Counterion-Induced Transformations of Cationic Surfactant Micelles

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Counterion-Induced Transformations of Cationic Surfactant Micelles Studied by Using the Displacing Effect of Solvatochromic Pyridinium N-Phenolate Betaine Dyes Nikolay O. Mchedlov-Petrossyan,*,† Natalya A. Vodolazkaya,† Anna A. Kornienko,† Eleonora L. Karyakina,‡ and Christian Reichardt§ Department of Physical Chemistry, V.N. Karazin Kharkov National University, 61077 Kharkov, Ukraine, Ukrainian State Research Institute of Refractories, 61024 Kharkov, Ukraine, and Department of Chemistry, Philipps University, D-35032 Marburg, Germany Received December 16, 2004 In this paper, we demonstrate that the behavior of a set of eight large-sized negatively solvatochromic pyridinium N-phenolate betaine dyes reflects the principle transformations, occurring in aqueous micellar solutions of three cationic surfactants. As surfactants, cetyltrimethylammonium bromide (CTAB), n-octadecyltrimethylammonium chloride (OTAC), and N-cetylpyridinium bromide (CPB) were used. Normally, for such probes coupled with micelles, a red shift of the vis absorption band is expected as a result of a hydrophobization (“drying”) of the micellar interface. However, under addition of electrolytes with anions such as tosylate, salicylate, and some n-alkanesulfonates or n-alkanecarboxylates to the micellar solutions, an unexpected effect was observed. Instead of a red shift, a blue shift of the vis absorption band of some of the dissolved betaine dyes was registered, as compared with the spectrum measured in pure aqueous micellar solutions of CTAB, OTAC, or CPB (∆λmax up to ca. 80 nm). This blue shift, indicating an increase in the polarity of the dye microenvironment, is explained by displacing the large dye dipoles from the thinned micelles toward the aqueous phase. The effect is well expressed at concentrations of C(betaine dye) ≈ 10-5 M, C(cationic surfactant) ≈ 0.001 M, and C(organic anion) ≈ 0.01 M. Transmission electron microscopy of dried samples confirms the distinct changes occurring in the studied micellar systems upon the addition of organic anions. The excess of inorganic salts [C(NaBr, KBr, or KCl) ) 0.5-4.0 M] restored the position of the vis absorption band or even shifted it toward the red. Moreover, some of the betaine dyes studied (i.e., the more hydrophobic ones) stay in the micellar pseudophase or precipitate under the aforementioned concentration conditions. The peculiarities of the behavior of these betaine dyes are in agreement with their molecular structure.

1. Introduction Micelles of ionic colloidal surfactants are of great interest for chemical kinetics, photochemistry, analytical chemistry, chromatography, and nanotechnology as useful media for versatile physicochemical processes and chemical reactions. Therefore, further studies of the properties of surfactant micelles and the ways of governing their structure are of importance. The properties of micellar pseudophases can be varied through modification of the self-assembled surfactant solutions by various additives, which influence the balance between hydrophobic and hydrophilic interactions. One of the useful tools for examination of the properties of modified micelles is the application of solvatochromic probes.1 The largest range of solvatochromism exhibits the so-called pyridinium N-phenolate betaine dyes,2 and, therefore, they are often used for such a purpose.3 Proceeding from NMR data, most of the researchers assume a proximity of the phenolate moiety of pyridinium N-phenolate betaines to the quaternary nitrogen atom of * To whom correspondence should be addressed. E-mail addresses: [email protected] (N.O.M.-P.); [email protected] (C.R.). † V.N. Karazin Kharkov National University. ‡ Ukrainian State Research Institute of Refractories. § Philipps University. (1) (a) Helburn, R.; Dijiba, Y.; Mansour, G.; Maxka, J. Langmuir 1998, 14, 7147. (b) Banerjee, D.; Das, P. K.; Mondal, S.; Ghosh, S.; Bagchi, S. J. Photochem. Photobiol., A 1996, 98, 183. (c) Mishra, A.; Behera, P. K.; Behera, R. K.; Mishra, B. K.; Behera, G. B. J. Photochem. Photobiol., A 1998, 116, 79.

