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Arenediazonium Salts: New Probes of the Interfacial Compositions of Association Colloids. 4.1-3 Estimation of the Hydration Numbers of Aqueous Hexaethylene Glycol Monododecyl Ether, C12E6, Micelles by Chemical Trapping† Laurence S. Romsted* and Jihu Yao Department of Chemistry, Wright and Rieman Laboratories, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 Received September 5, 1995. In Final Form: February 1, 1996X Chemical trapping of water and terminal OH groups in nonionic micelles of hexaoxyethylene monododecyl ether, C12E6, by an aryl cation formed by spontaneous decomposition of an aggregate-bound 4-hexadecyl2,6-dimethylbenzenediazonium ion, 16-ArN2+, demonstrates that the interfacial region of a C12E6 micelles is “wet” and that hydration of interfacial ethylene oxide groups depends on both surfactant concentration and temperature. Product yields are used to estimate hydration numbers of C12E6 micelles by assuming that the selectivity of the reaction in micelles is the same as that of its short chain analog 2,4,6trimethylbenzenediazonium ion, 1-ArN2+, in aqueous oligooxyethylene glycol solutions. The hydration numbers are found to decrease gradually with increasing C12E6 concentration at 40 °C from 3.5 at 0.45% (0.01 M) C12E6 to 2.5 in 54% C12E6, just before the lamellar phase region, and to 0.84 in 82.5% C12E6, above the lamellar phase region. The hydration numbers also decrease linearly with increasing temperature in 0.01 M C12E6 from 4.2 at 20 °C to 2.9 at 60 °C, passing through the cloud point at 50 °C. The values of the hydration numbers are in good agreement with some estimates made from water self-diffusion measurements. Chemical trapping is a rapid, straightforward method for estimating hydration numbers of aggregates of nonionic surfactants that requires no information about their size and shape and that can be used in any fluid region of their phase diagrams. Potential applications are briefly discussed.
Introduction Two-component systems of water and nonionic surfactants such as polyethylene glycol monoalkyl ethers, CmEn, form a number of aggregate structures and phases that depend on surfactant structure and on the hydration of its ethylene oxide, EO, chains.4-6 Below their cloud points (also called the lower critical consolute boundaries7 or lower critical solution temperatures8) nonionic surfactants form a number of isotropic phases with different structures such as spherical and spheroidal micelles in dilute solutions and lamellar and hexagonal liquid crystal and cubic phases in more concentrated solutions.4 Above their cloud points nonionic surfactants form opaque suspensions which eventually separate into water-rich and surfactantrich phases.4 Clouding is also observed when aqueous solutions of polyethylene oxide, PEO, polymers are heated,6,9 and the phase diagrams of both nonionic surfactants and aqueous PEO polymer solutions are characterized by a closed-loop of coexistence,10,11 which † Dedicated to Professor Donald B. Denney on the occasion of his retirement. * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, April 1, 1996.
(1) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351. (2) Chaudhuri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362. (3) Yao, J.; Romsted, L. S. J. Am. Chem. Soc. 1994, 116, 11779. (4) Mitchell, D. J.; Tiddy, G. J. T.; Waring, L.; Bostock, T.; McDonald, M. P. J. Chem. Soc., Faraday Trans. 1 1983, 79, 975. (5) Nilsson, P.-G.; Wennerstrom, H.; Lindman, B. Chem. Scr. 1985, 25, 67. (6) Lindman, B.; Carlsson, A.; Karlstrom, G.; Malmsten, M. Adv. Colloid Interface Sci. 1990, 32, 183. (7) Kjellander, R. J. Chem. Soc., Faraday Trans. 2 1982, 78, 2025. (8) Brown, W.; Johnsen, R.; Stilbs, P.; Lindman, B. J. Phys. Chem. 1983, 87, 4548. (9) Jonstromer, M.; Jonsson, B.; Lindman, B. J. Phys. Chem. 1991, 95, 3293. (10) Lang, J. C.; Morgan, R. D. J. Chem. Phys. 1980, 73, 5849. (11) Saeki, S.; Kuwahara, N.; Nakata, M.; Kaneko, M. Polymer 1976, 17, 685.
