Relation between surfactant structure and properties of spherical

Pyrophosphate-Based Gemini Surfactants. David A. Jaeger, Yapin Wang, and Richard L. Pennington. Langmuir 2002 18 (24), 9259-9266. Abstract | Full Text...
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Langmuir 1991, 7, 2089-2096

2089

Relation between Surfactant Structure and Properties of Spherical Micelles. 1-Alkyl-4-alkylpyridinium Halide Surfactants Jan Jaap H. Nusselder and Jan B. F. N. Engberts' Department of Organic Chemistry, University of Groningen, Nijenborgh 16, 9747 AG Groningen, The Netherlands Received January 15, 2991. In Final Form: May 13, 1991 This paper describes a detailed study of the properties of spherical micelles formed from 18 l-alkyl4-alkylpyridinium iodides. Structural variations in the surfactants include (i) branching of the 4-alkyl chain while keeping the number of carbons in the chain invariant and (ii) variation of the lengths of the 1-and 4-alkyl groups. It is found that the free energy of transfer of a methylene group from water to a spherical micelle depends on the position of the CH2 group in the aggregate (Stern region vs core) and on whether the methylene group is located in the main or in the side chain. Interestingly,the thermodynamic stability of a micelle appears to be little affected by changes in the packing of the alkyl chains in the core of a micelle. The micropolarity in the Stern region of the spherical micelles is ethanol-like and is not affected by (i) the shape of the surfactant monomer, (ii) the length of the 4-alkyl chain, and (iii) the hydrophobicityof the 1-alkyl chain. The catalytic efficiency of a spherical micelle in the decarboxylation does not depend on the alkyl chain length and is barely dependent of 6-nitrobenzisoxazole-3-carboxylate on the degree of branching of the 4-alkyl chain. The rate constant in the micellar phase points also to an ethanol-likemicropolarity in the Stern region. In contrast, the catalytic efficiency is strongly influenced by the hydrophobicityof the 1-alkyl chain and the substitution pattern of the pyridinium ring. Different degrees of shielding of the probe molecule from water by part of the surfactant may account for this effect. The dependence of the catalytic efficiency on the morphology of the aggregate stems probably from different locations of the probe in different aggregates. The small changes in surface potential reflect minor variations in the surface charge density and criticalmicelle concentration upon alkyl-chainbranching. Finally, a fast and efficient route for the synthesis of branched surfactants, starting from lactones, is presented.

Introduction In the past 3 decades a considerable amount of research effort has been directed toward the determination of the physicochemical properties of self-assembled surfactant aggregates, especially spherical micelles.1*2 The relation between the architecture of the surfactant molecule and various properties of spherical micelles formed from these surfactants has been widely studied. The results were of direct relevance in attempts to design structural models for spherical micelles.' In this context, it is surprising that few systematic studies have been made of the effect of alkyl chain branching on the properties of simple surfactant aggregates. Our previous work3 has shown that alkyl chain packing in the micellar core and the dynamics of a spherical micelle are substantially affected by chain branching. The morphologyof the aggregate is also greatly dependent on the precise structure of the surfactant. l-Methyl-4-(C12-alkyl)pyridiniumiodides can associate into spherical micelles, rodlike (or wormlike, no stiffness is implied) micelles, or vesicles (closed bilayer membranes) mainly depending on the shape of the surfactant monomer." Herein we present a detailed study of the effect of structural variations in the surfactant on the properties (1) Lindman, B.; WenneretrBm, H. Top. Curr. Chem. 1980,87,1. (2) Grieeer, F.; Drummond, C. J. J. Phys. Chem. 1988,92,5580. (3) Nueselder, J. J. H.; Engberta, J. B. F. N. J.Phys. Chem. 1989,93, 6142. ._._ (4) Nusselder, J. J. H.; Groot, T.-J. de; Trimbos, M.; Engberta, J. B. F. N. J. Org. Chem. 1988,53, 2423. (5) Nusselder, J. J. H.; Engberta, J. B. F. N.J.Am. Chem. SOC.1989, -1-1 -1 ,. -R -O-O-.. (6) Nuelder, J. J. H.; Engberta, J. B. F. N. J. Org. Chem. 1991,56, 5522-5527.

of spherical micelles formed from 1-alkyl-4-alkylpyridinium iodides (1-22). The headgroup of these surfactants shows a long wavelength charge-transfer absorption band which can be used as an intrinsic micropolarity probe.' This intrinsic probe has definite advantages over extrinsic probes, since probe-induced perturbations of the aggregate are absent and the location and orientation of the probe are well-defined. The properties of the spherical micelles which we examined include the critical micelle concentration (cmc), the degree of counterion binding (@),the micropolarity in the Stern region of the micelles, the surface potential, and the catalytic activity of the micelle toward the unimolecular decarboxylation of 6-nitrobenzisoxazole-3-carboxylate8 (a popular kinetic probe for the microenvironment at the micellar surfaces). In the surfactant molecule the following structural features have been varied: (i) the length of the alkyl chain in a series of 1-methyl-4-alkylpyridiniumiodide surfactants, 1-5; (ii) the degree of branching of the alkyl chain in l-methyl-4-(C12-alkyl)pyridinium iodides, 4 and 6-12; (iii) the counterion type in 1-methyl-4-n-dodecylpyridinium halides, 4 and 21; (iv) the substitution pattern of the pyridinium ring, 4 and 22; (v) the hydrophobicity of the 1-alkyl chain in a series of 1-alkyl-4-n-dodecylpyridinium iodide surfactants, 4 and 14-20. Interestingly, the relatively small effects which alkyl chain branching exert on the properties of spherical micelles studied in this work, contrast with those found by (7) SudhBlter, E. J. R.; Engberta, J. B. F. N. J. Phys. Chem. 1979,83, 1854. (8) Kemp, D. S.; Paul, K. G . J. Am. Chem. SOC.1975, 97, 7305. (9) Bunton, C. A,; Savelli, G . Ado. Phys. Org. Chem. 1986, 27, 213.

