542
Langmuir 1990, 6, 542-547
mixtures, i ~ ~ ( a t / ~ ~ ~ ) ~ ~ ethanol ~ ~ ~ ~ ~ one a ~observes that the values of NF cal(A21 culated from (A2) are in good agreement with those cal2~~~~(aa,~/ax,') culated from (Al). Therefore, one can conclude that the approximation where h is the wavelength used and aZathe activity of N" = 1/x2* used in our evaluation of 0 is correct in the the solute. dilute concentration range. Using the values of R, given by Wood" for waterRegistry No. 2-Butoxyethanol, 111-76-2; ethanol, 64-17-5; (18)Parfitt, G. D.;Wood, J. A. Trans. Faraday SOC.1968,64,2081. tert-butanol, 75-65-0; 2,6-lutidine, 108-48-5. NF
=
Determination of Microviscosity and Micropolarity of Lyomesophases Utilizing a Fluorescent Probe R. Parthasarathy and M. M. Labes* Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122 Received April 21, 1989
The excimer to monomer emission intensity ratio (E/M) of an intramolecular excimer-forming probe, 1,3-bis(l-pyrenyl)propane(DPyP), has been studied in dilute solutions as well as lyomesophases of potassium laurate (KL), sodium decyl sulfate (SDecS), and myristyltrimethylammonium bromide (MTAB). E/Ms in the mesophases are much lower than in dilute solution due to the combined effects of viscosity, orientational order, and specific host-guest interactions. The phase transitions from the disk-like N4 phase to the rod-like N, phase to the isotropic phase are each accompanied by increases in this ratio. Microviscosities of MTAB, SDecS, and KL are approximately 120, 70, and 60 cP, respectively. The extent of vibrational structure in the monomer emission of DPyP is sensitive to polarity, so DPyP provides a simultaneous measure of both micropolarity and microviscosity.
Introduction Microviscosity is a somewhat vague concept used to describe the short-range spatial constraints that limit molecular rearrangement over short time scales (C1ms). Estimations of microviscosity, in principle, are particularly helpful as a means of characterizing micellar domains that are expected to be important solubilization sites. Since Zachariasse' demonstrated about a decade ago that the ratio of excimer to monomer emission intensities in an intramolecular excimer-forming probe, 1,3-bis(lpyreny1)propane (DPyP), was sensitive to microviscosity, there have been many investigations utilizing this p r ~ b e and ~ - ~its chemical analogues (such as the diarylpro pane^'.^ and higher alkyl homologues' of DPyP). The technique requires a simple measurement of the intensities of excimer and monomer emission; their ratio is translated into an estimate of microviscosity by comparison with values obtained in calibrant mixtures of known macroscopic viscosity. Alternative approaches (such as fluorescence depolarization measurements) are more com(1)Zachariasse, K. Chem. Phys. Lett. 1978,57, 429. (2)Lianos, P.;Lang, J.; Strazielle, C.; Zana, R. J. Phys. Chem. 1982, 86,1019. (3)Viriot, M. L.; Bouchy, M.; Donner, M.; Andre, J. C. Photobiochem. Photobiophys. 1983,5 , 293. (4)Melnick, R. L.;Haspei, H. C.; Goldenberg, M.; Greenbaum, L. M.; Weinstein, S.Biophys. J. 1981,34,499. (5) Lianos, P.; Viriot, M. L.; Zana, R. J.Phys. Chem. 1984,'88,1098. (6) Turro, N. J.; Okubo, T. J. Am. Chem. SOC.1981,103,7224. (7)Turro, N.J.; Aikawa, M.; Yekta, A. J. Am. Chem. SOC.1979,101, 772. (8)Anderson, V. C.; Weiss, R. G. J . Am. Chem. SOC.1984,106,6628.
