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Decarboxylation and Dephosphorylation in New Gemini Surfactants. Changes in Aggregate Structures Lucia Brinchi,† Raimondo Germani,† Laura Goracci,† Gianfranco Savelli,*,† and Clifford A. Bunton‡ Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto, 8, I-06123 Perugia, Italy, and Department of Chemistry, University of California, Santa Barbara, California 93106 Received March 12, 2002. In Final Form: June 20, 2002 Dicationic gemini surfactants, [CH3-(CH2)m-Me2N+CH2ArCH2N+Me2-(CH2)m-CH3]2Br-, where CH2ArCH2 is the spacer, with Ar ) 2,5-(MeO)-C6H2, have been prepared with bromide as counterion and with m ) 11, 13, and 15, pXMo(DDA)2, pXMo(MDA)2, and pXMo(CDA)2, respectively; their physical properties have been compared with those of bis-(2,5-n-dodecyloxy)bis(trimethylammonium)benzene, pXDo(TA)2, and a surfactant with an unsubstituted spacer, m ) 11 and ArdC6H4, pX(DDA)2. Solubilities of pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, and pXDo(TA)2 are much higher than that of pX(DDA)2 in water, and critical micelle concentrations, cmc’s, are lower than those of the corresponding monocationic surfactants. Micelles of pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, and pXDo(TA)2 significantly accelerate decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion and dephosphorylation of 2,4-dinitrophenyl phosphate dianion. Rate constants increase and become constant as substrates become fully micellar bound and then increase at surfactant concentrations which are greater than the cmc’s by factors of ca. 25. These subsequent increases are ascribed to changes in micellar morphologies, consistent with changes in 1H NMR line widths.
Introduction Considerable attention has recently been devoted to gemini surfactants and the assemblies that they form.1 The name is generally applied to amphiphiles possessing two hydrophobic tails and two polar, or ionic, headgroups, linked by a spacer group, which may be rigid or flexible.1-3 These surfactants with various spacers, hydrophobic tails, and headgroups have structures and properties which may be very different from those of the corresponding singletailed surfactants and are said to be “unique to the world of surfactants”.1 They can be an order of magnitude more surface active and have lower critical micelle concentrations (cmc’s) relative to corresponding conventional surfactants.1,3 Gemini surfactants can aggregate to give micelles of different morphologies (spherical, disklike, rodlike, and wormlike) but also vesicles and gels, depending on the nature of the spacer and the length of the hydrophobic tails.1,3 Most papers on gemini surfactants have focused on their specific structural properties, with few studies of their effects upon reaction rates.4,5 In this paper, we report on structural and kinetic studies with new gemini surfactants pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, and pXDo(TA)2 (Chart 1). Surfactants pXMo(DDA)2, pXMo(MDA)2, and pXMo(CDA)2 are similar to a gemini surfactant, pX(DDA)2, studied earlier (Chart 1),6 except that two methoxy groups are in positions 2 and 5 of the aromatic ring. Gemini pXDo(TA)2 was synthesized to investigate effects of changes in positions of the long chains, with a constant spacer length. † ‡
Universita` di Perugia. University of California.
(1) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (2) Menger, F. M.; Littau, C. A. J. Am. Chem. Soc. 1993, 115, 10083. (3) Rosen, M. CHEMTECH 1993, 23, 30. (4) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G. Langmuir 1999, 15, 2631. (5) Bunton, C. A.; Minch, M. J.; Hidalgo, J.; Sepulveda, L. J. Am. Chem. Soc. 1973, 95, 3262. (6) Menger, J.; Littau, C. A. J. Am. Chem. Soc. 1991, 113, 1451.
Chart 1. Structure of the Gemini Surfactantsa
a pXMo(DDA)2: R ) OCH3, R′) (CH2)11CH3; pXMo(MDA)2: R ) OCH3, R′ ) (CH2)13CH3; pXMo(CDA)2: R ) OCH3, R′ ) (CH2)15CH3; pXDo(TA)2: R ) O(CH2)11CH3, R′ ) CH3; pX(DDA)2: R ) H, R′ ) (CH2)11CH3.
