Acid-catalyzed hydrolysis and monolayer properties of ketal-based

David A. Jaeger, Jamshid Mohebalian, and Philip L. Rose. Langmuir , 1990, 6 (3), pp 547–554. DOI: 10.1021/la00093a005. Publication Date: March 1990...
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Langmuir 1990,6, 547-554

Conclusions The relative intensities of excimer and monomer emission of DPyP have been examined in lyotropic mesophases. The relative intensity of excimer (with respect to monomer) emission is higher in anionic surfactant solutions than in their cationic counterparts but is strongly suppressed in the mesophases. The presence of orientational order and t h e higher microviscosity in the mesophase, together with possible host-guest interactions, contribute to decreased excimer emission intensities. The relative intensities of vibrational structure in the monomer emission seem to be sensitive to solvent

547

polarity, as in the case of pyrene. Preliminary data suggest that decreased polarity results in a decrease of relative vibrational intensities in DPyP. However, the reduced symmetry of DPyP vis-a-vis pyrene renders large variations as a function of solvent polarity somewhat unlikely. Acknowledgment. This work was supported by the National Science Foundation-Solid State Chemistry under Grant No. DMR84-04009. Registry No. KL, 10124-65-9;SDecS, 142-87-0;MTAl3,111997-7; DPyP, 61549-24-4.

Acid-Catalyzed Hydrolysis and Monolayer Properties of Ketal-Based Cleavable Surfactants David A. Jaeger,*'? Jamshid Mohebalian,? and Philip L. Rose**$ Department of Chemistry, University of Wyoming, Laramie, Wyoming 82071, and the Department of Chemistry, Duke University, Durham, North Carolina 27706 Received June 5, 1989. I n Final Form: September 1, 1989 The hydrolytic reactivities and monolayer properties of several ketal-based, cleavable surfactants were determined in 0.10 M hydrobromic acid at 50 "C and on a pH 7.5 buffer subphase at 25 "C,respectively. The hydrolyses of diastereomeric cis- (la) and trans-[(2-heptadecyl-2-methyl-1,3-dioxolan-4-yl)methyl]trimethylammoniumbromide (lb) were studied in homogeneous and mixed micellar forms. A mixture of the shorter chain analogues,cis- (2a) and trans-[(2-octyl-2-methyl-l,3-dioxolan-4-yl)methyl]trimethylammonium bromide (2b), was examined in unaggregated and micellar forms. [ (2,2-Diheptadecyl-1,3-dioxolan-4-yl)methyl]trimethylammonium bromide (3) and [(2,2-dimethyl-1,3-dioxolan-4-yl)methyl]trimethylammoniumbromide (4) were studied in vesicular form and as a nonaggregating analogue, respectively. Relative to the reactivities of 4 and 2 in unaggregated form, those of micellar l and 2 and vesicular 3 were about 2 orders of magnitude less. The observed pseudo-first-order rate constants, k , for la and l b decreased on going from homogeneous to mixed micelles, and the ratio of k , values ako changed. The ratio of k , values for 2a and 2b changed on going from unaggregated to mixed micellar form. The monolayer characteristics of la and lb were identical but slightly different than that of a 50:50 mixture of the two. Introduction

We and others have previously reported the synthesis, characterization, and application of several series of cleavable (destructible) surfactants, which can be used for various purposes and then converted to nonsurfactant compounds by reaction at labile linkages separating their major lipophilic and hydrophilic units.' Cleavable surfactants also present the opportunity for study of the dependence of functional group reactivity on aggregate morphology and surfactant stereochemistry. Herein we report a study of the acid-catalyzed hydrolyses of a series of +

ketal-based, cleavable surfactants that includes 1,2, and 3. The first two form micelles and the third vesicles; 4

University of Wyoming.

* Duke University.

(1)For examples, see: (a) Jaeger, D. A.; Jamrozik, J.; Golich, T. G.; Clennan, M. W.; Mohebalian, J. J. Am. Chem. SOC. 1989,111,3001.(b) Jaeger, D.A.; Chou, P. K.; Bolikal, D.; Ok, D.; Kim, K. Y.; Huff, J. B.; Yi, E.; Porter, N. A. J. Am. Chem. SOC. 1988,110,5123. (c) Jaeger, D. A.; Ward, M. D.; Dutta, A. K. J. Org. Chem. 1988,53,1577.(d) Cuomo, J.; Merrifield, J. H.; Keana, J. F. W. J. Org. Chem. 1980,45,4216. (e) Epstein, W. W.; Jones, D. S.; Bruenger, E.; Rilling, H. C. Anal. Biochem. 1982,119,304. (f) Hayashi, Y.;Shirai, F.; Shimizu, T.; Nagano, Y.; Teramura, K. J. Am. Oil Chem. SOC. 1985,62,555 and references therein.

was included as a nonaggregating analogue. The hydrolysis of 1 is illustrated in eq 1. Additionally, the monolayer properties of diastereomeric la, lb, and their mixtures have been determined. Romsted and co-workers' (2)Armstrong, C.; Gotham, W.; Jennings, P.; Nikles, J.; Romsted, L.; Versace, M.; Waidlich, J., submitted for publication.

0743-7463/90/2406-0547$02.50/0 0 1990 American Chemical Society

548 Langmuir, Vol. 6, No. 3, 1990 have reported a study of the acid-catalyzed hydrolysis of several neutral hydrophobic ketals in micellar C,,H,,N+Me, Br-/Cl-.

5

are tentative and are based on comparisons of their 13C and 'H NMR spectra and their TLC characteristics. Pertinent 13Cand 'H NMR chemical shifts are summarized in Figure 1. For the CH, group on carbon-2 of the dioxolane ring, both chemical shifts for 8b are downfield from those for 8a. Also, for the CH, group on the same carbon, both values for 8a are downfieid from those for 8b. These downfield chemical shifts are consistent with cis1,3 steric interactions of the CH, and CH, groups with CH,Br on carbon-4., Analogous chemical shift patterns were found for l a and l b (see Experimental Section). These stereochemical assignments are consistent with 'H NMR chemical shifts for a series of substituted 1,3dioxolanes of known c~nfiguration.~

88

8b

Of the three substituents on the dioxolane rings of 8a and 8b, the CH,Br and C17H35 groups are the largest. In 8a they are cis, and only CH, occupies the opposite face of the ring. In 8b, one large group occupies each face. Thus the bottom face of 8a is more sterically accessible than either face of 8b, and it should interact more strongly with silica gel in TLC with a resultant lower R, for the former. The R, values were 0.20 and 0.25 for the two diastereomers, and structure 8a was assigned to that with the lower value, consistent with the NMR chemical shift comparisons above. Individually, 8a and 8b were converted to l a and lb, respectively, by reaction with Me,N in MeOH (eq 2). Also, a mixture of the two bromo ketals gave a 44:56 mixture of l a and lb, as determined by 13C NMR analysis (see Experimental Section). A 53:47 mixture of 2a and 2b, 3, and 4 were prepared by analogous procedures starting with 7 and 2-decanone, 18-pentatriacontanone, and Me,CO, respectively. No attempt was made to separate the diastereomeric bromo ketal precursors of 2. The stereochemical assignments for 2a and 2b are consistent with those for l a and lb. No significance can be attached to the different diastereomer ratios for 1 and 2, since they are for recrystallized materials. MesN

8 + 1 MeOH

(2)

(3) (a) Wehrli, F. W.; Wirthlin, T. Interpretation of Carbon-I3 NMR Spectra; Wiley: New York, 1983, Chapter 2. (b)Jackman, L. M.; Sternhell, s. Applications of Nuclear Magnetic Resonance Spectroscopy in Organic Chemistry, 2nd ed.; Pergamon: New York, 1969; Chapter 2-2. (4) Wiley, W. E.; Binsch, G.; Eliel, E. L. J. Am. Chem. SOC.1970,92,

5394.

