining the micellar shape and size [13,17].
2.3. Steady fluorescence measurements The fluorescence emission spectra of pyrene were carried out by an Edinburgh FL/FS920 TCSPC fluorescence spectrophotometer (Edinburgh, UK) at an excitation wavelength of 335 nm. The intensity ratio of the first peak (I1 at 372 nm) to the third peak (I3 at 383 nm) was used as an index of the polarity of the pyrene solubilizing medium [20]. Such emission spectra were also used to determine whether excimers of pyrene were formed [21]. 3. Results and discussion
2. Experimental 2.1. Reagents
3.1. Characteristic parameters of pyrene solubilization in C12 -s-C12 ·2Br micelles
Alkanediyl-α,ω-bis-(dimethyldodecylammonium bromide), referred to as C12 -s-C12 ·2Br (s = 2, 3, 4, 6), was synthesized in our laboratory [18]. The molecule structure is shown as follows:
In terms of the Lambert–Beer law, absorbance intensity (A) at the designated wavelength is proportional to the concentration (CP ) of pyrene in C12 -s-C12 ·2Br aqueous solutions, A = εbCP ,
Dodecyltrimethylammonium bromide (C12 TABr, Sigma) was recrystallized three times in ethyl ether–acetone mixed solvent. In this paper, the solubilization of pyrene in aqueous solutions of C12 TABr is used for comparison since C12 TABr is generally regarded as the corresponding monomer of C12 -s-C12 ·2Br. The characteristic parameters for the micellization of these compounds in aqueous solution are listed in Table 1. Pyrene (Fluka) was recrystallized three times in ethanol. All solutions were prepared with Milli-Q water. 2.2. Pyrene solubilization and absorption spectrogram determination We prepared a series of C12 -s-C12 ·2Br solutions with different concentrations in test tubes (5 ml per tube) and put ca. 0.5 mg pyrene in each test tube. The test tubes were vibrated 48 h at constant 30 ± 0.1 ◦ C to make sure of the solubilizing equilibrium of pyrene in the micelles and then centrifuged to remove excess pyrene. The absorbance of each solution was measured at λmax = 276 nm by a PE Lambda900 spectrograph using a quartz cell of path length 1 cm.
where ε and b are the molar extinction coefficient and the absorption path length, respectively, and ε is considered as a constant [5,8,10]. Fig. 1 shows the variation of A with the surfactant concentration (CS ), from which one can observe a rapid increase in A after the breakpoints of the curves. The breakpoint obviously characterizes the critical micelle concentration (cmc) since the solubilization of pyrene occurs in the micelles. Assuming respectively Acmc and At as the absorbance intensity at the cmc and at arbitrary CS beyond the cmc, CPcmc and CPt as the concentrations of pyrene corresponding to those points, one obtains the following expressions by the Lambert–Beer law, At − Acmc CPt − CPcmc CPM = = V, Acmc CPcmc CP
Surfactants cmc, mmol L−1 [18] N (at CS = 2.5 mmol L−1 ) [18] K0 [18] ηrel [19]
C12 -s-C12 ·2Br
C12 TABr
s=2
s=3
s=4
s=6
0.95 25.0 0.83 3.11
0.96 21.0 0.80 2.68
1.17 18.4 0.76 1.93
1.07 16.0 0.70 1.92
14.6 55.1∗ 0.70
Note: cmc, critical micelle concentration; N , aggregation number of micelle; ∗ , N of C TABr determined at C = 37.5 mmol L−1 ; K , degree of coun12 S 0 terion association on the micelle; ηrel , the microviscosity of C12 -s-C12 ·2Br micelles relative to that of C12 ·TABr micelles.
(2)
where CPM and CPV are the concentrations of pyrene in the micelle and in water, respectively. According to the stepwise association equilibria model brought out by Moroi and coworkers [8], the relation between CPM /CPV and the micellar concentration (CS − cmc) can be yielded as CPM CPV
Table 1 The characteristic parameters of C12 -s-C12 ·2Br (s = 2, 3, 4, 6) and C12 TABr micelles at 30 ◦ C
(1)
=
K1 (CS − cmc), N
(3)
where K1 is the first stepwise association constant between the pyrene molecule and the vacant micelle and N is the aggregation number of a micelle. Fig. 2 shows the variation of CPM /CPV with (CS − cmc). All curves indeed display good linearity and K1 can be obtained from the slope (data shown in Table 2). The free energy change (G◦ ) during the process of pyrene molecule incorporation into vacant micelles can also be calculated by the equation G◦ = −RT ln K1 .
