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Langmuir 1998, 14, 768-772
Relation between the Infrared Spectrum of Water and Decarboxylation Kinetics in Cetyltrimethylammonium Bromide in Dichloromethane Pietro Di Profio,† Raimondo Germani,† Giuseppe Onori,‡ Aldo Santucci,‡ Gianfranco Savelli,*,† and Clifford A. Bunton§ Dipartimento di Chimica, Universita` di Perugia, Via Elce di Sotto, 8, I-06123 Perugia, Italy, Istituto per la Fisica della Materia, Unita` di Perugia and Dipartimento di Fisica, Universita` di Perugia, Via A. Pascoli, I-06123 Perugia, Italy, and Department of Chemistry, University of California, Santa Barbara, California 93106 Received February 24, 1997. In Final Form: October 15, 1997 The structure of water in cetyltrimethylammonium bromide (CTABr) reverse micelles in dichloromethane has been studied as a function of W ) [H2O]/[surfactant] by using the IR absorption due to O-H stretching modes in the 3800-3000 cm-1 range. We restricted our analysis to the region of small amounts of water (0 < W < 20) where the properties of the system depend strongly upon W. The IR spectra can be represented as a sum of contributions from interfacial and bulklike water, with a minor contribution from water in dichloromethane. The fractions of water in the two “regions” within the reverse micelle were evaluated as a function of W. Decarboxylation of 6-nitrobenzisoxazole-3-carboxylate ion in the reverse micelles is strongly inhibited by an increase in W, and first-order rate constants level off at W > 5. The rate effects can be ascribed to an increase in polarity and hydration of the reaction region as water is added and are related quantitatively to changes in the IR spectrum.
Introduction In nonpolar organic solvents surfactants often form aggregates, which can solubilize large quantities of water in oil and are described as “water-pool” reverse micelles when the number of water molecules per surfactant (W) is smaller than 10-15. Formation of reverse micelles of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) has received particular attention in recent years. The state of the water within AOT reverse micelles and the influence of W on the micellar properties have been investigated by many techniques.1-5 These studies indicate that the properties of surfactant-trapped water differ from those of bulk water and change strongly with water content if W is smaller than 6-10. At larger W values water behaves like bulk water. The anomalous behavior of water at low W has been attributed to the local interactions of water molecules with Na+ counterions and the sulfosuccinate ion. In previous papers we described IR investigations of water/ AOT/CCl46 and water/AOT/n-heptane7 reverse micelles in the O-H stretching region. The IR spectra are expressed as the sum of contributions from interfacial and bulklike water. The fraction of water in the two “regions” within the water pool was evaluated as a function of W, and the results are explained in terms of a continuous equilibrium between water molecules present in the two * To whom correspondence should be addressed. † Dipartimento di Chimica, Universita ` di Perugia. ‡ Istituto per la Fisica della Materia, Unita ` di Perugia and Dipartimento di Fisica, Universita` di Perugia. § University of California, Santa Barbara. (1) Eicke, H. F. Top. Curr. Chem. 1980, 87, 85. (2) Luisi, P. L.; Magid, L. J. CRC Crit. Rev. Biochem. 1986, 20, 409. (3) Chevalier, Y.; Zemb, T. Rep. Prog. Phys. 1990, 53, 279. (4) Goto, A.; Yoshioka, H.; Manabe, M.; Goto, R. Langmuir 1995, 11, 4873. (5) Varshney, M.; Maitra, A. Colloids Surf., A: Physicochem. Eng. Aspects 1995, 96, 165. (6) Onori, G.; Santucci, A. J. Phys. Chem. 1993, 97, 5430. (7) D’Angelo, M.; Onori, G.; Santucci, A. Nuovo Cimento 1994, 16D, 1601.
