446
Langmuir 1992,8, 446-451
NMR Study of Cyclodextrin Inclusion of Fluorocarbon Surfactants in Solution Wen Guo, B. M. Fung,' and S. D. Christian Department of Chemistry and Institute for Applied Surfactant Research, University of Oklahoma, Norman, Oklahoma 73019-0370 Received September 3,1991 The association of a-,8-, and y-cyclodextrins with sodium perfluorobutanoate, sodium perfluoroheptanoate, sodium perfluorooctanoate, and sodium perfluorononanoate has been studied by F-19NMR. First, the results suggest that the association of a-cyclodextrin with various fluorocarbon surfactants is fairly weak. The cavity of the host cyclodextrin is too small to accommodate the fluorocarbon chain to form an inclusion complex, but there exists an appreciableassociation between the tail of the surfactant and a-cyclodextrin. Secondly,the cavityof 8-cyclodextrinis large enoughto accommodatethe fluorocarbon chain, but the latter is forced to adopt an extended, possibly all-trans conformation to fit into the cavity. For pertluorosurfactantshaving a short fluorocarbonchain,a 1:l complex is formed. When the fluorocarbon chain is longer, 2:l complexation becomes significantfor higher concentrationsof 8-cyclodextrin. Thirdly, y-cyclodextrin forms a 1:l complex with fluorocarbon surfactanta. The complexationis stronger than that in the a-cyclodextrinlsurfactantsystem but weaker than that in the &cyclodextrinlsurfactant system. The association constants for the 1:l complexes have been calculated from the dependence of the F-19 NMR chemical shift on the concentration of cyclodextrin.
Introduction Cyclodextrins(abbreviated as c d s hereafter) area family of macrocyclic oligosaccharides consisting of six (a-cd), seven (8-cd), or eight (7-cd) D-(+)-glucopyranose units linked by a(1,4) interglucose bonds and shaped like a truncated cone with a relatively hydrophobic hollow cylindrical cavity.1*2 They are known to form so-called inclusion compounds with many hydrophobic or amphiphilic species by capturing different guest molecules inside their cavities. A number of groups have reported the results of studies of the association of surfactants with cd.3-14 For surfactant-cd complexes, a 1:l stoichiometry has usually been assumed, and the association constants for a number of surfactants have been reported.13 The formation of 2:l type cd-surfactant complexes has also been suggested in some Park and Song have reported the 2:l association constant for a few sodium salts of n-alkanesulfonates and n-alkyl sulfates based on fluorescence-probe studies.12 In general, the association constants between hydrocarbon surfactants and either acd or 8-cd are of the same order of magnitude. Literature data on y c d are very scarce14because the larger cavity of y c d gives weaker associations, and also because 7-cd has been very expensive and hard to obtain in the past. When all the C-H bonds in the hydrocarbon surfactants are replaced by C-F bonds, the association between a-cd and surfactant is reduced tremendously because the cross(1)Saenger, W. Angew. Chem., Int. Ed. Engl. 1980,19, 344. (2) Hamai, S. J . Phys. Chem. 1990,94, 2595. (3) Okubo, T.; Kitano, H.; Ise, N. J . Phys. Chem. 1976,80, 2661. (4) Okubo, T.; Maeda, Y.; Kitano, H. J . Phys. Chem. 1989,93,3721.
