Solvatochromism in Cationic Micellar Solutions: Effects of the

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Langmuir 2001, 17, 652-658

Solvatochromism in Cationic Micellar Solutions: Effects of the Molecular Structures of the Solvatochromic Probe and the Surfactant Headgroup Erika B. Tada, Luzia P. Novaki, and Omar A. El Seoud* Instituto de Quı´mica, Universidade de Sa˜ o Paulo, C.P. 26077, 05513-970, Sa˜ o Paulo, S.P., Brazil Received August 7, 2000. In Final Form: October 26, 2000 The solvatochromic behavior of 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (RB), 2,6-dichloro4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (WB), 1-methyl-8-oxyquinolinium betaine (QB), and sodium 1-methyl-8-oxyquinolinium betaine 5-sulfonate (QBS) has been studied as a function of increasing the length of R in the series C12H25N+R3Br- (R ) methyl, ethyl, n-propyl, and n-butyl). The microscopic polarity of water at the solubilization site of the micelle-bound probe, ET in kcal/mol, has been calculated from the position of its intramolecular charge-transfer band in the UV-vis region. Calculated polarities depend on the length of R and the probe structure and charge. This is attributed to gradual “dehydration” of the interfacial region as a function of the increasing length of R, and different (average) solubilization sites of the probes. Thus, hydrophobic RB and WB are located in a less polar environment than hydrophilic QB and QBS. These conclusions have been confirmed by measuring 1H NMR chemical shifts of the discrete protons of both surfactant and probes. The “effective” water concentration at the probe solubilization site, [water]interfacial, has been calculated from solvatochromic data in bulk aqueous 1-propanol and aqueous 1,4-dioxane. Both reference binary mixtures gave consistent [water]interfacial; our data also agree with those based on the use of a micelle-bound arenediazonium ion.

Introduction Aqueous micelles affect the rates and equilibria of chemical reactions that occur in the micellar interfacial region.1,2 An understanding of the nature of this region and the forces that come into play therein is, therefore, of prime importance for a better understanding of micellemediated phenomena. Cationic surfactants are candidates for obtaining such information because the properties of their organized assemblies can be “fine-tuned” by varying the counterion, the length of the hydrophobic tail, and the structure of the hydrophilic group. For example, increasing the headgroup hydrophobicity of cetyltrialkylammonium halides (from trimethyl to tri-n-butyl) enhances the rates of SN2 and SNAr reactions, probably due to a concomitant “dehydration” of the interfacial region.3 Heating aqueous solutions of alkyltri(n-butyl)ammonium bromides results in spontaneous demixing into dilute and concentrated conjugate phases because of temperature effects on waterwater interactions.4 Therefore, understanding the properties of interfacial water is important for rationalizing several solution properties and applications of surfactantorganized assemblies. An important property of this water is its microscopic polarity, measured by the ET scale, which is calculated * To whom correspondence should be addressed. Fax: +55-113818-3874. E-mail: [email protected]. (1) (a) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (b) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (c) Bunton, C. A. J. Mol. Liq. 1997, 72, 231 and references therein. (2) El Seoud, O. A. Adv. Colloid Interface Sci. 1989, 30, 1. (3) (a) Germani, R.; Ponti, P. P.; Romeo, T.; Savelli, G.; Spreti, N.; Gerichelli, G.; Luchetti, L.; Mancini, G.; Bunton, C. A. J. Phys. Org. Chem. 1989, 2, 553. (b) Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5062. (c) Bacaloglu, R.; Blasko, A.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990, 94, 5068. (d) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C. A. J. Phys. Chem. 1990, 94, 5331. (e) Broxton, T. J.; Christie, J. R.; Theodoridis, D. J. Phys. Org. Chem. 1993, 6, 535. (4) Buckingham, S. A.; Garvey, C. J.; Warr, G. G. J. Phys. Chem. 1993, 97, 10236.

from the position of the longest-wavelength intramolecular charge-transfer absorption band of a solvatochromic probe (hereafter denoted as “probe”) by the relationship5

ET (kcal/mol) ) 28591.5/λmax (nm)

(1)

