Langmuir 1990, 6, 1750-1757
1750
Polarized Fluorescence Emission Measurements in Mixtures of 2-Butoxyethanol, Cetyltrimethylammonium Bromide, and Water D. J. Jobe and R. E. Verrall* Department of Chemistry, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 0 WO Received October 27, 1989. I n Final Form: May 9, 1990 Polarized fluorescence emission and fluorescence lifetime measurements are reported for two fluorescent potassium salt (2,6probes, diphenylbutadiene (DPB) and 2-(p-toluidino)naphthalene-6-sulfonate, TNS), dissolved in binary water-surfactant systems for several different surfactants. As well, data for the probe DPB in ternary solutions of 2-butoxyethanol (BE)-cetyltrimethylammonium bromide (CTAB)water are presented. The use of the probe 2,6-TNS to investigate alkyltrimethylammoniumbromide surfactants was found to be impractical because of the occurrence of a strong complexation reaction with the surfactant. Estimates of the local microviscosity ( 9 ) around the probe were calculated, and, because of the solvent-dependent emission properties of DPB, it was possible to infer the relative polarity and solubilization site of the probe. For ternary solutions containing 5 mol 50 BE, the addition of CTAB has no effect on the microviscosity around DPB. This result is interpreted as arising from the breakdown of the CTAB micelles which occurs from the swelling of the micelle upon solubilization of BE. Water-rich regions of these systems also were studied by using the fluorescent probe dimethylaminonaphthalene sulfonate (1,8-DNS),sodium salt. The emission wavelength maximum for 1,8-DNSis sensitive to the polarity of the surrounding solvent, and its negative charge makes it a suitable probe to investigate the surface of cationic micelles. Studies of these systems show that initial solubilization of BE occurs at the micelle surface with concomitant swelling of the micelle. Further addition of BE leads to micelle breakdown. These results are in agreement with previously reported ultrasonic absorption and light-scattering studies of these systems. Introduction Although there are a number of reported studies dealing with surfactant-alcohol-water mixtures,l-12few have dealt with the composition region where the alcohol (cosurfactant) is in large e x c e ~ s . ~ - 3The ~ 6 ~latter ~ situation is often the case in the formation of oil field microemulsions where the concentration of the cosurfactant can be as much as 10 times greater than that of surfactant, thus making the cosurfactant the major component in these systems.13 Previous studies at compositions where the alcohol is in large excess have focused mainly on changes in the micelle due to the presence of the cosurfactant. However, at sufficiently high compositions, it has been proposed3J4-16 that some alcohols will self-aggregate in aqueous solution as well as form clusters with water. If such alcohols were used in the formation of microemulsions, then it would (1) Kato, S.; Jobe, D. J.; Rao, N. P.; Ho, C. H.; Verrall, R. E. J . Phys. Chem. 1986,90,4167. (2) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986,90, 5193. (3) Quirion, F.; Desnoyers,J. E. J.ColloidInterface Sci. 1987,125,196. (4) Lang, J.; Zana, R. J. Phys. Chem. 1986,90,5258. (5) Ben-Shaul, A.; Rorman, D. H.; Hartland, G. V.; Gelbart, W. M. J . Phys. Chem. 1986,90,5277. (6) Leung, R.; Shah, D. 0. J . Colloid Interface Sci. 1986, 113, 484. (7) Rao, 1. V.; Ruckenstein, E. J. Colloid Interface Sci. 1986,123,374. (8)Yamashita, F.; Perron, G.; Desnoyers, J. E.; Kwak, J. C. T. J . Colloid Interface Sci. 1986, 113, 548. (9) Turro, N. J.; Tanimoto, Y. Photochem. Photobiol. 1981, 34, 157. (10) Lianos, P.; Zana, R. J. Colloid Interface Sci. 1984, 101, 587. (11) Chen, V.; Warr, G. G.; Evans, D. F.; Prendergast, F. G. J . Phys. Chem. 1988, 92, 768. (12) a n a , R.; Yiv, S.; Strazielle, C.; Lianos, P. J . Colloid Interface Sci. 1981, 80, 208.
(13)Desnoyers, J. E.; Quirion, F.; Hetu, D.; Perron, G. Can.J . Chem. Eng. 1984, 61, 672. (14) Ito, N.; Saito, K.; Fujiyama, T. Bull. Chem. SOC.Jpn. 1981, 54, 991. (15) Ito, N.; Fujiyama, T.; Udagawa, Y. Bull. Chem. SOC.Jpn. 1983, 56, 379. (16) Kato, T. J . Phys. Chem. 1985,89, 5750.
