3582
J. Phys. Chem. 1987, 91, 3582-3585
Cryptand 222 Complexation of Anionic Surfactant Counterions. Drastic Decrease of the Critical Micelle Concentration of Sodium Dodecyl and Sodium Decyl Sulfates Pablo A. Quintela, Richard C. S. Reno, and Angel E. Kaifer* Chemistry Department, University of Miami, Coral Gables, Florida 331 24 (Received: December 15. 1986)
The addition of equimolar amounts of cryptand 222 to solutions containing either sodium dodecyl sulfate (SDS) or sodium decyl sulfate (SdecS) was shown to lower the critical micelle concentration (cmc) of both surfactants by a factor of nearly 5 , as measured by surface tension plots and changes in the fluorescence characteristics of pyrene. The cmc values for C222Na+DS- and C222Na’decS- were calculated to be 1.4 and 6.8 mM, respectively. In addition, cryptand complexation of the sodium counterions diminishes the hydrophobicity of the micellar hydrocarbon core. These findings were interpreted as the result of differences in the counterion location: the sodium counterions reside in the outer side of the micelle head-group region while the more hydrophobic sodium cryptate counterions are interspersed among the head groups, acting like spacers, decreasing interionic repulsions in the Stern layer, and favoring a greater extent of water penetration into the micellar structure.
Introduction The importance of polyether macrocyclic ligands is well documented in the numerous research papers’ devoted to the study of their properties, especially those related to their ability to form stable complexes with alkali and alkaline earth metal cations in weakly donor solvents. In aqueous media, the binding constants are usually very low due to strong ion solvation by water molecules which effectively compete with the polyether ligand.* An exception to this is found with crypt and^,^ a class of macrobicyclic ligands first synthesized by Lehn and co-workers, which are capable of encapsulating metal cations in their cavities, yielding inclusion complexes (cryptates) with large stability constant^.^ The prearranged tridimensional configuration of donor atoms in cryptands results in remarkably stable alkali and alkaline earth cation complexes even in aqueous media. Thus, the stability constants in water are in the range of lo4 to lo6 L/mol provided that there is a reasonably good fit between the cation radius and the cryptand’s cavity size.3 Macrocyclic ligands have been frequently used to model carrier-mediated transport of cations across membra ne^.^ Micelles are considered to be membrane mimetic agents5 because of the similarities of the microenvironments created by these assemblies with those found in the proximity of biological lipid membranes. Therefore, the study of systems containing micelles and macrocyclic ligands can shed some light on the basic interactions of cation carriers and membranes and provide new insight about the factors involved in transport mechanisms. Several authors have reported on the effects of alkali metal salts on the aggregation properties of nonionic surfactant containing crown ether moieties.6 We were intrigued by the effects of counterion complexation on the aggregation properties of simple anionic surfactants. In this work, we report the effects of cryptand 222 on the cmc’s of sodium dodecyl sulfate (SDS) and sodium decyl sulfate (SdecS) resulting from the complexation of sodium ion (the counterion in both surfactant systems) by this bicyclic ligand. (1) For a review see: De Jong, F.; Reinhoudt, D. N. Stability and Reacfiuifyof Crown-Ether Complexes; Academic: London, 1981. (2) Dishong, D. M.; Gokel, G.W. J. Org. Chem. 1982, 47, 147. (3) (a) Lehn, J. M.; Sauvage, J. P. J. Am. Chem. SOC.1975,97,6700. (b) Kauffmann, E.; Lehn, J. M.; Sauvage, J. P. Helu. Chim. Acra 1976,59, 1099. (4) (a) Lamb, J. D.; Christensen, J. J.; Izatt, S. R.; Bedke, K.; Astin, M. S.; Izatt, R. M. J. Am. Chem. SOC.1980, 102, 3399. (b) Lamb, J. D.; Christensen, J. J.; Oscarson, J. L.; Nielsen, B. L.; Asay, 8. W.; Izatt, R. M. J . Am. Chem. SOC.1980, 102, 3399. (5) Fendler, J. H. Membrane Mimetic Chemistry; Wiley: New York, 1982; Chapter 2. (6) (a) Moroi, Y.;Pramauro, E.; Gratzel, M.; Pelizzetti, E.; Tundo, P. J. Colloid Interface Sci. 1979, 69, 341. (b) Okahara, M.; Kuo, P. L.; Yamamura, S.;Ikeda, I. J . Chem. SOC.,Chem. Commun. 1980, 586. (c) Kuo, P. L.; Tsuchiya, K.; Ikeda, I.; Okahara, M. J. Colloid Interface Sci. 1983, 92, 463. (d) Turro, N. J.; Kuo, P. L. J . Phys. Chem. 1986, 90, 837.
