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Langmuir 1996, 12, 699-705

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H and 13C NMR Studies of Mixed Counterion, Cetyltrimethylammonium Bromide/ Cetyltrimethylammonium Dichlorobenzoate, Surfactant Solutions: The Intercalation of Aromatic Counterions P. J. Kreke,† L. J. Magid,* and J. C. Gee‡ Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996-1600 Received July 5, 1995. In Final Form: October 23, 1995X One- and two-dimensional 1H NMR studies of mixed counterion CTAB/CTA3,5 and CTAB/CTA2,6 surfactant systems indicate that, in their respective micelles, the dichlorobenzoate counterions intercalate among the surfactants’ trimethylammonium headgroups. Shielding effects observed in 1D 1H NMR studies of the N-CH3 and the R-CH2 through -CH2 protons suggest that, on average, the 3,5-dichlorobenzoate counterions insert further into the interface of their rodlike micelles than the 2,6-dichlorobenzoate counterions insert into the interface of their spherical micelles. 13C chemical shifts for the carbonyl carbons of the dichlorobenzoate counterions in their respective micellar systems also support this assertion.

Introduction Cetyltrimethylammonium (CTA+) mono- and dichlorobenzoate systems and their solutions have been characterized via small-angle neutron scattering (SANS),1 light scattering,2 cryo-transmission electron microscopy,3 and rheological studies.1c The substitution pattern of the aromatic counterion influences the size, shape, and flexibility of the micelles as well as the properties of their solutions. For example, while meta and para chlorobenzoate counterions induce growth to rodlike micelles whose solutions exhibit viscoelasticity, solutions of CTA+ with ortho chlorobenzoate counterions are not viscoelastic and, in the absence of added salt, contain globular micelles with aggregation numbers of 100 ( 5 at concentrations up to 60 mM. At higher concentrations, however, modest micellar growth to prolate ellipsoidal micelles of modest aspect ratio is observed. It has been suggested that the preferred loci of the aromatic counterions at or within the cationic micellar interface influences the morphology of the micelles formed. On a molecular level, the details of this effect are not well understood. Specifically, it has been proposed that the meta- and para-substituted counterions intercalate among the headgroups within the interface, and the ortho chlorobenzoates prefer loci tangential to the micellar interface.1a,4 Both of these locations allow for the counterions to screen partially the repulsive interactions among the headgroups. Our intent is to use one- (1D) and two-dimensional (2D) NMR techniques to investigate the average loci of the counterions within the micellar interface (intercalation) or * Author to whom correspondence should be addressed. E-mail address: [email protected]. † Current address: Mount St. Mary’s College, Emmitsburg, MD. ‡ Current address: Chevron Chemical Company, Kingwood, TX. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) (a) Magid, L. J. Colloids Surf. 1986, 19, 129. (b) Magid, L. J. In Ordering and Organization in Ionic Solutions; Ise, N., Sogami, I., Eds.; World Scientific Publishing Co. Pte Ltd.: Singapore; 1987; p 288. (c) Carver, M.; Smith, T. L.; Gee, J. C.; Delichere, A.; Caponetti, E.; Magid, L. J. Langmuir 1996, 691. (2) Butler, P. D.; Magid, L. J.; Hayter, J. B. In Structure and Flow in Surfactant Solutions; Herb, C. A., Prud’homme, R. K., Eds; American Chemical Society: Washington, DC, 1994; p 250. (3) Magid, L. J.; Gee, J. C.; Talmon, Y. Langmuir 1990, 6, 1609. (4) (a) Underwood, A. L.; Anacker, E. W. J. Colloid Interface Sci. 1985, 106, 86. (b) Johnson, I.; Olofsson, G. J. Colloid Interface Sci. 1985, 106, 222. (c) Kalus, J.; Hoffmann, H.; Reizlein, K.; Ulbricht, W. Ber. Bunsen-Ges. Phys. Chem. 1982, 86, 37. (d) Carver, M. T. Ph.D. Dissertation, University of Tennessee, Knoxville, Tennessee, 1986.

