Aggregation of ω-Hydroxy Quaternary Ammonium Bolaform

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Aggregation of ω-Hydroxy Quaternary Ammonium Bolaform Surfactants Tim W. Davey,†,‡ William A. Ducker,*,§ and Alan R. Hayman† Chemistry Department, University of Otago, P.O. Box 56, Dunedin, New Zealand, and Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212 Received December 1, 1997. In Final Form: November 23, 1999 ω-Hydroxy quaternary ammonium bolaform surfactants form much smaller micelles in solution than the corresponding non-hydroxy (conventional) surfactants. Aggregation numbers for micelles formed by the ω-hydroxy surfactants, determined using 1H NMR spectroscopy and steady-state fluorescence quenching, are very small (N ) 5-18). The micelle size is consistent with aggregates of about half the diameter of a conventional surfactant. This implies that both the quaternary ammonium and the terminal hydroxyl are positioned at the micelle/water interface. The critical micelle concentrations of the bolaform surfactants, determined using 1H NMR and the conductivity method, are between 3 and 9 times greater than those of the corresponding non-hydroxy surfactants.

Introduction Conventional surfactants comprise a hydrophilic and a hydrophobic group. The difference in interaction of these two groups with water causes surfactants to aggregate into micelles and other nanometer-scale structures in aqueous solution. In recent years work has begun on characterizing R,ω-type (bolaform) surfactants that have two identical hydrophilic groups separated by a hydrophobic spacer chain,1-7 as well as the structurally similar gemini surfactants.8-10 Little work has been done on the micellization properties of asymmetric bolaform surfactants.11-14 A surfactant with a hydroxyl group on the 12th carbon of an 18 carbon chain (sodium R-12-hydroxyl-cis-9-octadecanoate) has been shown to form small micelles in solution and to occupy a large area at the air-water interface.15 Our interest is in compounds with an ω-hydroxy quaternary ammonium structure, prepared for surface and micellization studies. Similar compounds have been prepared for micellization studies,14 biological studies,16 wetting properties,17 hair * To whom correspondence should be addressed. E-mail: [email protected]. † University of Otago. ‡ Now at Department of Chemistry, University of Sydney. § Virginia Tech. (1) Zana, R.; Yiv, S.; Kale, K. M. J. Colloid Interface Sci. 1980, 77, 456. (2) Shimizu, T.; Masuda, M.; Shibakami, M. Chem. Lett. 1997, 267. (3) Saremi, F.; Maassen, E.; Tieke, B.; Jordan, G.; Rammensee, W. Langmuir 1995, 11, 1068. (4) Muzzalupo, R.; Ranieri, G. A.; La Mesa, C. Langmuir 1996, 12, 3157. (5) Satake, I.; Morita, T.; Maeda, T.; Hayakawa, K. Bull. Chem. Soc. Jpn. 1997, 70, 761. (6) Menger, F. M.; Mounier, C. E. J. Am. Chem. Soc. 1993, 115, 12222. (7) Ikeda, K.; Khan, A.; Meguro, K.; Lindman, B. J. Colloid Interface Sci. 1989, 133, 192. (8) Rosen, M. J. CHEMTECH 1993, 30. (9) De, S.; Aswal, V. K.; Goyal, P. S.; Bhattacharya, S. J. Phys. Chem. 1996, 100, 11664. (10) Pinazo, A.; Diz, M.; Solans, C.; Pe´s, M. A.; Erra, P.; Infante, M. R. J. Am. Oil Chem. Soc. 1993, 70, 37. (11) Hayakawa, K.; Nagahama, T.; Satake, I. Bull. Chem. Soc. Jpn. 1994, 67, 1232. (12) Menger, F. M.; Chow, J. F. J. Am. Chem. Soc. 1983, 105, 5501. (13) Abid, S. K.; Hamid, S. M.; Sherrington, D. C. J. Colloid Interface Sci. 1987, 120, 245. (14) Jaeger, D. A.; Li, G.; Subotkowski, W.; Carron, K.; Bench, M. W. Langmuir 1997, 13, 5563. (15) Shinde, N.; Narayan, K. S. J. Phys. Chem. 1992, 96, 5160-5165.