0022-3654/87/2091-3582$01.50/0
P o ’
‘ o q
Cryptand 222
Experimental Section Materials. Sodium dodecyl sulfate (Baker) and sodium decyl sulfate (Kodak) were washed with diethyl ether, recrystallized twice from absolute ethanol, and dried in vacuo at 80 OC. This procedure yielded materials of satisfactory purity as judged from the absence of minima in the surface tension vs. concentration plots for the purified surfactants. Pyrene (Sigma) was also purified by recrystallization from ethanol. Cryptand 222 was obtained from Merck and used without further purification. All solutions were prepared with distilled water circulated through a fourcartridge Barnstead Nanopure water reagent system. Methods. Steady-state fluorescence spectra were recorded on an Aminco-Bowman spectrophotofluorimeter using narrow (0.5 mm) slits. All the surfactant solutions for fluorescence measurements were prepared with water previously saturated with pyrene. The excitation wavelength was 332 nm. Surface tension measurements were performed on a Fisher Model 20 tensiometer using the du Nouy method with platinum-iridium rings. Results Figure 1 shows the surface tension plots obtained with solutions containing varying concentrations of anionic surfactant and cryptand 222 (C222). All solutions were prepared with equimolar amounts of surfactant and cryptand. The shape of the surface tension plots is similar to that expected for the pure anionic surfactants. However, the observed surface tension values reach a plateau at lower concentrations than those for the corresponding pure anionic surfactant. Equimolar mixtures of C222 and SDS reach the horizontal surface tension region at 1.8 mM and equimolar mixtures of C222 and SdecS do so at 7.5 mM. The corresponding values for pure SDS and SdecS are 8.2’ and 33 mM,8 respectively. Indeed, this implies a decrease by a factor of more than 4 in the apparent cmc of both surfactant systems upon addition of equimolar amounts of the cryptand. To the best (7) (a) Williams, R. J.; Phillips, J. N.; Mysels, K. J. Trans. Faraday SOC. 1955, 51, 728. (b) Elworthy, P. H.; Mysels, K. J. J. Colloid Interface Sci. 1966, 21, 331. (8) (a) Mysels, K. J.; Kapauan, P. J . Colloid Sci. 1961, 16, 481. (b) Mukerjee, P.; Kapauan, P.; Meyer, H. G. J. Phys. Chem. 1966, 70, 783.
0 1987 American Chemical Society
Effect of Cryptand 222 on Cmc of SDS
The Journal of Physical Chemistry, Vol. 91, No. 13, 1987 3583
60
1.4-
50
1.2
-
1.0
40
0.8
30 3x;~-4
ik3
I
3x;oA3
1A-2 0.8-
CONCENTRATION, M
Figure 1. Concentration dependence of the surface tension of solutions containing equimolar mixtures of sodium alkyl sulfate and cryptand 2 2 2 ( 0 )SDS + C222; (7) SdecS + C222.
X N t
1.4 1 O-'
10-3
10-2
10-1
1
INI+I
Figure 4. Calculated fraction of complexed sodium ion as a function of total sodium concentration. The calculations were performed assuming a 1:l complexation process (log K, 3.9). The ligand concentration was considered to be equal to that of sodium throughout the plotted range.