6 39 02 S 1 58

88

ab

Figure 1. 13C and 'H NMR chemical shifts, respectively, for the indicated nuclei of 8a and 8b.

6

Results Syntheses. The reaction of ketone 5 with bromo diol 7 gave a mixture of diastereomeric bromo ketals 8a and 8b, which was separated by flash chromatography on silica gel. The stereochemical assignments for 8a and 8b

7

d 23 56 6 1 30

Aggregate Characterization. The critical micelle concentrations (cmc values) of l a , lb, and the 44:56 mixture of the two, determined in 0.010 M NaHCO, a t 25 "C, are (8 f 1) X lo4, (7 f 1) X lo4, and (8 f 1) X lo4 M. Thus, there is no dependence of cmc on surfactant stereochemistry within the accuracy of the measurements. The micellar systems were also examined by dynamic laser light scattering (DLLS) a t 35 "C. Distribution analysis of the autocorrelation function indicated that each system contained at least two size distributions, but from run to run, the analysis for each varied (see Experimental Section). However, reproducible mean volumeweighted diffusion coefficients ( D ) were obtained from cumulant analysis of the autocorrelation function. The values of D were (3.26 f 0.07) X (3.29 f 0.02) X and (3.03 f 0.02) X cmz/s for 0.0166 M solutions of la, lb, and the above l a / l b mixture, respectively, in H,O containing 0.010 M NaHCO, and 0.040 M NaBr. Thus, even though meaningful micelle sizes were not determined, the values of D suggest that the mixed micellar system is different than homogeneous micellar la and lb. The cmc of the 53:47 mixture of 2a and 2b is 0.010 f 0.002 M under the same conditions used for 1. As reported previously, double-chain surfactant 3 forms small unilamellar vesicles on sonication, which have a hydrodynamic diameter of 64 nm.la At 50 "C, the temperature of the kinetic studies below, the cmc's of 1 and 2 should be somewhat but not significantly higher. For example, the cmc's for C,H,,+,N+Me, Br- are as follows: for n = 16,9.2 X lo-* and 13.2 X m a t 25 and 55 oC;5afor n = 12, 1.6 X lo-' and 1.9 X m at 25 and 60 oC;5band for n = 10, 7.0 X lo-' and 8.0 X lo-' m at 25 and 60 "C, re~pectively.~~ Monolayer Studies. The surface pressure (n)vs area ( A )characteristics for monolayers cast from pure l a and l b as well as their mixtures are given in Figure 2. Both surfactants form stable (-20 dyn/cm) expanded monolayers at 25.0 "C on a pH 7.5 buffer subphase. The compression and expansion cycles are indicated by the directions of the arrows. Both pure l a and l b have limiting molecular areas of 90 A2/molecule, which is seemingly large for a single-chain compound. However, the doublechain analogue 3 also has a limiting molecular area of 90 A2/molecule, suggesting that the headgroup is restricted to the water surface. The relationships between the area (A2/molecule) and the percent composition of l b in the binary film at 2.5, 5.0, 10.0, 15.0, and 20.0 dyn/cm are given in Figure 3. The area additivities were obtained for 0, 25, 50, 75, and 100 mol '70 l b in the mixed system. The equilibrium spreading pressures (ESPs) for pure la and l b on the pH 7.5 buffer subphase a t 25.0 "C were 33.90 f 1.27 and 32.93 f 2.60 dyn/cm, respectively.

-

N

(5) (a) Czerniawski, M. Pol. J. Chem. 1966, 40, 1935. (b) Scott, A. B.; Tartar, H. V. J . Am. Chem. SOC.1943, 65, 692.

Langmuir, Vol. 6, No. 3, 1990 549

Ketal-Based Cleavable Surfactants

I

\

\

.- - _ 0 -

I

lZ5

1

*

l

I

l

0

I

T

- - -+ n = 10.0 dynicm 3 n = 15.0 dynicm -In = 20.0 dynicm

--

50:

I

.

, 25

.

, 50

.

I

.

75

tively. With an assumed 1a:lb ratio after reaction equal to that determined in entry 3 without added NaBr, the k, values for la and l b with 0.30 M NaBr were 2.2 X s-' and with 0.50 M NaBr 2.5 X and 2.7 X and 3.1 X s-', respectively. In entries 6 and 7, the kinetics of the hydrolysis of the 2a/2b mixture both above and most likely below the cmc (see Discussion) were determined with procedures similar to those used for the l a / l b mixture. Thus at the appropriate time, the reaction mixture was adjusted to pH 8-9 and lyophilized. The residue was dissolved in D20,and the resultant solution of unreacted 2 and 6 was analyzed by 'H NMR to give the overall extent of hydrolysis. Then the relative amounts of 2a and 2b were determined by 13C NMR analysis. The k values for 2a and s" at [surfactant] = and 170 X 2b were 270 X 0.0110 M and 26 X and 10 X s-' at [surfactant] = 0.280 M, respectively. The kinetics of the hydrolysis of vesicular 8.55 X lo-, M 3 were determined in entry 8 as follows. A mixture of 3 and H 2 0 was sonicated at 50-55 "C, adjusted to pH 1.00 with 6 M hydrobromic acid, and held at 50 "C for 6 h. Then it was adjusted to pH 8-9 and lyophilized. The residue was slurry-extracted with CDCl,, and the resultant solution of unreacted 3 and 18-pentatriacontanone was analyzed by 'H NMR to give the extent of hydrolys-'. sis and k, = 0.73 X The kinetics of the hydrolysis of 0.0328 M 4 were determined with the same procedures and conditions as used for 2. A D20 solution of the unreacted 4 and 6 remaining after lyophilization was analyzed by 'H NMR to give the extent of hydrolysis; k, values were 190 X and s-' in entries 9 and 10, respectively. Controls 170 X verified the accuracy of this and the other analytical methods used in the kinetic runs. Discussion

I

100

% Composition of 1b In the Monolayer

Figure 3. Area additivity relationships showing deviation from ideality for film composition of 50:50 1a:lb at surface pressures of 2.5,5.0, 10.0, 15.0, and 20.0 dyn/cm. The dashed lines represent the ideal area additivity cases and the error bars the 95% confidence limit. Surfactant Hydrolysis. The kinetics of the hydrolyses of 1-4 were studied in 0.10 M hydrobromic acid at 50 "C, and the results are summarized in Table I. In entries 1-3, runs were made with l a , lb, and the 4456 mixture of l a and l b with [surfactant] = 0.0174 M, which is well above the respective cmc's. After the uniform 6h reaction time, the reaction mixture was adjusted to pH 8-9 and lyophilized. The residue was slurry-extracted with CDCl,, and the resultant solution of unreacted 1 and 5 was analyzed by 'H NMR to give the extent of hydrolysis. Also, for the mixed micellar system, the relative amounts of l a and l b after hydrolysis were determined by 13C NMR analysis. For the individual runs with l a and lb, one-point, pseudo-first-order rate cons-l, respectively, stants, k,, of 2.1 X lo-' and 1.7 X were calculated directly from the 'H NMR analyses. For the mixed micellar system, k, values of 1.0 X and 1.6 X s-' for l a and lb, respectively, were determined from the extent of hydrolysis of the system as a whole and the 1a:lb ratios before and after hydrolysis. In entries 4 and 5 of Table I, hydrolysis of the l a / l b mixture was studied under the same conditions as above but with the addition of 0.30 and 0.50 M NaBr, respec-

There are no detectable differences in the energetics of compression for monolayer films cast from either la or lb; their isotherms are exactly coincident for at least six repetitions of each diastereomer. The II/A curve for the compression of films cast from both l a and l b exactly retraces the curve for the expansion process. This is an indication that the time scale for relaxation in the monolayer is faster than the time scale of the experiment. The same is true for all mixtures of l a and lb, suggesting that there is no effect of stereochemistry on hysteresis. There is also no diastereomericdiscrimination in the ESPs for surfactants l a and l b since the above values are within experimental error. Since both l a and l b form stable monolayer films, they make ideal candidates for studying their properties as mixed monolayers. We have shown that mixed monolayers can be used to elucidate the effect of stereochemistry on intermolecular interactions for a series of diastereomeric double-chainsurfactants consisting of straightchain fatty acids bonded together at various points along the hydrocarbon chains by a carbonyl group.6 By comparing excess free energies of mixing (AG"") for various structural and stereoisomers, we were able to assign which portion of the double-chaindiacid surfactant most affected intermolecular interactiom6 From Figure 2, it can be seen that there is no detectable difference in the energetics of compression for mixtures of l a and l b below 50 mol % of either diastereomer. There is, however, a small diastereomeric dis~~~

(6) Arnett,

~

E.M.; Harvey, N.; Rose, P.L.Langmuir

1988,4, 1049.

