5953
J. Phys. Chem. 1994,98, 5953-5955
Prototropic Equilibrium of Some Benzimidazoles in Anionic and Nonionic Micelles Subit K. Saha, Pankaj K. Tiwari, and Sneh K. Dogra' Department of Chemistry, Indian Institute of Technology. Kanpur, Kanpur 208 016, India Received: September 30, 1993; In Final Form: March 18, 1994"
The pKa values of the monocation-neutral equilibrium of five benzimidazoles have been studied in anionic micelle (sodium dodecyl sulfate, SDS), nonionic micelle (Tween-80), and dioxane-water mixtures of different compositions. The disagreement between the pKa value observed in water and in Tween-80 is due to smaller dielectric constant (&) at the interface, whereas in the case of SDS micelles it is due to smaller &, surface potentials, and specific molecular interaction between cationic proton and sulfate head group. Orientation of the heterocyclic nitrogen atom, in the case of 2-phenylbenzimidazole also, plays a role in the above discrepancy.
Introduction
Materials and Method
The importance of interfacial region between aqueous and nonpolar region of the self-assembled lipid phase is very well recognized in biological membranes. It is also well-known that characteristics of this region are different from those of bulk aqueous phase and interior of the lipid phase or detergent which can mimic the properties of lipids. Absorption and fluorescence spectroscopic probes are used to calculate the effective dielectric constants,l-9 m i c r o v i s c o s i t i e ~ surface , ~ ~ ~ ~ potential~,~J~-21 etc. One of the reactions involved in finding the above properties is the observed pKa values of the weak acid-base indicators. In ionic micelles it is well recognized that p g b s can be related to the surface potential by the following equation2,'1-13~17-23
BI, DMBI, TMBI,PBI, and MPBI were procured from Aldrich Chemical Co. (U.K.) and were purified as described earlier.30 Tween-80 (Aldrich), AnalaR hydrochloric acid, and NaOH (BDH) were used as received. Dioxane (E. Merck) was further purified as described in the l i t e r a t ~ r e . Triply ~ ~ distilled water was used for making aqueous solutions. Stock solutions of concentration le3M of each BI's were prepared in water if soluble; otherwise it was prepared in dioxane. Sample solutions of each benzimidazole of desired composition of dioxane-water solution and detergent concentrations were prepared by injecting 0.1 mL of the stock solution with the help of micropipet. The pH of each solution was adjusted by adding small amount of concentrated HCl or NaOH solution and was measured with Toshniwal pH meter Model CL 46. The UVvisible spectrum was recorded on Shimadzu spectrophotometer Model 190UV, equipped with chart recorder Model 135. The pKa values of the monocation-neutral equilibrium of each BI's
p g b s = pK', - e$,/2.303kbT
(1)
where pK', is the intrinsic pKa value, e the charge of the electron, kb Boltzmann's constant, +O the surface potential, and T the temperature in absolute scale. It has recently been recognized that pPa is also a function of effective dielectric constant of the interface (De& salt effect, and the specific interaction of the indicator species with the head groups of the surfactants.13 In the nonionic micelles, p c b s will be equal to pK', and also to the pK1, calculated in ionic micelles, provided the Deff,salt effect, and the molecular interactions are the same in both kinds of micelles. Although Fernandez and Fromherz,' using 4-heptadecyl-7hydroxycoumarin and 4-octacycloxy-1-naphthoic acid as acidbase indicators, have shown that Deff of ionic micelles can be equated to the Deff of the interface of nonionic micelles of surfactants with poly(ethy1ene oxide) head groups, Drummond et a1.I1J3J6have shown that this assumption is not valid in many cases and p c b sof weak acid-base equilibrium in ionic micelles can be explained on the bases of four factors listed above. Recently, we have been studying the effect of ionic and nonionic detergents on the acid-base properties of some groups likes N,24*25 )NH,24,25 -NH2,26-28and -OH. The fluorescence spectra of the monocations formed by protonating 3 N in benzimidazoles (BI's) have shown the tendency to form premicellar aggregates with sodium dodecyl sulfate (SDS) detergent m0lecules,2~indicating some kind of interaction with the sulfate group of SDS detergent molecules and the proton of the cation of BI's. In order to confirm this fact, we have carried out this study to compare the pK, values of few BI's in SDS and nonionic micelles withpoly(ethy1eneoxide) head groups, i.e., Tween-80. The BI's used are benzimidazole (BI), 5,6-dimethylbenzimidazole (DMBI), 2,5,6-trimethylbenzimidazole (TMBI), 2-phenylbenzimidazole (PBI), and 1-methyl-2-phenylbenzimidazole(MPBI). e Abstract published in Aduunce ACS Abstrucrs, May 1, 1994.