0 1991 American Chemical Society

Nusselder and Engberts

2090 Langmuir, Vol. 7, No. 10, 1991

19 -

n.c, , H 1 ,cw cn, ) , H3C(CH1)1

-

-

c N + - c H ,

21 -

1".CIlHlS

-

B;

0

I'

I

22

CHI

Varadaraj et al.1° These authors conclude that chain branching in their @-branchedGuerbet surfactants leads to a considerable increase in both the water penetration into the micellar core and the micropolarity in the Stern region of the micelle. Menger et a1.l1 and Lewis et al.12 have examined how the properties of bilayer membranes depend on the degree and position of branching in the alkyl chain of vesicle-forming surfactants. Experimental Section General Procedures. Melting points were measured on a Koffler hotstage and are uncorrected. The distillations were performed bulb-to-bulb in a Kugelrohr (BQchi). The boiling points are distillation temperatures and are uncorrected. 1H NMR spectra (solutions in CDCla) were determined on a Bruker WH-90-Ds (90 MHz) or a Varian VXR-300 (300 MHz) instrument. 13C NMR spectra (CDCls) were run on a Varian XL-100/5 (25 MHz), a Nicolet NT-200 (50 MHz), or a Varian VXR-300 (75 MHz) instrument. Elemental analyses were performed in the Microanalytic Department of this laboratory by Mr. H. Draayer, Mr. J. Ebels, and Mr. J. E. Vos. The water used in all experiments was demineralized and distilled twice in an all-quartz distillation unit. Synthesis. Surfactants4,16,' 12,'and 13'have been described previously. The novel Surfactants were all prepared from the (10) Varadesaj, R.; Bock, J.; Valint, P.; Brons, N. Langmuir 1990,6, 1376. Comparisonofthe data presentedin this study with thoseobtained by Varadaraj et al. for Guerbet sulfate and ethoxy sulfate surfactants reveals that surfactant properties depend not only on the nature of the hydrocarbon chain but may ale0 critically respond to the type of headgroup attached to it. See Varadaraj, R.; Bock,J.; Valint, P.; Zushma, s.; Thomas, R. J.Phys.Chem. 1991,96,1671. Varadaraj, R.; Bock, J.; Valint, P.; Zuehma, S. J. Phys.Chem. 1991,95, 1682. (11) Menger, F. M.; Wood, M. G.,Jr.; Richardson, S.; Zhou, Q.; Elrington, A. R.; Sherrod, M. J. J. Am. Chem. SOC. 1988,110,6797. (12) Lewis, R. N. A. H.; Sykes, B. D.; McElhaney, R. N. Biochemistry 1987,26,4036. Lewis, R. N. A. H.; McElhaney,R. N. Biochemistry 1985, 24,2431. Lewis, R. N. A. H.; McElhaney, R. N. Biochemistry 1985,24, 4903.

corresponding alkyl-substituted 1-bromoalkanes (vide infra) according to the general procedure reported previously.' Thus, the anion of 4-methylpyridine (in ether at -30 "C) was reacted with the relevant alkyl bromide13 to provide the long-chain pyridine. Quarternization was performed with the alkyl halide" in acetone. The surfactants were crystallized from THF-ether mixtures at low temperature. All novel compounds showed satisfactory elemental analyses and were fully characterized by 1H and 1Bc NMR spectral data (availableon request). Many of the surfactants showed thermotropic phase behavior which will be discussed separately.l8 Synthesisof Branched 1-Bromoalkanes(SchemeI). The synthesis of 1-bromo-5-ethylnonane (lla) (R = n-butyl, R' ethyl, and n = 4) will be described as an example. Similar conditionswere used for the preparations of the other branched bromides (7a, 8a, and loa). 5-Ethyl-4-nonenyl Acetate (lld). A solution of n-butylmagnesium bromide (0.25 mol) in 150 mL of ether (ca. 1.67 M) was slowly added to a stirred solution of 6-valerolactone (lle, 0.25 mol) in 150 mL of ether (ca. 1.67 M). After addition was completed, the mixture was refluxed for 2 h. After cooling to room temperature, ethylmagnesium bromide (0.25 mol) in 150 mL of ether (ca. 1.67 M) was added and the mixture was refluxed again for 2 h. To the cooled suspension, a mixture of acetic anhydride (1mol) and acetic acid (1mol) was carefully added. After evaporation (under vacuum) of the ether, the slurry was refluxedfor 24 h. TheHOAcAczOmixturewas partly evaporated under vacuum and the residue was poured into ice-water. After neutralization, the crude alkene (actually a mixture of isomeric alkenes) could be isolated in 85% yield by extraction with ether (three 150-mLportions) as an almost colorless oil. The mixture of alkenes was purified by distillation under reduced pressure (bp 130 "C at 0.1 mmHg). 1H NMR 6 = 0.77 (6 H, distorted triplet), 1.11-1.86 (10 H, b), 1.92 (3 H, e), 3.91 (2 H, t), 4.99 (1 H, m) ppm. 13C NMR 6 = 13.65 (q), 13.83 (q), 20.73 (d), 22.26 (t), 22.62 (t),22.99 (t), 25.89 (t),26.93 (t),27.08 (t), 29.49 (t), 29.58 (t),30.16 (t),30.26 (t),30.50 (t),32.79 (t),36.21 (t),36.39 (t),64.10 (t), 118.68 (d),125.24 (a), 125.45 (d), 137.72 (e), 137.87 (s), 142.66 (81, 170.84 ( 8 ) ppm. 5-Ethylnonanyl Acetate (llc). The crude mixture of alkenes (0.22 mol), dissolved in 150 mL of acetic acid, was hydrogenated in the presence of 5 g of 10% palladium-coated carbon (14h; 2-3 bar Hs pressure). The product was filtered and the fiitrate was concentrated and distilledunder reduced pressure (bpof l l c 130°Cat0.1mmHg)toyield75%of l l c (overallyield from lle). 1H NMR 6 = 0.90 (6 H, distorted triplet), 1.1-1.8 (15 H, b), 2.07 (3 H, s), 4.07 (2 H, t) ppm. 13C NMR 6 = 13.96 (q), 20.80 (q), 22.94 (t),25.62 (t), 28.72 (t),29.50 (t),33.04 (t), 36.86 (t),64.87 (t),170.98 ( 8 ) ppm. 1-Hydroxy-5-ethylnonane(1lb). The acetate l l c (0.094 mol) was refluxed for 2.5 h in 250 mL of ethanol containing 25 g (0.45 mol) of KOH. After evaporation (under vacuum) of ethanol, addition of water (250 mL), and neutralization with concentratedHC1,thealcoholl l b wasextractedwithether(three 150-mLportions). l l b was isolated in 85% yield. However, l l b could be isolated almost quantitatively (90-99% yield)when the ester bond of l l c was cleaved with lithium aluminum hydride." 1HNMR 6 = 1.0 (6H, distorted triplet),1.13-1.72 (15H, b), 3.67 (2 H, t) ppm. 13C NMR 6 = 14.01 (q),23.01 (t),28.78 (t),29.78 (t), 29.48 (t), 29.79 (t),33.17 (t),37.08 (d), 63.24 (t) ppm. 1-Bromo-5-ethylnonane(lla). PBr3 (35.4 g, 0.13 mol) was added slowlyto l l b (0.13mol) at 0 "C. The mixture was stirred for 2.5 h at 60 "C and subsequently poured onto ice. After neutralization with 4 N NaOH, extraction with ether (three 75mL portions), and distillation under reduced pressure (bp 135 "C at 12 mmHg), l l b (0.09 mol) was obtained in 69% yield. lH (13) 6-Bromoundecane ( 9 4 was prepared in two steps from Cundecanone, according to general literature procedures.17 (14) 1-Iodo-3-hydroxypropaneand l-iodc~3-methoxyetewere prepared from the corresponding chloro isomers according to ref 15. (15) Pattison, F. L. M.; Brown, G. M. Can. J. Chem. 1956, 34,879. Jones, L. W.; Powers, D. H. J. Am. Chem. SOC.1924,46,2518. (16) Nusselder, J. J. H.; Engberts, J. B. F. N.; Doren, H. A. van. To be submitted for publication to Liquid Cryst. (17) Organic Synth. 1973,53,70. Aizpurua, J. M.;Cassio, P.; Palomo, C. J. Org. Chem. 1986,51, 4941.