shows that water undergoes similar interactions with the EO units in the nonionic micelles and PEO polymers. The pronounced, cooperative structural and phase transitions of nonionic surfactants depend on a delicate balance of inter- and intraaggregate interactions.12 Estimating the amount of water associated with EO chains of aggregates of nonionic surfactants is crucial for understanding their structural and phase behavior.7,13-16 A number of methods are used to obtain information on the amount of water “bound” to surfactant aggregates including light scattering,17 sedimentation equilibrium,18 dielectric19 and ultrasonic relaxation,20 small angle neutron scattering,21,22 water (D2O) self-diffusion by NMR,5,9,13,14 and 17O magnetic relaxation.12 All these methods monitor a change in a bulk property of the system, and estimates of the amount of bound water depend upon the model used to interpret the data; i.e., the method may sense only water hydrating the EO groups or all water entrapped within the aggregate or all water that diffuses with but is “outside” of the aggregate.22 Comparisons of interfacial hydration estimates obtained by methods sensitive to the collective properties of the system with (12) Carlstrom, G.; Halle, B. J. Chem. Soc., Faraday Trans. 1 1989, 85, 1049. (13) Nilsson, P. G.; Wennerstrom, H.; Lindman, B. J. Phys. Chem. 1983, 87, 1377. (14) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1983, 87, 4756. (15) Jonsson, B.; Nilsson, P.-G.; Lindman, B.; Guldbrand, L.; Wennerstrom, H. In Surfactants in Solution; Mittal, K. L., Lindman, B., Ed.; Plenum: New York, 1984; Vol. 1, p 3. (16) Funari, S. S.; Holmes, M. C.; Tiddy, G. J. T. J. Phys. Chem. 1994, 98, 3015. (17) Streletzky, K.; Phillies, G. D. J. Langmuir 1995, 11, 42. (18) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977, 81, 1555. (19) Cheever, E.; Blum, F. D.; Foster, K. R.; Mackay, R. A. J. Colloid Interface Sci. 1985, 104, 121. (20) Tiddy, G. J. T.; Walsh, M. F.; Wyn-Jones, E. J. Chem. Soc., Faraday Trans. 1 1982, 78, 389. (21) Schefer, J.; McDaniel, R.; Schoenborn, B. P. J. Phys. Chem. 1988, 92, 729. (22) Zulauf, M.; Weckstrom, K.; Hayter, J. B.; Degiorgio, V.; Corti, M. J. Phys. Chem. 1985, 89, 3411.
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The chemical trapping method has been used to estimate counteranion exchange constants,23 interfacial Br-, Cl-, and H2O concentrations in cationic micelles,1 binding constants of medium chain length alcohols to aqueous microemulsions2 and water-in-oil cationic microemulsions,3 and estimates of alcohol/water ratios that mark the o/w to bicontinuous and w/o to bicontinuous structural transitions in four-component cationic microemulsions.3 Estimating Hydration Numbers by Chemical Trapping
Figure 1. Cartoon of a small section of the interfacial region of a nonionic micelle which includes the arenediazonium ion, 16-ArN2+. No attempt is made to represent the actual distributions of 16-ArN2+, interfacial water, or polyoxyethylene chains. Scheme 1
those from aggregate-bound molecular probes should provide a clearer picture of the hydration of nonionic micelles. We have developed a novel chemical trapping method for estimating the compositions of aggregate interfaces on the basis of product yields from reactions between an aggregate-bound arenediazonium ion and aggregatebound weakly basic nucleophiles (Scheme 1 and Figure 1).1-3,23-25 Interfacial concentrations are estimated from product yields without making estimates of or assumptions about aggregation numbers or about the sizes or shapes of the aggregates. Here we show that products from chemical trapping of the aryl cation, 16-Ar+, of 4-hexadecyl-2,6-dimethylbenzenediazonium ion, 16-ArN2+ (Scheme 1), in micelles of hexaoxyethylene monododecyl ether, C12E6, are consistent with the interfacial region of C12E6 micelles being “wet” and with the terminal OH groups of C12E6 being distributed throughout the interfacial region. The method also provides reasonable estimates of the hydration numbers at any surfactant concentration within the isotropic solution or L1 region of the phase diagram and across a temperature range that includes the biphasic region of the aqueous and L1 phases.4 We selected C12E6 because its phase diagram is well characterized,4 because its critical micelle concentration is very low, cmc ) 8.7 × 10-5, and virtually all added surfactant is in the micellar form,8 and because experiments could be carried out easily at temperatures below and above its cloud point at 50 °C.8 The experimental protocol is straightforward, and the only instruments required are an HPLC and a constant temperature bath once samples of reactants and products have been prepared and characterized. (23) Loughlin, J. A.; Romsted, L. S. Colloids Surf. 1990, 48, 123. (24) Chaudhuri, A.; Romsted, L. S. J. Am. Chem. Soc. 1991, 113, 5052. (25) Chaudhuri, A.; Loughlin, J. A.; Romsted, L. S. Atualidades de Fisico-Quimica Organica-1991; Florianopolis: Brazil, 1991; p 176.