Recently, we have studied solvatochromism in pure organic solvents and in binary mixtures of water with alcohols and with dipolar aprotic solvents.6 The probes employed were 2,6-diphenyl-4-(2,4,6-triphenyl-1-pyridinio)1-phenolate (RB), 2,6-dichloro-4-(2,4,6-triphenyl-1-pyridinio)-1-phenolate (WB), 1-methyl-8-oxyquinolinium betaine (QB), and sodium 1-methyl-8-oxyquinolinium betaine 5-sulfonate (QBS). Their structures are shown below, along with the designation of the protons that are relevant to our 1H NMR experiments, vide infra. The corresponding solvatochromic scales are ET(30), ET(33), ET(QB), and ET(QBS), for RB, WB, QB, and QBS, respectively. Except for WB, the solvatochromic behavior of these probes has been studied in the presence of aqueous micelles of halides (bromides and chlorides) of dodecyltrimethylammonium, dodecyldimethylbenzylammonium, cetyltrimethylammonium, and cetyldimethylbenzylammonium ions. Calculated ET values depend on the structure and charge of the probe, length of the surfactant hydrophobic tail, and structure of its headgroup.7 Although solvatochromism has been investigated in many ionic and nonionic micellar solutions,8-12 no infor(5) (a) Reichardt, C. Solvents and Solvent Effects in Organic Chemistry; VCH: New York, 1988; see also references therein. (b) Reichardt, C. Chem. Soc. Rev. 1992, 147. (c) Reichardt, C. Chem. Rev. 1994, 94, 2319. (6) (a) Novaki, L. P.; El Seoud, O. A. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 648. (b) Novaki, L. P.; El Seoud, O. A. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 105. (c) Novaki, L. P.; El Seoud, O. A. Ber. BunsenGes. Phys. Chem. 1997, 101, 902. (d) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. J. Phys. Org. Chem. 2000, 13, 679. (7) (a) Novaki, L. P.; El Seoud, O. A. PCCP 1999, 1, 1957. (b) Novaki, L. P.; El Seoud, O. A. Langmuir 2000, 16, 35.

10.1021/la001135l CCC: $20.00 © 2001 American Chemical Society Published on Web 01/13/2001

Solvatochromism in Cationic Micellar Solutions

mation is available on the effects of increasing the surfactant headgroup size on polarity. Therefore, we extended our studies to aqueous micellar solutions of C12H25N+R3Br-, R ) methyl, ethyl, n-propyl, and n-butyl. The polarities measured depend on the structure and charge of the probe and the length of R. Micellar ET values were transformed into interfacial water concentrations by using aqueous 1-propanol and aqueous 1,4-dioxane as reference solvents. Experimental Section Materials. The surfactants used were dodecyltrialkylammonium bromides, alkyl group ) methyl, DoMe3ABr; ethyl, DoEt3ABr; n-propyl, DoPr3ABr; and n-butyl, DoBu3ABr. The probes and surfactants were available in pure form (microanalyses for all compounds, critical micelle concentrations (cmc’s) for surfactants) from previous studies.6d,13 Spectrophotometric determination of ET. All micellar solutions were prepared in 10-3 mol L-1 NaOH, instead of water. The final probe concentrations in micellar solutions were (2-5) × 10-4 mol L-1. UV-vis spectra were recorded at 25.0 ( 0.1 °C with a Beckman DU-70 UV-vis spectrophotometer. Accurate λmax values were determined from the first derivative of the absorption spectrum and then used to calculate the corresponding ET from eq 1. Uncertainties in ET are 0.6, 0.3, and 0.1 kcal/mol for RB,14 WB, and QB (or QBS),6 respectively. (8) (a) Minch, M. J.; Shah, S. S. J. Org. Chem. 1979, 44, 3252. (b) Donchi, K. F.; Robert, G. P.; Ternai, B.; Derrick, P. J. Aust. J. Chem. 1980, 33, 2199. (c) Schanze, K. S.; Mattos, T. F.; Whitten, D. G. J. Am. Chem. Soc. 1982, 104, 1733. (d) Plieninger, P.; Baumga¨rtel, H. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 161. (9) (a) Zachariasse, K. A.; Phuc, N. V.; Konzanklewicz, Ku¨hnle, W. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 1, p 565. (b) Whitten, D. G.; Shin, D. M. J. Phys. Chem. 1988, 92, 336, 2945. (c) Kessler, M. A.; Wolfbeis, O. S. Chem. Phys. Lipids 1989, 50, 51. (10) (a) Warr, G. G.; Evans, D. F. Langmuir 1988, 4, 217. (b) L’Heureux, G. P.; Fragata, M. Biophys. Chem. 1988, 30, 293. (c) Mac¸ anita, A. L.; Costa, F. P.; Costa, S. M. B.; Melo, E. C.; Santos, H. J. Phys. Chem. 1989, 93, 336. (d) Melo, E. C.; Costa, S. M. B.; Mac¸ anita, A. L.; Santos, H. J. Colloid Interface Sci. 1991, 141, 439. (e) Berr, S.; Jones, R. R. M.; Johnson, J. S., Jr. J. Phys. Chem. 1992, 96, 5611. (11) (a) Sarpal, R. S.; Belleteˆte, M.; Durocher, G. J. Photochem. Photobiol., A 1995, 88, 153. (b) Saroja, G.; Samanta, A. Chem. Phys. Lett. 1995, 246, 506. (c) Banerjee, D.; Das, P. K.; Mondal, S.; Ghosh, S.; Bagchi, S. J. Photochem. Photobiol., A 1996, 98, 183. (d) Vitha, M. F.; Weckwerth, J. D.; Odland, K.; Dema, V.; Carr, P. W. J. Photochem. Photobiol., A 1996, 100, 18823. (12) (a) Mishra, A.; Patel, S.; Behera, R. J.; Mishra, B. K.; Behera, G. B. Bull. Chem. Soc. Jpn. 1997, 70, 2913. (b) Zana, R.; In, M.; Le´vy, H. Langmuir 1997, 13, 5552. (c) Datta, A.; Mandal, D.; Pal, S. K.; Das, S.; Bhattacharyya, K. J. Mol. Liq. 1998, 77, 121. (d) Karukstis, K. K.; Savin, D. A.; Loftus, C. T.; D’Angelo, N. D. J. Colloid Interface Sci. 1998, 203, 157. (13) Bazito, R. C.; El Seoud, O. A.; Barlow, G. K.; Halstead, T. K. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 1933.

