Excited state intramolecular proton transfer of 2-(2º-hydroxyphenyl)benzimidazole in non-ionic micelles : Brijs Someskumar Das and Sneh K. Dogra* Department of Chemistry, Indian Institute of T echnology Kanpur, Kanpur-208016, India Excited state intramolecular proton transfer (ESIPT) of 2-(2@-hydroxyphenyl)benzimidazole (2-HPBI) has been studied in four diÜerent Brijs, i.e. Brij-35, Brij-58, Brij-78 and Brij-99. In comparison to water, —uorescence maxima of the tautomer band is red shifted by B20 nm, whereas that of normal —uorescence is not aÜected much.The —uorescence quantum yield of the tautomer band increases and that of the normal band decreases. The eÜective relative permittivity of the site where neutral 2-HPBI is (eeff) present is 20^2, whereas of the sites where monocation»neutral and neutral»monoanion equilibria are taking place are eeff higher. The critical micelle concentration (c.m.c.) of the Brijs at pHB7 decreases with increase of Brij number.At pH B1.0 (ionic strength 0.3 M), the c.m.c. of the Brijs are larger, by nearly two orders of magnitude than at pH 7. The values of the pKa prototropic reactions are determined in diÜerent Brijs and are discussed. 1 Introduction Among the non-ionic surfactants, Triton X-100 (TX-100) has received the most attention, as it forms micelles in aqueous solution1h6 and –nds wide applications in biochemical studies involving membranes and protein puri–cation7,8 and lipolytic enzymes.9 A number of studies have been carried out to establish its size, shape and hydration.5,6 Based on the molecular weight and viscosity data,6 it is shown that if distinct polar and apolar regions of micelles exist, the micelle structure must be an oblate ellipsoid. However, a Raman spectroscopic study5 has shown the structure of TX-100 micelles to be spherical.In the latter model, the hydrophilic oxyethylene groups penetrate into the hydrophobic core of the micelles and do not exhibit clear distinct polar and apolar zones.The other classes of non-ionic surfactants, known as Tweens and Brijs, are shown in Scheme 1. Both these surfactants, similar to TX-100, contain poly(oxyethylene) groups as the polar part. The main diÜerences between the Tweens and Brijs are that : (i) the poly(oxyethylene) moiety of the Tweens is highly substituted and (ii) the Tweens are the esters of fatty acids of diÜerent chain lengths whereas the Brijs are the corresponding ethers.Unlike TX-100, the characteristics of these surfactants have not been investigated in detail, except for a few studies on the Tweens.10h13 These studies have shown that the hydrophobicity and the aggregation number of Tweens increase and the critical micelle concentration (c.m.c.) of Tweens decreases with an increase in the Tween number. Similar studies are not available for the Brijs except for Brij-35,11 where the prototropic equilibria of the weak acids or bases have been used to –nd the polarity of the site where the above equilibria are established.In the present study, we have tried to use the spectral characteristics of 2-(2@-hydroxyphenyl)benzimidazole (2-HPBI) and prototropic reactions [monocation (MC)»neutral (N) and neutral»monoanion (MA) equilibria] to –nd out the characteristics of four Brijs, viz. Brij-35 [C12H25(OCH2CH2)23OH], Brij-58 Brij-78 [C16H33(OCH2CH2)20OH], and Brij-99 [C18H37(OCH2CH2)20OH] The reasons for using this probe [C18H35(OCH2CH2)20OH].