Hydration numbers are estimated by chemical trapping from dediazoniation product yield ratios and the selectivity of the dediazoniation reaction toward water and the terminal OH groups of C12E6. The aryl cation, z-Ar+, formed by spontaneous loss of N2 from z-ArN2+ (Scheme 1) is a particularly suitable trapping reagent for several reasons. The heterolytic mechanism for dediazoniation is well established.26 Observed rate constants for dediazoniation are extraordinarily insensitive to medium effects and show remarkably low selectivities toward different weakly basic anionic and neutral nucleophiles1,25-27 because aryl cations are highly reactive, short-lived intermediates1,28,29 and because the arenediazonium ion and the aryl cation have very similar charge distributions, as indicated by recent ab initio calculations.30 Thus, little reorganization of the molecules and ions within the immediate vicinity of z-ArN2+ occurs during the transition through the aryl cation to product, and the overall product distribution reflects the equilibrium distribution of ions and molecules around the ensemble of diazonio groups in their ground states.1 Product distributions from spontaneous decomposition of 16-ArN2+ in aqueous solutions of C12E6 micelles are proportional to interfacial nucleophile compositions because the system is in dynamic equilibrium, i.e. the diffusion rate of the components in solution is orders of magnitude faster than the dediazoniation rate, and because the hexadecyl tail of 16-ArN2+ ensures that it is strongly bound to surfactant aggregates such that virtually all of the reactive diazonio groups are located in the interfacial region (Figure 1).1 The aryl cation intermediate 16-Ar+ traps water and terminal OH groups of C12E6 to give stable products that are analyzed by HPLC. Molar ratios of water to terminal OH groups are estimated from product yields by assuming that the selectivity of 16-Ar+ toward these nucleophiles is the same as that of its short chain analog, 1-Ar+, toward water and terminal OH groups in aqueous solutions of tetraethylene, E4, and hexaethylene, E6, glycols. 16-ArN2+ is also a good interfacial probe because its products absorb in the ultraviolet, so it can be used in very low concentrations, which minimizes perturbation of aggregate structure. In these experiments, the final 16-ArN2+ concentration is about 1 × 10-4 M, 100 times smaller than the lowest C12E6 concentration. Hydration numbers are defined by
hydration number )
NMW nNROH
(1)
where NMW is the moles of micellized water and NROH is the moles of a CmEn micellized surfactant with a PEO (26) Zollinger, H. Diazo Chemistry I: Aromatic and Heteroaromatic Compounds; VCH Publishers, Inc.: New York, 1994; p 1. (27) Swain, C. G.; Sheats, J. E.; Harbison, K. G. J. Am. Chem. Soc. 1975, 97, 783. (28) Ambroz, H. B.; Kemp, T. J. Chem. Soc. Rev. 1979, 8, 353. (29) Scaiano, J. C.; Kim-Thuan, N. J. Photochem. 1983, 23, 269. (30) Glaser, R.; Horan, C. J. J. Org. Chem. 1995, 60, 7518.
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chain of length n; i.e., n ) 6 in C12E6 micelles. The molar ratio is obtained from the dediazoniation product yields by using eq 2:
NMW %16-ArOH ) SROH W NROH %16-ArOR′
(2)
is the selectivity of the aryl cation intermewhere SROH W diate toward the terminal OH group of C12E6 compared to water and %16-ArOR′ and %16-ArOH are respectively the measured yields of the ether and phenol products is estimated by assuming that it is (Scheme 1). SROH W equal to SEOH (eq 3), the selectivity of 1-Ar+ toward water W and the terminal OH groups of E4 and E6:
) SEOH W
%1-ArOR′ NW %1-ArOH 2NE
(3)
where NW is the moles of water and NE is the moles of E4 and E6. The factor 2 corrects for the two terminal OH groups contributed by each glycol molecule. %1-ArOH and %1-ArOR′ are respectively the product yields from reaction with water and E4, water and E6, and their mixtures. Results Characterization of Dediazoniation Reaction Products. Dediazoniation of 16-ArN2+ in C12E6 micelles and its short chain analog 1-ArN2+ in aqueous solutions of E6 and E4 gives five products. All products were identified in HPLC chromatograms by spiking experiments using independently synthesized and characterized compounds. The products used to estimate hydration numbers are 4-n-hexadecyl-2,6-dimethylphenol, 16-ArOH, and dodecylhexaethylene glycol 4-n-hexadecyl-2,6-dimethylphenyl ether, 16-ArE6C12, in C12E6 micelles and 2,4,6trimethylphenol, 1-ArOH, and tetraethylene and hexaethylene glycol mono-2,4,6-trimethylphenyl ethers, 1-ArE4 and 1-ArE6, respectively, in aqueous E6 and E4 solutions. The side products (4-n-hexadecyl-2,6-dimethylfluorobenzene, 16-ArF, n-hexadecyl-3,5-dimethylbenzene, 16-ArH, and N-(4-hexadecyl-2,6-dimethylphenyl)acetanilide, 16ArNHAc, in micelles and their short chain analogs, 1-ArF, 1-ArH, and 1-ArNHAc, formed in aqueous E4 and E6 solutions) are present in variable but generally small amounts. Typical yields are as follows: 16-ArH, 0.61.9%; 16-ArF, 0.06-0.5%; 16-ArNHAc, 1-15%; 1-ArH, 0.2-1%; 1-ArF, 1.6-2.1%; and 1-ArNHAc, 0.6-1.8%. Because the side products are generated from competing reactions, their formation does not affect the values of the hydration numbers. See the Appendix for details on the formation of these side products. Total yields were always g85%. Fluctuations in the total yield are caused by variation in the small amounts of z-ArN2BF4 in MeCN stock solutions which are injected into aqueous C12E6, E4, and E6 solutions to initiate reaction.1 To correct for these fluctuations, normalized product yields are used to estimate selectivities and hydration numbers. All data, including HPLC peak areas, calibration curves, and measured and normalized product yields, are given in the Supporting Information. Determination of SEOH W . Figure 2 shows a plot of yield ratios obtained for a series of NW/NE molar ratios of water and E4, water and E6, and aqueous mixtures of E4 and E6 at 18 and 40 °C and, in one experiment, with added HCl (0.01 M). The molar ratios of H2O to E6 and to E4 ranged from about 20/1 to 100/1. The product yield ratios are directly proportional to the NW/NE molar ratios, showing that the selectivity of 1-Ar+ toward H2O and
Figure 2. Effect of increasing H2O concentration, expressed as the molar ratio of water to oligoethylene oxide, NW/NE, on the % 1-ArOH/% 1-ArE6 product yield ratios from dediazoniation of 1-ArN2+ in aqueous E6 (n ) 6) and E4 (n ) 4) solutions and their mixtures at two temperatures and with added HCl (0.01 M).