regions. Most studies on reverse micelles have focused on the three-component AOT/water/oil system. Little is known about the physicochemical properties of other surfactant systems, in particular as to the state of the solubilized water. The cationic surfactant cetyltrimethylammonium bromide (CTABr) has been known for several years to be capable of forming reverse micelles in various solvents such as n-hexanol, chloroform, and dichloromethane.8-12 There is infrared13 and 1H NMR12,14 spectral evidence for a change of the properties of water in these cationic reverse micelles with an increase in W, and 13C chemical shifts of the surfactant support the proposed change in local structure. However, NMR spectroscopy only provides evidence on average structure when species are equilibrating rapidly on the NMR time scale. Insofar as we planned to relate kinetic to structural evidence, we used CTABr, whose aggregates have a high affinity for the 6-nitrobenzisoxazole-3-carboxylate ion (6NBIC), rather than an anionic surfactant such as AOT. We also investigated the spontaneous decarboxylation of 6NBIC as a kinetic probe (Scheme 1). This reaction of 6NBIC is extremely sensitive to changes in polarity and (8) Ekwall, P.; Mandell, L.; Fontel, K. J. Colloid Interface Sci. 1969, 29, 639. (9) Seno˜, M.; Sawada, K.; Araki, K.; Iwamoto, K.; Kise, H. J. Colloid Interface Sci. 1980, 78, 57. (10) Fletcher, P. D. I.; Galal, M. F.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1985, 81, 2053. (11) Lang, J.; Mascolo, G.; Zana, R.; Luisi, P. L. J. Phys. Chem. 1990, 94, 3069. (12) Germani, R.; Savelli, G.; Cerichelli, G.; Mancini, G.; Luchetti, L.; Ponti, P. P.; Spreti, N.; Bunton, C. A. J. Colloid Interface Sci. 1991, 147, 152. (13) (a) Sunamoto, J.; Iwamoto, K.; Nagamatsu, S.; Kondo, H. Bull. Chem. Soc. Jpn. 1983, 56, 2469. (b) Kondo, H.; Miwa, I.; Sunamoto, J. J. Phys. Chem. 1982, 86, 4826. (14) (a) Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Cipiciani, A.; Cerichelli, G.; Bunton, C. A.; Si, V. J. Colloid Interface Sci. 1990, 138, 443. (b) 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.
S0743-7463(97)00202-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/27/1998
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Scheme 1
hydration of the reaction medium15 and has been used extensively as a kinetic probe for studying the properties of both normal and reverse micelles.14 Decarboxylation of 6NBIC is accelerated by aqueous cationic and zwitterionic micelles and hydrophobic ammonium ions. Decarboxylation is much faster in aprotic organic solvents than in water or polar hydroxylic solvents. But in CHCl3 and CH2Cl2 it is significantly accelerated by hydrophobic ammonium ions and cationic surfactants, and first-order rate constants decrease significantly on addition of water and formation of “water-pool” reverse micelles. This rate decrease correlates with a change in the structure of water, as shown by changes in the 1H NMR chemical shifts, toward that of normal water at interfaces of aqueous cationic micelles. Hydrogen-bond donation to the carboxylate residue of 6NBIC (Scheme 1) strongly inhibits decarboxylation,15 and a decrease in the amount of water, or its ability to donate hydrogen bonds, accelerates reaction. The anionic substrate in the interior of a reverse micelle interacts with the CTA+ headgroup, which assists charge delocalization in the transition state (Scheme 1), and reactions in CH2Cl2 and quaternary ammonium salts are faster than those in water by factors of approximately 105.14 These interactions weaken with increasing water, and in the limit of high W 6NBIC is extensively hydrated. This initial state stabilization strongly inhibits decarboxylation,15 and in this limit reaction in reverse micelles is faster than that at the surface of a normal CTABr aggregate by approximately 1 order of magnitude.14 In the present work we used IR spectroscopy to study the state of water in CTABr reverse micelles in CH2Cl2. Spectra are shown and discussed for the O-H stretching region of H2O (3800-3000 cm-1) at room temperature as a function of W. We have restricted our investigation to the region of small amounts of water (0 < W < 20), where the properties of the systems change strongly with the water content. Experimental Section CTABr (99%, Fluka) was recrystallized from acetone containing a small amount of methanol and dried in an oven at 60 °C. Triethylamine (Et3N) was redistilled and dichloromethane was distilled over P2O5 and stored in the dark over molecular sieves. Decarboxylation of 1 was followed spectrophotometrically at 25.0 °C and 410 nm by using HP-8452 diode-array spectrophotometers. Reaction was started by adding substrate as the acid in CH2Cl2, as described,12,14 with 3 × 10-5 mol dm-3 6NBIC and 3 × 10-3 mol dm-3 Et3N in the reaction solution. Deprotonation had been shown to be rapid under these conditions. These rate measurements extended earlier work,14 and experiments were focused on a narrower range of W than previously. Values of the first-order rate constant, kobs, agreed well with those reported earlier.14 CTABr has a low solubility in pure CH2Cl2, but the surfactant and water mutually enhance each other’s solubility to a considerable extent. For this reason measurements in the CTABr/H2O/CH2Cl2 system were performed at W > 1.5. H2O/ CH2Cl2 solutions of CTABr are still clear with W > 25.12 In the present paper, W represents the ratio of molar concentration of total water to that of surfactant molecules. (15) Kemp, D. S.; Paul, K. G. J. Am. Chem. Soc. 1975, 97, 7305.
