218
Langmuir 1989,5, 218-221
changes in the hemimicellar environment. To summarize, this work reports the excited-state Raman spectrum of R u ( b p ~ ) , ~in+ the adsorbed layers of a surfactant on a solid at a solid-liquid interface under in situ equilibrium conditions. This opens the general possibility of observing the adsorption phenomenon by yet another sensitive technique to provide basic information
on adsorbed layers a t solid-liquid interfaces.
Acknowledgment. P.S. and J.T.K. thank the NSF and DOE for financial support. N.J.T. and J.K.B. thank the NSF, AFOSR, and NIH for fhancial support. Registry No. SDS, 151-21-3;R~(bpy),~+, 15158-62-0; alumina, 1344-28-1.
Binding Constants of &Cyclodextrin/Surfactant Inclusion by Conductivity Measurements Ramamurthy Palepu and Joyce E. Richardson Department of Chemistry, Uniuersity College of Cape Breton, Sydney, Noua Scotia, B l P 6L2 Canada
Vincent C. Reinsborough* Department of Chemistry, Mount Allison University, Sackuille, New Brunswick, EOA 3CO Canada Receiued July 12, 1988. I n Final Form: September 13, 1988 Binding constants for p-CDlsurfactant inclusion complexes were obtained through conductivity for 20 anionic surfactants having different polar heads, different tail configurations, and the same Na+ counterion. Three cetyl-chained cationic surfactants were similarly investigated. The predominant complex in all cases was composed of an equimolar mixture of surfactant and cyclodextrin. Unusually large binding constants could be justified by the goodness of fit of the surfactant within the p-CD cavity or by high charges on the polar head. Two-tailed surfactantsappear to have only one tail encapsulated by @-CDin the 1:l complex.
Introduction When ionic surfactants form inclusion complexes with cyclodextrins, the amphiphilicity of the former often leads to strong and unique associative species that dramatically affect solution conductivities. Although largely unexploited, conductivity measurements can be used to elucidate not only the stoichiometry but also the binding constants of cyclodextrin (CD) inclusions with surfactants.14 With alkyl sulfate and alkane sulfonate surfactants, 1:l complexes predominate with p-cyclodextrin (pCD), which has an annulus diameter of 7 With a-CD (diameter 5 A), inclusion compounds displaying stoichiometry involving two cyclodextrins per included species are f ~ r m e d . ~With ? ~ y C D (diameter 9 A), the stoichiometry of the complexes is similar to those obtained with p-CD, but the binding constants are not as large. In fact, of the three cyclodextrins, 6-CD binds most strongly with surfactants. Presumably, the hydrocarbon tail of the surfactant fits most snugly into the p-CD cavity, but the polar head group (sulfate or sulfonate) is accommodated most efficiently by a-CD,4 with the 2:l complex involving an encapsulation of the surfactant monomer at both ends by CDs. To test this hypothesis, a variety of surfactants with various tail and polar head configurations was placed in solution with @-CD,and from the measured conductivities binding constants were derived. Head group variation in this context was studied by using sodium alkyl
carboxylates, disodium dodecylphosphate, and two-headed disodium 4-alkyl 3-sulfonatosuccinates as surfactants. Variation in the tail component of the surfactant was achieved by using sodium dodecyl benzenesulfonate, sodium perfluorooctanoate, two-tailed sodium dialkyl sulfosuccinates, sodium cyclohexyl carboxylates, and sodium cyclohexyl sulfamate. All these anionic surfactants had Na+ as the counterion. Three cationic surfactants with cetyl tails were also included for comparison.