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plicated, accounting for the popularity of excimer spectral studies. The main assumptions underlying the use of excimerforming guests to estimate host microviscosities are as follows: (i) the rate of molecular rearrangement required for excimer formation is dictated by solvent viscosity; (ii) the solvent does not affect any other photophysical properties of the probe. Both assumptions have been questioned?,lo but little doubt remains that the excimer/ monomer emission intensity ratio of DPyP in a given sample does reflect changes in matrix cohesion at least as a function of external variable^.^^^^ The ratio, E/M, of the intensities of excimer (E) and monomer (M) emission in the fluorescence spectra of DPyP has been investigated in simple micellar systems (anionic as well as c a t i o n i ~ ) ~and ' ~ ~emulsions2 *~~ to examine the effect of additives on their organization. Phase transitions in membranes, synthetic surfactant vesicles, and thermotropic liquid crystals have been detected by changes in E/M.394 It is known that E / M decreases monotonically (linearly at viscosities greater than 10 cP) as viscosity increases, but chemical considerations often influence the precise form of this equation, even among homo(9) Henderson, C. N.; Selinger, B. K.; Watkins, A. R. J. Photochem. 1981,16,215. (IO) Snare, M. J.; Thistlethwaite, P. J.; Ghiggino, K. P. J.Am. Chem. SOC.1983,105,3328. (11)Zachariasse, K.A.; Duveneck, G.; Busse, R. J. A m . Chem. SOC. 1984,106,1045. (12) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1982,88,594. (13)Lianos, P.; Lang, J.; Zana, R. J. Colloid Interface Sci. 1983,91,
276.
0 1990 American Chemical Society
Langmuir, Vol. 6, No. 3, 1990 543
Microviscosity and Micropolarity of Lyomesophases g e n e o u s solvent^.^^^^^^ E s t i m a t e s of micellar microviscosities using excimeric probes must take into account factors such as probe size, differences in the probe rotamer distributions, and possible chemical interactions between host and guest. However, intramolecular excimer formation is independent of details of the diffusive process, which plays a vital role in studies of their intermolecular counterparts. A study of the literature on micellar aggregates reveals that the fluorescence of DPyP in lyotropic mesophases has not been examined even though extensive data have been obtained on cognate phases. An investigation that could elicit information about the organization of the lyotropic mesophase appears to be particularly opportune in view of the sensitivity that these phases exhibit in modulating reaction rate constants.14 In this paper, we report our investigation of the fluorescence spectra of DPyP in lyomesophases comprising cationic (myristyltrimethylammonium bromide (MTAB)) and anionic (sodium decyl sulfate (SDecS) and potassium laurate (KL)) surfactants. The corresponding dilute micellar solutions have also been studied, primarily to isolate micelle-DPyP interactions from the orientational constraints imposed by the ordered (meso)phase. We have also explored the effect of altered micellar structure and size on the spectra of DPyP in electrolyte (NaC1)-containing dilute solutions of SDecS.
Experimental Section DPyP was obtained from American Tokyo Kasei and used as received. Pure SDecS was obtained from Research Plus and used as received while MTAB (from Aldrich Chemical Co.) was recrystallized from an ethanol-water mixture. KL was prepared as previously described.'* Methylcyclohexane (MCH) was a spectroscopicgrade obtained from Kodak. Quantitiesof DPyP in CHCl, required for a 5 x 10*-1 x M solution were pipetted into sample holders, and the solvent was evaporated under reduced pressure. In the case of the dilute micellar solutions, surfactant and triply-distilled water were then added to the DPyP and stirred for at least 48 h to solubilize the hydrophobic probe.I6 With MCH, stirring for 2 h was sufficient. The water used to make up the liquid crystals was flushed with prepurified N, gas for ca. 30 min, and the components were then stirred in an N, atmosphere for 48 h in screwtop bottles that were securely sealed with Teflon tape and Parafilm. Prior to recording the spectra, we flushed the dilute micellar samples with prepurified N, for at least 30 min. The liquid crystalline samples were withdrawn under N, by using a syringe but were not further flushed with inert gas since the loss of even small quantities of water resulted in precipitation of the surfactant. Uncorrected fluorescence spectra were acquired on a PerkinElmer MPF-66 spectrofluorimeter using 1-mm path length quartz cells with front surfaceexcitation. Emission spectra were obtained with slit widths of 4 nm (reduced to 2 nm in the case of the liquid crystals to avoid detector saturation), an excitation wavelength of 340 nm, and scanning speeds of 60 nm min-'. Excitation spectra employed 4-nm slit widths and an emission wavelength of 377.8 nm. A thermoelectric cell holder was used to maintain the cell temperature constant to f O . l deg. Samples were allowed to equilibrate for about 10 min at a given temperature, and the E/M ratios reported are averages of at least three scans. The dilute surfactant solutions were susceptible to oxygen recontaminationwithin the quartz cell (as reflected by changing E/M ratios), so measurements with a given sample were completed in 20 min during which time the ratio was constant to within 3%. E/M ratios in the mesophases were stable over 24 h, permitting variable-temperaturemeasurements to be made, in accord with previous reports that quenching by atmospheric 0, is negligible when sample viscosities exceed about 100 c P . ~ (14)Ramesh, V.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1987,152, 57; J. Am. Chem. SOC.1988, 110, 738. (15) Kano, K.; Ishibashi, T.;Ogawa, T. J.Phys. Chem. 1983,87,3010.