Figure 1. Molar extinction coefficient of water dissolved in CH2Cl2. The solvent mixtures were prepared by weight. IR spectra were recorded at room temperature on a Shimadzu Model 470 infrared spectrophotometer equipped with a variable path length cell and CaF2 windows. Typical path lengths employed were 50-800 nm. Spectra of bulk water were taken with shorter path lengths. The molar-extinction coefficient of water is given by ) A/(c‚d), where A is the absorbance, c is the water concentration in moles per cubic decimeter, and d is the cell path length in centimeters.
Results and Discussion IR Spectra in the O-H Stretching Region. Reverse micelles of 0.1 mol dm-3 CTABr in CH2Cl2 and increasing amounts of H2O were examined by IR spectroscopy. The solvent, CH2Cl2, although dried carefully, always contains traces of water. The IR spectra detect this water dissolved in the solvent by the absorptions at 3718 and 3629 cm-1 assigned to the antisymmetric and symmetric O-H stretching vibrations, respectively (Figure 1). The molar extinction coefficient of water dissolved in the solvent was calculated from the absorption spectra of solvent containing different amounts of water (measured by a Karl Fischer titrator). Water solubilized in CH2Cl2 containing CTABr exhibits two sets of overlapping absorption bands in the 39003000 cm-1 region (Figure 2). One of these is that of water dissolved in the bulk phase (Figure 1); another very broad band arises from the absorption due to stretching vibrations of water solubilized in the reversed micelles and appears in the range 3800-3000 cm-1. Although the bands of water dissolved in the bulk phase and entrapped water are overlapped (Figure 2), it is possible to separate them quantitatively in the CTABr/H2O/CH2Cl2 systems (Figure 3). Assuming that the molar extinction coefficient of water dispersed in CH2Cl2 is not affected by the presence of CTABr microemulsions, the concentration of H2O in the organic solvent can be evaluated, and the derived value turns out to be constant with increasing W and approximately equal to the CH2Cl2-saturating concentration. The amount of water entrapped into reverse micelles can therefore be calculated by subtraction from the total water concentration (Figure 3). The total peak area of the O-H stretching band of entrapped water increases linearly with its molar concentration, as predicted by Beer’s law with ) 3.7 × 104 L‚mol-1‚cm-2, in agreement with previously estimated values for the AOT/H2O/CCl46 and AOT/H2O/ n-heptane7 systems. This value of is equal within experimental error to that of bulk water.
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Figure 2. (s) IR spectrum of water in the H2O/CTABr/CH2Cl2 system at W ) 4, [CTABr] ) 0.1 mol dm-3: Contributions due to water dissolved in the solvent (- - -) and water solubilized in the reverse micelle (‚‚‚).
Di Profio et al.
Figure 4. Spectra of the H2O/CTABr/CH2Cl2 system. Molar extinction coefficient of the O-H stretching absorption band for bulk water and water in reverse micelles at selected values of W. The values of W indicated follow the order (top to bottom) of the peaks.
IR Characterization of Interfacial Water. The presence of an isosbestic point for the normalized O-H bands (Figure 4) is in strong support of an equilibrium between two absorbing species having concentrationindependent band shapes and extinction coefficients. A two-state equilibrium has been identified by IR spectroscopy6,7,18,19 and other techniques4,20,21 for water confined in AOT reverse micelles, and a distinction has been made between bound and bulklike water. Thus, we tried to express each spectrum of water confined in CTABr reverse micelles (ν,W) as a linear combination
(ν,W) ) Xbulk(W)‚bulk(ν) + Xbound(W)‚bound(ν) (1)
Figure 3. Concentration of water dissolved in the solvent (0) and water in the reverse micelles (O) as a function of W for the H2O/CTABr/CH2Cl2 system.