(2-HPBI) are : (i) the molecule exhibits dual —uorescence, the normal and tautomer bands, the former has a small Stokes shift, the band maximum is insensitive but the —uorescence quantum yield is sensitive to the solvent polarity, whereas the latter has a large Stokes shift and the band maximum is sensitive to the solvent polarity ; and (ii) the molecule possesses acidic and basic centres that are close to each other and this may provide better information about the site of these prototropic reactions.Both these aspects of 2-HPBI have previously been investigated thoroughly.14h21 2 Experimental 2-HPBI was synthesized by re—uxing o-phenylenediamine with o-hydroxybenzoic acid in polyphosphoric acid media and was puri–ed as described in the literature.22 AnalaR grade dioxane (E. Merck) was further puri–ed as suggested in the literature.23 All the Brijs (Aldrich Chemical Company) were used as received. AnalaR grade HCl, and NaOH H2SO4 (BDH) were used as received.Triply distilled water was used for making aqueous solutions. The instruments used to measure the absorption, —uorescence intensities and lifetimes of the excited singlet state, the preparation of solutions and adjustment of their pH, the procedure to correct the —uorescence spectra and the calculation of are all the same as described elsewhere.12,24h27 A /fl pH\7 required in the measurement of the apparent pKa values in the state was adjusted by addition of HCl.S0 However, was used for the adjustment in the measure- H2SO4 ment of the —uorescence intensities because halide ions quench the —uorescence intensities.28,29 The values of the MC»N and N»MA pKa (pKaI) (pKaII) equilibria were determined by using the procedure as described by Drummond et al.11 H2PBI`HHPBI]H` (I) HPBIHPBI~]H` (II) The respective equations used are as follows : pKam\B]log(UH 0 )[logA [HPBI] [H2PBI`]B[log cHPBI cH2PBI` (1) pKa1\pKa[logm cB (2) pKa0\pKaobs] eW0 2.303kbT (3) J.Chem. Soc., Faraday T rans., 1998, 94(1), 139»145 139Scheme 1 Structures of the non-ionic surfactants studied where and are the values of the indica- pKam, pKai pKaobs pKa tor (2-HPBI) determined in dioxane»water mixture, the intrinsic values and the values determined in the micellar pKa pKa solutions, respectively. is the mean ionic activity coeffi- mcB cient of HCl in the particular medium, is the apparent pKa0 value if the surface potential of the surfactant is zero.pKa (W0) B is the pH meter reading and is the correction factor log UH0 to be applied to the pH meter reading to depict the actual hydrogen ion concentration in dioxane»water solution, e is the charge on the electron, is the Boltzmann constant and kb T is the temperature in Kelvin. The other implicit assumptions involved in case of the above relations have been discussed by Drummond et al.11 and will not be discussed here.For convenience of discussion, the following relations are de–ned: *pKam\pKam[pKaw (4) *pKai\pKai[pKaw (5) *pKa0\pKa0[pKaw (6) where and are the values of the respective pKaw pKa0 pKa equilibrium in water and in non-ionic micelles. 3 Results 3.1 Spectral characteristics of neutral species Absorption band maxima —uorescence band maxima (jmax, ab), of the normal (norm) and tautomer (taut) bands, —uo- (jmax, fl) rescence quantum yields in diÜerent micelles at 298 K (/fl) and pH 7 are compiled in Table 1.The absorption spectra are shown in Fig. 1 and the —uorescence spectra in Fig. 2. Similar data in dioxane, 55% v/v dioxane»water mixture, water, SDS, CTAB, TX-100 and Tweens from our earlier studies20,30 are also given in Table 1 for comparison purposes. is jmax, ab Fig. 1 Absorption spectra of 2-HPBI in diÜerent micelles at pH 7 and 298 K. Surfactant concentration\0.02 M. [2-HPBI]\1]10~5 M. A, water; B, Brij-35 ; C, Brij-78 ; D, Brij-99. Table 1 Absorption band maxima, —uorescence band maxima, BWHMH, —uorescence quantum yield, (108 s~1), of 2-HPBI (1]10~5 qfl , kr knr M) and relative permittivity (e) of diÜerent micelles at pH 7 (pH 4 for CTAB) and 298 K jmax, fl/nm system jmax, ab/nm norm taut BWHMH/103 cm~1 e /fl qf/ns kr/108 s~1 knr/108 s~1 dioxane 318, 331(sh) » 467 2.83 2.3 0.56 4.20 1.33 1.05 55% dioxane 316, 330(sh) 353 447 2.81 28.0 0.79 4.50 1.76 1.46 water 313, 325(sh) 353 430 4.0 78.0 0.33 3.00 1.10 2.23 SDS (0.05 M) 315, 328(sh) 353 444 2.96 42.0 0.45 3.70 1.20 1.51 CTAB (0.001 M) 320, 334(sh) » 450 2.99 16.5 0.75 4.80 1.56 0.52 Brij-35 (0.02 M) 317, 331(sh) 353 448 3.05 22.5 0.54 4.97 1.09 0.92 Brij-58 (0.02 