J. CHEM. SOC. FARADAY TRANS., 1994, 90(6), 869-873 Cationic Micellar Effect on the Kinetics of the Protolysis of Aromatic Carboxylic Acids studied by the Ultrasonic Absorption Method Teruyo Isoda,t Miyuki Yamasaki and Hiroshige Yano* Daiichi College of Pharmaceutical Sciences, 22-1,Tamaga wa-cho, Minami-ku, Fukuoka 815,Japan Takayuki Sano Department of Materials Science, Faculty of Science, Hiroshima University, Higashi-hiroshima 724, Japan Shoji Harada Hiroshima Bunkyo Women 3 College, Kabehigashi, Asakita-ku, Hiroshima 731-02,Japan The protolysis of carboxylic acids has been kinetically studied by the ultrasonic absorption method in the pres-ence of tetradecyltrimethylammonium bromide (TTAB) micelles in aqueous solution. The carboxylic acids studied were classified into two categories, one capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form the bond, namely the benzoic acid deriv- atives (BAD). The rate constant (y2k,, kb),the apparent dissociation constant (Ka),and the volume change of the reaction (AV) were obtained.Different K, dependences of the rate constants observed for SAD and BAD are discussed in relation to the effect of intramolecular hydrogen bond. pK, dependences were also observed for AV of SAD and BAD. These dependences are larger than those in aqueous solution. This result was attributed to the change of arrangement of water molecules around the solute in micellar solution and aqueous solution. A number of intramolecular hydrogen bonds in the protein are contributing not only to the stabilization of its structure, but also to the control of the reactions of the dissociative groups.We can generally admit that the intramolecular hydrogen bond increases the dissociation of acid' and decreases the volume change of the reaction2 (AV) in aqueous solution. By the ultrasonic absorption method, we have reported that the micelles promote the dissociation of acids and bases, and catalyse their rapid ionization reactions3 -6 in the same manner as in the slow reactions, e.g. the hydrolysis of esters, amides, and Schiff In the present work, we measured the ultrasonic relaxation absorption based on the protolysis of BAD and SAD in TTAB micellar solutions and aimed to examine the effect of intramolecular hydrogen bonds on the rate constants and AV in the micellar solution.Experimental Benzoic acid (BA), m-nitrobenzoic acid (m-NO,BA), m-chlorobenzoic acid (m-ClBA), p-hydroxybenzoic acid (p-OHBA), p-toluic acid (p-CH,BA), salicylic acid (SA), 4-methylsalicylic acid (4-CH3SA), 5-methylsalicylic acid (S-CH ,SA), 5-chlorosalicylic acid (5-ClSA) and 5-bromosalicylic acid (5-BrSA) purchased from Nakarai and 3-methylsalicylic acid (3-CH3SA) and 3-hydroxy-4-methylbenzoic acid (3-OH-4-CH3BA) from Aldrich, and TTAB from Tokyo Kasei were all reagent grade and used without further purification. Ultrasonic absorption measurements were performed with the pulse technique over the frequency range 5-105 MHz.Details of the apparatus have been described elsewhere.' The velocity of sound was measured by the sing-around even in the saturated solution. When the acids were solu- bilized in the TTAB micelles, however, the ultrasonic absorp- tion was observed in spite of their low concentrations. All of the spectra were characterized by a single relaxation equation' '-I3 M-' = A/{1 + (flfJ') + B (1) where s( is the absorption coefficient,fis the frequency,S, is the relaxation frequency, and A and B are the relaxation and non-relaxation absorptions, respectively. The absorption parameters,f,, A and B, were determined by a least-squares fit of the experimental data to eqn. (1). Experimental errors were within 9, 6 and 6% of the corresponding values, respec- tively.Representative ultrasonic absorption spectra are shown in Fig. 1. Experimental conditions and the absorption parameters obtained are summarized in Table 1. The concen- tration, acidity of acids and pH in the solution, greatly affect the ultrasonic absorption ; similar dependences have been observed for the protolysis of carboxylic acids in cationic micellar systems in our previous ~orks.