6454
J Phys. Chem. 1989, 93, 6454-6458
Associiatlon of Anionic Surfactants with 6-Cyclodextrin. Fluorescence-Probed Studies on the 1:l and 1:2 Complexation Joon Woo Park* and Hye Jin Song Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea (Received: December 6, 1988: In Final Form: March 28. 1989)
Formation of inclusion complexes between anionic surfactants, alkanesulfonates (n = 5-8, 10, 12) and alkyl sulfates (n = 8, 10, 12, 14, 16, 18), and P-cyclodextrin was studied by a fluorometricmethod using 1-anilinonaphthalene-8-sulfonate (1,8-ANS) and 2-anilinonaphthalene-7-sulfonate(2,7-ANS) as probes. The association constants for 1:1 complexes of 1,8-ANS and 2,7-ANS with p-CD were determined as 85 and 1500 M-’, respectively, at 25 “C. The first and second association constants of the surfactants with P-CD were calculated from competitive binding data of the surfactants and the ANS’s with p-CD. The 2:l P-CD-surfactant complexes were formed with alkanesulfonates of n 1 10 and alkyl sulfates of n 1 8. The surfactant-0-CD association was accompanied by a large increase in entropy, and hydrophobic binding appeared to contribute to a great extent to the association. Self-association of 1:l complexes of surfactants of long hydrocarbon chains was also suggested.
Introduction Cyclodextrins (CD) form inclusion complexes with a variety of hydrophobic and amphiphilic species. Because of this characteristic, they are widely used as biomimetic systems and as novel media for chemical, photophysical, and photochemical studies.’ A number of groups have reported the results of studies on the association of surfactants with CD.2-8 Also, formation of ternary complexes of aromatic hydrocarbons with surfactant-CD complexes has been For surfactant-CD complexes, a 1:l stoichiometry has usually been assumed. However, in some cases, formation of 2: 1-type CDsurfactant complexes has been s ~ g g e s t e d ,but ~ . ~the association constants for such complexes are not known. Since neither C D nor surfactant molecules bear a chromophore, the usual spectroscopic methods cannot be applied to determine the association constants of surfactants with CD. Instead, conductometric methods with nonlinear data fitting have been sed.^^,^ However, the reported association constants differ widely among investigators. Moreover, the strong dependency of the association constant on the concentration of surfactant has been reported for 8-CD with surfactants having hydrocarbon chains longer than the depth of the cavity of P-CD.’ These anomalies seem to arise from the presumption of 1 :1 stoichiometry for the complexes and from the small difference in electric conductivity between the associated and unassociated surfactant ions, which makes it difficult to evaluate the association constants with reasonable accuracy. Therefore, a more reliable and convenient method for determining association constants is required. In this paper, we present results of fluorescence studies of the association of anionic surfactants with p-CD. The fluorescent anilinonaphthalenesulfonateswere used as competitive inhibitors for the association.12 The 1:l and 2:lb-CD-surfactant association ( I ) For reviews, see: (a) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: New York, 1977. (b) Fendler, J. H. Membrane Mimefic Chemistry; Wiley-Interscience: New York, 1982; pp 194-201, (c) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems: Academic Press: New York, 1987; pp 300-317. (2) Okubo, T.; Kitano, H.; Ise, N. J. Phys. Chem. 1976, 80, 2661. (3) Satake, 1.; Ikenoue, T.; Takeshita, T.; Hayakawa, K.; Maeda, T. Bull. Chem. SOC.Jpn. 1985, 58, 2746. (4) Satake, 1.; Yoshida, S.; Hayakawa, K.: Maeda, T.; Kusumoto, Y . Bull. Chem. SOC.Jpn. 1986, 59, 3991. (5) Hersey, A.; Robinson, 8.H.: Kelly, H. C. J . Chem. SOC.,Faraday Trans. 1 1986, 82, 127 I . (6) Georges, J.; Desmettre, S. J . Colloid Interface Sci. 1987, 118, 192. (7) Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1988, 66, 325. ( 8 ) Jobe, D. J.; Verrall, R. E.; Palepu, R.; Reinsborough, V. C. J . Phys. Chem. 1988, 92, 3582. (9) Edwards, H. E.; Thomas, J. K. Carbohydr. Res. 1978, 65, 173. (IO) Hashimoto, S.; Thomas, J. K. J . Am. Chem. SOC.1985, 107, 4655. ( I 1 ) Kusumoto, Y.; Shizuka, M.; Satake, I. Chem. Phys. Left. 1986, 125, 64. (12) Tabushi. 1.: Kurcda, Y . ; Mizutani, T. J . Am. Chem. SOC.1986, 108, 45 14.