(1)Satake, I.; Ikenoue, T.; Takeshita, K.; Hayakawa, K.; Maeda, T. BuEl. Chem. SOC.Jpn. 1986,58, 2746. (2) Satake, I.; Yoshida, S.; Hayakawa, K.; Maeda, T.; Kusumoto, Y. Bull. Chern. SOC.Jpn. 1986,59, 3991. (3) Hersey, A.; Robinson, B. H.; Kelly, H. C. J. Chern. SOC.,Faraday Trans. 1 1986,82, 1271. (4) Palepu, R.; Reinsborough, V. C. Can. J. Chern. 1988,66,325. (5) Okubo, T.; Kitano, H.; Ise, N. J. Phys. Chem. 1976, 80, 2661.
where AA is the decrease in molar conductivity of the surfactant occasioned by adding P-CD, A i the difference in the ionic con-
A.1123415
Experimental Section A Radiometer CDM83 conductance bridge was used to measure solution conductivitiesat 25.0 “C. 0-CD was obtained from Sigma and dried before use. The sodium alkyl sulfoacetates and the dialkyl sulfosuccinates were prepared as described previously6 or purchased from Diagnostics, where the compounds were prepared by the same procedure. The disodium 4-alkyl 3sulfonatosuccinate preparations have also been described elsewhere,’ as have the sodium cyclohexyl sulfamate surfactant syntheses.s All the other surfactants were of the best grade that was commercially available. The stoichiometries of the inclusion complexes were deduced from the breaks in the molar conductivity versus [p-CD]curves. The association constants K for 1:l complexation were obtained by applying a nonlinear least-squares regression to the equation of Satake et al.:lJ AA =
(6) Jobe, D. J.; Reinsborough, V. C. Can J. Chem. 1984,62, 280. (7) Jobe, D. J.; Reinsborough, V. C. Aust. J. Chem. 1984, 37, 303. (8) Skauge, A.; Spitzer, J. J. J. Phys. Chern. 1983, 87, 2398.
0743-7463/89/2405-0218$01.50/0 0 1989 American Chemical Society
Binding Constants of p-CyclodextrinlSurfactant
1
U
i0 5
{ 604
5 0
X O O O
++,
O
O
O
++* 5
5
10
15 1
[P-CDl/rnmol drn"
Figure 1. Dependence of molar conductivity of 5.00 mmol dm-3 sodium alkyl carboxylate solutions on &CD concentration at 25.0 OC: 0 , sodium hexanoate; X, sodium heptanoate; 0,sodium octanoate; +, sodium dodecanoate. ductivities of the unassociated and associated surfactant ions, C, the surfactant concentration, and C, the &CD concentration.
Results Sodium Alkyl Carboxylates. The sodium alkyl carboxylates represent a change in surfactant polar head from the sodium alkyl sulfates and sulfonates investigated previously! The molar conductivities of 5.00 mmol dm-3 sodium alkyl carboxylate solutions with added 0-CD were measured at 25.0 "C ([p-CD] = 0-15 mmol dm-3). The members investigated were the hexyl, heptyl, octyl, nonyl, decyl, and the dodecyl homologues. The dependence of the conductivity upon p-CD concentration is shown for the three shortest members and the longest member in Figure 1. The nonanoate and the decanoate members were omitted for the sake of clarity. The changes in ionic conductivity (AX) and the association constants for all six systems are given in Table I. The binding of sodium hexanoate with 6-CD is not pronounced, but a discernible break in the conductivity curve occurs at 5.0 mmol dm-3 0-CD, indicating that the stoichiometry of the inclusion complex is chiefly equimolar. With sodium heptanoate, 8-CD addition decreases the solution conductivity to a greater extent, and intersection of the two linear portions at the beginning and end of the curve yields a ratio of 1.3 8-CD to surfactant, suggesting that even though the predominant species is 1:l some 2 1 j3-CDlsurfactant complexation is also occurring. With sodium octanoate, the molar conductivity is decreased even further with 0-CD addition, and the same deduction about the stoichiometry of the complexes can be made. The trend in increasingly lowered conductivity continues with the higher homologues of the sodium alkyl carboxylates. However, evidence for higher complexes than the equimolar aggregate diminishes as the curves more clearly define two straight lines, wherein the intersection at 5 mmol dm-3 8-CD becomes sharper, viz., the sodium dodecanoate curve in Figure 1. Increasing the length of the hydrocarbon tail leads not only to a stronger binding of the surfactant with p-CD (cf. K values in Table I) but to a diminishing of the relative abundance of complexes other than the 1:l complex. Disodium Dodecylphosphate. p-CD binds strongly with disodium dodecylphosphate with its doubly charged