preparations,18 pyrolysis studies,19,20 studies of interactions with polyelectrolytes21 and polypeptides,11 and studies as synthetic intermediates.22 In this paper we report the critical micelle concentration (cmc) and aggregation numbers of a range of ω-hydroxy quaternary ammonium bromide bolaform surfactants (Table 1), studied using 1H NMR spectroscopy, conductivity, and steady-state fluorescence. We aim to determine whether these surfactants form small micelles with both terminal polar groups positioned on the micelle exterior, as has been shown previously for symmetrical R,ω-type bolaform surfactants.1 ω-Hydroxy quaternary ammonium surfactants typically have a Krafft temperature about 30 °C higher than the corresponding non-hydroxy surfactants.23 For this reason our work focuses on triethyl surfactants, which have much lower Krafft temperatures than trimethyl surfactants.23 Experimental Section Bolaform surfactants are named as outlined in Table 1. Conventional surfactants are named along similar lines: TMCn refers to an alkyltrimethylammonium bromide; TECn refers to an alkyltriethylammonium bromide; PyCn refers to an alkylpyridinium bromide; DMC16 refers to hexadecyldimethylammonium bromide. Most of the surfactants have bromide counterions; chloride surfactants are indicated by a “.Cl” suffix. The following chemicals were obtained from Aldrich: D2O (99.9% D), dioxane, pyrene, PyC12.Cl, PyC16.Cl, TMC12, TMC16. Water was distilled, then filtered through a Milli-Q RG water purification system consisting of a charcoal ion-exchange cartridge and a 0.2 µm filter. Pyrene was recrystallized from ethanol. The preparation of the bolaform surfactants has been reported (16) Barrass, B. C.; Brimblecombe, R. W.; Rich, P.; Taylor, J. V. J. Pharmacol. 1970, 39, 40. (17) Blackman, L. C. F.; Harrop, R. J. Appl. Chem. 1968, 17, 187. (18) Kuriyama, S.; Okahara, Y. Japanese Patent 61,229,813. Kuriyama, S.; Okahara, Y. Chem. Abstr. 1987, 106 (89950n). (19) Barbry, D.; Hasiak, B.; Glacet, C. C. R. Acad. Sci., Ser. C 1975, 281, 889. (20) Barbry, D.; Hasiak, B. Collect. Czech. Chem. Commun. 1983, 48, 1734. (21) Hayakawa, K.; Fukuda, K.; Maeda, T.; Satake, I. Bull. Chem. Soc. Jpn. 1993, 66, 2744. (22) Ukawa, K.; Imamiya, E.; Yamamoto, H.; Mizuno, K.; Tasaka, A.; Terashita, Z.; Okutani, T.; Nomura, H.; Kasukabe, T.; Hozumi, M.; Kudo, I.; Inoue, K. Chem. Pharm. Bull. 1989, 37, 1249. (23) Davey, T. W.; Ducker, W. A.; Hayman, A. R.; Simpson, J. S Langmuir 1998, 14, 3210-3213.

10.1021/la971303i CCC: $19.00 © 2000 American Chemical Society Published on Web 02/08/2000

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Table 1. Bolaform Surfactants Used in This Study Me3N+(CH2)nOH BrEt3N+(CH2) nOH Br-

TMCnOH TECnOH PyCnOH

(n ) 12, 15, 16) (n ) 12, 15, 16) (n ) 15, 16)

Me2HN+(CH2) nOH BrMe3N+(CH2)15CO2H Br-

DMCnOH TMC15COOH

(n ) 15, 16)

elsewhere.24 Ethanol was distilled over magnesium and iodine.25 The surfactants TEC12, TMC15, PyC15, and DMC16 were prepared from the corresponding bromoalkane and amine as reported elsewhere.24 All surfactants used were analytically pure as determined by elemental analysis. The structures were confirmed by 1H NMR spectroscopy. NMR spectra were recorded on a Varian Gemini 200 or Varian VXR-300 spectrometer in D2O using an insert of dioxane in D2O as an external reference (3.700 ppm). Typical settings for the VXR-300 are the following: spectral width, 1532 Hz; number of points, 32 768; number of transients, 50-500. These settings gave a digital resolution of 0.05 Hz and an accuracy in the chemical shift of (0.7 ppb. The temperature was controlled using a standard Varian VT unit, with an accuracy of (1 °C. Steady-state fluorescence spectra of dissolved pyrene in surfactant solutions were acquired on a Perkin-Elmer LS50B fluorescence spectrophotometer at 25 °C. Pyrene was added to the fluorescence cell as an ethanol solution, and the solvent was evaporated using a stream of compressed air. Solid surfactant was then weighed into the cell, and finally, 3 mL of argonsaturated H2O was added. The final pyrene concentration was 10 µmol L-1. Pyrene was excited at 335 nm, and the fluorescence intensity was recorded at 374 nm. Quencher (PyC12.Cl or PyC16.Cl) was added as a concentrated solution such that the total volume added was no more than 3% of the initial volume of the surfactant/pyrene solution. The concentrations of TEC12OH and TEC12 were 193.5 and 43.5 mmol L-1, respectively. Solution conductivity was measured at 25.0 ( 0.1 °C with a Philips PR 9500 conductivity bridge. Solid surfactant was weighed into a sample tube, and water (13 mL) was added. Solutions with lower surfactant concentrations were prepared by sequential dilution with water.