0743-7463/96/2412-0699$12.00/0

tangential to the interface (but still in the Stern layer). If these extremely different counterion loci exist, large differences in the average environment of the system nuclei as detected via NMR would be expected. Environmental changes experienced by both the counterion and the CTA+ chain may indicate the preferred solubilization sites of the aromatic counterion via the shielding and deshielding effects experienced by the 1H and 13C (carbonyl) nuclei.5,6 CTA+ chain and headgroup protons in the proximity of the shielding cone associated with the aromatic ring current become shielded. Not only do these upfield shifts of the CTA+ 1H resonances in the headgroup region indicate the possible intercalation of the aromatic ion but the degree of the upfield shift may suggest the average depth of the individual proton’s intercalation into the micelle. For example, Bacaloglu et al.7 noted that, in separate CTA+ micellar solutions, methyl naphthalene-2-sulfonate and naphthalene-2-sulfonate anion show different shielding effects on the CTA+ chain protons. From this and from the chemical shifts of the aromatic groups, it was suggested that the naphthalene ester, on average, inserts more deeply into the micelle than does the anion. In past studies of chlorobenzoates,8 hydroxybenzoates,9,10 and other ionic11 and nonionic aromatics,12 it was inferred that an upfield shift of an aromatic resonance in the presence of CTAB (CTA+ bromide) micelles indicated the intercalation of that proton into the micelle. Since the micellar interior is less polar than water, a proton in that environment will be more shielded. Downfield shifts which have been noted for some ortho resonances are believed to indicate that the proton (5) Eriksson, J. C.; Gillberg, G. Acta Chem. Scand. 1966, 20, 2019. (6) (a) Menger, F. M.; Jerkunica, J. M.; Johnston, J. C.; J. Am. Chem. Soc. 1978, 100, 4676. (b) Menger, F. M.; Aikens, P.; Wood, M. J. Chem. Soc., Chem. Commun. 1988, 180. (c) Schmidt, C. F.; Barenholz, Y.; Huang, C.; Thompson, T. E. Biochemistry 1977, 18, 3948. (7) Bacaloglu, R.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1989, 93, 1490. (8) (a) Smith, Bryan C. Ph.D. Dissertation, The Ohio State University, Columbus, OH, 1992. (b) Smith, B. C.; Chou, L. C.; Zakin, J. L. J. Rheol. 1994, 38, 73. (9) Manohar, C.; Rao, U. R. K.; Valaulikar, B. S.; Iyer, R. M. J. Chem. Soc., Chem. Commun. 1986, 379. (10) Rao, U. R. K.; Manohar, C.; Valaulikar, B. S.; Iyer, R. M. J. Phys. Chem. 1987, 91, 3286. (11) (a) Broxton, T. J.; Christie, J. R.; Chung, R. P.-T. J. Org. Chem. 1988, 53, 3081. (b) Bachofer, S. J.; Turbitt, R. M. J. Colloid Interface Sci. 1990, 135, 325. (c) Mishra, B. K.; Samant, S. D.; Pradhan, P.; Mishra, S. B.; Manohar, C. Langmuir 1993, 9, 894. (12) Suckling, C. J.; Wilson, A. A. J. Chem. Soc., Perkin Trans 2 1981, 1616.

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is at the micelle/water interface. Other researchers have utilized 13C chemical shift changes of carbonyl carbons to explore the micellar interface.6 In the present work, the carbonyl carbons of the dichlorobenzoates are used to probe the interfacial environment experienced by the carboxylate region of the counterion in the micelle. Two-dimensional homonuclear NMR experiments, such as ROESY13 and NOESY,13c can reveal information about internuclear distances and molecular conformations. The NOESY experiment has been utilized in studies on surfactant chain conformations within micelles14 and on the loci of counterions15 and solubilizates.16 This study, however, utilizes the ROESY pulse sequence which was designed for small and medium size molecules. In both ROESY and NOESY experiments, the cross peak intensity reflects the extent of magnetization transfer between nuclei and is proportional to their internuclear distance. The ROESY experiments presented in this paper show that the dichlorobenzoate counterions intercalate among the trimethylammonium headgroups at the interface in both the rod-like and the spherical micelles investigated. The systems investigated here include CTA+ 3,5dichlorobenzoate (CTA3,5) whose viscoelastic solutions contain rodlike micelles, and CTA+ 2,6-dichlorobenzoate (CTA2,6), whose Newtonian solutions contain globular micelles. Inferences about the counterion effect on micellar morphology are made from comparisons of the chemical shift changes and 2D ROESY interaction peaks observed for these systems. To avoid line-broadening effects observed in the NMR spectra of rodlike micelles even at low surfactant concentrations,17 these studies deal with mixed CTAB/CTA3,5 and CTAB/CTA2,6 solutions. This study differs from most NMR work dealing with related systems because binary solutions are used with no supporting electrolyte. The 10 mM surfactant concentration used exceeds the cmc values of CTA2,6 and CTA3,5 by factors of 20 and 100, respectively. Thus, contributions from monomers may be neglected. Micrographs of CTAB/CTA3,5 solutions show that a sphere- to rod-transition occurs when the mixture contains 20-30% CTA3,5.18 SANS, at values of the scattering vector’s reciprocal (Q-1 on the order of 0.1-1 nm) sufficiently small to be in the Porod limit, can be used to provide information on the local environment (via the area per headgroup (Ahg) at the micellar surface). The following format will be used to classify the different solution components. The percentage of CTAB in the solution appears first followed by the percentage of the CTA+ chlorobenzoate. Thus, “90/10 CTA2,6” refers to a 10 mM solution which is 9 mM CTAB and 1 mM CTA2,6. (Note that all of the solutions contain 10 mM CTA+ ionssso that, in fact, only the counterion concentrations vary.) Comparisons will be made between the chemical shifts of the CTAB/CTA2,6 and CTAB/CTA3,5 solutions and, respectively, those of 10 mM CTAB, sodium 2,6-dichlorobenzoate, and sodium 3,5-dichlorobenzoate. The change in 1H (or 13C) chemical shift from that of CTAB is calculated via the following equation: (13) (a) Bothner-By, A.; Stephens, R. L.; Lee, J.; Warren, C. D.; Jeanloz, R. W. J. Am. Chem. Soc. 1984, 106, 811. (b) Summers, M. F.; Marzilli, L. G.; Bax A. J. Am. Chem. Soc. 1986, 108, 4285. (c) Kessler, H.; Gehrke, M.; Griesing, C. Angew. Chem. 1988, 27, 490. (14) Zhao, J.; Fung, B. M. J. Phys. Chem. 1993, 97, 5185. (15) Bachofer, S. J.; Simonis, U.; Nowicki, T. A. J. Phys. Chem. 1991, 95, 480. (16) Kohelmainen, E. Magn. Reson. Chem. 1988, 26, 764. (17) Ulmius, J.; Wennerstrom, H.; Johansson, L.; Lindblom, G.; Gravsholt, S. J. Phys. Chem. 1979, 83, 2232. (18) Gee, J. C. Ph.D. Dissertation, University of Tennessee, Knoxville, Tennessee, 1991.