(4)
The saturated solubility of pyrene in water, here namely CPV , was known to be 5.3 × 10−7 mol L−1 at 30 ◦ C [8]. Thus the
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(a)
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(b)
Fig. 1. Absorbance intensity (A) in pyrene–surfactant aqueous solutions as a function of the surfactant concentration (CS ) at 30 ◦ C: (a) C12 -s-C12 ·2Br and (b) C12 TABr shown for comparison.
(a)
(b)
Fig. 2. Variation of the ratio of pyrene concentration CPM in the micelle to CPV in water with the micellar concentration (CS − cmc) at 30 ◦ C: (a) C12 -s-C12 ·2Br and (b) C12 TABr shown for comparison.
numbers (Rpm ) of pyrene molecules per micelle when reaching saturated solubilization and the average numbers (RC ) of pyrene molecules corresponding to each dodecyl chain in the micelle can be further calculated by the expressions Rpm = K1 CPV
(5)
and Rpm . (6) mN The m is 2 for C12 -s-C12 ·2Br and 1 for C12 TABr, respectively. All data are listed in Table 2. RC =
3.2. Discussion of solubilization The values of K1 , Rpm , and RC for C12 -s-C12 ·2Br (s = 3, 4, 6) are higher than those of C12 TABr. This indicates that gemini surfactant micelles have a stronger ability to solubilize pyrene than conventional cationic surfactant micelles.
Table 2 Thermodynamic parameters of pyrene solubilization in aqueous micellar solutions of C12 -s-C12 ·2Br at 30 ◦ C Surfactants C12 -2-C12 ·2Br C12 -3-C12 ·2Br C12 -4-C12 ·2Br C12 -6-C12 ·2Br C12 TABr
K1 × 10−5 (L mol−1 ) 9.31 17.7 17.6 16.6 12.6
G◦ (kJ mol−1 ) −34.6 −36.3 −36.2 −36.1 −35.4
Rpm
RC
ds (nm)
0.49 0.94 0.93 0.88 0.67
0.0098 0.022 0.025 0.027 0.012
0.381 0.508 0.635 0.889 −
Note. The data of C12 TABr are also listed for comparison.
Here the data for s = 2 seem to be unusual compared with those of other s; for example, Rpm is nearly half of the corresponding value for s = 3. What makes the great disparity among the solubilizing amounts? Some valuable information about the aggregation state of pyrene in the micelles can be obtained from its steady fluorescence emission spectra as shown in Fig. 3. For example, a distinct broadband appears at 480 nm
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Fig. 3. Fluorescence emission spectra of pyrene in the aqueous micellar solutions of C12 -s-C12 ·2Br at excitation wavelength of 335 nm and at 2.5 times cmc for each surfactant. C12 TABr aqueous solution is also shown for comparison.
in the cases of s 3 and the intensity increases gradually with s increasing from 3 to 6. This means that some pyrene molecules form the excimers (excited dimers) in the micelle [21] and the content increases with s. In the s = 2 case, however, one almost cannot observe the band at 480 nm. In other words, pyrene solubilized in these micelles is mainly in single-molecule form. We attribute the phenomenon to different solubilization microenvironments caused by gemini surfactants with different length of s. The stretched length of the spacer of C12 -s-C12 ·2Br, ds , can be calculated by the formula ds (nm) = 0.127(s + 1) [22] (the data listed in Table 2). These ds s are smaller than the electrostatic equilibrium distance between the two heads of C12 TABr packed in the micelle, which was estimated to be ca. 0.9 nm [22]. With s decreasing, the two alkyl chains of the C12 s-C12 ·2Br molecule are drawn close and as a result the density of alkyl chains in a molecule increases. This enhances the hydrophobic interaction among surfactant molecules and makes the alkyl chains and heads of surfactants more closely packed within the micelles. This has been characterized by increasing values in both the relative microviscosity and the counterion association degree of the micelles, as summarized in Table 1. Gadelle and his colleagues concluded that loose micelles could solubilize aromatic hydrocarbons more easily than compact micelles [23]. Thus we could comprehend the present differences in the solubilizating amount and the pyrene dimer formation for different s, which come from different compact extents of the micelles in structure. Generally, the micelles initially formed are spherelike and the micelle volume V can be approximately estimated by the length of the alkyl chain as the radius of the spherical core. Tanford [24] suggested an equation to calculate the length (lC ) of alkyl chain having fully stretched conformation, lC (nm) = 0.15 + 0.1265nC .
