13500
J. Phys. Chem. 1995,99, 13500-13504
Location of Hydrophobic Solutes in Sodium Dodecyl Sulfate Micelles and the Effect of Added Tetraalkylammonium Counterions on the Structure of the Head-Group Region. Pulse Radiolytic Study on Scavenging of Hydrated Electrons Ewa Szajdzinska-Pietek* and Jerzy L. Gebicki Institute of Applied Radiation Chemistry, Technical University of Eodi, Wrbblewskiego 15, 93-590 Lbdi, Poland Received: February 23, 1995; In Final Form: May 3, 1995’
Pulse radiolysis has been used to examine scavenging of hydrated electrons (e-aq) by 3,3’-dimethylbiphenyl (DMBP) and hexadecylpyridinium chloride (HPC1) in anionic micellar solutions of sodium dodecyl sulfate (NaDS) in the presence of the electrolytes with tetramethyl- and tetrabutylammonium cations (TMA+ and TBA’, respectively) and, for comparison, in NaDS/NaCl solution. The obtained kinetic data for eCaqand the absorption spectra and decay of the radical-anion DMBP- are discussed, taking into account location of the scavengers in micelles and electrostatic and structural effects of added counterions. It is concluded that the DMBP molecule is embedded below the head-group region, while the pyridinium ring of the HPCl molecule (the reactive center) remains anchored on the micellar surface. The hydrated electron does react with the surface-located acceptor, in spite of electrostatic repulsions between e-aqand the aggregates. The rate constant of the reaction, ca. 7 times lower in neat NaDS than in homogeneous aqueous solution, increases versus concentration of the added electrolyte; the effect is much more pronounced in the presence of hydrophobic tetraalkylammonium counterions than in the presence of hydrophilic Na’. Micellized DMBP is not accessible for hydrated electrons in neat NaDS and in NaDS/NaCl solutions at [NaCl] I0.3 M. The reaction becomes possible, however, with addition of TMA+ and TBA+ counterions. The observed specific effect of tetraalkylammonium counterions on the reactivity of e-aq toward micelle-bound scavengers indicates a more disordered structure of the surface region of the aggregates due to intercalation of the hydrophobic cations between surfactant heads, in accord with the structural model of sodium and tetramethylammonium dodecyl sulfate micelles inferred earlier from the studies by ESR and ESEM methods.
Introduction The structure and properties of ionic micelles are known to depend in part on the properties of counterions.’ Hydrophobic counterions are inherently interesting, for instance as charge carriers or quenchers in biomembranes and membrane photochemistry. The earlier study of one of us, performed in Kevan’s group by means of electron spin resonance (ESR) and electron spin echo modulation (ESEM) techniques, revealed that substitution of tetramethylammonium cation for sodium cation in dodecyl sulfate micelles leads to a decrease in the compactness of the polar head-group region and increased surface roughness, resulting in a deeper penetration of H20 molecules into micelles. At the same time the internal micellar structure is affected; alkyl chains are less packed and have more gauche and even bent conformations in the presence of the more hydrophobic counter ion^.*-^ These conclusions were further supported by Hiromitsu and Kevan5 and Berr et aL6 A more recent study of Bonilha et al.’ indicates that the more “wet” and more disordered micelle of tetramethylammonium dodecyl sulfate, in comparison to the corresponding assembly containing the Na+ counterion, is simultaneously a poorer solvent of organic cations added as fluorescence quenchers. The present work is aimed to gain additional evidence on the above counterion effects from pulse radiolytic investigation. Scavenging of hydrated electrons (e-aq, produced radiolytically in the aqueous phase) by hydrophobic acceptors embedded in
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, August 1, 1995.
sodium dodecyl sulfate (NaDS) micelles is followed versus concentration of tetramethyl- and tetrabutylammonium counterions (TMA+ and TBA+, respectively), added as halides or hydroxides, and the results are compared to those obtained for the NaDS/NaCl system and for homogeneous aqueous solution. Two kinds of scavengers are used: hardly soluble in water 3,3’dimethylbiphenyl (DMBP) and well soluble in water, amphiphilic hexadecylpyridinium chloride (HPC1). The scavengers have been studied earlier in cationic micellar and from the kinetic data we have deduced that the DMBP molecule and the pyridinium ring (reactive center) of the HPCl molecule are both located in the head-group region of the micelles. The additional objective of the present work is to examine if e-aq behavior in anionic micellar solutions can also be reconciled with such location of the scavengers. Preliminary results of this study have been reported.”