the cationic surfactant within the “normal” micelles of colloidal surfactants.3b,i,j The addition of excess amounts of inorganic salts, particularly salts with organic anions (e.g., salicylate, tosylate, and some others) to aqueous micellar solutions of cationic surfactants is well-known to result in sphere f rod transitions of the micelles. These facts have been well-documented by numerous experimental methods, namely, by electron microscopy,4 viscosimetry,4,5 NMR,6 (2) (a) Reichardt, C. Kharkov University Bulletin 1999, 437 (Chemistry, Issue 3; 26), 9; Chem. Abstr. 2000, 132, 180091f. (b) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry, 3rd ed.; WileyVCH: Weinheim, 2003; Chapter 6, pp 332-334, and Chapter 7, pp 416-429. (3) (a) Zachariasse, K.; Van Phuc, N.; Kozankiewicz, B. J. Phys. Chem. 1981, 85, 2576. (b) Plieninger, P.; Baumga¨rtel, H. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 161; Justus Liebigs Ann. Chem. 1983, 860. (c) Drummond, C. J.; Grieser, F.; Healy, T. W. Faraday Discuss. Chem. Soc. 1986, 95. (d) Kessler, M. A.; Wolfbeis, O. S. Synthesis 1988, 635; Chem. Phys. Lipids 1989, 50, 51. (e) Kriwanek, J.; Miller, R. Colloids Surf., A 1995, 105, 233. (f) Mchedlov-Petrossyan, N. O.; Plichko, A. V.; Shumakher; A. S. Chem. Phys. Rep. 1996, 15, 1661. (g) Vitha, M. F.; Carr, P. W. J. Phys. Chem. B 1998, 102, 1888. (h) Seeboth, A.; Kriwanek, J.; Vetter, R. J. Mater. Chem. 1999, 9, 2277. (i) Novaki, L. P.; El Seoud, O. A. Phys. Chem. Chem. Phys. 1999, 1, 1957; Langmuir 2000, 16, 35 and references cited therein. (j) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. J. Phys. Org. Chem. 2000, 13, 679. (k) Fuguet, E.; Ra´fols, C.; Bosch, E.; Rose´s, M. Langmuir 2003, 19, 55. (l) Mchedlov-Petrossyan, N. O.; Vodolazkaya, N. A.; Timiy, A. V.; Gluzman, E. M.; Alekseeva, V. I.; Savvina, L. P. http://preprint.chemweb.com/physchem/0307002; Chem. Abstr. 2003, 139, 524451. (m) Vodolazkaya, N. A.; MchedlovPetrossyan, N. O.; Heckenkemper, G.; Reichardt, C. J. Mol. Liq. 2003, 107, 221. (4) Wolff, T.; Emming, C.-S.; von Bunau, G.; Zierold, K. Colloid Polym. Sci. 1992, 270, 822.

10.1021/la0401361 CCC: $30.25 © 2005 American Chemical Society Published on Web 07/02/2005

Micelle Transformation by Dye Displacing Effect Chart 1. Molecular Structure of Solvatochromic Pyridinium N-Phenolate Betaine Dyes

and electron spin resonance spectroscopy.7 In the case of n-dodecylpyridinium chloride (or bromide), the catalytic properties of cationic micelles decrease markedly along with the increasing addition of sodium hydrosalicylate.7 It was shown that salicylate, tosylate, and other more hydrophobic anions exchange Cl- or Br- ions in the Stern layer of micelles.8 As a rule, such micellar conversions lead to a weakening of the micellar surface hydration. If this is indeed the case, then such a “drying” effect should be confirmed using solvatochromic probes. For pyridinium N-phenolate betaine dyes with a negative solvatochromism, the dehydration (hydrophobization) of the micellar surface should have led to a red shift of the longwavelength intramolecular charge transfer (CT) vis absorption band. However, instead of a red shift, an unexpected blue shift of the vis absorption band of the standard betaine dye 1, 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate (Chart 1), was surprisingly registered by us in numerous experiments with dye 1 in a variety of aqueous micellar solutions of cationic surfactants (C ) 0.001 M) under addition of a 10-fold excess of organic anions.9 The largest band shift (∆λmax ≈ 75 nm) was observed in the case of tosylate and the anions H2n+1Cn-SO3- (n ) 6-10). The pH value of the micellar solutions was kept near 12, (5) (a) Barnes, H. A.; Eastwood, A. R.; Yates, B. Rheol. Acta 1975, 14, 53. (b) Heidl, A.; Strnad, J.; Kohler, H.-H. J. Phys. Chem. 1993, 97, 742. (c) Hartmann, V.; Cressely, R. Colloids Surf., A 1997, 121, 151. (6) Bachofer, S. J.; Simonis, U.; Nowicki T. A. J. Phys. Chem. 1991, 95, 480. (7) Kudryavtsev, D. P.; Zakharova, L. Ya. Surfactants. Synthesis, Properties, Application; Tver State University: Tver, Russia, 2001; pp 43-48. (8) (a) Jansson, M.; Jonsson, B. J. Phys. Chem. 1989, 93, 1451. (b) Imae, T.; Kohsaka, T. J. Phys. Chem. 1992, 96, 10030. (c) Cassidy, M. A.; Warr, G. J. Phys. Chem. 1996, 100, 3237. (d) Magid, L. J. J. Phys. Chem. B 1998, 102, 4064. (e) Buwalda, R. T.; Stuart, M. C. A.; Engberts, J. B. F. N. Langmuir 2000, 16, 6780. (f) Aswal, V. K. J. Phys. Chem. B 2003, 107, 13323. (9) (a) Timiy, A. V.; Mchedlov-Petrossyan, N. O.; Glaskova, E. N.; Pinchukova, N. A.; Zhyvotchenko, O. E. Kharkov University Bulletin 1998, 420 (Chemistry, Issue 2), 235; Chem. Abstr. 2000, 132, 98516g; (b) Mchedlov-Petrossyan, N. O.; Vodolazkaya, N. A.; Reichardt, C. Colloids Surf., A 2002, 205, 215 and references therein.