Langmuir, Vol. 17, No. 3, 2001 653 1H NMR Study of Solubilization of Probes in Micellar Solutions. In the first set of experiments, the chemical shift, δ, of the surfactant protons (R ) methyl and n-butyl) was measured as a function of probe concentration (WB, QB, and QBS). The surfactant stock solution was 0.1 mol L-1 in 10-3 mol L-1 NaOD/ D2O. Stock solutions of QB and QBS in water and of WB in acetone were pipetted into 1 mL volumetric tubes, the solvent was evaporated, the surfactant stock solution was added, and the mixture was agitated (Labquake tube rotator) until complete dissolution of the probe. In the second set of experiments, δ of the probe protons (0.002 mol L-1) was measured in different media, D2O (QB, QBS), 20% CH3OD in D2O (WB), pure CH3OD, and 0.1 mol L-1 surfactant solution (R ) methyl and n-butyl) in D2O (QB, QBS) or in 20% CH3OD in D2O (WB). Use of aqueous methanol was necessary because of the very low solubility of WB in water, 3.7 × 10-4 mol L-1.6d 1H NMR spectra were recorded at 30 °C with a Bruker DRX500 (operating at 500.13 MHz for proton), at a digital resolution of 0.06 Hz/data point. Chemical shifts were measured relative to internal 1,4-dioxane (5 × 10-3 mol L-1) and then transformed into the TMS scale, by using δdioxane ) 3.53 ppm.15

Results and Discussion Choice of the Solvatochromic Probes. In an ionic organized assembly, the (average) solubilization site of a substrate depends on its charge and hydrophilic/hydrophobic character and on the structure and charge of the surfactant. Therefore, we employed RB as a model hydrophobic substrate (solubility in water 7.2 × 10-6 mol L-1),5 which is expected to penetrate into the micelle, and is probably anchored to the cationic interface by its phenoxide oxygen.16 QB is much more hydrophilic than RB. The negative charge of QBS makes it still more hydrophilic than QB, without affecting its solvatochromic behavior; i.e., ET(QB) ≈ ET(QBS), both in pure solvents and in binary solvent mixtures.6 Anionic QBS is expected to exchange with the surfactant counterion, Br-, which should lead to different micellar solubilization sites for QB and QBS. All probes used are weak acids and can be present in both cationic and zwitterionic (conjugate base) forms. The latter is the solvatochromic species, whose concentration in the pseudophase depends on the (i) micelle-induced pKa shift, due to a combination of “medium” and electrostatic effects,2,17 (ii) interfacial concentration of OH- (e.g., in CMe3ABr, C ) cetyl, K for [OH-bulk]/[Br-counterion] ) 0.024),18 and (iii) solubilization site of the probe within the micelle. An experimental problem arises when the pKa of the micelle-bound probe is increased, because the micellar solution should be made basic enough to generate the solvatochromic form. Indeed, in previous work we have used (bulk) pH 11 with RB in cationic micelles.7 In the present work, WB has been used because we were unable to carry out the experiment of RB in micellar DoBu3ABr. At bulk pH 11, the concentration of zwitterionic RB was negligible. Increasing the pH to 13 did not solve the problem because the solution became cloudy, probably due to an electrolyte-induced phase separation.4 (14) Dawber, J. G.; Ward, J.; Williams, R. A. J. Chem. Soc., Faraday Trans. 1 1988, 84, 713. (15) Derome, A. Modern NMR Techniques for Chemistry Research; Pergamon Press: Oxford, 1987. (16) (a) Drummond, C. J.; Grieser, F.; Healy, T. W. Faraday Discuss. Chem. Soc. 1986, 81, 95. (b) Grieser, F.; Drummond, C. J. J. Phys. Chem. 1988, 92, 5580 and references therein. (17) Soldi, V.; Keiper, J.; Romsted, L. S.; Cuccovia, I. M.; Chaimovich, H. Langmuir 2000, 16, 59. (18) Araujo, P. S.; Aleixo, R. M. V.; Chaimovich, H.; Bianchi, N.; Miola, L.; Quina, F. H. J. Colloid Interface Sci. 1983, 96, 293.