We used decarboxylation of 6-nitrobenzisoxazole-3carboxylate ion (6-NBIC) and hydrolysis of 2,4-dinitrophenyl phosphate dianion (2,4-DNPP2-) as kinetic probes of the properties of these new micellized surfactants. These spontaneous reactions are useful indicators of structures of association colloids because with a fully bound substrate, rate constants relative to those in water, that is, changes in the relative free energies of the initial and transition states, depend wholly on properties of the colloid.7 The situation is more complicated for nonspontaneous bimolecular reactions where distributions of two species have to be considered, and there are questions regarding the meaning of “concentration” in the colloidal submicroscopic region. These uncertainties disappear with spontaneous reactions which are overall first order, and rate constants in micellar pseudophase can be defined unambiguously. The pseudophase treatment, which predicts that values of kobs will increase (or decrease) monotonically and become constant ((k′M)) with fully bound substrate, fits extensive data for spontaneous reactions.7 The spontaneous decarboxylation of 6-NBIC (Scheme 1) is accelerated by a variety of aqueous colloidal assemblies that provide submicroscopic reaction media.5,7,8 Rate constants in homogeneous (7) (a) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (b) Savelli, G.; Germani, R.; Brinchi, L. In Reactions and Synthesis in Surfactant Systems; Texter, J., Ed.; Marcel Dekker: New York, 2001; p 175.
10.1021/la020250o CCC: $22.00 © 2002 American Chemical Society Published on Web 09/11/2002
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Scheme 1. Decarboxylation of 6-Nitrobenzisoxazole-3-carboxylate Anion (6-NBIC)
Brinchi et al. Table 1. Properties of Gemini Surfactantsa pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, pXDo(TA)2, and pX(DDA)2 surfactant
104cmc, Mb
pXMo(DDA)2 6.6 (6.4) pXMo(MDA)2 1.2 (1.1) pXMo(CDA)2 0.35 (0.34) pXDo(TA)2 4.4 (4.2) pX(DDA)2 10d
Rc 0.32 0.43 0.41 0.40
solubility in solubility in H2O, M 0.01 M NaOH, M 0.06 0.06 0.01 0.05 7.5 × 10-4
0.08 0.07 0.015 0.05 0.001
a At 25 °C if not otherwise specified. b From conductivity, in parentheses from surface tension. c From ratios of slopes above and below cmc of linear plots of conductivity against [surfactant], see text and ref 13. d At 50 °C, ref 2.
Scheme 2. Dephosphorylation of Dianionic 2,4-Dinitrophenyl Phosphate (2,4-DNPP2-)
media increase sharply with decreasing polarity of the solvent, due to dispersion of charge in going from the carboxylate ion to the charge-delocalized transition state.9 Rate effects in colloidal assemblies are sensitive to the nature of the counterion, the charge type of the amphiphile, and the headgroup bulk, giving information about structural variations of the submicroscopic reaction environments.7 For example, increases of rates of decarboxylation are ascribed to decreases in hydrogen bonding and polarity in the submicroscopic reaction medium which destabilizes the initial state, relative to the transition state (Scheme 1). Spontaneous hydrolyses of acyl and aryl phosphate dianions are faster at micellar surfaces than in water,10 and consistently rate constants increase with a decrease in solvent polarity.11 Solvent and micellar effects upon these phosphate ester hydrolyses are qualitatively similar to those upon anionic decarboxylation of 6-NBIC, but with some significant differences; for example, dephosphorylation involves at least one water molecule in the overall reaction. In the pH-independent region11(pH ) 6-12), dephosphorylation of dianionic 2,4-DNPP2-, Scheme 2, can be regarded as a spontaneous elimination of metaphosphate ion, PO3-, consistent with kinetic solvent and substituent effects.11a-c However, there is inversion of configuration at phosphorus and free metaphosphate ion is not an intermediate in aqueous media.11d Results and Discussion Surfactant Characterization. Solubilities of the new surfactants in water and aqueous alkali at 25 °C are in Table 1. Gemini surfactants are typically less soluble than (8) (a) Bunton, C. A.; Minch, J. M. Tetrahedron Lett. 1970, 44, 3881. (b) Germani, R.; Ponti, P. P.; Savelli, G.; Spreti, N.