Kreke et al.

∆HCTAB ) δsurfactant - δCTAB

(1)

For comparisons involving the aromatic region of the spectra, the sodium salts are used as the reference and “CTAB” is replaced by “2,6” or “3,5” in the notation. Experimental Section NMR. A Bruker AMX400 spectrometer with an X32CPU3 computer and a wide bore Oxford/Spectrospin magnet of 9.4 T field strength was used for all of the reported experiments. Field strengths of 400.13 and 100.62 Mhz were used to obtain the 1H and 13C spectra, respectively. Typical 1H parameter settings include temperature (TE) ) 305 K, spectral width (SW) ) 4400 Hz, 45° pulse ) 6 µs; relaxation delay ) 2-5 s; number of scans (NS) ) 32, time domain (TD) ) 32K, and size (SI) is set for one zero fill. The standard reference was the N-methyl (δ 3.115) signal on an external sample of 10 mM CTAB. Experimental reproducibility of the chemical shifts of samples that do not experience appreciable line broadening is within (0.01 ppm. Typical 13C parameter settings include TE ) 305 K, SW ) 23 800 Hz, 45° pulse ) 3.5 µs; relaxation delay ) 4 s, NS varied with concentration from 28 000 transients for the 90/10 solutions to 300 transients for the sodium dichlorobenzoate solutions, TD ) 16 K, and SI is set for one zero fill. The composite pulse decoupling program, WALTZ16, was used. The middle peak of the chloroform triplet (δ 77.00) in an external sample of o-dichlorobenzene/CDCl3 was used as the reference. Experimental reproducibility of the chemical shifts of the carbonyls is within (0.05 ppm for the samples of concentration e20% CTA3,5 or CTA2,6 and (0.02 ppm for all other concentrations. The ROESY pulse sequence utilized was the Bruker program roesyprtp (ROESY with continuous wave pulse for ROESY spin lock, phase sensitive using TPPI (time-proportioned phase increment), and presaturation of the water signal) which is τ190-τm; τm is the mixing time. All ROESY experiments were performed using a proton selective probe. To minimize t1 noise, the samples were not spun. Typical parameters used include TE ) 305 K, SW ) 4400 Hz, 90° pulse ) 6.7 µs, relaxation delay ) 2 s, τm ) 110 ms, NS per experiment ) 48, dummy scans ) 64, in the F2 dimension, TD ) 1K; SI is set for one zero fill, in the F1 dimension, number of experiments ) TD ) 500; SI is set for ca. one zero fill, and B1 field strength (for spin lock) ) 1.4 kHz. The apodization function utilized was the sine (or, when necessary, the sine2) function where SSB ) 2. To avoid distortion from the residual water peak, separate (automatic) base line corrections were applied to the data on either side of the δ 5.04.0 region. A base line subtraction technique was utilized to subtract t1 noise from the F2 dimension. The mixing time, 110 ms, was chosen to optimize cross peaks in another region of the spectra.19 Note that the CTAB/CTA3,5 solutions used in the 2D experiments were actually 92/8 CTA3,5 and 62/38 CTA3,5 concentrations (referred to as 90/10 CTA3,5 and 60/40 CTA3,5, respectively) while the CTAB/CTA2,6 solutions were 90/10 and 50/50. This allows comparisons of the systems at concentrations both above and below the sphere to rod transition of the CTAB/ CTA3,5 system. SANS. The SANS measurements were performed on the W. C. Koehler 30m SANS facility20 at Oak Ridge National Laboratory. Surfactant solutions in D2O were contained in quartz spectrophotometric cells, mounted in a thermostated cell holder held at 25.0 ( 0.1°C. The raw scattering data were corrected for detector background and sensitivity and for scattering from the empty cell; the intensities were placed on an absolute scale using precalibrated secondary standards provided by the SANS facility. The maximum accessible Q [(4π/λ) sin Θ] at λ ) 0.475 nm was 3.8 nm-1. After subtraction of a residual (Q-independent) incoherent background, B, the scattering curves were extrapolated to Q ) 0 and Q ) 10 nm-1 prior to computing the invariant, Q*: (19) Kreke, P. J.; Magid, L. J. In preparation. (20) Koehler, W. C. Physica (Utrecht) 1986, 137B, 320.

