Langmuir 1993,9,1213-1219
1213
Effect of the Presence of B-Cyclodextrin on the Micellization Process of Sodium Dodecyl Sulfate or Sodium Perfluorooctanoate in Water E. Junquera,’ G. Tardajos, and E. Aicart Departamento de QuEmica Fkrica I, Facultad de Ciencias Qutmicas, Universidad Complutense de Madrid, 28040 Madrid, Spain Received July 6, 1992. I n Final Form: February 10,1993 The encapsulation processes of sodium dodecyl sulfate (SDS) or sodium perfluorooctanoate (SPFO) monomers into the cavity of 8-Cyclodextrin (8-CD)and its effect in the micellizationprocess of the surfactant itself have been analyzed by measuring the speed of sound, u, at 298.15 K (a) as a function of [surfactant] in the presence of various constant concentrations of B-CD and (b) as a function of [P-CDI at different surfactant constant concentrationsboth in the premicellar and micellar regions. The predominant complex formed (8-CD:surfactant)in both cases has a stoichiometry of 1:1 and the association constants K have been determined from speed of sound measurements by using a semiempirical model proposed by us previously. The apparent critical micellar concentration, cmc*, is found to increase upon the addition of cyclodextrin,for both systems. However,the concentrationof free surfactant availablefor the micelliiation process in the postmicellar region when the cyclodextrin is present, [surflf, remains constant in the case of SDS + 8-CD and presents an overall increase in the case of SPFO + 8-CD. Introduction Cyclodextrins are cyclic carbohydrates consisting of six, seven, or eight a-D-glUCOpy”iOSe units and respectively called a-,8-, and y-cyclodextrins. They are produced by the enzymatic conversion of starch, followed by an ellaborate purification process. Cyclodextrins have a unique spatial configuration, with a hydrophilic outside and a hydrophobic inside. The hydrophobic cavity forms an ideal harbor in which poorly water-soluble molecules can shelter their most hydrophobic parts. Contact between such a poorly soluble compound and a cyclodextrin in aqueous environment can result in complexation. Due to the hydrophilic outside of the cyclodextrin, such a complex is a soluble entity on its own. Thus, cyclodextrins (or CD’s) have been well-known to form inclusion compounds with a variety of molecular species1p2by several kinds of driving forces: where hydrophobic interaction has been found to play an important role.'^"^ A number of studies such as, thermodynamic, X-ray, and UV absorption have been carried out in order to clarify the mechanism of inclusion complex formation and they are well reviewed by S a e n g e ~ ~ Of the three most important cyclodextrins, 8-CD (with a cavity diameter of 6 . H . 4 A), is the cyclodextrin of most interest because ita cavity size allows for the best spatial fit for many common guest moieties such as surfactant hydrocarbon tails.8~9 Ita use in the food, drug, and agricultural industries is growing rapidly as an encapsulating agent to protect sensitive molecules in hostile environmenta,”’-16 and it would be of special interest to (1)Cramer, F.; Hettler, H. Naturwissenschaften 1967, 54, 625. (2) Thoma, J. T.; Steward, L. Starch: Chemistry and Technology; Whistler, R. L., Paschall, E. F., Eds.; Academic Press: New York, 1965; Vol. 1, p 209. (3) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: Berlin, 1978. (4) Komiyama, M.; Bender, M. L. J.Am. Chem. SOC. 1978,100,2259. (5) Saenger, W. Angew. Chem., Znt. Ed. Engl. 1980, 19, 344. (6) Saenger, W. Inclusion Compounds; Atwood, J. L.; Davies, J. E. D.; Mac Nicol, D. D., Eds.; Academic Press: London, 1984; Vol. 2. (7) Tanford, C. The Hydrophobic Effect. Formation of Micelles and Biological Membranes, 2nd ed.;John Wiley & Sons: New York, 1980. (8) Clarke, R. J.; Coates, J. H.; Lincoln, S. F. Adu. Carbohydr. Chem. and Biochem. 1988,46,205. (9) Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1988, 66, 325.