550 Langmuir, Vol. 6,No. 3, 1990

Jaeger et al.

Table I. Surfactant Hydrolysis in 0.10 M HBr at 50.0 f 0.2 OC surfactant nature concn, M time, min % hvdrolvsis" la 0.0174 360 36.3 f 1.0 (6) lb 0.0174 360 30.0 f 2.0 (6) la/lb' 0.0174 360 25.0 f 1.6(5)d la/ 1b' 0.0174 360 41.3 i 1.3 (6)' la/lb' 0.0174 360 45.8 f 0.8 (4)' 2a f 2bh 0.0110 5 48.2 f 1.8 (6)' 27.3 f 0.9 (3)' 2af 2bh 0.280 30 3 0.00855 360 14.5 f 1.3 (4) 68.5 f 0.5 (2) 4 0.0328 10 4 0.0328 30 95.0 f 0.8 (5)

entry 1 2

3 4e

58 6

I 8 9

10

105k*,* 8-1 2.1 f 0.1

1.7 f 0.2 1.0 f 0.2, 1.6 f 0.4 2.2 f 0.5, 2.7 f 0.7 2.5 f 0.6, 3.1 f 0.8 270 i 40, 170 f 40 26 i 5, 10 f 2 0.73 f 0.07 190 f 10 170 f 10

"By 'H NMR analysis. For each entry, the limit of error is the average deviation for the number of individual runs given in parentheses. For entries 3-7, the two values correspond to diastereomers a and b, respectively. The limits of error for entries 1, 2, and 8-10 are average deviations, and those for entries 3-7 were determined by propagation of errors. 'By "C NMR analysis, the 1a:lb ratio was 44:56. By 13C NMR analysis, the average 1a:lb ratio for recovered 1 was 47:53. e The reaction mixture contained 0.30 M NaBr. 'The 1a:lb ratio for recovered 1 of entry 3 was used to calculate the k, values for la and lb. g The reaction mixture contained 0.50 M NaBr. By 13C NMR analysis, the 2a:Zb ratio was 53:47. ' By 13C NMR analysis, the average 2a:Zb ratio for recovered 2 was 4654 in both entries 6 and 7.

crimination in the monolayer film cast from a 5050 mixture of la and lb. Figure 2 shows a slight expansion of the film cast from a 50:50 mixture, indicating a dependence of packing on stereochemistry as the film is compressed (see also Figure 3). This discrimination is small and amounts to no more than 15 cal/mol at any given pressure. The diastereomeric discrimination manifested in la/ l b mixtures is much more subtle than the chiral molecular recognition found in other chiral surfactant systems.' An analysis of AGxS was performed for mixtures of la and l b at 2.5, 5.0, 10.0, 15.0, and 20.0 dyn/cm. The AGxs was 0 within experimental error for the propagation of errors at the 95% confidence limit for both graphical integration and area measurements. The surface pressure vs area characteristics for mixtures l a / 3 and l b / 3 were also measured (not shown). There were no differences in the energetics of compression between monolayers cast from mixtures of la and 3 and those from l b and 3. Based on literature analogy: the hydrolyses of 1-4 are expected to proceed with specific acid catalysis and a ratedetermining step involving cleavage of the protonated ketal group. For each compound, there are two possible protonated species, e.g., la' and la" for la, whose relative involvement in hydrolysis is unknown. Each k, for H

1a'

1a '

entries 1-5, 7 , and 8 of Table I represents a weighted average of rate constants for unaggregated and micellar/ vesicular surfactant according to eq 3, wherein k, and k, are the rate constants for the former and latter, respectively, and n is the mole fraction of aggregated surfactant. Furthermore, each k, and k, represents a composite value, including the rate constant for the ratedetermining step and the dissociation constant of the ketal.' (7)(a) Harvey, N.; Rose, P. L.; Huff, J. B.; Porter, N. A.; Arnett, E. M. J. A m . Chem. SOC.1988,110,4395. (b) Harvey, N.G.; Mirajovsky, D.; Rose, P. L.; Verbiar, R.; Arnett, E. M. J. Am. Chem. SOC.1989,111, 1115. ( c ) Arnett, E. M.; Harvey, N.; Rose, P. L. Acc. Chem. Res. 1989, 22, 131. (8) Cordes, E. H.; Bull, H. G. Chem. Reu. 1974,74, 581. (9)Fife, T. H.; Hagopian, L. J. Org. Chem. 1966,31,1772.

k, = (1- n)k, + nk, (3) The hydrolytic reactivity of 4 represents that of an unaggregated system. Entry 6 was performed under nonmicellar conditions since the concentration of the 2a/2b mixture was probably below the cmc a t 50 O C . Thus 2a and 2b in unaggregated form" displayed reactivities that are comparable to that of 4. Fife and Hagopian have reportedg that the rates of hydrolysis for a series of 2methyl-2-alkyl-1,3-dioxolanes in 1:l (v/v) 1,4-dioxanewater at 30 "C do not vary much with the nature of the alkyl group (Me, Et, LPr, t-Bu). At a concentration significantly above the cmc in entry 7 , the reactivities of 2a and 2b were substantially less than those in entry 6. These differences reflect the lesser reactivity of micellar relative to unaggregated surfactant (k, > km). The exact micellar fraction, n, in entry 7 is unavailable because the cmc was not determined under the reaction conditions, but it is estimated to be ca. 0.95. The reactivities of 2a and 2b are indeed expected to be less in micellar than in unaggregated form, since the positively charged Stern layer electrostatically repels H30+.11 Thus the [H,O+] experienced by micellar 2 will be less than that experienced by unaggregated 2 in the aqueous phase. Also, there is probably a medium effect on the hydrolysis of micellar 2. In general, the rate constant for hydrolysis of an acetal/ ketal in water-organic solvent mixtures decreases with an increase in the relative amount of the latter." The Stern layer of a micelle has an effective dielectric constant less than that of water and about that of methanol/ethanol.13 In the limiting case, micellar 2 does not react, but this is unlikely given the discussion below of results obtained with 1 and added NaBr in entries 4 and 5. Different hydrolytic reactivities are expected for 2a and 2b, since they are diastereomers. However, it is noteworthy that the 2a:2b reactivity ratio changed from 1.6:l to 2.6:l on going from unaggregated to micellar conditions (10)The presence of premicellar aggregates cannot be discounted. For example, see Hioka, N.; Politi, M. J.; Chaimovich, H. Tetrahedron Lett. 1989,30, 1051. (11)For examples, see: (a) Mukerjee, P.; Banerjee, K. J. Phys. Chem. 1964,68,3567. (b) Bunton, C. A.; Robinson, L. J.Phys. Chem. 1969,73,4236. (b) Behme, M. T. A.; Cordes, E. H. J . Am. Chem. SOC. 1965,87, 260. (12) (a) Cordes, E. H. Progr. Phys. Org. Chem. 1967,4,1. (b) Wolford, R. K. J.Phys. Chem. 1963,67,632. (13!(a) Mukerjee, P.; Cardinal, J. R.; Desai, N. R. In Micellitation, Solubtltzation, and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977;Vol. 1, p 241. (b) Sudholter, E. J. R.; van de Langkrius, G. B.; Engberts, J. B. F. N. Recl. Trau. Chim. Pays-Bas 1980,99,73.