Results & Discussion cmc. NMR. Previous workers have shown26-28 that 1H NMR chemical shifts may be used to determine the cmc of a surfactant. Below the cmc, the observed chemical shift (δobs) is the chemical shift of the monomer (δmon). Above the cmc, we assume that the rate of exchange of monomers between the micelle and aqueous bulk solution is faster than the NMR time scale, so δobs is the weighted average of the monomer and micelle (δmic) chemical shifts:

( )

δobs ) δmon

( )

Cmon Cmic + δmic CT CT

(1)

where Cmon, Cmic, and CT are the concentrations of surfactant molecules existing as monomers, in micelles, and in total in the solution. If we also assume that the monomer concentration is constant above the cmc, then

δobs ) δmic -

( )

cmc (δmic - δmon) CT

(2)

Therefore, a plot of δobs vs 1/CT should yield a straight line above the cmc. Intersection of this line with a horizontal (24) Davey, T. W.; Hayman, A. R. Aust. J. Chem. 1998, 51, 581-586. (25) Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell, A. R. Vogel’s Textbook of Practical Organic Chemistry, 5th ed.; Wiley: New York, 1989. (26) Lee, Y. S.; Woo, K. W. Bull. Korean Chem. Soc. 1993, 14, 392. (27) Das, S.; Bhirud, R. G.; Nayyar, N.; Narayan, K. S.; Kumar, V. V. J. Phys. Chem. 1992, 96, 7454. (28) Zhao, J.; Fung, B. M. Langmuir 1993, 9, 1228.

Figure 1. Chemical shift (relative to monomer) versus 1/CT for NCH2 headgroup protons of TEC12 (2) and TEC12OH (9) at 25 °C in D2O.

line drawn through the points below the cmc yields the cmc. δmic is obtained by extrapolation of the line above the cmc to 1/CT ) 0. Examples of such plots are shown in Figure 1 for the surfactants TEC12 and TEC12OH. Table 2 lists the cmcs we have obtained by this method for bolaform and conventional surfactants, along with literature values for some of the non-bolaform analogues. Each value listed in Table 2 is the average of the cmcs obtained using all resolved 1H chemical shifts. TMCnOH, PyCnOH, DMCnOH, and TMC15COOH have high Krafft temperatures,23 so Table 2 lists their cmcs at 65 °C. The plot of δobs vs 1/CT for TEC12OH shows that the chemical shift is constant up to about 0.06 mol L-1, then the chemical shift increases with concentration. We interpret the change in chemical environment as the formation of micelles. For conventional micelles, the change in chemical shift is quite abrupt. The gradual transition observed for TEC12OH shows that the fraction of molecules in micelles increases more slowly than for conventional micelle-forming surfactants, and therefore, the micelles are small. This can be seen from the expression29

N h )

∂ ln(mole fraction of surfactant in micelles) ∂ ln(mole fraction of surfactant in monomers) (3)

where N h is the mean aggregation number. Therefore, we use the term “critical micelle concentration” somewhat loosely with respect to the bolaform surfactants. The cmc measurements using NMR were in D2O solvent, not H2O, so it is worthwhile considering the effect of substituting H2O for D2O on the cmc. El Seoud30 states that cmc of a surfactant in D2O is expected to be slightly lower than in H2O because D2O is a more structured liquid. Berr et al.31 note that the cmc of TMC16 (CTAB) in D2O is lower than in H2O (0.82 compared to 1.00 mmol L-1), while Kaler32 found that the cmcs of sodium alkyl sulfates are about 8% lower in D2O. The values of the cmc obtained by us for TMC12, TMC16, TEC12, and PyC16 by NMR in D2O are in good agreement with those measured by other workers using different techniques in H2O. For a homologous series of surfactants the measured cmcs often fit the equation ln cmc ) A + Bn.39 The plots of ln cmc vs n for TMCn(OH) and TECn(OH) are shown in (29) Isrealachvilli, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 1976, 72, 1525. (30) Okano, L. T.; El Seoud, O. A.; Halstead, T. K. Colloid Polym. Sci. 1997, 275, 138. (31) Berr, S. S.; Caponetti, E.; Johnson, J. S., Jr.; Jones, R. R. M.; Magid, L. J. J. Phys. Chem. 1986, 90, 5766. (32) Chang, N. J.; Kaler, E. W. J. Phys. Chem. 1985, 89, 2996.