0743-746319312409-1213$04.OO/ 0
determine the fundamental processes at work when this particular cyclodextrin plays host to other molecules in solution. Surfactant molecules, which have an ionic head group as well as a large hydrocarbon chain of varying hydrophobicity, are expected to form complexes with CD’s by the inclusion of the hydrophobic chain of the surfactant into the apolar cavity of the cyclodextrin, affecting the micellization process of the surfactant itself. However, the overwhelming majority of publications in this field has usually involved conventional micelles composed of hydrocarbon backbones. Few studies of any type have been reported for micelles built up from perfluorinated carbon ~ h a i n s . l ~ -Recent ~l work has shown that fluorocarbon surfactants (with cross sectional diameter of 6 A) fit more snugly into the 8-CD cavity than the a-CD cavity.17@ In this connection, a detailed study of the inclusion of two anionic surfactants (one of them perfluorinated, such as SPFO) by the 8-CD in solution was deemed appropriate to shed light on this field. On the other hand, speed of sound measurements have been proved to be very powerful to detect even little structural changes involved in the formation of these inclusion complexes and, of course, in the parallel micel(10) Uekama, K. Pharm. Znt. 1985,6,61. (11) Szejtli, J. The Cyclodextrins and Their Inclusion Complexes; Academiai Kiado: Budapest, Hungary, 1982. (12) (a) VanEtten, R. L.; Sebastian, J. F.; Clowes, G. A.; Bender, M. L. J.Am. Chem. SOC. 1967,89,3242. (b) VanEtten, R. L.; Clowee, G.A.; Sebastian, J. F.; Bender, M. L. J. Am. Chem. SOC. 1967,89, 3253. (c) Perkin Trans. 2 1977, 432. (d) Kitano, H.; Okubo, T. J. Chem. SOC., Ihara, Y.;Nakanishi, E.; Nango, M.; Koga, J. Bull. Chem. SOC. Jpn. 1988, 59, 1901. (13) Straub, T. S.; Bender, M. L. J. Am. Chem. SOC. 1972,94,8875, 8881. (14) Cramer, F.; Kampe, W. J. Am. Chem. SOC. 1965,87,1116. (15) Komiyama, M.; Hirai, H. BdZ. Chem. Soc. Jpn. 1988,66,2853. (16) Breslow,R.;Campbell,P. J.Am. Chem.Soc. 1969,91,3085;Biorg. Chem. 1971,1,140. (17) Palepu, R.;Richardson, J. E.; Reinsborough,V. C.Langmir 1989, 5, 218. (18) Muller, N.; Simsohn, H. J. Phys. Chem. 1971, 75,942. (19) Muto, Y.; Yoda, K.; Yoahida, N.; Esumi, K.; Meguro, K.; BinanaLimbele, W.; h a , R. J. Colloid Interface Sci. 1989,130, 165. (20) Palepu, R.; Reinsborough, V. C. Can. J. Chem. 1989, 67, 1550. (21) Turro, N. J.; Lee, P. C. C. J. Phys. Chem. 1982,86,3367. (22) Jiang, X. K.; Gu, J. H.; Cheng, X. E.; Hui,Y. Z.Acta Chim. Sin. 1987,45, 159.
0 1993 American Chemical Society
Junquera et al.
1214 Langmuir, Vol. 9,No. 5, 1993 lization p r o c e ~ s .The ~ ~ ~characterization ~~ of the complexation phenomenom between SDS or SPFO and 8-CD, and its influence on micellar properties of surfactant itself, is the object of the present work. For that purpose, speed of sound of /3-CD + SDS or SPFO systems have been measured at 298.15 K in both senses, by forming micelles in the presence of complex and destroying them because of the inclusion process.