~,~ All of the results 6 100-'6100nr c -----------0 T, N method at 1.92 MHz. Density was measured by a pyc-nometer. The pH and ultrasonic measurements for the solu-tions were carried out under a dry nitrogen gas atmosphere. All the measurements were performed at 30.0 "C. Results and Discussion The acids used in this work are only slightly soluble in water and the ultrasonic relaxation absorption was not observed ~ t Nee Yamashita.V 1 I I 1 5 10 50 100 f/M Hz Fig. 1 Representative ultrasonic absorption spectra of acid aqueous solution (0.05 mol dm-3) in the presence of TTAB (0.30 mol dm-3) at 30.0"C : 3-methylsalicylic acid (0,pH = 1.93) and p-methylbenzoic acid (0,pH = 2.78) J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Ultrasonic absorption parameters for various pH of acids (0.05 mol drn-') in the presence of TTAB (0.30 mol dm-j) at 30.0 "C pH A/lO-" s2 cm-' B/10-17 s2 cm-' f,/MHz m-N0,BA1.88 45 31 15.0 2.04 71 30 11.3 2.17 76 30 10.8 2.36 63 31 11.6 2.57 42 31 13.2 rn-ClBA 1.89 28 30 13.5 2.11 44 29 10.5 2.36 68 29 8.3 2.66 48 30 9.7 2.82 29 30 12.7 BA 1.99 17 27 18.0 2.23 32 28 13.0 2.63 46 28 10.3 3.01 36 28 12.0 3.30 16 29 16.8 3-OH-4-CH3BA 2.10 17 32 20.0 2.67 45 31 11.4 2.99 44 31 10.5 3.30 25 31 14.6 3.54 12 31 21.7 p-OHBA2.09 11 30 20.0 2.23 27 29 12.5 2.99 46 29 9.4 3.19 26 30 12.6 3.39 15 30 16.8 p-CH3BA 2.28 16 29 15.5 2.49 28 29 11.2 2.78 48 29 8.5 3.14 26 28 11.2 3.39 18 29 13.4 mentioned above suggest that the relaxation absorption can be ascribed to the protolysis of carboxylic acid solubilized in the TTAB micelles. kt RCO; + H+ SRC02H (1) kb where R is incorporated into the micelle core.For reaction (I), the relaxation frequency and the maximum relaxation absorption per wavelength, are expressed by the fol- lowing equations' '-' 7cp u2 (a'4rnax = -(A V)2 -(3)2RT with r = [RCO,]-' + [Hf]-' + [RCO,H]-' (4) -K, 'C,[H+]-K,-'C, + (K,-'[H+] + 1)2 where C, is the total concentration of acid, a' is the excess absorption coefficient, R the wavelength, p the density, U the sound velocity, AV the volume change of the reaction, and the subscript max means the maximum value. When K,-'C, > 1 is satisfied, as in the present experiments, these equations predict that the minimum and the maximum of PH A/10-17 s2 cm-' B/lO-" s2 cm-' f,/MHz 5-BrSA 1.20 29 32 35.6 1.38 37 31 30.3 1.60 47 31 26.7 1.92 44 30 23.5 2.52 15 30 25.4 5-C1SA 1.19 20 33 35.6 1.29 29 33 30.0 1.42 36 33 25.2 1.64 45 32 22.3 1.94 33 32 20.0 2.11 26 32 21.4 2.35 16 32 23.0 3-CH3SA 1.11 14 29 43.1 1.50 53 29 21.0 1.93 102 31 13.9 2.33 62 29 14.8 2.54 36 31 16.8 5-CH3SA 1.07 18 31 30.7 1.54 42 30 20.0 1.92 89 31 13.4 2.27 44 31 15.9 2.74 27 30 18.0 4-CH3SA 1.15 13 31 38.4 1.57 50 31 20.0 2.04 94 31 14.5 2.42 50 30 18.0 3.21 14 30 25.0 SA 1.23 17 32 33.7 1.61 41 32 21.8 1.97 88 31 14.6 2.44 45 30 17.8 2.72 28 30 , 20.0 (27cf,)and r-[then (C~'A)~.J, respectively, appear at the defi- nite value of pH, i.e. pH*; pH* = -(log Co + log K,)/2 (6) Since C, is known, the value of K, can be determined from pH*.As shown representatively in Fig. 2 and 3, experimental data are in agreement with eqn. (2)and (3).The values of K, , y2k,, kb and AV were determined so as to give best fits for the experimental values of (27cf,) and (a'A)mm. All the values obtained are summarized in Table 2. As an index of the catalysis, the differences of the pK, values in aqueous solution and micellar solutions, denoted by ApK,, are listed in Table 2 together with those in the DAC3 and DPC solution^.