terminal OH groups is independent of solution composition and temperature, even in concentrated solutions of E4 and E6. For example, E6 is about 40 wt % of the total solution at NW/NE ) 24. The average value of the selectivity calculated from the slopes in Figure 2 using eq ) 0.60. 3 is SEOH W The experiments with E4 in 0.01 M HCl at 18 °C were used to determine if 1-ArH might be generated by a competing radical pathway, which is sometimes reported for other diazonium salts in the absence of added acid.26 1-ArCl is formed in 1-2% yield because of the competitive reaction with Cl-, but the yield ratios of all other products are the same as those run in the absence of added HCl, indicating that 1-ArH is also produced by a heterolytic pathway (see Appendix). Determination of Hydration Numbers. Table 1 lists normalized product yields of 16-ArOH and 16-ArE6C12 from dediazoniation of 16-ArN2+ in dilute aqueous C12E6 solutions at 40 °C. Each experiment was run four times to confirm the reproducibility of the method, and each reported yield is an average value obtained from three injections. The average deviation in the yield of 16ArE6C12 is about 3% for all five C12E6 concentrations, showing that the hydration of EO groups is essentially independent of C12E6 concentration in dilute solutions. In more concentrated C12E6 solutions (Table 2) the percent yield of 16-ArE6C12 increases to about 10% at about 82 wt % C12E6. At this concentration, the stoichiometric ratio of water to C12E6, Nw/NROH, is only about 5. The results in Table 3 show that in 0.01 M C12E6 the percent yield of 16-ArE6C12 increases gradually with temperature with a concomitant decrease in the percent yield of 16-ArOH. Figure 3 shows the change in the hydration number with increasing weight percent of C12E6 at 40 °C calculated from the percent yields in Tables 1 and 2 by using eq 2 ) SEOH ) 0.6. The hydration number and by setting SROH W W decreases gradually at higher C12E6 concentrations and approaches that of the stoichiometric molar ratio of water to EO units (solid line) near the lamellar phase boundary (dashed lines).4 The inset, which shows the increase in the fraction of aggregate bound water with added C12E6, is obtained from ratios of the hydration numbers, NMW/ 6NROH, to the stoichiometric ratios of water to C12E6 EO groups, NW/6NROH, at each percent weight C12E6. Figure
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Table 1. Normalized Product Yields of 16-ArOH and 16-ArE6C12 from Dediazoniation of 16-ArN2+ in Diluted Aqueous C12E6 Solutions at 40 ( 0.1 °Ca [C12E6]/M
% wt C12E6
% yield 16-ArOH
% yield 16-ArE6C12
0.01
0.45
97.2 97.2 97.2 97.2 〈97.2 ( 0.0〉b 97.2 97.2 97.2 97.2 〈97.2 ( 0.0〉b 97.1 97.3 97.3 97.1 〈97.2 ( 0.1〉b 97.1 96.9 96.9 97.0 〈97.0 ( 0.1〉b 97.0 97.0 96.8 96.9 〈96.9 ( 0.1〉b 〈97.1 ( 0.1〉c
2.81 2.77 2.78 2.78 〈2.79 ( 0.02〉b 2.81 2.78 2.78 2.77 〈2.79 ( 0.01〉b 2.89 2.74 2.75 2.86 〈2.81 ( 0.07〉b 2.94 3.12 3.07 2.96 〈3.02 ( 0.07〉b 3.03 3.00 3.19 3.10 〈3.08 ( 0.07〉b 〈2.90 ( 0.12〉c
0.05
2.25
0.10
4.5
0.15
0.20
6.75
9.0
Table 3. Normalized Product Yields of 16-ArOH and 16-ArE6C12 from Dediazoniation of 1 × 10-4 M 16-ArN2+ in Aqueous Solutions of 0.01 M C12E6 at Various Temperaturesa T/°C
% yield 16-ArOH
% yield 16-ArE6C12
20
97.7 97.7 97.7 〈97.7 ( 0.0〉b 97.4 97.4 97.4 〈97.4 ( 0.0〉b 97.2 97.2 97.2 〈97.2 ( 0.0〉b 97.1 97.1 97.0 〈97.1 ( 0.0〉b 96.7 96.7 96.6 〈96.7 ( 0.0〉b
2.32 2.33 2.31 〈2.32 ( 0.01〉b 2.62 2.56 2.63 〈2.60 ( 0.03〉b 2.77 2.78 2.78 〈2.78 ( 0.00〉b 2.91 2.90 3.00 〈2.94 ( 0.04〉b 3.30 3.33 3.41 〈3.35 ( 0.04〉b
30
40
50
60
a HPLC peak areas, observed percent yields, and calibration curves are in Table S3 in the Supporting Information. b Average yield and deviation at each temperature.
a HPLC peak areas, observed percent yields, and calibration curves are given in Table S1 in the Supporting Information. b Average yield and deviation for samples at the same concentration. c Average yield, deviation, and percent devriation for all experiments in Table 1.
