12592
J. Phys. Chem. 1996, 100, 12592-12599
Enolization of Benzoylacetone in Aqueous Surfactant Solutions: A Novel Method for Determining Enolization Constants Emilia Iglesias Departamento de Quı´mica Fundamental e Industrial, Facultad de Ciencias, UniVersidad de La Corun˜ a, 15071-La Corun˜ a, Spain ReceiVed: February 15, 1996; In Final Form: May 9, 1996X
The keto-enol tautomerism of benzoylacetone (1-phenyl-1,3-butadione) in aqueous acidic solution at 25 °C was investigated by studying the influence of anionic, cationic, and nonionic surfactants in the UV-vis spectra of benzoylacetone. The percentage of the enol tautomer was found to increase abruptly at surfactant concentration above the critical micelle concentration (cmc). A model is proposed to quantitatively interpret the experimental results, which allows for the determination of the enolization constant in water, KE, and in micellar, KEm, pseudophases; the binding constant of the enol tautomer to micelles, Ks; and the extinction coefficients of the ketonic and enolic forms, , of benzoylacetone. The formation of the enolate in the absence and presence of micelles in basic media was also studied. The acidity constants of the enolic, KEa , and ketonic, KKa , forms of benzoylacetone have been determined.
Introduction
SCHEME 1
The determination of keto-enol equilibrium constants has been at the center of physical organic studies of keto-enol tautomerism for many years.1,2 Many of the quantitative studies have dealt with β-dicarbonyl compounds, which are commonly enolized to such an extent that both keto and enol forms are present in measurable concentrations at equilibrium. Keto-enol tautomerism has been studied through either enol titrations by halogens3,4 or through a large variety of spectroscopic methods, including infrared spectroscopy,5-8 UV absorption,5-7,9,10 NMR,7,8,9-13 or HPLC (high performance liquid chromatography).14 Nevertheless, reliable data have been obtained only recently. NMR spectroscopy requires the use of very concentrated solutions, and it has often been shown that enol contents are extremely concentration-dependent, due to association;9 also, in using UV-absorption spectroscopy the extinction coefficients for pure enols must be estimated; furthermore, the titration method has the disadvantage that the end point is sometimes not sharp. Meanwhile, the majority of studies has been carried out with pure solvents, as is the case for most of the NMR studies, or with water-solvent mixtures or even with a pure dicarbonyl compound.11 The determination of enolization equilibrium constants of β-dicarbonyl compounds in water is scarce; nevertheless, these constants are often required for explaining mechanistic studies.15 1,3-Dicarbonyl compounds, as BZA, may exist in solution in three tautomeric forms (see Scheme 1). Open chain 1,3dicarbonyl compounds are observed in the trans-enolic form only in rare cases.14 In solution, they enolize practically exclusively to the cis-enolic form, in which case the enol exists in a cyclic form stabilized by intramolecular hydrogen bonding. In a symmetrically substituted β-dicarbonyl compound, e.g., acetylacetone, there is only one indistinguishable enolic form: the two valence bond structures that may be written for the cisenolic form contribute equally to the total structure of the enol. On the other hand, when the diketone is unsymmetrically substituted, e.g., benzoylacetone, two structures with unsymmetrical hydrogen bonds may exist, in which the fractions of X
Abstract published in AdVance ACS Abstracts, July 1, 1996.