0743-7463/90/2406-1750$02.50/0
be important to understand the impact of the surfactant on the coexisting aggregates of alcohol and alcohol/ water. Previously, we have presented the results of a study of the dynamical properties of 2-butoxyethanol (BE)water systems in both the presence and absence of the surfactant, cetyltrimethylammonium bromide (CTAB).' In t h a t study, aggregates consisting of BE-HzO and BE-CTAB-H20 were postulated to occur, and their tendency to form was found to be strongly dependent on the composition of the system. For the binary BE-HzO system, at low BE composition (Le., XBE< 0.02), it was proposed that large clusters or "clathrate-like" aggregates of alcohol in water are formed, in agreement with other reports.14-16 However, as the composition of BE is increased, these microstructures begin to merge t o accommodate the increasing alcohol concentration, and a t sufficiently high BE concentrations, the merged structures and the aggregates of alcohol become important features of t h e system. For t e r n a r y systems of BE-H20-CTAB, the results of the ultrasonic relaxation and photon correlation spectroscopy studies showed that addition of CTAB to BE-H20 solutions results in modified microstructures similar to those present in the binary system. Specifically, a t low compositions of BE, the addition of CTAB leads to the breakdown of the clathratelike structures of BE-HzO resulting in the formation of mixed micelles of BE and CTAB. However, for thoee compositions where BE is in large excess, the mixed micelles dissociate and form small, mixed clusters, perhaps similar to the merged clathrates, but also containing CTAB. This has been suggested to occur for other surfactantcosurfactant-water mixture^.^ BE aggregates are also present, as indicated by the presence of a second ultrasonic relaxation. Ternary systems of CTAB-HzO-alcohol and/or oil have @ 1990 American
Chemical Society
Langmuir, Vol. 6, No. 12, 1990 1751
Polarized Fluorescence Emission Measurements been studied by other techniques, most notably fluorescent probing.17J8 In particular, fluorescent depolarization has been extensively used to study micelles (normal and reversed), microemulsions,and membrane systems. These studies have provided important information about the effect of additives on micellar dynamics,*+21the detection of structural/phase changes in micelle^,^^^^^ and memb r a n e and ~ ~ have ~ ~ been ~ ~ used to calculate the microvisThe cosity (7) of the local environment of the latter parameter would seem to be an effective means of confirming and extending the interpretation proposed for the previous ultrasonic study of the CTAB-BE-HzO system. While polarized emission measurements can provide important information on the nature of the environment around the probe, it is also important to know the location of the probe in the micelle in order to correctly interpret the data.30 Lack of consideration of this factor has been attributed as the reason for the variation in the reported values of 7 for identical micellar systems using different probes.33 T h e probes anthracene and 9-methylanthracene have been used extensively to estimate microviscosities of micellar systems, and values of 15-35 cP19-21i26 have been reported. Other probes have given similar values, although larger values have been reported. For example, the method of using the lateral diffusion coefficient of pyrene excimer formation to calculate the microviscosity has led to values of 9 in the range 125-195 cP.~ However, ~ the calibration of the pyrene intensity using hydrocarbon solvents of known viscosity may be unreliable because the microenvironment of the hydrocarbon solvent is not necessarily the same as the interior of the micelle.12 Through the use of steady-state fluorescence polarization measurements ( P ) ,the microviscosity (7) around a probe molecule can be calculated by means of the Perrin equation:
(l/P- 1/3) = (l/Po-1/3)(1+ kTr/vV)
(1)