0022-3654/89/2093-6454$01.50/0
constants were obtained for various alkyl sulfates and alkanesulfonates, and the results were correlated with the chain lengths of the alkyl groups.
Experimental Section Materials. Surfactants used in this study are sodium salts of n-alkanesulfonates C,S ( n = 5-8, 10, 12) and n-alkyl sulfates C,OS ( n = 8, 10, 12, 14, 16, 18). The alkanesulfonates, with the exception of C5S (Sigma), were obtained from Aldrich. ClzOS was purchased from Fluka, and the other alkyl sulfates were from Merck. The highest purity chemicals from the respective sources were obtained. Except sodium dodecyl sulfate (SDS, C,20S), which was recrystallized from ethanol three times after it was washed with ether, the surfactants were used as received. p-CD was purchased from Aldrich, and concentrations of the 0-CD solutions were calculated from optical rotation data taken with a Jasco DIP-I40 polarimeter at 25 OC with [a]25D = 162.5°.1a Sodium salts of 1-anilino-8-naphthalenesulfonate(1,I-ANS) and 2-anilino-7-naphthalenesulfonate (2,7-ANS) were obtained from Molecular Probes and used without further purification. Water was deionized and distilled in glass. Fluorescence spectra were recorded on a Hitachi Model 65010s spectrofluorimeter at 25 “C, unless otherwise mentioned. The excitation wavelengths were 350 and 320 nm for 1,8-ANS and 2,7-ANS solutions, respectively. Association constants of ANS’s with /3-CD were determined from dependencies of fluorescence intensities of A N S on the concentration of P-CD.I3 For this, the concentrations of ANS were fixed at 1.0 X M for 1,8-ANS and at 1.0 X 10” M for 2,7-ANS. The concentration ranges of p-CD were (0.5-10) X M for 1,8-ANS and (0.1-1.0) X M for 2,7-ANS. The association constants of surfactants with p-CD were determined by following the fluorescence intensity of ANS-P-CD solutions as function of concentration of surfactants (see Results and Discussion for details). Results and Discussion Association Constants of ANS‘s with p-CD. Addition of p-CD to ANS solutions remarkably enhanced the fluorescence intensity of ANS’s, as shown in Figure la. This is consistent with previous report^'^,^^ and indicates the transfer of A N S molecules from aqueous medium into the apolar P-CD cavity by inclusion complexation. The solute entry and exit rate constants into and from p-CD are usually on the order of lo7 M-’ s-’ and lo4 s-’, respectively, while the rate constant for the decay of the excited state (13) Tabuchi, I.; Shimokawa, K.; Shimizu, N.; Shirakata, H.; Fujita, K. J . Am. Chem. SOC.1976, 98, 7855. ( 1 4) Cramer, F.; Saenger, W.; Spatz, H.-Ch. J . Am. Chem. SOC.1967.89,
14.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6455
Association of Surfactants with 0-Cyclodextrin
80
c
[S-CD],=lO
[ceos],, = o
mM
TABLE I: 1:l Association Constants ( K 3 of ANS’s with 8-CD and Enhancement of Fluorescence Intensity of ANS’s upon Complexation with 8-CD at 25 OC in Water ANS K‘, M-’ I c D / I w ANS K’,M-‘ I C D J I ,
mM
A
1,s-ANS
85“
2063‘
2,7-ANS
‘Reported value is 58 M-’.I3 bReported as nm. dMeasured at 460 nm.