Langmuir, Vol. 7, No. 10, 1991 2091

Properties of Spherical Micelles NMR: d = 0.88 (6 H, t), 1.13-1.48 (13 H,b), 1.83 (2 H,qi), 3.40 (2 H, t) ppm. I9C NMR 6 = 14.03 (q), 23.02 (t),25.20 (t),28.80 (t), 32.64 (t), 33.15 (t), 33.43 (t), 33.71 (t),37.10 (d) ppm. Conductivity Measurements. Conductivitywas measured by use of a Wayne-KerrAutobalanceUniversal Bridge B642 fitted with a Philips electrode,PW 9512101, with a cell constant of 0.71 cm-1. The solutions were thermostated in a cell for at least 15 min. The conductivity cell was equippedwith a magneticstirring device. Surfactant concentrations were varied by the addition (microsyringe)of 20-50-pL portions of a concentrated solution of the surfactant to the conductivity medium. Concentrations were corrected for volume changes. The degree of counterion binding is taken as the ratio of the slopes of the conductivityvs concentration curve above to that below the cmc. Optical Absorption Spectroscopy. Optical absorption spectra were recorded on a Perkin-Elmer A5 double beam spectrophotometer, equipped with a thermostated cell compartment. Quartz cuvettes had optical path lengths of 1.000 or 0.100 cm. Sampleswere thermostated at least 15 min before the recordings were started. Surface Potential Measurements. Both the preparation of the probe 1-hexadecyl-4-[oxocyclohexadienylidene)ethylene]1,4-dihydropyridine(HOED)and the measurementsof the pKlob values of HOED bound to a micelle were performed according to Drummond.18 Kinetic Measurements. 6-Nitrobenzisoxazole-3-carboxylate (6-NBZ)was prepared by following literature procedure^.^^ The first-order rate constants for the decarboxylation of 6-NBZwere measuredat 30.0i0.1O C bymonitoringtheincreaseinabsorption at 410 nm with a Varian Cary 210 or Perkin-Elmer A5 spectrophotometer. All reactions were followed for at least three halflives, and the rate constants (reproducible to within 2%) were calculated by the Guggenheimmethod.20In a typicalexperiment 5 p L of a freshly prepared stock solution of 6-NBZ in methanol (8 X 10-2 M) was added to 2.5 mL of the surfactant solution (pH 11.3, NaOH) in the cell.

Results and Discussion A General Route to the Preparation of AlkylSubstituted 1-Bromoalkanes. The preparation of branched monofunctional alkanes, especially the alkylsubstituted 1-bromoalkanes,is not trivial, although many of these compounds have been described in the literature. However, few general synthetic methodologies exist. Keil and co-workers21have synthesized alkyl-substituted 1-bromoalkanesusing malonic ester and a secondary alkyl bromide. Other possibilities for stepwise elongation of secondary alkyl bromides have been described.% However, these methods can be laborious. Whitemore and coworker~ have ~ ~ prepared branched alcohols by reaction of an alkyl Grignard reagent with an aldehyde or ketone. Various alcohols have been synthesized by using this concept, but the preparation of alkyl-substituted primary alcohols is not possible. The use of w-functionalized aldehydes or alkyl halides can solve this problem.24 Menger et al.l* used this basic idea and developed an elegant and general route to alkyl-substituted carboxylic acids. These acids can be transformed into the corresponding bromides by standard procedures. A totally different approach to prepare branched monofunctional alkanes involves the photolysis of unsymmetrical diacyl peroxide^.^^ ~

~

~~~~

~~

~~~~

(18) Drummond, C. J.; Crieser, F.; Healy, T. W. J. Phys. Chem. 1988, 92,2604. (19) Borsche, W. Chem. Ber. 1909,42,1316. Lindemann, H.; Cissee, H. Juetus Liebigs Ann. Chem. 1929,469,44. (20) Cuggenheim, E. A. Philos. Mag. 1926,2,473. (21) Keil, W. Hoppe-Seyler's 2.Physiol. Chem. 1947,282, 137. (22) Bergamasco, R.; Horn, D. H. S.; Nearn, R. H.; Wilkie, J. S. A u t . J. Chem. 1985,38,475. (23) Moersch, W.; Whitemore, F. C. J. Am. Chem. SOC.1949, 71,819. (24) Fuganti, C.; Graseelli,P.; Servi, S.;Hdgberg, H.-E.J. Chem. SOC., Perkin Trans. 1 1988,3061. (25) Feldhues, M.; Schafer, H. J. Tetrahedron 1985,41, 4213.

Scheme I 0

II 1) RHgX 2) R'HgX ______)

3) Ac,O/HOAc (80-904)

R'

x-1

d

e

b

a

Table I. Hydrophobicities of the 4-Alkyl Groups (C4), Cmc's, and Degrees of Counterion Binding (8) for l-Methyl-a-(or2-)alkylpyridiniumHalides in Water at 30 OC surfactant 1 2

3 4 5 6 7 8 12

13 21 22

Efi"

cmc,b mmol kg-1

fisc %

4.01 5.47 6.00 6.53

42.7 10.65 5.07 2.50 1.29 3.93 3.76 4.65d 4.35 13.0" 4.95 3.65

78 79 81 82 79 84

7.06 6.50 6.41 6.41 6.41 6.20 6.53 6.53

80 79 80 78d 71 84

See text. Experimental error 1-2 %. Experimental error 1%. d At

25

OC.