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to avoid the conversion of the zwitterionic betaine dyes into colorless cations by protonation of the phenolate oxygen atom. Variations in the concentration of surfactants and organic anions, especially their increase, led to the disappearance of the “strange” band in the region of λ ) 470 nm and to restoration of the band with λmax ≈ 540 nm. We explain these unusual results found in aqueous micellar systems with concentrations of cetyltrimethylammonium (or N-cetylpyridinium) and organic anions near C ) 0.001 M and C ) 0.01 M, respectively (“critical zone” of concentrations), by a dislocation of the zwitterionic pyridinium N-phenolate molecules toward outlying areas of the rodlike (or even wormlike) micelles;9 in pure water, the λmax value of betaine dye 1 equals 453 nm.2,9 We studied aqueous micellar solutions of cetyltrimethylammonium bromide (CTAB) and N-cetylpyridinium bromide (CPB), modified by addition of altogether 24 salts with various organic anions. The betaine dye concentrations were C ≈ 5 × 10-5 M.9b Some of these anions give rise to the aforementioned blue shift. In other cases this band shift was not so pronounced (∆λmax ≈ 10-15 nm) or the blue shift was not observed at all. Obviously, the internal structure of the dye + micelle complex is modified in various manners by different organic anions penetrating the assembly. Three pyridinium N-phenolate betaine dyes of different hydrophobicities (i.e., 1, 3, and 9) were used as negatively solvatochromic indicators. Only the relatively hydrophilic dye 3 is incompletely bound by CTAB micelles at a surfactant concentration of C ) 0.001 M, while the hydrophobic dye 9 precipitates from aqueous 0.001 M CTAB solutions on the addition of salicylate, tosylate, and other salts. The above results invite further investigation to obtain a better insight into the properties of surfactant micelles. The aim of the present study was to clarify to what extent the mentioned effects are of general character for solvatochromic pyridinium N-phenolate betaine dyes of different molecular structures and for different cationic surfactants. In this work, we examined a set of nine betaines presented in Chart 1. It should be pointed out that these dye molecules are rather large-sized. For example, the total van der Waals volume of standard dye 1, calculated using the scale given in ref 10), equals 0.83 nm3. According to ref 11 (AM 1 calculations), this molecule fits into a cage of 1.26 × 1.01 × 0.81 nm3 ) 1.03 nm3. However, the volume of a spherical CTAB micelle is typically about 70 times larger. In this study, we used the cationic surfactants CTAB, CPB, and n-octadecyltrimethylammonium chloride (OTAC) and the following organic anions (mostly as sodium salts): tosylate (Tos-, H7C7-SO3-), salicylate (HSal-), H5C6-CO2- (benzoate, Benz-), 4-nitrobenzoate (4-NO2Benz-), 2-sulfobenzoate (2-SO3Benz2-), n-heptanoate n-H13C6-CO2-, and the alkanesulfonates n-H11C5-SO3-, n-H13C6-SO3-, n-H17C8-SO3-, and n-H21C10-SO3-. The influence of inorganic ions such as hydroxide, silicate, chloride, and bromide was examined too. We studied by UV/vis spectroscopy the properties of aqueous micellar systems of cationic surfactants, modified by the addition of organic anions, by using the negatively solvatochromic pyridinium N-phenolate betaines 1-8 as indicator dyes, mainly within the “critical zone” of concentration, where the concentrations of surfactants and organic anions are about 0.001 and 0.01 M, respectively. In total, 127 micellar systems were studied. (10) Ershov, V. V.; Nikiforov, G. A.; Volodkin, A. A. Sterically Hindered Phenols; Khimiya: Moscow, 1972. (11) Mente, S. R.; Maroncelli, M. J. Phys. Chem. B 1999, 103, 7704.

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2. Experimental Section 2.1. Chemicals. CTAB (Merk, purity 99%), OTAC (Fluka, purity 98%), and CPB (Minkhimprom, USSR) were used as commercially obtained. 4-Toluenesulfonic acid was purified with trichloromethane according to a procedure recommended by Perron.12 Sodium n-alkanesulfonates (chromatographically pure) and ammonium 2-sulfobenzoate, 4-nitrobenzoic and n-heptanoic acid (analytical grade) were used without further purification. Sodium salicylate and benzoate were purified by recrystallization. Potassium and sodium bromide, as well as sodium silicate, were of analytical grade. The betaine dyes 4-(2,4,6-triphenylpyridinium-1-yl)-2,6-diphenylphenolate (× ca. 2H2O; 1); sodium {4[4-(4-carboxylatophenyl)-2,6-diphenylpyridinium-1-yl]-2,6-diphenylphenolate} (2); 4-(2,4,6-triphenylpyridinium-1-yl)phenolate (× ca. 6-12H2O; 3); 2,6-dichloro-4-(2,4,6-triphenylpyridinium1-yl)phenolate (4); 2,6-di-tert-butyl-4-(2,4,6-triphenylpyridinium1-yl)phenolate (5); 4-{2,4,6-tri[4-(trifluoromethyl)phenyl]pyridinium-1-yl}-2,6-di[4-(trifluoromethyl)phenyl]phenolate (6); 4-{2,4,6-tri[4-tridecafluorohex-1-yl)phenyl]pyridinium-1-yl}-2,6diphenylphenolate (× ca. 1H2O; 8); 4-[2,4,6-tri(4-tert-butylphenyl)pyridinium-1-yl]-2,6-di(4-tert-butylphenyl)phenolate (× ca. 3H2O; 9) were synthesized and purified as described earlier.3d,13 Betaine dye 7 was prepared in situ by deprotonation of 2,4,6triphenyl-N-[3,5-nonamethylene-4-hydroxyphenyl]pyridinium perchlorate. In solutions at pH 12, all betaine dyes exist exclusively in the zwitterionic pyridinium-N-phenolate form. Ethanol (95.6 mass %) of high quality was purified by distillation. Methanol was purified using standard procedures. The aqueous NaOH stock solution, prepared with CO2-free water, was kept protected from carbon dioxide. 2.2. Procedure. Stock solutions of dyes 1 and 5-8 were prepared with ethanol (95.6 mass %) as the solvent. Some experiments were also carried out with a stock solution of these dyes in methanol as well as in aqueous micellar CTAB solutions. In all cases, the final volume fractions of alcohols in the working solutions did not exceed 2%; the alterations of the properties of micellar surfaces of cationic surfactants caused by such admixtures are known to be negligible.3f Stock solutions of dyes 2-4 were prepared with water as the solvent. Dye 8 is only sparingly soluble even in 0.1 M cationic surfactant solutions. We managed to prepare the required solution by introducing a concentrated ethanolic solution of dye 8 in micellar CTAB solution and separating the formed precipitate by filtration. The λmax values of dyes 1-8 were measured in solutions with pH ≈ 12 (adjusted with aqueous NaOH) to ensure their presence in the colored zwitterionic form (in acidic media, all betaine dyes are reversibly protonated and the solvatochromic CT band disappears). Electrolytes containing organic anions were added to the aqueous solutions mainly as the corresponding sodium salts. In the cases of tosylate, 2-sulfobenzoate, 4-nitrobenzoate, and n-heptanoate, the corresponding acids were neutralized with an aqueous NaOH solution. The UV/vis absorption spectra were measured using a SP-46 spectrophotometer. The betaine dye concentrations were as a rule C ) 5 × 10-5 M, the path length of the cell was 5 cm. The λmax values of dyes 1-8 were determined as described in refs 9 and 13a,b; their reproducibility was ((1-2) nm. All solutions were prepared and measured at 25 °C, except the OTAC micellar solutions. For this surfactant, the experiments were made at 30 °C corresponding to the Krafft temperatures of this surfactant. For the transmission electron microscopy studies, an EMV 100 AK apparatus, operating at 75 kV, was used. These measurements were carried out by using the “suspension” method; that is, a drop of the micellar solution was placed on a structureless nitrocellulose support. After air-drying, the specimens were placed into the apparatus (vacuum 3 × 10-5 Torr) and examined.