(7)
Thus, the lC is 1.66 nm for an alkyl chain with nC = 12. As N is known, the occupied volume of each alkyl chain in the micelle (V12C ) can be calculated by V /2N for C12 -s-C12 ·2Br. Fig. 4 shows variation of V12C and RC with s. The fact that V12C
Fig. 4. The average numbers (RC ) of pyrene molecules corresponding to each dodecyl chain in the micelle and the volume (V12C ) of each alkyl chain occupied in the micelle as a function of the methylene number (s) in a spacer. Filled symbol 2 represents the twice actual values that is designated as 1 in the case of C12 -2-C12 ·2Br.
increases linearly with s demonstrates in another view that the micellar structure becomes gradually loose. If most of pyrene in the C12 -2-C12 ·2Br micelles also form dimers as in the cases of other s, the value of RC should be twice the actual value, here marked as 2 substituting for the original 1 as shown in Fig. 4. This point lies quite well in the RC ∼ s straight line. This again supports the view that pyrene solubilized in the micelles of C12 2-C12 ·2Br is mainly in single molecule form. 3.3. Solubilization sites of pyrene in the micelles 3.3.1. Characteristics of I1 /I3 As described in the experimental section, the I1 /I3 of pyrene is sensitive to the polarity of the medium located by the probe molecules. For example, the I1 /I3 of pyrene in water, ethanol, and n-dodecane, the last of which can be used as analogous environment of the micelle core consisted of dodecyl chains as in the present case, is 1.87, 1.18, and 0.59, respectively [20]. Yoshida and Moroi [10] determined the I1 /I3 of pyrene in the aqueous micellar solution of dodecyltrimethylammonium perfluorocarboxylate and got 1.25–1.33 of I1 /I3 during the temperature variation from 15 to 55 ◦ C. Thus they pointed out that the pyrene should be solubilized in the palisade layer of micelles. Heindl et al. [25] also confirmed the similar location of benzene or its derivatives in the hexadecyltrimethylammonium bromide micelles by NMR. The present I1 /I3 s are 1.24, 1.35, 1.36 and 1.37, for s = 2, 3, 4, and 6, respectively. This suggests that pyrene should also be located in the palisade layers of C12 s-C12 ·2Br micelles. In these cases, the I1 /I3 of pyrene in C12 -2-C12 ·2Br micellar solution is evidently lower than the others, which also supports the micelles of C12 -2-C12 ·2Br having more a compact structure than other micelles. 3.3.2. Characteristics of UV spectra Taking C12 -2-C12 ·2Br as an example, ultraviolet absorption spectra of pyrene in the aqueous micellar solution, n-dodecane,
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Fig. 5. The UV spectra of pyrene in (1) n-dodecane, (2) water, and (3) C12 2-C12 ·2Br micellar solution (CS = 2.5 mmol L−1 ), respectively.