M) 319, 333(sh) 353 449 2.98 19.5 0.54 5.14 1.05 0.89 Brij-78 (0.02 M) 318, 333(sh) 353 450 3.02 16.5 0.54 5.30 1.03 0.86 Brij-99 (0.02 M) 318, 333(sh) 353 449 3.06 19.5 0.49 5.17 0.95 0.97 Tween-20 (0.001 M) 318, 333(sh) 353 449 3.03 19.5 0.54 5.4 1.00 0.85 Tween-40 (0.001 M) 318, 333(sh) » 450 3.07 16.0 0.59 5.1 1.16 0.80 Tween-60 (0.001 M) 318, 333(sh) » 451 3.05 14.0 0.61 5.3 1.15 0.74 Tween-80 (0.001 M) 318, 333(sh) » 450 3.02 16.0 0.62 5.7 1.09 0.66 TX-100 (0.001 M) 318, 333(sh) 353 450 3.0 16.0 0.63 3.8 1.66 0.97 140 J.Chem. Soc., Faraday T rans., 1998, V ol. 94Fig. 2 Fluorescence spectra of 2-HPBI in diÜerent micelles at pH 7 at 298 K. A, water; B, Brij-35 ; C, Brij-78 ; D, Brij-99. Normal band intensities are enlarged by 10 times. nm.[2-HPBI] jexc\314 \1]10~5 M. Surfactant concentration 0.02 M. nearly 8»9 nm (3 nm in SDS at 0.05 M) red-shifted in all these micelles in comparison to those in water. Whereas of jmax, fl the normal band in all the micelles (except CTAB, Tween-40, Tween-60 and Tween-80, where the intensity is too small to notice) is the same as in water. The band width at half the maximum height (BWHMH) of the tautomer band is less in these surfactants [(2.96»3.0)]103 cm~1] in comparison to that in water (4.0]103 cm~1).As with that for nonionic micelles and unlike that for ionic micelles (SDS and CTAB), the —uorescence intensity of the tautomer band increases continuously with increasing surfactant concentration (Fig. 3) and becomes almost constant at a concentration of 0.001 M. The c.m.c. values obtained from the in—ection points are shown in Table 2. The values obtained using absorption spectra are not very accurate as the changes observed are not very large, but the order of magnitude is the same.The —uorescence intensity of the normal band decreases by a factor of 3»4 with increasing surfactant concentration, but does not tend to zero. This behavior is diÜerent from that observed in CTAB and Tweens but is similar to that in SDS. The relative with respect /fl, taut to water increases upto Brij-78 and decreases slightly for Brij- 99. A similar trend is also observed for the However, /fl, norm . decreases with increasing Brij number./fl, norm//fl, taut 2-HPBI was also excited using excitation wavelengths in the range of 315»340 nm, known as the red-edge eÜect,31 and observed in each case was similar. This indicates that jmax, fl the —uorescence occurs from the completely relaxed excited state and that the environments around the —uorophore in the Brijs are at equilibrium at room temperature. It is also evident that the solvent relaxation time of the medium around the Fig. 3 Plot of —uorescence intensities of the tautomer band vs.the logarithm of surfactant concentration. A, Brij-35 ; B, Brij-58 ; C, Brij- 78; D, Brij-99. nm for all Brijs. Intensities are at 448, 447, jexc\314 450 and 450 nm, respectively. —uorophore in micelles is shorter32,33 than that of the radiative decay time. Similar behavior has been observed earlier.34 A calibration curve (Fig. 4) was constructed for the —uorescence band maxima of the tautomer band and (l6 max, fl/cm~1) the relative permittivity of the medium, obtained by mixing diÜerent amounts of water»dioxane.20 By using this correlation diagram, it was found that the polarity at the binding site of 2-HPBI in each Brij, expressed as decreases from eeff , Brij-35 to Brij-78, and then increases slightly in Brij-99, which has much less water content.The values of so obtained for these micelles are much less than that of SDS20 but are slight- Fig. 4 Plot of —uorescence band maxima vs. relative per- (l6 fl/cm~1) mittivity of dioxane»water mixtures Table 2 Binding constants of 2-HPBI [neutral (N), monocation (MC) and monoanion (MA)] with diÜerent micelles (Brijs) and their c.m.c.