S0022-3654(96)00473-X CCC: $12.00
the two types of tautomers present at equilibrium depend on the nature of the substituent, and, possibly, on the medium in which the compound is located. The interconversion between both tautomers involves merely an intramolecular proton transfer with a concomitant change in the electron distribution of the molecule. The rate of this transfer is fast, even on the time scale of nuclear magnetic resonance frequency, and therefore the determination of this equilibrium constant is impossible to arrive at by classical chemical methods because the two enolic forms are in rapid dynamic equilibrium. In the case of BZA, Gorodestsky et al.16 determined that the oxygen adjacent to the phenyl ring is predominantly in the hydroxylic form, the equilibrium constant being K ) 1.29. The same conclusion has been obtained by other authors.17 When trans-enolic form is excluded, the keto-enol equilibrium constant is given by KE ) [EH]/[KH] (where EH means the enolic form and KH the ketonic form). As the diketo form is usually more dipolar than the chelated cis-enolic form, the keto/enol ratio often depends on solvent polarity; the internal hydrogen bonds between the hydroxy group and the carbonyl group help reduce the dipole-dipole repulsion of the carbonyl groups, which remains unreduced in the diketo form. Furthermore, enol stabilization due to intramolecular hydrogen bonding will be more pronounced when intermolecular hydrogen bonding with the solvent does not compete. Hence, keto-enol equilibria of β-dicarbonyl compounds are extremely solvent-sensitive, and the proportion of enol form is found to be much greater in nonpolar solvents, such as cyclohexane, than in polar solvents, such as alcohol or water. The present work reports a new method for determining enolization constants of β-dicarbonyl compounds. It is well © 1996 American Chemical Society
Enolization of Benzoylacetone in Aqueous Surfactant known that the presence of micelles can notably modify the equilibrium of a given reaction.18-20 In this investigation we have carried out a thorough study of the enolization of benzoylacetone (1-phenyl-1,3-butanedione, BZA) in aqueous micellar medium. The results enabled us to determine the enolization constant in water, and this method has been applied to the study of the keto-enol tautomerism of other β-dicarbonyl compounds.21 The advantages of this method can be summarized as follows: (i) the presence of micelles shifts the keto-enol equilibrium toward the enol form, which is trapped by the micelles, but the presence of micelles does not alter the equilibrium in the bulk water phase; (ii) small benzoylacetone concentrations (10-4-10-5 mol dm-3) were used, thus preventing the possibility of association; and (iii) it is not necessary to assume the extinction coefficient of the enol form. The only a priori assumption required is that the extinction coefficient of the enol in the micellar interface be the same as that in water, a very recurrent situation, as has been seen with weak acids18 and amines20 and as will be confirmed in the results here. Contrarily, with the exception of acetylacetone, there have been relatively few studies on the enolization of β-diketones in aqueous solution. Experimental Section Materials. Benzoylacetone (1-phenyl-1,3-butanodione, BZA), an Aldrich product of the maximum purity, was used as supplied. Surfactants, sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTABr), hexadecyltrimethylammonium bromide (CTABr), polyoxyethylene 20-cetyl ether (C16E20), and polyoxyethylene 9-dodecyl ether (C12E9), all from Sigma or Aldrich and of the highest available purity, were used without further purification. Tetradecyltrimethylammonium chloride (TTACl) was prepared through the ion exchange of a solution of TTABr, using an Amberlite IRA-400(Cl) ion exchange resin, followed by elution with distilled water. All other reagents were supplied by Merck and were used as received. All the solutions were prepared with doubly-distilled water obtained from a permanganate solution. Methods. BZA was dissolved in dioxane (spectrophotometric grade). From this stock solution, the working solution was daily prepared by diluting 0.40 mL in 25 mL final volume to give an aqueous solution of BZA of 5.3 × 10-4 mol dm-3. From this latter solution 0.40 mL were placed in a 1.0 cm quartz cell to a final volume of 3.0 mL in order to carry out the UVvis absorption measurements. The percentage of dioxane in the final sample mixture was less than 0.2 vol %. Ultraviolet absorption spectra were recorded with a KontronUvikon (Model 941) double-beam spectrophotometer with a thermostated cell holder. The scans were monitored by working with four cells (two samples + two references). Each scan was taken after 15 min thermostating. The reference cell contained all the reagents except for the BZA. All measurements were performed at 25 °C. Results and Discussion A micelle can be regarded as a microreactor that influences reaction rates and equilibria by taking up reactants and providing a medium different from that of the bulk solvent. The observations of the micellar effects on chemical reactivity and equilibria lead to the generalization that aqueous micelles could be regarded as submicroscopic reaction or solubilization media.23,24 Most works carried out on the influence of micelles on equilibrium reactions deal with the dissociation constants
J. Phys. Chem., Vol. 100, No. 30, 1996 12593
Figure 1. Absorption spectra of benzoylacetone dissolved in cyclohexane (1) and in water (2) at benzoylacetone concentration of (s) 8.2 × 10-5 and (- - -) 4.1 10-5 mol dm-3.