Ketal-Based Cleavable Surfactants in entries 6 and 7, respectively. The origin of the change is unclear but may be associated with different time-averaged conformations of the octyl group in nonmicellar and micellar forms. In the latter, an octyl group would likely be more extended as the result of its participation in hydrophobic interactions with other octyl groups, whereas in the former it would tend to coil in order to minimize its interaction with water. The enhanced 2a:2b reactivity ratio could also derive from the formation of mixed micelles that are enriched in 2b relative to the composition of the initial 2a/2b mixture. Under these conditions, a greater fraction of 2a than of 2b would be in the more reactive unaggregated form. However, this is an improbable contributor to the change in reactivity ratio since 2a, 2b,and the 2a/2b mixture probably have the same cmc, as is the case for the corresponding species in the 1 system as noted above. Porter et al.lb found that the ratio of rates for epimerization of diastereomeric double-chain amphiphilic ketones changed on going from unaggregated to micellar conditions. Micellar diastereoselectivity has also been observed in other system~.~* The very low cmc's for 1 prevented a study of the hydrolysis kinetics of this system in unaggregated form. All runs were performed at a concentration well above the cmc's. The reactivity of la in homogeneousmicellar form in entry 1 is greater than that in mixed micellar form in entry 3. Furthermore, the 1a:lb reactivity ratio changed from 1.2:l for the homogeneous micellar systems of entries 1 and 2 to 0.63:l for the mixed micellar system. The dependence of ketal reactivity on micelle type suggests that the surfactant-surfactant interactions in the la/ lb mixed micelles are different than those in the homogeneous micelles. The DLLS results compliment this interpretation, and it is interesting to note that a difference between the individual surfactants and their 50:50 mixture was also detected in the monolayer study. In entries 4 and 5, the reactivities of la and lb in the mixed micellar system increased in the presence of added NaBr. The rate enhancements are consistent with at least two interpretations. T h e first derives from t h e pseudophase ion-exchangemodel15and parallels that used by others to describe the effects of micelles on .the reactions of organic substrates with co-ions.2"6 Relative to entry 3, the additional Br- in entry 4 displaces some of the OH- from the Stern layer of micellar la/lb. As a result, [H,O+] increases according to the ion product of water in the Stern layer, which is in fact unknown. Overall, the greater [H,O+] in the Stern layer, the presumed site of reaction, results in larger k, and k, values, and the increased [NaBr] of entry 5 leads to an even greater k, as expected. The second interpretation derives from an analysis of the interaction of ionic micelles and ions based on the Poisson-Boltzmann equation." In entries 4 and 5, the added Br- reduces the micellar electrical surface potential. As a result, the Coulombic repulsion of H,O+ is decreased, and the increased [H,O+] leads to greater k , and k , values. The added salt may also effect a change in the shape of the la/lb micelles; it is known that added salts can cause a sphere to rod transition." However, (14) For example, see: Moss, R. A.; Chiang, Y . 4 . P.; Hui, Y. J. Am. Chem. SOC.1984, 106, 7506. (15) (a) Romsted, L. S.In ref 13a; Vol. 2, p 509. (b) Bunton, C. A. CataE. Reu.-Sci. Eng. 1979, 20, 1. (16) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Martins-Franchetti, S. M.; Chaimovich, H. In Solution Behavior of SurfactantsTheoretical and Applied Aspects; Mittal, K. L., Fendler, E. J., Eds.; Plenum Press: New York, 1982; Vol. 2, p 1125 and references therein. (17) Bunton, C. A.; Moffatt, J. R. J . Phys. Chem. 1988, 92, 2896.

Langmuir, Vol. 6, No. 3, 1990 551 such a change is probably not an important factor in the rate enhancements because it does not decidedly alter the nature of the micelle-water interface.lg In fact, a rate decrease would be expected to accompany the transition from spherical to rod-like micelles because the electrical surface potential of the latter should be greater than that of the former." Romsted and co-workers obtained analogous rate enhancements for ketal hydrolyses in micellar C16H,,N+Me3 Br-/C1- on the addition of NaBr/NaCl.' The rate enhancements of entries 4 and 5 support the implicit assumption above that micellar surfactant, as well as unaggregated surfactant in the aqueous phase, undergoes hydrolysis. If reaction occurred only through the latter, k, would have dropped on the addition of NaBr, reflecting an expected decrease in the cmc of la/lb2' with a concomitant lesser fraction, 1 - n,of reactive, unaggregated material. The greater reactivities for 2a and 2b in entry 7 compared to those for la and lb in entry 3 reflect the larger cmc's for the former. Greater fractions of 2a and 2b are in the more reactive unaggregated form. Also, the mixed micelles of 2a/2b should be smaller and less organized than those of la/ lb,since the alkyl chain of 2 is shorter. As a result, the average head group-head group separation will be greater in the 2a/2b mixed micelles with a resultant lesser micellar electrical surface potentialz2Thus the Coulombic repulsion of H30+ is smaller, and the increased [H,O+] leads to greater k, and k, values. The lesser reactivity of vesicular 3 in entry 8 compared to homogeneous and mixed micellar la and lb in entries 1-3 is similarly explained. Since the critical vesicle concentration of 3 should be less than the cmc's of ,',l a lesser fraction of 3 is in unaggregated form. Furthermore, the electrical surface potential of vesicular 3 might be greater than that of micellar 1 since the aggregate-water interfaces of the former have less curvature." Electrical surface potentials of +157 and +148 mV have been reported for vesicular (ClzHz5j2N+Me2 Brand micellar c,,H,,N+Me, Br-, respectively. The phase transition temperature (TJof vesicular 3 is 39 "C.la Therefore, at 50 "C 3 is in the liquid crystalline rather than the more ordered gel state, which exists below T,.

Summary The hydrolytic reactivities of aggregated cationic surfactants 1,2,and 3 were uniformly less than that of unaggregated 4 in aqueous acid as the result of microenvironmental effects in the Stern layers involving electrostatic depletion of H30+ and a lower polarity relative to the aqueous phase. The reactivity of the ketal group had no appreciable dependence on aggregate morphology since the k , values for micellar 1 and vesicular 3 were comparable. The 1a:lb reactivity ratio changed on going from homogeneous to mixed micelles, and a difference between the individual surfactants and their 50:50 mixture was (18) Harada, S.;Fujita, N.; Sano, T. J . Am. Chem. SOC.1988, 110, 8710 and references therein. (19) Bunton, C. A.; Savelli, G . Adu. Phys. Org. Chem. 1986,22, 213. (20) Lindman, B.; Wennerstrom, H. In Topics in Chemistry; Springer-Verlag: New York, 1980; Vol. 87, p 1. (21) (a) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975, Chapter 2. (b) Heckmann, K.; Schwarz, R.; Strnad, J. J . Colloid Interface. Sci. 1987, 120, 114. (22) Hoyer, H. W.; Marmo, A. J . Phys. Chem. 1961,65, 1807. (23) For example, see: Kunitake, T.; Okahata, Y.; Ando, R.; Shinkai, S.;Hirakawa, S.J. Am. Chem. SOC.1980,102,7877. (24) (a) Fernandez, M. S.;Fromhen, P. J. Phys. Chem. 1977,8I, 1755. (b) Lukac, S.J . Phys. Chem. 1983,87, 5045.