Table 2. Normalized Product Yields of 16-ArOH and 16-ArE6C12 from Dediazoniation of 1.00 × 10-4 M 16-ArN2+ in Concentrated Aqueous C12E6 Solutions at 40 ( 0.1 °Ca run no.
NW/NROHb
% wt C12E6
% yield 16-ArOH
% yield 16-ARE6C12
1 2 3 4 5 6 7
101 40.6 30.1 20.9 20.9c 5.42 5.30c
19.8 38.2 45.4 54.5 54.5 82.3 82.5
97.1 96.9 96.6 96.1 95.8 89.4 89.5
2.90 3.13 3.39 3.87 3.70 10.6 10.5
a HPLC peak areas, observed percent yield, and calibration curves are given in Table S2 in the Supporting Information. b The molar ratio of water to C12E6. c The solution also contained 0.01 M HCl solution.
Figure 3. Effect of increasing the weight percent of C12E6 on the hydration number, NMW/6NE, of C12E6 micelles. The solid line shows the change in the stoichiometric H2O per mole of ethylene oxide units, NW/6NE. The arrows mark the boundaries of the lamellar phase region. The inset shows the fraction of bound water, NMW/NW, with increasing weight percent of C12E6.
4 shows that in 0.01 M C12E6 the hydration number decreases linearly with temperature through the cloud point. Discussion The product yields in Tables 1-3 demonstrate several important properties of the interfacial regions of nonionic micelles. The results at 40 °C in Tables 1 and 2 are consistent with the interfacial region of C12E6 micelles being “wet”5 and not almost “dry”.31 The major reaction product, 16-ArOH, is formed in 95-97% yield from dilute, 0.45 wt % (0.01 M) C12E6, up to the lamellar phase boundary, 54.5 wt % (1.2 M) C12E6. Even at 82 wt % (1.8 M) C12E6, above the lamellar phase region (Figure 3), 16ArOH is formed in about 90% yield, showing that 16ArN2+ always senses water. The formation of a small but very reproducible amount of 16-ArE6C12 shows that a significant fraction of the terminal OH groups of C12E6 (31) Baglioni, P.; Bongiovanni, R.; Rivara-Minten, E.; Kevan, L. J. Phys. Chem. 1989, 93, 5574.
Figure 4. Effect of increasing temperature on the hydration number of C12E6 micelles in 0.01 M C12E6. The arrow marks the cloud point. The line is drawn to aid the eye.
reacts with 16-ArN2+, consistent with the idea that EO chains and their terminal OH groups in nonionic micelles are flexible and move freely within the interfacial re-
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Hydration Numbers of Nonionic Micelles
gion.13,15,18,31 The small but reproducible decrease in 16ArOH yield (and the concomitant increase in 16-ArE6C12 yield) with increasing temperature through the cloud point (Table 3) is consistent with gradual dehydration of the EO groups with increasing temperature. The values of the hydration numbers estimated from the results in Tables 1-3 depend upon (a) the reproducibility of the product yields, (b) the assumption that the selectivity of the dediazoniation reaction of 1-ArN2+ determined in aqueous E4 and E6 solutions is the same as the selectivity of 16-ArN2+ in the interfacial region of C12E6 micelles, and (c) the assumption that 16-ArN2+ is completely micellar bound with its reactive diazonio group located in the interfacial region. The product yields listed in Tables 1-3 are very reproducible, e(2% for triplicate experiments. The results in Figure 2 show that the selectivity of the dediazoniation reaction in aqueous E4 and E6 solutions toward water versus terminal OH groups is almost one (SEOH ) 0.6) and is independent of solution W composition and temperature, consistent with the small selectivity of dediazoniation reactions toward alcohols (SAW ∼ 0.3)1 and anionic nucleophiles compared to water.1,26,27 The assumption that the selectivities of the dediazoniation reaction in bulk solution and at the micellar surface are the same is equivalent to assuming that the interaction of 1-ArN2+ with water and EO chains in H2O/ E4 and H2O/E6 mixtures is the same as the interaction of 16-ArN2+ with water and EO chains in the interfacial region of C12E6 aggregates. We note that PEO polymer hydration is often used to model hydration of EO groups in nonionic micelles.7,11,13-15 The fraction of the interfacial region sampled by 16-ArN2+ is uncertain. C12E6 micelles are assumed to be dynamic aggregates having a hydrocarbon core surrounded by an interfacial region composed of EO chains.9 Because 16-ArN2+ is hydrophobic and a surfactant, we assume that it is completely micellar