3. Results and Discussion 3.1. Water-Soluble Probes: Betaine Dyes 2-4. In aqueous micellar solutions of cationic surfactants, a (12) Perron, R. Bull. Soc. Chim. Fr. 1952, 966. (13) (a) Dimroth, K.; Reichardt, C.; Siepmann, Th.; Bohlmann, F. Justus Liebigs Ann. Chem. 1963, 1. (b) Reichardt, C.; Harbusch-Go¨rnert, E. Justus Liebigs Ann. Chem. 1983, 721. (c) Osterby, B. R.; McKelvey, R. D. J. Chem. Educ. 1996, 73, 260. (d) Libor, T. Diploma Thesis, University of Marburg, Marburg, Germany, 1995.

Figure 1. Vis absorption spectra of betaine dye 2, measured in aqueous solutions at pH ) 12 (NaOH) and 25 °C: 1, in pure water; 2, 0.01 M n-C10H21SO3Na added; 3, 0.001 M CTAB added; and 4, 0.001 M CTAB + 0.01 M n-C10H21SO3Na added.

hypsochromic shift of the CT band, induced by adding Tos-, HSal-, Benz-, n-H13C6-CO2-, n-H11C5-SO3-, n-H13C6-SO3-, and n-H21C10-SO3-, was registered for betaine dyes 2-4, as found earlier for standard dye 1.9b The vis spectra are typified in Figure 1. In the “displaced” state, λmax ) 463 nm for dye 2, which is equal to the λmax value found for pure aqueous dye solutions. The absorption maximum of dye 2 in micellar CTAB solutions with no organic anions is 541 nm. Though dye 2 is certainly more hydrophilic as compared with standard dye 1, it is fixed at the micellar pseudophase due to electrostatic attraction. Note that in the aqueous system (0.001 M CTAB + 0.01 M n-H11C5-SO3-), as well as in other systems where the displacing effect is registered, the micelles retain their positive charge. This is demonstrated by both electrophoretic studies and determination of the apparent dissociation constants of different acidbase indicators coupled with micelles.9b On the other hand, the response of the vis absorption spectra of the same dye 2 on the alterations in micellar shape and size agrees with the well-known effects3a,c observed for standard dye 1. For instance, the red shifts of the CT band of the dye 2 are registered after both an increase in the CTAB concentration from 0.001 to 0.1 M (∆λmax ) 9 nm) and adding inorganic electrolytes such as 0.01 M NaBr (∆λmax ) 6 nm) or 4 M KCl (∆λmax ) 33 nm). These data are presented in Figure 2 and in Table 1. Hence, our approach distinguishes between anisometrical micelles being formed in concentrated surfactant solutions (or in the presence of inorganic electrolytes) and those appearing after addition of tosylate, salicylate, and so forth. Electron microscopy (see subsection 3.4.4) confirms these findings. Dye probes 3 and 4 are completely bound by the micelles only if the cationic surfactant concentration reaches C ) 0.01 M. In this event, the expected bathochromic shift of the CT band is fixed in a “bound” state as compared to the “aqueous” λmax value, in line with literary data.3b,d Therefore, the “displacing” impact of Tos- and other anions is difficult to observe. Indeed, in the “critical” concentration region (ca. 0.001 M of surfactant) a substantial fraction of the probe dipoles are located in the bulk (aqueous) phase, even without addition of organic anions. Hence, the λmax values of dye 3 in water and in 0.001 M solutions of CTAB or CPB are equal to 410 and 416 nm, respectively. The