0
I
550
500
450
1
I
550
I
500
I
450
X /nm Figure 1. Fluorescence spectra of 1.0 X lo-’ M 1,S-ANS solutions at 25 “C in water: (a) at various concentrations of @-CDin the absence of surfactant and (b) at various concentrations of n-octyl sulfate in the presence of 10 mM @CD. The excitation wavelength was 350 nm. The numbers in the spectra correspond to concentrations of @-CD(a) and the surfactant (b) in millimolar units.
-1 3 2
4
10
I
\
8 t
1
O\
+y&.-77:, OO
20
40
60
80
AI Figure 2. Benesi-Hildebrand-type plots of ANS-0-CD complexation data: ( 0 ) 1,8-ANS-@-CDand (0)2,7-ANS-@-CD. The highest concentrations of @-CDwere 10 mM for 1,s-ANSand 1.0 mM for 2,7-ANS.
of A N S is on the order of IO8 s-I.l5 Thus, there is little chance for the excited ANS molecules to enter or exit p-CD during their lifetimes. Therefore, we can soundly assume that the enhancement of fluorescence intensity of ANS solutions by the addition of p-CD reflects ground-state association of ANS’s with p-CD. If ANS’s form 1:l-type complexes with 0-CD, the increase in fluorescence intensity of A N S solutions, AI, by the addition of p-CD is related to the concentration of uncomplexed fl-CD, [pCD], by a Benesi and Hildebrand type equation16
where K’is the association constant and AI, is the maximum change in the fluorescence intensity when all of the ANS molecules form the complex. When the total concentration of p-CD, [pCD],, is much higher than the total concentration of ANS, [ANSIo, [p-CD] can be replaced by [p-CD],. The plots of experimental data according to eq 1 are shown in Figure 2 and give (15) Turro, N. J.; Okubo, T.; Chung, C.-J. J . Am. Chem. SOC.1982, 104, 1789. (16) Benesi, H. A.; Hildebrand, H. J . Am. Chem. SOC.1949, 71, 2703.
1500
12d
CMeasuredat 500
good straight lines with a coefficient of correlation better than 0.998 under the experimental conditions of [p-CD] I10 mM for 1,8-ANS and [p-CD] I1 mM for 2,7-ANS. When the concentration of @-CDexceeded 3 mM, the Benesi-Hildebrand-type plot of 2,7-ANS-B-CD showed significant deviation from linearity and the spectral peak shifted gradually to a shorter wavelength. 2-@-Toluidinyl)naphthalene-6-sulfonate (2,6-TNS) and 24Nmethylanilino)naphthalene-6-sulfonate (2,6-MANS) showed trends similar to those observed with 2,7-ANS above 0-CD concentrations of 1 and 4 mM, respectively. These are indications of the formation of higher complexes with the ANS’s at high concentrations of p-CD. The second association constant of 2,6-TNS with p-CD was reported as being in the range of 3-14 M-I from fluorescent methods.8 We cannot rule out the possibilities of the formation of a 1:2 complex of 1,8-ANS with p-CD and the presence of a 1:2 complex of 2,7-ANS-P-CD at higher concentrations of p-CD. However, the second association constants of the ANS’s with 0-CD might be too small, and thus, the fraction of ANS’s present as higher complexes would not be significant enough to influence the determination of 1:1 association constants and the analysis of surfactant-P-CD complexation data (see the following section) under the experimental conditions. The good linearity of the Benesi-Hildebrand plots shown in Figure 2 supports these views. The 1:l association constants and AI, were calculated from the plots. The results are summarized in Table 1. (Details on the complexation between ANS’s and p-CD will be reported elsewhere.) Association Constants of Surfactants with p-CD. The addition of the anionic surfactants to the ANS-P-CD solutions decreased the fluorescence intensity. However, the spectral position and shape of a spectrum taken at a high concentration of p-CD with surfactant were not different from those of a spectrum obtained at a lower concentration of p-CD without surfactant, whenever the intensity of the two spectra is the same. Figure l b shows this for n-octyl sulfate. No effect of surfactants on the fluorescence spectra of ANS is observed when p-CD is absent from the solution~.