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1767. (c) Germani, R.; Ponti, P. P.; Romeo, T.; Savelli, G.; Spreti, N.; Cerichelli, G.; Luchetti, L.; Mancini, G.; Bunton, C. A. J. Phys. Org. Chem. 1989, 2, 553. (d) Di Profio, P.; Germani, R.; Savelli, G.; Cerichelli, G.; Spreti, N.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1996, 1505. (9) (a) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1970, 92, 2553. (b) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7305. (c) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7312. (10) (a) Bunton, C. A.; Fendler, E. J.; Sepulveda, L.; Yang, K.-U. J. Am. Chem. Soc. 1968, 90, 5512. (b) Buist, G. J.; Bunton, C. A.; Robinson, L.; Sepulveda, L.; Stam, M. J. Am. Chem. Soc. 1970, 92, 4072. (c) Del Rosso, F.; Bartoletti, A.; Di Profio, P.; Germani, R.; Savelli, G.; Blasko`, A.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1995, 673. (11) (a) Di Sabato, S.; Jencks, W. P. J. Am. Chem. Soc. 1961, 83, 4395. (b) Bunton, C. A.; Fendler, E. J.; Fendler, J. H. J. Am. Chem. Soc. 1967, 89, 1221. (c) Kirby, A. J.; Varvoglis, A. G. J. Am. Chem. Soc. 1966, 88, 1823. (d) Buchwald, S. L.; Friedman, J. M.; Knowles, J. R. J. Am. Chem. Soc. 1984, 106, 4911.
cationic analogues, probably because of their higher molecular weights, and solubilities increase slightly on addition of NaOH. As with single-chain surfactants, solubilities decrease with increasing chain length.7 Solubilities are increased by introduction of two methoxy groups into the skeleton of the gemini pX(DDA)2, by ca. 2 orders of magnitude. Critical micelle concentrations, determined by conductivity (Table 1), are significantly lower than those of corresponding surfactants of equivalent chain length,12 consistent with extensive evidence.1 Increasing the chain length by 3 carbon atoms decreases the cmc by a factor of ca. 20 (Table 1), as for conventional surfactants, although for other gemini surfactants a larger effect of chain length had been observed (by up to 2 orders of magnitude for an increase of 4 carbon atoms1). Degrees of ionization, R, were determined conductometrically from ratios of slopes, above and below the cmc, of the linear plots of conductivity against [surfactant].13 Values for gemini surfactants are larger than for cationic micelles (Table 1) but by less than the factor of 2 expected statistically, indicating extensive charge neutralization by Br-. (Values for monocationic bromides are ca. 0.25.)14-16 Rate-Surfactant Profiles for Kinetics of 6-NBIC and of 2,4-DNPP2-. Variations of kobs for decarboxylation of 6-NBIC with concentrations of surfactants pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, and pXDo(TA)2 are in Figure 1. We also used the surfactant pX(DDA)2, but only at low concentration because of its low solubility (Table 2), and values of kobs are similar to those in solutions of pXMo(DDA)2. As noted earlier, values of kobs for spontaneous reactions generally increase monotonically with increasing surfactant concentration and become constant when the substrate is fully micellar bound.7 But, in the present case, values of kobs increase to a constant value over a range of surfactant concentration and then increase at higher [surfactant], without leveling off. Values of kobs for spontaneous reactions typically become approximately constant over concentration ranges for which micellar structures do not change. Although examination of the behavior of surfactant pXMo(CDA)2 is limited by its low solubility (Table 1 and Figure 1), concentrations at which rate constants change decrease as the chain length of the surfactant increases. This kinetic behavior is also seen with surfactants pXMo(DDA)2 and pXDo(TA)2 in dephosphorylation of 2,4(12) Mukerjee, P.; Mysels, K. L. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1970. (13) (a) Zana, R. J. Colloid Interface Sci. 1980, 78, 330. (b) Evans, H. C. J. Chem. Soc. 1956, 579. (14) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (15) Brady, J. E.; Evans, D. E.; Warr, G. G.; Grieser, F.; Niham, B. W. J. Phys. Chem. 1986, 90, 1853. (16) Bunton, C. A.; Nome, F.; Romsted, L. S.; Quina, F. H. Acc. Chem. Res. 1991, 24, 357.