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Table 2. Critical Micelle Concentrations (cmc mmol-1 L-1) and Aggregation Numbers for Surfactantsa number of carbons, n 12 TMCn TECn TMCnOHc TECnOH PyCn PyCnOHc DMCnb DMCnOHc TMCn-1COOHc

15

cmc

N

cmc

15, 21.9b-d 13.3-15.8b,e 13, 13.6b,g 75 65 11-11.5b,e

53-65b,e 44 ( 7b,h 5(1 7(1 86,b,f 58b,k

2.3, 3.8c 1.6b,i 15 13 1.36b,l 17

16 N

5(2 9(1

cmc

N

0.86, 0.8-1,b,e 1.7,c 1.55b-d 0.73b,g 9.4 6.2 0.75, 0.67,b,d 1.3c 5.9 2.6 12 9.7

88-104,b,e 61b,f 43 ( 3b 5 ( 2j 11 ( 2 95b,c,m 9(2 5(2 6(4

a Measurements are in D O at 25 °C unless stated otherwise. b H O. c 65 °C. d Reference 52 (from conductivity). e Reference 33 (from 2 2 surface tension and conductivity). f Reference 53. g Reference 54 (from conductivity). h Fluorescence, quencher ) PyC16.Cl. i Reference 55 (from potentiometry). j r2 ) 0.94. k Reference 56 (from light scattering). l Reference 57 (from surfactant selective electrode). m In 0.0175 M NaCl.

Figure 2. Change in cmc for surfactant homologues TMCn (2) in H2O and TMCnOH (9) in D2O at 65 °C, and TECn (4) in H2O and TECnOH (0) in D2O at 25 °C. TMCn data are from ref 52 except TMC15 (D2O, this work). TECn data are from ref 54 except TEC15 from ref 55. Table 3. Values of A and B from the Relationship ln cmc ) A + Bn for TMCn(OH) and TECn(OH) a TMCn TMCnOH TECn TECnOH a

temperature (°C)

A

-B

65 65 25 25

10.7 ( 0.8 11 ( 2 11.3 ( 0.7 11 ( 3

0.64 ( 0.06 0.52 ( 0.15 0.73 ( 0.05 0.6 ( 0.2

Errors are 90% confidence intervals.

Figure 2, and the collected values of A and B are listed in Table 3. The similarity of the B values for the conventional and bolaform surfactants shows that the energy to transfer a methylene unit from solution to the micelle is similar for the two types of micelle.29 This implies that the environment in the interior of the two micelles is similar. The alcohol (bolaform) surfactants have higher cmcs than those of their nonalcohol analogues, which can be explained by the fact that the hydrophilic hydroxyl group increases the solubility of the alcohol surfactant (33) Van Os, N. M.; Haak, J. R.; Haak, L. A. M. Physico-chemical Properties of Selected Anionic, Cationic, and Nonionic Surfactants; Elsevier: New York, 1993. (34) Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes, 2nd ed.; Tanford, C., Ed.; Wiley: New York, 1980; p 17. (35) Persson, B.-O.; Drakenberg, T.; Lindman, B. J. Phys. Chem. 1976, 80, 2124. (36) Batchelor, J. G.; Prestergard, J. H.; Cushley, R. J.; Lipsky, S. R. Biochem. Biophys. Res. Commun. 1972, 48, 70. (37) Menger, F. M.; Dulany, M. A.; Carnahan, D. W.; Lee, L. H. J. Am. Chem. Soc. 1987, 109, 6899-6900. (38) Gunther, H. NMR Spectroscopy, 2nd ed.; Gunther, H., Ed.; Wiley: Chichester, 1995; p 114. (39) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; Evans, D. F., Wennerstro¨m, H., Eds.; VCH: New York, 1994; Chapter 4.