~’These observations suggest that neither binary complexes between ANS’s and the surfactants nor ternary complexes of ANS-P-CD-surfactant are formed in the experimental conditions,” presumably due to electrostatic repulsion between the negatively charged A N S and the surfactants. The variations of the fluorescence intensity at 500 nm with the concentrations of surfactants for various 1,8-ANS-p-CD systems are shown in Figure 3. The effect of the surfactants on the fluorescence intensity of ANS in solutions containing p-CD can be understood by complex formation of the surfactants with p-CD, which depletes free p-CD and thus results in dissociation of the ANS-P-CD complexes. The equilibrium concentrations of uncomplexed p-CD at various surfactant concentrations were calculated from eq l with A I values and the parameters of ANSp-CD associations given in Table I. From this, the concentrations of (3-CD associated with the surfactants, [p-CD],, were calculated. The results are plotted as functions of the total concentration of surfactant [SIoin Figure 4. This figure clearly shows that, with the exception of alkanesulfonates of n 5 8 among the surfactants (17) Enhancement of the fluorescence intensity of ANS by the addition of surfactants was observed above the cmc of the surfactants. This accords with the reports of the binding of ANS on anionic micelles.l8 The conductometric titration of @-CDsolutions with surfactants under the condition of [surf] < [@-CD]did not reveal any evidence of the formation of micelles. This agrees well with a report of negligible surface activity of CTAB when the concentration of CTAB is lower than that of ,T-CD.2 (18) Birdi, K. S.; Singh, H. N.; Dalsager, S . U. J . Phys. Chem. 1979,83, 2733, and references cited therein.
6456
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989
60
-u. 4 0 9
20
5 c o n c. / mM
10
Figure 3. Plots of emission intensity of 1.0 X M 1,8-ANS solutions at 500 nm as a function of [p-CD] ( 0 )and as functions of the concentration of surfactants at a fixed concentration of 8-CD, 10 mM.
Park and Song Two different approaches are made to determine the association constants. Method 1. This method uses prior assumptions on the magnitude of values of association constants and considers only one complexation step. If one assumes that the higher association constants K2 and K3 are very small, or that only the 1:l complex can be formed because of geometrical reasons, the values of K, are calculated from eq 2 by substituting [p-CD], for [p-CD-SI and [SIo- [P-CD], as [SI. Each AIvalue obtained for a different surfactant concentration gives a calculated K, value. Consistency of the association constant for a given surfactant-0-CD system, regardless of concentrations of surfactant and p-CD, indicates validity of the prior assumptions. This was the case for alkanesulfonates with n I8. This implies that only 1:1 complexation is indeed involved in these surfactant-p-CD systems. For C,OS, good agreement among K1 values calculated from different surfactant concentrations at constant [p-CD], was observed only when the concentration of p-CD is low, e.g., 1 mM. The other systems with surfactants of longer hydrocarbon chains did not yield constant KI values: the calculated value of K, depended strongly on the concentrations of both surfactant and p-CD. This suggests that complexations other than the presumed 1:l complexation also take place in the systems. For surfactants with long hydrocarbon chains, the K, value might be large enough to ensure that all surfactant molecules are associated with p-CD when [SI, > 1. If we consider up to 2: 1 complexation, the second association constant K2 in eq 3 can be rewritten as eq 5 under these conditions. For solutions of [p-CD], K2 =
[p-cDlb - [SI0 (2[Sl, - [P-cDlb)[P-CDl
(5)
= 10 mM, reasonably constant K2 values were obtained when [SI, was less than 3 mM for CloS, CI2S,and ClOOS. However, for alkyl sulfates of longer alkyl chain length, the calculated values of K2 varied widely with the concentration of the surfactants and consistent K2 values were obtained only at lower p-CD concentration, 1 mM, when [SI, < 0.3 mM. Two possible causes can be suggested for the failure to obtain constant Kz values by this method at a high concentration of p-CD, even though the prior condition of K1[p-CD] >> 1 is more closely met at the situation. One is the formation of complexes with more than two @-CD molecules per surfactant molecule. The other is self-association of the 1:l complexes, which is discussed later. Method 2. When eq 2-4 are combined and the mass balances of the surfactants and 6-CD are used, the total surfactant concentration is related to [p-CD] by eq 6. The association constants
c
5
[SI0 = 1 + K,[P-CD]
10
[SIo I mM
Figure 4. Plots of the concentrationsof surfactant-bund p-CD against the total concentrations of the surfactants. The total concentrations of 0-CD were 10 mM for alkyl sulfates and 7.0 mM for alkanesulfonates.