552 Langmuir, Vol. 6, No. 3, 1990 also detected i n monolayer studies. The reactivity ratios for diastereomeric 2a and 2b were different in unaggregated and mixed micellar forms, p e r h a p s reflecting a change in t h e time-averaged conformation of the octyl group.

Experimental Section General Procedures and Materials. 'H (270 MHz) and 13C (67.8 MHz) NMR spectra were recorded in CDC1, with Me,Si as internal standard unless noted otherwise. High-resolution mass spectra were obtained at the Midwest Center for Mass Spectrometry, a National Science Foundation Regional Instrumentation Facility (Grant No. CHE 8211164). The cmc measurements were made as before2, a t 25 "C in 0.010 M NaHCO,. The pH measurements of solutions in 5-mm NMR tubes were made with a 3-mm (diameter) Ag/AgCl combination electrode (Sargent-Welch S-30070-05). Hydrobromic acid (47-49%, J. T. Baker) was used as received, and 0.10 M solutions were standardized with Na,CO, and bromcresol green as indicator. T H F was distilled from benzophenone potassium ketyl, and Me,CO, CHCl,, and CDCl, were stored over Na,CO,. Silica gel (Merck 9385, 60 A, 230-400 mesh) was used for flash chromatography. TLC analyses of surfactants and nonsurfactanta were performed on 0.25-mm aluminum oxide (Merck 57313) and 0.25-mm silica gel plates (Merck 5714-3) with 5 vol % EtOH in CHCl, and 3.4 vol % EtOAc in hexane as eluants, respectively. All melting points are uncorrected. Elemental analyses were performed by Atlantic Microlab, Atlanta, GA.

Jaeger et al. cis-[(2-Heptadec yl-2-methyl- 1,3-dioxolan-4-yl)methylltrimethylammonium Bromide (la). A mixture of 1.05 g (2.50 mmol) of 8a, 50 mL of 25% (w/v) Me,N-MeOH (Kodak), and 50 mL of MeOH was held a t 85-90 "C in an autoclave for 6 days. The MeOH was removed by rotary evaporation to leave a crude product that was recrystallized from Me,CO (5 "C) to give 0.76 g (64%) of la: mp 188-192 "C (dec); 'H NMR 6 4.584.77 (m, 2 H), 4.28-4.36 (m, 1 H), 3.60-3.75 (m, 1 H), 3.56 ( 8 , 9 H, (CH,),N+), 3.16-3.28 (m, 1 H), 1.67 (m, 2 H, CH,CO), 1.32 and 1.25 (2 s, 33 H total, CH,CO and (CH2)15,respectively), 0.88 (t, 3 H, CH,); 13C NMR 6 113.31, 69.74, 68.60, 67.32, 54.74, 39.76, 31.85, 29.70, 29.62, 29.27, 24.16, 23.23, 22.61, 14.04; IR (KBr) 2919 (s),2851 (s), 1468 (m), 1377 (w), 1241 (w), 1150 (w), 1098 (w), 1074 (w), 1047 (w), 967 (w), 919 (w), 721 cm-' (w). Anal. Calcd for C,,H,,BrNO,: C, 62.74; H, 10.95. Found: C, 63.02; H, 11.11. This acceptable analysis was obtained when the sample was dried a t 75 "C.

trans-[(2-Heptadecyl-%-met hyl-1,3-dioxolan-4-yl)methyl]trimethylammonium Bromide (lb). The above procedure for la was used to prepare lb (63%) from 8b: mp 188-

192 OC (dec); 'H NMR 6 4.50-4.68 (m, 2 H), 4.26-4.35 (m, 1H), 3.64-3.75 (m, 1 H), 3.56 (s,9 H, (CH,),N+), 3.33-3.47 (m, 1H), 1.60 (m, 2 H, CH,CO), 1.39 and 1.25 (2 s, 33 H total, CH,CO and (CH2)15,respectively), 0.88 (t,3 H, CH,); 13C NMR 6 113.37, 70.26,68.57,67.21,54.68,38.67,31.82,29.73,29.59,29.24,24.64, 24.13, 22.58, 13.99; IR (KBr) 2922 (s), 2849 (s), 1491 (m), 1468 (m), 1378 (m), 1241 (m), 1148 (m), 1075 (m), 971 (m), 937 (m), 922 (m), 853 (m), 830 (m), 724 cm-' (m). Anal. Calcd for C,,H,,BrNO,: C, 62.74; H, 10.95. Found: cis- and trans-(2-Heptadecyl-2-methyl-1,3-dioxolan-4- C, 62.91; H, 10.97. This acceptable analysis was obtained when the sample was y1)methyl Bromide (8a and 8b). According to the published dried a t 150 "C; when dried a t 75 or 100 "C, it analyzed as the procedures, stearic acid (Aldrich, 99+%) was converted to 2hemihydrate. nonadecanone ( 5 ) (70%), mp 53-54 "C (lit.26mp 55.5-56 "C), cis- and trans-[ (2-Heptadecyl-2-methyl-1,3-dioxolan-4by reaction with MeLi,,' and epibromohydrin (Sigma) was conyl)methyl]trimethylammonium Bromide (la and lb). The verted to 3-bromo-1,2-propanediol(7) (76%),bp 86-88 "C (0.5 above procedure for la was used to prepare a mixture of l a mmHg) (lit.,' bp 106-110 "C (4 mmHg)), by p-toluenesulfonic and lb (64%) from the above mixture of 8a and 8b: mp 188acid catalyzed hydrolysis." By a published pr~cedure,~'the 192 "C (dec). reaction of 2.80 g (10.0 mmol) of 5 , 7.80 g (50.0 mmol) of 7, and 100 mg of p-toluenesulfonic acid monohydrate in 60 mL of Anal. Calcd for C,,H,,BrNO,: C, 62.74; H, 10.95. Found: C,H, gave 3.30 g (79%) of crude product, mp 41-43 "C. This C, 62.83; H, 11.13. material was recrystallized from MeOH (5 "C) to give a mixBy 13C NMR analysis (see below), the 1a:lb ratio was (44 i ture of 8a and 8b, mp 47-48 "C, that was separated by flash 2):56. chromatography. Typically, 1.1g of the mixture was chromatocis- and trans-(2-Octyl-2-methyl-1,3-dioxolan-4-y1)graphed on a 50 mm X 23 cm column of silica gel with 3.5 vol methyl Bromide. By a published procedure," the reaction of 3.20 g (20.5 mmol) of 2-decanone (Aldrich), 7.80 g (50.0 mmol) % EtOAc in hexane as eluant and collection of 46 fractions of ca. 18 mL each. From this, 380 mg of 8a (Rf = 0.20), 460 mg of of 7, and 50 mg of p-toluenesulfonic acid monohydrate in 100 mL of C,H, yielded 6.00 g of crude product that was distilled 8b (Rr= 0.25), and 250 mg of a mixture of the two were obtained. Each diastereomer was further purified by recrystallization from to give 5.40 g (92%) of a mixture of the title compounds: bp 106-109 "C (0.8 mmHg); 'H NMR 6 4.24-4.40 (m, 1 H), 4.10MeOH (5 "C) to give 8a, mp 43-44 "C, and 8b, mp 36-37 "C. For 8a: 'H NMR 6 4.31-4.40 (m, 1 H), 4.09-4.18 (m, 1 H), 3.804.18 (m, 1 H), 3.78-3.88 (m, 1 H), 3.39-3.50 (m, 1 H), 3.25-3.35 3.87 (m, 1 H), 3.39-3.47 (m, 1 H), 3.25-3.31 (m, 1 H), 1.66 (m, (m, 1 H), 1.55-1.72 (m, 2 H, CH,CO), 1.37, 1.30, 1.27 (3 S, 15 H 2 H, CH,CO), 1.30 and 1.25 (2 s, 33 H total, CH,CO and total, 2 CH,CO and (CH,),, respectively), 0.88 (t, 3 H, CH,); (CH2)15,respectively), 0.87 (t, 3 H, CH,); 13C NMR 6 112.07, 13C NMR 6 112.07, 111.88,75.37,74.96,68.55,68.30,39.92,38.94, 32.64,32.56, 31.80, 29.76, 29.46, 29.16,24.86,24.07, 23.88,23.42, 75.07, 68.44,40.03, 32.72, 31.96, 29.81, 29.73, 29.59, 29.37, 23.99, 22.58, 14.02; IR (neat) 2927 (s), 2856 (s), 1467 (m), 1377 (m), 23.56, 22.71, 14.12; IR (KBr) 2917 (s), 2849 (s), 1470 (s), 1379 (m), 1231 (m), 1161 (m), 1145 (m), 1029 (m), 901 (m), 824 cm-' 1249 (w), 1213 (m), 1105 (m), 1057 (m), 883 cm-' (w). E1 HRMS calcd for Cl,H,~9/81Br0, 279.0783 and 277.0804, (m). Anal. Calcd for C,,H,,BrO,: C, 62.99; H, 10.33. Found: C, found 279.0779 and 277.0800; calcd for C,H~9/81Br0,180.9687 63.13; H, 10.37. and 178.9708, found 180.9684 and 178.9702. For 8b: 'H NMR 6 4.25-4.35 (m, 1 H), 4.09-4.18 (m, 1 H), These compositions correspond to fragment ions from the 3.80-3.87 (m, 1 H), 3.41-3.49 (m, 1 H), 3.27-3.35 (m, 1 H), 1.58 loss of CH, and C8H17,respectively; M" was not observed. (m, 2 H, CH,CO), 1.37 and 1.24 (2 s, 33 H total, CH,CO and cis- and trans-[(2-Octyl-2-methyl-1,3-dioxolan-4-y1)(CH,)',, respectively), 0.87 (t, 3 H, CH,); 13C NMR 6 112.20, methyl]trimethylammonium Bromide (2a and 2b). A mix75.45,68.63,39.02,32.77, 31.93,29.84,29.70,29.59, 29.37,24.94, ture of 2.36 g (8.05 mmol) of the above bromide mixture, 50 24.18, 22.69, 14.12; IR (KBr) 2917 (s), 2849 (s), 1471 (s), 1377 mL of 25% (w/v) Me,N-MeOH, and 50 mL of MeOH was held (m), 1249 (m), 1234 (m), 1154 (m), 1099 (m), 1074 (m), 1048 a t 75-80 "C in an autoclave for 7 days. The MeOH was removed (m), 890 (m), 720 (m), 645 cm-' (m). by rotary evaporation to leave crude product that was chroAnal. Calcd for C,,H,,BrO,: C, 62.99; H, 10.33. Found: C, matographed on a 25 mm X 25 cm column of alumina (J. T. 63.10; H, 10.40. Baker 0537-5; pH 6.6) packed in CHCl,. The column was eluted with 150 mL of CHCl, and then with 450 mL of 1:9 (v/v) EtOHCHCl,. Each fraction was ca. 18 mL, and a total of 2.70 g of (25) Jaeger, D. A.; Robertson, R. E. J . Org. Chem. 1977, 42,3298. (26) Saville, W. B.; Shearer, G. J. Chem. SOC.1925, 591. hygroscopic 2a and 2b eluted together in fractions 12-25. This (27) Jorgenson, M. J. Organic Reactions; Dauben, W. G., Ed.; Wiley: material was dried a t 140 "C (0.1 mmHg) and recrystallized in New York, 1970; Vol. 18, p 1. a drybox under N, from 1:9 (v/v) anhydrous Et,O-CHC1, to (28) Winstein, S.;Goodman, L. J . Am. Chem. SOC.1954, 76, 4368. give 2.35 g (83%) of a mixture of 2a and 2b: mp 190-195 "C; (29) Jaeger, D. A,; Martin, C. A,; Golich, T. G. J . Org. Chem. 1984, 'H NMR 6 4.46-4.78 (m, 2 H), 4.25-4.36 (m, 1 H), 3.18-3.72 (m 49. 4545.