Experimental Section Materials. Sodium dodecyl sulfate (SDS) was Fluka puriss with purity of 99 mol % or greater. Sodium perfluorooctanoate (SPFO) was purchased from PCR, Inc., and j3-cyclodextrin (8CD) was purchased from Aldrich. All of them were used without further purification. j3-CD was found to consist of 13.5% water, by a thermogravimetric analysis (TG). Solutions were prepared with bidistilled, deionized (taken from a Millipore Super-Q system), and degasified water. Speed of Sound Measurements. A puke-echo-overlap technique of fiied path type at a frequency of 10 MHz was used to measure the speed of sound u in aqueous solutions of j3-CD + SDS and j3-CD + SPFO. This technique operates in a multipleecho mode with broad-band pulses, and the experimental procedure, as well as the details of the equipment, have been fully described in a previous paper.25 The mixtures were formed in a successivedilution cell and the second component was added by using a 655 Dosimat Metrohm buret with a precision of 2 X 10-9 m3. The buret cylinder is thermostated by a recirculation water circuit from the bath where the dilution cell is immersed and whose temperature is maintained at 298.15 K. Calibration of the distance between the transducer and the reflector was made from the speed of sound in pure water (1496.74 m d ) reported by Kroebel and MahrtSz6The reproducibility of u data is kO.02 mwl, and temperature stability is f l mK. The u measurements were made for aqueous solutions of surfactant and j3-CD in two complementary studies: (1)study I, speed of sound as a function of surfactant concentration for different constant j3-CD concentrations; (2) study 11, speed of sound as a function of j3-CD concentration for various constant surfactant concentrations, both in the premicellar and micellar zones. In study I, the range of [surfactant] goes from premicellar to micellar region, while the range of [B-CD] goes up to ita solubility limit in water (approximately 1.85mg/100 mL, which represents 0.016 M). In this study, the formation of micelles in the presence of the corresponding inclusion complexes is studied. In study 11, when [surfactant] < cmc (case a, premicellarregion)the formation of the complex is studied. When [surfactant] > cmc (case b, micellar region), the destruction of the micelles because of the complex formation is analyzed. Since in these cases (a and b) j3-CD is added to an initial surfactant solution, the cyclodextrin is always in the complex form, being then possible to scan a larger [P-CD] range.
3
Figure 1. Speed of Bound u for the systems j3-CD i SDS + HzO as a function of surfactant concentration at a constant [P-CDI [B-CDI = O.OO0 M; 0, [j3-CDl = 0.003 M; 0, at 298.15 K: [B-CD] = 0.009 M; A, [O-CD] = 0.012 M; 0,[B-CD] = 0.015 M.
*,
1505
1
1501
1497
** 4 1493
(23)Junquera, E.;Aicart, E.; Tardajos, G. J. Phys. Chem. 1992,96, 4533. (24)Junquera, E.;Tardajos, G.; Aicart, E. J. Colloid Interface Sci., in press.
(25)Tardajos, G.; DIaz PeAa, M.; Aicart, E. J. Chem. Thermodyn. 1986,18,683. (26)Kroebel, W.; Mahrt, K. H. Acustica 1976,35, 154.