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Figure 1. Dependence of Eobs T on surfactant concentration for RB, WB, QB, and QBS. The points are experimental data, and the curves were generated by using an exponential decay equation, as given in Table 1. The vertical line shows the surfactant cmc in water. a Table 1. Regression Coefficients for the Dependence of Eobs T on Surfactant Concentration

surfactant

probe

Nc

A

B

C

D

E

∑Qd

DoMe3ABrb

RB WB QB QBS RB WB QB QBS RB WB QB QBS RB WB QB QBS

10 11 12 14 13 13 18 18 9 18 18 18

53.33 62.66 62.51 63.80 53.15 61.83 59.32 63.76 52.92 61.86 62.99 62.94

0.457 38.52 2.37 0.714 729.98 39.4 5.19 0.90 68.62 -42.72 1.50 1.00

0.019 0.0049 0.81 0.048 0.002 0.007 2.012 0.075 0.003 0.0025 0.431 0.034

0.55 44.4 -0.32 0.43 896.19 0.134

0.286 0.005 0.725 0.002 0.065

0.26 0.797 25.75

2.209 0.028 0.008

0.005 0.1891 0.0013 0.0035 0.0601 0.227 0.023 0.029

1.005

0.994

23 19 19

62.00 62.78 62.86

6.238 0.766 1.044

2.723

0.029

1.078

0.649

DoEt3ABr

DoPr3ABr

DoBu3ABr

0.023 0.193 0.019

0.0528 0.0105 0.0792 2.81 0.0112 0.0426

At 25 °C. All data were adjusted with an exponential decay of the type ET ) A + B exp(-[surfactant]/C) + D exp(-[surfactant]/E). b Data for RB, QB, and QBS in DoMe ABr were taken from ref 7b. c Number of surfactant concentrations used in the regression analysis. 3 The maximum surfactant concentration was 0.1-0.3 mol L-1. d ∑Q ) sum of the squares of the residues. a

WB is an interesting probe because it is structurally related to RB, and its pKa in water is much lower (4.78 and 8.63, respectively).9c Additionally, the susceptibilities of RB and WB to solvent dipolarity/polarizability and “acidity” (i.e., H-bond donation) are similar, despite the large difference in their pKa values!6d This similarity, however, is very convenient because differences between the solvatochromic behavior of both probes should be due to factors other than differences between the energetics of their solvation, e.g., distinct solubilization sites in the micellar pseudophase. Solvatochromic Behavior in Micellar Solutions. Figure 1 the shows representative dependence of Eobs T , the observed microscopic polarity, on surfactant concentration. The points are experimental, and the curves were generated by equations that gave the best data fit, as indicated by the sums of the squares of the residues, ∑Q, Table 1.