(Ks) values at 298 K determined by eqn. (7) (the values given in parentheses indicate the c.m.c. values determined from the surfactant concentration vs. absorbance or —uorescence intensity plots) Ks/dm3 mol~1 c.m.c./M tautomer, normal, tautomer, normal, probe micelle absorbance —uorescence —uorescence absorbance —uorescence —uorescence N, pH 7 Brij-35 500 690 2600 8.4]10~5 8.0]10~5 (10.0]10~5) 8.7]10~5 (9.0]10~5) Brij-58 1800 2750 4200 3.65]10~5 3.3]10~5 (3.0]10~5) 3.0]10~5 Brij-78 2050 1680 4200 1.85]10~5 (0.45]10~5) 1.5]10~5 (1.25]10~5) 1.4]10~5 Brij-99 1770 1370 2700 0.9]10~5 (0.6]10~5) 1.2]10~5 (1.3]10~5) 0.9]10~5 MC, pH 1.2 Brij-35 150 » » 1.75]10~3 » » Brij-78 98 » » 1.50]10~3 » » Brij-99 95 » » 1.40]10~3 » » MA, pH 13.2 Brij-35 138 » » 2.25]10~3 » » Brij-78 128 » » 1.0]10~3 » » Brij-99 147 » » 0.5]10~3 » » J.Chem. Soc., Faraday T rans., 1998, V ol. 94 141ly larger than those of CTAB, TX-100 and Tweens. In other words SDS is more hydrophilic than the Brijs, whereas the Brijs are slightly more polar than the Tweens, of the same hydrocarbon chain, as well as TX-100 and CTAB. 3.2 Spectral characteristics of the ionic species The spectral characteristics of 2-HPBI have been studied over pH 1»13 at diÜerent surfactant concentrations. The relevant data are shown in Table 3.It is clear from the data that the values for the monocation of 2-HPBI (pH 1.2) in Brijs jmax, ab are red-shifted by 6 nm in comparison to that of monocation in water and also by 6 nm with respect to the of the jmax, ab neutral species in Brijs. On the other hand, the values jmax, ab of the monoanion (pH 13) in Brijs are red-shifted by 3»6 nm in comparison to that of the monoanion in water, but it is red-shifted by 15»18 nm with respect to that of the neutral in micelles and by 25 nm when compared to the neutral species in water.The values of the monoanion in micelles are jmax, fl red-shifted by 2»3 nm to that in water at pH 13, but are largely blue-shifted in comparison to that in water at pH 13. It is largely blue-shifted in comparison to the 450 nm and redshifted in comparison to the 350 nm —uorescence bands of neutral 2-HPBI in Brijs. The of the tautomer band is jmax, fl blue-shifted by 7»8 nm as the acid concentration increases. The —uorescence intensity of the 442 nm band reaches its maximum by pHB3 and starts decreasing at pH\3.At the same time, the structured normal Stokes-shifted band is replaced by a broad band with very low —uorescence intensity at pH\3. The assignment of the species in the state, i.e. mono- S0 cation at pHB1 and monoanion at pH[11, are consistent with the spectral changes previously observed for a similar system.35 In the state, assignment of the 413 nm band to S1 monoanion at pHP11 and the 376 nm band to monocation at pHO2 are again consistent with previous spectral changes.35 The assignment of the 442 nm band to the zwitterion band, which is formed by the reorganization of charge transfer leading to structure VI, can be made on two grounds: (i) the band maxima observed for the various species are in the order36 and agree with our jzwitterion[janion[jcation[jneutral observation ; and (ii) the increase in the acidity and basicity of the wOH group and the xNw atom, respectively, in the S1 state is such that the order of prototropic reactions in the S0 state are changed upon excitation. Similar observations have been made for molecules containing both electron donating and electron attracting functional groups.35 The blue-shift observed in the —uorescence spectrum of the tautomer band at pH 6»2 could have been due to the increase in the polarity of the micelles when the acid concentration is increased.A similar behavior is observed when the polarity of the pure solvents increases.14 But this is rejected on the grounds that : (i) an increase in the polarity or in the hydrogen bond formation capacity of the solvents leads to an increase in the —uorescence Table 3 Absorption band maxima and —uorescence (jmax, ab/nm), band maxima of monocation and monoanion (pH 13) of (jmax, fl/nm) 2-HPBI in aqueous and micellar media ([Brijs]\0.02 