where POis the limiting polarization for the probe, 12 is the Boltzmann constant, T is the absolute temperature, and V and 7 are the molecular volume and fluorescence lifetime of the probe, respectively. Application of this technique to mixed micelles and microemulsion systems has been shown to be useful, as any changes in microstructure are reflected by changes in the values of P and 9. Turro et (17) Atik, S.S.;Thomas, J. K. J. Am. Chem. SOC.1981, 103, 4367. (18) Atik, S.S.;Thomas, J. K. J. Am. Chem. SOC.1982,104, 5868. (19) Shinitzky, M.; Dianoux, A. C.; Gitler, C.; Weber, G. Biochemistry 1971,10,2106. (20) Turro, N. J.; Tanimoto, Y. Photochem. Photobiol. 1981,34,157. (21) Grtitzel, M.; Thomas, J. K. J . Am. Chem. SOC.1975,95, 6885. (22) Galla, H.-J.;Sackmann, E. Biochim. Biophys. Acta 1974,339,103. (23) Kunitake, T.; Kimizuka, N.; Higashi, N.; Nakashima, N. J.Am. Chem. SOC.1984, 106,1978. (24) Nagmura, To;Mihara, S.;Okahata, Y.; Kumitake, T.; Matauo, T. Ber. Bunsen-Ces. Phys. Chem. 1978,82,1093. (25) Turley, W. D.; Offen, H. W. J.Phys. Chem. 1986,90, 1967. (26) Shinitzky, M. Isr. J . Chem. 1974, 12, 879. (27) Pownall, H. J.; Smith, L. C. J . Am. Chem. SOC.1975, 95, 3136. (28) Aoudia, M.; Rodgers, M. A. J.; Wade, W. H. J . Colloid Interface Sci. 1984, 101, 472. (29) Reed, W.; Politi, M.; Fendler, J. H. J. Am. Chem. SOC.1981,103, 4591. (30) Blatt, E.; Ghiggino, K. P.; Sawyer, W. H. J . Chem. Soc., Faraday Trans. 1 1981, 77, 2551. (31) Blatt, E.; Sawyer, W. H.; Ghiggino, K. P. Aust. J . Chem. 1983, 36. - -, 1079. - - .-. (32) Blatt, E.; Ghiggino, K. P.; Sawyer, W. H. J . Phys. Chem. 1982, 86, 4461. (33) Blatt, E.; Ghiggino, K. P. Chem. Phys. Lett. 1985, 114, 47. (34) Zinsli, P. E.; Blankert, T. H. Tag. Schwerz. Phys. Cesellschaft 1977,28,1162. (35) Jobe, D.J.; Verrall, R. E.; Palepeu, R. M.; Reinsborough, V. C. J. Phys. Chem. 1988,92, 3582.
al.9 have used several fluorescent probes to examine how the addition of alcohol to CTAB micelles affects the micellar microviscosity. However, the concentrations of alcohol used were dilute, so that mixed micelles predominated. Chen et a1.l1 have used polarized fluorescence measurements to study microemulsions of surfactant, oil, and water and have shown that the polarization is very sensitive to curvature a t the interfacial region. More recently, using the probe 2-(p-toluidino)naphthalene-6sulfonate, potassium salt (2,6-TNS),we have used polarized fluorescence measurements to examine the mechanism of inclusion of 2,6-TNS molecules in the cavities of cyclodextrin molecules and to calculate the microviscosity in the cavity.35 It has been previously shownmo that, like 2,6-TNS, the fluorescence lifetimes and quantum yields of diphenylbutadiene (DPB) are very dependent upon the solvent polarity. Generally, lifetimes between 450 and 800 ps have been found for DPB in nonpolar solvents36 whereas lifetimes of 60-420 ps have been found for DPB in polar solvents (e.g., alcohols).37 Therefore, based on the magnitude of the lifetime, the relative polarity of the environment of the probe can be obtained, and assignment of the solubilization site can be made. The results of our ultrasonic study indicated that the interior of the micelle can be penetrated by BE and that sufficiently high alcohol concentrations cause the breakdown of the micelle. Such a result should cause a change in the polarity around the solubilization site of DPB and reinforce our previous interpretation. Such changes have been used in the past to explore structural features of micelles, membranes, and other microemulsion system^.^^^^^^^^ The mole ratio of water to BE was found to be a crucial factor in the formation of any microstructures.' DPB is hydrophobic in nature and would only be useful as a probe of hydrophobic structures. A more hydrophilic probe would be better suited to provide information about the water-rich regions. Members of the aminonaphthalenesulfonate family of compounds, of which 2,6-TNS is a member, have shown large changes in their quantum yields, wavelength of emission maxima, and fluorescence lifetimes when the solvent polarity is ~ h a n g e d . 4 ~One 1 ~ ~member of this family, l-(dimethylamino)-8-naphthalenesulfonate, sodium salt (l,&DNS),has been used successfully to detect the critical micelle concentration (cmc) of surfactants.28 Since l,8-DNS is negatively charged, it is well suited for use as a hydrophilic probe of cationic micelles. In this paper, we report the results of fluorescence emission studies of DPB, 1,8-DNS, and 2,6-TNS fluorescence probes dissolved in aqueous micelle solutions and BE-H20-CTAB systems. For the probes DPB and 2,6TNS, the results are used to estimate the microviscosity around the probe. The conclusions reached from these studies reaffirm the conclusions drawn from our previous study' concerning the nature of the aggregates formed. More importantly, they appear to provide a better (36) Velsko, S. P.; Fleming, G. R. J. Chem. Phys. 1982, 76, 3553. (37) Keery, K. M.; Fleming, G. R. Chem. Phys. Lett. 1982, 93,322. (38) Waldeck, D. H.; Lotahaw, W. T.;MacDonald, D.B.; Fleming, G. R. Chem. Phys. Lett. 1982,88, 297. (39) Birch, J. S.;Imhof, R. E. Chem. Phys. Lett. 1982,88,243. (40) Fleming, G. R.; Velsko, S.P.; Waldeck, D. H. Springer Ser. Chem. Phys. 1982,23, 239. (41) Allen, M. T.; Miola, L.; Whitten, D. G. J. Am. Chem. SOC.1988, 110, 3198. (42) Suddaby, B. R.; Brown, P. E.; Russell, J. C.; Whitten, D. G. J. Am. Chem. Soc. 1986, 107, 5609. (43) Robinson, G. W.; Robbins, R. J.; Fleming, G. R.; Morris, J. M.; Knight, A. E.; Morrieon, R.J. S.J . Am. Chem. SOC.1978,100,7145. (44) Edelman, G. M.; McClure, W. 0. Acc. Chem. Res. 1968,1,65.
1752 Langmuir, Vol. 6, No. 12, 1990
Jobe and Verrall
understanding of how the aggregation processes depend on the relative molar amounts of the components. Experimental Section Materials. The surfactants cetyltrimethylammonium bromide (CTAB), tetradecyltrimethylammonium bromide ("AB), cetylpyridinium bromide (CPYB), and sodium dodecyl sulfate (SDS) were obtained from Sigma (=99%). CTAB, TTAB, and SDS were twice recrystallized from ethanol while CPYB was recrystallized from hot water. l,8-DNS, 2,6-TNS (Sigma), and DPB (Aldrich) were recrystallized from ethanol. The concentration of the probes in all solutions was 2.0 X 10-6M. This ensured that the mole ratio of probe to micelle never exceeded 0.9 and in most cases was C0.3. Therefore, probe-probe interactions could be neglected. B E (Aldrich = 99%) was distilled under reduced pressure (22 mm of Hg, only the middle fraction being used) and was further purified by passing it over a silver nitrate impregnated silica gel column (Fisher, mesh 80-200) in order to remove any remaining fluorescent impurities. All solutions were prepared by using millipore Super-Q water, and the absorption spectra of the surfactant solutions in the absence of DPB or l&DNS were checked to verify that no impurities were present that may absorb at the excitation wavelength. Some measurements involving DPB showed a n intensity dependence with time after prolonged exposure to the laser emission. For these solutions, a 1 mm X 1 mm fluorescence flow cell with a flow rate greater than 0.1 mL min-' was used,& otherwise a 1-cm path length fluorescence cell was used. Mole percent B E in this study refers to percent B E in the binary BE-HzO solvent system. Methods. Lifetime measurements were made by using the single-photon counting technique. The excitation source was a synchronously pumped, cavity-dumped, frequency-doubled rhodamine 6G picosecond dye laser exciting at a wavelength of 295 nm.46 Polarization bias on the lifetime measurements was eliminated by using an emission polarizer oriented at 54.F to the exciting light. Steady-state fluorescence polarization (P) measurements were made by using a microprocessor-controlled Spexfluorolog spectrofluorimeter. For all measurements involving DPB and 2,6-TNS, both time resolved and steady state, the excitation wavelength was 295 nm while the emission was collected a t 420 and 500 nm, respectively. For the studies using DNS, the excitation wavelength was 320 nm. The absorption spectra were obtained on a Cary 120C spectrophotometer. The fluorescence decay curves were analyzed by using the pulse shape mimic techniquean and were fitted to the expression n
I(t)= ZAie-'lri i=l
where I ( t ) is the intensity of fluorescence at time t , Ai is the preexponential factor for the fraction of intensity from species i with a fluorescence lifetime ~ iand , n is the number of fluorescing species present. Data analysis was carried out by using a nonlinear least-squares fitting program based on the Marquardt algorithm. The analysis program provided values of the residuals, autocorrelation, and the reduced x 2 as well as the calculated values of Ai and 7;. The steady-state polarization emissions were collected by using the L-format and were calculated from the polarization ratios (Pr) by means of the relations
P = (Pr - l ) / ( P r + 1)
(3)
where I,, is the intensity for the emission when the excitation ( x ) and emission Cy) polarizers are in either the vertical (v) or horizontal (h) positions. These intensity values were averaged over 60 s. The values of Po = 0.442 and PO= 0.331 for DPB and (45) Demmer, D. R.; James, D. J.; Steer, R. P,;Verrall, R. E. Photo-
chem. Photobiol. 1987,45, 39.
