Langmuir 1989,5, 1005-1008 between the enantiomers of SSME is given in Table 111. Although the experiments presented here do not explicitly take into account the contribution of the subphase water, possible changes in the structure of the vicinal water layer, or the degree of headgroup solvation, they do reduce all possible interactions between film components to a set of well-defined composition variables which differ only through the stereochemistry of the SSME headgroup. Taken as a whole, the I I / A isotherms, monolayer stability limits, area/composition diagrams, surface shear viscosities, and equilibrium spreading pressures indicate that any detectable chiral discrimination in these multicomponent films must arise from direct contact between chiral headgroups (Figure 1B). Conclusions By every criteria we have devised, multicomponent films of palmitic acid and enantiomeric and racemic SSME demonstrate chiral molecular recognition only when the chiral component is in excess and when the film system is in a condensed or even collapsed state. No chiral recognition could be detected in any viscoelastic or thermodynamic monolayer property under conditions where the films were distinctly fluid monolayers. The chiral discrimination as detected in these chiral/achiral systems is therefore due to a segregation of the less stable enantiomeric film component from the fatty acid matrix and
1005
to a slow collapse to a different, solidlike surface state. The experiments presented here have demonstrated several phenomena unprecedented in the study of chiral interactions in ordered systems. It has been shown here for the first time that the flow properties of enantiomeric and racemic monolayers may differ drastically and that these flow properties may be altered by the addition of other surfactant components while maintaining a stereoselective packing pattern. In addition, the dependence of the surface shear viscosities on composition indicates that the added surfactant is capable of breaking the two-dimensional lattice of the enantiomeric films much in the same manner as does the addition of the opposite enantiomer. It has also been demonstrated by ESP measurement that a prespread film at the air/water interface may act as a two-dimensional "solvent", enhancing the spreading of a monolayer of SSME from the crystal; this dilution effect is different for racemic and enantiomeric crystals, as is often the case in three-dimensional systems. Acknowledgment. This work was supported by a generous grant from AT&T. We thank Philip L. Rose and Jonathon Heath for helpful discussions and criticisms and Marjorie Richter for technical assistance. Registry No. SSME, 118319-49-6;palmitic acid, 57-10-3; stearic acid, 57-11-4; arachidic, 506-30-9.
Reverse Micelles of Aerosol-OT in Benzene. 4. Investigation of the Micropolarity Using 1-Methyl-8-oxyquinoliniumBetaine as a Probe M. Uedal and Z. A. Schelly* Center for Colloidal and Interfacial Dynamics, Department of Chemistry, The University of Texas at Arlington, Arlington, Texas 76019-0065 Received October 24, 1988. I n Final Form: February 21, 1989 The aggregation and the micropolarity of Aerosol-OT reverse micelles in benzene are investigated by betaine (QB)at 25 "C, as a function of the using a new absorption probe, 1-methyl-8-oxyquinolinium surfactant concentration and the water content of the solutions. QB is preferentiallypartitioned (>500:1) in the aqueous pool of the aggregates, and its transition energy EQBis shown to have a linear relationship with Kosower's 2 values and Dimroth et al.'s ET(30)values. Introduction Solutions of surfactants in nonpolar solvents always contain at least a small amount of water which cannot be neglected because it promotes the association of the amphiphile to reverse micelles and accumulates as a pool in the polar core of the aggregates. With increasing water content of the solution, the pools swell, which successively leads to the formation of swollen reverse micelles and w/o microemulsions. Due to the rather peculiar chemical and physical properties2of the polar interior of reverse micellar aggregates, substantial efforts have been focused on the investigation (1) R.A. Welch postdoctoral fellow. On leave from the Osaka Municipal Technical Research Institute, Japan. (2) Fendler, H. J. Membrane Mimetic Chemistry;Wiley: New York, 1982.