( 5 ) Satake, I.; Ikenoue, T.; Takeshita, T.; Hayakawa, K.; Maeda, T. Bull. Chem. SOC.Jpn. 1985,58, 2746. (6) Satake, I.; Yoshida, S.; Hayakawa, K.; Maeda, T.; Kusumoto, Y. Bull. Chem. SOC.Jpn. 1986,59,3991. (7) Hersey, A.; Robinson, B. H.; Kelly, H. C. J . Chem. Soc., Faraday Trans. 1 1986,82, 1271. (8) Georges, J.; Desmettre, S. J . Colloid Interface Sci. 1987,118,192. (9) Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1988, 66, 325. (10) Palepu, R.; Richardson, J. M.; Reinsborough, V. C. Langmuir 1989, 5, 218. (11) Palepu, R.; Reinsborough, V. C. Can. J . Chem. 1989, 67, 1550. (12) Park, J. W.; Song, H. J. J . Phys. Chem. 1989, 93, 6454. (13) Saint Aman, E.; Serve, D. J . Colloid Interface Sci. 1990,138,365. (14) Saenger, W.; Muller-Fahrnow, A. Angew. Chem., Int. Ed. Engl. 1988, 27, 393.
section of the fluorocarbon chain is too large to be included in the a-cd cavity.13 On the other hand, the association constanta between 8-cd and fluorocarbon surfactants have been found to be significantly larger than those for hydrocarbon surfactantsof the same chain length.13 This is because a fluorocarbon chain can fit the cavity of 8-cd more tightly." Since neither cd nor most surfactant molecules bear a chromophore, the usual spectroscopic methods cannot be applied. Therefore, most published results are obtained by the conductometric method. However, the difference in electric conductivity between associated and unassociated surfactant ions is not always very significant,12especially when higher order association is present. Recently, the use of a visible spectral displacement method has also been developed to study the complexing between 8-cd and aliphatic alcohols or surfactants.15J6 Palepu et al.l0 have measured the F-19 chemicalshift changes of the fluorocarbon chains in sodium perfluorooctanoate with a-cd and 8-cd at a 1:lmixing ratio. They found that in 8-cd-surfactant mixtures, the chemical shifts for fluorine atoms in the middle of the chain change more significantly, while in a-cd-surfactant mixtures, the fluorine atoms a t the end of the chain change more significant1y.l' This result indicates the CF3 group in sodium perfluorooctanoate is not strongly affected by the association with b-cd, while the CF2 groups near the carboxylate head group are not strongly affected by the association with a-cd. Since the observed F-19chemical shift is the average value of the shifts for the monomer and the complex, ita value is dependent on the configuration of the complex, the association constant, the mixing ratio, and the conformation of the fluorocarbon chain. However, these effects have not been considered in the work of Palepu et al.ll To understand the problem at the molecular level, we have conducted a detailed NMR investigation of the association of four anionic fluorocarbon surfactants with a-, p-, and y-cd's. The chemical shift changes of the fluorine nuclei in the surfactant chain provide direct (15) Sasaki, K. J.; Christian, S. D.; Tucker, E. E. Fluid Phase Equilib. 1989, 49, 281.
(16)Sasaki, K. J.; Christian, S. D.; Tucker, E. E. J . Colloid Interface Sci. 1990, 134, 412.
0743-7463/92/2408-0446$03.00/00 1992 American Chemical Society
NMR Study
Langmuir, Vol. 8, No. 2, 1992 447
of Cyclodextrin 1
Table I. Cmc's and Association Constants of the 1:l Complexes Formed with a-,B-, or 7-Cd and Some Fluorinated Surfactants. at 298 K surfactant K,,, M-l K.E, M-' K8.*, M-l cmc, mM 11 f 2 >lo00 C&COONa 29 f 2 (25)b 202 f 12 (69Wb CgllCOONa (20)b 80f 2 Cfi1&OONa 26 f 1 78oof480 420f40 32f1 CTFlsCOONa 28 f 1 (15)b 21800 f 500 750f70 (21500),b (780Lb (4500)' (64O)C 7200 f 510 1100 f 300 9.2 f 0.2 (0.68)b ~8600P (0.22)b (64"
C81,COONa 23 f 1 Cfi1,SO&i CsFlgCOOLi (10)b a Surfactant concentration 5 mM. From ref 13,with surfactant concentration 2.5 mM. From ref 11,with surfactant concentration 5 mM. 0
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40
45
Concentration of total a-cd, mM Figure 2. F-19 chemical shift difference A ~ =F (6,b - 6,) plotted against the totalconcentration of a-cd in a-cd-C,F&OONa with a fixed surfactant concentration of 5.0 mM. 0,the a-CF2; +, the b-CF2; X, the 7-CFz; 0 ,the 6-CFz; A,the c-CF2; the A-CF,; and 0,the o-CF3. The dashed lines are the calculated A ~ from F eqs
+,
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Figure 1. F-19 spectra for the fluorocarbon surfactants used in this work.