M) monocation monoanion medium pH jmax, ab jmax, fl a jmax, ab jmax, fl water 2.5 320, 332(sh) 364, 442 345 408 SDS (0.02 M) 4.4 325, 337(sh) 376, 443 345 410 CTAB (0.01 M) » » » 348 418 Brij-35 (0.02 M) 1.6 323, 338(sh) 376, 442 348 412 Brij-58 (0.02 M) 1.2 324, 338(sh) 376, 442 351 418 Brij-78 (0.02 M) 1.1 323, 338(sh) 376, 442 351 413 Brij-99 (0.02 M) 1.2 323, 338(sh) 376, 442 351 413 a Zwitterion. intensity of the normal Stokes-shifted band,14 which is not observed in our case ; and (ii) the values of observed at two Ks diÜerent pH values, i.e. 7 and 1.2, are diÜerent (see later), e.g. ca. 10~4 dm3 mol~1 at pH 7 and ca. 100 dm3 mol~1 at pH 1.2. 3.3 Binding constants The binding constants at pH 7 and 298 K, in all the Brijs (Ks) have been determined using eqn. (7) :37 f 1[f \KsA[D]t[ [S]t f B[Ks[c.m.c.] (7) where and [D]t (\[S]m][D]m]c.m.c.) [S]t (\[S]a][S]m) are the total surfactant and substrate concentrations (a and m stand for aqueous and micellar phase respectively) and f is de–ned as : f\ A[Aa Am[Aa (8) where A, and are the absorbances (or the —uorescence Aa Am intensities) in surfactant, in water and when the —uorophore is completely solubilized in micelles, respectively.The values of f lie in the range from 0.0 to 0.8 to minimize the errors. A plot of f/1[f vs. is a straight line. The slope gives ([D]t[[S]t/f ) the value of and the intercept The absorption Ks Ks/[c.m.c.]. spectra, tautomer and normal —uorescence spectra are used to determine the and c.m.c. The relevant data are given in Ks Table 2.The value of the c.m.c. observed in each of the three cases for each micelle agrees with each other as well as with the literature values.38 The values of obtained using absorbance or —uorescence Ks data of the tautomer band agree with each other whereas those obtained using the normal —uorescence band are higher. The value of increases up to Brij-78, but decreases slightly Ks in Brij-99. One other aspect worth considering is that the values of for the species giving normal —uorescence are dif- Ks ferent from those obtained from the tautomer band.This clearly suggests that the two species have diÜerent structures and thus are interacting diÜerently with the micelles. The above conclusions are consistent with earlier results,17 which suggest that : two ground state conformers are present in the state ; conformer II, which leads to normal —uorescence, is S0 present in the relatively polar region ; and conformer I, which leads to the tautomer band, is present in the less polar region of the micelles.The observation of the tautomer band also suggests that the formation of tautomer (III) is a very fast step even in micellar medium. The values of for the species present at pHB1.2 Ks (monocation) and at pHB13 (monoanion) were determined using absorption data. The relevant data along with the c.m.c. values are also given in Table 2. The values of and c.m.c. Ks for both the species could not be determined with the help of the —uorescence data because the changes in the intensities are not very large. Unlike the eÜect of ionic strength on the ionic micelles, the c.m.c.of the non-ionic micelles increases with increasing ionic strength. Similar behavior has been observed previously for TX-100 and Tweens.39 Furthermore, no de–nite trend is observed in the values of for monocations or Ks monoanions as seen for neutral species. 