1.o
1
b=
CONCENTRATION, M
Figure 2. Change in the fluorescence characteristics of pyrene (M) as a function of SDS concentration: (0)SDS; (V) SDS + C222 equimolar mixtures.
of our knowledge, this cmc lowering ability constitutes a previously unreported2' effect of macrocyclic polyether ligands which results from their strong affinity to metal cations. In order to corroborate these results we used a different method for the determination of cmc values. The relative intensities of the vibronic bands in the emission spectrum of pyrene were used for this purpose due to their sensitivity to microenvironment polaritye9 In particular, the intensity ratio of the first and third lines (11/13) has been shown to behave as a linear function of the solvent's dielectric constant.1° Furthermore, Ananthapadmanabhan et al." have recently demonstrated that the aggregation properties of surfactants are unchanged by the addition of pyrene to their solutions due to the extremely low solubility of this compound in water. Thus, plots of 11/13vs. surfactant concentration display sigmoidal shapes with clearly defined transition (9) (a) Kalyanasundaram,K.; Thomas, J. K. J. Am. Chem. Soc. 1977,99, 2039. (b) Turro, N . J.; Barentz, B. H.; Kuo,P.L.Macromolecules 1984,17, 1321. (10) Glushko, V.;Thaler, M. S. K.; Karp, C. D. Arch. Biochem. Biophys. 1981, 210, 33. ( 1 1 ) Ananthapadmanabhan,K. P.; Goddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, I , 352.
regions whose center is taken as the cmc.]' Figure 2 shows two such plots for pure SDS and equimolar mixtures of SDS and C222. Both graphs show similar shapes with well defined transition regions. It is clearly evident that the presence of C222 decreases the cmc value to 1.7 mM. In its absence, the cmc value determined from the data in Figure 2 was 8.0 mM in good agreement with literature values.' The corresponding plots for SdecS appear on Figure 3. Both graphs display sigmoidal shapes with evident cmc regions. Again, the presence of C222 lowers considerably the cmc. For the pure surfactant a value of 30 mM was obtained from the graph, while 7.5 mM was the observed cmc in the presence of equivalent concentrations of C222. The agreement of the cmc values obtained from the fluorescence characteristics of pyrene and those measured from surface tension plots is excellent. It is also noteworthy that the 11/13values measured in the micellar region for the pure surfactant are lower than those measured with the solutions containing equimolar concentrations of surfactant and cryptand above the apparent cmc. This effect was clearly observed for both surfactants (see Figures 2 and 3), and suggests that the pyrene environment is less hydrophobic in the micelles formed in cryptand-containing solutions. Discussion
Lehn and Sauvage reported a value of 7900 L mol-I (log K , = 3.9) for the sodium complexation constant of C222 in water.3 This value does not warrant complete complexation of sodium ion at the concentration levels used in this work. A plot of molar fraction of complexed sodium ion vs. sodium ion concentration is shown in Figure 4. The graph was calculated assuming 1 :1
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The Journal of Physical Chemistry, Vol. 91, No. 13, 1987
TABLE I: Critical Micelle Concentrations for Several Alkyl Sulfate Surfactants surfactant cmc. mmol/L temp, "C ref 1.4 this work C222Na'DS25 7 8.2 Na'DS25 6.8 this work C222Na'decS25 8 25 Na'decS33 18 Li*DS8.8 25 19 K'DS7.8 40 18 5.4 Me4NtDS25 20 Et4N'DS4.5 30 Bu,N'DS1.3 20 30
complexation (log K, = 3.9) and equimolar amounts of ligand and sodium ion in the solution. At concentrations in the range 10-3-10-2 M, where the apparent cmc of the surfactant-cryptand mixtures are observed, the molar fraction of complexed sodium ion ranges from 0.70 to 0.89. It is therefore evident that the majority of the sodium ions are complexed by the bicyclic ligand and that the behavior of the anionic surfactant-cryptand mixtures must reflect the properties of C222Na+DS- and C222Na+decSsurfactant salts in which the sodium counterions have been replaced by sodium cryptate complexes. However, at these concentration levels the fraction of uncomplexed sodium ion, albeit small, is not negligible. In order to obtain the cmc of the sodium cryptate surfactants, the observed data can be treated as corresponding to mixed micelles. Holland and Rubingh12 have recently proposed an equation to correlate the observed cmc (C*) of a mixture of n surfactants with the cmc's (Ci) of the individual surfactants and their molar fractions in the mixture ( c y i ) ; that is n
1/c* =
E .,/f;c,
i= 1