of acids and bases. Nevertheless, to our knowledge, no study has been performed on the effects of micelles on the ketoenol equilibria, a nonionic equilibrium. 1. Enol Formation. As we have pointed out, keto-enol equilibria of β-dicarbonyl compounds are extremely solvent sensitive. In principle, on the dissolution of β-dicarbonyl compounds in solvents of low polarity the percentage of the cis-enolic form increases, whereas polar solvents displace the equilibrium toward the diketo form. When BZA is dissolved in water, the spectrum exhibits strong absorption at 250 nm (due to π f π* transition) and weak absorption at 312 nm (due to n f π* transition). On the contrary, for cyclohexane solvent, strong absorption appears at 312 nm (λmax ) 306 nm), and the weak band appears at 250 nm (λmax ) 245 nm); see Figure 1. We establish that the absorption band near 250 nm is due mainly to the ketonic form (although the enol contributes substantially to the absorption at this point) and that at a higher wavelength, near 312 nm, the band is due to the enolic form, in accordance with the greater conjugation in the BZA molecule and the observed solvent effects, since it is attributed to the n f π* and π f π* transitions.6,25,26 At BZA concentrations in the range (2-25) × 10-5 mol dm-3, the absorbance of the aqueous and cyclohexane solutions was measured at 250 and 312 nm using a bandwidth of 2 nm. The absorbance measurements in water were also determined in the presence of 0.033 mol dm-3 hydrochloric acid. In cyclohexane solvent all BZA are present in the enolic form. Therefore, the absorption bands near 312 nm and 250 nm must result from the enol. On the contrary, a high percentage of the ketonic form is in equilibrium with the enolic form in water. Assuming that the absorbance at 312 nm is due only to the enol form,27 and that [BZA]t ) [KH] + [EH], eqs 1 and 2 may be obtained to express
EHlKE [BZA]t 1 + KE
(1)
(KH + ′EHKE) l[BZA]t 1 + KE
(2)
Ab312 ) Ab250 )
the variation of the absorbance of the solution as a function of
12594 J. Phys. Chem., Vol. 100, No. 30, 1996
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Figure 2. Relationship between the absorbance of the (- - -) cyclohexane and (s) water solutions at (2) λ ) 312 nm and (O) λ ) 250 nm and the benzoylacetone concentration. (4) Ab250 - Ab312 in water.
TABLE 1: Slopes of the Plots of Absorbance Readings at the Wavelength Indicated in Parentheses against the BZA Concentration (See Eqs 1 and 2) medium
slope1a (λ)
slope2a (λ)
(slope250 slope312)a
cyclohexane cyclohexane H2O H2O/HClc
15500 ( 40 (312)b 16150 ( 80 (306)b 5400 ( 20 (312)b 5330 ( 20 (312)b
7270 ( 40 (250) 7491 ( 29 (245) 10560 ( 40 (250) 10510 ( 40 (250)
5160 ( 40 5160 ( 40
a
In mol-1 dm3. b λ in nm. c At [HCl] ) 0.033 mol dm-3.
[BZA] at λ ) 312 and 250 nm, respectively. In both expressions l ()1.0 cm) is the path length of the light and EH, ′EH, and KH are the extinction coefficients of the enol at 312 and 250 nm and that of the keto form at 250 nm, respectively. In each case, a perfectly linear correlation between Ab and [BZA] was observed (Figure 2), thereby indicating negligible association of the BZA molecules in both the enol and keto forms. Least-squares fitting of the data to eqs 1 and 2 yielded the results listed in Table 1 for the slopes of the corresponding lines. In the case of cyclohexane as solvent, these slopes are equal to the extinction coefficient of the enol at the wavelength where the absorbance readings were taken, i.e. EH at 312 nm and ′EH at 250 nm; for the measurements carried out in water, one applies eqs 1 and 2. The effect of surfactants on the keto-enol equilibrium of BZA was investigated by measuring absorbance at λ ) 250 and 312 nm. The changes in the absorption spectrum with varying concentration of surfactants in aqueous solutions containing 7.0 × 10-5 mol dm-3 of BZA are shown in Figure 3 for the typical cases of CTABr and C12E9. The existence of the keto-enol equilibrium is corroborated by the presence of the two isosbestic points at λ = 231 and 269 nm: the absorption maximum at 312 nm, which is due to the enol form, increases with the surfactant concentration above the cmc (critical micelle concentration); meanwhile, the absorption band around 250 nm, due to the ketonic form, decreases as the [surfactant] increases. This means that as the enol form is taken up by the micelle (a less polar medium), the keto-enol equilibrium in water displaces toward the enol formation. It is then possible to propose Scheme 2. In this scheme subscripts w and m refer to water and micellar pseudophases respectively; [Dn] represents
Figure 3. Absorption spectrum of BZA (7.0 × 10-5 mol dm-3) at [HCl] ) 0.033 mol dm-3 as a function of (a) [C12E9] of (1) 0.0; (2) 1.78; (3) 3.57; (4) 7.13; (5) 10.7; (6) 14.3; (7) 21.4; (8) 42.8; (9) 85.6 mmol dm-3; and (b) [CTABr] of (1) 0.0; (2) 0.83; (3) 1.67; (4) 3.33; (5) 5.0; (6) 10; (7) 16.7; (8) 33.3, and (9) 100 mmol dm-3.