3.4 Lifetimes in the excited state Lifetimes of neutral 2-HPBI were measured in all Brijs at 0.02 M concentration at pH 7 and 298 K.The excitation wavelength used was 313 nm and the decay emission was recorded at 450 nm. The decay curves followed a single exponential, indicating that the —uorophore is completely solubilised and is present at only one site of the micelles. This is consistent with the large values of the for each Brij. The values of the Ks radiative and non-radiative decay constants can be (kr) (knr) 142 J. Chem.Soc., Faraday T rans., 1998, V ol. 94calculated from the following relations : kr\/fl/q knr\1/qf[kr The values of and the decay constants are given in qf , /fl Table 1, along with the values of similar parameters in water, dioxane, a 55% v/v dioxane»water mixture, 0.05 M SDS, 0.001 M CTAB, 0.001 M TX-100 and diÜerent Tweens for comparison. The agreement between the literature values is very good.14 The data in Table 1 clearly show that the rate of the radiationless process is accelerated by water molecules.The other point worth mentioning is that the values of knr observed in the Tweens and CTAB are less than those determined in the Brijs or TX-100. This clearly indicates that the interior of TX-100 or the Brijs are relatively more hydrated than the Tweens and CTAB. 3.5 Proton transfer reactions of HPBI in the state S0 The 2-HPBI molecule contains an wOH group that can act as an acid and an xNw moiety that can act as a base. Hence both MC»N and N»MA equilibria can be studied with this molecule, as given in Scheme 2.Furthermore, as pointed out earlier, the dioxane»water mixtures adequately represent the interfacial region of the non-ionic micelles composed of poly(oxyethylene) groups.40,41 Therefore, the values of pKa the MC»N and N»MA equilibria of HPBI have been determined in 1, 11, 21, 31, 41, 51, 61, 71 and 81% v/v dioxane» water mixtures. The values of and in these organic pKam pKai solvent»water mixtures were calculated by using eqn.(1) and (2), respectively. The values of and have been log UH0 log mcB taken from the work of Drummond et al.11 as mentioned above. The relative permittivities for the dioxane»water solutions were obtained from the work of Critch–eld et al.42 The other assumption made in these calculations is that in the MC»N and in N»MA log(cHPBI/cH2PBI`) log(cPBI~/cHPBI) equilibria are very small and therefore can be neglected. The values of for the MC»N and N»MA equilibria in 1% v/v pKaw dioxane»water mixture agree with our earlier work.14 The Scheme 2 values of and obtained by using eqn.(4) and (5), *pKam *pKai respectively, for the two equilibria of 2-HPBI vs. relative permittivities of various dioxane»water mixtures are plotted in Fig. 5 and 6, respectively. The apparent values of the MC»N and N»MA equi- pKa libria of 2-HPBI have been determined at various concentrations of Brijs and are given in Table 4. The concentrations of the micelles were selected in such a way that at least 99% of the indicator is solubilized in the micelles and this concentration is found to be less than 0.02 M.The value found for eeff the sites of the micelles, at which the prototropic reactions are occurring, from the correlation diagrams (shown in Fig. 5 and 6) are also given in Table 4. It is evident from the data in Table 4 that the values of the MC»N equilibrium pKa decrease, whereas that of N»MA increase with increasing surfactant concentration.This is consistent with the fact that the polarity in the micelles is much less than that of aqueous phase. In other words, in the micellar phase the equilibrium will shift towards the direction where the neutral species are stable. Hence, high acid concentration is required for the former equilibrium and high base concentration for the latter. It is also clear from the data of Table 4 that the values of at the site at which the prototropic reactions are occurring eeff decrease with increasing surfactant concentration and almost levels oÜ at a surfactant concentration of 0.02 M.This indicates that nearly complete micellization of 2-HPBI is achieved at this concentration and hence the prototropic reactions only Fig. 5 Plot of and of 2-HPBI monocation»neutral *pKam *pKai equilibrium vs. relative permittivity of dioxane»water mixtures. A, B, *pKam; *pKai. Fig. 6 Plot of and of 2-HPBI neutral»monoanion *pKam *pKai equilibrium vs.relative permittivities of dioxane»water mixtures. A, B, *pKam; *pKai. J. Chem. Soc., Faraday T rans., 1998, V ol. 94 143Table 