used, the amounts of p-CD complexed with the surfactants are greater than the total concentration of surfactant added when [SI, < [p-CD],. Moreover, the [p-CD], to [SI, ratio exceeds 2 for CI6OSand C180S. This can be taken as an evidence of formation of complexes with more than one p-CD per surfactant molecule. Therefore, multiple equilibria between p-CD and surfactant molecules are considered:
(3)
(4)
+ K1K2[/3-CDlZ+ K,KZK3[/3-CDl3 ([P-CDIo K,[P-CD] + 2K1K2[p-CDlZ + 3K1K2K3[P-CDI3 [P-CDI) ( 6 )
K,, K2, and K3 can be calculated by nonlinear least-squares regression analysis. On the basis of the results from method 1 and the geometries of the surfactants and 8-CD molecules, we assumed K2 = K3 = 0 for surfactants with n II and K3 = 0 for 8 In I 14. We also attempted to fit the data without these prior assumptions and obtained the association constants, which were assumed 0, as less than 10 M-l. This value seems too small to have physical meaning. Also, the fitting of experimental data was much better with the prior assumptions. For the alkanesulfonates used ( n I12) and C,OS, the calculated K2values did not depend on the initial concentration of p-CD. However, for the alkyl sulfates of n L 10, the calculated K2 values from data taken in the presence of 1 mM p-CD were always greater than the values calculated from data obtained in the presence of 10 mM p-CD (see Table 111). The values of K2 from the former condition, which agreed well with those from method 1 , are considered more reliable and are listed in Table 11. For alkanesulfonates of n = 5-8, the values of K1 determined in this study agree well with the results of a conductometric study
The Journal of Physical Chemistry, Vol. 93, No. 17, 1989 6457
Association of Surfactants with 0-Cyclodextrin TABLE 11: Association Constants of Anionic Surfactants with &CD at 25 OC n
KI,O M-I this work
lit. value
K2,0 M-’ this work
5 6 7 8 10 12
C~H~~+ISOJ” 45 (43 f 8) 60b 177b 163 ( I 6 0 & 12) 436b 434 (464 & 25) 1 180 (1 050 f 200) 1O3Ob 16 (20 8) 5360 4O9Ob 40 (43 & 19) 16100 4340b
8 IO 12
2560 8750 25600
*
14 48200 16 56300 16 g
C,H~,+IOSO~N~ 13 56Y 58 2310‘ 200 300;d 356c 1380-7230‘ 3630b 604 1560 2860
(54 f 7) (220 & 90)
430
I
’I 4 t
-
.p
110
I i
It(560 & 150) (1 400 i 240)
‘Numbers in parentheses were determined by method I . bReference 4. cReference 7. dReference 6. CReference 2. ’Formation of 3:l complex with K3 0 100 M-I was also suggested. KToo large to be determined accurately.