(46) James, D. J.; Demmer, D. R.; Verrall, R. E.; Steer, R. P. Rev. Sci. Instrum. 1983,54, 1121.
2,6-TNS, respectively, were determined experimentally by obtaining the polarization values for these probes in propylene glycol a t -20 "C. The value of the molar volume, V, used for DPB was 205 A3 and was calculated by using a symmetric prolate ellipsoid model with an axial ratio of 3.2:l. This is the same as the value used by Fleming and co-workers= in their polarization studies with DPB. A value of 450 A3 was used for 2,6T N S and was calculated by using eq 1 (P= 0.214, T = 60 p ~ , 3 ~ and 7 = 0.89 cP).
Results Aqueous Surfactant Solutions. Values of the fitted parameters to eq 2, P and v, obtained for DPB and 2,6TNS in the various surfactant solutions, are shown in Table I. The values are in the range of ca. 15-25 cP, in agreement with values obtained previously by using other probes.ls2l* The most notable feature of Table I is the requirement of a multiexponential function to fit the data for some of the solutions, Le., for DPB in CTAB micelles at concentrations of CTAB < 0.10 M. Multiple-exponential behavior for other probes dissolved in micellar systems has been reported p r e v i o ~ s l y .Anthroyloxy ~~~~~ fatty acid derivatives yield double-exponentialdecays in micellar SDS systems30 as well as in phosphatidylcholine vesicles (PHQ31 However, these probes did not show doubleexponential behavior in CTAB systems.32 In the systems reported here, where multiple exponential decays are observed, an average microviscosity value was calculated by using an average value of the lifetime (( T)) given by n
( 7 )= CfiTi i=l
(5)
where n
fi =
A ~ ~ ~ / C A(6)~ ~ ~ i=l
and f i is the weighted fractional intensity of light from each emitting species. BE-HzOXTAB Solutions. Table I1 shows the values for Ai, 7i, P, and rl (calculated according to eq 1) for the BE-H20-CTAB solutions. The lifetime value for DPB in water could not be obtained experimentally due to its low solubility and low quantum yield as well as the lower limit that fluorescence lifetimes could be determined with confidence (ca. 95-100 ps). However, based on the values of T for DPB in other polar solvents, it is expected that 7 < 60 ps. Single exponentials were found for most of the solutions studied except for those solutions having concentrations > 0.075 M CTAB and containing 0.9 mol % BE, where two exponentialswere required to adequately fit the data. Values of the viscosity of BE-H20 mixtures obtained by using an Ostwald viscometer also are shown in Table 11. A comparison of these values with those obtained by using DPB and the fluorescence polarization technique shows good agreement. Figure 1shows a plot of the fluorescence lifetime versus the concentration of CTAB at various compositionsof BE. There are three notable features about this graph (1)Two fluorescent lifetimes occur for DPB a t high CTAB concentrations when the mole percentage of BE is >0.9%. (2) For a fixed concentration of CTAB, the value of T generally decreases with increasing composition of BE. (3) A t high mole percentage of BE, there is very little change in the fluorescence lifetime with increasing CTAB concentration, such that the value of T in the binary BE-HzO solutions remains unchanged upon addition of CTAB.