Ketal-Based Cleavable Surfactants

+ s a t 3.56, 11 H total, 2 CH, (CH,),N+,

respectively), 1.521.70 (m, 2 H, CH,CO), 1.40, 1.32, 1.27 (3 s, 15 H, 2 CH,CO and (CH,),, respectively), 0.88 (t, 3 H, CH,); 13C NMR 6 113.26, 113.15,70.18,69.63,68.41,67.19,67.10,54.57,39.62,38.59,31.66, 29.57, 29.35, 29.08, 24.59, 24.02, 23.15, 22.47, 13.96; IR (neat) 2929 (s), 2857 (s), 1468 (m), 1379 (m), 1245 (m), 1216 (m), 1108 (m), 1075 (m), 971 (m), 927 (m), 843 (w), 755 (s), 622 cm-' (w). FAB HRMS calcd for C,,H,,NO, (cation) 272.2589, found 272.2590. By 13C NMR analysis, the 2a:2b ratio was (53 f 2):47. [ (2,2-Dimethyl-1,3-dioxolan-4-yl)methyl]trimethylammonium Bromide (4). A solution of 7.00 g (0.121 mol) of Me,CO, 15.5 g (0.100 mol) of 7, and 100 mg of p-toluenesulfonic acid monohydrate in 125 mL of C,H, was refluxed under a Dean Stark trap for 24 h and then washed with 100 mL of 5% aqueous NaHCO,. The organic layer was dried over K,CO, and rotary evaporated to leave crude product that was distilled to give 17.0 g (87%) of (2,2-dimethyl-l,3-dioxolan-4-yl)methyl bromide, bp 53-55 "C (9 mmHg) (lit.,' bp 78.5 "C (24 mmHg)). A solution of 9.7 g (50 mmol) of this bromide in 70 mL of 25% (w/v) Me,N-MeOH was allowed to stand a t 25 "C for 2 days. Then an additional 40 mL of Me,N-MeOH was added, and the solution was refluxed under a dry ice-Me,CO condenser and rotary evaporated to leave an oily residue that was dried a t 25 "C (0.1 mmHg). The resultant crude product was washed with 10 mL of Me,CO ( 5 "C) and then recrystallized from 100 mL of Me,CO (5 "C) to give 7.1 g (56%) of 4 after drying a t 100 "C (0.1 mmHg): mp 196-198 "C; 'H NMR 6 4.55-4.70 (m, 1 H), 4.40-4.50 (m, 1 H), 4.20-4.29 (m, 1 H), 3.62-3.70 (m, 1 H), 3.49 (6, 9 H, (CH ) N+), 3.29-3.40 (m, 1 H), 1.40 (s, 3 H, CH,), 1.30 (s, 3 H, CH,; ,C ' NMR 6 111.44,70.07,68.60,67.19,54.71,26.68, 25.30; IR (neat) 3009 (m), 2977 (m), 1485 (m), 1371 (m), 1232 (m), 1215 (m), 1167 (m), 1070 (s), 980 (m), 941 (m), 856 cm-' (m). FAB HRMS calcd for C,H,,NO, (cation) 174.1494, found 174.1493. DLLS. Measurements were made on a Nicomp Model 370 submicron particle analyzer (scattering angle 90") for 0.0166 M solutions of la, lb, and a 4456 mixture of la and lb in H,O containing 0.010 M NaHCO, and 0.040 M NaBr. Each micellar solution was filtered through either a Millipore Millex HV, filter unit (contains a 0.45-wm Durapore membrane) or a Nuclepore 0.20-wm polycarbonate membrane into a 6 mm X 50 mm culture tube (Kimble 73500-650), which was inserted into the particle sizer a t 35.0 f 0.1 "C. As noted in the Results, a distribution (Nicomp) analysis of the autocorrelation function for each solution indicated the presence of a t least two size distributions, but from run to run, the analysis varied. Typically, in the volume-weighted distribution of apparent micelle diameters, one population was grouped a t ca. 7 nm (83% volume) and another a t ca. 32 nm (17% volume). However, reproducible values of the mean volume-weighted diffusion coefficient were obtained for each solution from cumulant analysis of the autocorrelation function: (3.26 f 0.07) X (3.29 f 0.02) X cm2/s for the above solutions of and (3.03 f 0.02) X la, lb, and the la/lb mixture, respectively.