Experimental Section The following chemicals were used as received: NaDS (99%, Sigma), tetramethylammonium chloride, TMACl (purum, >98%, Fluka), tetrabutylammonium bromide, TBABr (99%, Aldrich), tetrabutylammonium hydroxide, TBAOH (purum, 40% in water, Fluka), NaCl (Wako Pure Chemical Industries), DMBP (>99%, Aldrich). HPCl (purum, >98%, Fluka) was purified by repeated crystallization from spectroscopic grade ethanol. Micellar solutions were prepared in triply distilled water adjusted to pH = 11 with sodium hydroxide. The component concentrations were as follows: 0.1 M NaDS, 10.3 M TMACl or NaCl, 10.15 M TBABr or TBAOH, 4 mM DMBP, and 0.2
0022-365419512099-13500$09.0010 0 1995 American Chemical Society
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or 1 mh4 HPC1. The DMBP concentration was only a few times lower than its solubility limit in 0.1 M NaDS solution, which we estimated spectrophotometrically as 13 mM. Scavenger/ micelle concentration ratios were 2.7 for DMBP and 0.7 or 0.13 for HPCl, as estimated using the aggregation number 62 and the critical micelle concentration, cmc, 8 mM.’” Complementary mesurements were also done for homogeneous aqueous solutions saturated with DMBP, in the absence and in the presence of TBAOH (0.1 M concentration). DMBP was dissolved by magnet stirring of the samples at ca. 50 “C for a few hours, and the nonmicellar solutions were centrifuged to separate an excess of the solute. The DMBP concentration in the saturated aqueous solution was determined spectrophotometrically as 1.5 x M, assuming that the absorption coefficient is the same as in the presence of NaDS. All the solutions were kept overnight before the pulse radiolysis experiments were performed. The linear electron accelerator, ELU-6E, and the equipment for pulse radiolysis with optical detection have been described.’* Electron pulses of 7 ns delivered the dose ca. 10 Gy, which produced ca. 3 pM hydrated electrons. Some experiments for DMBP were done using 17 ns pulses (dose ca 4 times higher), and the obtained rate constants of eCaqdecay were the same, within uncertainty 5 15%, as for the lower dose, although the fitting to the kinetic equation was poorer. The mesurements were performed at room temperature (ca. 23 “C) for samples deaerated by careful bubbling with argon (at least 20 min).
-
Results The decay of hydrated electrons in the presence of DMBP has been followed at the wavelengths 500 and 525 nm, where contribution from the scavenging product, the radical anion DMBP-, is negligible. For HPCl as a scavenger the wavelengths 600 and 650 nm have been used instead (signals more intense and signallnoise ratio higher), since no absorption of the product is observed in this range.8 The decay curves were well described by the pseudo-first-order kinetic equation, as expected for scavenger concentrations much higher than that of e-aq. The kinetic results for DMBP are shown in Figure 1 as the relative pseudo-first-order rate constant, with respect to NaDS solution containing no additional counterions, versus concentration of added cations. The data represent average results obtained from multiple shots in at least two experimental sessions, with the use as co-ions of either Br- or OH- in the
Figure 2. Dependence of the rate constant for e-aq decay in the NaDS micellar solution containing 0.2 mM (+,0)or 1.O mM (0)HPCl versus concentration of added counterions. The error bars, given as an example for one set of the data only, represent standard deviations from multiple measurements.