SCHEME 2
the micellized surfactant concentration, i.e. [Dn] ) [surfactant] - cmc; KE is the enolization constant; Ks is the binding constant of the enolic form of BZA to the micelles; and Km E represents the enolization constant in the micellar pseudo-phase, i.e. Km E ) KE Ks.
Enolization of Benzoylacetone in Aqueous Surfactant
J. Phys. Chem., Vol. 100, No. 30, 1996 12595 at 250 nm through eqs 3 and 4, respectively.
Ab312 ) lEH([EH]w + [EH]m)
(3)
Ab250 ) lKH[KH]w + l ′EH([EH]w + [EH]m)
(4)
From the expressions of KE and Ks given in Scheme 2, and taking into account that [BZA]t ) [KH]w + [EH]w + [EH]m, one may obtain the following equations to represent the concentration of each form of BZA:
[KH]w )
[EH]w )
[EH]m )
{1/(1 + KE)}[BZA]t KEKs 1+ [Dn] 1 + KE
( )
{KE/(1 + KE)}[BZA]t KEKs 1+ [Dn] 1 + KE
( )
{KE/(1 + KE)}[BZA]tKs[Dn] KEKs 1+ [Dn] 1 + KE
( )
(5)
(6)
(7)
The combination of eqs 3 and 4 with eqs 5-7 produces the variation absorption at the two maximums as expressed in eqs 8 and 9, respectively;
Ab0312(1 + Ks[Dn]) Ab312 ) KEKs 1+ [Dn] 1 + KE
( )
Ab250 )
Figure 4. Plot of the variation of the absorbance at (b) λ ) 312 nm and (2) λ ) 250 nm of aqueous solutions of BZA as a function of [SDS] at [BZA] ) 8.0 × 10-5 mol dm-3 and [HCl] ) 0.033 mol dm-3; and of [TTACl] at [BZA] ) 7.0 × 10-5 mol dm-3 and [HCl] ) 0.033 mol dm-3. The insert shows the cmc determination. Solid lines fit eqs (b) 8 and (2) 9; for parameters see Table 2.