CTAB/CTA+ Surfactant Solutions Q* )

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∫Q I(Q) dQ 2

(2)

From the relationship between the Porod limit, lim{Q4[I(Q) B]}, and Q*:

lim{Q4[I(Q) - B]} )

∑Q* πφd(1 - φd)

(3)

where φd is the volume fraction of dispersed phase and Σ is the total internal surface area per unit volume of sample, the area per headgroup can be calculated21 according to Ahg ) ∑/nsurf. Ahg values were determined for CTAB/CTA3,5 mixtures at φd ) 0.01 in order to confirm the effect of the sphere-to-rod transition. Figure 1a shows the Porod plots for these mixtures. For CTA2,6, which does not form rodlike micelles below ca. 200 mM,1c SANS measurements were made on CTA2,6 in aqueous NaCl, solutions which do contain rodlike micelles. Figure 1b compares the Porod plots for 20 mM CTA3,5, 20 mM CTA2,6, and 20 mM CTA2,6 in 1 M NaCl. Table 1 tabulates the Ahg values derived from the scattering data. Materials. The 2,6-dichlorobenzoic and 3,5-dichlorobenzoic acids were obtained from Aldrich, dissolved in methanol, and stirred over charcoal in excess of 24 h. After filtration, they were recrystallized three times from acetonitrile; titration with 3 N NaOH followed. The 2,6 and 3,5-dichlorobenzoate surfactants were synthesized by Ms. Terri Smith using a silver salt method described by Gee.18 A small amount of CTA+ hydroxide was present in the solutions containing CTA3,5. The CTAB obtained from Sigma was recrystallized three times from an 80/20 (vol %) mixture of methanol and acetone. D2O was obtained from Aldrich.

Results 1H

NMR Data. The 1D 1H NMR spectrum of 10 1D mM CTAB is shown in Figure 2. The chemical shift assignments are in Table 2. The R-CH2 appears furthest downfield (δ 3.35). Traditionally, its multiplet structure has been attributed to 14N-1H coupling in addition to 1H1H coupling to the β-CH . Recently, Cheng et al.22 2 suggested that this interesting resonance pattern arises from the magnetic nonequivalence of the two 1H’s on the R carbon due to hindered rotation about the C1-C2 bond. The large narrow N-methyl (headgroup) singlet appears slightly upfield (δ 3.12) from this multiplet. Moving upfield, the broad hump next encountered (δ 1.72) is the β-CH2 signal. Next, a shoulder peak (δ 1.32) overlapping a broad, more intense signal appears. COSY experimental results from Rao et al.10 show this peak coupling to both the β-CH2 and the main chain peak, indicating that the protons of the γ-CH2 contribute to this resonance signal. Integration of this resonance equates its intensity to six 1 H’s, so, in fact, the γ, δ, and  methylenes are responsible for this peak. For simplicity, it will be referred to as γ′. The large broad peak at δ 1.24 is the combined resonances of the (CH2)6-15 methylenes of the CTA+ chain and will be referred to as the “main chain” peak. The last peak (δ 0.84), a triplet, is the ω-CH3 resonance. In the 100% CTA2,6 solutions (10 and 40 mM) and in 90/10 CTAB/ CTA2,6 data, the γ′ peak splits into separate resonances. This is apparent in the 10 mM CTA2,6 solution (see Figure 3). At this concentration, the total integration of the peaks between δ 1.21 and 0.93 equates to 10 1H’s, indicating that the γ′ peak(s) now includes the ζ and η methylenes. These assignments have been confirmed via a COSY experiment on the 40 mM CTA2,6 solution. Table 2 also contains the chemical shift information for the CTAB/ CTA2,6, CTAB/CTA3,5, and sodium 2,6-dichlorobenzoate (21) Barnes, I. S.; Hyde, S. T.; Ninham, B. W.; Derian P.-J.; Drifford, M.; Zemb, T. N. J. Phys. Chem. 1988, 92, 2286. (22) Cheng, J.; Xenopoulos, A.; Wunderlich, B. Magn. Reson. Chem. 1992, 30, 917.