NaDS/TBA+ system and C1- in the other two systems. The standard deviation does not exceed 15%. We note that in blank solutions, containing no scavenger, the rate of e-aq decay in the presence of the additives was not higher than that in neat NaDS, ensuring that the observed effects are not due to possibly introduced impurities. Thus, the rate constant of electron scavenging evidently increases with addition of more than 0.05 M TMAf and more than 0.025 M TBA+; in the latter case a 2-fold increase is observed at 0.15 M concentration, which is the solubility limit of tetrabutylammonium cations in the examined NaDS solution. For Naf, up to 0.3 M concentration, the counterion effect is not higher than the accuracy of our experiments. Figure 2 presents similar data for HPCl as a scavenger in the NaDS/TBAOH and NaDS/NaCl systems. In this case the influence of added counterions is stronger than that for DMBP. A significant acceleration of e-aq decay is caused even by hydrophilic Naf . For hydrophobic TBA+ the relative rate constant increases linearly in the range of low concentrations of the cation and reaches the value of about 3 at the maximum TBA’ content. It is also seen from Figure 2 that the effect is somewhat more pronounced for the higher concentration of the scavenger. The scavenging product could be conveniently observed for DMBP. The low-wavelength absorption spectra of the DMBPradical-anion in NaDS solutions, in the absence and in the presence of hydrophobic counterions, are shown in Figure 3 and compared to that observed earlier in the cationic micellar solution of dodecyltrimethylmonium bromide (DTABr).8 The absorption maximum shifts from 400 nm in neat NaDS to 405 nm in the NaDS/TMAf system and to 415 nm in the NaDS/ TBA+ and DTABr systems. The intensity of the maximum (normalized to the same dose) is higher in the presence of hydrophobic cations than in neat NaDS (or NaDSmaCl) solution. Complementary experiments for homogeneous aqueous solutions saturated with DMBP (the results not shown in Figure 3) have revealed that in the absence of micelles the absorption maximum of DMBP- occurs at 400 nm and its position is not affected by addition of TBAf cations. Finally, in Figure 4 the decay of DMBP- at the absorption maximum is shown for a NaDYO.1 M TBA’ solution (415 nm) and for a neat NaDS solution (400 nm). It is clearly seen that the radical-anion is more stable in the presence of tetraalkylammonium counterions. It can be noticed from Figure 4 that when the decay of DMBP- is completed, a small long-lived
13502 J. Phys. Chem., Vol. 99, No. 36, 1995
Szajdzinska-Pietek and Gebicki
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absorption remains which may be due either to solvent-derived radicals or to product radicals. These long-lived absorptions were not investigated further in the present study.
Discussion Reactions of hydrated electrons, produced radiolytically in the aqueous phase, with hydrophobic solutes in anionic micellar solutions are slowed down with respect to those in the homogeneous solution, due to solute incorporation into the micelles and electrostatic repulsion between e-aq and negatively charged aggregates. In the extreme case of an acceptor unsoluble in water and embedded deeply in the micellar core, the reaction may be completely inhibited.I3 For the DMBP/NaDS solution (in the absence of tetraalkylammonium counterions) the observed rate of e-aq decay under our experimental conditions was not much higher than that in blank NaDS solution. But formation of the radical-anion DMBP-, clearly seen from Figure 3, indicates that the hydrated electron does react with the scavenger. The apparent secondorder rate constant of the reaction, estimated using the analytical DMBP concentration, is on the order of lo7 M-' s-I. For the homogeneous solution (containing no NaDS) the respective value is 2 orders of magnitude higher, ca. lo9 M-' s-I. Thus, the inhibiting effect of anionic micelles is very strong and may suggest that micellized DMBP is hardly accessible for e-aq. Taking into account that the position of the absorption maximum