Figure 4 shows the molar extinction coefficient of the O-H stretching absorption band for bulk water and surfactant-entrapped water at selected values of W. The IR spectrum of surfactant-entrapped water differs significantly from that of bulk water, indicating that water solubilized in reverse micelles lacks the normal hydrogenbonded structure of bulk water. Figure 4 clearly shows the existence of an isosbestic point at ν ) (3350 ( 5) cm-1, which is indicative of the existence of two species of water with different absorbances. There is a decrease of the intensity of the low-frequency part of the O-H absorption band as the water content is reduced. This behavior is qualitatively similar to that observed on increasing the temperature of bulk water. The low-frequency part of the O-H absorption band is ascribed to water molecules in regular structures with unstrained H bonds.6,7,16,17 Hence, the observed behavior indicates a minimal amount of water involved in a tetrahedral array of hydrogen bonds in the micellar-bound water at low water content and shows that two types of water coexist in reverse micelles. (16) Walrafen, G. E. In Water, a Comprehensive Treatise; Franks, F., Ed.; Plenum Press: New York, 1972; Vol. 1, pp 151-214. (17) D’Arrigo, G.; Maisano, G.; Mallamace, F.; Migliardo, P.; Wanderling, F. J. Chem. Phys. 1981, 75, 4264.
where bulk and bound are the extinction coefficients of “bulk” and “bound” water, respectively. Xbulk and Xbound ) 1 Xbulk represent the mole fraction of “bulk” and “bound” water, respectively, in the reverse micelles. Usually, analysis in terms of a two-state equilibrium is performed by assuming that the signal obtained at W f 0 is that of “bound” water. This assumption seems not to be justified because at very low values of W, that is, as the “bound” region is forming, changes in the spectral properties of “bound” water are to be expected.7 In any event, the IR spectra for the CTABr/H2O/CH2Cl2 system have been recorded at W > 1.5 (see Experimental Section), thus avoiding the problems arising with previous procedures. Here values of Xbound and bound were determined by a procedure previously employed to describe the development of the O-H band of water in the AOT/H2O/CCl4,6 AOT/H2O/n-heptane,7 and perfluoropolyether/H2O18 systems. In brief: (i) The bulk water spectrum (assumed to be governed by bulk) was subtracted with an increasing contribution from a given experimental spectrum (ν,W). The subtraction was stopped at the value of X‚bulk giving a negative absorption in some spectral region, and the corresponding (18) D’Angelo, M.; Martini, G.; Onori, G.; Ristori, S.; Santucci, A. J. Phys. Chem. 1995, 99, 1120. (19) Giammona, G.; Goffredi, F.; Turco Liveri, V.; Vassallo, G. J. Colloid Interface Sci. 1992, 154, 411. (20) Hauser, H.; Haering, G.; Pande, A.; Luisi, P. L. J. Phys. Chem. 1989, 93, 7869. (21) Maitra, A. J. Phys. Chem. 1984, 58, 5122.
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Figure 5. Molar extinction coefficient for surfactant-bound water in H2O/CTABr/CH2Cl2.
X value was assumed to give Xbulk. This procedure gives Xbulk and Xbound values which are uncertain within 0.01 units. (ii) bound was calculated as
bound )
(ν,W) - Xbulk‚bulk 1 - Xbulk
Figure 6. Number of bound water molecules per surfactant molecule as a function of W.
(2)
The bound values obtained in this way (Figure 5) did not depend on the selected spectrum, that is, they are practically independent of W. As compared with the broad band associated with the O-H stretching vibrations of “bulk” water, the band associated with bound is more symmetric, narrower, and shifted toward higher frequency [νmax ) (3430 ( 5) cm-1]. Generally, O-H stretching frequencies decrease with increasing strength of Hbonding, so this result indicates a weakening of the hydrogen bonds of water molecules in the “bound” region. NMR data show for both AOT21,22 and CTABr12 in CD2Cl2 reverse micelles an upfield 1H chemical shift with decreasing water content, and formation of hydrogen bonds usually gives a downfield 1H shift. Hence, NMR and IR results are consistent with evidence of a minimum amount of water involved in a tetrahedral array of hydrogen bonds in the micellar water phase at low water content. The number of bound water molecules per CTABr was calculated by using the relation Wbound ) Xbound × Wm (where Wm denotes the overall fraction of micellar associated, i.e., bound plus bulklike, water); the values so obtained are plotted in Figure 6 as a function of W. When W is increased, Wbound increases steeply at low W, attaining a value of Wbound = 1 at W ∼ 5 and then increases slowly at higher values of W. The observed behavior significantly differs from that for AOT reverse micelles,6,7 where IR measurements indicate that Wbound increases steeply at low W values and approaches a near-constant value close to 3 for W > 6. Kinetics. Decarboxylation of 6-nitrobenzisoxazole-3carboxylate ion in CTABr plus water in CH2Cl2 or CHCl3 is much faster than that in water or normal CTABr micelles. Addition of water slows the reaction (Figure 7). There is a marked decrease in kobs up to W ≈ 5, and then kobs becomes approximately constant at higher W, indicating no further change in the properties of the water pool as a kinetic medium. There is a strong correlation between the spectral and kinetic data (Figures 6 and 7) with a close connection between the relation of kobs to W (Figure 7) and the properties of water in the reverse micelle (Figure 6). We propose that the carboxylate ion (1) in the interior of the water pool competes with water for the surfactant (22) Wong, M.; Thomas, J. K.; Nowak, T. J. Am. Chem. Soc. 1977, 99, 4730.