Figure 1. (a) Porod plots for CTAB/CTA3,5 mixtures. (b) Porod plots for 20 mM CTA3,5, 20 mM CTA2,6, and 20 mM CTA2,6 in 1 M NaCl. Table 1. Areas per Headgroup (Ahg) for CTA+ Micelles surfactant CTA3,5

CTA2,6

CTAB/CTA3,5a

concn, mM 5.5 8.0 14.0 20.0 20.0 20.0 20.0 20.0 27.4/0 21.9/4.2 16.5/8.4 13.7/10.5

[NaCl], M

0.1 0.5 1.0

Ahg, nm2 0.42 ( 0.04 0.36 ( 0.03 0.35 ( 0.03 0.31 ( 0.03 0.98 ( 0.06 1.12 ( 0.08 0.78 ( 0.05 0.58 ( 0.04 0.59 ( 0.04 0.61 ( 0.04 0.53 ( 0.04 0.48 ( 0.03

a These solutions contain 10 g/L surfactant and on a wt/wt basis are 0, 20, 40, and 50% CTA3,5.

(Na2,6), sodium 3,5-dichlorobenzoate (Na3,5) solutions investigated. Since the formation of rodlike micelles induces visible line broadening in the aliphatic region of the 1H spectra at concentrations higher than 30/70 CTAB/ CTA3,5, these chemical shifts are not presented. Line broadening of the aliphatic resonances hides any possible resolution of ζ or η peaks in solutions of higher CTA3,5 concentrations. In the aromatic region, the chemical shift assignments were confirmed via integrations of the resonances. In the aliphatic region of both the CTAB/CTA2,6 and CTAB/CTA3,5 systems, the R, headgroup, β, and γ′ peaks all move upfield relative to their resonance positions in CTAB with the γ′ peak experiencing the most noticeable

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Figure 2.

1H

NMR spectrum for 10 mM CTAB.

Table 2. solutions CTAB Na2,6 Na3,5 10 mM CTAB/CTA2,6 90/10 70/30 60/40 50/50 30/70 10 mM CTA2,6 40 mM CTA2,6 10 mM CTAB/CTA3,5 90/10 80/20 70/30 60/40 50/50 30/70 10 mM CTA3,5 a

Kreke et al.

1H

Chemical Shift Data for CTAB, CTAB/CTA2,6, CTA2,6 and CTAB/CTA3,5 Solutionsa R

headgroup

β

3.35

3.12

1.72

γ′ 1.32

mc

ω

1.24

0.84

ortho

meta

para

7.32

7.19 7.49

7.18 7.17 7.17 7.17 7.16 7.16 7.15

6.99 6.97 6.97 6.97 6.96 6.97 6.95

7.66 3.31 3.22 3.20 3.16 3.10 3.04 3.01

3.09 3.05 3.04 3.02 2.98 2.96 2.96

1.68 1.60 1.58 1.54 1.49 1.42 1.41

1.27 1.19 1.17 1.14 1.09 1.03, 1.11 0.94, 1.02, 1.11

1.23 1.24 1.26 1.27 1.27 1.29 1.30

0.82 0.83 0.85 0.85 0.86 0.87 0.89

3.30 3.25 3.19 3.15 3.10 3.05

3.09 3.08 3.05 3.03 3.01 2.99

1.67 1.62 1.56 1.51 1.47 1.42

1.22, 1.26 1.12, 1.20 1.12 1.07 1.02, 1.13 0.98, 1.06, 1.15

1.24 1.25 1.26 1.26 1.28 1.29

0.83 0.84 0.85 0.86 0.87 0.88

7.83 7.81 7.79 7.77 7.76 7.74 7.65

7.20 7.20 7.19 7.18 7.17 7.17 7.13

Units ) ppm.

Figure 3.

1H

NMR spectrum for 10 mM CTA2,6.

shift. Figure 4 compares the ∆HCTAB values of the CTAB/ CTA2,6 and CTAB/CTA3,5 surfactant systems relative to 1 H atom positions for the 90/10 and 50/50 solutions. In the 90/10 CTAB/CTA2,6 solution, both the meta and para resonances of the chlorobenzoate counterions shift upfield from their resonance positions for Na2,6 (Figure 5). At higher concentrations of CTA2,6, however, the chemical shifts show no appreciable change. The data in Figure 5 also show that, in the 90/10 CTAB/CTA3,5

Figure 4. Comparison of ∆HCTAB for CTAB/CTA2,6 and CTAB/CTA3,5 solutions. Here -1 refers to the N-methyl 1H’s, the γ, δ,  peak is shown at its median value, 4, and the main chain peak also appears at its median value, 10.

solution, the ortho peak shifts downfield relative to its position in Na3,5. However, at higher concentrations

CTAB/CTA+ Surfactant Solutions

Langmuir, Vol. 12, No. 3, 1996 703

Figure 5. Comparison of ∆H2,6 and ∆H3,5 for CTAB/CTA2,6 and CTAB/CTA3,5 solutions, respectively. Table 3. Chemical Shifts for Carbonyl Carbonsa

a

solution

2,6

3,5

Na+ 10% 20% 30% 40% 50% 60% 70% 90% 100%

175.25

174.92 171.54 171.51 171.59 171.60 171.58

172.46 172.54 172.60 172.69 172.74 172.92 172.98

Units ) ppm.