of DMBP- in micellar solution is the same as in neat water, we conclude that only a minor fraction of the scavenger, which remains in the bulk aqueous phase, reacts with hydrated electrons. Our kinetic data would indicate that the fraction of nonmicellized scavenger is not higher than O S % , i.e. 5 2 x M, while from the solubility data (see Experimental Section) the value can be estimated as 0.5 x M.I4 For the HPCVNaDS system the apparent second-order rate constant, determined from the data for the two examined scavenger concentrations, is (1.15 f 0.15) x lo9 M-' s-l , only ca. 7 times lower than that reported by Gratzel et al. for homogeneous HPCl solution, below the critical micelle concentration of this amphiphilic compound (0.8 mM).I5 If the micellized HPC1, similarly as DMBP, did not scavenge hydrated electrons, the kinetic data could only be accounted for assuming that as much as ca. 15% of the solute remained free in bulk water. This fraction can hardly be reconciled with the cationic and amphipbilic nature of the scavenger. It is commonly accepted that HPCl becomes comicellized with the host surfactant molecules and its pyridinum ring is anchored in the headgroup layer.8.9,16.'7Since the cmc value of HPCl itself is lower than that of NaDS, it may be anticipated that the amount of nonmicellized scavenger is negligible. Furthermore, the results of the fluorescence quenching study" indicate that in NaDS solution the rate constant for HPCl exit from the aggregate is lower than105 s-], so the probe can be regarded as immobile on the time scale of our experiments, where the observed rate constants (pseudo-first-order) for e-aq decay were above this limit value. We thus conclude that, in spite of electrostatic repulsion of the hydrated electron from micelles, it does react with bound HPC1, although the reaction is inhibited with respect to that in homogeneous solution. To explain the different behavior of the two scavengers in neat NaDS solution, we postulate that the DMBP molecule is located deeper in the micelle than the pyridinium ring of HPC1. We note that in cationic micellar solutions scavenging of e-aq (in this case catalyzed with respect to the homogeneous solution) occurs only 3 times faster in the presence of HPCl than in the presence of DMBP, and the effect has been assigned to higher effectiveness of the former (cationic) scavenger rather than to different locations of the reactive moieties of the solutes in micelles.* Thus, DMBP seems to be buried deeper in anionic , NaDS micelles than in cationic micelles of alkyltrimethylammonium halides. This conclusion would be consistent with the observation that interactions of aromatic solutes with water molecules are usually weaker in the former s y ~ t e m ~ .and ~*-~~ with the ability of complex formation between arenes and quatemary ammonium groups . 2 1 - 2 3 We stress the relevance of this conclusion for all the studies in which aromatic solutes are used as probes of micellar structure and dynamics. The primary effect of an electrolyte addition to the solution of ionic micelles is an increased counterion binding and neutralization of the surface charge. In the examined system of anionic micelles this leads to a decrease of electrostatic repulsions between the aggregates and hydrated electrons, and one can expect higher rates of e-aq scavenging by a micellized solute. We do observe such a kinetic salt effect with NaCl addition to the HPCliNaDS solution, but there is no influence of NaCl on e-aq decay in the DMBP/NaDS solution, cf. Figures 1 and 2. These results are in support of the postulated different locations of the two scavengers in micelles. It is logical that although the salt-induced lowering of the micellar potential speeds up e-aq reaction with the surface located acceptor (pyridinium ring of HPCl), the effect, even at 0.3 M NaCl
Sodium Dodecyl Sulfate Micelles concentration, may be not sufficient to promote electron transfer below the Stem layer, where the DMBP molecule is located. In the NaDSDMBP system the electrolyte effect is detected only for tetraalkylammonium additives. More efficient e-aq scavenging in the presence of hydrophobic cations is suggested by the enhanced rate constant for its decay and by higher absorbance of DMBP-, cf. Figures 1 and 3. Since at the same time the position of the DMBP- absorption maximum shifts toward that observed in cationic micellar systems, we conclude that in NaDS/TMA+ and NaDS/TBA+ solutions, contrary to neat NaDS solution, hydrated electrons are able to react with the scavenger molecules embedded in the micelles. The red shift of the DMBP- absorption maximum, in going from water to the micellar phase, reflects the lower polarity of the product microenvironment in the latter case.24 An increased lifetime of the reaction product, cf. Figure 4, seems consistent with the above conclusion. As reported earlier, the aromatic radical anions formed by scavenging of e-aq most probably decay via hydrolysis, giving second transients which are the hydrogen adducts of the aromatic molecule^.