bound1 with its hexadecyl tail imbedded in the hydrocarbon core of the C12E6 micelles and its diazonio group reacting with water and the terminal OH groups of C12E6 in the interfacial region of the aggregates. Determining the actual distribution the diazonio group within the interfacial region, by NMR for example, requires careful study because of the low 16-ArN2+ concentrations used in these experiments and because 16-ArN2+ decomposes spontaneously. The hydration numbers shown in Figures 3 and 4 are within the ranges reported by a variety of methods for different CmEn surfactants obtained under different experimental conditions. Some representative examples of hydration numbers for the isotropic, L1, phase listed by surfactant(s), hydration number, and method are as follows: C8E4, 5.5, water self-diffusion;32 C8E5, 2.6, viscosity;33 C12E6, C16E8, between 1 and 2.5, neutron scattering;22 C12E8, C12E5, C12E4, 1000) on a GC/MS system (HP5890 Series II GC interfaced with a HP5971 mass selective detector) using a 12 m × 0.2 mm × 0.33 µm HP-1 capillary column (cross-linked methyl silicone on fused silica). IR spectra were recorded on a Mattson Genesis Series FT-IR spectrophotometer. Product yields were determined on a Perkin-Elmer LC-235 HPLC equipped with a diode array detector fitted with a C-18 reverse phase column (5 µm, 4.6 mm i.d. × 25 cm), using 20 and 150 µL loops, a LC-235 diode array detector, a PE-Nelson 900 series interface attached to a Ultra PC computer, and a Hewlett-Packard Laser Jet III printer. HPLC chromatograms were analyzed by using PE Nelson Turbochrom 3 software. Melting points were determined on a Mel-Temp apparatus and are uncorrected. Dediazoniation Reactions. Dediazoniation was initiated by injecting freshly prepared, ice cold, stock solutions of 16-ArN2BF4 or 1-ArN2BF4 (ca. 0.01 M in CH3CN as needed to give final concentrations of about 1 × 10-4 M) into thermally equilibrated solutions containing all other reagents. Because arenediazonium ions react with MeCN (see Appendix), stock solutions were used as soon as possible. Dediazoniation reactions ran for at least 10 half-lives (t1/2) at all temperatures, except reactions at 18 °C, which are quite slow and were run for only 5t1/2 (3.2 days, 97% reaction). In aqueous 0.02 M C12E6, t1/2 for 16-ArN2+ is 913 min (18 °C), 191 min (30 °C), and 47.5 min (40 °C) compared to 45 min (40 °C) in cationic micelles.1 In aqueous E6 at NW/NE ) 15, t1/2 for 1-ArN2+ is 758 min (18 °C) and 25 min (40 °C) compared to 30 min (40 °C) in aqueous 0.01 HCl.1 In each dediazoniation reaction of 1-ArN2+ in aqueous E4 and E6 solutions, cyclohexane (50 µL) was layered on top of each solution to prevent loss of 1-ArOH by vaporization into the head space of the volumetric flask.1 Product Yields. In HPLC analyses, the mobile phase was 64% MeOH/36% i-PrOH for separating products from 16-ArN2+
Romsted and Yao and 78% MeOH/22% H2O for separating products for 1-ArN2+, with flow rates of 0.8 mL/min. Typical retention times in minutes for products of 16-ArN2+ are 16-ArNHAc, 6.2; 16-ArOH, 8.14; 16-ArF, 15.4; 16-ArH, 16.4; 16-ArE6C12, 22.2; and those for products of 1-ArN2+ are 1-ArNHAc, 4.8; 1-ArOH, 7.9; 1-ArE4, 8.1; 1-ArH, 27.6; 1-ArF, 29.1. Absorbances were monitored at 220 nm, and reported peak areas are averages of triplicate injections. Product concentrations were obtained from peak areas from standard calibration curves using independently prepared products, and percent yields are based on the concentration of added 16-ArN2BF4 or 1-ArN2BF4. Materials. Hexaethylene glycol monododecyl ether, C12E6 (Fluka, >98%, mol wt ) 450), and reagent grade (Aldrich) MeCN, 2,4,6-trimethylphenol, 1-ArOH (99%), 1,3,5-trimethylbenzene, and 1-ArH were used as received. The preparations of 4-nhexadecyl-2,6-dimethylbenzenediazonium tetrafluoroborate, 16ArN2BF4, 2,4,6-trimethylbenzenediazonium tetrafluoroborate, 1-ArN2BF4, and 4-n-hexadecyl-2,6-dimethylphenol, 16-ArOH, have been described.1 Tetraethylene glycol, E4, and hexaethylene glycol, E6 (Aldrich, 97%), were vacuum distilled before use. All aqueous solutions were prepared in distilled water which was passed over activated carbon and deionizing resin and then redistilled. Hexaethylene Glycol 4-Hexadecyl-2,6-dimethylphenyl Mono Ether, 16-ArE6. Freshly distilled E6 (50 g, 180 mmol) and 16-ArN2BF4 (1.5 g, 3.4 mmol) were placed together in a 100mL round bottom flask and stirred overnight at 40 °C. The solution was extracted with Et2O (3 × 50 mL), and the combined ether extracts were washed with water (3 × 100 mL), dried over MgSO4, and rotoevaporated to give 1.6 g of a yellow oil which showed several peaks by HPLC. The product mixture was chromatographed on silica gel (150 g; 70-230 mesh, Aldrich) and eluted first with ethyl acetate (500 mL) to remove byproducts and then with MeOH (500 mL) to remove 16-ArE6, which was a pale yellow solid after isolation. Two consecutive Norit treatments in MeOH gave 0.9 g (43%) of white crystals, mp 34 °C. 1H NMR (CDCl3) δ (ppm): 0.88 (3 H, t, J ) 8.0 Hz, RCH3), 1.26 (26 H, br s, (CH2)13), 1.54 (2 H, br, CH2), 2.23 (6 H, s, o-ArCH3), 2.46 (2 H, t, J ) 8.0 Hz, p-ArCH2), 3.00 (1 H, s, br, OH), 3.504.00 (24 H, m, Ar-(OCH2CH2)6), 6.79 (2 H, s, ArH). Hexaethylene Glycol Dodecyl 4-Hexadecyl-2,6-dimethylphenyl Ether, 16-ArE6C12. To prepare the alkoxide ion, 16ArE6 (0.5 g, 0.82 mmol) in toluene (10 mL) was placed in a 50-mL two-neck round bottom flask fitted with a condenser. Pieces of Na (0.1 g, 4.3 mmol) were added, and the mixture was refluxed under dry N2 until the Na dissolved. The solution was cooled, C12H25I (0.8 g, 2.7 mmol), dissolved in toluene (10 mL), was added, and refluxed for 2 days. The solution was cooled, a small amount of water was added to neutralize unreacted alkoxide ion, and then the solution was transferred to a separatory funnel containing 100 mL of Et2O and 100 mL of 0.01 M HCl. The water layer was extracted twice with Et2O, and the combined Et2O extracts were washed twice with 0.01 M HCl and twice with water, dried over MgSO4, and rotoevaporated to give a yellow oil. Chromatography on silica gel (100 g; 70-230 mesh, Aldrich) eluted with a hexane/ethyl acetate gradient (100:0 f 80:20 f 60:40 f 40:60 f 20:80 f 0:100, in 300 mL/step) gave 0.2 g (31%) of 16-ArC12E6 as white crystals, mp 33 °C. 1H NMR (CDCl3) δ (ppm): 0.87 (6 H, t, J ) 8.0 Hz, RCH3), 1.25 (44 H, br s, (CH2)13 and (CH2)9), 1.56 (4 H, br, CH2 and CH2), 2.33 (6 H, s, o-ArCH3), 2.47 (2 H, t, J ) 8.0 Hz, p-ArCH2), 3.42 (2 H, t, J ) 8.0 Hz, OCH2R), 3.50-4.00 (24 H, m, Ar-(OCH2CH2)6), 6.79 (2 H, s, ArH). 4-Hexadecyl-2,6-dimethylfluorobenzene, 16-ArF. 16-ArF was synthesized by the Schiemann reaction. 16-ArN2BF4 (1 g) was placed in a 50-mL breaker, hexane (30 mL) was added, and the solution was warmed overnight at 40 °C. The solution was extracted with water (3 × 30 mL), dried over MgSO4, and rotoevaporated to give white crystals. Pure 16-ArF (0.5 g, 63%, mp 47 °C, m/z ) 348) was obtained by chromatography on silica gel (60 g; 70-230 mesh, Aldrich) eluted with hexane. 1H NMR (CDCl3) δ (ppm): 0.90 (3 H, t, J ) 8.0 Hz, RCH3), 1.27 (26 H, br s, (CH2)13), 1.56 (2 H, br, CH2), 2.23-2.24 (6 H, d, o-ArCH3), 2.49 (2 H, t, J ) 8.0 Hz, p-ArCH2), 6.78-6.82 (2 H, d, ArH). 1-Hexadecyl-3,5-dimethylbenzene, 16-ArH. 16-ArN2BF4 (1 g, 2.2 mmol) dissolved in cold CH3CN (20 mL) was added to an aqueous 0.01 M NaOH solution (500 mL). The mixture was stirred overnight at 40 °C and then extracted with Et2O (3 × 100
+
+
Hydration Numbers of Nonionic Micelles mL). The combined ether extracts were washed with 0.01 M HCl (2 × 100 mL) and with water (2 × 100 mL), dried over MgSO4, and rotoevaporated to give a yellow solid. White crystals of 16ArH (0.15 g, 20%, mp 41 °C, m/z ) 330) were obtained by chromatography on silica gel (100 g; 70-230 mesh, Aldrich) eluted with hexane. 1H NMR (CDCl3) δ (ppm): 0.87 (3 H, t, J ) 8.0 Hz, RCH3), 1.25 (26 H, br s, (CH2)13), 1.56 (2 H, br, CH2), 2.27 (6 H, s, o-ArCH3), 2.50 (2 H, t, J ) 8.0 Hz, p-ArCH2), 6.80 (2 H, s, ArH). N-(4-Hexadecyl-2,6-dimethylphenyl)acetamide, 16-ArNHAc. This compound was prepared by two methods. Method A: 4-Hexadecyl-2,6-dimethylaniline,1 16-ArNH2 (1 g, 2.9 mm), was dissolved in cold, anhydrous pyridine (40 mL), 0.8 mL of acetyl chloride was added slowly with stirring, and the solution was warmed to room temperature for about 1 h. Cold water (20 mL) was added, and the precipitate was collected and recrystallized twice from CH3CN (0.8 g, 71%). Method B: 16-ArN2BF4 (0.1 g, 0.25 mmol) was dissolved in CH3CN in a 10-mL volumetric flask containing 0.1 mL of water. The