The regression parameters given in this table permit calculation of Eobs T at any surfactant concentration. The on the latter concentration is a dependence of Eobs T consequence of partitioning of the probe into the micelle. It is clear from Figure 1 that this partitioning is incomplete, especially for QB, up to quite high surfactant concentrations. That is, reporting microscopic polarities at a single, low surfactant concentration, typically 0.05 mol L-1,8,9c,10-12 is unjustified, at least for certain probes. The dependence of the following quantities on the structures of the probe and the surfactant headgroup is given in Table 2: probe-micelle association constant, -1 Kassoc, Eobs T at [surfactant] ) 0.05 mol L , and microscopic polarities calculated for micelle-bound probes, Emic T . into a dimensionless, normalized Conversion of Emic T , by use of eq 2, renders scales of different polarity, Enorm T

Solvatochromism in Cationic Micellar Solutions

Langmuir, Vol. 17, No. 3, 2001 655

Table 2. Dependence on Surfactant Structure of the Probe-Surfactant Association Constants, Kassoc, Observed ET at 0.05 mol L-1 Surfactant, Eobs T , Interfacial Water Polarities Measured by Micelle-Bound Probes, mic a,b ET , and Their Normalized Counterparts, Enorm T surfactant

probe

Kassoc, mol L-1

d Eobs T , kcal/mol

e Emic T , kcal/mol

,e Enorm T kcal/mol

DoMe3ABrb

RB WB QB QBS RB WB QB QBS RB WB QB QBS WB QB QBS

18 000 9 000 2.7 19 35 000 12 000 4 24 44 000 10 000 6.5 30 3 500 13 100

53.9 62.7 64.4 64.4 53.1 62.1 64.4 64.4 53.0 62.1 64.3 64.1 63.1 64.4 64.0

53.6 62.7 62.8 64.0 52.7 61.8 61.9 63.7 52.2 61.2 61.5 63.5 60.7 61.3 63.2

0.707 0.761 0.902 0.974 0.682 0.725 0.850 0.929 0.664 0.712 0.827 0.923 0.693 0.815 0.899

DoEt3ABrb

DoPr3ABr

DoBu3ABrf

a At 25 °C. b See the Calculations for details. The uncertainty in Kassoc is typically (10%. c Data for RB, QB, and QBS in DoMe3ABr were taken from ref 7b. d Determined at [surfactant] ) 0.05 mol norm TMS L-1. e Emic ) (Emic )/(Ewater T was calculated from eq 4, ET T - ET T f Instability of the absorbance reading precluded deter- ETMS ). T mination of the microscopic polarity by RB in micellar solution of this surfactant; see the text.

Figure 2. Dependence of ∆δsurfactant ) δsurfactant - δsurfactant/probe on probe concentration for the discrete protons of DoMe3ABr, part I, and of DoBu3ABr, part II. The symbols refer to the following surfactant protons: 9, A; b, B; 2, C; 1, D; +, E; and [, -N+Me3. Due to the equivalence of δ of protons on both sides of the quaternary ammonium ion of DoBu3ABr, the same symbol refers to two protons, i.e., 9, A and A′; 2, B and B′; 2, C, and C′. Interfacial -N+(CH2)3CH3 is designated by a solid star. Table 3. Dependence of the Slopes of Plots of ∆δsurfactant ) δsurfactant - δsurfactant+probe on Probe Concentration for the Discrete Surfactant Protonsa slope,e Hz/(mol L-1)

Surfactant/proton

probes readily comparable (see the Calculations for details).

Enorm ) T

TMS Emic T - ET Ewater - ETMS T T

(2)

mic In all cases, Eobs T > ET ; the difference is largest for QB. Except for one case (WB in DoBu3ABr), Kassoc for the same probe increases as a function of increasing headgroup bulk. Note that Kassoc is rather sensitive to the parameters used in eq 3 (see the Calculations); especially the cmc19 and the probes may change these concentrations slightly. Additionally, the order of binding to any micelle increases as a function of increasing hydrophobicity of the zwitterionic probe, RB > WB > QB, and is higher when the probe and interface carry opposite charges, QBS > QB. For all probes, Emic T decreases as a function of increasing length of the surfactant headgroup. Two alternative explanations can be advanced to rationalize the dependence of Emic T on the length of R: (i) The interfacial region becomes “drier”. (ii) Probe penetration into the micelle increases with the length of R. According to explanation i, the probe solubilization site in the interfacial region does not change appreciably as a function of increasing length of R. The main reason for the observed decrease of Emic T is that as the length of R increases, water is displaced from the interfacial region with a concomitant decrease in polarity.3 Explanation ii implies that the polarity is not appreciably dependent on the length of R. An increase of the latter, however, leads to deeper penetration of the probe into the micelle, i.e., into a less polar environment. Both alternatives should be considered because increasing the size of R is associated with several changes in micellar properties (e.g., surface charge density, aggregation number, and surface area/

(19) (a) Possidonio, S.; Siviero, F.; El Seoud, O. A. J. Phys. Org. Chem. 1999, 12, 325. (b) Possidonio, S.; El Seoud, O. A. J. Mol. Liq. 1999, 80, 231.