where thef,'s stand for the activity coefficients of the individual surfactants. If these are assumed to be unity, the surfactants are said to form ideally n ,xed micelles. This case was also treated by Clint'3 and by Lange and Beck.I4 For two different surfactants forming ideally mixed micelles, eq 1 simplifies to l/C* = (YA/CA+ a g / C g (2) where the subscripts A and B identify each of the two surfactants. This equation applied to the cmc of cryptand-anionic surfactant mixtures permits the determination of the cmc's for sodium cryptate decyl and dodecyl sulfates. The resulting values (given in Table I) are close to those experimentally obtained with the equimolar mixtures (see Results section) reflecting the predominance of cryptate counterions over free counterions at the concentrations in the cmc range. It must be pointed out that the effective sodium binding constant of the cryptand in these solutions might be somewhat larger than the aqueous value due to the presence of the micellar aggregates. Although this micellar effect is probably small because, on the average, most of the sodium counterions actually reside in the bulk aqueous phase, the calculated cmc values can be best viewed as lower limits. In addition, the small concentrations of free cryptand in these solutions could assist in the nucleation of micelles, thus lowering the observed cmc values. These factors reflect the complexity of all the interactions involved. The model chosen to evaluate the cmc of the cryptate alkyl sulfate surfactants is quite simple and does not take into account some of these interactions. Therefore, the calculated cmc's should only be regarded as a first approximation to the actual values. The decrease in the cmc values of sodium cryptate alkyl sulfates as compared to those for uncomplexed sodium alkyl sulfates is attributable to structural differences in the micellar head-group regions. The accepted model for the formation of micelles5 envisions this process as the result of two opposing forces: (a) the tendency of the hydrocarbon chains to aggregate in aqueous medium, and (b) the electrostatic repulsions among the ionic head (12) Holland, P. M.; Rubingh, D. N . J. Phys. Chem. 1983, 87, 1984. ( 1 3 ) Clint, J. J. Chem. SOC.1975, 71, 1327. (14) Lange, H.; Beck, K. H. Kolloid Z . Z . Polym. 1973, 251. 424.
Quintela et al. groups which counteract the hydrophobic tendency toward aggregation. Indeed, counterion binding to the head-group region decreases the latter repulsive interactions and stabilizes the micelles. Mukerjee et al.15 have demonstrated that the degree of counterion binding to dodecyl sulfate micelles is about the same for tetramethylammonium cations as for sodium cations. Therefore, it does not seem likely that complexation of the sodium counterion by the cryptand ligand (with its associated increase in counterion hydrophobicity) might result in significant changes of counterion binding to the micelle surface. Our results can be best explained through the postulation of different average locations for sodium and cryptate sodium counterions in the charged micelle surface. Thus, micelle-bound sodium ions would reside in the outer side of the head-group region so that a compact head-group structure is maintained. Conversely, micelle-bound sodium cryptate ions, being more hydrophobic, would reside in the inner side of the head-group region enlarging the distances between adjacent head groups, increasing the curvature of the micelle surface, and, thus, stabilizing the micelle through a reduction of repulsive electrostatic interactions amond the head groups. This accounts for the experimentally observed decrease of cmc values upon cryptand complexation of the sodium counterions. Similar arguments have been recently used by Szajdinska-Pietek et a1.I6 to explain differences in behavior between tetramethylammonium and sodium dodecyl sulfate micelles, and by Almgren and Swarupl' to account for the decrease in micelle aggregation number observed upon addition of tetraethylammonium chloride and other hydrophobic salts into SDS micellar solutions. Further support for this interpretation can be found in the cmc data in Table I. A comparison of cmc values for lithium, sodium, and potassium dodecyl sulfates reveals only a slight dependency of the surfactant cmc upon counterion size. In contrast to this, the cmc values for sodium, tetramethylammonium, tetraethylammonium, and tetrabutylammonium dodecyl sulfates display a significant trend: increases in the counterion hydrophobic nature and size are translated into decreased cmc values. Interestingly, the cmc of