Figure 7. (a) Expansion of 90/10 CTA2,6 ROESY spectrum which shows the aromatic-CTA+ cross peaks. (b) Same region for the 50/50 CTA2,6 solution. Figure 6. Comparison of the chemical shift changes for the carbonyl carbons.

(>90/10), the 3,5-ortho signal shifts upfield from its resonance position for the 90/10 solution. The para 1H of the 3,5-dichlorobenzoate also shifts upfield relative to its resonance in Na3,5 in the 90/10 CTA3,5 solution (and solutions of higher CTA3,5 concentration). While appreciable line broadening occurs in the aliphatic region of solutions of higher CTA3,5 concentrations, it should be noted that the aromatic resonances in these solutions remain relatively sharp. This can be attributed to the mobility of the counterions due to rapid exchange on the NMR time scale. The chemical shifts of the counterions of course represent the average of these two environments. 13C NMR Data. 13C chemical shift information for the carboxylate carbon in the Na2,6, Na3,5, CTAB/CTA2,6, and CTAB/CTA3,5 solutions is given in Table 3, and the ∆C2,6 and ∆C3,5 are shown in Figure 6. Incorporation of the dichlorobenzoate anion into the CTA+ micellar system initially (90/10 solutions) results in the increased shielding of the carbonyl carbon for both the 2,6 and 3,5 solutions. With increased 2,6 concentration, the shielding decreases

slightly. At higher 3,5 concentrations, ∆C3,5 shows little change from the 90/10 CTA3,5 value. Expanded spectral regions of the 2D ROESY spectra show the aromatic-CTA+ chain cross peaks in Figures 7 and 8 for the CTAB/CTA2,6 and CTAB/CTA3,5 solutions, respectively. This expansion shows the area between ca. 0.6 and 3.6 ppm in the F2 dimension and, for the CTAB/ CTA2,6 solution spectra, ca. 6.8 and 7.5 ppm, or, for the CTAB/CTA3,5 solution spectra, ca. 7.0 and 8.0 ppm in the F1 dimension. The expanded plot of the 90/10 CTAB/ CTA2,6 solution (Figure 7a) shows the cross peaks between the 2,6-dichlorobenzoate meta and para protons and the CTA+ main chain peak as well as a cross peak between the meta protons and the headgroup protons in the 90/10 CTAB/CTA2,6 solution. In the 50/50 CTAB/CTA2,6 solution (Figure 7b), meta-γ′ and para-γ′ cross peaks are present in addition to those which appear in the 90/10 CTAB/CTA2,6 solution. Additionally, there are weaker cross-peaks (which are difficult to plot) between the metaR, meta-β, and para-headgroup resonances. In the 90/10 CTAB/CTA3,5 solution (Figure 8a), ortho-headgroup, ortho-main chain, and a para-main chain cross peaks are present. Figure 8b shows the expanded plot for the 60/40 CTAB/CTA3,5 solution, which contains six cross peaks

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Kreke et al. Table 4. Summary of Aromatic-CTA+ Cross Peaks Present in Various Solutionsa aromatic 1H meta 10mM CTAB/CTA2,6 90/10 70/30 50/50 10 mM CTA2,6 meta 40 mM CTA2,6 meta para 10 mM CTAB/CTA2,6 90/10 70/30 50/50 10 mM CTA2,6 (para) 40 mM CTA2,6 (para) ortho 10 mM CTAB/CTA3,5 90/10 75/25 60/40 para 10 mM CTAB/CTA3,5 90/10 75/25 60/40

R

headgroup

β

γ′

main chain

w w w x

w w x x x

w w w w

x x xb xb

w x x x x

w

x x xb xb

w x x x x

x x

x x x

x x

x x x

w w w w

w

x x x

w

w w

a x signifies that a crosspeak is present. w signifies that there is a weak cross peak present. b Cross peaks appear with several of the resolved γ′ peaks.