0022-3654/94/2098-5953!§04.50/0
HBI'
BI + H+
(2)
were determined by using the procedure described by Drummond et al.13 The respective equations used are as follows:
p p = p g b s + e$,/(2.303kbT)
(5)
where p c , pPa, p g " are the pKa values of the BI's indicator determined in dioxane-water mixture, intrinsic pKa values, and pKa values determined in the micellar solution, respectively. my* is the mean ionic activity coefficient of HCl in the particular medium, p e is the apparent PKa value if the surface potential of the micelle is zero. B is the pH meter reading and log v", is the correction factor to be applied to pH meter reading to depict the actual hydrogen ion concentration in dioxane-water solutions. Other constants have already been defined earlier. The values of log @, and log HT+ have been taken from the work of Drummond et al.I3 The other implicit assumptions involved in each of the above relations have been discussed by Drummond et al." and thus will not be described here. For convenience of discussion, the following relations are defined: 0 1994 American Chemical Society
5954 The Journal of Physical Chemistry, Vol. 98, No. 23, 1994
Saha et al.
-
Q
a
where p c is the pKa value of the monocation-neutral equilibrium of BI’s in water.
Discussion Using p e values from the Table 1 and Figure 2, Dcffat the interface between the micelle Tween-80 and aqueous phase was calculated. The values so obtained are also compiled in Table 1 . It is clear from the data of Table 1 that, for each benzimidazole, the observed valueof D,ffof the site where the prototropic reaction is taking place decreases with the increase of detergent concentration. The value of D,ff a t the interfacial microenvironment found to be 40 5, when each of the methyl substituted BI’s is solubilized at least 90%, whereas for 2-phenyl-substituted benzimidazole, D,ffis smaller than that noticed for methyl-substituted BI’s. It can thus be inferred that (i) microenvironments of the interfacial region of nonionic micelles comprising surfactant molecules with poly(ethy1eneoxide) group can be represented by the organic solvent (dioxane)-water mixture, (iii) log (&/&+) term does not contribute very much to the above monocation-neutral equilibrium, and (iii) there does not seem to be any specific molecular interaction between the conjugate acidbase species. All these inferences are based on our results and similar results obtained by others.lJI-13
*
C-C-O
@-e-~ P B I MPBl
Results pK, values of the five BI’s have been determined in 0, 10,20, 30, 50, 60, and 80% dioxane-water mixtures. p c and pPa values in these organic solvent-water solutions have been determined by using eqs 3 and 4, respectively. The values of log pHand log ,,Ti have been taken from the work of Drummond et al.13 as mentioned earlier. The dielectric constants for the mixtures were obtained from the work of Critchfield et al.3z Further assumption involved in this work is that log ( Y B I / Y H ~ I + ) is negligibly small. The values of p c obtained in this work agree with thoseof literaturedata30Jz within limitsofexperimental error. The values of A p e and ApK‘, versus dielectric constants have been plotted in Figures 1 and 2 respectively. The apparent pKa values of monocation-neutral equilibrium of all the BI’s have been determined at various concentrations of Tween-80, and the values so obtained are compiled in Table 1 . The apparent pKa values of the monocation-neutral equilibrium of DMBI and PBI were also determined in 0.04 M Brij-35 detergent, to compare our results with those of Drummond et al.13 The values of the apparent pK, values were found to be 4.9 and 3.9, respectively. The value of Dcfffound for Brij-35 site where the prototropic reaction is taking place is a little higher than that observed by Drummond et aLI3 and it will be discussed later. The apparent pKa values of the BI’s for the same equilibrium determined in 0.02 M SDS have been taken from the earlier work of this lab0ratory.2~ The apparent pKa values for BI and DMBI in SDS have also been determined upto 0.2 M c o n c e n t r a t i ~ n . ~ ~ The data have indicated that the maxima in p g b ”have reached at nearly 0.04 M SDS concentration (Figure 3). The maximum values in p g b sfor BI and DMBI observed 7.2 f 0.1 and 8.4 f 0.1, respectively. Based on the values of binding constants of other BI’s used,29 it can be concluded that the maximum in p g b s for other BI’s will also be nearly at the same micellar concentration. p e , calculated in 0.02 M SDS taking +o equal to the -140 and -195 mV, respectively, have also been compiled in Table 1.
81 DMBI TMBl
-8
b
I
I
10
i
I 20
I
I 30
I
I 40
I
I
50
I
I 60
I
I
70
I
I
80
- D
Figure 1. Plot of A p e versus dielectric constant of dioxane-water
mixtures.