by Satake et aL4 These surfactants form only 1:l complexes. However, for the other surfactants that bear longer alkyl chains and thus also form 2:l p-CD-surfactant complexes, the values of K , determined in this study are much larger than those reported by others from the analysis of conductometric titration data assuming only 1:l-type association. It can be also noticed from Table I1 that the value reported for K l for CI2OS (SDS) with 0-CD varies from 300 to 7230 M-I. Furthermore, Palepu and Reinsborough showed from conductometric titration of CI2OS with p-CD that the apparent K l value increased from 1380 to 7230 M-I as the initial concentration of C 1 2 0 Swas lowered from 5 mM to 1 mM.7 Also, an apparent dependence of the difference in electric conductance between unassociated and associated surfactants on the [C120S]owas shown. These observations cannot be explained from a thermodynamic point of view, unless one assumes multiple equilibria between the surfactant and p-CD. As [C120S]ois lowered, the formation of the 2:l complex becomes less significant and the value of K l calculated on the assumption of 1:1 complexation approaches the value defined in eq 2. The trend of the apparent KI vs [CI2OS],, reported by Palepu and Reinsborough’ seems to support the validity of the values of the association constants determined in this study. Thermodynamics and Alkyl Chain Length Dependence of the Associations. We have determined the standard free energy, enthalpy, and entropy of the 1:l complexation of one surfactant, C7S, with P-CD from the dependence of the association constant on temperature: the values are -15.1 kJ/mol, -4.1 kJ/mol, and +36.9 J/K per mol, respectively, at 25 OC. This is in fairly good agreement with reported values4 and suggests that the inclusion complexation is largely an entropy-driven process. The 1:l association constant of C7S with p-CD was decreased to a great extent by the addition of N,N-dimethylformamide (DMF), a well-known chaotropic agent that reduces the hydrophobic attractive force: the value dropped to 137 M-’ in 5.0% (v/v) aqueous D M F solution from 434 M-’ in water. These results support earlier conclusions that the major driving force for the complex formation of amphiphilic molecules with CD is apolar hydrophobic binding.I9 The inclusion of the polar ionic head groups, sulfonate and sulfate, of the surfactant in the cavity of p-CD is expected to be disfavored by the large desolvation energy.20 This view is sup(19) (a) Komiyama, M.; Bender, M. L. J . Am. Chem. Soc. 1978, 100, 2259. (b) Schneider, H.; Kramer, R.;Simova, S.; Schneider, U. J . Am. Chem. SOC.1988, IIO, 6442. (20) Harata2’ reported that inclusion of the sulfonate group of benzenesulfonate in a-CD is also energetically favorable but that the complex is less stable than the one in which the apolar benzene group is included. This estimation has been criticized as too oversimplified.”
10
6
14
18
n Figure 5. Dependence of the surfactant-@-CD association constants and the standard free energy change on the chain length of the hydrocarbon tail of the surfactants: ( 0 )K , for alkyl sulfates; ( 0 )K1 for alkanesulfonates; and (0) K2 for alkyl sulfates.