sodium cryptate dodecyl sulfate is very close to that of tetrabutylammonium dodecyl sulfate. Indeed, the tetrabutylammonium cation has been shown to be a good model for cryptate complexes owing to similarities in solvation enthalpy and entropy Another interesting feature of our experimental results resides in the pyrene's 11/13ratios observed in the micellar region. These were higher for the sodium cryptate surfactant than for the corresponding sodium surfactant for both alkyl sulfates surveyed. These results undoubtedly indicate that the microenvironment sensed by the pyrene probe is less hydrophobic in the cryptandcontaining solutions. The structural differences in the head-group region invoked before also provide a consistent explanation for this fact. Thus, as cryptate counterions reside in the inner side of the micelle surface, the Stern layer is thicker than in the more compact, cryptate-free micelle, giving rise to more surface roughness and, hence, more water penetration in the cryptate surfactant micelles. The micelle-solubilizedpyrene molecules thus sense a less hydrophobic environment (as reflected by the larger Z1/Z3 ratios) than they do in sodium alkyl sulfate micelles. The cmc of a surfactant system constitutes a relative measure of the stability of its micelles, that is, the lower the cmc value the more negative the free energy of micellization. The results of this work clearly suggest that cryptand complexation of the sodium counterions of anionic alkyl sulfate micelles results in the stabilization of the micellar aggregates. Some structural arguments (15) Mukerjee, P.; Mysels, K. J.; Kapauan, P. J. J. Phys. Chem. 1976, 71, 4166. (16) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. M. J . Am. Chem. SOC.1984, 106,4675. (17) Almgren, M.; Swarup, S. J . Phys. Chem. 1983, 87, 876. (18) Mysels, K. J.; Prineen, L. H. J. Phys. Chem. 1959, 6 3 , 1696. (19) Meguro, K.; Kondo, T.; Yoda, 0. J . Chem. SOC.Jpn. 1956, 77, 1236. (20) Meguro, K.; Kondo, T. J . Chem. SOC.Jpn. 1959, 80, 823. (21) While this paper was being reviewed similar results were reported: Evans, D. F.; Sen, R.; Warr, G. G. J . Phys. Chem. 1986, 90, 5500.
J . Phys. Chem. 1987, 91, 3585-3588 have been given here to account for this stabilization. However, a complete thermodynamic model accounting for the decreased free energy of the cryptate micelles cannot be developed without first assessing the thermodynamics of all the interactions involved in these systems.
Conclusions We have shown that the complexation of C222 of sodium counterions in SDS and SdecS micelles leads to a sizable decrease in the cmc values of both surfactants. This trend is consistent with reported cmc values for hydrophobic counterion-dodecyl sulfate surfactants which clearly indicate that increases in the hydrophobic character and size of the counterion translate into lowered cmc values. The sodium cryptate alkyl sulfate micelles seem to have a less hydrophobic core (greater extent of water
3585
penetration) than the corresponding sodium alkyl sulfate micelles. All these results can be interpreted as the reflection of different average locations of the counterions in the Stern layer. Thus, the sodium cryptate counterions reside in the inner side of the micellar head-group region whereas the sodium counterions are predominantly located in the outer side of this region. The charged micelle surface is then noticeably less compact in the former case resulting in more water penetration and reduced repulsive interactions among the ionic head groups.
Acknowledgment. We thank Brenda Santiago for performing some preliminary experiments. This work was partially supported by the University of Miami Research Council. Registry No. SDS, 151-21-3; SdecS, 142-87-0; cryptand 222, 23978-09-8;Na, 7440-23-5.
Formation of Gas-Phase Methylallyl Radicals during the Oxidation of 1-Butene and Isobutylene over Bismuth Oxide Daniel J. Driscoll, Wilson Martir, and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: December 22, 1986)
Using a matrix isolation-electron spin resonance (MIESR) technique it has been shown that Biz03 is capable of generating gas-phase 1-methylallyland 2-methylallyl radicals from 1-buteneand isobutylene, respectively. Previous results had suggested that the rate of surfacegenerated gas-phase radical formation over this material was governed by C-H bond strength differences only. These new results, however, indicate that other factors such as the availability of abstractable hydrogen atoms and the reaction stereochemistry also may be important in determining the amount of gas-phase radicals produced.