0
10
20
40
30
50
60
70
80
-D
Figure 2. Plot of A p g versus dielectric constant of dioxane-water
mixtures. The very high value of D,ff observed in case of BI’s at low detergent concentration is due to low solubilization of these molecules in the detergent. Actually it represents the average behavior of the prototropic reaction occurring in aqueous plus micellar interface. The slightly higher value of Dcff (45 f 5) found by us in case of Brij-35 than that obtained by Drummond et al.I3 can be explained on the same lines as has been done in case of Tween-80. On the other hand, the low value of Dcfffound in case of PBI is due to the orientation of the molecule. Since phenyl group is more hydrophobic, it will be located toward the core of the micelle. Because of this, the heterocyclic nitrogen atom will not be pointing toward the Stern layer, as can be seen in the case of methyl-substituted BI’s and reaction may be occurring at low Dcffvalues. This has also been reflected by the binding constants of the BI’s to the S D S 9 and cetyltrimethylammonium bromidez5(CTAB), as well as the neutral-monoanion equilibrium of these BI’s in CTAB micelles. SDS Micelles The p$ calculated from the pg’” values in SDS micelles taking $0 = -140 mV do not agree with the p$ values obtained in Tween-80, whereas little better agreement between the p e in SDS with $0 = -195 mV and p e in Tween-80 is observed. But the latter agreement could be a coincidence because the highest value of $0 calculated for SDS is -195 mV and this is based on the theoretical calculations~4involving an unmodified nonlinearized Poisson-Boltzmann equation for a spherical SDS micelles with 64 charged surface sites in aqueous solution with no added electrolyte. The recent studies34 have shown that some of the counterions are bound to the ionic head groups. Due to this, a surface potential lower than -195 mV is expected. Thus it may be concluded that the disagreement between the p e values determined in SDS and Tween-80 could not be due to the inaccurate value of the surface potential. Similar observations have been noticed in the protonation reactions involving the similar groups.I3q35
Prototropic Equilibrium of Some Benzimidazoles
The Journal of Physical Chemistry, Vol. 98, No. 23, 1994 5955
TABLE 1: p c , p g b s (in 0.02 M SDS), and p e for SDS Micelles (#o = -140 and -195 mV), p e (Different Tween-80 Concentrations) and Respective A p e and D . of Tween-80 Observed from Acid-Base Equilibrium of BI’s SDS = -140 mV
$0
$0
= -195 mV
Tween-80
compd BI
P C
PGb
P e
Ape
P e
Ape
P e (cone (M))
Ape
5.7
7.0
4.63
-1.07
3.7
-2.0
DMBI
5.9
8.3
5.93
+0.03
5.0
-0.9
TMBI
6.6
8.2
5.87
-0.73
4.9
-1.7
PBI
5.1
8.2
5.87
+0.77
4.9
-0.2
MPBI
5.4
7.8
5.47
+0.07
4.5
-0.9
5.4 (0.01) 5.1 (0.02) 3.7 (0.15) 5.2 (0.02) 4.6 (0.1) 6.2 (0.01) 5.9 (0.02) 4.1 (0.02) 3.8 (0.05) 3.4 (0.1) 4.5 (0.02) 3.7 (0.05) 3.4 (0.1)
-0.3 -0.6 -2.0 -0.7 -1.3 -0.4 -0.7 -1.0 -1.3 -1.7 -0.9 -1.7
-2.0
DCii 60 45 30 55 40 58 50 50 45 37 50 45 37
in water and SDS is due to (i) low Den, (ii) surface potential, and (iii) specific molecular interaction between the proton of the monocations of BI’s and the sulfate head group of SDS micelles. The lower values of pKa observed in the case of 2-phenylbenzimidazole is due to the location of the heterocyclic nitrogen atom in both the micelles.
,
I
0.02
0.04
0.06
I
I
I
0.08
I
0.1
[SDS] ( M )
Figure 3. p q b versus [SDS] for the molecules BI and DMBI.