of the state of water in the pools. For this purpose, absorption and fluorescence probes such as TNS? ANS; pyranine derivatives: vitamin BI2,6acridine orange: picric acid?yQTCNQ,'OJ1 pyrene derivatives,12 etc., have been commonly used. In utilizing probe molecules, ideally, one (3) Menger, F. M.; Donohue, J. A.; Williams, R. F. J. Am. Chem. SOC. 1973, 95, 286. (4) Wong, M.; Thomas, J. K.; Gratzel, M. J. Am. Chem. SOC.1976,98, 2391. (5) Kondo, H.; Miwa, I.; Sunamoto, J. J. Phys. Chem. 1982,86, 4826. (6) Fendler, J. H.; Nome, F.; VanWoer, H. C. J.Am. Chem. SOC.1974, 96, 6745. (7) Herrmann, U.; Schelly, Z. A. J. Am. Chem. SOC.1979, 101, 2665. (8) Tamura, K.; Schelly, Z. A. J. Am. Chem. SOC.1981, 103, 1013. (9) Tamura, K.; Schelly, Z. A. J. Am. Chem. SOC.1981, 103, 1018. (10) Muto, S.; Meguro, K. Bull. Chem. SOC. Jpn. 1973, 46, 1316. (11) Harada, S.; Schelly, Z. A. J. Phys. Chem. 1982,86, 2098. (12) Verbeeck, A.; Galade, E.; DeSchryver, F. C. Langmuir 1981, 2, 448.
0743-7463/89/2405-1005$01.50/00 1989 American Chemical Society
1006 Langmuir, Vol. 5, No. 4,1989
expects the probe not to significantly perturb the system it is to measure. It should have the proper location within the aggregate, and it should be sensitive enough to relevant changes in the system to be able to provide meaningful information. The purpose of the present study was to investigate the micropolarity of the interior of Aerosol-OT reverse micelles in benzene. The relatively low aggregation number of the surfactant ( f i = 11.7) demands a small probe molecule. In addition to great sensitivity to polarity, it is also required that the probe not introduce free counterions into the micelles. The requirements are met by 1-methyl-8-oxyquinolinium betaine (QB), which we introduce as a probe for the aqueous pool of reverse micelles.
Ueda a n d Schelly
r
E0
,-
s
?
z X
w
A. nm Figure 1. Absorption spectra of QB in water (a), acetonitrile (b), and benzene (c), at 25 OC.
It is one of the smallest organic optical probes that have been used for the investigation of such systems. Due to its zwitterionic structure, QB is strongly hydrophilic (its partition coefficient between benzene and water is 15500); thus, its preferential location is expected to be the aqueous core of the aggregates. Moreover, the absorption spectrum of betaine is highly sensitive to the polarity of its local environment as exemplified by the deep purple benzene solution compared to orange in water.13 This negative solvatochromic behavior can be closely related to the behavior of Kosower's 2 c ~ m p o u n d 'or ~ Dimroth's ET(,?, c ~ m p o u n d , and ' ~ it can be utilized to determine the micropolarity of the pools of reverse micelles. In this paper, the relationship between the transition energies of absorption of QB and Kosower's 2 values, as well as Dimroth's ET(30)values, are presented. Subsequently, the use of QB as an absorption probe for the state of aggregation and the micropolarity of the core of Aerosol-OT reverse micelles in benzene are reported. Experimental Section Chemicals. 