DoMe3ABrb -N+Me3 A B C D E DoBu3ABr -N+(CH2)3CH3 A and A′ B and B′ C and C′ D E

WB

QB

QBS

2350.1 4165.2 4775.2 4231.1 2494.8 2119.9 WB 823.5 1293.7 1251.4 1039.4 1922.9 1441.8

25.5 57.8 72.0 115.3 18.6 -78.6 QB 13.9 45.3 44.4 78.9 -85.1 -140.4

507.7 497.0 475.0 326.8 27.0 -74.9 QBS 440.5 496.9 440.4 291.4 -80.8 -213.2

a Experiments were carried out at 500.13 MHz and 30 °C. b Data for RB, QB, and QBS in DoMe3ABr were taken from ref 7b. c The uncertainty in these slopes is typically (4%.

headgroup)4,13 whose resultant effect both on the polarity of interfacial water and on the position of the probe in the aggregate cannot be predicted. To choose between these alternatives, 1H NMR has been used to determine the (i) dependence of δ of the discrete surfactant protons on probe concentration and (ii) medium effects on δ of the probe protons. Considering the first set of experiments, Figure 2 shows some typical plots of chemical shift differences (vide infra) versus probe concentration; the slopes (Hz/(mol L-1) probe) are given in Table 3 for WB, QB, and QBS in the two limiting surfactants, namely, DoMe3ABr and DoBu3ABr (see the proton designation below):

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First, we comment on the 1H NMR spectra of the surfactants. Virtual coupling results in the appearance of several methylene groups (e.g., D protons) as a single broad band. As given elsewhere, the 1H NMR peaks of the methylene protons on both sides of the quaternary ammonium ion of DoBu3ABr have the same δ and are, therefore, unresolvable.3c,13 For clarity, we have plotted our data as chemical shift differences, i.e., ∆δsurfactant ) δsurfactant - δsurfactant+probe. All plots are linear; the slope depends on the extension of the π electron system of the probe (which causes diamagnetic shielding/deshielding of the surfactant protons), its residence time within the micelle, and its average orientation relative to a discrete surfactant group.20 In the following, we are concerned with |∆δsurfactant|. Consider the effects of solubilization of QBS on ∆δsurfactant. As shown in Table 3, this probe affects protons at the interface (i.e., N+Me3 and N+(CH2)3CH3) as well as protons A, B, and C, whereas the effect is much smaller for protons D and E. The most important result is that ∆δsurfactant values for these groups decrease only slightly on going from DoMe3ABr to DoBu3ABr. Both results agree with QBS being solubilized in the outer part of the interfacial region because it exchanges with the surfactant counterion. For zwitterionic WB and QB, the order of |∆δsurfactant|, for every surfactant segment is DoMe3ABr > DoBu3ABr. Additionally, the order of |∆δsurfactant| for QB is the same in both surfactants, namely, C > B > A > (N+Me3 or N+(CH2)3CH3). We discuss these chemical shift orders in terms of penetration of each probe into both micelles. The length of WB is 12.7 Å; i.e., it covers ca. 87% of the length of the DoMe3ABr monomer, 14.6 Å.7b That is, the results of this probe are not expected to offer a clear answer to the question addressed, because there is not much room left in the micelle for further probe penetration. The length of QB is, however, smaller, 4.8 Å;7b i.e., it covers ca. one-third of the length of the surfactant monomer. That is, if penetration of QB into DoBu3ABr micelles were noticeably deeper than into DoMe3ABr micelles, one should have observed different orders of |∆δsurfactant| for the segments of both surfactants. Our results, Table 3, show that this is not the case; this indicates that the (average) solubilization site of QB is little dependent on the length of R. Finally, the observed order of |∆δsurfactant|, DoMe3ABr > DoBu3ABr, probably reflects different orientations and residence times of the probe in both micelles.7b,20 In the second experiment, the probe concentration was kept constant and the chemical shifts of its protons were measured in different media. First we determined medium polarity effects on δ, outside the micellar domain. The transfer of probes (WB, QB, and QBS, respectively) from a polar solvent (aqueous methanol and water, respectively) to a less polar one (methanol) resulted in upfield shifts (i.e., toward TMS) of all protons of the three probes; i.e., ∆δprobe/solvent ) δprobe/aqueous methanol - δprobe/methanol for WB and δprobe/water - δprobe/methanol for QB and QBS were positive.21 We have then determined the effect of micellar solubilization of the probe on its δ, and calculated ∆δprobe/micelle ) δprobe/20% aqueous methanol - δprobe/micelle,20% aqueous methanol for WB and δprobe/water - δprobe/aqueous micelle) for QB and QBS, as shown in Figure 3. (20) Chachaty, C. Prog. NMR Spectrosc. 1987, 19, 183 and references therein. (21) Values of ∆δprobe/solvent (Hz, at 500.13 MHz) were found to be 567.3, 757.5, 727.0, 755.5, and 771.4 for WB, protons a, b, c, d, and e, respectively, 542.9, 554.7, 532.4, 560.7, 565.0, 573.3, and 472.0 for QB, protons H2, H3, H4, H5, H6, H7, and -N+CH3, respectively, and 417.8, 585.4, 557.1, 568.2, 518.6, and 481.3 for QBS, protons H2, H3, H4, H6, H7, and -N+CH3, respectively.