information about the cd-surfactant complexing, and the results are reported in this paper. Experimental Section Sodium perfluorobutanoate, sodium perfluoroheptanoate, sodium perfluorooctanoate,and sodium perfluorononanoate were prepared by neutralizing the corresponding acid with NaOH and recrystalliiing the salt from 1-butanol. The critical micelle concentrations (cmc) of these surfactants have been measured by surface tension and F-19 NMR, as listed in Table I. For this work, the surfactant concentrations were kept at 5.0 mM unless otherwise indicated. Such a value is chosen to keep the concentrations of all the surfactants the same and below the cmc and comparable to the results reported by other researchers.llJ3 The a-cd and &cd were purchased from Aldrich, and 7-cd was purchased from Sigma Chemical; they were used without further purification. All NMR experiments were performed on a Varian VXR-500 spectrometer. The F-19 NMR chemical shift was referred to CF3COOH as an external reference (6 = -79.45 ppm). All solutions were prepared with 50% DzO (for field frequency lock) in distilled water. The F-19NMR spectra of the surfactants below their critical micelle concentration (cmc) are shown in Figure 1. The peak assignmentswere obtained from COSY experiments." For C&,COONa,the y,6, and c peakssuperimpose. When this compound is mixed with different cd's, the peaks may cross-over and be superimposed depending on the mixing ratio and the association constants. For other surfactants, all the F-19 peaks are well resolved both in the monomer and in the complexed state. ~~
(17)Guzman, E. K.M.S.Thesis, University of Oklahoma, Norman, OK,1989.
Results and Discussion Association between a-Cd and Fluorinated Surfactants. The change in the F-19 chemical shift, namely, the difference between the shift for the mixed system and that for the monomeric fluorocarbon surfactant, is denoted F each CFZgroup and the CF3 by A ~ F .The values of A ~ for group in C7F16COONa are plotted against the concentration of a-cd in Figure 2; those for other surfactants are similar.18 The data in Figure 2 show that A&, in general, decreases along the chain starting from CF3 to the head group for all the fluorocarbon surfactants. For other fluorocarbon surfactants, the quantitative changes in A ~ F ( w are ) almost the same regardless of the chain length, but A~F'sfor a-CFz and j3-CFz decrease as the chain length increases.18 The cavity of a-cd is small (diameter ca. 0.5 nm) and cannot provide enough space to accommodate the fluorocarbon chain." Therefore, the mode of association may be such that the fluorocarbon chain is located a t the opening of the cone, which is basically composed of hydrocarbon and hydroxyl groups. Such an environment has asmaller polarizabilitycompared to that of the aqueous phase, so that the van der Waals interaction between the fluorocarbonchain and the environment is reduced, which would result in an increase in A6p19 On the basis of the fact that the chemical shifts change in the order of A ~ F ( w ) > A ~ F ( w - ~>) AWw-2) > ...> Ab~(j3)> A&(a), we suggest that the CF3 group prefers to locate at the opening of the cavity while the rest of the chain can move around. The lack of dependence of A ~ F ( w )on the chain length also supports this model. For long chain surfactants, the anionic head group may extend into the aqueous phase and be nearly unaffected by the association. In a mixture of cd and surfactant, the observed chemical shift ( 6 ~ )is a weighted average of that for the monomer (18)Guo, W.Ph.D. Thesis, University of Oklahoma, Norman, OK, 1991. (19)Emsley, J. W.;Feeney, J.; Sutcliffe, L. H. Progress in Nuclear Magnetic Resonance Spectroscopy; Pergamon Press: New York, 1971; Vol. 7,p 11.