Langmuir, Vol. 6, No. 12, 1990 1753
Polarized Fluorescence Emission Measurements Table I. Values of A h
i j
(Eq 2). P,and
r)
(Eq 1) for CTAB, TTAB, SDS,and CPYB*
DPB surfactant CTAB
SDS
TTAB
CPYB
concn, M 0.010 0.025 0.050 0.075 0.100 0.200 0.300 0.010 0.025 0.050 0.075 0.100 0.010 0.025 0.050 0.075 0.100 0.010
71, PS
446 313 238 204
72, PS 680 642 592 539 500 525 502
Ai
A2
X2
P
0.620 0.648 0.618 0.574
0.380 0.352 0.382 0.426
1.80 1.92 1.71 1.87 1.34 1.57 1.33 1.64 1.50 1.49 1.58 1.64 1.73 1.41 1.68 1.70 1.53 1.33
0.297 0.304 0.313 0.315 0.317 0.317 0.319 0.324 0.314 0.297 0.294 0.292 0.312 0.323 0.333 0.334 0.342 0.344
P 0.121 0.025 0.207
298 362 387 405 410 341 373 385 387 392 468
cp 19.9 18.6 18.0 19.1 22.1 23.1 22.7 14.5 15.6 14.0 14.1 14.0 14.3 17.8 20.7 21.0 23.4 28.2
tlt
2,6-TNS surfactant CTAB TTAB a
concn, M 0.010 0.0005 0.10
ns 3.55 3.19 3.55
T1
ns 6.24 7.81 6.08
Ai
A2
X2
0.528 0.332 0.760
0.472 0.668 0.240
1.18 1.79 1.26
72,
tl, c p
27.1 4.7 18.9
Obtained by using DPB and 2,6-TNS as fluorescent probes at 25 "C. Table 11. Fluorescence Emission Properties for DPB in Mixed Micelles of BECTAB-H20
9% BE 0.9
2
5
20
30
[CTAB], mol dm-3 0.010 0.025 0.050 0.075 0.100 0.000 0.010 0.025 0.050 0.075 0.100 0.000 0.010 0.025 0.050 0.075 0.100 0.000 0.010 0.025 0.050 0.075 0.100 0.000 0.010 0.025 0.050 0.075 0.100
PS 251 296 328 256 233 95b 134 210 227 289 307 l00b 141 168 179 180 168 141 132 134 133 157 150 159 142 139 138 144 141
71,
72,
PS
Ai
A2
X2
1.49 1.56 1.81
501 506
0.768 0.709
0.232 0.291
1.74 1.83 3.06 1.89 1.33 1.42 1.79 1.43 2.03 1.45 1.75 1.69 1.71 1.73 1.91 1.65 1.80 1.37 1.05 1.60 1.80 1.35 1.39 1.92 2.01 1.82
P 0.274 0.262 0.248 0.257 0.271 0.203 0.226 0.326 0.323 0.319 0.319 0.297 0.270 0.262 0.253 0.236 0.246 0.289 0.297 0.300 0.299 0.290 0.297 0.292 0.275 0.286 0.291 0.300 0.313
tl,o
7.15 7.52 7.74 8.38 9.95 1.41 2.46 10.5 10.9 13.2 13.9 2.29 3.86 4.29 4.18 3.60 3.68 4.66 4.72 4.98 4.87 5.24 5.36 5.14 4.13 4.49 4.70 5.37 6.04
cp
(1.27)
(2.43)
(4.52)
(4.45)
I, Values given in parentheses are the viscosity values found for the binary BE-HzO systems by using an Ostwald viscometer. b These values are expected to have a higher uncertainty than the others as they are very close to the lower limit T can be measured with confidence. Since the lifetime of DPB is sensitive to the local environment, these trends in T give an indication of the environmental changes that are occurring in these solutions with increasing BE and CTAB composition. The values for T in the aqueous micellar CTAB solutions are relatively longer than the value expected for DPB in H20, indicating the solubilization of DPB in a nonpolar region of the micelle. The steady decrease in T with increasing BE composition indicates that DPB is experiencing an increasingly polar environment due to the penetration of
BE into the micelle. At high compositions of BE, the invariance of T with increasing CTAB concentration may indicate the complete breakdown of the micelles, as the value of T is similar to the value found in the binary BE-H20 system. This suggests that the surfactant containing aggregates may be similar in nature to the aggregates present in the binary BE-H20 system. Figure 2 shows the dependence of 7 with increasing CTAB concentration at various mole percentages of BE. For solutions containing 0.0,0.9, and 2 mol 5% BE, there
Jobe and Verrall
1754 Langmuir, Vol. 6, No. 12, 1990 51 0
500 n
E
C
W
E 6
c
200
490
480
470 I
0.00
CTAB
i
0.02
"
0.04
CTAB
(mol dm-')
Figure 1. Plot of the fluorescence lifetimes of DPB versus the concentration of CTAB at fixed mole percentages of BE (0) 0.0% (71); (m) 0.0% ( 7 2 ) ; (0) 0.9% (71); ( 0 )0.9% ( T 2 ) ; (+) 2 % ; (A)5 % ;
-
0.06
r
0.08
0.10
q
0.12
(mol dm-3)
Figure 3. Plot of the fluorescence emission maximum of 1,BDNS versus the concentration of CTAE! at fixed mole percentages of BE: (0) 0.0%; (0)0.9%; (+) 2 % ; (A)10%; ( 0 )20%.