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Figure 3. Effects of micellar DoMe3ABr (b) and DoBu3ABr (9) on 1H NMR chemical shifts of the discrete protons of WB, QB, and QBS. ∆δprobe/micelle ) δprobe/20% aqueous methanol δprobe/micelle,20% aqueous methanol for WB and δprobe/water - δprobe/aqueous micelle for QB or QBS.

Several pieces of information can be extracted from this figure. (i) Taking into account that ∆δprobe/solvent is positive, and that the transfer of the probe from the bulk solvent to the micelle is equivalent to its transfer from a more polar to a less polar medium, one expects positive ∆δprobe/micelle for protons of all probes, provided that no other mechanism is affecting the chemical shift. Figure 3 shows that this is not the case for most protons; i.e., other contributing mechanisms should be considered. (ii) The electrostatic attraction between the surfactant quaternary ammonium ion and the probe phenoxy oxygen16 should lead to a decrease of the electron density in the phenoxide aromatic ring, with concomitant changes in δ due to a decrease of the electron density around the attached protons (downfield shift) and a decrease of the diamagnetic deshielding of these protons by the ring π electron cloud (upfield shift). The contribution to δ of diamagnetic anisotropy of the phenoxide ring is expected to be larger than that due to changes in electron density around the relevant protons.15,20 Consequently, one expects positive ∆δprobe/micelle for the phenoxide ring protons, as shown for H6 and H7 of QBS in both micelles and H6 and H7 of QB in DoMe3ABr. (iii) Likewise, there can be an electrostatic attraction between the probe quaternary ammonium ion and the surfactant counterion. This should decrease the electron deficiency in the heterocyclic ring, with changes in δ due to an increase of electron density around the attached protons (upfield shift) and an increase of diamagnetic deshielding of the same protons by the ring π electron cloud (downfield shift). On the basis of the abovementioned relative importance of these two effects, one expects negative ∆δprobe/micelle for the heterocyclic ring protons (H2, H3, and H4), as shown in Figure 3. In summary, our 1H NMR data show the following. (i) The effects of [QBS] on δ of the surfactant protons are very similar for DoMe3ABr and DoBu3ABr, which indicates similar solubilization sites in the outer part of the interfacial regions of both micelles. (ii) For QB, the order of δ is the same for DoMe3ABr and DoBu3ABr, which is compatible with the same (average) solubilization site in both micelles. (iii) For each probe, the sign of ∆δprobe/micelle for its discrete protons is the same for both micelles, indicating similar probe-surfactant headgroup interactions due to similar solubilization sites. (iv) ∆δprobe/micelle values for the discrete protons of WB are remarkably independent of the structure of the

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Langmuir, Vol. 17, No. 3, 2001 657

Table 4. Molar Concentration of Water at the Average Solubilization Sites of the Probes, [water]interfacial, Calculated from Solvatochromic Data in Bulk Binary Aqueous Solvents [water]interfac ial, mol L-1 surfactant

probe

1-propanol

1,4-dioxane

DoMe3ABra

RB WB QB QBS RB WB QB QBS RB WB QB QBS WB QB QBS

29.5 17.7 39.8 44.5 22.3 9.5 29.9 40.6 13.9 6.5 24.6 37.9 4.3 21.6 33.8

28.1 28.3 41.8 46.9 23.9 24.6 34.0 44.0 22.1 22.5 30.3 42.1 20.6 28.4 39.4

DoEt3ABr

DoPr3ABr

DoBu3ABr

a

Data for RB, QB, and QBS in DoMe3ABr were taken from ref

7b.