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Figure 1. Values of kobs for the reaction of 6-NBIC in pXMo(DDA)2 (b), pXMo(MDA)2 ([), pXMo(CDA)2 (9), and pXDo(TA)2 (3). Lines are drawn only to guide the eye. Table 2. Decarboxylation of 6-NBIC and Dephosphorylation of 2,4-DNPP2 in Gemini pX(DDA)2a 2,4-DNPP2-
6-NBIC
103[surfactant], M 104kobs, s-1 103[surfactant], M 104kobs, s-1 0 0.1 0.2 0.3 0.5 0.6 1.0
0.03 0.547 1.65 2.55 4.30 4.97 6.34
0 0.1 0.2 0.5 0.75b
0.083 0.145 0.205 1.73 2.47
a At 25 °C. b Maximum solubility in these conditions, NaOH 3.3 × 10-4 M.
Figure 2. Values of kobs for the reaction of 2,4-DNPP2- in pXMo(DDA)2 (b) and pXDo(TA)2 (3). Lines are drawn only to guide the eye.
DNPP2- (Figure 2), although the effect is less evident than for decarboxylation, especially for surfactant pXDo(TA)2. However, surfactant concentrations where values of kobs are constant are similar for reactions of the two substrates. Reactions in Dilute Surfactants. The simple pseudophase treatment predicts that for spontaneous reactions values of kobs will be constant below the cmc, increase monotonically as micelles form, and become constant with fully bound substrate. However, it is often difficult to quantify effects of concentrations below the cmc, due to solubility problems with hydrophobic substrates, reactant-
Figure 3. Values of kobs for the reaction of 6-NBIC in pXMo(DDA)2 (b) and for the reaction of 2,4-DNPP2- in pXMo(DDA)2 (O) and pXDo(TA)2 (3) in the region of dilute surfactant.
induced micellization, and interactions with nonmicellized surfactant.7 Some ionic, water-soluble substrates, for example, derivatives of 2,4-DNPP2-, form insoluble salts with very dilute cationic surfactants.17 Furthermore, when reactions in dilute surfactants are followed, values of kobs often increase monotonically below the cmc in water, due to reactant-induced micellization, especially with polyvalent counterions.7 In a few cases, with very hydrophobic substrates, values of kobs reach maxima in dilute surfactant, below the cmc, due to reaction in so-called premicelles, as for hydrophobic derivatives of 6-NBIC and 2,4DNPP2-, with long alkyl substituents.17,18 With both kinetic probes, we could use the dilute gemini surfactant, down to molar ratios of substrate/surfactant of 1, without solubility problems: this kind of experiment has not been possible in monocationic surfactants because of precipitation due to ion-pair formation.19 No precipitation occurs with gemini surfactants either for monoanionic substrate 6-NBIC or for dianionic substrate 2,4-DNPP2-. There is no rate effect of very dilute surfactant (Figure 3), and values of kobs increase only at concentrations close to the cmc in water (Table 1); thus there is minor substrateinduced micellization or reaction in premicelles, as the pseudophase treatment predicts. Reactions in Micelles. At surfactant concentrations where kobs becomes approximately constant, the limiting values give the rate constants, k′M, in the micellar pseudophase (eq 1), and the values are in Table 3, with comparisons for other micellar systems. For decarboxylation of 6-NBIC, values of 104k′M, s-1, in gemini surfactants pXMo(DDA)2, pXMo(MDA)2, and pXMo(CDA)2 are in the range of 7.8-10.8, but the values are lower for surfactant pXDo(TA)2. Gemini surfactants pXMo(DDA)2 and pXDo(TA)2 have the same spacer and alkyl chain lengths, but the different positions of the alkyl tails should provide different reaction microenvironments, and reactivities of both substrates indicate a more aqueous environment for pXDo(TA)2 than for pXMo(DDA)2. Decarboxylation had been investigated earlier5,9c in gemini surfactants 1,3-bis(N-cetyl-N,N-dimethylamino)alkane dibromide, (CDA)2Cn2Br, with variations in the (17) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.; Tugliani, M.; Bunton, C. A. Langmuir 2000, 16, 10101. (18) Brinchi, L.; Di Profio, P.; Germani, R.; Giacomini, V.; Savelli, G.; Bunton, C. A. Langmuir 2000, 16, 222. (19) Germani, R. Unpublished results.