Figure 8. (a) Expansion of 90/10 CTA3,5 ROESY spectrum which shows the aromatic-CTA+ cross peaks. (b) Same region for the 60/40 CTA3,5 solution.

corresponding to interactions between the ortho-headgroup, ortho-main chain, ortho-γ′, para-headgroup, paramain chain, and para-γ′ protons. Also present are weak ortho-R and ortho-β cross peaks. A summary of cross peaks resolved at several concentrations is shown in Table 4. It should be noted that the resolution of some cross peaks may be decreased and cross peaks of weaker interactions may disappear under various operating conditions (i.e. unstable magnetic field, temperature fluctuations). The relative integration of the stronger interaction cross peaks, however, should remain constant. Discussion 1

In the 1D H NMR data (Table 2), the upfield shifts of the headgroup, R, β, and γ′ resonances indicate that each of these protons experiences, on average, the shielding cone portion of the aromatic ring current from nearby dichlorobenzoate counterions. From this it can be inferred that both the 2,6- and 3,5-dichlorobenzoate counterions are preferentially located at the interface and intercalate among the trimethylammonium headgroups and the first few methylenes of the CTA+ chain. As the concentrations of CTA2,6 and CTA3,5 increase in the mixed counterion solutions, the shielding of these atoms also increases,

indicating the intercalation of the additional aromatic counterions. Though the resonances of the two systems undergo similar qualitative shifts, a comparison of their respective ∆HCTAB values in Figure 4 reveals several interesting differences. First consider both the 90/10 CTAB/CTA3,5 and CTA2,6 chemical shift changes. Since only spheres are observed in micrographs of both the CTAB/CTA2,6 and CTAB/ CTA3,5 systems below their 80/20 concentrations, the similarity of the ∆HCTAB values for their headgroup, R, β, main chain, and ω protons at this concentration is not surprising. However, there already exists a substantial difference in the environment experienced by their γ′ protons at this concentration. A comparison of their chemical shift changes at the 50/50 CTAB/CTA3,5 and 50/50 CTAB/CTA2,6 solution concentrations shows that the affected CTA+ peaks (except the headgroup peak) are more shielded in the 50/50 CTAB/ CTA3,5 system, such that the extent of environmental change experienced by the γ′ methylenes (and, to a lesser degree, the R and β protons) is much greater than any experienced by the 50/50 CTAB/CTA2,6 aliphatic protons. At the 50/50 concentration, the chemical shift change for the N-CH3 is independent of the micellar morphology. This indicates that the interfacial environment, as it is experienced by the headgroups, is very similar in the 50/ 50 systems investigated. Also note that there is very little difference in the environments experienced by the main chain and ω protons for the systems when the CTAB/ CTA3,5 and CTAB/CTA2,6 solutions are of similar concentrations. Similar chemical shift effects on the headgroup and, to a lesser degree, R-CH2 peaks indicate that the extent of counterion binding is approximately the same in the CTAB/CTA2,6 and CTAB/CTA3,5 systems. (This is supported by measurements which indicate that the 2,6 and 3,5 counterions displace the Br- similarly at 90/10, 80/20, and 60/40 CTAB/CTA2,6 and CTAB/CTA3,5 concentrations.23 ) However, the greater magnitude of the ∆HCTAB (23) Magid, L. J.; Weber, R. Unpublished results.

CTAB/CTA+ Surfactant Solutions

for the R, β, and γ′ resonances of the CTAB/CTA3,5 systems indicates that, on average, the 3,5-dichlorobenzoate counterion inserts further into the micelle than does the 2,6-dichlorobenzoate counterion. Insertion into the micelle’s nonpolar environment causes the aromatic 1H’s of the dichlorobenzoate counterions to be shielded. Thus, the shielding of the 1H’s at both the meta and para positions (Figure 5) of the CTAB/CTA2,6 solutions suggests that these protons insert into the micelle. The more negative ∆H2,6 of the para 1H indicates that it experiences a less polar environment, on average, than the meta 1H. Since these two protons are on the same counterion, it may be inferred that the para 1H intercalates more deeply into the micelle. All of the micellized CTAB/CTA3,5 counterion protons also experience significant chemical shift changes of their aromatic resonances at the 90/10 concentrations (Figure 5). The para resonance in the 90/10 CTA3,5 solution shifts upfield, indicating insertion into the core. The deshielding effects experienced by the ortho CTAB/CTA3,5 protons can be attributed to their average loci at the micellar interface. Both of the aromatic protons in the CTAB/CTA3,5 solutions experience shielding effects as the concentration moves from the 90/10 to 70/30 CTAB/CTA3,5. This effect increases in solutions of higher 3,5 content. In comparison, the meta and para signals of the CTAB/CTA2,6 solutions shift only slightly (if at all) at higher CTA2,6 content, from the chemical shifts of the 90/10 CTAB/CTA2,6 solution. These differences between the ∆H3,5 and ∆H2,6 with respect to counterion concentration can be attributed to a changing environment within the CTA3,5 micellar core as it undergoes the sphere-to-rod transition and the rodlike micelles continue to grow. It has been suggested that an average gradient of polarity exists within a micelle due to the micellar palisade regions and the dynamic equilibrium between bound and free states of the counterions and the micellized and free surfactants, all of which influence the extent of water-hydrocarbon contact.24 A less rough micellar surface for the rodlike micelles may contribute to a different gradient of polarities within the micelle. This polarity gradient may drive the 3,5dichlorobenzoate counterion to insert, on average, more deeply into the micellar core.25 The SANS-derived areas per headgroup presented in Table 1 for the very long rodlike micelles of pure CTA3,5 in the concentration range 5.5-20 mM and for the shorter micellar rods of pure 20 mM CTA2,6 in 1 M NaCl versus the globular micelles of CTA2,6 in pure water support these inferences concerning the rougher micellar surfaces in the case of globular micelles. For the case of CTAB/ CTA3,5 mixtures, the declining Ahg values track well the sphere-to-rod transition which occurs above the composition of 80/20. It should be noted that the two dichlorobenzoate counterions are sufficiently weak bases in water that they show negligible protonation to their conjugate acids.26 In addition, cationic surfactants (such as CTAB) are known to increase the acidity of phenols12 and aromatic carboxylic (24) Heindl, A.; Strnad, J.; Kohler, H.-H. J. Phys. Chem. 1993, 97, 742. (25) Surface “roughness” may be less for rodlike micelles because of lower average Ahg values, less water-hydrocarbon contact at the micelle-water interface, and less chain protrusion. (26) The pKa values of the two dichlorobenzoic acids are 3.45 for 3,5-dichlorobenzoate and 1.59 or 1.82 (via two different methods of determination) for 2,6-dichlorobenzoate. Reported in Ionisation Constants of Organic Acids in Aqueous Solutions. International Union of Pure and Applied Chemistry Commission on Equilibrium Data; Pergamon Press: Oxford, York, 1979.