Figure 5. Representative plots of eq 7 showing the dependence of Eobs T on fmic for RB in WB in DoBu3ABr (A) and QBS in DoEt3ABr (B).

in aqueous 1-propanol and aqueous 1,4-dioxane is satisfactory (Table 4). For QBS, our [water]interfacial values agree with those calculated for C16H33N+R3Br-, 46 ( 2 mol L-1 at 40 °C and 0.2 mol L-1 surfactant.17 This is satisfying in view of the very different approaches employed, namely, solvent dependence of an intramolecular charge-transfer band of a solvatochromic probe, and chemical trapping of the surfactant counterion by an aggregate-bound arenediazonium ion. Conclusions

Figure 4. Relationship between [water]interfacial and the number of carbon atoms in R of C12H25N+R3Br- for WB (9), QB (b), and QBS (2) in aqueous 1,4-dioxane and aqueous 1-propanol.

surfactant headgroup, indicating that this probe penetrates deep within the micelle (vide supra). (v) Our results agree, therefore, with previous arguments (inferred from kinetic data),3 namely, that, as the length of R increases, the substrate experiences a drier microenvironment because water is displaced from the interfacial region by the bulky headgroup. Interfacial water has been modeled by binary aqueous solvents, e.g., aqueous 1,4-dioxane and aqueous alcohols. As shown in the Calculations, we have transformed ETmic into the “effective” water concentration in the interfacial region from solvatochromic data of these probes in bulk aqueous 1-propanol and aqueous 1,4-dioxane. Table 4 shows the dependence of [water]interfacial on the structures of the probe and surfactant headgroup. Figure 4 shows plots of these concentrations versus the number of carbon atoms in R. The corresponding equations for QBS were used to calculate ∆[water]interfacial/CH2, i.e., the decrease of [water]interfacial per each additional methylene group of R, 3.5 and 2.4 mol L-1 for data obtained from aqueous 1-propanol and aqueous 1,4-dioxane, respectively. Considering the different nature of both binary mixtures, and the fact that solvatochromism is affected by the phenomenon of preferential solvation,5,6 the agreement between [water]interfacial calculated from solvatochromism

The microscopic polarity of interfacial water depends on the structure and charge of the probe employed and the length of R, and is related to the probe solubilization site in the micellar pseudo-phase. Results of two different 1H NMR experiments are compatible with gradual dehydration of the interfacial region as a function of increasing length of R, rather than increased penetration of the probe into the micellar pseudophase. [water]interfacial can be conveniently calculated by using bulk aqueous 1-propanol or aqueous 1,4-dioxane as a model for interfacial water. The results of QBS agree with those determined by an independent approach. Calculations

Enorm , T

Eq 2. ETMS values (39.3, 47.2, Calculation of T and 48.0 kcal/mol for WB, QB, and QBS, respectively), from eq 2, have been which are required to calculate Enorm T obtained from the corresponding value of RB (30.7 kcal/ mol)5 and the linear correlation between ET(30) and the appropriate polarity scale, ET(33), ET(QB), and ET(QBS), respectively.6 Probe-Micelle Association Constant, Kassoc. Equation 3 describes the observed absorption of the probe, Aobs, in terms of absorption of the species present in solution, Kassoc, and the concentration of the micellized surfactant,22

Aobs ) Awater + {(Amic - Awater)/ (1/(Kassoc([surfactant] - cmc)n + 1)} (3) where Awater and Amic refer to absorbance in the absence of the surfactant and that due to the micelle-bound probe, respectively, n is the number of surfactant monomers associated with the probe (n ) 1 for QB and QBS, and n (22) Perussi, J. R.; Yushmanov, V. E.; Monte, S. C.; Imasato, H.; Tabak, M. Physiol. Chem. Phys. Med. NMR 1995, 27, 1.

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) 2 for RB and WB), and the remaining symbols are those previously defined. Polarities of Interfacial Water As Determined by Micelle-Bound Probes. These were calculated from7 water water Eobs + fmic(Emic ) T ) ET T - ET

(4)

A plot of Eobs T versus fmic should be a straight line, Figure 5, whose slope is then used to calculate Emic T . The fraction of micelle-bound indicator, fmic, was calculated from the

probe absorbances at different λ values, as given elsewhere.7 [water]interfacial was obtained by determining the composition of the binary mixture which had, for the same 6b-d probe, ET equal to Emic T . Acknowledgment. We thank the FAPESP for financial support and a postdoctoral fellowship to L.P.N. and the CNPq for a research productivity fellowship to O.A.E.S. LA001135L