4 Apparent values for the monocation»neutral and neutral»monoanion equilibria of 2-HPBI and the relative permittivities at the pKa binding site of the micelles at diÜerent concentration Brij-35 Brij-58 Brij-78 Brij-99 equilibrium [D]/M pKa e pKa e pKa e pKa e MC»neutral 0.00 5.1 78.0 5.1 78.0 5.1 78.0 5.1 78.0 0.000 05 4.9 70.0 » » » » » » 0.0001 4.8 63.5 5.0 44.0 » » » » 0.0005 4.6 55.0 » » 4.0 42.0 » » 0.001 4.15 45.0 4.02 42.0 3.85 40.0 3.9 40.0 0.01 3.6 35.0 3.4 32.5 3.25 31.0 3.35 32.5 0.02 3.4 33.0 3.15 30.0 3.10 29.5 3.08 29.5 0.05 3.35 32.0 » » » » neutral»MA 0.00 8.9 78.0 8.9 78.0 8.9 78.0 8.9 78.0 0.0005 9.1 67.0 » » 9.05 69.0 » » 0.001 9.45 56.5 9.65 54.5 9.8 40.0 9.7 52.5 0.01 10.45 33.0 10.67 25.0 10.67 25.0 10.6 34.5 0.02 10.5 31.0 10.82 21.0 10.85 20.0 10.8 21.5 occur at one site of the micelles.This is consistent with the high values of and the —uorescence intensity following Ks single exponential decay. The values of observed using the eeff apparent values for the MC»N equilibrium of 2-HPBI pKa are found to be 32^2, with a slight decreasing trend with increasing Brij number, whereas the value of found from eeff the N»MA equilibrium is always less than that obtained from the MC»N equilibrium for the same Brij. The value of eeff\ observed for Brij-35 agrees with the literature11 data; 35 eeff values are not available for the higher Brijs.The values of eeff obtained from the —uorescence band maxima of the tautomer band vs. the relative permittivity plot (Fig. 4) are lower than those obtained from the prototropic reactions. 4 Discussion As stated earlier,5,6 con—icting views are available for the shape, size and geometry of TX-100. If a spherical shape is assumed for TX-100, it is necessary that some of the oxyethylene groups are present in the core of the hydrophobic region of the micelles.This view will allow the outer portion to oÜer more channels for the seepage of water, suggesting the presence of 1.2 Hence for this model a sharp gwater gsurfactant ~1 . boundary does not exist between the hydrophobic interior and the polar oxyethylene chains. On the other hand, if a classical model (the presence of a hydrophobic core, followed by hydrophilic oxyethylene chains) is considered, then the preferred structure of TX-100 micelles ought to be oblate.Based on our results in TX-100,20 diÜerent values of eeff have been predicted, i.e. the —uorescence data for the tautomer band predict the value of to be 16, whereas the values eeff pKa for the MC»N and N»MA equilibria predict to be 35 and eeff 18, respectively. This disagreement between values in eeff TX-100, as determined by diÜerent techniques, can be rationalized as follows. The value determined from the —uores- eeff cence data is obtained from the correlation diagram, drawn between the tautomer —uorescence band maxima and the relative permittivity (Fig. 4). The tautomer species is a neutral molecule and may not have a large dipole moment in the S1 state. In reality the blue-shifts observed in the tautomer —uorescence with the increase of polarity re—ect the decrease in the dipole moment of this species (III) and are supported by theoretical calculations.17 On the other hand, for the N»MA equilibrium, although the monoanion is an ionic species, it has a closed ring structure, V, as shown in Scheme 2.The bond between the O~ and the H of is very strong. This is also Ü åN manifested by the fact that the for the deprotonation pKa reaction of NHwO~ in 2-HPBI is not observed even up to in aqueous medium,14 even though the value H~\16 pKa for the similar deprotonation reaction of in Ü åNH benzimidazole43 is 13.4. Because this charge on the phenoxyphenyl ion might be distributed over the complete molecule, thereby reducing the ionic character, it might resemble the phototautomer and be located inside the non-ionic micelles.On the other hand, in the MC»N equilibrium the ionic species monocation is an open structure IV