^^ One can expect that the hydrolytic decay of the DMBP- produced in the bulk aqueous phase (in NaDS solution) occurs faster than that of micellized DMBP- (in NaDS/TBA+ solution), the more so that the latter species may be complexed by added quaternary ammonium It should be noted here that the second transient, i.e. the DMBPH' radical, is presumably responsible for the absorption maximum seen in Figure 3 at ca. 325 nm. The significantly lower intensity of this band in the presence of TBAOH, as compared to TBABr, may be reconciled with the higher pH of the solution in the former case and with a possible contribution of other mechanisms of DMBP- decay.25 The observed effect of hydrophobic counterions on e-aq scavenging by DMBP is in accord with the structural models inferred for TMADS and NaDS aggregates from ESR and ESEM Due to intercalation of TMA+ cations between dodecyl sulfate heads, the surface of the TMADS micelle is more rough and more permeable for H20 molecules than that of NaDS micelles. Consequently, it is also more permeable for hydrated electrons, as indicated by the present results. More bulky TBA+ counterions appear to be even stronger spacers of the surfactant heads. In the NaDS/TBA+ system the decay of e-aq is mainly due to reaction with the micellized scavenger, since the position of the DMBP- absorption band is the same as in cationic micellar solution, where hydrated electrons are attracted by the aggregates and the process is catalyzed with respect to that in homogeneous solution. Since the reactive center of the HPCl molecule is located at the micellar surface, we have not expected a pronounced increase of e-aq decay rate due to opening up the head-group region by addition of hydrophobic counterions. However, the experiments reveal that the kinetic effect of TBA+ is much stronger than that of Na+, cf. Figure 2. This could be explained by the higher degree of counterion binding in the former case, although we have to mention that there is no agreement in the literature whether the fraction of neutralized head-groups does increase with substitution of tetraalkylammonium counterions for Na+.6,7.26But even if the micellar charge remains unchanged, the charge density is probably lower in the presence of hydrophobic counterions, as a result of the structural modifications of the head-group region discussed above, and consequently, the electrostatic barrier for e-aq to reach the surface-located acceptor may be lower. It seems, however, that another phenomenon should also be taken into account to explain the results obtained for the NaDS/TBAf/HPC1 system.
J. Phys. Chem., Vol. 99, No. 36, 1995 13503
As suggested by Bonilha et al.,' the partition coefficient of organic cations (used as luminescence quenchers) between micelles and the aqueous phase is lower for tetraalkylammonium dodecyl sulfate than for NaDS. Thus, one can expect that in the examined system there is a competition between TBA+ and the scavenger cations for negative surfactant heads. While in neat NaDS practically all hexadecylpyridinium cations are micellized, with addition of TBA+, part of them likely exit into bulk water, where they scavenge e-aq with the higher rate constant. We note that even at [HPCl] = 1 mM, which is above cmc value for cationic HF'Cl micelles, the apparent second-order rate constant of the reaction in the NaDWO.15 M TBA+ (solubility limit) solution is lower than that for homogeneous HPCl; Le., an equilibrium exists between micellized and free scavenger and there is no aggregation of HPCl itself.
Conclusions From the kinetic data for electron scavenging by hydrophobic solutes in anionic micellar solutions and from the absorption spectra and decay of the reaction product, we have inferred the location of the solutes in micelles and gained support for the previously suggested effect of tetraalkylammonium counterions on the structure of the head-group region. The pyridinium ring of HCPl is anchored in the head-group region of NaDS micelles and does scavenge hydrated electrons, in spite of electrostatic repulsions between e-aq and the anionic aggregates; the rate constant of the reaction increases with electrolyte addition, especially if the electrolyte contains TBA+ cations. The DMBP molecule is buried below the Stem layer and remains unreactive toward e-aq, even if the micellar potential is diminished by addition of NaCl in concentrations up to 0.3 M. However, the micellized DMBP becomes accessible for hydrated electrons in the presence of tetraalkylammonium cations. The observed specific effect of hydrophobic counterions is assigned to their intercalation between surfactant heads, which leads to an increased roughness of the micellar surface.