Figure 3. 1H δdiff vs carbon number for TMC16 (2) and TMC16OH (9) in D2O at 65 °C. The alkyl chain is numbered from C1 to C16, beginning at the carbon bearing the nitrogen. The headgroup methyl group is denoted as C-1. Note that the H8 proton chemical shift is taken as the broad singlet at about 1.25 ppm and is the average of 10 CH2 units comprising the middle section of the polymethylene spacer chain.

monomer. The cmcs of the bolaform surfactants are between 3 and 9 times greater than those of their conventional anaologues. Similar ratios have been measured by other workers.14,15 The addition of the alcohol group increases the cmc by the same amount as removing two methylene groups, as seen by Shinde et al. for the difference in cmcs of sodium ricinoleate and sodium oleate.15 Changes in the headgroup of the bolaform surfactants have an effect on their cmcs. For example, TMC16OH has a lower cmc than DMC16OH. This is probably because substitution of a proton for a methyl group allows water to form shorter and thus stronger ion-dipole bonds, or a hydrogen bond, with the DM surfactant. In addition, DMC16OH has a higher charge density than TMC16OH because two methyl groups would have a smaller inductive effect. Both these effects serve to make the DMC16OH monomer more water-soluble, thus increasing its cmc. The same effect is seen for the conventional surfactants. Pyridinium surfactants have lower cmcs than their trimethylammonium analogues,33 as seen for PyC16 and TMC16. This is also the case for the bolaform surfactants PyC16OH and TMC16OH. Substitution of an ω-CH2OH for ω-COOH does not change the cmc. This is consistent with previous estimations of similar hydrophobicity for these two groups.34 The change in chemical shift on micelle formation reflects the change in chemical environment. Figure 3 shows a plot of 1H δdiff (defined as δmic - δmon) vs carbon number for TMC16 and TMC16OH. The δdiff values for the

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Langmuir, Vol. 16, No. 6, 2000 2433

bolaform and normal surfactants are similar for carbons 2-15, indicating that the change in chemical environment is similar when bolaform or conventional aggregates form. Similar plots were obtained for other trimethyl and triethyl surfactants and have been observed for other surfactants.26,28 The changes in the chemical shifts of the ω protons are different; the methyl resonance of TMC16 moves to lower frequency, while the carbinol resonance of TMC16OH moves to higher frequency. Micellization affects the methyl signal in the same way as the methylene signals and is consistent with the methyl being in the liquid interior of the micelle. The contrary shift of the carbinol signal without the concomitant shift of the adjacent methylene (H15) signal indicates that the shielding is due to a decrease in the inductive effect of the alcohol. A reasonable mechanism for this is from electrostatic interactions of the alcohol with the headgroup, counterion, and the micelle solvation sphere. This evidence suggests that the ω-alcohol is on the micelle exterior. Shinde et al.15 observed negative δdiff values for the protons of sodium ricinoleate in the vicinity of the alcohol and alkene groups, which supported their model of looping of the surfactant in the micelle. Coupling Constants. The vicinal coupling constants, 3J, in a RCH2CH2R′ spin system have been shown38 to depend on four factors: the dihedral angle, the electronegativity of the attached substituents (R and R′), the C-C bond length, and the valence angles. We were interested to see if any of these factors would be sufficiently altered on micellization to produce a measurable effect on 3J, either for the CH2OH protons of the bolaform surfactants or the CH3 protons of conventional surfactants. We hypothesized that two effects could give rise to changes in 3J. First, it has been suggested35-37 that the alkyl chains of surfactants adopt an increased proportion of trans conformations upon micellization. This factor would alter the average dihedral angle in the above systems, thus affecting the observed coupling constant 3 J. Second, if the alcohol is located near the quaternary nitrogen or bromide counterion in the micelles formed by the bolaform surfactants, the ion-dipole interaction would alter the electron-withdrawing effect of the alcohol on the adjacent CH2-CH2 system, thereby affecting 3J. Owing to fast exchange processes between the micelle and bulk solution, the observed coupling constant, 3Jobs, would be a weighted average of the monomer and micelle coupling constants, 3Jmon and 3Jmic, respectively. Using the arguments presented for eq 2, we can write 3

Jobs ) 3Jmic -

( )

cmc 3 ( Jmic - 3Jmon) CT

(4)