and hence can not be located inside the micelles but can be present at the interface of the micellar and aqueous phase. In other words, the MC»N equilibrium re—ects the of the interface between the micel- eeff les and water or the more polar region of the micelles, whereas the N»MA equilibrium re—ects the site away from the interface and towards the core of micelle TX-100.Although the Ks values of monocation IV and monoanion V of 2-HPBI have not been determined in TX-100, those determined in the Brijs suggest that the monocation dm3 mol~1) should be (KsB100 more polar than the monoanion (V, dm3 mol~1). KsB150 Two ground state conformers (I and II) are established17 and the values of observed support this. Based on the values of Ks it can be predicted that conformer II, which leads to the Ks formation of monocation IV, is present near the interface, whereas conformer I, which leads to the formation of tautomer III, is located inside the core. The high value of for the knr tautomer band in TX-100 indirectly supports the presence of oxyethylene groups in the core, which can interact with the —uorophore. This is supported by the fact that with the similar hydrocarbon chain-length of CTAB, the value of is much knr less than that in TX-100 because the CTAB micelles do not contain oxyethylene groups.The main diÜerence between TX-100 and the Brijs is the presence of a long hydrocarbon chain, i.e. instead of a p-(1,1,3, 3-tetrabutyl)phenoxy group in TX-100, the minimum chainlength in the Brijs is and that is linear. Thus the length of C12 the hydrophobic core will be much larger (1.67»2.43 nm) than that of TX-100 (1.172 nm).3 Although our results cannot predict the shape, size or geometry of the Brijs, some speculations can be made.Based on the results observed in the Brijs, it can be concluded that the two conformers I and II of 2-HPBI are present in the Brijs. Based on the diÜerent values of observed from the tautomer and normal —uorescence Ks bands, the former is present away from the interface towards the core, as supported by the values for N»MA equi- pKa librium, and the latter is present near the interface, as supported by the value of the MC»N equilibrium.The values pKa of observed for the tautomer band also suggests the pres- knr ence of oxyethylene moieties in the hydrophobic core. The slightly smaller values of in the Brijs, in comparison to knr TX-100, could be due to the presence of oxyethylene groups (not too deep) as might be present in TX-100, because of the longer hydrocarbon chain-length. In other words, the Brijs 144 J. Chem. Soc., Faraday T rans., 1998, V ol. 94might also represent a similar shape and geometry as that observed for TX-100.Lastly, by comparing the results observed in the Brijs and in the Tweens, i.e. Tween-20, -40, -60 and -80, the following diÜerences are observed: (i) for the equal hydrocarbon chainlength in the Brijs and Tweens, the values of predicted for eeff the Tweens, from the —uorescence band maxima of the tautomer and the value of the N»MA equilibrium, are slightly pKa lower than those observed for the Brijs ; (ii) the values of in knr the Tweens are less than those observed in the Brijs ; and (iii) the normal Stokes-shifted —uorescence band is absent in the Tweens.The above points clearly suggest that the interiors of the Tweens are more hydrophobic than those of the Brijs. This suggests that the number of oxyethylene groups present in the core of the Tweens will be less than those present in TX-100 or the Brijs. This could be due to : (i) the highly substituted furan ring in the Tweens, which might oÜer steric hindrance and/or (ii) the presence of an ester group in between the furan and hydrocarbon chain in the Tweens.Conclusion The above study has revealed that : (i) two kinds of conformers are present in 2-HPBI, conformer I is present more towards the core than is conformer II. 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