Guo et al.
448 Langmuir, Vol. 8, No. 2, 1992
(6,) and that for the complex (&), which depends on the mixing ratio and the association constant (K). The quantitative description for a 1:l complex follows:
cd + F =,cd-F
(1)
FT = [F] + [cd-F]
(3)
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(4)
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[FIGJFT
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(5)
where cd, F, and cd-F denote the monomer cd, monomer surfactant, and the 1:lcomplex, respectively. FT denotes the total concentration of the surfactant, and CT denotes that of cd. The activity coefficient of each species is assumed to be unity here because the solution is dilute and no extra electrolyte is added. Then, the value of K can be obtained by the use of a nonlinear least-squares program to fit the observed chemical shift 6p as a function of CT at a fixed value of FT. In the present case, all the 6~’sfor different peaks are processed as one single data set to further minimize random errors. For all Cd-CnF2,+1COONa systems, compounds with n = 3 and 6-8 were studied; those with n = 4 and 5 were not available commercially. The values of K calculated for the a-cd systems are about 10-30 M-l and do not change appreciably with the fluorocarbon chain length (Table I). For a-cdC7F15COONa, the A6& calculated from eq 5 with the K value thus obtained are shown as dashed lines in Figure 2, together with the experimental data. The A ~ values F for some CF2 groups in other systems are not significant. Then the data were not included in the nonlinear leastsquares calculation, and no dashed lines are displayed. For comparison, the K values reported in literature are also listed in Table I, which shows that the agreement between the two sets of data is very good. The small values of K indicate that the association is very weak; the fact that the K s do not change much with the increase of the chain length is in accordance with our previous suggestion that the association with a-cd is through the CF3 group of the surfactants. We noticed that the a-cd-C7F&OONa system has been studied by Palepu et a1.l1 and by Saint Aman et al.13 using the conductometric method. From their results, the decrease in the conductance of the mixed solution with added a-cd is much less significant than that in 8-cd-surfactant or y-cd-surfactant mixed systems. However, from Figure 2, it is seen that the A~F’S are rather appreciable, especiallythose for the CF3 and the CF2 groups near the end of the chain. On average, as it will be seen in the following sections, the values of A ~ F ( o ) and Aap(w-1) are considerably larger than the maximum change of A ~ in F the y-cd-CnF2n+lCOONa systems with the same concentrations and mixing ratios. Association between B-Cd and Fluorocarbon S u r factants. The inside diameter of the cavity of 8-cd is ca. 0.7 nm.ll The diameter of a CF2 group is estimated to be 0.5 nm, and that of a CF3 group 0.7 nm.22 Therefore, a fluorocarbon chain would be able to fit snugly inside the cavity. Since the interior of the cavity is less polar than the aqueous phase, the inclusion of the fluorocarbon chain into such a cavity reduces the contact between the fluorocarbon chain and the aqueous environment, which makes such inclusion energetically favorable. There are basically three types of interactions involved
in such an inclusion which would affect the F-19 chemical shift. First, for an anionic surfactant, the negatively charged head group should stay outside the cavity, forming hydrogen bonds with the cd hydroxyl groups and water near the top or bottom of the host cavity. As a result, there would be charge migration away from the negatively charged head group so that the shielding on the a-CF2 group would be reduced and the chemical shift would increase. Second, the limited size of the cavity is likely to force the guest fluorocarbon chain to adopt an extended, possibly all-trans conformation inside the cavity. Although a fluorocarbon chain is less flexible than a hydrocarbon chain, it is by no means rigid. In the monomer state, the chain can have 3n-2 conformations with the highest probability for the all-trans conformation. When the chain is forced to adopt an all-trans conformation in the cavity of @-cd,the F-F distance between neighboring CF2 groups is maximized, and the intramolecular shielding of the fluorine atoms is reduced. This factor would cause the chemical shift to increase. Third, because there are seven bridged oxygen atoms in the middle of the cavity in O-cd, all facing inward, the van der Waals interaction between the fluorocarbon chain and the