( 8 )2OC< ; ( X I 3Or(.
5
0
.
CTAB
72
(mol dm-3)
Figure 2. Plot of the calculated microviscosity versus the concentration of CTAB a t fixed mole percentages of BE: (0) O.O''r; (0) 0.9''r; (+) 2'0; (A)5 " ; ; ( 8 )20%;( X ) 30%. is an initial increase in 7 with small additions of CTAB.
For those solutions where the fraction of BE is much higher, there is very little change in 7 with increasing CTAB composition. Figure 3 shows plots of A,, of 1,8-DNS vs the concentration of CTAB a t various compositions of BE. It is clear from the small changes in ,A, that micelle formation (indicated by a sharp decrease in ,A, with the addition of CTAB) is more gradual in ternary systems having >0.9 mol p ( BE than for those solutions containing less BE. This transition in micelle character occurs at a lower composition of BE than was reflected in Figure 2. Plots of 7 vs CTAB concentration show a loss of micelle character (indicated by a decrease in 7 with the addition of CTAB) a t compositions of BE > 5 mol One reason for this apparent disagreement may be due to the potentially different sites occupied by l,&DNS and DPB in these systems. Discussion Aqueous Surfactant Solutions. The values for 7 (using DPB) given in Table I are in agreement with values
obtained by using other fluorescent The most notable feature of Table I is the lifetime data for DPB a t concentrations KO.100 M CTAB. Although the values of 71 and 7 2 are only separated, on average, by a factor of ca. 2, as observed in a previous s t ~ d y , ~the ~ residual ?~l and x 2 values showed very poor fits when a single-exponential function was used t o fit these data. James and coworkers&have measured decays using the mimic technique for a mixture of noninteracting species with lifetimes separated by a factor of 4 and have shown that it is possible to analyze the data to give results within 2 5% for the values of 7 and the fractional composition of the pure components. Given the lower factor of separation and the time scale of the measurements reported here, higher uncertainties would have to be assigned to these values. This undoubtedly is the reason for the larger values of x2 obtained for these systems. The origin of the multiple-exponential behavior has been e ~ p l a i n e dpreviously ~ ~ ~ ~ l as being due to the different solubilization sites of the probe. If the absolute values of the lifetime are used to rationalize the solubilization site, then the shorter lifetime species can be assigned to a more polar environment, probably closer to the surface of the micelle, whereas the longer lifetime would represent a site deeper within the micelle. The values found here for A1 and A2 (for both 2,6-TNS and DPB) are similar in magnitude to those reported for anthroyloxy fatty acid derivatives in SDS micelles and PHC ve~icles.3~~3~ If one assumes that quantum yields are proportional to the lifetimes, then the ratio Az/A1 can be taken as the ratio of the percentages of DPB occupying the two sites. Assuming that A1 represents the surface site, then it is estimated for CTAB that approximately 60% of the DPB molecules are nearer the surface while the remainder may be deeper in the micelle. From surface tension measurements of benzene in heptane, Mukerjee and Cardinal4' have calculated that 885% of solubilized benzene molecules in a SDS micelle would be surface "adsorbed". The authors indicated that this fraction is probably too high, so the magnitude reported here for DPB is not inconsistent with their estimate. A t CTAB concentrations >0.075 M, the multiexponential behavior is lost and a single lifetime of ca. 500 ps is (47) Mukerjee, P.; Cardinal, J. R.J. Phys. Chem. 1978,82, 1620.