The synthetic methodology we have used to synthesize alkyl-substituted 1-bromoalkanes is based upon two successive Grignard reactions on a lactone (Scheme I). The tertiary alcohol is directly dehydrated and the primary hydroxyl group is protected with a mixture of acetic acid/ acetic anhydride to inhibit the formation of cyclic ethers. After hydrogenation and deprotection, the obtained alcohol can be transformed into the corresponding bromide. The lactone, if not commercially available, can be prepared from the corresponding ketone by a BaeyerVilliger oxidation. This synthetic approach is a relatively quick (the total synthesis takes 4-5 days), efficient (ca. 60% overall yield), and cheap method to prepare alkylsubstituted 1-bromoalkanes. The ring opening reaction of lactones to prepare branched monofunctional alkanes has been described previously.26 Cmc and Counterion Binding of l-Methyl-4-(or 2-)alkylpyridinium Halide Surfactants. The cmc, counterion binding (p), and the hydrophobicity of the 4-alkyl group, expressed as the sum of Rekker's hydrophobic of 1-8,12-13, and 21-22 are fragmental constants (Cfi)27 listed in Table I. As anticipated, elongation of the 4-alkyl group (1-5) decreases the cmc and slightly increases 8. Alkyl chain branching (6-8 and 12) increases the cmc. Although the number of carbon atoms is constant, the hydrophobicity of 6-8 and 12 is slightly influenced since the branched surfactants have somewhat smaller surface areas exposed to water (vide infra). As a result the (26) BystrBm, S.; HBgberg, H.-E.; Narin, T. Tetrahedron 1981, 37, 2249. (27) Rekker, R. F. The Hydrophobic Fragmental Constant; Eleevier: Amsterdam, 1977; p 350-355.

2092 Langmuir, Vol. 7, No. 10,1991

Nusselder and Engberts

Table 11. Free Energies of Transfer of Various CHI Groups from Water to Different Parts of a spherical Micelle surfactant AGmc, kJ mol-' A@, kJ mol-' A@-, kJ mol-' A C M , kJ mol-' ref l-alkyl-4-alkylpyridinium iodides -3.19 f 0.03 -2.42 k 0.3 -0.60 0.02 this work -2.99 -1.81 b sodium l-(alky1)alkyl sulfates decylammonium carboxylates -1.08 -3.29 C decylammonium dicarboxylates -0.55 d dodecylalkyldimethylammoniumbromides ca. -0.39 -1.95 35 phosphatidylcholines -2.74 -1.80 e n ref 0 See text. * Calculated from the cmc's given in ref 31. Calculated from the cmc's given in ref 33. Calculated from the cmc's given i 35. e Calculated from the cmc's given in ref 37.

hydrophobiceffect upon micelle formation is smaller which explains the slightly higher cmc's. Thus, differences in alkyl chain packing3 are of minor importance. In contrast, a much higher cmc is found for 13,in which a rigid segment has been introduced in the center of the alkyl chain, whereas the hydrophobicity of the 4-alkyl chain is only slightly affected (6Cfi = -0.33) by the presence of the triple C-C bond. Therefore, it must be concluded that the stiffness of this alkyl chain hampers effective alkyl chain packing in the micellar core and, concomitantly, hampers micellization. The lower value of fl is also indicative for a less well packed micelle. It appears that this less efficient chain packing is not easily rationalized in terms of the surfactant-block mode128 in which it is assumed that the alkyl chains possess an all-trans configuration. Changing the counterion from iodide (4)into bromide (21)results in a 2-fold increase of the cmc and a decrease in the degree of counterion binding. The presence of the more strongly hydrated bromide ion lessens the ability of the counterion to approach closely to the headgroup at the micellar surface. The resulting increase in the effective charge of the headgroup will increase the free energy of micellization. The higher cmc of 22 compared to that of 4 originates probably from the less favorable packing of monomers of 22 in a micelle because of the less symmetric nature of this surfactant. Differences in charge distribution in the pyridinium ring cannot account for the observed change in ~ m c . ~ 9 The change in the cmc as a function of the alkyl chain length of a surfactant can be described by the Shinoda equation log cmc = an, + c (1) in which n, represents the number of carbon atoms in an unbranched hydrocarbon chain and the cmc is expressed in mole fraction units. For 1-5 indeed a linear correlation is found (r = 0.9999) with a = -0.306, a value often found for ionic surfactants,' and c = 0.668 at 30 OC in water. The value of c is characteristicfor the kind of headgroup of the surfactant. The free energy for transfer of one methylene group in the main chain of a surfactant from water into the hydrocarbon core of a micelle can be calculated from eq 3, which is a combination of eqs 1and 2. Equation 2 stems

+

AGmic= (1 B)RT In cmc AG"" = 2.3(1

+ B)RTa

(2)

(3)

from the mass-action model.30 A constant fl (0.8)is chosen and small changes in the counterionbinding upon variation (28) Fromherz, P. Ber. Bunsen-Ges. Phys. Chem. 1981,85,891; Chem. Phys. Lett. 1981, 77, 460. (29) Jacobs, P. T.; Anacker, E. W. J. Colloid Interface Sci. 1973,44,

Table 111. Hydrophobicities of the 1-Alkyl Groups (Cfi), Cmc's, and Degrees of Counterion Binding (8)of l-Alkyl-4-n-dodecylpyridinium Iodides in Water at 30 OC surfactant

Zfi"

cmc,) mmol k g l

8,' 7%

4 14 15 16

0.702 1.232 1.762 1.639 2.292 0.099 0.181

2.50 2.21 1.91 1.93 1.54 2.45 1.99

82 79 79 76 79 80 78

17 19 20 a

See text. Experimental error 1-2 %

.