interior of the cavity is stronger than that in the aqueous environment. This factor would make the environment of the guest molecule more shielded and decrease its chemical shift. The values of A ~ for F the @ - C ~ - C , F ~ ~ + ~ Csystem OON~ are plotted in Figure 3. For n = 3, the changes follow the order A8p(a)> A&(@) > A~F(o)= 0. The increase in A 6 ~ ( a ) is likely due to the first type of interaction described above. The effect of charge migration would also cause Ab@) to increase, but to a lesser extent. Since the chain length of C3F7COONa is very short, the CF3 group may not be able to penetrate deeply inside the cavity. It is possible that the deshielding effect from the exterior of the cavity and the shielding effect inside the cavity approximately cancel each other, so that the net value of A ~ F ( w )is very insignificant (Figure 3a). Because of the the short length of the C3F7 chain, the effect of conformational changes of Abp is not likely to be important for all these groups. For n = 6-8, with the exception of the a-CFz group, A6p decreases along the chain (Figure 3b-d). In this case, the change in A6p(a) is caused by the same effects as discussed previously. The reduction in thevalue of A6p(r~),compared to that shown in Figure 3a, probably indicates that the head group extends slightly further away from the host cavity, so that the host cavity can better accommodate the more hydrophobic tail. On the other hand, the deshielding effect on the P-CF2, caused by the stretching of the fluorocarbon chain to form the all-trans conformation, is much stronger in these cases. Since the head group protrudes into the solution, the o-CF2 would also be close to the opening of the cavity and the shielding effect inside the cavity should not affect Asp(@) considerably. Furthermore, the top and bottom of the cavity are less polarizable as mentioned earlier, and would cause the shielding on the P-CF2 to decrease. These are the main reasons to explain the large quantitative increase in the Asp(@). The other CF2 groups in the chain all experience both the second and the third types of interactions. However, the effect of the forced all-trans conformation may be much greater so that the apparent shielding on the CF2 groups decreases as the result of the inclusion. Since the influence of the conformational change on the CF3 group is very small and the CF3 group would be outside or at most near the secondary hydroxyl side of the cavity, the effect of both the second and the
NMR Study
Langmuir, Vol. 8, No. 2,1992 449
of Cyclodextrin 1
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Concentration of total P-cd, mM Concentration of total P-cd, mM Figure 3. F-19 chemical shift difference A ~ plotted F against the total concentration of 0-cd with a fixed surfactant concentration of 5.0 m M (a) n = 3, (b) n = 6,(c) n = 7, and (d) n = 8 with A, the a-CF2; +, the B-CF2; 0 , the yCF2; X, the 6-CF2; a, the c-CF2; +, the A-CF2; and 0,the w-CF3. third kinds of interactions on the CF3 group should be ) close to zero. very small, and A ~ F ( w is The formation of complexes between a cd and a surfactant is dependent on the mixing ratio in the cd-surfactant mixture, which is defined as f = [cdl/ [surfactant] (6) For the ~ - C ~ - C , F ~ , + ~ C Osystem O N ~ with n = 6-8, A ~ F for all fluorine nuclei in the fluorocarbon chain changes almost linearly with increasing [P-cdl until f = 1,or [p-cd] = 5 mM, is reached. After that, Asp levels off or even decreases as [fl-cdl further increases. This implies that the 1:l complexation is strong and close to completion when the stoichiometric mixing ratio cf = 1)is reached. Assuming that higher order complexation is negligible for f < 1, the association constant was calculated using the A6p - [O-cdl relation for [b-cdl 5 5 mM (dashed lines in Figure 3b-d), and the values of K thus obtained are listed in Table I. The basic driving force for the complexing between cd and surfactant is the minimization of the contact of the fluorocarbon chain with the aqueous environment through hydrophobic interaction."P2l Therefore, when the chain length is greater than the depth of the cavity, the hydrophilic group of the surfactant will be pushed out slightly in order to accommodate the maximum length of the fluorocarbon chain. Because the association between the carboxylate head group and the cavity can be related to the change of the shielding on its neighboring groups, the values of A ~ F ( ( Yand ) A6~(/3)at a 1:l mixing ratio can be used to compare the strength of such a complex. (20) Nemethy, G.; Scheraga, H. A. J . Chem. Phys. 1962, 36, 3401.