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(dye 5) and 28 nm (dye 6) as compared with the λmax value of standard dye 1 in the same systems. A comparison of the λmax values of different betaine dyes can be made using the linear correlations between their molar electronic transition energies, ET (kcal/mol) ) hcν˜ NA ) 28 591/λmax (nm), measured in solvents of different polarities.2 The ET(1) values of standard dye 1 are also known as the empirical ET(30) parameter of solvent polarity (the formula number of dye 1 corresponds to no. 30 in ref 13a). For betaine dyes 6 and 1, the following linear correlation is valid for n ) 26 solvents of different polarities:14

ET(6) ) 1.38 + 0.961ET(1)

(1)

(n ) 26, correlation coefficient r ) 0.995)

Figure 2. Vis absorption spectra of betaine dye 2, measured in aqueous solution at pH ) 12 (NaOH) and 25 °C: 1, 0.1 M CTAB added and 2, 0.001 M CTAB + 4 M KCl. Table 1. Vis Absorption Maxima of Betaine Dye 2,a Measured in Aqueous Micellar Solutions of CTAB or CPB (0.001 M) with Various Organic Anions X- (0.01 M) at pH ) 12 (NaOH) and 25 °C λmax, nm

b

organic anion X-

CTAB

no addition Brno additionb Cl- c H7C7-SO3H21C10-SO3H13C6-SO3HSalH13C6-CO2H11C5-SO3H5C6-CO2-O S-H C -CO 3 5 6 2

541 547 550 574 463 463 470 471 473 487 534 543

In aqueous 0.001 M CTAB solutions, λmax ) 534 nm for probe 1, hence, for probe 6 eq 1 leads to a calculated value of λmax ) 541 nm. The experimental value of λmax ) 568 nm for 6 indicates a less polar (more dehydrated) microenvironment surrounding dye 6 as compared with that of dye 1, probably due to a deeper penetration of molecules of 6 into the pseudophase. The situation with dye 5 is less clear. The corresponding correlation obtained from vis spectral data measured in n ) 47 solvents of different polarities

ET(5) ) 4.940 + 0.781ET(1)

(2)

(n ) 47, r ) 0.943) CPB

543 546

463 turbidity 484 478

541

a λ max value in pure water ) 463 nm (ref 13c, λmax ) 467 nm). 0.1 M CTAB. c 4 M KCl.

latter value decreases to λmax ) 412 nm under addition of 0.01 M n-pentane-, n-hexane-, or n-decanesulfonate. In the case of dye 4, λmax ) 409 nm in water and 421 nm in a 0.001 M CTAB solution. Under addition of 0.01 M Tos-, Benz-, and n-hexanesulfonate to the surfactant solution, λmax drops to 410 nm, while the addition of HSal- and 2-SO3Benz2- causes a band shift up to λmax ) 413-415 nm. Under conditions approaching complete binding, the λmax value of dye 3 equals 460-463 nm (0.01-0.22 M of cationic surfactant), and that of dye 4 is 458 nm (0.01 M of CTAB). 3.2. Probes Precipitating within the “Critical Zone” of Concentrations: Betaine Dyes 5 and 6. The phenolate group of the probe 5 is well-protected from hydrogen bonding with hydrogen-bond donor () HBD) solvents because of the two bulky tert-butyl groups in 2,6positions, while the hydrophobicity of probe 6 is essentially higher as compared with that of betaine 1. The λmax values of dyes 5 and 6 in aqueous micellar CTAB (or CPB) solutions are equal to 600 and 568 nm, respectively. That is, the bands are shifted toward the red by ∆λmax ≈ 60 nm

leads for 5 to an expected λmax value of 611 nm, while utilization of the data either for 16 non-HBD solvents or for 17 HBD solvents results in λmax values of 634 and 583 nm, respectively.13d Thus, if we regard the micelle/water interface as a kind of HBD solvent, then the experimental value of λmax ) 600 nm for 5 is 17 nm larger than the expected one, and probe 5 must be considered to exist in a less polar (less hydrated, more dried) surrounding than standard probe 1. The “displacing” effect in the vis spectra of dye 5 was not found in the aqueous CTAB (or CPB) solutions with H7C7-SO3-, n-H13C6-SO3-, n-H17C8-SO3-, 2-SO3Benz2-, Benz-, HSal-, or SiO32- anions. In all cases, dye 5 is pushed out of the modified micelles into the aqueous phase and precipitates in the form of highly dispersed particles, because its solubility in water is very small. An analogous picture was observed with dye 6 and earlier with dye 9.9b Only in the aqueous system probe 5 + CTAB + 2-SO3Benz2- were we able to measure a λmax value of 582 nm. Hence, the modification of cationic micelles with organic anions results only in a small blue shift of ∆λmax ) 18 nm of the solvatochromic CT absorption band of betaine dye 5. 3.3. Probes with Poorly Expressed Vis Spectral Effects: Betaine Dyes 7 and 8. Betaine dye 8, only sparingly soluble in micellar solutions, was studied only in two systems: in aqueous 0.001 M CTAB solution with λmax ) 654 nm and in the same surfactant solution containing 0.01 M Tos- with λmax ) 652 nm. The absence of a “displacing” effect can be explained by the peculiar molecular structure of dye 8: the C6F13 groups of dye 8 cause a deep penetration into the micelles. Such location cannot be influenced even by an alteration of the micelles’ size and shape. For betaine dye 7 a poorly expressed “displacing” effect was observed only in the case of cetyltrimethylammonium tosylate micelles: in aqueous (14) Calculated by Yu. V. Isaenko from the data of Reichardt, C.; Eschner, M.; Scha¨fer, G. J. Phys. Org. Chem. 2001, 14, 737.