Figure 7. Rate constant (kobs) of decarboxylation of 1 in the H2O/CTABr/CH2Cl2 system as a function of W. [CTABr] ) 0.1 mol dm-3.
headgroups. On increasing the amount of solubilized water, interactions of 1 with CTA+ are replaced by interactions of water with CTA+; kobs decreases until hydration around the cationic headgroups is almost complete. Added water then solvates the substrate and inhibits reaction by stabilization of the initial state. In Figure 8 kobs is plotted as a function of Wbound. There is a linear decrease of kobs on increasing Wbound up to ∼1. At Wbound > 1, kobs becomes almost constant. This result indicates that the decrease of kobs is ascribable essentially to the association between CTABr and the first water molecule; the bonding of a second water molecule, characterized by a smaller association constant (see Figure 6), does not affect kobs, because water molecules become freer to hydrate 6NBIC. We propose the following model: The carboxylate ion (6NBIC) is solubilized in the reverse micelles, and its reactivity depends upon the properties of water and the interfacial region. Hydrogen bonding and an increase in the local dielectric constant of the reaction region inhibit reaction,15 which is fastest in the hypothetical situation in which the substrate is in contact with CTA+ in the absence of water.12,14a This conclusion is consistent with evidence that reaction is faster with nonamphiphilic quaternary ammonium ion, which should not form water-pool micelles in CH2Cl2, than with CTABr at the same, low, water concentrations.12
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in this reaction kobs continues to decrease with increasing W and eventually becomes higher than that in normal micelles of CTABr in water by approximately 1 order of magnitude.14a Although these reactions are similar in that hydrogen-bonding stabilization of the anionic initial state strongly inhibits reaction, there is evidence for nucleophilic attack by water in dephosphorylation,24 whereas this intervention is unimportant in the spontaneous decarboxylation of monoanionic 6NBIC.15
Figure 8. Rate constant (kobs) of decarboxylation of 1 in the H2O/CTABr/CH2Cl2 system as a function of Wbound. [CTABr] ) 0.1 mol dm-3.
This “two-environment” kinetic model of reaction for the region Wbound < 1 is described by
kobs ) k0(1 - f) - kW f ) k0 - f(k0 - kW)
(3)
where k0 and kW are first-order rate constants in the interfacial region for water-free and water-associated CTABr, respectively, and f is the fraction of waterassociated CTABr. In our case Wbound ∼ f when Wbound e 1. The values of kobs plotted in Figure 8 show a linear trend of kobs with Wbound for Wbound e 1. A fit of kobs ) a - bWbound in the range 0 < Wbound < 0.8 gives a ) b ) 0.05, according to all the evidence that k0 . kW. We note that a similar dependence of kobs upon W was observed in the dephosphorylation of 2,4-dinitrophenylphosphate dianion in moist CH2Cl2/CTABr.23 However, (23) 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.
Conclusions The properties of water solubilized by CTABr in CH2Cl2 have been studied by kinetics and infrared spectroscopy. Results from both approaches emphasize the existence of two kinds of aqueous domains in reverse micelles which are organized by surfactant: a bound water domain with water molecules hydrating the interfacial headgroups of the surfactant and a bulklike water domain. The results show that IR spectra can be expressed as a sum of contributions from interfacial and bulklike water. The fractions of water in the two “regions” within the water pool were evaluated as a function of W. The data indicate that there is a continuous variation in the water properties inside the micellar core rather than a two-step hydration mechanism. The same picture results from kinetics of decarboxylation and dephosphorylation in that the inhibition of both reactions can be ascribed to an increase in polarity and hydration of the reaction (i.e., headgroup) region as water is added; rate constants then level off at W > 5, indicating that further added water does not affect the interfacial region. Acknowledgment. This work was supported in part by contributions from the Consiglio Nazionale delle Ricerche (CNR, Rome) and the Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica (MURST, Rome). LA970202H (24) Jencks, W. P. In Nucleophilicity; Harris, J. M., McManus, S. P., Eds.; Advances in Chemistry Series; American Chemical Society: Washington, DC, 1987; p 155.