Unlike the results of Fernandez and Fromherz,’ it has been shownlIJ3aJsthat the Dewof SDS interface is not the same as that of the nonionic micellar interface comprising poly(ethy1ene oxide) head groups. The variation in the Dcffa t the interface of SDS micelles lies between 16and 67, obtained by different techniques.16 Taking D,ff to be the highest limit, Le., 67 and $0 = -140 mV, the pKa values calculated for the BI’s are found to be higher than those obtained in pure aqueous medium which is impossible. This reason can also be rejected. In the light of above discussion it may be concluded that the main discrepancy between the p c observed in SDS and Tween80 micelles could be due to the specific molecular interaction between the monocations of BI’s and the sulfate head groups of SDS micelles. This interaction is through the proton attached to the heterocyclic nitrogen atom of the monocations of BI’s. This is further manifested by the fact that the premicellar aggregation of SDS detergent molecules is predicted based on our results of the fluorescence spectra of the monocations of BI’s;29 Le., the fluorescence intensity of the monocations of each BI decreases, reaches the minimum value at 6 X 10-3 M SDS, and then starts increasing with the increase in the SDS concentration. The very large discrepancy of p e for 2-phenyl-substituted BI’s in Tween-80 and SDS at $0 = -140 mV can be explained on the same lines as has been done earlier for Tween-80, i.e., orientation of the molecules toward the Stern layer of the micelles. Based on our results and the results of other workers,l3~35~36 it may be concluded that specific molecular interaction between the monocations, formed by the protonation of heterocyclic nitrogen atom, and negative polar head groups exists through the hydrogen atom. Conclusions The difference between the pKa values of monocation-neutral equilibrium of the BI’s observed in water and Tween-80 is due to smaller D,ffof Tween-80 interfacial region, and D,aof Tween80 equal to 40 f 5 can be assigned to the interface of Tween-80, when a t least 90% of each BI’s is solubilized in the micelles, whereas the difference in the pKa values of the above equilibrium
Acknowledgment. The authors are thankful to the Department of Science and Technology, New Delhi, for the financial support to project no. SP/SI/H-19/91. References and Notes (1) Fernandez, M. S.; Fromherz, P. J. Phys. Chem. 1977, 81, 1755. (2) Kalyanasundram, K.; Thomas, J. K. J. Phys. Chem. 1977,81,2176. (3) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micellization,
Solubilization and Microemulsions, Mittal, K. L., Ed.;Plenum Press: New York, 1977; Vol. 1, p 241. (4) Mukerjee, P.; Cardinal, J. R. J. Phys. Chem. 1978,82, 1620. (5) Zachariasse, K. A.; Van Phua, N.; Kozankiewcz, B. J. Phys. Chem. 1981.85, 2676. (6) Law, K. Y. Photochem. Photobiol. 1981,33, 799. (7) Ramachandran, C.; Pyter, R. A.; Mukerjee, P. J . Phys. Chem. 1982, 86, 3198. (8) Lianos, P.; Viriot, M. L.; Zana, R. J . Phys. Chem. 1984,88, 1098. (9) Handa, T.; Matsuzaki, K.; Nakagaki, M. J. Colloid. Interface Sei. 1987, 116, 50. (10) Blatt, E.; Ghiggino, K. P.; Sawyer, W. H. J . Phys. Chem. 1982,86, 446 1. (11) Drummond, C. J.; Grieser, F.; Healy, T. W. J. Phys. Chem. 1988, 92. 2604. (12) Drummond, C. J.; Grieser, F.; Healy, T. W. Faraday Discuss. Chem. soc. 1986. ai. 95. (13) Drummond, C. J.; Grieser, F.; Healy, T. W. J . Chem. Soc., Faraday Trans. 1989, 85, 521, 537, 551, 561. (14) Mukerjee, P. In Solufion Chemistry of Surfactanrs; Mittal, K. L., Ed.; Plenum Press: New York, 1979; Vol. 1, p 153. (15) Turro, N. J.; Aikawa, M.; Yokta, A. J. Am. Chem. SOC.1979,101, 772. (16) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988,92, 5580 and references listed therein. (17) Lucas, S.J. Phys. Chem. 1983, 87, 5045. (18) Mukerjee, P.; Banerjee, K. J . Phys. Chem. 1964, 68, 3567. (19) Moller, J. V.; Kragh-Hansen, U. Biochemisrry 1975, 14, 2317. (20) Castle, J. D.; Hubell, W. L. Biochemisfry 1981, 20, 2208. (21) Ehrenberg, B.; Berezin, Y. Biophys. J . 1984, 45, 663. (22) Hartley, G. S.;Roe, J. W. Trans. Faraday SOC.1940, 35, 101. (23) Davies, J. T. Ado. Carol. 1954, 6, 56. (24) Nigam, S.; Dogra, S. K. J . Photochem. Photobiol. 1990,54A, 219. (25) Nigam, S.;Dogra, S.K. 2.Phys. Chem., in press. (26) Sarpal, R. S.; Dogra, S. K. J . Chem. Soc., Faraday Trans. 1 1992, 88. 2715. (27) Sarpal, R. S.; Dogra, S. K. J . Photochem. Photochem. Photobiol. 1993,69, 1993. (28) Sarpal, R. S.;Dogra, S. K. Indian J . Chem. 1984, 33A, 111. (29) Nigam, S.;Sarpal, R. S.;Dogra, S.K. J . Colloid. InterfaceSci. 1994, 163, 152. (30) Krishnamurthy, M.; Phaniraj, P.; Dogra, S.K. J. Chem. Soc., Perkin Trans. 2 1986, 1917. (31) Riddick, J. A.; Bunger, W. B. Techniques of Organic Chemistry, Vol. 2, Organic Solvents; Wiley Interscience: New York, 1970; p 592. (32) Critchfield, F. W.; Gibson, J. A,; Hall, J. L. J . Am. Chem. SOC.1953, 75, 592. (33) Mishra, A. K.; Dogra, S. K. Spectrochim. Acta 1983, 3 9 4 609.