Langmuir, Vol. 12, No. 3, 1996 705

acids.27 It is therefore unlikely that the observed trends in chemical shifts can be attributed to partial neutralization of the counterions associated with the micellar surfaces. The two dichlorobenzoate anions show similar 13C chemical shift effects (i.e. shielding of the carbonyl) upon inclusion in their respective micellar systems. The counterion’s insertion into the micelle decreases its interaction with the aqueous solvent. This leads to less hydration of the carboxyl group which, in turn, results in the upfield shift of the carbonyl carbon peak. Thus, the shielding effects seen for the carbonyl carbon in all of the solutions support the inference that both the 2,6 and the 3,5-dichlorobenzoate counterions intercalate into the their respective micelles. A comparison of the data in Table 3 and Figure 6 shows that these effects are quantitatively different, with the CTAB/CTA3,5 carboxylate carbons being more shielded than those in their respective CTAB/ CTA2,6 systems. Thus, the data do suggest that the 3,5 counterion inserts into a less polar environment than does the 2,6 counterion. This agrees with observations from the 1H data. The cross peaks between the aromatic and the aliphatic regions of the NMR spectra, shown in Figures 7 and 8, indicate that both the 3,5-dichlorobenzoate and the 2,6dichlorobenzoate counterions interact significantly with the CTA+ chain. The presence of para-main chain cross peaks in the two CTAB/CTA2,6 solutions and the weak (or undetected) interactions of the para proton with the headgroup, R, β, or, in the 90/10 CTAB/CTA2,6 solution, the γ′ protons, suggests that the average loci are those in which the 2,6-dichlorobenzoate counterion intercalates into the micellar interface rather than remaining tangential. Similar observations can be made concerning the cross peaks visible in the 90/10 CTAB/CTA3,5 spectrum. In the case of the 60/40 CTA3,5 solution (Figure 8b), however, a para-headgroup cross peak is present. Note that the intensity of this cross peak is visibly much less than those of the other para-CTA+ cross peaks. Since the distance between the two atoms involved in the interactions is proportional to the I-6 (where I refers to the integration of the atoms’ cross peaks),28 one can infer that intercalation of the 3,5-dichlorobenzoate counterion is preferred in this case as well. Conclusions Both 2D ROESY and 1D 1H NMR experimental data indicate that, in their respective systems, both the 2,6and 3,5-dichlorobenzoate counterions intercalate among the trimethylammonium headgroups at the micellar interface. Different polarities experienced by the aromatic protons and carbonyl carbons as well as the upfield shifts of CTA+ chain 1H resonances point to different insertion depths of the two counterions. The 3,5 counterion experiences an average environment that is less polar than that of the 2,6 counterion, indicating that it inserts further into the micelle’s core. While differences between the average loci of the 3,5- and 2,6-dichlorobenzoate counterions exist, they appear to be more subtle than previously thought. Acknowledgment. P.J.K., L.J.M., and J.C.G. thank the National Science Foundation (NSF CHE-9008589) for financial support. LA9509662 (27) (a) Wolff, T.; Auck, T. A.; Emming, C.-S.; von Bunau, G. Prog. Colloid Polym. Sci. 1987, 73, 18. (b) Bunton, C. A.; Minch, M. J. J. Phys. Chem. 1974, 78, 1490. (28) Kessler, H.; Bats, J. W.; Griesinger, C.; Knoll, S.; Will, M.; Wagner, K. J. Am. Chem. Soc. 1988, 110, 1033.