-
* *+
'A
*
OO 0
1485
0
I
I
I
40
20
I
80
60
[SPFO]
100
/ mM
+
Figure 2. Speed of sound u for the system j3-CD SPFO + HzO as a function of surfactant concentration at a constant [B-CD] at 298.15 K: *, [b-CDI = O.OO0 M; 0,[B-CDI = 0.003 M 0 [j3-CD] = 0.006 M;A, [O-CD] = 0.009 M; 0,[@-CD]= 0.013 M; X, [j3-CD] = 0.018 M. Table I. Values of Apparent Critical Micellar Concentration (cmc*), [SDSIf, Stoichiometry of the Complex @-CD/SDS(A), and Association Constants ( K ) Dewnding on IB-CDl (Study I) tB-CDI, cmc*, tSDSlf, mM mM mM A K, M-l ~
Results and Discussion The ultrasonic velocities, u, of the systems HzO + 8-CD + SDS and HzO + 8-CD + SPFO as a function of surfactant concentration are shown in Figures 1 and 2. As can be clearly seen in these figures, there can be drawn two straight lines with different slopes, corresponding the point of intersection to the apparent critical micellar concentration, cmc*, defined as the cmc for the system surfactant + cyclodextrin. These cmc* values, shown in Table I and 11,were clearly observed to increase upon the addition of 8-CD. In the absence of 8-CD, both surfactants present cmc values in good agreement with literat~re.18,~~ The
**
O.OO0
3.273 8.900 12.050 15.083
8.44 10.79 15.32 17.36 20.30
~~
8.44 8.24 7.84 7.66 8.57
~
1.28 1.20 1.26 1.29 A = 1.3 f 0.1
~
~~
557 414 526
K - 500f 100
cmc for SPFO (32.5 X 10-3 M, Table II), compared with the value for the correspondinghidrocarbon counterpart, sodium octanoate (0.35M28),reveals the well-known fact that, with the number of carbon atoms the same, the fluorinated surfactants present a more hydrophobic character and consequently form micelles at lower concentration than their hydrocarbon homologous. (27) Miura, M.; Kodama, M. Bull. Chem. SOC.Jpn. 1972,45,428. (28)Mukerjee, P.; Mysels, K. J. CMC of Aqueous Surfactant Systems; NSRDS-NBS 36;US.Government Printing Office: Washington, DC, 1971.
1.5
-
(4 [b-CD]
-
1.0
= 0.003 N
3.0
[b-CD]
2.5
p
0.009 L1
0
-
a
'5
20
0
E
0
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.s
1
0
4 0.s
-
0
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0
0.5
0
0
0.0
*
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4
1
f
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0.0
(4
(4 [B-CD]
0.012 M
(29) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Young, C. Y.; Carey, M. C. J. Phys. Chem. 1980,84,1944. (30) Verrall, R. E.; J o b , D. J.; Skalski, B. Private communication.
[b-CD]
3:
0.015 L1
function-of [B-CDI, with [SPFOI constant and equal to 0.006M, plotted in Figure 4a (study IIa). In this case, the cyclodextrin is added to an initial solution of monomeric
Junquera et al.
1216 Langmuir, Vol. 9, No. 5, 1993 [SPFO]
-
0.001
u
4
0-
**
e*
** e**
4-
[SPFO]
-
.
*=*\
[SPFO]
0.054 Y
0.
I
lb
I [p-W]
-
0.088 Y
3b /
I
sb
di
Figure 4. (a) Experimental points and calculated values (solid line) of Au vs [p-CD] in the premicellar region at constant [SPFO] = 0.006 M. Experimental data of Au vs [B-CD] in the micellar region at constant [D-CDI: (b) [p-CD] = 0.041 M; (c) [P-CD] = 0.054 M;(d) [@-CD]= 0.068 M.
SPFO, resulting in the encapsulation of these monomers into the CD cavities. The stoichiometry of this complex, defined as the ratio A = [B-CD/SPFO], can be obtained in Figure 4a as the intersection point of the two straight lines which fit these Au experimentalvalues. The resulting A value ia approximately 1.21 B-CDSPFO, indicatingthat in this case the chief inclusion complex formed is mainly 1:las well, but the contribution of a possible 2:l complex is approximately 20%. In both cases, the agreement with literature value (obtained mainly from conductometrictechniques) is quite satiafactory.*J7 Once the B-CDlsurfactant inclusion complexes have been characterized through the stoichiometries A, the micellization process in the presence of B-CD can be fully analyzed. The cmc* values have been evaluated from data plotted in Figures 1and 2 (study I),and the concentration of free surfactant available for the micellization process, [surflf,can be also calculated using the expression [surf], = cmc* - [surfl,,
= cmc* - [@-CDI/A (1)