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0.001 M CTAB solution, λmax ) 543 nm, and in aqueous CTAB (0.001 M) + H7H7SO3- (0.01 M) solution, λmax ) 534 nm. Thus, the observed blue shift is, with ∆λmax ) 543534 ) 9 nm, rather small. A decrease in the Tosconcentration (0.005 M, 0.001 M) leads to a restoration of the λmax value at 543 nm. In other cases, that is, after addition of 0.01 M n-hexanesulfonate, 2-SO3Benz2-, Benz-, or HSal- to aqueous 0.001 M CTAB solutions, the CT band of dye 7 stays unchanged at λmax ) 541-545 nm. Again, the peculiar molecular structure of 7 is of importance: a displacing of dye 7 toward the more hydrophilic (hydrated) peripheral micellar region is hindered by the hydrophobic loop, which is certainly located in the region of the surfactant’s hydrocarbon chains and, thus, serves as an “anchor”. 3.4. Specificity of Anions and the Nature of Micellar Transformations. 3.4.1. Classification of Anions. Based on the results of the present study and of previous experiments,9b we propose the following classification of anions. According to their ability to cause a blue shift of the solvatochromic CT band of betaine dyes dissolved in aqueous 0.001 M solutions of cationic surfactants, the anions studied can be divided into three groups: (i) anions Tos- and n-H2n+1Cn-SO3- (n ) 6-10), with blue shifts up to ∆λmax ) 70-80 nm against the band position in pure aqueous cationic surfactant solutions; (ii) anions n-H11C5-SO3-, n-H13C6-CO2-, Cl3C-CO2-, F3CCO2-, HSal-, Benz-, 2-ClBenz-, 2-SO3Benz2-, and 2phthalate2- which cause only modest but still blue shifts with ∆λmax ≈ 15 nm; (iii) eventually, anions such as acetate, 5,5-diethylbarbiturate-, 4-NO2phenolate-, and sulfanilic acid anion which display no band shift at all; the nitrobenzoate anion induces even a red shift. The λmax values of betaine dye 2 are given in Table 1 as a representative set from a much larger body of data. The similarity among the anions’ influence, detected by using different betaine probe dyes, distinctly reflects some peculiarities of the corresponding modified surfactant micelles; numerous data describing such peculiarities are available in the literature.6-8 For anions of group ii, the effect of “displacing” seems to be intermediate between “normally bound” and “strongly displaced” states. Anions of group iii certainly cause some alterations in the micellar structure as well, however, of that kind which does not cause a displacement of the solvatochromic dye dipoles toward the more hydrated region. Evidently, the molecular structure and the properties of the organic counterions determine the peculiarities of the observed micellar transformations. Further detailed information is available in the literature concerning the structure of cationic surfactant micelles in the presence of carboxylates (H2n+1Cn-CO2-, up to n ) 5),8a 5-ethylsalicylates,8e and hydroxybenzoates.6,7,8b,c,f Our division of organic anions into three groups is only based on their influence on the cationic micelles within the “critical zone” of concentrations, that is, C(cationic surfactant) ) 0.001 M of and C(anion) ) 0.01 M. For anions of group i, the “displacing” effect is probably caused by an enlargement of the micelles, which, at low concentrations of surfactant (0.001 M), results in a sharp decrease in the numerical micelle concentration. The ratio of dye:micelle becomes g10, thus, hindering a complete binding of the dye. Simultaneously, the conversion of spherical micelles into rod- or even wormlike ones makes them less suitable for the binding of such large-sized molecules as the betaine dipoles. At higher concentrations, the number of modified micelles is fairly sufficient for binding the dyes and the effects under discussion are undetectable. But the changes in the micelles’ size and shape in this concentration range