Experimental error 1 %

of n, (Table I) are ignored. AGmc amounts to -3.19 kJ mol-' for l-methyl-4-n-alkylpyidinium iodide surfactants at 30 "C This value is in good accord with literature data (Table 11). Alkyl chain branching, while keeping the number of carbon atoms in the alkyl chain constant (6-8and 121, increases the cmc. Apparently, the transfer of a methylene group in a side chain of an alkyl group from water to the core of a micelle (AGBC)is less favorable than the transfer of a CHz group in the main chain (AGmc). Introduction of this observation into eq 1 results in log cmc = an,

+ bnb + c

(4)

in which nb is the number of carbon atoms in the side chain. A linear dependence of log cmc on nb is, indeed, found (r = 0.973, b = -0.232, and c = -0.633). A constant value for a (-0.306) was chosen, derived from the changes in cmc with n, for unbranched surfactants, since AGmcis considered to be not affected by the presence of side chains.31 The effects due to the introduction of a side chain are totally accounted for in the side chain constant ( b ) ,which acts as a kind of uexcessfunction". As expected, the value of c remains invariant within experimental error. A value for AGm of -2.42 kJ mol-' has been calculated from the value of b, in good accord with values calculated from literature data (Table 11). The smaller AGmcompared to AGmcis generally ascribed to the less unfavorable hydration of the branched surfactant monomer, because less apolar area is exposed to ~ a t e r . ~ lThe 2 ~ resultant smaller hydrophobic effect is the main reason for the higher cmc of micelles formed from branched instead of unbranched surfactants. 1-Alkyl-4-n-dodecylpyridinium Iodide Surfactants. The hydrophobicity of the 1-alkyl group (Cfi),27as well as the cmc and the degree of counterion binding (8) for micelles formed from 4, 14-17, and 19-20, are listed in Table 111. The decrease of the cmc per CH2 group in the l-alkyl chain is much less compared to that for a CH2 group in the 4-alkyl chain. Furthermore, the cmc is independent of the degree of branchingof the l-alkylchain; compare 15 and 16. Introduction of an oxygen atom into the l-alkyl group (19and 20)increases the cmc. The degree

505. _._.

(30) Steniue, P.; Backlund, S.; Ekwall,P. IUPAC Chem. Data Ser.

1980,28, 295.

.

(31) Evans,H. C. J. Chem. SOC.1956,579. (32) Funasaki, N.; Hada, S. J. Colloid Interface Sci. 1978, 68, 454.

Properties of Spherical Micelles

Langmuir, Vol. 7, No. 10, 1991 2093

Table IV. Values for a and b in Equation 6, Correlation of counterion binding is only slightly dependent on the Coefficients (r),and Micropolarities in the Stern Region hydrophobicity of the l-alkyl group. Expressed in 2 Values for Micelles Formed from A l-alkyl chain with less than four carbon atoms will be l-Alkyl-4-(or2-)alkylpyridinium Iodides located in the Stern region of a mi~elle.~-~9~3 From the data in Table 111,we calculate that AGSbm,the free energy surfactant a b r kcal mol-' of transfer of a methylene group from water into the Stern l-alkyl-4-alkylpyridinium 0.856 -5.31 0.9994 80.5b region, is -0.60 kJ mol-'. This value is much smaller than iodidesa AGmcbecause of the incomplete dehydration of these me19 1.114 -28.21 0.9954 84.8 thylene groups in the Stern region. -2.81 0.9959 20 0.818 79.6 Interestingly, for decylammonium carboxylates two 22 0.882 -6.06 0.9997 79.7 regions in the plot of log cmc vs the number of carbon atoms of the carboxylate (nd) can be d i ~ t i n g u i s h e d . ~ ~ - ~ ~0 Surfactants 1-12 and 14-18. b Surfactants 2-5,7,8, 12, 14, 16, and 17. For 6, .Wesue = 78.5 kcal mol-', and for 16, Pmb = 79.3 kcal For nd I 3, AGSbm is -1.08 kJ mol-'. For nd 1 4, a much mol-'. larger dependence of log cmc on nd is found. This is indicative of penetration of (part of) the alkyl chain of the the surfactant molecules in an aggregate. However, carboxylate into the core of the micelle. From these data, changes in Am are observed when a heteroatom is present AGbCk (-3.27 kJ mol-') can be calculated, where AGback is in the l-substituent (19 and 20) or when the substitution the free energy of transfer of a methylene group into the pattern of the pyridinium ring is changed (22). The micellar phase for a backfolding alkyl chain. As expected, transition energies (ET= 2869OAm-' kcal mol-'; Am in nm) AGbCk is rather similar to AGmc. However, for n-dodeare linearly correlated with the Kosower 2 valuem cylalkyldimethylammoniumbromides AGhckequals -1.95 kJ a value much smaller than that for the decylZ=aET+b (5) ammonium carboxylates, but of the same order as AGsc (Table 11). Apparently the alkyl chains of n-dodecylalTable IV lists the constants a and b for 1-12,14-20, and 22. The micropolarity in the Stern region of spherical kyldimethylammonium bromides already interact in the micelles formed from 2-8, 12, 14-17, 19-20, and 22 monomericstate,because both chains are covalently bound expressed in Z values is also given. An ethanol-like Stern to the same headgroup. As shown above, interaction of region is found for all spherical micelles studied, except alkyl chains leads to a decrease in the free energy of transfer for 19 in which the micropolarity is methanol-like. These from water to an apolar environment. In contrast, for results are in good accord with those of Drummond and decylammonium carboxylates interaction between the deGrieser; where ethanol-like Stern regions for micelles cy1 chain and the alkyl chain of the carboxylate is very formed from a large variety of surfactants were found. unlikely in the monomeric state, consistent with an The more polar Stern region of micellar 19 may originate undisturbed free energy of transfer. from partial deprotonation of the hydroxyl group, caused The backfolding of the l-alkyl group of 18 accounts for by the positively charged micellar surface nearby. The the higher stability of the aggregates formed from 18 than easy transition of these micelles into vesicles supports this anticipated from extrapolation of the data of 4 and 14-17. conclusion.6 We contend that this higher stability does not originate Surfactants 8 (above 18mmol kg-l), 9-1 1,17 (above 4-5 from the deviating aggregate morphology of 18 (monomers mmol kg-l), and 18 associate into bilayers, which can be of 18 associate into vesicles, those of 4 and 14-17 into easily transformed into vesicles.6 The maximum of the spherical micelle# since the free energy of transfer of a CT absorption band of these aggregates is hardly shifted methylene group from water into the core of an aggregate (AGmc)is independent on the aggregate m o r p h ~ l o g y . ~ ? ~ (Am ~ = 286-290 nm) compared to that of micelles. This observation is in contrast to earlier work of Sudhiilter et Calculations of the cmc's of 19 and 20 using eq 2 and alS4lwho found Am = 358 nm for vesicular l-methyl-4the data in Table I1 do not provide satisfactory estimates. (17-tritriacontany1)pyridiniumiodide, pointing to a dichloThus the hydrophobicity of the surfactant alone does not romethane-like Stern region. However, this absorption determine the stability of an aggregate when heteroatoms maximum most likely stems from 13-:2 which is generated are present in the alkyl chain of a surfactant. Most likely during the vesicle formation using the sonication procespecific interactions with water will play an important dure. Thus the micropolarity of the Stern region of role. vesicular l-alkyl-4-alkylpyridiniumiodides is ethanol-like, Micropolarity in the S t e r n Region. We find that too. Interestingly the micropolarity of the Stern region changes in the structure of the l-alkyl and 4-alkyl group is independent on the morphology of the aggregate, in l-alkyl-4-alkylpyridiniumiodides have no effect on the although the alkyl chain packing in the various aggregates position of the charge-transfer (CT) absorption maximum varies considerably. (Am). This applies both for monomers in solution and for Surface Potentials of Spherical Micelles. The solvatochromic acid-base indicator l-hexadecyl-4-[(oxo(33) Lianos, P; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983, 91, 276. Verrall, R. E.; Milioto, S.; h a , R. J. Phys. Chem. 1988,92,3939. cyclohexadienylidene)ethylene]-1,4-dihydropyridine (34) Jansson, M.; Jbnsson, B. J. Phys. Chem. 1989,93, 1451. (HOED)has been utilized to measure the potential of (35) Li,P.; Janeeon, M.; Bahadur, P.; Stilbe, P. J. Phys. Chem. 1989, spherical micelles according to the method of Drummond 93. 6458. (36)Zana, R. J. Colloid Interface Sci. 1980, 78, 330. et a1.18 Table V lists the maximum of the solvatochromic (37) Lin, T.-L.; Chen, S.-H.; Roberta, M. F. J. Am. Chem. SOC. 1987, absorption band of HOED ( A d , the polarity a t the binding 109,2321. Tausk, R. J. M.; Esch, J. van; Karmiggelt, J.; Voordouw, G.; site (@), the value of pKaOb"of HOED, the value of pKai Overbeek, J. T. G. Biophys. Chem. 1974,1,184. Lin, T.-L.; Chen, S.-H.; Gabriel, N. E.; Roberts, M. F. J. Am. Chem. SOC. 1986,108,3499. King, of HOED,and the potential (\kelp) for micelles formed M. D.; Marsh, D. Biochemistry 1987,26, 1224. from 2, 4, 6-8, 12, 15, 20, and 21. For all micelles isos(38) From the experimental values of the CVC of l-methyl-4-n-alkox-9