Acknowledgment. This work has been supported by a grant from the Polish Committee of Scientific Research (KBN project 2 2462 91 02). References and Notes (1) See for example the following papers and references therein: (a) Mukerjee, P.; Mysels, K. J.; Kapauan, P. J. Phys. Chem. 1967, 71, 4166. (b) Almgren, M.; Swamp, S. J. Phys. Chem. 1983, 87, 876. (c) Romsted, L. S.; Yoon Ch.-0. J. Am. Chem. SOC.1993, 115, 989. ( 2 ) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J. Am. Chem. SOC. 1984, 106, 4675. (3) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M.; Coleman, M. J. J. Am. Chem. SOC.1985, 107, 784. (4) Szajdzinska-Pietek, E.; Maldonado, R.; Kevan, L.; Jones, R. R. M. J. Am. Chem. SOC.1985, 107, 6467. (5) Hiromitsu, I.; Kevan, L. J. Phys. Chem. 1986, 90, 3088. (6) Ben, S. S.; Coleman, M. J.; Jones, R. R. M. J. Phys. Chem. 1986, 90, 6492. (7) Bonilha, J. B. S.; Georgetto, R. M. Z.: Tedesco, A. C.; Miola, L.; Whitten, D. G. J. Phys. Chem. 1989, 93, 376. (8) Szajdzinska-Pietek, E.; Gebicki, J. L.; Kroh J. Radiaf. Phys. Chem. 1992, 39, 117. (9) Szajdzinska-Pietek, E.; Lubis, R.; Gebicki, J. L. Radiaf. Phys. Chem. 1992, 40, 541. (10) Szajdzinska-Pietek, E.; Gebicki, J. L.; Kroh J. J. Colloid Interface Sci., accepted for publication. (11) Szajdzinska-Pietek, E.; Gebicki, J. L.; Kroh, J. Pure Appl. Chem. 1993, 65, 1617. (12) (a) Karolczak, S.; Hodyr, K.; tubis, R.; Kroh, J. J. Radioanal. Nucl. Chem. 1986, 101, 177. (b) Karolczak, S.; Hodyr, K.; Polowinski, M. Radiat. Phys. Chem. 1992, 39, 1. (13) Gebicki, J. L.; Szajdzinska-Pietek, E.; Kroh, J. Radiar. Phys. Chem. 1990, 36. 113; and references therein.
13504 J. Phys. Chem., Vol. 99, No. 36, 1995 (14) Although the agreement is reasonable (given the data uncertainty), we cannot definitely exclude a possibility, indicated by a reviewer, that a part of DMBP- is formed in micelles but the species instantaneously (on the time scale of microseconds) exit into the aqueous phase. Such an explanation, however, could hardly be reconciled with the relatively deep location of the DMBP molecule in the aggregate, cf. further discussion. (15) Gratzel, M.; Thomas, J. K.; Patterson, L. K. Chem. Phys. Lett. 1974, 29, 393. (16) Patterson, L. K.; Gratzel, M. J. Phys. Chem. 1975, 79, 956. (17) Malliaris, A,; Boens, N.; Luo, H.; Van Der Auweraer, M.; De Schryver, F. C.; Reekmans, S. Chem. Phys. Lett. 1989, 155, 587. (18) Ghosh, S.; Petrin, M.; Maki, A. H. J. Phys. Chem. 1986, 90,5210. (19) Maldonado, R.; Kevan, L.; Szajdzinska-Pietek, E.; Jones, R. R. M. J. Chem. Phys. 1984, 81, 3958. (20) Rivara-Minten, E.; Baglioni, P.; Kevan, L. J. Phys. Chem. 1988, 92, 2613. (21) Almgren, M.; Grieser, F.; Thomas, J. K. J. Am. Chem. Soc. 1979, 101, 219. (22) Almgren, M.; Medhage, B.; Mukhtar, E. J. Phorochem. Photobiol. A 1991, 59, 323.
Szajdzinska-Pietek and Gebicki (23) Lianos, P.; Viriot, M.-L.; Zana, R. J. Phys. Chem. 1984,88, 1098. (24) In the preliminary report" we have ascribed the red shift of the DMBP- absorption maximum to an increased micropolarity, but the present results for an homogeneous aqueous solution indicate that such an interpretation was incorrect. We note that the UV absorption peak of the DMBP molecule also shifts from 252 nm in pure water to 256 nm in micellar solutions. (25) Fendler, J. H.; Gillis, H. A,; Klassen, N. V. J. Chem. SOC., Faraday Trans. 1 1974, 70, 145. (26) A reviewer has pointed out that not only a better binding of counterions but also a smaller aggregation number would lead to a diminished net charge of the micelle in the NaDS/TBAf system as compared to that in the NaDS/Na+ system. A decrease of NaDS aggregation number with addition of small amounts of tetraalkylammonium salts was reported by Almgren and Swarup.Ib It should be noted, however, that Berr ef aL6 deduced a smaller degree of aggregation and higher net charge of TMADS micelles in comparison to NaDS micelles. JP9505284