where [surfl,is the concentration of surfactant forming the complex and [B-CD] the concentration of cyclodextrin kept constant in each case. The calculated [surflf values are presented in Tables I and 11. In Figures 4(b-d) and 5, experimental values of Au are plotted as a function of [B-CDI for different micellar solutions for both systems. The addition of cyclodextrin to the micellar solution resulta in the formation of the complex, decreasing the concentration of micelles. When there is not surfactant left to form micelles in the presence of this complex, a change in Au can be observed. There
can be drawn two straight lines whose intersection corresponds to this particular 18-CD] at which all the micellesare broken. The resulting cmc* and [surflfvalues have been determined by using also expression 1and are summarized in Table 111. In Figures 6 and 7, both quantities are plotted vs [B-CD] together with the corresponding values obtained from study I, for the systems B-CD + SDS and 8-CD + SPFO, respectively. As can be clearly seen, the apparent critical micellar concentration cmc* increases with [&CD] for both surfactanta. However, [SPFOIf shows firstly a slight decrease at low 18-CDI to increase as long as 18-CDI is larger than ita solubility limit, while [SDSIfremains almost constant, indicating that the presence of cyclodextrin does not modify the amount of monomeric SDS available for the micellization process. In other words, cmc* increases with [B-CD] because until all the cyclodextrin present is complexed, there is not enough surfactant to form micelles. From expression 1, if [SDSlf is constant, an increase in [SDSI,, resulta in the increase in cmc*. Anyway, in both Figures 6 and 7, extrapolation to zero cyclodextrin concentration, for both lines,yields the cmc for pure surfactants,in good agreement with the value measured by us (Tables I and 11)and also with the literature.18*27 On the other hand, it can be observed that forming micelles in the presence of cyclodextrin and/or complex (study I) or destroying them because of this complex formation (study IIb) drives us to the same resulta. The values of the binding constant K of these inclusion complexesare nowadays the center of a great controversy. Assuming that the inclusion process proceeds via a single
Effect of 8-CD on Micellization
e
(4
Langmuir, Vol. 9, No.5, 1993 1217
[SDS]
I
-
[SDS]
0.018 Y
-
0.024 Y
6
i
s
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I
.
I
1
0
I
2
I
4
8
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mn . Figure 5. Speed of sound Au for the systems 8-CD + SDS + H20 as a function of 8-CD concentration at a constant micellar [SDS]: (a) [SDSI = 0.012 M; (b) [SDSI = 0.015 M, (c) [SDSI = 0.018 M (d) [SDSI = 0.024 M. /
Table 111. Values of Apparent Critical Micellar Concentration (cmc*) and [surflf, Calculated from Study I1 ( u vs [&CD] at [surfactant] Constant) WCDI, mM cmc*, mM [SDSIf, mM 6.51 9.54 12.32 18.90
12.109 15.072 17.634 23.315
--I 28
7.11 7.76 8.17 8.78
[&CDl, mM
cmc*, mM
[SPFOlf,mM
14.9 23.1 33.6
41.031 53.900 68.314
28.6 34.2 40.3
step, as many others authors have propo8ed,31*32 the binding constant K can be expressed either by
considering f the extent of reaction with respect to the initial 8-CD concentration c h (C, is the total surfactant concentration) or
considering f the extent of reaction with respect to the (31)Satake, I.; Ikenoue, T.; Takeehita, T.; Hayakawa, K.;Maeda, T. Bull. Chem. Sac. Jpn. 1988,58,2746. (32)Okubo, T.; Kitano, H.;Ise, N. J . Phys. Chem. 1976,80,2661.
'-1 i
Figure 6. Apparent critical micellar concentration, cmc*, and [SDSlr in the postmicellar region 88 a function of [p-CDI: 0, results from study I; A,results from study 11.
initial surfactant concentration C. ( c h is the total B-CD concentration in this case). In a previous paper,= we proposed a semiempirical model to determine these associationconetanta from speed of sound measurements. On the basis of the tangential functionality of Au with f as follows Au = a2tan(a&
and the dependency off with C,,
ch,
(3)
and K
1218 Langmuir, Vol. 9, No. 5, 1993
Junquera et al. Table V. Values of the Association Constant K for Various CD +Surfactant Systems Obtained by Different Researchers KIM-'
1
system
I
60
SDS + j3-CD
-
our values
literature
500
356,32363034(C, = 2.6 mM)a 300,337230-23109 (1 mM < C, < 5mM)a 248035(C, = 2.5 m M p 2100037 (c, = 5 m ~ ) 111,32112031 (C, = 2.7 mM)a 160,3*7549 (C, = 2 mM)" 4500'7 (C, = 5 mM)a
\
SDS + CY-CD
" - 0
"
SPFO + j3-CD DTAB + j3-CD 4-BN-DTAB j3-CD CloSO3Na + j3-CD
+
01
I
333 39423
59039
180,38409034(C, = 2.5 m M p
Obtained with Satake's model.