ycarbonylpyridinium iodides a value of -3.20 kJ mol-' for the free energy of transfer of a CHz group from water into the core of the vesicle can be calculated. This value is in excellent agreement with AGme (-3.19 kJ mol-l).e (39) Tanford, C. The Hydrophobic Effect; 2nd ed; Wiley: New York, 1980; Chapter 11.

(40) Kosower, E. M. J. Am. Chem. SOC. 1958,80, 3253. (41) Sudh6lter, E. J. R.; Engberta, J. B. F. N.; Hoekstra, D. J. Am. Chem. SOC. 1980,102,2467. (42) Chang, J. H.; Ghno,M.; Esumi, K.; Meguro, K. Colloid Surf. 1989, 40, 219.

Nusselder and Engberts

2094 Langmuir, Vol. 7, No. 10,1991 Table V. Absorption Maxima of HOED (A=), Dielectric Constants at the Micellar Interface (&), pK,l and pK.Ob Values of HOED, and the Surface Potentials ( W x D ) of Micelles Formed from 1-Alkyl-4-alkylpyridinium Iodide Surfactants.

surfactant

Xm,nm

2

492 495 493 493 491 492 497 493 493

4

6 7 8 12 15b 20* 21 0

~ht

30.1 27.4 29.0 29.0 30.7 30.1 25.7 29.0 29.0

pK.‘

pK.ob

WP,mV

9.57 9.64 9.61 9.61 9.56 9.57 9.68 9.61 9.61

7.94 7.32 7.54 7.44 7.64 7.54 7.59 7.43 7.44

97 f 3 137 f 2 122 f 3 128 f 3 114 0.4 120* 1 124 129 129 f 0.4

*

[Surfactant]= cmc + 2.5-6mmol kg-1; T = 25 O C . b At 30 O C . 1

.e 4J

I:

QI

LI

‘0 a

t I

I

(Y

u O re L

3 0)

-4.5

-4.0

-3.5

log CCMCI

Figure 1. Experimental (m) and calculated potentials ( 0 )of micelles formed from 2,4, 6-8, and 12 a~ a function of log cmc.

bestic points were found, indicating that there is no specific interaction of the probe with the surfactants. In all experiments the surfactant concentration was 2.5-6 mmol kg-l above the cmc. It has been demonstrated that in this concentration range the (surface) potential is independent of small changes in the surfactant c ~ n c e n t r a t i o n . ~ ~ The micropolarity at the binding site can be obtained from a comparison of A, for HOED in micelles and in various 1,4-dioxane-water mixtures.18 This micropolarity of the micelles lies between that of methanol (e = 32.6) and ethanol ( 6 = 24.3). This result is in full accord with that obtained by using the intrinsic CT absorption band (vide supra). The pKai’sof HOED in the micelles can be derived from A, and literature data.18 These pKai values differ slightly, but significantly, between the different micelles. The surface potential decreases when the 4-alkyl chain is branched (6-8), when the 4-alkyl chain is shortened (2), when the counterion is changed from I- to Br- (21), or when the 1-alkyl chain is varied (15,20) (Table V). The experimental potential is found to be linearly dependent on log cmc (slope -62 mV, r = 0.9813, Figure 1). Healey et al.44have analyzed the effect of the addition of salt on the surface potential of micelles. They found a slope of +59 mV for the plot of the change in surface potential vs the salt-induced change in log cmc. These authors have derived theoretically the following relation (43)Frahm,J.;Diekmann, S.; H w e , A. Ber. Bunsen-Ges. Phys. Chem. 1980,84, 566. (44)Healy, T.W.;Drummond, C. J.; Grieser, F.; Murray, B. S. Langmutr 1990, 6, 506.