^ In order to evaluate this, the electro- static effect and the polarity effect of the micelle should be examined.Since the cationic atmosphere of the micelle stabil- izes the carboxylate anion, the electrostatic effect promotes the dissociation of the acids. The polarity of the micelle was evaluated from the ratio of ketonic and enolic forms of benz~ylacetoanilide'~solubilized in the micelles. Fig. 4 shows the UV spectra of benzoylacetoanilide in the micellar solu- tions together with that in the aqueous solution. This figure shows that the polarity of the micelle is lower than that in water, and the TTAB micelle provides the most hydrophobic field. The decrease of polarity induces the depression of the dissociation, then the two opposite factors affect the pK, of acid in the TTAB micellar solution. All the ApK, values are J. CHEM. SOC. FARADAY TRANS., 1994, VOL.90 87 1 positive and indicate that the dissociation is promoted in micellar solution, which means that the electrostatic effect is 1II superior to the hydrophobic effect. The largest value of ApK,I in DAC solution is easy to understand from a considerationI II of the largest electrostatic effect induced by the exposure of I I, the charged centre outside of the Stern layer. The equivalentI I value of ApK, in the TTAB and DPC micellar solutions andI2 I I the superior hydrophobicity in the former, known from the5 UV spectra, indicates that the electrostatic effect is larger in0 the TTAB solution. cI( The acids in this work can be divided into two groups, one3 capable of formation of intramolecular hydrogen bond, namely the salicylic acid derivatives (SAD) and the other which cannot form a bond, namely the benzoic acid deriv-atives (BAD).The values of ApK, of SAD are about two times larger than those of BAD; this can be attributed to the stabilization of COY by intramolecular hydrogen bond for-:mation in the hydrophobic environment of the TTAB O1 2 3 4 PH Fs2 Plots of 2nf, us. pH for acid aqueous solution (0.05 rnol dm-j) in the presence of 'TTAB (0.30 rnol dm-j) at 30.0"C:3-methylsalicylic acid (0)and p-methylbenzoic acid (a) O01234 PH Fig. 3 Plots of (a'A)-us. pH for acid aqueous solution (0.05 mol dm-3) in the presence of TTAB (0.30 mol dme3) at 30.0"C:3-methylsalicylic acid (0)and p-methylbenzoic acid (0) micelle.15.16 Acidity dependences of the rate constants for BAD are plotted in Fig.5A and the relationships are expressed by eqn.(7)and (8) for the aqueous solution, and eqn. (9) and (10) for the micellar solution. y2kf/dm3mol-'s-' = 10*o*6 (7) k&-' = 10'0.6Ka (8) y2kf/dm3 mol- s-' = 108*3K-0*44 (9) a kds-1 = 108.JK0.56 (10) In the micellar solution, the rate constants at pK, = 0 are more than two orders of magnitude smaller than that in aqueous solution. This indicates that the TTAB micelle causes the reaction mechanism to vary widely from that in aqueous solution. Another feature is the lack of dependenceof ;t2kf on K, in aqueous solution while both rate constants are dependent on K,in micellar solution. The protolysis of carboxylic acids has often been inter-preted by the followingmechanism (iii) Table 2 Kinetic parameters for the protolysis of acids (0.05mol drn-') in the presence of TTAB micelle at 30.0"C y2kf/109 dm3 mol-' s-l W1O6s-I AV/cm3 mol-' pK, TTAB DAC DPC BAD m-N0,BA 5.1 4.7 11.8 3.03 0.5 1.2 0.4 m-C1BA 6.0 2.4 12.1 3.40 0.4 1.2 0.3 BA 12 1.2 17.3 4.01 0.2 1.2 0.2 3-OH, 4-CHSBA 16 1.o 1 7.0 4.19 0.2 p-OHBA p-CH,BA 14 17 0.93 0.85 17.8 17.0 4.21 4.31 0.4 0.1 SAD SBrSA 2.9 36 9.4 1.90 0.7 5-ClSA 2.7 30 8.1 1.94 0.7 3-CH3SA 3.2 11 12.2 2.44 0.6 SCH3SA 3.3 10 11.6 2.49 0.7 CCH3SA 3.7 10 12.6 2.56 0.6 SA 3.9 9.9 12.5 2.60 0.4 200 300 400 A/nm Fig.4 Absorption spectra of benzoylacetoanilide in aqueous solu- tion (-), in the solutions of TTAB (-* -), DAC (---) and DPC (. .. ..) where (i) ==(ii) is a diffusion process and (ii) e(iii) is a proton-transfer process and (ii) is a steady-state intermediate. Since the index of K, in eqn. (7)-(10) indicates the degree of proton transfer at a transition state," it is concluded that the rate- determining step is the diffusion process in aqueous solution and the proton-transfer process in micellar solution. These results are similar to those of BAD in DAC and DPC solu-tion~,~,~but the catalytic effect is much larger in TTAB solu-tion. In the case of SAD, the situation is a little different. Sum- marizing