(21) Tabushi, I.; Kuroda, Y.; Mizutani, T. J. Am. Chem. Soc. 1986, 108,4514.
(22) Tiddy, G. L. T. In Morden Trend of Colloid Science in Chemistry and Biology; Eicke, H. S . , Ed.; Birkhauser: Basel, 1985; p 148.
Table 11. Values of &(a) and A&@) for &CdCnFzn+iCOONawith Different n Values at 1:l Mixing Ratio surfactant C&'&OONa C,F&OONa C81,COONa
A a ~ ( 4ppm , 0.32 0.20 0.15
A~F(B),ppm
1.14 0.80 0.63
As is shown in Table 11, the absolute change of A ~ F ( ( Yand ) A b ~ ( 8at ) a 1:l mixing ratio decreases significantly as the chain length increases from n = 6 to n = 8. On the other hand, the 1:lassociation constants are close to each other, with the one for n = 7 being the largest. Therefore, as the chain length increases until it exceeds the depth of the cavity, the deshielding effect due to charge migration from the carboxylate head group is reduced, indicating that the average distance between the carboxylate group and the cavity is increased. After most of the surfactant molecules are included in the cd cavities, a second 8-cd molecule may be able to stack on top of the 1:l complex to form a tunnel structure including the fluorocarbon chain. Since the chain in CsF13COONa is not long enough, higher order association would not be significant. Therefore, there is little change in A& beyond the f = 1mixing ratio. The fluorocarbon chain is longer for C,Fl&OONa and CsFITCOONa, and the formation of a 2:l complex may be appreciable for f > 1.The 2:l complex would be more stable if the fluorocarbon chain could extend further into the cavity of the second 8-cd molecule. The deeper penetration of the chain into the "tunnel" would pull the head group back from the cavity opening of the first 8-cd molecule. Since the a-and &CF2 groups do not extend as far into the aqueous environment as in the 1:l complex, they are in the less shielded environment of the cavity opening. Therefore, instead of
Guo et al.