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are proved by other methods, for instance, by light scattering8b,d and small-angle neutron scattering.8f In the aqueous micellar CTAB + Tos- system, with C(CTAB) ) 0.022 M and C(anion) ) 0.025 M, λmax ) 548 nm,9a and at C(CTAB) ) 0.044 M and C(Tos-) ) 0.05 M, a gelation of the system takes place and the solution does not run out after turning over the flask, in agreement with the high viscosity of this system reported in the literature.5a 3.4.2. Influence of 4-Nitrobenzoate and Silicate Ions. Some additional experiments were performed with probe 1 in aqueous CTAB micellar solutions by addition of 4-NO2Benz- and SiO32- ions. In a 0.001 M CTAB solution, the CT band of dye 1 shifts markedly toward the red after addition of 0.01 M of 4-NO2Benz- ions (∆λmax ) 19 nm), probably due to the relatively high hydrophobicity of the surface of cetyltrimethylammonium 4-nitrobenzoate micelles. An analogous effect was registered by us earlier after the addition of 3-NO2Benz- ions.9b The addition of silicate salts to micellar solutions of cationic surfactants is known to result in alterations in the micelles’ size and shape, and, consequently, in the viscosity of micellar systems.15 We examined the solvation properties of aqueous CTAB micellar solutions with SiO32additives, within a concentration range of C(CTAB) ) 0.001-0.022 M and C(SiO3-) ) 0.001-0.025 M, using standard probe 1. Probably, cetyltrimethylammonium silicate micelles are formed. In the “critical zone” of surfactant (C ) 0.001 M) and anion (C ) 0.01 M) concentrations the solution becomes turbid. In aqueous 0.001 M CTAB solutions with 0.001 and 0.005 M SiO32the λmax values are 523 and 535 nm, respectively. At high concentrations of CTAB and SiO32- (C ≈ 0.02 M), the λmax value is near 540 nm. So, a “displacing” effect in the CTAB + silicate system is not observed. 3.4.3. Lengthening of the Cationic Surfactant Tail from C16 to C18. The properties of micelles of another cationic surfactant, OTAC, without and with addition of organic counterions, were studied at 30 °C, using standard probe 1, within the “critical zone” of concentrations. The λmax value of betaine dye 1 in aqueous OTAC micellar solution (C ) 0.001 M) equals 535 nm. In the presence of 0.01 M Tos- or n-H13C6-SO3-, λmax ) 470 nm (in pure water 453 nm), with HSal- and Benz-, λmax ) 500-507 nm, and with 2-SO3Benz2-, λmax ) 540 nm. Thus, the “displacing effects” are approximately the same as those found in the CTAB systems, modified with different organic anions. In the case of aqueous micellar n-octadecyltrimethylammonium systems, the λmax values are somewhat higher. 3.4.4. Electron Microscopy. Disturbing Effect of Betaine Dyes upon Surfactant Micelles. Typical electron micrographs are presented in Figure 3. During evaporation of the solvent, the initial CTAB + water system passes through the stage of a concentrated solution, where rodlike and wormlike micelles certainly appear. However, a comparison of Parts a and b of Figure 3 demonstrates principal differences between the pure CTAB system and the CTAB + organic counterion system. Note that in the presence of dye 1 the picture is also somewhat different (Figure 3c), reflecting the alterations in micellar structure caused by the large dye dipole. In general, the application of acid-base indicators or solvatochromic dyes for the examination of liquid systems is connected with a more or less pronounced disturbance of the systems under study. In common solutions, the influence of small amounts of an added acid-base indicator (15) Iler, R. K. The Chemistry of Silica; Wiley: New York, 1979; Vols. 1, 2.

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Figure 4. Vis absorption spectra of betaine dye 1, measured in aqueous solution at pH ) 12 (NaOH) and 25 °C: 1, in pure water, λmax ) 457 nm (ref 13b, λmax ) 453 nm); 2, 0.001 M CTAB added, λmax ) 534 nm; 3, 0.001 M CTAB + 0.01 M n-C6H13SO3Na added, λmax ) 474 nm; 4, 0.01 M n-C6H13SO3Na added, λmax ) 458 nm; 5, 0.001 M CTAB + 0.01 M n-C6H13SO3Na + 0.5 M NaBr added, λmax ) 548 nm; 6, 0.001 M CTAB + 0.01 M n-C6H13SO3Na + 2 M NaBr added, λmax ) 550 nm; and 7, 0.003 M CTAB + 4 M KCl added, λmax ) 564 nm.

Figure 3. Electron micrographs of (a) 0.001 M CTAB + 0.01 M NaOH; (b) 0.001 M CTAB + 0.01 M H7C7-SO3- + 0.02 M NaOH; and (c) 0.001 M CTAB + 0.01 M H7C7-SO3- + 0.02 M NaOH + betaine dye 1.

on the equilibrium state of buffer systems is minor or even negligible. However, in micellar systems a disturbing effect is practically inevitable. Even if the region of location of the dye probe used can be reliably stated, the information obtained refers always to a new dye + micelle complex rather than to a pure surfactant homomicelle. As it is known,16 critical micelle concentration (cmc) values determined by means of dyes are sometimes erroneous due to the formation of dye-induced mixed surfactant + dye micelles, which can appear at surfactant concentrations below the cmc of homomicelles. From this viewpoint, the results of the present study refer not precisely to the cationic surfactant systems with organic counterions but rather to the corresponding micelles additionally containing betaine dyes. However, the data obtained reflect at least the general trends in the influence of added organic anions on the structure of micelles studied. There is some analogy between the displacing effect, described in this communication, and the well-known phenomenon of deviations from the Henry’s adsorption law. Indeed, if the micelles of cationic surfactants are enlarged and prolonged as a result of conversion from CTA+Br- micelles to CTA+Tos- or CTA+HSal- micelles, there is a lack of pseudophase volume appropriate for solubilization of the betaine dyes. 3.4.5. Restoring Influence of Inorganic Salts Excess. It is necessary to emphasize that the “displacing effect” is of a reversible (equilibrium) nature. The vis spectra of dye 1, presented in Figure 4, demonstrate, in a first (16) (a) Mukerjee, P.; Mysels, K. J. J. Am. Chem. Soc. 1955, 77, 2937. (b) Ananthapadmanabhab, K. P.; Goddard, E. D.; Turro, N. J.; Kuo, P. L. Langmuir 1985, 1, 352.