Thus, a plot of 3Jobs vs 1/CT should give a straight line above the cmc. In Figure 4 the coupling constants, 3J, for the ωCH2OH triplet of TEC12OH and the ω-CH3 triplet of TEC12 are plotted against 1/CT. Although there is scatter in the plots, there are breaks that are much greater than the digital resolution of 0.05 Hz. The cmc is not resolved in plots of 3J vs 1/CT for other protons in TEC12OH and TEC12 except for the quadrupolar coupling constant (between N and CH3). The cmc determined from coupling constants has a large error, but for TEC12 the cmc is consistent with values determined by NMR spectroscopy and conductivity measurements (see Table 4). The cmc obtained for TEC12OH is larger than the cmc obtained from other measurements (see Table 4). If the difference in 3Jobs were only due to changes in the dihedral angle, then our results would show a change in

Figure 4. Coupling constant 3J vs 1/CT for CH3 protons of TEC12 (2) and CH2OH protons of TEC12OH (9) in D2O at 25 °C. Table 4. Critical Micelle Concentrations of TEC12OH and TEC12 Measured Using Various Methods at 25 °C cmc (mmol L-1) method

solvent

TEC12OH

TEC12

NMR 3J coupling NMR quadrupolar coupling NMR chemical shift conductivity

D2O D2O D2O H2O

61 90 68 65

16 13 13 13

angle of 1.3° for TEC12OH and 1.7° for TEC12 (using the Karplus equation38). Because both bolaform and conventional surfactants experienced changes in 3Jobs on micellization, any alteration due to the electronegativity of the ω-alcohol in TEC12OH cannot be distinguished from conformational effects. Conductivity. Plots of conductivity vs CT for TEC12 and TEC12OH in H2O show that the molar conductivities are the same below the cmc and that TEC12OH has a higher molar conductivity above the cmc (see Supporting Information). Changes in the slopes of these plots are due to a change in charge and/or size, which are usually interpreted as a sign of aggregation.39 A large change in slope reveals a cmc of 13 mmol L-1 for TEC12, and a small change in slope reveals a cmc of 69 mmol L-1 for TEC12OH. These values are in reasonable agreement with those obtained using 1H NMR chemical shifts in D2O. The small difference in slope above and below the cmc for TEC12OH shows that the bolaform surfactant is small, highly charged, or both. Using the procedure of Evans,40 we calculated the degree of dissociation, β, of the micellar surfactants as 0.22 ( 0.03 for TEC12 and 0.49 ( 0.05 for TEC12OH. This method estimates the viscous drag on the micelle in terms of the aggregation number, N, which we obtained from NMR and fluorescence measurements (see below). The much greater dissociation of the bolaform micelle indicates a lower density of quaternary ammonium groups on the micelle exterior. Aggregation Numbers. To ascertain whether our bolaform surfactants form small aggregates with both the quaternary ammonium and the alcohol groups at the micelle/water interface, we have measured the aggregation number, N, of both the bolaform and the corresponding normal surfactant using techniques that are sensitive to small N (NMR)26,29,41 or sensitive to large N (fluorescence).42 NMR. Previous workers have shown that NMR chemical shifts may be used to calculate micelle aggregation numbers.26,30,41 In a plot of δobs vs 1/CT, the breadth of the transition at the cmc is inversely proportional to the (40) Evans, H. C. J. Chem. Soc. 1956, 579. (41) Muller, N.; Plakto, F. E. J. Phys. Chem. 1971, 75, 547. (42) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951.

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Figure 6. Schematic of micelle cross section: (a) conventional surfactant; (b) proposed structure of micelles formed by ω-hydroxy quaternary ammonium surfactants.

Figure 5. (a) Chemical shift of CH2OH protons (relative to monomer) vs 1/CT for TEC12OH in D2O at 25 °C. (b) Chemical shift of CH3 protons (relative to monomer) vs 1/CT for TEC12 in D2O at 25 °C.

aggregation number.42 Figure 5 shows the chemical shift of the CH2OH protons of TEC12OH and of the CH3 protons of TEC12 vs 1/CT. The curvature in the plot for TEC12OH indicates that small micelles are present. The size of the micelle can be quantified using the singlestep equilibrium model (the closed association model):

NA H AN and K )

[AN] [A]N

(5)

where A represents a monomer, AN a micelle, and K is the equilibrium constant. From a plot of δobs vs 1/CT, δmon and δmic are obtained. When δmon ) 0, it has been shown that43

ln(CTδobs) ) N ln[CT(δmic - δobs)] + ln N + (1 - N) ln δmic + ln K (6) Thus, a plot of ln(CTδobs) vs ln[CT(δmic - δobs)] has a slope of N. Previous studies have shown that aggregation numbers calculated using 13C NMR chemical shifts are usually lower than those obtained using other techniques because of the assumption in the mass action model (eq 5) that K and N are constant over the concentration range studied.43 The chemical shifts of all protons in the surfactant may also be used to obtain N. The average value of N ) 7 ( 1 is obtained for all plots with r2 > 0.98. The aggregation numbers obtained using the above method for bolaform and conventional surfactants are listed in Table 2, along with some literature values for conventional surfactants in H2O. Although the errors in determining N are large, it is clear from Table 2 that the bolaform surfactants have (43) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 183.