Table VI. Experimental and Calculated Surface Potentials of Micelles Formed from 1-Methyl-4-alkylpyridiniumIodides surfactant \Iraxp,mV q*p mV 2 97 177 4 137 216 6 122 193 7 128 196 8 114 182 12 120 195 a See text.

between the surface potential (90) and the activity of the surfactant ion in water (aaq) q o = (59.16/~)log aaq+ (l/ze)baqo + pmic0+

dip)

(6)

Herein are paqoand pmicothe standard potentials of a surfactant molecule in aqueous solution and in the micellar state, respectively, pdjp the chemical potential due to dipoles, z the valency of the charged surfactant, and e the unit of charge. Provided that the set of terms occurring after the first term in eq 6 does not change appreciably with a change in electrolyte concentration, it follows that for univalently charged surfactants dlqol/d log aaq= 59.16 mV (7) By plotting A1901 vs A log cmc the effect of the different surfactants is masked. Only the effects of the salt-induced change in the cmc on I q o l is revealed. According to Healey et al.44the surface potential is being changed by the addition of salt “not because the ionic strength is being changed but because the surfactant ion is the potential determining ion”, and the concentration of this surfactant ion depends on the electrolyte concentration. This analysis is, however, not feasible for a study of the change in the surface potential with the surfactant structure since it is in this case invalid to assume that the latter terms in eq 6 remain constant when different surfactants are compared. Therefore, potentials of micelles formed from 2,4,6-8, and 12 have been calculated (Table VI) accordingto the dressed micelle model, which is based on the nonlinear Poisson-Boltzmann equation.46 These calculated potentials must be regarded as surface potentials (\kdC), since no counterions are taken into account in that part of the calculation. The potentials are larger than the measured ones by a constant factor (Table VI). The slope of qCdc vs log cmc is -55 mV, a value close to that of @ e x p ~log s cmc (Figure 1). The difference between the experimental and calculated potentials may originate from the use of an inaccurate dielectric constant in the calculation. It appears, however, that HOED probes a surface potential ( q o ) and that this surface potential can be accurately described by the nonlinear Poisson-Boltzmann equation. The surface potential of micelles is thus a function of both the charge density at the surface and the ionic strength of the solution. The decrease of the surface potential upon alkyl chain branching stems both from a slight decrease of the surface charge density and from an increase in the cmc (the concentration of free surfactant in solution) which results in a higher ionic strength. Further work is certainly needed to strengthen these conclusions. The relation between the surface potential and the cmc remains an intriguing and delicate problem, which is still not sufficiently understood. Influence of the Surfactant Structure on the Micelle-Catalyzed Decarboxylation of 6-Nitrobenzisox(45) Evans,D. F.; Mitchell, D. J.; Ninham, B. W. J . Phys. Chem. 1984, 88, 6344.

Langmuir, Vol. 7, No. 10, 1991 2095

Properties of Spherical Micelles Scheme I1

18

10

0 17

16 -

Table VII. Values for km and K/nfor the Decarboxylation of 6-NBZCatalyzed by Micelles Formed from l-Alkyl-4-(or 2-)alkylpyridinium Halide Surfactants. surfactant

1 2 3 4

6 7 8 9 10 11 12 13 14 15 16 17 19 20

21 22

k,- X l@, s-l 3.2 3.3 3.7

3.5 4.2 4.6 4.0 17 >llb >lob

K l n X lo-*,M-I 0.6 7.3

12 12 14 9.9 9.3

1.7

3.9

17

4.1 5.2 6.2 8.6 9.8 5.6 5.1 4.4 7.2

15 17 8.8 6.0 12 12 18 5.8

2.0

0 At 30 "C in aqueous solution; pH = 11.25.b kat. at high surfactant concentrations;it is aasumed that the probe is completely bound to the aggregate.

azole-3-carboxylate.The decarboxylation of 6-nitrobenzisoxazole-3-carboxylate(6-NBZ) is an appropriate and popular model reaction to elucidate the structure and properties of surfactant aggregate^.^ The mechanism of this reaction8*&(Scheme 11)can be viewed as an example for many decarboxylation processes, including those occurring in nature.47 This reaction is markedly sensitive to solvent effects. Rate enhancements of ca. 108are found on going from water to hexamethylphosphoramide.8 The first-order rate constants determined at different surfactant concentrations were analyzed according to the Menger-Portnoy pseudophase modeL48 Rate constants in the micellar phase (k,) and values for K / n ( K is the binding constant of 6-NBZ to a micelle and n is the micellar aggregation number) are listed in Table VII. We find that k m does not depend on the alkyl chain length (1-4,k, = (3.4f 0.2) X lo4 s-l). The magnitude of k m points to a micropolarity at the binding site which lies between that for methanol and ethanol ( ~ M ~ O=H2.5 X lo4 s-l, k ~ t = o 10 ~ X lo4 s-l).8 This micropolarity is in good accord with that revealed by the CT-absorption band measurements. The value of K / n ,and concomitantly K , decreases dramatically upon shortening of the alkyl chain. A detailed understanding of this common obser~ a t i o is n ~still ~ lacking. It is noted that km is slightly larger for spherical micelles formed from branched surfactants (6-8 and 12)than for those formed from the unbranched 4. The value of K/n varies little, but the actual binding constant will be decreased, since the aggregationnumber of micelles formed (46) Lardet, D.; Thomalla, M. Bull. SOC.Chim. Fr. 1988,524. (47) Headley, G. W.; O'Leary, M. H. J. Am. Chem. SOC.1990, 112, 1894. (48) Menger, F. M.; Portnoy, C. E. J . Am. Chem. SOC.1967,89,4698. (49) Bacaloglu, R.;Blaeko, A.; Bunton, C. A.; Cerichelli, G.; Ortega, F. J. Phys. Chem. 1990,94,5062.

"

14

-

12

-

I

I

I

-8 I VI

t'

-6

2X

l4

I

Y

6

,,/,Ci',

,

.I6

,

,

*' 2X E Y

m17

4 0

2

1

3

If i

Figure 2. K (m) and k, ( 0 )as a function of the hydrophobicity of the 1-aikyl group (see text).