Langmuir, Vol. 5, No. 1, 1989 219 Table I. Binding Constants for 1:l Surfactant/&CD Complexation at 5.00 mmol dm-s- Surfactant Concentration conducbinding tance constant, difference, dm3 surfactant dm3 mol-' mol-' 1. Na hexanoate 23.3 34 2. Na heptanoate 20.1 110 16.7 3. Na octanoate 370 4. Na nonanoate 14.8 640 16.5 5. Na decanoate 740 11.0 6. Na dodecanoate 1600 7. di-Na dodecylphosphate 25.0 3900 40.2 8. di-Na 4-hexyl 3-sulfonatosuccinate 210 9. di-Na 4-heptyl 3-sulfonatosuccinate 34.0 490 37.4 10. di-Na 4-octyl 3-sulfonatosuccinate 760 31.1 1130 11. di-Na 4-nonyl 3-sulfonatosuccinate 12. di-Na 4-decyl3-sulfonatosuccinate 32.0 1400 13. Na dodecyl benzenesulfonate" 5.9 1.5 x 104 14. Na perfluorooctanoate 4500 13.3 15. Na dihexyl sulfosuccinate 11.2 340 16. Na dioctyl sulfosuccinate" 6.1 2800 17. Na cyclohexyl acetate 490 16.6 18. Na cyclohexyl propanoate 15.1 1400 14.4 1700 19. Na cyclohexyl butanoate 20. Na cyclohexyl sulfamate 17.7 230 21. cetylpyridinium chloride 7.5 5 x 104 22. cetylpyridinium bromide 7.4 4 x 104 2 x 104 23. cetyltrimethylammonium bromide 8.0
" Surfactants 13 and 16 were at 0.500 mmol dm-3 concentration; 21, 22, and 23 were at 0.208 mmol dm4. -m
135L
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phosphate head group. The stoichiometry of the complex is indisputably equimolar at 5.00 mmol dm-3 surfactant (Figure 2). The value of the association constant is 3 times those of sodium dodecylsulfate and sodium dodecanoate, indicating that the doubly charged phosphate group strongly promotes binding, possibly through hydrogen bonding with OH groups on the outer rim of the entrance to the p-CD torus.g There is, however, accumulating evidence that the encapsulated surfactant not only retains its counterions but immobilizes them to some extent in the entranceway of the 0-CD torus, thus promoting synergistically the binding of the surfactant with p-CD. A double (9) Cromwell, W. C.; Bystrijm, K.; Eftiik,M. R J.Phys. Chern. 1986, 89, 326.
Palepu et al.
220 Langmuir, Vol. 5, No. 1, 1989
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Figure 3. Dependence of molar conductivity of 5.00 mmol dm-3 disodium 4-alkyl 3-sulfonatosuccinateson P-CD concentration at 25.0 O C : 0,heptyl; 0 , nonyl; X, decyl. charge on the surfactant head might be expected to be more effective in enhancing this binding. Disodium 4-Alkyl 3-Sulfonatosuccinates. Disodium 4-alkyl 3-sulfonatosuccinatemolecules have two negatively charged polar heads: a carboxylate and a sulfonate. When these surfactants form micelles, both head groups experience an aqueous environment.' In this investigation, conductivity data were collected for solutions of each of the C6-Cl0 homologues at 5.00 mmol dm-3 surfactant with varying amounts of 0-CD. Figure 3 displays this data for the heptyl, nonyl, and decyl members. Again, the binding behavior appears to be similar to that observed in the carboxylate systems except that the association constants are roughly 2-4 times greater. The stoichiometry is predominantly 1:1, especially with the longer chained members. Sodium Dodecyl Benzenesulfonate. The hydrocarbon tail of a surfactant can be varied by introducing an aromatic group into it as with sodium dodecyl benzenesulfonate. The concentration of this surfactant was set at 0.500 mmol dm-3 to avoid being above the critical micelle concentration. The stoichiometry of the complex with @-CDis decidedly 1:1, as determined from conductivity data, but the binding is much stronger than with other surfactants with dodecyl tails, as revealed by the 10-fold increase in the value of K. The benzene moiety with its diameter of 6.5 A fits snugly into the 0-CD activity,lo thus enhancing the overall stability of the complex. Sodium Perfluorooctanoate. Sodium perfluorooctanoate represents another variation in the surfactant tail where all the hydrogen atoms are replaced by fluorine atoms. The @-CDsodium perfluorooctanoatecomplex also exhibits 1:l stoichiometry. The binding might at first appear surprisingly very strong (K= 4500) given that the carbon chain is that of an octanoate (Figure 4); however, fluorocarbons are more hydrophobic than their hydrocarbon counterparts. Moreover, the greater cross section of the fluorocarbon tail ensures a closer fit in the @-CD cavity, resulting in a greatly increased stability for the complex." (10) Konda, H.; Nakatani, H.; Hiromi, K. J.Biochern. 1976, 79,393.