Monolayer Studies. a. Preparation and Purification of Materials. Procedures for the preparation and handling of solutions of pure la and lb and their mixtures in purified hexanesEtOH have been described in detail.31 All experiments were carried out with a subphase of pH 7.5 [tris(hydroxymethyl)aminolmethane-HC1 buffer. The subphase H,O was triply distilled, [tris(hydroxymethyl)amino]methane(Schwarz/Mann, ultra pure) was recrystallized 3 times from triply distilled H,O, and concentrated hydrochloric acid (Mallinckrodt) was used without further purification. b. Spreading Solutions. The proper amount of each surfactant to be delivered to the surface was determined in "scouting" runs performed by delivering 4.403 X lo', molecules to a typical area of 6.84 X 10" A'. Solutions that formed highly expanded films a t greater than 120 A2/molecule were spread (30) Sugai, S.; Ikawa, H.; Hasegawa, Y.; Yoshida, S.; Kutsuma, T.; Akaboshi, S. Chem. Pharm. Bull. 1984,32, 967. (31) Arnett, E. M.; Chao, J.; Kinzig, B.; Stewart, M.; Thompson, 0.; Verbiar, R. J . Am. Chem. Soc. 1982, 104, 389.

Langmuir, Vol. 6, No. 3, 1990 553 by using smaller aliquots of spreading solution until the entire isotherm from n = 0 to approximately n = 40 dyn/cm could be recorded. Solutions of mixtures were prepared by delivering aliquots of solutions of the pure components to a stoppered 2-mL test tube by an Agla microliter syringe. Special care was taken to ensure that each drop of solution was delivered to the bottom of the tube rather than to the wall in order to avoid concentration errors due to evaporation of the hexanes. Every third solution of a mixture was prepared twice to alleviate random errors in the mixing process. The surface pressure vs area isotherms obtained from these mixtures agreed to within f 2 A2/molecule a t any given surface pressure. c. Langmuir Film Balance Techniques. The Langmuir film balance used in this study employs the floating barrier/ torsion head system for the detection of surface pressure and is sensitive to changes as small as 0.005 dyn/cm. Its construction, cleaning, preparation, calibration, and technical specifications have been described in detail.31 The rate of compression for each monolayer film was held constant a t 19.24 A2/molecule per min. The temperature of the subphase was maintained a t 25.0 f 0.1 "C by using a constant-temperature bath connected to a serpentine glass coil placed in the buffer subphase. The Langmuir film balance was housed in a Puffer-Hubbard-UniTherm cabinet. Analysis of each surfactant or surfactant mixture was reproduced 5-10 times. The stability of the films was checked by halting compression for 1 min and monitoring the loss in surface pressure. The film was deemed stable if there was a surface pressure drop of no more than 0.1 dyn/cm per min. The validity of the film balance calibration was checked every 12 h with the well-known stearic acid i~otherm.~' d. Equilibrium Spreading Pressures. ESPs of the surfactants were determined on the pH 7.5 buffer subphase a t 25.0 "C by Du Nouy ring tensiometry using either a Fisher Autotensiomat or a manual Cenco Du Nouy Tensiometer (No. 70535). Before each measurement, the ring was cleaned by flaming, and the surface tension of the freshly aspirated subphase taken. In each experiment, 0.5-1.0 mg of surfactant crystals, far in excess of the amount needed to form a monolayer, was delivered carefully to the subphase surface of a 6.8-cm (diameter) Teflon dish (Autotensiomat) or a 6.5-cm (diameter) Pyrex T-cup (Cenco). The crystals were allowed to equilibrate for a minimum of 12 h, with successive experiments performed at 18 and 24 h. Equilibration was assumed when the readings changed by no more than 0.2 dyn/cm in a 4-h period. Each experiment was repeated a t least 3 times with fresh subphase and crystals. e. Area Additivity Relationships. The area additivities were calculated from the II/A isotherms for the mixtures of la and lb. f. Data Analysis. All reported data were analyzed a t the 95% confidence limit using the "student t" for 5-10 repetitions for II/A isotherms and 3-6 repetitions for ESPs. The free energy changes of compression for pure la and lb and their mixtures were obtained from graphical integration from II = 0 (area = 500 A2/molecule) to the surface pressure in question. Area additivity relationships for mixtures of la and lb are presented a t the 95% confidence limit for 5-10 repetitions. Hydrolysis of the Mixture of la and lb. To 0.60 mL of 0.10 M hydrobromic acid a t 25 "C in a 5-mm NMR tube, cut to a length of 10 cm, was added 5.00 mg (0.0105 mmol) of a 44:56 mixture of la and lb. The surfactant did not dissolve completely but did so immediately when the capped tube was placed in a water bath thermostated a t 50.0 f 0.2 "C. After 6 h, the tube was removed from the bath, and the reaction mixture, now cloudy due to the formation of 5 , was cooled to 15 "C and adjusted to pH 8-9 with 20% aqueous NaOH, as determined with a pH meter. The resultant mixture was transferred to a lyophilizing flask, and the tube was rinsed with 1 mL of H,O and then with 1mL of CHCl,. The combined reaction mixture and washes were lyophilized (0.01 mmHg), and the resultant residue was suspended in 0.75 mL of CDCl, containing 0.01% Me,Si and transferred to a 5-mm NMR tube. (32) Gaines, G. L. Insoluble Monolayers Wiley: New York, 1966; p 220.

at Liquid-Gas Interfaces;