ported by X-ray crystallographic studies on the benzenesulfonate-a-CD2’ and 4-[4-(dimethylamino)phenylazo]benzenesulfonate (methyl orange)-cu-CD22complexes, which revealed that the sulfonate groups protrude from the P-CD cavity. Consequently, it can be reasonably assumed that the apolar hydrocarbon chains of the surfactants are included in the p-CD cavity in the surfactants-P-CD complexes. Both the internal diameter and the depth of the cavity of p-CD are about 7 A.Ia Thus, the volume of the cavity of p-CD is about 270 A). The length ( L ) and the volume (V) of the fully extended hydrocarbon chain CnHWlare estimated as L / A = 1.5 + 1.265(n - 1) and V / A 3 = 27.4 26.9(n - l).23 Each kink created by two gauche connections reduces the length by 1.25 A but increases the volume by 20-50 A3.” Thus, the maximum number of carbon atoms (n) of an aliphatic hydrocarbon chain that can be accommodated inside the cavity of p-CD is 8. Since the ether oxygen of the sulfate group usually behaves as an extra methylene group in the hydrophobic interaction, the number would be less by one for alkyl sulfates. This estimation is in good agreement with the observations of the formation of 2:l complexes only with alkanesulfonates of n 2 10 and the alkyl sulfates of n I 8.25 These 2: 1-type complexes very probably have a channel structure built by the stacking of P-CD molecules with the hydrocarbon tails of the surfactants located in the channel. Variations of the association constants and of the standard free energy change of the association with carbon numbers of hydrocarbon chain of the surfactants are shown in Figure 5. For the alkanesulfonates of n I8, the free energy change varies linearly with n and follows
+
AGO = 3.7 - 2.67n (kJ/mol)
(7)
This increment in AGO of about -2.67 kJ/mol per methylene group is slightly less than that observed in micellization processes of aliphatic surfactants (about -3 kJ/mol). Interestingly, Figure (21) Harata, K. Bull. Chem. Soc. Jpn. 1976, 49, 2066. (22) Harata, K. Bull. Chem. SOC.Jpn. 1976, 49, 1493. (23) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Wiley: New York, 1980; pp 51-53. (24) Reference lb, p 122. (25) The formation of 2:l a-CDalkyl sulfate complexes was reported by Hersey et al.’ from kinetic methods based on competition experiments. The stability constants of the complexes ( K I K 2in this paper) ranged from 2.9 X IO’ (for n = 8) to 1.4 X IO9 M-’ (for n = 14). These values are 2-3 orders of magnitude larger than the corresponding values for j3-CD complexes determined in this study. This is consistent with an observation of higher K, values for alkanesulfonates of n 8 with a-CD than with j3-CD.’ A possible explanation for this is the difference of the cavity size of the host molecules as suggested by a reviewer: the hydrocarbon tail of a surfactant fits snugly into a-CD but rattles around inside the j3-CD cavity.
6458
J . Phys. Chem. 1989, 93. 6458-6463
TABLE 111: Comparison of the Apparent Second Association Constants of Alkyl Sulfates with j3-CD at Different Concentrations of B-CD, Calculated by Nonlinear Least-Squares Regression K2, MV1 n [P-CDln = 10 mM [P-CD], = 1 mM 10 12 14 16 18
69 290 810 1510
200 600 1560 2860
5 does not show saturation phenomena. Rather the 1:l association constants and thus the standard free energy change (-AGO) of the association increase steadily with the length of hydrocarbon chain, beyond the maximum length that can be included in a (3-CD cavity. However, the increment in the -AGO per methylene group becomes progressively less as the length exceeds that for n = 8. A plausible explanation for this observation is interaction of the protruding hydrocarbon tail with the external hydrated surface of p-CD. This interaction might be energetically favored over the interaction with bulk water, which forms entropically disfavored clusters. However, we cannot rule out the possibility of a contribution from some contraction in the length of the hydrocarbon chain by trans to gauche conformational changes as n increases. Association of 1 1 1 Complexes. Table I11 shows the apparent second association constants Kzof the alkyl sulfates of n 1 8 with p-CD determined by nonlinear least-squares regression of data taken at two different total p-CD concentrations. The disagreement between the two sets of results implies that, in addition to the simple successive associations of p-CD with the surfactant molecules as defined in eq 2-4, another equilibrium is also involved