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Table 3. Values of 104k′M (s-1) for Decarboxylation of 6-NBIC and for Dephosphorylation of 2,4-DNPP2at 25 °C
a
surfactant
6-NBIC
2,4-DNPP2-
pXMo(DDA)2 pXMo(MDA)2 pXMo(CDA)2 pXDo(TA)2 (CDA)2C32Br (CDA)2C42Br (CDA)2C62Br DTABr CTABr DDDABr
7.8 9.6 10.8 3.4 7.7a 10b 12b 2.2c ≈3.3c ≈21
4.6
Reference 9c. b Reference 5. c Reference 25.
Table 4. Values of lw (Hz) for Gemini Surfactants pXMo(DDA)2, pXMo(MDA)2, and pXDo(TA)2 at Concentrations below and above the Transition to Larger Aggregatesa pXMo(DDA)2
pXMo(MDA)2
pXDo(TA)2
2 mM 50 mM 1.7 mM 30 mM 2 mM 50 mM 1.7
1.5 1.8d ≈7d d
Reference 23.
Chart 2. Gemini 1,3-Bis(N-cetyl-N,N-dimethylamino)alkane Dibromide, with Variation of na
a (CDA)2C32Br (n ) 3), (CDA)2C42Br (n ) 4), and (CDA)2C62Br (n ) 6).
spacer (n, with n ) 3, 4, and 6, Chart 2), and values of 104k′M, s-1, depend on the spacer length, vary between 7.7 and 12, and increase modestly with spacer length (Table 3). These rate constants are similar to those in our gemini surfactants, pXMo(DDA)2, pXMo(MDA)2, and pXMo(CDA)2, and reaction is faster than in monocationic surfactants of equivalent chain length by factors of ca. 3. This trend is general, for example, for dephosphorylation: the value of k′M is higher by a factor of ca. 3 in gemini pXMo(DDA)2 than in monocationic dodecylammonium bromide, DTABr (Table 3). There is a similar relative rate increase for cyclization of o-(3-bromopropyloxy)phenoxide ion4 in gemini surfactant (CDA)2C42Br, as compared with a monocationic surfactant: values of 104k′M, s-1, are 4.1 and ca. 8 in CTABr and gemini (CDA)2C42Br, respectively.4,20 We may therefore conclude that generally, except for gemini surfactant pXDo(TA)2, dicationic surfactants are better catalysts than the corresponding monocationic surfactants, probably because the spacer decreases the extent of water penetration at the aggregate surface. Decarboxylation of 6-NBIC, dephosphorylation of 2,4DNPP2-, and cyclization are all assisted by a decrease in the water content of the reaction environment. Menger et al.21 have used chemical trapping to estimate concentrations of H2O and Br- at surfaces of gemini micelles and conclude that proximity of the positive charges increases anion binding at the expense of binding of H2O, which provides a ready explanation of our kinetic data. However, the change from an aromatic to an aliphatic spacer has little effect on kobs although it has a major effect on solubilities. For both conventional and gemini surfactants, values of k′M increase with increasing alkyl chain length. A longer alkyl chain may lead to larger aggregates with decreased interfacial water content, although Engberts et al. found that changes in alkyl chain length of 4-alkyl(20) Cerichelli, G.; Luchetti, L.; Mancini, G.; Muzzioli, M. N.; Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Bunton, C. A. J. Chem. Soc., Perkin Trans. 2 1989, 1081. (21) Menger, F. M.; Keiper, J. S.; Mbadugha, B. N. A.; Coran, K. L.; Romsted, L. A. Langmuir 2000, 16, 9095.
-N+CH3 -OCH3 Ar-CH2-N+ H-Ar
6.5 3.5 11 4.5
14 9.5 47 14
8 5 18 6
26 19 ca. 55 25
6
14
ca. 11 8
32
b
a Spectra in D O at 25 °C. b Signal as a shoulder of, almost 2 collapsed with, the signal of HOD.