Table IV. Statistics of the Fit of A u vs [surfactant] with Expressions 3,4a, and 4b [8-CDI, mM 8.900 12.050 15.083 6.179
K
a ] ,M-I
a2
j3-CD + SDS 0.86 f 0.01 557 f 42 414 33 1.50 f 0.01 526 f 42 1.33 f 0.01 8-CD + SPFO 333 f 70 2.72 f 0.09
an
10%
Y2
1.56 f 0.02 2.35 121 1.53 f 0.02 2.50 132 1.47 f 0.02 2.33 109 1.23 f 0.01 3.71 145
case A
or case B
&AC,Ch)1/21 (4b) the experimental values of Au vs concentration (Figures 3 and 4a) were fitted using a nonlinear least-squares method with a Marquardt algorithm, being a2, a3, and K (=ad, the fit coefficients. For the complexes fl-CD/SDS(case A) and fl-CDISPFO (case B),the association constants K have been determined for each initial concentration, being the resulting mean values 500 f 100 and 333 f 70 M-l, respectively. The statistics of the fits are shown in Table IV and the calculated curves are plotted (solid line) together with experimental values in Figures 3(b-d) and 4a. The agreement between experimental and calculated curves is quite satisfactory over the whole premicellar range of C,, indicating the validity of the scheme. From the results of Table I for the system &CD + SDS, it can be observed that, within experimental error, the association constant is independent of fl-CDconcentration,as might be expected from a proper thermodynamic standpoint. In Table V we have summarized K values for various inclusion complexes with CD's, obtained by several researchers from different experimental techniques, mostly conductivity measurements. For the system 8-CD + SDS, Okubo et al.32and Georges et al.33have obtained a value of K = 356-300 M-l, by using the definition of the equilibrium constant. They only (33) Georges, J.; Desmettre, S. J.Colloid Interface Sei. 1987,118,192.
assume that [surflf is constant with [fl-CDI and equal to the cmc of the pure surfactant. This assumption is not completely correct, although in this case it does not imply a big error because [SDSIf is constant with [fl-CDI (see Figure 6). The resulting K values are similar to the ones obtained by us with our model and, in any case, independent with the concentration of the present species. On the other hand, Satake et Reinsborough et al.? and Saint Aman et al.35have determined K values, which range from 7230 to 2310 M-l, depending on surfactant concentration. These groups have based their calculations on a model which fit experimental conductivity values, AA,,,, linearly with the extent of reaction,f. As was fully analyzed elsewhere,23this assumption implies that, whether the monomer is free or complexed with the cyclodextrin, the counterion mobility is exactly the same, which is a mistake as they have probed from their own emf20vs measurements. Recently, Wyn-Joneset al,,37fromemfmeasurements and assuming a multiple step mechanism, have reported a value of K = 21 OOO M-l, much higher than the other values. For the rest of the systems shown in Table V, the same discrepancy can be observed. Besides, Okubo et working with a stop-flow conductivity technique (but not using Satake's model) and Turro et al.3efrom phosphorescence measurements have determined K values of the same order of magnitude as ours. In a previous paper,z3 the formation of the inclusion complex between decyltrimethylammonium bromide (DloTAB) and fl-CD and how this equilibrium affects the micellization process were studied. There are several similarities on the behavior of this system and the ones analyzed in the present work. Thus, stoichiometry A was found to be 1.1:l for the complex ,&CD/DloTAB (where DloTAB is a 10 carbon atom surfactant), 1.3:l for fl-CD/ SDS (where SDS is a 12 carbon atom surfactant), and 1.2:l for @-CD/SPFO(where SPFO is an 8 carbon atom surfactant). These values can be explained considering that the longer the surfactant