Typical results of the variation in the absorbance of aqueous BZA solutions with the surfactant concentration are shown in Figure 4. Considering that the entire amount of BZA taken up by the micelles is in the enolic form, and assuming that the extinction coefficient of the enolic form in the micellar pseudophase is the same as that in water, a situation frequently observed,18,20 one may describe the increase in absorbance of the aqueous micellar BZA solution at 312 nm and the decrease in absorbance
Ab0250 + {Ab∞250KEKs/(1 + KE)}[Dn] KEKs 1+ [Dn] 1 + KE
( )
(8)
(9)
with Ab0 being the corresponding values of absorbance in the absence of surfactant (see eqs 1 and 2), and Ab∞250 ()′EHl[BZA]t) the absorbance at high surfactant concentration, when all the substrate is in the enolic form. Solid lines on Figure 4 correspond to the theoretical fits of eqs 8 (the ascending curve) and 9 (the descending curve) to the experimental points of Ab Vs [Dn] at λ ) 312 and 250 nm, respectively. The values of Ks, KEKs/(1 + KE), and Ab∞250 were calculated by fitting the data for the variation of Ab with [Dn] to the above equations, using the three values as adjustable parameters and Ab0 as input. The correspondence between the data and the Ab-[Dn] functions generated by the different surfactants was excellent in each case. The results obtained for the optimized parameters are given in Table 2. The experimental conditions and the calculated values of KE, and the extinction coefficients EH, ′EH, and KH determined from the values of KE and Ks together with the expressions of Ab∞312, Ab∞250, and Ab0250, respectively, are also reported in Table 2. The values of the cmc used to calculate [Dn] were determined experimentally as the minimum surfactant concentration necessary to observe a variation in the absorption spectrum28 (see insert of Figure 4). Several conclusions may be drawn from the results in Table 2. One can see from the data on Figures 1 and 3 that on increasing the solvent polarity and hydrogen-bonding solutesolvent interactions (on going from cyclohexane to water), a bathochromic shift, of both π f π* and n f π* transitions, occurs. Nevertheless, this effect is not observable on going from
12596 J. Phys. Chem., Vol. 100, No. 30, 1996
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TABLE 2: Values of the Initial Absorbance (Ab0) and Cmc Used To Fit the Experimental Data Ab-[Dn] to Eqs 8 (λ ) 312 nm) and 9 (λ ) 250 nm) and the Values of the Optimized Paremeters for the Binding Constant of the Enol to Micelles (Ks), the Enolization Constant (KE), and the Molar Absorption Coefficients (E) Obtained by Studying the Influence of [surfactant] on the Absorption Spectra of BZA in the Presence of 0.033 mol dm-3 of Hydrochloric Acid Data of Ab312-[Dn] Fitted to Eq 8 surfactant
Ab0
312
103
Cmca
Ksb
KEKsb/(1 + KE)
KEmb
KE
EHc
224 ( 3 330 ( 13 320 ( 10 459 ( 7 259 ( 4 455 ( 6 208 ( 4
89 ( 2 127 ( 6 110 ( 4 172 ( 3 94 ( 2 164 ( 3 77 ( 2
148 185 178 275 148 255 123
0.66 0.63 0.61 0.60 0.57 0.56 0.59
13 630 13 920 13 820 14 320 14 700 14 880 15 140
SDSd TTABrd TTACle CTABrd C12E9e C16E20d SDS-C12E9e
0.4310 0.4294 0.3645 0.4294 0.3735 0.4290 0.3770
surfactant
Ab0250
103cmca
KEKsb/1 + KE
Ab∞250
KE
′EHc
KHc
0.8653 0.8804 0.7605 0.9081 0.7630 0.8803 0.7605
2.2 0.8 1.1 0.1 0.08
87 ( 5 108 ( 5 113 ( 8 159 ( 6 98 ( 2 161 ( 6 81 ( 3
0.533 ( 0.005 0.484 ( 0.004 0.443 ( 0.006 0.509 ( 0.004 0.407 ( 0.002 0.477 ( 0.005 0.407 ( 0.002
0.68 0.59 0.63 0.53 0.57 0.55 0.60
6688 6050 6293 6363 5814 5961 5814
13 580 13 850 13 750 14 290 13 790 13 560 14 710
2.2 0.8 1.1 0.1 0.08 0.1
Data of Ab250-[Dn] Fitted to Eq 9 d
SDS TTABrd TTACle CTABrd C12E9e C16E20d SDS-C12E9e
0.l1
In mol dm-3. b In mol-1 dm3. c In mol-1 dm3 cm-1. d At [BZA] ) 8.0 × 10-5 mol dm-3. e At [BZA] ) 7.0 × 10-5 mol dm-3, and at [C12E9]/ [SDS] ) 1. a