from branched surfactants is usually lower than that for micelles formed from unbranched ones.5o The catalytic effect is insensitive to the presence of a rigid segment in the center of the alkyl chain (13). The value of K / n , however, strongly decreases. This seems to indicate that the binding of &NBZ to micelles of 13 is reduced because the structure of the micellar surface is more open, but in that case a smaller k, would also have been expected. The nearly identical values of k m can be rationalized in terms of the almost constant degree of counterion binding to the different micelles and, thus, a constant micellar surface charge. Previously, it has been found that k, is inversely dependent on the micellar charge of cetyltrimethylammonium mi~elles.5~ Additional support for this view is derived from the slightly higher k, for micelles formed from 21. The degree of counterion binding of these micelles is, indeed, significantly lower than that for 4 (vide supra). The higher K l n for 21 compared to that for 4 stems probably from a higher K , pointing to a stronger electrostatic interaction between the kinetic probe and the micelles. Variation of the substitution pattern of the pyridinium ring (22) decreases K / n and increases 12,. These effects may be caused by a shift of the charge to locations deeper inside the micellar core. The micellar rate enhancements are also sensitive to changes in the hydrophobicity of the 1-alkyl group of the associated surfactants (Table VII). Clearly k, increases upon increasing the hydrophobicity of the 1-alkyl moiety (see Figure 2). A similar dependence was found for micelles formed from cetyltrialkylammonium bromides.52 The enhanced catalytic effect is, according to Savelli and co-workers,52not due to changes in the bulkiness of the headgroup but rather stems both from a lower extent of hydration of the kinetic probe and from a further reduction of the polarity of the binding site in the Stern region. However, for 1-alkyl-4-alkylpyridinium iodide surfactants it was shown on the basis of the CT absorption band that (50) Caponetti, E.; Triolo, R.; Ho, P. C.; Johnson, J. S., Jr.; Magid, L. J.; Butler, P.; Payne, K. A. J. Colloid Interface Sci. 1987, 116, 200. (51) Germani, R.;Paolo Ponti, P.; Savelli,G.; Spreti,N.; Cipiciani,A.; Cerichelli, G.; Bunton, C. A. J. Chem. SOC.,Perkin Trans. 2 1989,1767. (52) Germani, R.; Paolo Ponti, P.; Romeo, T.; Savelli, G.; Spreti, N.; Cerichelli, G.; Luchetti, L.; Manchini, G.; Bunton, C. A. J. Phys. Org. Chem. 1989,2, 553.

2096 Langmuir, Vol. 7, No. 10, 1991 the micropolarity in the Stern region remains constant upon changing the hydrophobicity of the l-alkyl group. Therefore we suggest that the higher k m value is caused by a decreased hydration of 6-NBZ. This may be caused by shielding of the probe molecules from water due to the presence of alkyl groups at the binding site. Additional evidence emerges from the higher k, value of micellar 16, in which the l-alkyl group is a bulky isopropyl moiety, compared to that of 15 in which the l-alkyl group is n-propyl. The hydrophobicity (Cfi) of both groups is nearly identical.27 The high k m for micelles formed from 17 can be explained by backfolding of the l-alkyl group into the core, resulting in a further dehydration of the binding site. Introduction of an oxygen atom in the l-alkyl group (19 and 20) results in a higher k, than expected in view of the hydrophobicity of the l-alkyl group (see Figure 2). The polarizability of the oxygen atom may account for this effect. For micellar 4 a binding constant of 9.3 X lQ4 M-I can be calculated, taking into account the aggregation number of 80.63 The aggregation number of micelles formed from l-alkyl-4-n-dodecylpyridiniumiodides is likely to be constant, provided that the l-alkyl group does not fold back.33 The kinetic probe binds stronger to micelles when the hydrophobicity of the l-alkyl group is increased (Figure 2). Again 16, 17, 19, and 20 show a deviating behavior. Backfolding (17) or branching (16) decreases K. Steric hindrance may lie a t the origin of these effects. Finally, the effect of the morphology of the aggregate on the catalytic efficiency is revealed by the data in Table VII. The catalytic efficiency of vesicles of 9-11 is much higher than that of micelles. In the case of 9, k m = 17 X lo4 s-l, pointing to a carbon tetrachloride like binding site.8 s-l can be For 10 and 11 values for k, of at least estimated. The rate enhancement for the decarboxylation of 6-NBZ in rodlike micelles is larger, compared to that in spherical micelles! The increased size of the aggregate cannot be the cause of the higher k,.54 Instead, these higher k m values seem to point to a less polar Stern region of vesicles and rodlike micelles compared to that of spherical micelles. This conclusion is, however, in contrast with previous conclusions (vide supra). Different locations of the probe in spherical micelles and vesicles, which leads to differences in the micropolarity at the binding site and to differences in the shielding of the probe from water, can, however, explain the d i s c r e p a n ~ y . ~ ~ (53) Binana-Limbele,W.;Zana, R.; Nusselder,J. J. H.; Engberta, J. B. F. N. To be submitted for publication. (54) Biresaw, G.; Bunton, C. A. J. Phys. Chem. 1986,90,5854. (55) Shobha, J.; Srinivae, V.; Balasubramanian, D.J. Phys. Chem. 1989,9417. Shin, D.-M.: Schanze, K. S.: Whitten. D.G. J.Am. Chem. SOC.1989,111,8494.

Nusselder and Engberts

Conclusions Studies of the effect of alkyl chain branching on various properties of spherical micelles contribute significantly to an understanding of the precise structure of micelles. Particularly interesting in the present study is the finding that the free energy of micellization (as expressed in the cmc) is not significantly affected by alkyl chain branching, even though the packing of the alkyl chains is significantly altered. Only a small destabilization of the micelle upon branching is observed. This stems solely from the slightly less unfavorable hydration of branched surfactant monomers compared to that of the unbranched one. The micropolarity in the Stern region (as deduced from the position of the charge-transfer absorption band of the pyridinium iodide headgroup) is ethanol-like and does not depend on the shape of the surfactant and the hydrophobicity of the 4-alkyl and the l-alkyl chain. In principle the same micropolarity should be obtained by using a kinetic probe. However, the catalytic efficiency of the spherical micelles toward the decarboxylation of 6-nitrobenzisoxazole-3-carboxylatedoes depend on the hydrophobicity of the l-alkyl group. Therefore, one must be careful in using kinetic probes to delineate the micropolarity in the Stern region since slight differences in binding sites are strongly revealed in the catalytic effect. Clearly the use of a surfactant containing an intrinsic probe is preferred for the determination of the micropolarity in the Stern region. Surface potentials of spherical micelles reflect small changes in the surface charge densities upon alkyl chain branching. These potentials can be estimated from the charge density and the cmc by using the ’dressed micelle” model based on the nonlinear Poiason-Boltzmann relation. Thus, it is found that spherical micelles formed from branched surfactants may be described by the same ‘dressed micelle” model and are well-defined aggregates whose structure, size, and Stern region are determined by the balance between the hydrophobic effect (mainly London-dispersion interactions) and headgroup repulsion. The results of this study strongly support the micelle model of Gruen.=

Acknowledgment. The investigations were supported by the Netherlands Foundation for Chemical Research (SON)with financial aid from the Netherlands Foundation for Scientific Research (NWO). We thank Mr.K. Hovius and Mr. A. Kluppel for technical assistance with the synthesis of the branched surfactants, Dr. L. Grierson for the preparation of HOED, and Dr. C. J. Drummond for sending detailed data on the relation between the polarity of 1,4-dioxane-water mixtures and Am of HOED. (56) Gruen, D.W. R. b o g . Colloid Polym. Sci. 1988, 70,6.