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Concentration of total pcd, mM Figure 4. F-19 chemical shift difference A& of the 6-CF2groups plotted against the total concentrationof &cd +, FT = 5 mM, 0 , FT = 2 mM; and X, FT = 1 mM. staying constant as in C813COONa (Figure 3a), A ~ F increases with f > 1. The increases of A6&) and for n = 8 are more significant than those for n = 7 because the 2:l complex would be more stable for a longer chain. On the other hand, the values of A& for other -CF2 groups and the -CF3 group decrease with the increase of [P-cd] when f > 1. This is due to an increase in the shielding effect when the tail of the chain goes deeper into the cavities of the two 8-cd molecular “tunnel”. The decreases for n = 8 are also larger than those for n = 7, in accordance with the corresponding increases for the a-and B-CF2 groups. For such a 2:l complex, the observed chemical shift can be expressed as 6, = [Fl6JFT + [CF]G,/FT + [CzF]62/FT (7) where [CzFI and 62 denote the concentration and the chemical shift of the 2:l complex, respectively; other symbols are the same as in eq 5. Although it is in principle possible to calculate the association constant of the 2:l complex by the use of the chemical shift data, the data thus obtained are not precise enough to allow determination of all the parameters involved in the mathematical analysis. However, the existence of such a 2:l complex is quite plausible. For more substantiation, the D-cd-C7F15COONa mixed system was studied at two other surfactant concentrations, [ C ~ F I ~ C O O N=~1.0 I and 2.0 mM, respectively. As an example, the A ~ -F [cdl curves for the 6-CF2 group with different surfactant concentrations are shown in Figure 4. Those for other CF2 groups also resemble the corresponding A ~ F [cdl curves shown in Figure 3c, with a sharp transition point at the mixing ratio off = l . l 8 The slopes of the A ~ -F [cdl curves beyond the 1:l mixing ratio are not much affected by the total surfactant concentrations. This helps to confirm that when f < 1, the 1:l complexation is very strong and dominates the association, whereas higher order complexation exists whenf > 1. The association constants obtained with these two data sets cf < 1)are 20 500 f 600 and 22 100 f 200 M-’ for [C7F15COONal = 1.0 and 2.0 mM, respectively, which are of the same order of magnitude as the constant for [ C ~ F I ~ C O O N=~ 5.0 I mM (Table I). The results indicate that there is no significant dependence of K on concentration for this system. The values of K a t these three surfactant concentrations are comparable to that reported in ref 13 ( [ C ~ F I ~ C O O N=~ ]2.5 mM) but considerably larger than that in ref 11([C7F15COONal = 5.0 mM). Complexationbetween 7-Cd and Fluorinated Surfactants. Since 7-cd has a large cavity (ca. 0.9 nm),” the
-0.64 0
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Concentration of total y-cd, mlM Figure 5. F-19 chemical shift difference A ~ in F C,F&OONa plotted against the total concentration of 7-cd for with a fiied surfactant concentration of 5.0 mM. The symbols are the same as those in Figure 2.
included fluorocarbon chain is not forced to adopt the all-trans conformation to fit inside the cavity. Therefore, deshielding due to conformational changes of the chain is no longer significant. The values of A ~ for F the y-cd-C3F&OONa system are very small and follow the order A 6 ~ ( a > ) A ~ F ( u )= A ~ F @ ) . Because the chain length is very short and the y-cd cavity is too large to hold it tightly, the surfactant molecule is held only loosely in the cd cavity. The 1:l association constant calculated by the use of eqs 2-6 is 11 f 2 M-l (Table I), which is considered to be very small. For ~-cd-CnF2,+lC0ONa with n = 6-8, the fluorocarbon chain becomes much more hydrophobic so that the complexation with cd is energetically more favorable compared F for y-cd-C,F15with the case for n = 3. The A ~ data COONa are displayed in Figure 5; those for the other systems are similar.18 In these systems, the transitions near the point of [y-cdl- 5 mM, at which the mixing ratio f = 1,are not as sharp as those in the B-cd-CnF2,+1COONa mixtures. This suggests that the association follows a 1:l stoichiometry, and no significant concentration of 2 1 complexes is formed. Otherwise, sharp breaks in the plots of A& versus the concentration of cd would be observed, such as those shown in Figures 3 and 4. This is not surprising because the large cavity of y-cd allows the fluorocarbon chain