approximation, the restoration of the initial absorption band at a 200-fold excess of added Br- ion (curve 6). Such a high concentration of inorganic anions induces a reformation of the CTAB micelles, which are, however, not spherical, but of rodlike form.8d,17 The sphere f rod transition is known to lead to some “dehydration” of the micellar interface. Actually, for dye 1, λmax is equal to 552 nm, while in CTAB micelles without salt background, λmax ) 534 nm. Moreover, the vis spectra of betaine dye 1 demonstrate even some differences between the cationic micelles in the presence of 2 M KBr and those in the presence of 4 M KCl (Figure 4). When NaBr was used instead of KBr, the results are the same ((2 nm). The large excess of Br- or Cl- ions [ca. (200-400)-fold] ensures a practically complete subtracting of organic anions from the Stern region of cationic micelles. This, as well as the opposite effects observed with a 10-fold excess of Tos- and other organic ions, follows from the values of the “selectivity parameters” Si, terms close to ion-exchange constants.9b,18 For instance, the Si values for H13C6-SO3-, H7C7-SO3-, and HSal- in the CTA+ system, as determined using the influence of added salts on the apparent dissociation constants of indicators, are 14 ( 2, 23 ( 5, and about 60,9b,18 respectively; here Br- is chosen as a reference ion with SBr- ≡ 1. It should be mentioned that the Si values resulting from the cmc values19 are somewhat lower.9b Because the working pH value was mostly equal to 12, some Br- or Cl- ions can be substituted in the Stern region by HO- ions. However, even at lower pH values, in the presence of corresponding organic anions, the discussed displacing effect takes place as well. On the other hand, without these organic anions the λmax values remain unaltered both with and without addition of NaOH. Hence, the observed “displacing” effects cannot be con(17) Ness, J. N.; Moth, D. K. J. Colloid Interface Sci. 1988, 123, 546. (18) Mchedlov-Petrossyan, N. O.; Timiy, A. V.; Vodolazkaya, N. A.; Pinchukova, N. A. Kharkov University Bulletin 1999, 454 (Chemistry, Issue 4; 27), 235. (19) Underwood, A. L.; Anacker, E. W. J. Phys. Chem. 1984, 88, 2390.

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nected with a (partial) transformation of CTAB micelles into CTA+HO- ones. We also made an attempt to reveal corresponding “displacing” effects in “symmetrical” situations with anionic surfactants and hydrophobic organic counterions. We carried out a study with probe 1, using sodium n-dodecanesulfate and sodium cetyl sulfate as surfactants and tetra-n-butylammonium and N-(n-butyl)pyridinium as organic counterions. Hence, in the anionic surfactant systems studied the “displacing” effect is not revealed distinctly. These results are available in Supporting Information. Conclusions In the aqueous systems (cationic surfactant + organic anion), with C(surfactant) ≈ 0.001 M and C(anion) ≈ 0.01 M, the vis absorption spectra of negatively solvatochromic pyridinium N-phenolate betaine dyes reflect alterations in the micellar structure. The changes are more pronounced in the case of added tosylate and alkanesulfonate anions, n-H2n+1Cn-SO3- (n ) 6-10). The influence of the anions varies due to the peculiarities of their structure and the character of their incorporation into the micellar pseudophase. In the cases of added hydrosalicylate, benzoate, and some other organic anions the vis spectral effects are less expressed. The solvatochromic probe dyes 1-6 and 9 indicate the possibility to study micellar transformations, leading to unfavorable conditions for dye binding. In the case of dyes 1 and 2, addition of Tos- and the aforementioned alkanesulfonates to aqueous micellar CTAB, CPB, and OTAC solutions exhibit blue shifts of the solvatochromic CT bands up to ∆λmax ) 80 nm as compared with the band positions in “pure” aqueous surfactant systems. This effect of displacing solvatochromic probes toward the more polar (more hydrated) regions reflects micellar transitions. More hydrophilic betaine dyes 3 and 4 are to a less degree coupled with cationic micelles, whereas more hydrophobic dyes 5, 6, and 9 move from the micelles, modified by organic

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anions, into the aqueous phase in the form of precipitating aggregates. Betaine dyes 7 and 8 are strongly fixed to the micelles, because their hydrophobic portions are situated in the region of the surfactant’s hydrocarbon chains; thus, the vis spectral effects are worse expressed in these cases. The addition of a large excess of inorganic salts to the systems with the blue-shifted absorption of probe 1 restores the position of the CT band to that found in the “pure” cationic surfactant system. At C(NaBr, KBr, or KCl) ) 2-4 M, red shifts up to ∆λmax ) 18 nm are observed, reflecting the well-known sphere f rod transition of micelles. Hence, the λmax values are sensitive to micellar transformations, but the band shifts found after addition of inorganic counterions are opposite to those found in the case of organic ones. The anisometrical character of cationic surfactant micelles modified by organic anions (prolonged, rodlike, wormlike, and other structures) manifests itself in strong vis-spectral band shifts only within a narrow concentration range, and only with the large-sized betaine dyes. However, both vis-spectroscopic and electron-microscopic data indicate principal differences between the prolonged micelles of such type and of those formed by “pure” cationic surfactants in concentrated aqueous solutions or in the presence of an excess of inorganic counterions. The formation of anisometrical (i.e., rodlike or wormlike) micelles as a result of addition of organic counterions to aqueous CTAB, CPB, and ODTAC micellar solutions can be easily proved by vis spectroscopic observations and can be watched even by the naked eye. Supporting Information Available: The λmax values of solvatochromic betaine dyes in aqueous micellar solutions of cationic surfactants CTAB, CPC, and OTAC in the presence of various anions compiled in 10 tables. Description of experiments with betaines in aqueous solutions of anionic surfactants in the presence of organic cations. This material is available free of charge via the Internet at http://pubs.acs.org. LA0401361