1-Methyl-8-oxyquinolinium betaine (QB) was prepared by a modified procedure of Saxena et al.13 A solution of 8-hydroxyquinolinein acetone was refluxed with methyl iodide for 3 h. After filtration, the crude 1-methyl-8-hydroxyquinoliiium iodide obtained was recrystallized twice from ethanol-acetone (1:2 v/v). The quinolinium iodide was then dissolved in 2 N aqueous K2C03,and the resulting betaine was extracted with chloroform. The chloroform solution was dried with inhydrous K2C03 and concentrated under reduced pressure. The solid betaine was precipitated from the chloroform solution by the addition of cyclohexane. The betaine was then recrystallized twice from ethyl acetate. The solid anhydrous betaine was'obtained as violet needles after drying at 80 "C in vacuo over P2O5. Its purity was checked by TLC (on silica gel with acetone-water 1:l v/v R, = 0.2, with acetic acid-water 1:l v/v R, = 0.7). Aerosol-OT (AOT, bis(2-ethylhexy1)sodium sulfosuccinate; Nikarai Chemicals Ltd., 97.4%) was purified by a method16that was derived from the procedures of others."J8 It was dried to constant weight in vacuo over P,05. The water content of the "dry" AOT was found to be 10 mol % by Karl-Fischer titration. The absence of acidic impurity was checked by using QB as an indicator in a manner similar to that reported by Magid et al.19 (13) Saxena, J. P.;Stafford, W. H.; Stafford, W. L. J. Chem. SOC. 1959,
1579. (14) Kosower, E. M. J . Am. Chem. SOC. 1958, 80, 3253. (15) Dimroth, K.;Reichardt, C.; Siepmann, T.; Bohlmann, F. Ann. Chem. 1963,661, 1. (16) Ueda, M.; Schelly, Z. A. Langmuir 1988, 4, 653. (17) Kunieda, H.; Shinoda, K. J . Colloid Interface Sci. 1979, 70,577. (18) Martin, C.A,; Magid, L. J. J . Phys. Chem. 1981, 85, 3938.
E,, kcal mol-' Figure 2. Correlation between EQSand Kosower's 2 values: A, H2O; B, CH,OH; C, CZH5OH; D, CH3CN; E, CH3COCH3; F, CHCl,; I, 90% v/v aqueous C2H50H; J, 80% v/v aqueous C2H50H;K, 70% v/v aqueous C2H50H. Benzene was distilled over sodium. The water content of the dried solvent was found to be 0.015% w/w by Karl-Fischer titration. Water was double-deionized and distilled. All other chemicals used were of reagent grade. Absorption Measurements. The UV-vis absorption spectra were recorded on a Cary 14 spectrophotometer at 25 "C by using 1-cm and 10-cm path length cells. In the concentration range studied, the absorption at 587 nm of QB in benzene solutions obeys the Bouguer-Lambert-Beer law, with the molar decadic extinction coefficient o f t = 1.41 X lo3 M-' cm-'. Vapor Pressure Osmometric Measurements. An HP-302B instrument was used for the determination of the effect of the presence of M QB on the aggregation number in a 0.1 M AOT-benzene solution. The calibration and experimental procedures have been described previously.16,m
Results and Discussion Absorption Spectra of QB in Pure Solvents. The solvent dependence of the absorption spectrum of QB is shown in Figure 1. The longer wavelength absorption band is due to the transition from a predominaptly dipolar ground state to an excited state of considerably reduced polarity. With increasing polarity of the solvent, the ground state becomes more stable, which leads to an increase in the transition energy, i.e., negative solvatochromism. The transition energy (expressed in kcal mol-') (19) Magid, L.;Kon-no, K.; Martin, G. J. Colloid Interface Sci. 1981,
..". .
R.7 --, 107
(20) Ueda, M.; Schelly, Z. A. J . Colloid Interface Sci. 1988,124, 673.
Langmuir, Vol. 5, No. 4 , 1989 1007
Reverse Micelles of Aerosol-OT in Benzene
-
56
e@
7
2
54
2
52
e
z l
500
600
700
h nm Figure 4. Absorption spectra of lo4 M QB in "dryn AOTbenzene solutions at 25 "C. Concentration of AOT a, 1.25 X M; b, 4 x M; c, 3.2 X M; d, 1.28 X M; e, 5.12 X
10-4
103
10-2
10-1
[AOT] M
Figure 3. Correlation between E and Dimroth et al.'s ET(%) values: A, H20;B, CH,OH; C, C$&H; D, CH3CN;E, CH3COCH,; F, CHCl,; G, CH3COOC2H5;H, C,H,; I, 90% v/v aqueous CzH50H;J, 80% v/v aqueous C,H,OH; K, 70% v/v aqueous C2H5OH.