a small amount of kinetic information on the intra- molecular hydrogen bonding acids in aqueous solution,' 9-20 following K, dependences of the rate constant of SAD were obtained and shown by the dotted line in Fig.5B. y2kf/dm3 mol-' s-' = loge6 (11) kds-l= 109.6~~ (12) 5 I I I 1 0 1 2 3 4 PK, J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 The smaller k, of SAD compared with eqn. (8) is explained in the following way. The reaction species of the protolysis is the non-bonding COY group (open form) which is in rapid equilibrium with the bonding one (closed form) as expressed by the following equation. where (i') is the closed form and (i) is the open form. Since (i)' s(i) is much faster than (i) e(ii), the relaxation equation for the overall reaction is given by + [i] + [i']) + k, = Y,(~)([H+]1 + K43 Here, the value of k, might be similar to those of ordinary acids and of the order of 10" dm3 mol-' s-'.Since K,, is much larger than 1, however, the apparent rate constant y2k; becomes much smaller than the rate constant of protolysis of ordinary carboxylic acids. K, dependences of the rate constants of SAD in the TTAB micellar solution are plotted in Fig. 5B and given by y2k,/dm3 mol-' s-1 = 109.1Ka-0.18 (14) As seen in the figure, the values of the intercept and the slopes are very close to those in aqueous solution. These results indicate that the reaction mechanism and the energy levels of the protolysis of SAD are not much different from those in aqueous solution; then, in reaction (11), (i)e(ii) 11 B 10 9 8 7 6 I E 5 J 0 1 2 3 4 PK, Fig.5 A, pK, dependence of the rate constants of the protolysis of acid in the presence of 0.30 mol dm-3 TTAB (large Circles, data of BAD) and in the absence of micelle (small circles, data from ref. 17). B, pK, dependence of the rate constants of the protolysis of acid in the presence of 0.30 mol dm-3 TTAB (large Circles, data of SAD) and in the absence of micelle (A, SAD; 0,lactic acid). (a)log(y2k,) and (b)log k,. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 20 I 1 1 t-0 ,E 10 E I-d 0012345 PK, Fig. 6 pK, dependence of AV of the ionization of acid in the pres-ence of 0.30 mol dm-3 TTAB (0,SAD; a,BAD system) and in the absence of micelle (a,data from ref.21-23) might be much slower than (ii)e(iii). If this is the case, the K, dependences of the overall rate constants are expected to be the same as those in the aqueous solution, and the index K, will be 0 and 1 in eqn. (14) and (15), respectively. Detailed discussion on this problem has been developed in our pre- vious ~aper.~.~ As seen in Table 2, AV is larger in the micellar solution and dependent on K, as shown in Fig. 6. Similar tendencies have been observed for BAD in DAC and DPC solutions. With increase of pK,, localization of charge density of the carboxylate anion increases and hydration is promoted. While the molar volume might not be greatly affected in the non-ionic form, then AV increases with pK,. Hepler studied the protolysis of acids and amines in aqueous solution and observed a linear relationship between AV and AS.24If this is the case, a linear relationship is expected between AV and pK, and the volume change is mainly attributed to the change of the arrangement of water molecules around the reaction species.The slope of the linear relationship between AV and pK, in TTAB solution is larger than that in aqueous solution ; this indicates that the compactness of water mol-ecules around the non-ionic form in the micellar solution decreases compared to it in the aqueous solution. References 1 CRC Handbook of Chemistry and Physics, ed. R. C. Weast, CRC Press, Cleveland, 1989. 2 R. J. H. Clark and A. J. Ellis, J. Chem. SOC., 1960, 247. 3 S.Harada, T. Yamashita, H. Yano, N. Higa and T. Yasunaga, J. Phys. Chem., 1984,88, 5406. 4 S. Harada, H. Yano, T. Yamashita, S. Nishioka and T. Yasu-naga, J. Colloid Interface Sci., 1986, 110, 272. 5 S. Harada, H. Okada, T. Sano, T. 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