water to the micellar pseudophase. This fact indubitably indicates that BZA-enol is not dissolved in the micellar core but in the micellar interface, where the hydration or the presence of water molecules is very important.22-24,29 This is a general observation whatever the nature of the surfactant head groups (anionic, cationic, or nonionic), indicating that the higher solubilization of the BZA-enol in the micelle is due to hydrophobic effect. As we have pointed out, the formation of intramolecular hydrogen bonds in a cyclic structure makes the enolic form of BZA less polar and then more hydrophobic. This constitutes an example in which a change in the chemical structure of the solute strongly affects the interfacial partition coefficient by altering interface-solute and water-solute intermolecular forces (the enthalpic part of the free energy of transfer). In line with the model of solvation proposed by of Abraham et al.30,31 (see also the work of Quina et al.32), the main parameters which would control the value of the association constant of BZA-enol to micelles are the phase hydrophobicity and the solute hydrogen-bond basicity and acidity. The phase hydrophobicity increases with the hydrocarbon chain length of the surfactant. In this sense, Figure 5 shows the good correlation between ln(Ks) and N0, the number of C atoms in the hydrocarbon chain of the surfactant. Additionally, for the SDS the hydrogen bond basicity of the micellar solubilization environment is comparable to that of bulk water, and hydrogen bond acidity of the BZA-enol makes little or no contribution to micellar solubilization. For cationic and nonionic surfactants, solute hydrogen bond acidity favors incorporation into the micelles. That explain the higher association constant of BZAenol to C12E9 micelles as compared with SDS micelles or SDSC12E9 mixed micelles (equal molar ratio, i.e., [C12E9]/[SDS] ) 1). The practical constancy of the values found for the enolization constant, KE, with the varying surfactant nature, means that the enolization of BZA in water is not altered by the presence of micelles, which lends credibility to the method described here. The mean value obtained for KE ) 0.60 implies a 37.5% enolization of BZA in water. Morton et al.25 found that 36.7% of the BZA is enolized in water. On the other hand, the high magnitude found for the enolization constant in the micellar medium, Km E (it varies between 123, with the less hydrophobic
Figure 5. Correlation between association constants of BZA-enol to the indicated surfactant micelles and the number of C atoms of the alkyl chain of the surfactant.
micelles, to 275 with the more hydrophobic micelles), indicates that BZA is practically 100% enolized at high micelle concentration. Finally, the magnitude of EH reported in Table 2 (entry 8) refers to the extinction coefficient of the BZA-enol in the micelle; thus, it has been determined from the value of Ab∞312 ()EHl[BZA]t ) Ab0312 Ks/{KEKs/(1 + KE)}). The mean value calculated for EH ) 13 900 mol-1 dm3 cm-1 agrees perfectly with that of 13 940 mol-1 dm3 cm-1 obtained in water from the slope of the line of Ab312 Vs [BZA] (see eq 1 and Table 1) and the value of KE. This confirms the opening hypothesis that the extinction coefficient of the enol change in the micellar medium would not vary from that in water. The same concordance can be observed with KH. On the other hand, the values of EH ) 15 500 and of ′EH ) 7270 mol-1 dm3 cm-1 determined at 312 and 250 nm, respectively, from the LambertBeer law validity in cyclohexane are slightly greater than those determined in water, nevertheless, the substantial change in the solvent may cause such minor effects.
Enolization of Benzoylacetone in Aqueous Surfactant
J. Phys. Chem., Vol. 100, No. 30, 1996 12597
Figure 7. Spectrophotometric titration curve for the acid ionization of BZA (6.2 × 10-5 mol dm-3) in aqueous solution of the buffers HPO42-/H2PO4-, morpholine-morpholinium chloride, HCO3-/CO32and NaOH at 0.035 mol dm-3 and 25 °C. The insert shows the fit of the data Ab320-[H+] to eq 13.
SCHEME 3
Figure 6. Absorption spectra of BZA (7.0 × 10-5 mol dm-3) in (a) water as a function of the pH: (1) no buffer, (2) pH ) 7.67, (3) 8.39, (4) 8.56, (5) 8.79, (6) 9.03, (7) 9.27, (8) 9.82, (9) 11.98; and (b) at (1) [TTABr] ) 0.0132 mol dm-3, no buffer; (2) [TTACl] ) 0.011 mol dm-3, [NaCl] ) 0.133 mol dm-3, [buffer] ) 0.015 mol dm-3 of pH 7.45 (morpholine-morpholinium chloride); (3) [TTACl] ) 0.0088 mol dm-3, no buffer; (4) [TTACl] ) 0.0132 mol dm-3, [buffer] ) 0.015 mol dm-3 of pH 7.45 (morpholine).