and water are respectively shown in Fig. 5. It is well known that the character of electronic spectra of aromatic nuclei is connected with the polarity of the medium. As pyrene is incorporated into the palisade layer of the micelle, the maximum absorption wavelength λmax ought to lie between those of ndodecane and water. But one actually observes an obvious red shift of λmax in Fig. 5. Similar phenomenon also occurs in the s = 3, 4, 6, and C12 TABr cases. In fact, besides the medium properties, λmax of the aromatic compound is related to whether the π electronic cloud of the aromatic nucleus is disturbed. Obviously, it is a case that cations are close to aromatic. Recent investigations have revealed so-called cation–π interaction in some cationic surfactants and aromatic additives mixtures. For example, Huang et al. [26] investigated the interaction between aromatic counterions and the micelles of C12 TABr and observed an obvious red shift of the characteristic wavelength of aromatic counterions. However, similar phenomenon was not found in the aqueous micellar solutions of anionic surfactants. They suggested that aromatic counterions might incorporate into the palisade layer of the micelle and interact with the quaternary ammonium head groups through a cationic–π effect, which resulted in a red shift of the UV absorption spectrum. In the present case, the observed red shift rather than the expected blue shift means a strong cation–π interaction between pyrene and quaternary ammonium head groups. Thus, one can come to the conclusion that pyrene molecules must be close to quaternary ammonium head groups. In other words, those probe molecules are indeed located in the palisade layer of the micelle. Fig. 6 shows the variation of the shift (λ) of maximum absorption wavelength (λmax ) of pyrene in C12 -s-C12 ·2Br micellar solutions corresponding to that in water with the spacer length s. With increasing s, the charge density of the heads of C12 -s-C12 ·2Br decreases, which weakens the cation–π interaction between pyrene and the heads of C12 -s-C12 ·2Br. Therefore, decrement of λ is reasonable. 4. Conclusion The solubilization ability of C12 -s-C12 ·2Br (s = 3, 4, 6) micelles for pyrene is greater than that of C12 TABr micelles. The
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Fig. 6. The difference (λ) between the maximum absorption wavelength (λmax ) of pyrene in C12 -s-C12 ·2Br micellar solutions and that in water as a function of the spacer length (s).
pyrene molecules are incorporated into the palisade layer of C12 -s-C12 ·2Br micelles and interact with cationic heads, resulting in a striking red shift in the UV spectrum. With increasing the spacer length of C12 -s-C12 ·2Br, the micelles become more adaptable to solubilizing pyrene. Because of the more compact structure of C12 -2-C12 ·2Br, pyrene solubilizes in the micelles in the single-molecule form while it forms a dimer in the micelles of s = 3, 4, and 6. Acknowledgment Support from the National Sciences Fund Foundation of China (No. 20173010) is gratefully acknowledged. References [1] R.A. Kroc, R.L. Kroc, G.D. Whedon, W. Garey, Hepatology, The Chemistry of Bile in Health and Disease, vol. 4 (5), Williams & Wilkins, Baltimore, 1984. [2] F. Grieser, C.J. Drummond, J. Phys. Chem. 92 (1988) 5580. [3] P. Lianos, M.L. Viriot, R. Zana, J. Phys. Chem. 88 (1984) 1098. [4] Y. Moroi, R. Matuura, J. Colloid Interface Sci. 125 (1988) 463. [5] Y. Moroi, K. Mitsunobu, T. Morisue, Y. Kadobayashi, M. Sakai, J. Phys. Chem. 99 (1995) 2372. [6] M. Take’uchi, Y. Moroi, Langmuir 11 (1995) 4719. [7] Y. Moroi, T. Morisue, J. Phys. Chem. 97 (1993) 12668. [8] T. Morisue, Y. Moroi, O. Shibata, J. Phys. Chem. 98 (1994) 12995. [9] M. Take’uchi, Y. Moroi, J. Colloid Interface Sci. 197 (1998) 230. [10] N. Yoshida, Y. Moroi, J. Colloid Interface Sci. 232 (2000) 33. [11] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1993) 1451. [12] F.M. Menger, C.A. Littau, J. Am. Chem. Soc. 113 (1993) 10083. [13] R. Zana, Adv. Colloid Interface Sci. 97 (2002) 205. [14] T.S. Choi, Y. Shimizu, H. Shirai, K. Hamada, Dye Pigments 45 (2000) 145. [15] Th. Dam, J.B.F.N. Engberts, J. Karthauser, S. Karaborni, N.M. van Os, Colloids Surf. A 118 (1996) 41. [16] F. Devinsky, I. Lacko, T. Imam, J. Colloid Interface Sci. 143 (1991) 336. [17] R. Zana, J. Colloid Interface Sci. 248 (2002) 203. [18] Y. You, O. Zheng, Y. Qiu, Y.H. Zheng, J.X. Zhao, G.B. Han, Acta Phys. Chim. Sin. 17 (2001) 4 [in Chinese]. [19] R. Zana, M. In, H. Levy, G. Duportail, Langmuir 13 (1997) 5552. [20] D.C. Dong, M.A. Winnik, Can. J. Chem. 62 (1984) 2560. [21] B. Valeur, Molecular Fluorescence, Principles and Applications, Wiley– VCH, New York, 2001. [22] D. Danino, Y. Talmon, R. Zana, Langmuir 11 (1995) 1448.
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