Figure 4. Effect upon molar conductivity of surfactant/@-CD solutions from variation in surfactant carbon chain character: 0 , sodium dioctyl sulfosuccinate (0.500 mmol dm-3);0,sodium cyclohexyl butanoate (5.00 mmol dm-s); X, sodium perfluorooctanoate (5.00 mmol dm-3).
Sodium Dialkyl Sulfosuccinates. Both dihexyl and dioctyl homologues of the two-tailed sodium dialkyl sulfosuccinates have been examined as including species. The critical micelle concentration for the dioctyl member is 0.7 mmol dm", so 0.500 mmol dm-3 was selected as an appropriate concentration for this surfactant. The dependence of the conductivity of both surfactants on @-CD concentration is not much different from the behavior noted for single-tailed surfactants of similar length. The association is predominantly 1:l (cf. Figure 4 for the dioctyl member). Sodium Cyclohexyl Carboxylates and Sulfamate. The cyclohexyl group is sufficiently hydrophobic in itself to bestow surfactant properties upon any cyclohexane molecule that has a polar head group attached to it? This rather unique hydrocarbon tail modification might be expected to make accomodation within the torus of a cyclodextrin more problematical. The first three members of the sodium cyclohexyl carboxylates and sodium cyclohexyl sulfamate were subjected to the usual conductometric study. With sodium cyclohexyl ethanoate as with sodium cyclohexyl sulfamate, the inclusion complex with p-CD is weakly formed, and the principal species is equimolar in substrate and cyclodextrin. With the propanoate, binding is much stronger ( K = 1380) and the stoichiometry more clearly defined at 1:l. With the butanoate, the association constant is only slightly higher, but any question about the stoichiometry of the complex is removed by the sharpness of the direction change in the conductivity curve at 5 mmol dm-3 0-CD (Figure 4). The dimensions of the cyclohexyl group as with benzene are correct to promote the stability of the resultant complex. Cationic Surfactants. Previous work had shown that cationic Surfactants did not appear to behave much differently than their anionic counterparts of comparable carbon chain length, strengthening the argument that the surfactant chain is within the @-CDcavity in the complex s p e ~ i e s .This ~ was further examined with Surfactants cetyltrimethylammonium bromide (CTAB), cetylpyridinium chloride, and cetylpyridinium bromide at 2.08 X lo4 mol dm-3. Despite the greater error involved in working at such (11) Jiang,
X.-K.; Gu, J.-H.; Cheng, X.-E.; Hui, Y.-Z. Acta Chim. Si-
nica 1987, 45, 159.