Introduction Recent studies on the oxidative dimerization of simple alkanes and alkenes have suggested that radicals, formed on a metal oxide surface, may be released into the homogeneous gas phase where they then undergo subsequent coupling reactions to produce the final, stable products.I4 Allyl radicals, for example, are readily formed during the reaction of propylene and oxygen over Biz03 and may be detected by using a matrix isolation-electron spin resonance (MIESR) t e ~ h n i q u e . ' , ~If radical recombination is allowed to occur, 1,5-hexadiene is observed in amounts which confirm that gas-phase radical coupling is indeed a major channel for the catalytic oxidative dimerization of propylene. In addition, free gas-phase methyl radicals have been detected with the MIESR technique during the catalytic reaction of methane and oxygen over MgO, Li-doped MgO,Zf and La203$ and mechanisms have now been proposed in which coupling of these species is the primary pathway for C2 product formation. Subsequent experiments have demonstrated that the efficiency of a surface for radical generation depends both on the strength of the reactant hydrocarbon C-H bond and on the nature of the active surface oxygen ion.6 Bismuth oxide contains active sites (probably oxide ions in sites of low coordination) which are capable of promoting radical formation from hydrocarbons having C-H bond strengths 189 kcal/mok7 By contrast, Li-promoted MgO (1) Martir, W.; Lunsford, J. H. J . Am. Chem. SOC.1981, 103, 3728. (2) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. Chem. SOC.1985, 107, 58. (3) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J . Am. Chem. SOC. 1985, 107, 5062. Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721.
(4) Lin, C.-H.; Campbell, K. D.; Wang, J.-X.; Lunsford, J. H. J . Phys. Chem. 1986, 90, 534. (5) Driscoll, D. J.; Lunsford, J. H. J . Phys. Chem. 1983, 87, 301. (6) Driscoll, D. J.; Lunsford, J. H. J . Phys. Chem. 1985, 89, 4415.
is capable of generating methyl radicals from CH4, which has a C-H bond strength of 104 kcal/mol. The activity of the Li/MgO surface is attributed to surface 0- ions, which are known to be effective in hydrogen atom abstraction In the present study the ability of Bi203 to generate gas-phase allylic-type radicals has been further investigated. Surface-generated gas-phase radical production was examined during the reactions of propylene (DC+,= 89 kcal/m01),~1-butene (DC+, = 83 kcal/mol),'O and isobutylene (DC+,= 86 kcal/mol)" in the presence of oxygen. The results suggest that the relationship between bond energy and the rate of radical formation over Bi203 may not be as straightforward as was previously indicated.6 Other factors such as the availability of abstractable hydrogen atoms, steric considerations, and the occurrence of secondary surface reactions may also be important.
Experimental Section The apparatus for the MIESR experiment has been described in detail elsewhere.' Briefly, the reactant gases are passed through the heated catalyst bed in a low-pressure (- 1 Torr) quartz flow reactor. The effluent gases are then carried downstream into a Torr) collection region and frozen on low pressure (-2 X (7) This value was based on a DC-., = 89 kcal/mol for propylene: Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 62nd ed; CRC Press: Boca Raton, FL, 1982; p F-191. More recent data suggest a slightly lower value of DC-"= 86 kcal/mol: McMillen, D. F.; Golden, D. F. Annu. Rev. Phys. Chem. 1982, 33, 493. Both values have an estimated uncertainty of f l
kcal/mol.
(8) Aika, K.; Lunsford, J. H. J . Phys. Chem. 1977, 81, 1383. (9) Bohme, D. K.; Fehsenfeld, F. C. Can. J . Chem. 1969, 47, 2717. (10) O'Neal, H. E.; Benson, S. W. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol. 11, p 275. ( 1 1) Trenwith, A. B.; Rigley, S . P. J . Chem. SOC., Faraday Trans. 1 1977, 73. 817.
0022-3654/87/2091-358~$01.50/0 0 1987 American Chemical Society