/ I
360
I 400
I
\
I
I
1 480
440
I 520
I
nm
Figure 1. Emission spectrum of DPyP in methylcyclohexane at room temperature. See text for details.
At the conclusion of the variable-temperatureruns, sample integrity was confirmed by the absence of change in the room temperature spectra. Mesophases and their transition temperatures were identified by using a Mettler FP82 hotstage in conjuction with a polarizing microscope.
Results and Discussion The emission spectrum recorded for DPyP in MCH is shown in Figure 1. Monomer fluorescence extends from 370 to 430 nm with a pair of prominent peaks at 377.8 and 397.1 nm that we denote by M, and M,, respectively. Vibrational structure is evident between the monomer peaks and around 370 nm. Excimer fluorescence occurs as a broad and featureless band extending from 430 to 540 nm. Our value for E/M,, evaluated as the ratio of the intensities at 480 and 395 nm (following Melnick et al.4), was found to be 2.97 after correction for monomer/excimer absorption, close to the value reported4 (2.89 at a DPyP concentration of lo4 M). However, if intensities a t the peak maxima at 397 and 485 nm are used, the corrected ratio is 2.75. It is worth noting that the spectrum in Figure 1 depicts better resolved structure than the spectrum published in ref 4. The structure of the monomer emission of DPyP (bands at 377.0, 383.5, 388.0, and 397.1 nm with shoulders at 394.3 and 368.8 nm) is reminiscent of that of pyrene (vibrational structure peaks a t 372.5,378.95,383.03,388.55and 393.09 nm, denoted I-V by Kalyanasundaram and Thomas''). The ratio, E/M, is proportional to the ratio of the quantum yields of excimer ($E) and monomer (4M)fluorescence. It is also related to kfhl(E),kiM(E), k,, and k d (which represent rate constants for monomer (excimer) fluorescence, monomer (excimer) internal quenching, excimer formation, and excimer dissociation, respectively) by4 Q'(k,/k,)(l + ( k i E + kJ/km)-' $E/$M a E/M where a is the fraction of DPyP that exists in the conformation state separated from the excimer by a single hindered rotation. If (kiE + k d ) / k , SDecS > MTAB. The value in MTAB is similar to that measured in MCH. 13/11ratios in pyrene are larger in nonpolar as compared to polar solvents, but our data in DPyP suggest the opposite: thus, values in MCH are lower than those in micellar hosts, in contrast to the data for pyrene reported by Kalyanasundaram and Thomas.'' These authors concluded that the solubilization site of pyrene was more hydrophobic (i.e., 13/11was greater) when the surfactant comprising the micelle had a more compact headgroup-as is the case, generally, with anionic amphiphiles. M,/M, of DPyP is smaller in MTAB than in KL or SDecS micelles but similar to that in MCH, which leads us to infer that the solubilization site is more hydrophobic in the cationic micelles. It is, however, not certain that pyrene and DPyP are solubilized at different sites. Decreasing M,/Ml as the SDecS concentration increases (Figure 4) suggests that with improved surfactant packing DPyP molecules find themselves increas(19) Tanford, C.
J.Phys. Chern. 1974, 78, 2469; 1972, 76, 3020.