to retain its flexibility,so that the chain is less extended than the case of 0-cd. The association constants for y c d with fluorocarbon surfactants of different chain length are calculated from data points throughout the whole range of [cdl, and are listed in Table I. Except for C3F7COONa, the values of Ks,r are all significantly larger than the corresponding values of but smaller than those of Ks,,+,and the increase of K,,, with the increase of chain length is also significant. Quantitatively, in the y-cd-CnF2,+lC0ONa systems, A b increases for a- and /3-CF2 (near the head group) and decreases for y- and 6-CF2 (in the middle of the chain). A ~ for F CF3 (terminal of the chain) decreases slightly for n = 6, does not change significantly for n = 7, and increases slightly for n = 8. Obviously, the migration of the charge of the head group is still the major cause of a decrease of the shielding on its neighboring a-CF2, and the same is
NMR Study of Cyclodextrin true for the change in A6~(0). On the other hand, those CF2 groups in the middle of the chain are located near the bridge oxygens in the middle part of the cavity and become F decrease upon commore shielded, so that the A ~ values plexation. With such an association configuration, the CF3 group for CJ?&OONa is probably near the opening of the cavlty so that the shielding from the cavity is less strong and the A ~ F ( w only ) decreases slightly upon association.18 For C ~ F I ~ C O O with N ~ , one extra CF2 group, the carboxylate group and the CFBgroup probably extend slightly further outside the cavity. ASF for the a-, j3-, y-, 6-,and eCF2 groups are of the same pattern as those in C$13) A&(@) are COONa, but the absolute values of A 6 ~ ( a and ) A b ~ ( 6 )increased reduced slightly and those for A ~ F ( Yand slightly, while A~F(X)and A ~ F ( ware ) essentially zero. The last finding indicates that the tailof the fluorocarbon chain is essentially in the aqueous environment, which does not differ from that of the uncomplexed surfactant. The trends of ASP’Sfor C8FnCOONa are similar to those for ita two homologous analogues.*8 The longer fluorocarbon chain would cause the carboxylate group to extend even more and reduce A b ~ ( a )and Agp(j3). On the other hand, A ~ F ( w )now increases slightly with the complexation instead of staying unchanged as in the case of C7F15COONa. The reason for this is probably as follows: With the carboxylate head group interacting with the CH20H group in y c d , the tail of the very long CgT17COONa chain now extends much beyond the cavity. Because of this and the large sue of the cavity in y c d , the chain could conceivably bend back toward the y c d opening in some of many possible conformations. Then, the CF3 group would interact with the 7-cd molecule to cause ita F-19 chemical shift to increase. This is to be compared with
Langmuir, Vol. 8, No. 2, 1992 451
the case of a-cd (Figure 21, for which the fluorocarbon cannot go inside the narrow cavity, but for which A ~ F ( w ) is quite appreciable.
Conclusion The mixed systems of a-, D-, and 7-cd with fluorinated carboxylic surfactants of different chain length have been systematically studied by F-19 NMR. For a- and y c d associated with fluorinated surfactants, our results agree with the 1:1complexation models proposed by many other researchers. For P-cd, the F-19 chemical shift dependence on the fluorocarbon chain strongly suggests a 2 1 stacking model when the fluorocarbon chain is long enough. The association constants for the 1:l complex are such that 8-cd > 7-cd >> a-cd, because the fluorocarbon chain cannot fit into the cavity of a-cd, fits snugly inside the cavity of 8-cd, and fits loosely inside the cavity of y c d . Because of the high resolution of high-field F-19 NMR and the sensitivity of the F-19 chemicalshift to the change in the environment of the fluorocarbon chain, this investigation has provided details of the molecular interactions that would be difficult to obtain by other techniques. Acknowledgment. We acknowledge the support of industrial sponsors of the Institute for Applied Surfactant Research, including E. I. du Pont de Nemours & Co., Kerr-McGee Corp., Sandoz Chemicals Corp., and Union Carbide Corp. We also thank Dr. R. Palepu for helpful discussions. Registry No. (u-cd-C$13COONa, 137917-31-8; a-cd-CgF17COONa, 137917-32-9;&cd-Cfl,COONa, 137917-33-0;&cd-C&’laCOONa, 137917-34-1; j3-cd-C$17COONa, 137917-35-2; y c d CaF7COONa, 137917-36-3; r-cd-CsF13COONa, 137917-37-4;7cd-C&’&OONa, 137917-38-5.