much smaller aggregation numbers than the corresponding non-hydroxy surfactants. Figure 6 shows our model for the cross section through a small bolaform micelle. The quaternary ammonium ion and the alcohol group are positioned at the micelle surface. The ω-hydroxy is drawn to the surface to hydrogen-bond with the solvent. The smallest dimension for a micelle composed of conventional surfactant must be less than or equal to twice the extended length of the surfactant. If the ω-hydroxyl of the bolaform surfactant is on the micelle surface, then the smallest dimension for the bolaform micelle must be less than or equal to the length of the bolaform molecule. If we assume that both the conventional and bolaform micelles are approximately spherical, then the volumes of micelles of equal n must be in the ratio 1:8 and the aggregation numbers should be in the same ratio. Our measured values of N for the bolaform and conventional surfactants (see below) give a similar ratio, suggesting that the alcohol of the bolaform surfactant does lie on the micelle surface. A micelle formed by a bolaform surfactant with half the radius of a micelle formed by the equivalent conventional surfactant will have twice the surface area available per headgroup, assuming both micelles are spherical. It is interesting that when the headgroup area doubles, the dissociation constant also doubles (0.49 for TEC12OH vs 0.22 for TEC12). It appears that in this case the counterion association is regulated to a constant charge per unit area. The placement of the quaternary ammonium and alcohol groups on the micelle exterior is consistent with previous observations for an ω-vic-diol surfactant,14 a chainsubstituted diol species,44 sodium ricinoleate,15 and other asymmetrical bolaform surfactants45 but in contrast to those of other chain-substituted species.46,47 Important evidence for the location of the alcohol on the micelle surface comes from work by Balasubramanian et al.45 in which they demonstrate that even mildy polar moieties reside on the micelle exterior because of the enormous surface-area-to-volume ratio of spherical micelles that amplifies even weak surface-active tendencies. We have no direct method for measuring the area occupied by the alcohol and the ammonium groups on the micelle exterior, but for comparison, we have calculated the surface excess at the air/solution interface by applying the Gibbs adsorption isotherm to surface tension mea(44) Jaeger, D. A.; Sayed, Y. M. J. Org. Chem. 1993, 58, 2619. (45) Shobha, J.; Belasubramanian, D. J. Phys. Chem. 1986, 90, 28002802. (46) Brown, J. M.; Schofield, J. D. J. Chem. Soc., Chem. Commun. 1975, 434. (47) Menger, F. M.; Jerkunica, J. M.; Johnston, J. C. J. Am. Chem. Soc. 1978, 100, 4676.