solution was kept at room temperature overnight and then at 40 °C for 6 h, giving an offwhite precipitate that was recrystallized twice from CH3CN to give white crystals (0.06 g, 69%). Melting points are 116-117 °C for both preparations, and they give the same retention times by HPLC. Additional analytical data for product from Method A: m/z ) 387; IR (Nujol) λmax (cm-1) 3293 (NH, m), 1657 (CONH, m); 1H NMR (CDCl3/DMSO, 1:3) δ (ppm) 0.85 (3 H, t, RCH3), 1.20 (26 H, br s, (CH2)13), 1.50 (2 H, br s, CH2), 2.07 (3 H, s, COCH3), 2.11 (6 H, s, o-ArCH3), 2.43 (2 H, t, p-ArCH2), 6.79 (2H, s, ArH), 8.74 (1 H, s, CONHAr). Hexaethylene Glycol Mono-2,4,6-trimethylphenyl Ether, 1-ArE6. Freshly distilled hexaethylene glycol, E6 (50 g, 180 mmol), and 1-ArN2BF4 (1.0 g, 4.3 mmol) were placed in a 100-mL round bottom flask and stirred overnight at 40 °C. The solution was extracted with Et2O (3 × 50 mL), and the combined ether extracts were washed with water (3 × 50 mL). The combined water extracts were reextracted with Et2O (3 × 50 mL), and the combined ether extracts were dried over MgSO4 and rotoevaporated to give a yellow oil. Pure liquid 1-ArE6 (0.5 g, 29%) was obtained by using the chromatography procedure described for 16-ArE6. 1H NMR (CDCl3) δ (ppm): 2.21 (3 H, s, p-ArCH3), 2.23 (6 H, s, o-ArCH3), 2.88 (1 H, s, br, OH), 3.50-4.00 (24 H, m, Ar-(OCH2CH2)6), 6.79 (2 H, s, ArH). Tetraethylene Glycol Mono-2,4,6-trimethylphenyl Ether, 1-ArE4. Pure liquid 1-ArE4 (m/z ) 312) was obtained in 40% yield from 1-ArN2BF4 in E4 by using the procedure described for 1-ArE6. 1H NMR (CDCl3) δ (ppm): 2.20 (3 H, s, p-ArCH3), 2.23 (6 H, s, o-ArCH3), 3.39 (1 H, s, br, OH), 3.50-4.00 (16 H, m, Ar-(OCH2CH2)4), 6.78 (2 H, s, ArH). 2,4,6-Trimethylfluorobenzene, 1-ArF. Pure liquid 1-ArF (m/z ) 138) was synthesized in 75% yield by using the procedure described for 16-ArF. 1H NMR (CDCl3) δ (ppm): 2.19-2.20 (9 H, ArCH3), 6.73-6.78 (2 H, d, ArH). N-(2,4,6-Trimethylphenyl)acetamide, 1-ArNHAc. 1-ArN2BF4 (1 g, 4.3 mmol) was dissolved in CH3CN in a 50-mL volumetric flask containing 0.1 mL of water. The solution was kept at room temperature overnight and then at 40 °C for 6 h. The crystals obtained after removing the solvent by rotoevaporation were recrystallized twice from CH3CN to give white crystals (0.5 g, 65%, mp 216-217 °C, m/z ) 177). IR (KBr) νmax (cm-1): 3236 (NH, m), 1646 (CONH, m). 1H NMR (CDCl3/DMSO, 1:1) δ (ppm): 2.01 (3 H, s, COCH3), 2.08 (6 H, s, o-ArCH3), 2.18 (3 H, s, p-ArCH3), 6.78 (2 H, s, ArH), 8.85 (1 H, s, CONHAr).
Acknowledgment. We are grateful to George Huber for obtaining the NMR spectra, to Heng-Xin Weng for obtaining the mass spectra, and to C. A. Bunton and Torsten Herbertz for helpful discussions. Support for this work was generously provided by the Center for Advanced Food Technology (CAFT Publication No. D10535-8-95), New Jersey Commission on Science and Technology Center at Rutgers University. Additional support from the NSF U.S.-Latin American Cooperative Programs Brazil, the National Science Foundation (Grant CHE9113331), and the Busch Fund of Rutgers University is appreciated.
Langmuir, Vol. 12, No. 10, 1996 2431 Scheme 2
Table 4. Effect of Added 16-ArOH on the Percent Yields of 16-ArOH, 16-ArE6C12, and the Byproduct 16-ArH for Dediazoniation of 1 × 10-4 M 16-ArN2+ in 0.01 M C12E6 Solutions at 40 ( 0.1 °C observed yieldsa 104[16-ArOH]/M
% 16-ArOH
% 16-ArE6C12
% 16-ArH
% totalb
0.25 0.50 1.00 1.50 2.00
95.2 92.9 75.8 56.7 38.9
2.62 2.42 2.11 1.71 1.39
3.48 6.35 15.6 23.4 31.2
105 108 109 105 103
a HPLC peak areas and calibration curves are in Table S7 of the Supporting Information. b %total ) %16-ArOH + %16-ArE6C12 + 2(%16-ArH).
Appendix The fluoro product, z-ArF, is probably formed by thermal dediazoniation (i.e. the Schiemann reaction) in the MeCN stock solutions, in C12E6 micelles (16-ArF), and in aqueous E4 and E6 solutions (1-ArF). This reaction does not affect the % z-ArOH/% z-ArE6C12 product ratios because formation of fluoro products in MeCN simply reduces the initial concentration of arenediazonium ion in solution and because they are formed in competing reaction with H2O and C12E6 in micellar and oligooxyethylene solutions. The origin of z-ArNHAc is more speculative. Initial experiments showed that stock solutions of z-ArN2BF4 in MeCN (