N-methylpyridinium iodide surfactants did not affect rates of decarboxylation of 6-NBIC.22 However, we note that the monocationic twin-tailed surfactant didododecylammonium bromide, DDDABr, which may not form normal micelles, is a better catalyst than single-tailed surfactants and dilute gemini surfactants, in the initial plateau region, for both decarboxylation and dephosphorylation (Table 3):23 with DDDABr the change in the hydrophilic/lipophilic balance should give a less aqueous interfacial region. Changes of Aggregate Structure. As noted, at high [surfactant], where the pseudophase model, eq 1, predicts constant values of kobs, we observe increases (Figures 1 and 2). This behavior is unusual but has recently been observed for cyclization of 2-(3-bromopropyloxy)phenoxide ion4 in the gemini surfactant (CDA)2C42Br. The maximum surfactant concentration (5 × 10-4 M) used earlier in decarboxylation of 6-NBIC was too low to see evidence of any breaks in plots of kobs against [surfactant]. The increase of rates of cyclization in micelles of (CDA)2C42Br was associated with a change of structure, and similar structural changes are probably responsible for the sharp breaks seen in Figures 1 and 2 for decarboxylation and dephosphorylation, respectively. The positions of the breaks shown in Figures 1 and 2 do not depend significantly on the nature of the substrates, indicating that they are due to changes in the aggregate structures, generating new, less polar, reaction microenvironments. We examined the 1H NMR spectra of surfactants pXMo(DDA)2, pXMo(MDA)2, and pXDo(TA)2 (Table 4) and saw no significant variations of chemical shifts with concentrations of surfactant but observed increases of line width (lw), Hz, of signals of aliphatic and aromatic hydrogens (Table 4). This is not so for 3 where a larger concentration range would be needed but there are solubility problems. These increases are higher than those of a conventional surfactant such as cetyltrimethylammonium bromide, CTABr, where lw is 1.2 Hz in water at 5 × 10-2 M at 25 °C4 even though CTABr micelles grow readily with increasing [surfactant]. We see these increases for all the signals of the gemini surfactants as their concentrations are increased. Relative lw value (lw(50 mM)/lw(2 mM)) can be used to discuss structural changes in the gemini skeleton. When you compare the relative lw value for the Ar-CH2-N+ or H-Ar segment with that of the N+CH3 segment, evidently the former relative value tends to become larger than the latter one, indicating that the mobility of the Ar-CH2-N+ or H-Ar segment may be further restricted at high gemini concentrations, as a consequence of changes in the micellar morphology. There is extensive evidence in the literature associating such broadening with a transition to larger aggregates, and we conclude that it increases kobs. We note that electrolytes (22) Engberts, J. B. F. N. Pure Appl. Chem. 1992, 64, 1653. (23) Bunton, C. A.; Dorwin, E. L.; Savelli, G.; Si, V. C. Recl. Trav. Chim. Pays-Bas 1990, 109, 64.
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Scheme 3. General Synthetic Route to Geminisa
a pXMo(DDA)2: R ) OCH3, R′ ) CH2-(CH2)10-CH3; pXMo(MDA)2: R ) OCH3, R′ ) CH2-(CH2)12-CH3; pXMo(CDA)2: R ) OCH3, R′ ) CH2-(CH2)14-CH3; pXDo(TA)2: R ) OCH2-(CH2)10-CH3, R′ ) CH3.