2. Enolate Formation. The acid dissociation constant of the enolic form of BZA was also determined in water by measuring the absorbance of an aqueous BZA solution of 6.2 × 10-5 mol dm-3 as a function of pH. The different pH values were obtained from NaH2PO4-Na2HPO4, morpholine-morpholinium chloride, and carbonate-bicarbonate buffer solutions, conveniently prepared to obtain the desired pH. In every case a [buffer] ) 0.035 mol dm-3 was used. Figure 6 displays the corresponding spectra. The band absorption near 312 nm increases with the pH, as expected, since the enolate ion should
absorb light much more strongly at this wavelength than does the enol. As we can see, the maximum at 312 nm, in consequence of the enolic form, shifts to 320 nm due to the enolate ion; in addition, the peak centered at 250 nm in the case of the enol also shifts to 245 nm. One can also observe the existence of two isosbestic points, not as well-defined as in the case of the enolization process, which appear near 240 and 281 nm, respectively. The dependence of the absorbance measurements on pH at 320 nm, Ab320, describes a sigmoid titration curve with an inflection point at pH ) 8.70 (see Figure 7). As determined in the previous section, 37.5% of BZA are present in the enolic form. Therefore, titration of an equilibrium mixture of KH and EH yields an apparent acid ionization constant whose value corresponds to the pH of the inflection point. Scheme 3 can be used to describe the system. This scheme leads to Ka(ap) ) [E-]t[H+]/([EH] + [KH]), then pKa(ap) ) pKEa + log(1 + 1/KE) and pKa(ap) ) pKKa + log(1 + KE). Thus, from the value of pKa(ap) ) 8.70 and the value of KE ) 0.60 one determines pKEa ) 8.27 and pKKa ) 8.50. On the other hand, the pKEa can be directly determined by fitting the experimental data Ab320 - [H+] to eq 13, which is obtained by taking into account that [BZA]t ) [KH] + [EH] + [E-] and that Ab320 ) lEH[EH] + lE[E-], where EH and E are the absorption coefficients of the enol and enolate ion at 320 nm, respectively (see insert of Figure 7).
Ab320 )
(EH[H+] + EKEa )[BZA]t (1 + 1/KE)[H+] + KEa
)
Ab∞EH [H+] + Ab∞EKEa (1 + 1/KE) [H+] + KEa (13)
12598 J. Phys. Chem., Vol. 100, No. 30, 1996
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To fit the experimental data to this eq the values of Ab∞E ) 1.289 (the absorbance of the solution at high pH when all [BZA] is in the form of enolate ion) and of KE ) 0.60 were introduced as known parameters, which anables one to obtain the values of KEa ) (5.0 ( 0.2) × 10-9 mol dm-3 and of Ab∞EH ) 0.908 ( 0.023 as the optimized parameters. These results give E ) 20 800 mol-1 dm3 cm-1 and KKa ) KEKEa ) 3.0 × 10-9 mol dm-3. All results are in perfect agreement with those determined above form the pKa(ap) and also with the values recently reported by Bunting et al.33 In the absence of HCl and in the presence of TTACl, the formation of the enolate ion was also observed. The [E-] increases with the [TTACl] above the cmc. This effect is not observed in the presence of TTABr micelles. Thus, the phenomenon could be interpreted as an ion-exchange process between the enolate ion E- and Cl- counterion of TTACl micelles, which is favorable to enolate formation in the case of Cl- counterions, but is unfavorable in the case of Br- counterions; this can be easily understood if one keeps in mind that Br ) 0.2034 and the maximum [E-] is 20-fold lower than KCl [Cl-] in the most favorable case of less [surfactant].35 In the presence of a phosphate buffer of pH ) 7.30, large amounts of TTACl are necessary to observe enolate formation. With the purpose of evaluating the ion-exchange constant KI between Cl- and E- ions, corresponding to the equilibrium Clm+ Ew- a Clw- + Em-, we measured absorbance at 360 and 362 nm of a solution of 7.0 × 10-5 mol dm-3 of BZA and 0.015 mol dm-3 buffer morpholine-morpholinium chloride with pH 7.45. At these wavelengths the absorption of the enol form of BZA is negligible; then Ab ) El[E-]t. Meanwhile, the apparent acid ionization constant of BZA in the presence of TTACl micelles, Km a (ap), can be defined by eq 14, where [H+]w represents the intermicellar concentration of H+ ions, which is kept constant by the use of the buffer morpholinemorpholinium chloride, whose acid form is repelled from the micellar surface, with the basic form not associating to the micelles.20b + Km a (ap) ) [H ]w
[E-]w + [E-]m [EH]w + [EH]m + [KH]w
(14)
Use of the ion-exchange constant, together with the binding constant of the BZA-enol to TTACl micelles (Ks) and the definition of Km a (ap) given in eq 14, provides for eq 15, which relates the experimental absorbance at 360 or 362 nm to the surfactant concentration.