Langmuir, Vol. 6, No. 12, 1990 1755
Polarized Fluorescence Emission Measurements
obtained. Also, beyond this concentration the microviscosity appears to increase; however, the change in 9 is within the probable uncertainty in 77, i.e., f3 cP. These changes, of real, may possibly be due to structural changes in the CTAB micelle, i.e., sphere to some form of ellipsoid or rod shape.*% It has been suggested that this transition may occur between 0.2 and 0.3 M CTAB. Recent studies,48.49 however, have shown that prior to this change there is an increase in the binding of Br- to the surface of the micelle. This would account for the steady decrease in 71 and' 7 2 with increasing CTAB concentration, since the increase in the bound Br- would quench the fluorescence lifetime. Furthermore, a change from a sphere to rod-shaped micelle would decrease the micelle curvature, and the resulting micelle could better accommodatesurface adsorption of DPB and lead to the observed loss of microheterogeneity. Examination of the relative magnitudes of the lifetimes also indicates a difference exists in the ability of the bromide ion to quench probes residing in the different sites. The value for 7 1 decreases by 54% upon changing the CTAB concentration from 0.010 to 0.075 M whereas 7 2 (the species in a more nonpolar environment) only decreases by 20%)in the same concentration range (cf. Figure 1). Since the surface-adsorbed DPB molecules would be more susceptible to quenching by the bromide ions, they would be expected to be quenched to a greater degree. An attempt was made to calculate the individual microviscosities of each of the individual sites occupied by the different DPB species in CTAB solutions < 0.100 M. This was done by assuming that the polarization values in these solutions are due to the sum of the weighted fractions of the individual polarizations of each of the species (Pi):
P = a l P 1+ ffzPoz
(7) where ( ~ and 1 a2 are the mole fractions of DPB in sites 1 and 2, respectively. Furthermore, if it is assumed that the quantum yields are proportional to the lifetimes, then ai = Ai and hence (8) PIA, = Po, + Az/AIPoz Therefore, a plot of PIA1 versus A2IA1 should be linear with a slope P o 2 and intercept Pol. Given that there are only four data points, a linear least-squares fit was used, and the calculated microviscosities that result from the slope and intercept are v01 = 9.6 f 2.4 CPand 77O2 = 24 f 8 cP. These values can be assigned to DPB at site 1,near the surface of the micelle, and site 2, deeper within the micelle, respectively. However, this analysis is based on a number of assumptions and should be regarded as only offering qualitative support for a two-site solubilization of DPB in CTAB micelles below 0.100 M. A comparison of the lifetime results obtained for DPB in SDS and TTAB micelles with those obtained in micellar CTAB solutions shows some interesting differences. The value of 7 increases with increasing surfactant concentration in SDS and TTAB systems, an unexpected result since counterion binding is expected to increase with increasing surfactant concentration. The increase in 7 for DPB in these surfactants, as opposed to the decrease in (48) Quirion, F.; Magid, L. J. J. Phys. Chem. 1986, 90,5435. (49) Quirion, F.; Desnoyers, J. J. Colloid Interface Sci. 1986,112,565. (Sa) Backlund,S.; Heiland, H.; Kvammen, 0.; Ijosland, E. Acta Chem. Scand. 1982, A36,698. (51) Reiss-Husson, F.; Luzzati, V. J. J. Phys. Chem. 1964, 68, 3504. (52) Rodgers, M. A. J., Da Silva e Wheeler, M. E. Chem. Phys. Lett. 1978, 53, 165. (53) Malliaris, A,; Lang, J.; Zana, R. J. Phys. Chem. 1986, 90,655.
and T Z for DPB in CTAB, may result from a deeper penetration of DPB into the SDS and TTAB micelles, rather than the redistribution of the probe between the surface/core regions, as appears to be the case for CTAB micelles. The surface solubilization of DPB is further supported by the results obtained with the CPYB-DPB system. For a given probe concentration, the fluorescence intensity of DPB is quenched more efficiently in 0.010 M CPYB than in 0.010 M CTAB. For example, the fluorescence intensity of DPB in CPYB is ca. 80% of that found in CTAB. Other studies have also shown t h a t CPYB is an efficient fluorescenceq u e n ~ h e r . Clearly, ~ ~ , ~ ~ the pyridinium group must be the cause of this effect, and, given that it is located in the headgroup region of the micelle, the DPB molecules must be located in or near the surface region of the micelle for such a high degree of quenching to occur. The use of 2,6-TNS as a probe of micellar systems led to unexpected difficulties. Since 2,6-TNS is negatively charged, studies in anionic SDS could not be made, as evidenced from the fact that time-resolved emission measurements for 2,6-TNS in SDS solutions were similar to those for 2,6-TNS in HzO. As well, the relative emission intensity was considerably lower than that obtained for 2,6-TNS in cationic micellar systems, also indicating that 2,6-TNS was not strongly included in the SDS micelle. For cationic CTAB and TTAB micellar systems, the fluorescence emission from 2,6-TNS is greatly enhanced over that in either water or SDS solutions. This phenomenon also has been observed for the 1,8 isomer of 2,6-TNS with CTAB.54 The association is quite strong since the enhanced 2,6-TNS fluorescence and multiexponential decay are observed even at concentrations as low as 5 X 10-4M TTAB, i.e.,