Langmuir, Vol. 6, No. 3, 1990 545
-N,
27.5'
?-NL
27.5'
I
N, 25.5'
F N c 30.0 '
Ll 40.0'
380
420
460
500
540
nm
Figure 5. Emission spectra of DPyP in mesophases with the indicated textures: MTAB (upper), KL (middle), and SDecS (lower) at the temperatures shown in degrees Celsius.
nm
Figure 6. Typical excitation spectra of DPyP in liquid crystalline phases. ingly protected from contact with the aqueous phase until there is no further change a t surfactant concentrations typical of the mesophase. The decrease of M,/M, with rising [NaCl] also has a similar explanation. The change in slope around [NaCl] = 0.2 M in the M,/M, vs log cmc plot (Figure 4) suggests that the headgroup repulsion, which is initially decreased as salt is added, reaches a constant value at higher [NaCl] possibly due to a change in micellar anisotropy. Overall spectral intensities show a marked increase, KL < SDecS < MTAB. The intensities also increase with increasing [NaCl]. Evidently, radiative deexcitation is favored over nonradiative means in the more viscous hosts, as would be expected. Mesophases. The emission spectra of DPyP in the various mesophases are depicted in Figure 5, and typical excitation spectra are shown in Figure 6. The excitation spectra, as before, reflect the absence of aggregation in the fluorophore. Monomer emission is significantly higher in the mesophases than in the corresponding dilute micellar solutions. There is little evidence of structure in the excimer peaks except, perhaps, in the alkyl sulfate mesophase, where a shoulder at 515 nm is perceptible. In contrast to the excimer peak, the monomer emission shows structure in all of the mesophases exam-
546 Langmuir, Vol. 6, No. 3, 1990
Parthasarathy and Labes
Table 11. Relative Emission Intensities of DPyP in Mesophases system
temp, "C
MTAB MTAB:H,O: (40:60) 25.5 44.3 MTAB:H,O:l-decanol:NH,Br 25.5 (30:60:3.8:6.2) 42.8 SDecS:H,O:l-decanol (35.4:59:5.6)
SDecS 25.5 30.0 40.0
Dhase"
N,
I N, I
N, N, I=B
E/M
MJM,
0.11 0.28 0.10 0.24
0.58 0.63 0.58 0.61
0.45 0.52 0.68
0.61 0.68 0.72
0.59 0.68 0.85
0.62 0.63 0.68
KL KL:H,O:l-decanol (28.4:65.5:6.1)
25.5 30.0 41.9
N,
N, I
The symbols N,, N,, I, and I = B represent nematic cylindrical, nematic lamellar, isotropic, and isotropic = batonnet phases, respectively.
ined. The values of E / M and M,/M, for the mesophases investigated are listed in Table 11. Microviscosities in terms of the ethanol/glycerol calibration curve referred to earlier turn out to be in excess of 120 CPfor the MTAB mesophases and around 70 and 60 CPfor SDecS and KL liquid crystals, respectively. Measurements of the macroscopic viscosity utilizing a capillary viscometer give values of about 120 CP for both of these anionic surfactants." There is an increase in the E / M value across the NL N, phase boundary. A more pronounced increase is noticeable across the mesophase/isotropic solution boundary, but the values even in the latter phase are substantially lower than those in dilute solution, presumably due to the higher surfactant concentrations. The appearance of structure in the monomer component of the spectrum akin to that seen in MCH suggests solubilization of the probe in a hydrophobic domain, a possibility strengthened by the absence of structure in the excimer emission. E / M decreases in the order KL > SDecS > MTAB, as noted before in the dilute solutions. The decreased excimer yield in the mesophases is certainly due to heightened microviscosity (which would decrease k,) and order-which would suppress excimergenerating molecular rearrangements (thus reducing CY in eq 1). Excimer-forming conformations with nearly coplanar arene moieties necessitate a kink in the alkyl chain. The energy barrier to such a conformation may reasonably be expected to be quite high when the probe is solubilized amongst long and ordered alkyl chains. The N, phase consists of disk-like micelles, whereas the N, phase contains cylindrical micelles. While mesophase formation with anionic surfactants requires the presence of cosurfactants such as decanol, these additives are required to form only the NL phase in the MTABwater system. Isothermal comparisons amongst the different mesophases in a single anionic surfactant solution cannot be made with the data at hand, but in these cases as well, the NL phase must be less disordered since it is stable a t lower temperatures than the N,. The lower value of E / M in the NL phase, in general, therefore appears to stem from a greater degree of order, especially since the addition of alcohol to surfactant solutions has been shown to reduce microviscosity (increase E/M) in multicomponent emulsions.' In this context, it should be noted that rate constants for a variety of reactions have
-
(20) Kuzma, M.; Hui, Y. W.; Labes, M. M. Mol. Cryst. Liq. Cryst. 1989, 272, 211.