Ab ) {Ab∞(1 + KI[Cl-]m/[Cl-]w)}/{(1 + KI[Cl-]m/ [Cl-]w) + {1 + KEKs[Dn]/(1 + KE)}[H+]w(1 + KE)/KEa KE} (15) The analytical concentrations of Cl-(m) and Cl-(w) can be described in terms of the degree of micellar charge neutralized, β ()0.84), as [Cl-]m ) β[Dn] - [E-]m and [Cl-]w ) (1 β)[Dn] + cmc + [Cl-]ad + [E-]m, [Cl-]ad being the chloride ion concentration added with the buffer. Since the [E-]m is negligible in the experimental conditions of the present work, the quotient [Cl-]m/[Cl-]w can be readily evaluated. (The same procedure has previously been applied to quantitatively estimate the equilibrium ion-exchange constant between piperidinium ions and the Na+ counterions of SDS20 or between mercaptide ions and the Br- ions of CTABr micelles18). The constant KI and the value of Ab∞ were calculated by fitting the data for the variation of Ab with [TTACl] to eq 15,
Figure 8. Variation of the absorbance at (b, O) λ ) 360 nm and (4, 2) λ ) 362 nm at [BZA] ) 7.0 × 10-5 mol dm-3 and [buffer morpholine] ) 0.015 mol dm-3 of pH 7.45 as a function of [TTACl] (solid points), and of [NaCl] (open points) at [TTACl] ) 0.011 mol dm-3.
using both constants as adjustable parameters and Ks, KE, KEa , [H+], [Cl-]m, and [Cl-]w as inputs. Figure 8 displays the adaptation of the experimental data to the model; also shown is the effect of the addition of NaCl, which, as we can see, decreases the absorbance due to enolate ions; i.e., as more Clions become present in the solution, competition with the enolate ions for the sites of the micellar surface leads to displacement of the latter from the micellar surface; and at this pH value they are converted into the enolic form (see Figure 6b). The results obtained using Ks ) 320 mol-1 dm3, KE ) 0.60, and pKEa ) 8.30 (all previously determined) were as follows: Ab∞ ) 0.325 and KI ) 115 from the results obtained at λ ) 360 nm, and Ab∞ ) 0.282 and KI ) 111, at λ ) 362 nm. From the values of Ab∞ and the Ab value corresponding to the maximum variation with [TTACl] (=0.020 mol dm-3) (Figure 8), one can calculate the maximum variation of pKa(ap) induced by the presence of TTACl micelles as ∆pKa(ap) ) pKm a (ap) (ap) equaling pH pKwa (ap), with pKwa (ap) equal to 8.70 and pKm a + log(Ab∞ - Ab)/Ab ) 7.18. The electrical contribution of the free energy of transfer of the enolate ions from water to TTACl micelles is then determined as (∆µ0E)e ) -2.303RT∆pKa(ap) ) 8.67 kJ/mol. This value implies a surface potential φ for the micelle of 90 mV, which is much lower than the value of 150 mV calculated for these micelles.36 Therefore, one may conclude, as expected, that the location of the enolate ion would be inside of the Stern layer, meaning that an important contribution to the association process by specific effects (hydrophobicity of the enolate anion) would take place. Br allow us to On the other hand, the constants KI and KCl calculated the ion-exchange constant between the enolate and Br Br-, KBr E ()KIKCl ) as =23. This low value together with the experimental conditions used, [BZA]