in the systems. The hydrocarbon chains of the surfactant molecules exhibiting the discrepancy are longer than the depth of the cavity of p-CD. Thus, the 1:l surfactant-/3-CD complexes can associate by themselves and possibly with free surfactant molecules as well as with another 0-CD molecule. The self-association is reminiscent of the formation of a 2:2 naphthalene-0-CD complex by association of the 1:l complexesz6 and the formation of three-component 1 :1: 1 complexes in pyrene-P-CD-surfactant systems.9-" Obviously, the possibility of these associations is greater at higher concentrations of surfactants and p-CD. When this is the case, the calculated values of Kzwould be smaller than those defined in eq 2. Our results in Table I11 seem to reveal this. In conclusion, it has been shown that the competitive binding of ANS's and anionic surfactants with 0-CD enables us to determine the association constants of surfactants with p-CD by a fluorescence method. Alkyl sulfates of n 1 8 and alkanesulfonates of n 1 10 form 2:l P-CD-surfactant complexes, in addition to 1:l complexes. The association is driven by the large increase in entropy accompanying hydrophobic binding. It may be expected that the procedure outlined in this paper can be applied to many other systems and conditions, provided that a proper probe is chosen. Acknowledgment. We thank the Ministry of Education of the Republic of Korea for support of this work through the Basic Research Institute Program. We are grateful to Prof. J. Edward of McGill University for reading the manuscript. Registry No. I,O-ANS, 1445-19-8; 2,7-ANS, 121472-43-3; C5S, 22167-49-3; CIS, 5324-84-5; CI$, 13419-61-9; C,2S, 2386-53-0; CBOS, 142-34-1; CiOOS, 142-87-0 C12OS, 151-21-3; C,,OS, 1191-50-0; Ci,OS, 1120-01-0; C,,OS, 11 20-04-3; P-cyclodextrin, 7585-39-9. (26) Hamai, S. Bull. Chem. SOC.Jpn. 1982, 55, 2721.
NMR Study of Organic Counterion Binding and Micellization of Decylammonium Dicarboxylate Surfactants Puyong Li,*?+Mikael Jamson,* Pratap Bahadur,s and Peter Stilbs' Department of Physical Chemistry, The Royal Institute of Technology, S-100 44 Stockholm 70, Sweden, Institute of Physical Chemistry, Box 532, S-751 21 Uppsala, Sweden, and Department of Chemistry, South Gujarat University, S w a t 395 007, India (Received: December 9, 1988; In Final Form: April 10, 1989)
Decylammonium surfactants with dicarboxylate counterions (malonate, succinate, glutarate, adipate, pimelate, and suberate) in D 2 0solutions were studied by IH NMR self-diffusion and I3CNMR chemical shift measurements. The degree of counterion binding, the cmc, and the free surfactant concentration are discussed in terms of electrostatic and hydrophobic interactions between the counterions and the micelle. The results found for the dicarboxylate counterions are compared with the results obtained for monocarboxylate counterions studied in an earlier paper (Jansson, M.; Stilbs, P. J . Phys. Chem. 1987, 91, 113 ) . The increasing hydrophobic character of the divalent counterions does not give rise to an increased counterion binding until more than four methylene groups are present in the counterion chain. The highest degree of counterion binding was found for the malonate ion whereas the lowest one was found for adipate ion. However, the cmc was found to be monotonically decreasing with the methylene chain length. The systems of the first five members in the homologous series were found to form spherical micelles while decylammonium suberate was found to form larger micelles.
1. Introduction The Fourier transform pulse gradient spin-echo (FT PGSE) N M R technique has proven to be a powerful method for the investigation of counterion binding to micelles,z The simultaneous monitoring of the self-diffusion coefficients of the components in
these systems2 provides direct access to the fraction of micellized counterions and amphiphiles. There have been many studies concerned with inorganic counterion binding to surfactant but only a few ( 1 ) Jansson, M.; Stilbs, P. J . Phys. Chem. 1987, 91, 113. (2) Stilbs, P. Prog. N M R Spectrosc. 1987, 19, 1.
'The Royal Institute of Technology. Institute of Physical Chemistry. *South Gujarat University. f
(3) Gunnarsson, G.; Jonsson, B.; Wennerstrom, H. J. Phys. Chem. 1980, 84, 3114. (4) Jonsson, B. Ph.D. Thesis, University of Lund, Sweden, 1981.
0022-3654/89/2093-6458$01.50/00 1989 American Chemical Society