400
0
105
E, kcal mol-'
0
e o
M.
of QB can be used as a polarity parameter, E,,, similar to Kosower's 2 value14and Dimroth et al.'s ET(30) value,15 which were derived from the spectral behavior of 1ethyl-4-carbomethoxypyridiniumcompounds and 2,6-diphenyl-4-(2,4,6-triphenylpyridinio)phenoxide,respectively. Figures 2 and 3 illustrate the relationship between EQB, 2, and ET(m) in various solvents as well as solvent mixtures. The relationships can be expressed as 2 = 2.520EQB - 68.4 (correlation coefficient = 0.9937) (1) E~(30= ) 1.712E~ -~ 49.1 (correlation coefficient = 0.9934) (2) Clearly, QB is eminently suitable as a probe for micropolarity. Use of QB in Reverse Micelles. The absorption spectra of lo4 M QB in dry AOT-benzene solution a t several different AOT concentrations are shown in Figure 4. With increasing surfactant concentration, the absorption maximum of QB shifts markedly to shorter wavelengths, and the spectra exhibit an isosbestic point at 540 nm. At surfactant concentrations higher than those used for the figure, the isosbestic point is obscured by the increasing light scattering of the micelles. The presence of the isosbestic point suggests that only two spectrophotometrically distinguishable states of QB exist in the solutions: one in a hydrophobic environment and the other solubilized in the polar interior of the reverse micelles. The independence of the E Q B values of the QB concentration is demonstrated in Figure 5: a 10-fold increase in
Figure 5. E , values in 'dry" AOT-benzene solutions at 25 O C . Concentration of QB: 0, M; 0 , lo4 M. the concentration of the probe leaves the polarity of its local environment unaffected. QB has apparently no influence on the mean aggregation number ii of AOT either. Vapor pressure osmometric measurements on 0.1 M AOT-benzene in the presence of 10"' M QB yielded the same ii = 11.7 as without the probe.20 These results suggest that in the concentration range 10-5-104 M of QB the presence of the probe has no measureable effect on the aggregation and solubilization behavior of the surfactant. Up to an AOT concentration of 3 X M, QB senses a micropolarity of that of pure benzene. Above this AOT concentration, E,, increases in a sigmoid fashion, indicating that reverse micelle formation and the solubilization of the probe are progressing over a wide AOT concentration range. As pointed out previ~usly,~ the conventional concept of the cmc, as it is used for aqueous surfactant solutions, is not generally transferable to nonaqueous systems. In contrast to aqueous systems, in nonpolar solvents the formation of surfactant aggregates is usually unsharp, and the mean aggregation number ii as well as the monomer concentration may be functions of the surfactant concentration. Thus, in nonpolar solvents the concept of cmc can only be used in an operational sense. The operational cmc is the lowest surfactant concentration a t which aggregation can be inferred by the particular method used. Hence, the operational cmc of the dry AOT-benzene system is 3 X M, as detected by the change of the transition energy EQB. This value can be contrasted with those obtained by other methods (4 X lo4 M by vapor pressure osmometry> 7 X lo4 M by acridine M by conorange base as an absorption probe,7 5 X ductivity measurements,2l and 9 X M by iodine as an absorption probelo). In previous studies>9 the bimodal monomer-6-mer14-mer association model was found to be the best description among ll models tested for the aggregation of AOT in dry benzene. A result of the bimodal association is that the 6-mer concentration becomes significant a t [AOT] = M whereas the 14-mer population becomes significant only above [AOT.] = M. In the AOT concentration range 10-3-10-2 M, the 14-mer population sharply increases. This may explain the steep rise of EQB in the same AOT concentration range (Figure 5) if one assumes that QB is preferentially solubilized by the larger aggregates. Above M concentration of AOT, E p attains a value that indicates a micropolarity of the environment of QB similar to that in pure acetonitrile. (21)Eicke, H.F.;Arnold, V. J. J.Colloid Interface Sci. 1974,46,101.
1008 Langmuir, Vol. 5, No. 4, 1989
Ueda and Schelly 61
60 -
-a
58
c
/
-
5
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wo 5 2 50 48
c 1, 105
Figure 6.