generally been found to be lower in NL phases than in N, in the three surfactant systems.14 Visual observation indicates that the macroscopic viscosity of MTAB mesophases is greater than that of SDecS or KL, in keeping with our finding that SDecS and KL mesophases exhibit similar values of E/M. Further indication that DPyP behaves anomalously, perhaps due to incomplete solubilization, in micellar solutions of KL is obtained by comparing the ratio of E / M in the micelles ((E/M),J to that in the mesophase ((E/MIme8). The values obtained are 4, 2.6, and 4 in MTAB, SDecS, and KL, respectively. That (E/M)mic/(E/M)me8 is higher in the KL liquid crystal than in SDecS, while the (E/M)mes values are comparable, is suggestive of anomalous behavior in dilute KL solution. It is quite possible that the arene moieties of DPyP in dilute KL solution adopt a nearly coplanar orientation (facilitating excimer formation), as in vesicles and membra ne^,^ so that unfavorable interactions with water may be avoided. If we assume that the effect of temperature on spectral properties of DPyP is similar in the three surfactants, the ratio of (E/M)i80(obtained in the orientationally disordered isotropic phase, around 40 "C) to (E/ M),,, (E/M a t room temperature) should highlight the role of orientational order in the mesophase. MTAB, SDecS, and KL exhibit values of 2.4 (NL) or 2.6 (N,), 1.5, and 1.4, respectively. (SDecS was observed to be biphasic a t 40 "C with the predominantly isotropic phase cont,aining some batonnet structure). The similar values that SDecS and KL display confirms that DPyP is adequately solubilized a t higher surfactant concentrations and that the mesophases of the two surfactants are ordered to about the same extent. There is no evidence that MTAB mesophases are more ordered than those of SDecS and KL, so larger (E/M)iso/(E/M)mesin MTAB may indicate that temperature affects the spectral properties of DPyP differently in this case and/or the presence of chemical interactions unique to DPyP and MTAB. Tables I and I1 show that M,/M, decreases in the three mesophases to a value of around 0.6 (at room temperature) from dilute micellar values of 0.94 and 1.15 (SDecS and KL) or 0.7 (MTAB) and increases slightly upon entering the isotropic phase. The similarity between the value in the mesophase and that in MCH solution suggests that the solubilization site of DPyP is less polar in the mesophase than in the micellar solution. To substantiate this inference, we turn to a study using visible spectroscopy of methyl orange (MO), a polar molecule.21 The absorption maxima of this probe molecule have been reported to show blue shifts in surfactant solutions, indicative of an increasingly hydrophobic environment, as surfactant concentration is increased between micellar solution and the mesophase.'l Especially intriguing is the finding that the blue shifts observed are greater in KL and SDecS mesophases than in MTAB.'l DPyP and MO are structurally quite dissimilar, but it is noteworthy that the change in MJM, between micellar solution and mesophase is also greater in the anionic surfactants. Evidently, increased surfactant concentration reduces contact with water for the micelle as a whole; the apparent difference between cationic and anionic amphiphiles merits further study. If the relative intensities of vibrational structure in DPyP monomer emission can be used as a polarity probe, the higher value of M,/M, in dilute KL (vis-a-vis SDecS) solution strengthens the argument for a polar DPyP solubilization site. (21) Ramesh, V.; Chien, Hai-Shan; Labes, M. M. J.Phys. Chem. 1987, 91, 5937.
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.
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