Binding Constants of 0-CyclodextrinlSurfactant
*I2 *5 L. *I0 24
.3
45
1
IO1
lo-'
I
lo-z
I
crnc / mol
lo-'
I
IO'
Figure 5. Relationship between the binding constant for surfactant inclusion in 8-CD and the cmc of the surfactant. Numbers relate to Table I except for 24, sodium octyl sulfate; 25, sodium decyl sulfate; and 26, sodium dodecyl sulfate; taken from ref 4.
a low concentration, this concentration was chosen so as to be below the critical micelle concentrations of these surfactants. As can be seen from Table I, these surfactants bind extremely strongly with j3-CD, and, as with the anionic surfactants in this study, the complexation was clearly 1:l. The strongest binding is with the pyridinium-containing surfactants, where undoubtedly this moiety ensures a closeness of fit of the surfactant within the j3-CD cavity.
Discussion These results affm that j3-CD forms predominantly 1:l inclusion complexes with all surfactants no matter how unusual the tail or head component. Head group alteration makes less difference in the strength of the binding than does tail modification. When the hydrophobicity of the surfactant tail is increased either by lengthening the chain, by replacing hydrogen with fluorine, or by inserting aromatic groups of the correct fit, the value of the binding constant increases, which argues for inclusion of the tail portion of the surfactant within the cavity of j3-CD. The conductance data for the two-tailed surfactants, the sodium dialkyl sulfosuccinates, suggest binding constants to be no different than those of their single-tailed counterparts despite the additional hydrophobicity component imparted by the extra tail. This suggests that either only one of the tails becomes encapsulated by j3-CD or that the
Langmuir, Vol.5, No.1, 1989 221
two tails are so entwined as to act as one. The latter scenario seems less likely from steric and hydrophobic contribution considerations. In the former case, the second tail may avoid encapsulation simply through steric restrictions. One measure of the hydrophobic character of a surfactant is its critical micelle concentration (crnc), which is determined by the interplay between the hydrophobic forces drawing the tails together to form the micelle core and the repulsive forces pushing the heads apart in the micelle surface region. For the same polar head, the greater the hydrophobicity of the surfactant the lower the cmc. Varying the polar head makes little difference to the cmc as long as the charge on the head is the same. Figure 5 shows the relationship between the j3-CDlsurfactant binding constants and the cmc of the anionic surfactants listed in Table I, where cmc values are known. The inverse proportionality, which accounts for most of the data, is further evidence that the hydrophobicity of the surfactant plays a major role in the strength of the association with j3-CD or, more specifically, the surfactant tail resides within the j3-CD cavity in the binary complex. Even doubly charging the polar head does not greatly alter this relationship. There are two discordant data points, but they correspond to the two-tailed sulfosuccinates whose binding constants are apparently too low by an order of magnitude. However, if only one tail were encapsulated by 8-CD, the effective hydrophobic force in the inclusion would be that of a single tail, and the discrepancy disappears.
Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for funding and the Department of Employment and Immigation of Canada for a job development grant to J.E.R. Registry No. P-CD, 7585-39-9; sodium hexanoate, 10051-44-2; sodium heptanoate, 10051-45-3; sodium octanoate, 1984-06-1; sodium nonanoate, 14047-60-0; sodium decanoate, 1002-62-6; sodium dodecanoate, 629-25-4; disodium dodecylphosphate, 7423-32-7; disodium 4-hexyl 3-sulfonatosuccinate, 18299-71-3; disodium 4-heptyl 3-sulfonatosuccinate, 18299-71-3; disodium 4-octyl 3-sulfonatosuccinate, 18299-70-2; disodium 4-nonyl 3sulfonatosuccinate, 18278-70-1; disodium 4-decyl 3-sulfonatosuccinate, 1827869-8; sodium dodecyl benzenesulfonate, 5746-33-8; sodium perfluorooctanoate, 335-95-5; sodium dihexyl sulfosuccinate, 3006-15-3; sodium dioctyl sulfosuccinate, 577-11-7; sodium cyclohexyl acetate, 54668-20-1; sodium cyclohexyl propanoate, 85454-69-9; sodium cyclohexyl butanoate, 61886-29-1; sodium cyclohexyl sulfamate, 139-05-9;cetylpyridinium chloride, 123-03-5; cetylpyridinium bromide, 140-72-7; cetyltrimethylammonium bromide, 57-09-0.