hydrocarbon chain is, the greater the contribution of the 2:l complex (and consequently the stoichiometry) results. On the other hand, taking into account that SPFO has the shortest surfactant tail, this result indicates that fluorinated surfactants are more hydrophobic than their hydrocarbon counterparts. (34) Satake, I.; Yoshida, S.; Hayakawa, K.; Maeda, T.; Kusumoto, Y. Bull. Chem. SOC. Jpn. 1986,59, 3991. (35) Saint Aman,E.; Serve, D. J. Colloid Interface Sci. 1990,138,365. (36) MacPherson, Y. E.; Palepu, R.; Reinsborough, V. C. Inclusion Phenomena and Molecular Recognition in Chemistry 1990, 9, 137. (37) Wan Yunus, W. M. Z.;Taylor, J.; Bloor, D. M.; Hall,D. G.; WynJones, E. J. Phys. Chem. 1992,96,8979. (38) Okubo, T.; Maeda, Y.; Kitano, H. J. Phys. Chem. 1989,93,3721. (39) Turro, N. J.; Okubo, T.; C h u g , C. J. J.Am. Chem. SOC. 1982,104, 1789.
Effect of 8-CDon Micellization
But anyway, the three surfactants form a complex with 8-CDpredominantly 1:1,as the majority of the surfactants3 independently of their polar head. With respect to the association constant, we have obtained values of 394 f 80 M-1 for the complex 8-CDI DloTAB, 500 f 100 M-' for 8-CDISDS, and 333 f 70 M-' (independent of the concentration). It looks like themore hydrophobic the surfactant chain is, the greater the association constant results. The binding might at first appear surprisinglystrong for 8-CDISPFO complex given that the carbon chain is that of an octanoate. Recent work17 has shown that the greater cross section of the fluorocarbon tail (cross section diameter is =6 A) ensures a closer fit in the 8-CD cavity, resulting in a greatly increased stability for the complex. The latter features confirm that all the parameters directly related with the complexation process depend mainly on the hydrophobicity of the surfactant chain and its length, revealing that this is the part of the surfactant which is included into the CD cavity, as might be expected. Thus, the character cationic or anionic of the surfactant appears to have little influence. Moreover, a very small aggregation number of SPFO micelleshas been determined from phosphorescence2land fluorescence quenching30 measurements, revealing that SPFO forms small supramolecular aggregates or "mini"micelles in contrast with surfactants containing typical hydrocarbon chains. This extraordinary small aggregation number, together with the short fluorocarbon
Langmuir, Vol. 9, No.5,1993 1219
chain, is expected to make "mini" SPFO micelles, loose and open to the interaction with surrounding water molecules. It is expected that even the fluorine atoms of the micellar core are in relative contact with water. This characteristic could explain the particular behavior found in this work for the system 8-CD + SPFO. Thus, in contrast with the systems 8-CD + DloTAB and 8-CD + SDS, where [surflf remains constant with 8-CD, [SPFOlf has been found to increase upon to the addition of cyclodextrin. In other words, in the presence of 8-CD and as long as the [@-CDIincreases, it is necessary for more SPFO monomers to form micelles. This particular behavior could be justified considering that micelles with such: a contact of the apolar core with the surrounding water might appear to be energetically favorable, while the apolar perfluorinated chain could be stabilized in solution by the hydrophilic outer surface of the cyclodextrin.
Acknowledgment. The authors are grateful to Professor A. Jerez for the thermogravimetric analyses of the 8-CD and also to MEC of Spain for financial support through DGICYTGrant No. PB89-0113. E.J. also thanks MEC for a scholarship from the FPI program. Supplementary Material Available: Tables of speed of sound data (20 pages). Ordering information is given on any current maaterhead page.