A
I
4 I
10-4
i
I
I
103
10-2
10"
[AOTI M EQBvalues in "wet" ( R > 10) AOT-benzene solutions
at 25 "C. Concentration of QB: A,
R Figure. 7. Effect of water content on EQBin 0.1 M AOT-benzene solutions at 25 "C.
M; A,
M.
Effect of Water Content on Aggregation and Micropolarity. Since the water content of dried benzene is M, and solid, dried AOT contains 10 mol 5% water, 7X the R [H,O]/[AOT] value of "dry" AOT-benzene solutions is very high at low AOT concentrations (e.g., R = 18 at 4 X 10"' M AOT). However, as the surfactant concentration is increased, the water content of the benzene becomes less significant compared to the amount of water introduced with the solid AOT, and R asymptotically approaches the value 0.1. This is the situation for the solutions used for Figure 5. Now, if additional water is dissolved in the solutions, a relatively constant, high R value can be maintained in a series of experiments. The results are shown in Figure 6. The solutions were prepared through the appropriate dilution with dry benzene of a 2 X lo-' M AOT-benzene stock solution of R = 10. (The water content at R = 10 is near the value above which phase separation occurs.) Thus, all the clear solutions used for Figure 6 had an R value of only slightly greater than 10. A t AOT concentrations up to 10"' M, QB reports the same micropolarity for the "wet" system as for the "dry" one (Figure 5), and the operational cmc is lowered only slightly. These results are not surprising since, as mentioned above, the R value of the dry system is already high at low AOT concentration, without the addition of water. However, in the M, EQB = 61 kcal concentration range of [AOT] h mol-', corresponding to a micropolarity of that in pure methanol or in a 70% v/v ethanol-water mixture. Our results are comparable with those of Wong et al.4 obtained on AOT-alkane systems by the use of ANS as a fluorescence probe. They found the Kosower's values of 60 ("dry" system) and 80 (at R = 50) which correspond to micropolarities of the reverse micellar interior ranging from that in pure chloroform to that in pure methanol, with increasing water content.
If the AOT concentration is kept constant at 0.1 M, and the R value is varied, one observes a clear break in the micropolarity at R = 0.5 (Figure 7). Also at around the same R value the heat of solubilization of water in AOTbenzene has a indicating a special significance of the ratio [H,O]/[AOT] = 1/2. With increasing water content up to R = 0.5, the micropolarity of the pools changes only slowly, but above R = 0.5 at a higher rate. This seems to suggest that the micropolarity of the pool can be significantly raised by the addition of water only after all pairs of AOT molecules are bridged by H20. It is interesting to note that at R = 10 EQB (= 61 kcal mol-') is significantly smaller than that (65 kcal mol-') in pure water. On the other hand, as our controlled partial pressure-vapor pressure osmometric (CPP-VPO) measurements have shown,20the water vapor pressure pw of the system at R = 10 is already more than 99% of the vapor pressure p o of pure water. The discrepancy is probably due to the distribution of QB between the reverse micellar cores and the bulk solvent, as well as to the distribution of the pool sizes.23 An alternative hypothesis, in which the QB is assumed to be a cosurfactant, seems to be less attractive, especially at R > 1. Namely, in that case the probe would be reporting on the micropolarity of the micellar interface which, however, should change little after R = 1 is reached. Evidently, the cosurfactant interpretation is inconsistent with the significant polarity changes with R (above 0.5) we observed. Acknowledgment. This work was partially sponsored by the R.A. Welch Foundation and the National Science Foundation (Grant No. CHE-8706345). Acknowledgement is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for additional support. This material is based in part upon work supported by the Texas Advanced Research Program under Grant No. 1766. Registry No. AOT, 577-11-7;QB, 22544-89-4; benzene, 71-43-2. (22) Kon-no, K.; Kitahara, A. J. Colloid Interface Sci. 1971,35, 409. (23) Nemeth, S.; Schelly, Z. A,, manuscript in preparation.