increase kobs for decarboxylation in CTABr, provided that they do not displace the anionic substrate from the micelles.5,7 This increase is most evident with a low charge density anion such as tosylate which enters the cationic micellar surface and induces growth. The bulky anion displaces Br- and probably interfacial water. An increase in the size of gemini micelles should generate the rate increases seen in Figures 1 and 2 at concentrations above those of the initial apparent plateaux. Decarboxylation of 6-NBIC and dephosphorylation of 2,4-DNPP2- are simple, reliable kinetic probe ions in aggregate structures. Experimental Section Materials. Substrates 6-NBIC and 2,4-DNPP2- were samples earlier prepared as reported in the literature.8a,24 Surfactants were prepared as shown in Scheme 3. Details of the preparations of pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, and pXDo(TA)2 are given in the Supporting Information, and preparation of pX(DDA)2 has been described.2 Kinetics. Decarboxylation and dephosphorylation were followed spectrophotometrically at 410 and 358 nm, respectively, at 25.0 °C ((0.1) in an HP 8452 diode array spectrophotometer. A freshly prepared solution of 6-nitrobenzoisoxazole-3-carboxylic acid in MeOH (30 µL) was added to 3 mL of reaction solutions containing 0.01 M NaOH, and [substrate] in the cuvette was 1 × 10-4 M. A freshly prepared solution of lutidinium 2,4dinitrophenyl phosphate in water was added to 3 mL of the reaction solution containing 3.3 × 10-4 M NaOH, and final [substrate] was 2 × 10-5 M. Solutions were made up in CO2-free redistilled water. Most values of kobs were averages of 2-5 data points which were within 5% of the mean. For decarboxyation in pXMo(DDA)2 (1-2 and 60-70 mM), some surfactant solutions were left overnight before starting the reaction by addition of 6-NBIC. Values of kobs were within 4% of those with freshly made up solutions, and there were no systematic variations in the values. Conductivity Measurements. Conductivities of surfactants were measured at 25 °C in an Orion Research instrument with a 1 cm dipping cell. Values of the cmc are from the breaks in plots of conductivity against [surfactant], and values of the ionization degree are estimated from ratios of slopes above and below the cmc.13 (24) Rawji, G.; Milburn, R. M. J. Org. Chem. 1981, 46, 1205. (25) Bunton, C. A.; Kamego, A. A.; Minch, M. J.; Wright, J. L. J. Org. Chem. 1975, 40, 1321.
Surface Tension Measurements. The surface tension was measured by a Fisher 20, du Nou¨y type, tensiometer at 25.0 °C. The cmc values are estimated as intersections of two linear plots, above and below the cmc, of surface tension versus -log[surfactant]. NMR Spectra. NMR spectra of synthesized materials were recorded on a 200 MHz Bruker spectrophotometer, generally in CDCl3, with 1H chemical shifts relative to internal TMS (Supporting Information). Other NMR spectra in D2O or in CD3OD (nonaggregating conditions) were recorded in a 400 MHz Bruker instrument, at 400,14 MHz with one pulse sequence. Chemical shifts are referred to HOD at 4.71 ppm or to CD3OH at 3.34 ppm, and spectra were recorded at 25 °C ((0.5). Typical parameters are as follows: aq, 1 s; sw, 6 kHz; resolution, 0.5 Hz. If aq is the delay time it is probably too short, especially when you measure lw in micelles. Some values should be obtained with a longer delay to check this point.
Conclusions Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion and dephosphorylation of 2,4-dinitrophenyl phosphate dianion are useful methods to investigate aggregates of gemini surfactants pXMo(DDA)2, pXMo(MDA)2, pXMo(CDA)2, and pXDo(TA)2. In fact, rate constants increase and become constant as substrates become fully micellar bound but then increase again at [surfactant] greater than the cmc by factors of ca. 25. These subsequent increases correspond to a transition to larger aggregates that is consistent with 1H NMR investigations. Acknowledgment. Support of this work by Ministero Dell’Istruzione, Universita` e Ricerca, Rome, (COFIN 2001) and by Consiglio Nazionale delle Ricerche, Rome, is gratefully acknowledged. Supporting Information Available: Preparation of dicationic gemini surfactants [CH3-(CH2)m-Me2N+CH2ArCH2N+Me2-(CH2)m-CH3]2Br-, where CH2ArCH2 is the spacer, with Ar ) 2,5-(MeO)-C6H2, with bromide as counterion and with m ) 11, 13, and 15, pXMo(DDA)2, pXMo(MDA)2, and pXMo(CDA)2, respectively. Preparation of bis-(2,5-n-dodecyloxy)bis(trimethylammonium)benzene, pXDo(TA)2. This material is available free of charge via the Internet at http://pubs.acs.org. LA020250O