554 Langmuir, Vol. 6, No. 3, 1990 The tube was capped and centrifuged upside down to leave undissolved material (61a and NaBr) within the cap. The extent of hydrolysis was determined by 'H NMR analysis of the solution of la, lb, and 5. The calculation was based on electronic integrals for the following signals: for 5 , the Me singlet a t 6 2.15 and the a-CH, triplet at 2.40; for la and lb, the overlapping one-proton multiplets a t ca. 4.35 and the overlapping two-proton multiplets at ca. 4.70. In five runs, the extents of hydrolysis were 23%, 26%, 25%, 2770, and 23%. By 13C NMR analysis, the 1a:lb ratios for recovered starting material in two runs were 4654 and 48:52. Control runs as follows with known amounts of 5 and the la/lb mixture verified the accuracy of the analytical procedure. To 0.60 mL of H,O in an NMR tube were added known amounts of the surfactant mixture and 5 (0.0103 mmol total). The system was shaken and adjusted to pH 8-9 with 20% aqueous NaOH. Thereafter, the procedure was identical with that used in the actual runs. In controls with mixtures containing 25% and 30% 5 , the analytical procedure gave 24% and 30% 5 , respectively. Another control demonstrated that 6 does not dissolve in CDC1, and therefore does not complicate the 'H NMR analysis in the actual runs. Individual Hydrolyses of la and lb. The procedure was the same as that used for the la/lb mixture. For la in six runs, the extents of hydrolysis were 37%, 36%, 37%, 35%, 3870, s-'. For lb in and 3570, which correspond to k = 2.1 X sixruns, thevalues were 31%, 27$0,32%, 33%, 28%,and 29%, which correspond to k , = 1.7 X s-'. In no run was la/lb interconversion detected by 'H NMR analysis. Hydrolyses of the Mixture of la and lb with Added NaBr. Runs with 0.30 and 0.50 M NaBr were made with the above procedure, including the addition of NaBr to the reaction mixture after that of la and lb. The former runs required 18.5 mg (0.180 mmol) of NaBr and gave 40%, 42%, 44%, 42%, 39%, and 41% hydrolysis, and the latter runs required 30.9 mg (0.300 mmol) of NaBr and gave 47 %, 45%, 45%, and 46% hydrolysis. Hydrolysis of the Mixture of 2a and 2b. Runs were made with 0.0110 and 0.280 M solutions of a 53:47 mixture of 2a and 2b, which correspond to concentrations below/at and above the cmc. To 0.60 mL of 0.10 M hydrobromic acid a t 25 "C in a 5mm NMR tube, cut to a length of 10 cm, was added the appropriate amount of the 2a/2b mixture. The capped tube was held a t 50.0 i 0.2 "C for a given time (see below), and then the reaction mixture was cooled immediately to 15 "C and adjusted to pH 8-9 with 20% aqueous NaOH, as determined with a pH meter. The resultant solution was transferred to a lyophilizing flask, and the tube was rinsed with two 1-mL portions of H,O. The combined reaction mixture and washes were lyophilized, and the resultant residue was dissolved in 0.70 mL of D,O, containing 0.05 M Na,CO, and ca. 0.1% Me,Si(CD,),CO,Na, and transferred to a 5-mm NMR tube. The extent of hydrolysis was determined by 'H NMR analysis of the D,O solution of 2a, 2b, and 6. The calculation was based on electronic integrals for the following signals: for 2a and 2b, the Me triplet a t 6 0.88 and the multiplet for CH,CO a t ca. 1.70; for 2a, 2b, and 6, the overlapping one-proton multiplets at ca. 4.27. At 0.0110 M 2a/ 2b in six 5-min runs, the extents of hydrolysis were 49%, 49%, 46%, 45%, 50%, and 50% and a t 0.280 M 2a/2b in three 30min runs 26%, 27%, and 29%. By 13C NMR analysis, the 2a:2b ratios for recovered starting material in two 0.0110 M runs were 45:55 and 47:53 and in two 0.280 M runs 45:55 and 46:54. Control runs with known amounts of 6 and the 2a/2b mixture verified the accuracy of the analytical procedure. The 2a/ 2b mixture and 6 (0.0327 or 0.293 mmol total) were added to 0.60 mL, of H,O in a 5-mm NMR tube. The resultant solution was adjusted to pH 8-9 with 20% aqueous NaOH, and thereafter, the procedure was identical with that used in the actual runs. In runs with mixtures containing 16% and 37% 6, the analytical procedure gave 15% and 33% 6, respectively. Hydrolysis of 3. A mixture of 6.00 mg (0.00855 mmol) of 3 and 1.00 mI, of H,O was sonicated (150 W, bath, 50-55 "C) for 1 h in a mm X 90 mm test tube capped with a rubber sep-

Jaeger et al. tum. At 50 "C, the resultant vesicular solution was adjusted to pH 1.00 by the addition of 6 M hydrobromic acid, as determined with a pH meter. Then the reaction mixture was held at 50.0 k 0.2 "C for 6 h, cooled to 25 "C, and adjusted to pH 8-9 with aqueous 20% NaOH. Thereafter, it was transferred to a lyophilizing flask, and the test tube was rinsed with 1 mL of H,O and then with 1 mL of CHCl,. The combined reaction mixture and washes were lyophilized, and the residue was suspended in 1 mL of CDC1, containing 0.01% Me,% and transferred to a 5-mm NMR tube. The capped tube was centrifuged upside down to leave undissolved material (6 and NaBr) within the cap. The extent of hydrolysis was determined by 'H NMR analysis of the solution of 3 and 18-pentatriacontanone. The calculation was based on electronic integrals for the following signals: for 18-pentatriacontanone,the a-CH, triplet at 6 2.36; for 3, the one-proton multiplet a t 4.30. In four runs, the extents of hydrolysis were 12%, 16%, 15%,and 15%, which gave ky = 0.73 X s-'. Hydrolysis of 4. To 0.60 mL of 0.10 M hydrobromic acid a t 25 "C in a 5-mm NMR tube, cut to a length of 10 cm, was added 5.00 mg (0.0197 mmol) of 4. The capped tube was held a t 50.0 i 0.2 "C for either 10 or 30 min, and then the reaction mixture was worked up with the procedure used for the 2a/2b mixture. The extent of hydrolysis was determined by 'H NMR analysis of the resultant D,O solution of 4 and 6. The calculation was based on electronic integrals for the following signals: for 4, the Me singlets a t 6 1.43 and 1.50; for 4 and 6, the overlapping one-proton multiplets a t ca. 4.30. In two 10-min runs, the extents of hydrolysis were 68% and 69%, which gave k , = 190 X s-'. In five 30-min runs, the extents of hydrolysis were 93%, 95%, 95%, 96%, and 96%, which gave k , = 170 X 10-5 s-1. Control runs as follows with known mixtures of 4 and 6 verified the accuracy of the analytical procedure. In a 5-mm NMR tube, 0.60 mL of 0.10 M hydrobromic acid was adjusted to pH 8-9 with 20% aqueous NaOH. Then known amounts of 4 and 6 (0.0191 mmol total) were added. The resultant solution was transferred to a lyophilizing flask, and the tube was rinsed with two 1-mL portions of H,O. Thereafter, the procedure was identical with that used in the actual runs. In multiple runs with mixtures containing 24% 6, the analytical procedure gave 25%, 25%, 27%, and 27% 6. Quantitative Analysis of la/lb and 2a/2b Mixtures by 13CNMR. The following carbons, identified by 13CNMR DEPT experiments, were used for the quantitation of la/lb mixtures: CH,CO (la, 6 23.23; lb, 24.64) and CH,CO (la, 39.76; lb, 38.67). By analogy, the following carbons were used for the quantitation of 2a/2b mixtures: CH,CO (2a, 6 23.15; 2b, 24.59) and CH,CO (2a, 39.62; 2b, 38.59). For a given mixture, a t least two inverse gated 13C NMR spectra (pulse delay = 5 s) were obtained for a surfactant solution in CDCl, (ca. 0.07 and 0.3 M for 1 and 2, respectively) containing 0.08 M chromium(II1) acetylacetonate, and the signals for the appropriate carbons were electronically integrated. In controls with a known 4555 mixture of la and lb the analytical procedure gave 1a:lb values of 4555 and 47:53.

Acknowledgment is made to the National Cancer Institute, DHHS (PHS Grant No. CA45769-01) (D.A.J.), the State of Wyoming Eagles (D.A.J.), and the Marathon Oil Co. (D.A.J.) for the support of this research. We thank Professor Edward. M. Arnett for guidance in the monolayer studies and Dr. Daniel A. Netzel and Ethan L. G. Brown for help in the I3C NMR quantitative analyses and the preparation of 1, respectively. Registry No. la, 124582-96-3; lb, 124582-97-4; lb.1/2H20, 124583-03-5; 2a, 124582-98-5; 2b, 124582-99-6; 3, 119296-58-1; 4, 124583-00-2; 5, 629-66-3; 7, 4704-77-2; 8a, 124583-01-3; 8b, 124583-02-4; stearic acid, 57-11-4; epibromohydrin, 3132-64-7; cis-(2-octyl-2-methyl-l,3-dioxolan-4-yl)methyl bromide, 124583bro04-6; trans-(2-octyl-2-methyl-1,3-dioxolan-4-yl)methyl mide, 124583-05-7; 2-decanone, 693-54-9; (2,2-dimethyl-1,3-dioxolan-4-y1)methyl bromide, 36236-76-7.