Bolaform Surfactants

surements48 (see Supporting Information). We find that the headgroup area is 60 nm2 for TEC12 and 112 nm2 for TEC12OH. The value for TEC12 is similar to that measured for other quaternary ammonium surfactants. The much larger area for TEC12OH suggests that both the quaternary ammonium and hydroxyl groups are at the air/water interface. If we were to calculate the packing parameter48 for TEC12OH using this value of the heagroup area and the Tanford formulas34 for the chain volume, then the minimum value of 1/3 (for a spherical micelle) is obtained by using a chain length of about 6. This is consistent with a 12-methylene chain with both the hydroxyl and the quaternary ammonium groups on the micelle surface. Fluorescence. Turro and Yekta42 showed that the aggregation number may be obtained by fluorescence measurements. This technique is most suited for high N values, so we have used it to obtain accurate values of N for conventional surfactants and as an approximate check of N for the bolaform surfactants. Fluorescence measurements require the use of micelle-soluble quencher molecules, so a major disadvantage of the technique is that the quencher may modify N. This is particularly likely for small micelles. Assuming that the ω-hydroxy quaternary ammonium surfactants form small micelles, as suggested by the NMR evidence discussed above, a suitable quencher must be chosen that will “fit” in the micelle. Alkylpyridinium chlorides are commonly used as fluorescence quenchers when studying cationic surfactants. Dodecylpyridinium chloride was chosen as a quencher of micelle-solublized pyrene to study micelles formed by TEC12OH, with a maximum quencher concentration that would correspond to about 1 quencher/micelle if the micelles had an aggregation number of 6. This quencher was also used by Ikeda et al.49 for their study of disodium 1,12-dodecane disulfate in which they obtained an aggregation number of 5. The aggregation number measured by fluorescence quenching for a 194 mmol L-1 TEC12OH (3 times the cmc) was 16 ( 4. This value is somewhat higher than that found at the cmc using NMR. This larger aggregation number may be due to micelle growth induced by the quencher (for an aggregation number of 16, this would correspond to about 3 quenchers/micelle) or the probe,15 or the micelles may be larger at higher surfactant concentrations. Previous work has shown that a switch from D2O to H2O does not strongly influence N.32 The aggregation of the conventional surfactant, TEC12, was examined using fluorescence with two different quenchers. An assumption of this technique is that the quencher resides exclusively in the micelle. Employing PyC12.Cl as quencher gave an aggregation number of 32 ( 2, but with PyC16.Cl as quencher, an aggregation number of 44 ( 7 was obtained. This difference demonstrates the problem resulting from greater partitioning of the shorter quencher into the aqueous medium, a problem that could (48) Evans, D. F.; Wennerstro¨m, H. The Colloidal Domain; Evans, D. F., Wennerstro¨m, H., Eds.; VCH: New York, 1994; Chapter 2. (49) Ikeda, K.; Esumi, K.; Meguro, K.; Binana-Limbele, W.; Zana, R.; Lindman, B. J. Colloid Interface Sci. 1989, 130, 290.

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also occur for TEC12OH. Despite the large errors in determining N, the results clearly demonstrate that N is much smaller for the bolaform surfactants. The vibronic structure of the fluorescence spectrum of monomeric pyrene is known to be sensitive to the local polarity. In particular, the ratio I1/I3, of the intensities of the first and third vibronic peaks increases on going to more polar solvents and may be used as a measure of the polarity of the local environment experienced by the micelle-solubilized pyrene.50 Lianos and Zana51 reported53 that I1/I3 values for alkyltrimethylammonium bromides show a slight decrease, from 1.58 to 1.42, as the alkyl chain increases in length from 10 to 16 carbons. For solvated pyrene, characteristic values of I1/I3 are 1.11, 1.33, and 1.59 in toluene, methanol, and water, respectively.50 Upon comparison of these values, it has been concluded that the pyrene was solubilized in the micelle palisade layer in contact with the aqueous medium.51 The I1/I3 ratios are 1.46 ( 0.04 for TEC12 and 1.50 ( 0.05 for TEC12OH. The similarity of the I1/I3 ratio for the bolaform surfactants to the I1/I3 ratio for the alkyltrimethylammonium bromides shows that the pyrene is in a similar environment in the solutions of conventional and bolaform surfactants. This provides further evidence for the existence of bolaform micelles. Conclusions ω-Hydroxy quaternary ammonium bolaform surfactants form micelles in solution. The cmcs are higher than those of their non-hydroxy analogues owing to the hydrophilicity of the alcohol group. 1H NMR spectroscopy and steadystate fluorescence show that the aggregation numbers for the ω-hydroxy surfactants are very small, indicating micelles that have approximately half the radius of the non-hydroxy analogues. This size is consistent with positioning of the alcohol and quaternary ammonium functionalities on the micelle surface and with a similar charge density for bolaform and conventional surfactants. 1H NMR coupling constants can be used to determine the cmc of surfactants. Supporting Information Available: Two figures illustrating the surface tension of TEC12OH and the conductivity of TEC12OH solutions. This material is available free of charge via the Internet at http://pubs.acs.org. LA971303I (50) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (51) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1981, 84, 100. (52) Evans, D. F.; Allen, M.; Ninham, B. W.; Fouda, A. J. Solution Chem. 1984, 13, 87. (53) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Fendler, J. H., Fendler, E. J., Eds.; Academic: New York, 1975; p 20. (54) Buckingham, S. A.; Garvey, C. J.; Warr, G. C. J. Phys. Chem. 1993, 97, 10236. (55) Fedchuk, T. M.; Tulyupa, F. M. Kolloidn. Zh. 1988, 50, 942. (56) Ford, W. P. J.; Ottewill, R. H.; Parrerira, H. C. J. Colloid Interface Sci. 1966, 21, 522. (57) Ben˜an, M.; Malavao´i, M.; Vesnaver, G. J. Chem. Soc., Faraday Trans. 1993, 89, 2445.