Chapter 20
Dynamic Fluorescence Quenching in Reverse Microemulsions in Propane Thomas S. Zemanian, John L. Fulton, and Richard D. Smith Chemical Methods and Separations Group, Chemical Sciences Department, Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352
Measurements have been made of micelle size and core exchange rates of reverse (water-in-oil) micellar systems of sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) or didodecyldimethyl ammonium bromide (DDAB) and water in propane at various pressures and temperatures. Ruthenium tris(bipyridyl) dichloride, soluble in the aqueous phase, was used as a fluorescent probe to determine surfactant aggregation numbers, and hence water core radii. Potassium ferricyanide was used as the water-soluble quencher in these experiments. Pressure and temperature effects are discussed as they pertain to bulk oil phase and surfactant properties.
Microemulsions are thermodynamically stable, optically clear fluid systems consisting of a nonpolar ("oil") phase and a polar, generally aqueous, phase separated by a monolayer of an amphiphilic compound (surfactant). These systems are of broad practical significance (from cosmetics to enhanced oil recovery), and are of scientific interest due to their structure and dynamic nature. Such systems can order themselves in various fashions, depending on the system temperature, pressure, and the molar ratios between the polar, nonpolar, and amphiphilic components. Three general structures may be identified. A stable microemulsion may contain (i) a continuous aqueous phase with interspersed microdroplets of oil surrounded by surfactant (micellar), (ii) a continuous oil phase containing microdroplets of water surrounded by surfactant (reverse micellar, or water-in-oil (w/o)), or (iii) a bicontinuous structure, in which both phases are continuous and intertwined. For this paper we shall focus on reverse micellar systems of water, iso-octane, liquid and supercritical propane, and didodecyl dimethyl ammonium bromide (DDAB) or sodium bis-(2-ethylhexyl) sulfosuccinate (AOT) as the surfactant. The use of compressed propane as the oil phase allows tuning of the system characteristics with pressure. Tingey et al. (I) have shown, from measurements of the system electrical conductivity, a transition from a bicontinuous system to a reverse micellar system over a narrow range of pressure for the DDAB/propane/water system.
0097-6156/93/0514-0258$06.00/0 © 1993 American Chemical Society
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Although microemulsions are thermodynamically stable, they are by no means static. The micelles and bicontinuous structures are constantly changing and reforming. Infelta (2) developed a method to measure the micelle size using a fluorescent probe molecule and a quencher at sufficiendy low concentration to ensure no more than one probe molecule per micelle. This technique may also be used in a time-resolved mode by monitoring the fluorescence decay after excitation by a sharp pulse of light. Such measurements allow determination of the rate at which the micelles coalesce and separate, and in particular the rate of mixing of the contents of their cores. (3,4) Atik and Thomas (3) give the canonical equation for the decay of fluorescence from such systems as ln(I/Io) = -A2t - A3O - exp(-A4t)) where I is the fluorescence intensity at time t after the laser pulse, Io is the fluorescence at time t=0, and A2, A3, and A4 are constants derived from kinetic arguments (5). If the assumption can be made that the rate of quenching is much greater than the rate of micelle core exchange, and that the quencher resides almost entirely in the micelle core, then A2 = ki + ke[Qt] A3 = [QtMM]
and
A4 = kq. In the preceding equations, ki is the inverse of the unquenched fluorescence lifetime (t), k is the bimicellar exchange rate constant, [Qt] is the molar concentration of the quencher in the microemulsion, [M] is the concentration of micelles, and kq is the rate of quenching within the micelle water core. From [M] and the known water and surfactant concentrations, one may deduce the water core radius and the surfactant aggregation number for the system. Taking the volume occupied by one molecule of water to be 30 Â (corresponding to 1.0 g/ml) and assuming the micelle core to be a sphere, e
3
R = 1.928 {[Η2θ]/[Μ]}(1/3) = 1.928 {[H2O] A3/[Q ]}( / ) 1
3
t
where [H2O] is the concentration of water in the microemulsion and R is the micelle water core radius, given in Angstroms. Use of this technique for the DDAB/propane/water reverse micellar system requires selection of a fluorescent probe that is water soluble and has a fluorescent lifetime significantly greater than the time scale on which the micelles mix. The ruthenium tris-bipyridyl dication (Ru(bipy)3^ ) is such a probe, and may be quenched using potassium ferricyanide (K3Fe(CN)6). Unfortunately, the fluorescent lifetime of Ru(bipy)3^ drops dramatically with increasing temperature.(6) This places an upper limit of approximately 100°C on the use of this probe for dynamic fluorescence quenching experiments. At higher temperatures the lifetime of the probe is insufficient to experience transfer in the excited state from one micelle to another, and hence one cannot probe the rate of micelle core exchange. Some short time data may be recovered in such situations, thereby yielding water core radius information. +
+
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Experiment The fluorescent probe, ruthenium tris-bipyridyl dichloride hexahydrate (Ru(bipy)3Cl2.6H20), was purchased from Chemical Probes, Inc., and was recrystallized from methylene chloride. A 5.35 m M aqueous solution was prepared using the purified probe and distilled, deionized water. This solution was then deoxygenated with nitrogen and sealed in a septum vial. The potassium ferricyanide (K3Fe(CN)6) was obtained from Alfa Products, Inc. and used as received. A 107 m M aqueous solution was prepared, deoxygenated, and sealed in a septum vial. The propane was obtained from Scott Specialty Gases (CP grade) and passed through an oxygen trap (Alltech, Oxy-Purge-N) prior to use. The D D A B was purchased from Eastman Kodak and was recrystallized prior to use. The A O T was purchased from Fluka and purified by the method of Kotlarchyk et al. (7) The iso-octane was from Aldrich, and was deoxygenated by bubbling with purified propane just prior to use. The apparatus used to measure the fluorescence quenching rates is shown in schematic diagram in Figure 1. An 8 ns pulse from a neodymium Y A G laser was used to excite the probe using the 532 nm wavelength. The beam was attenuated by using the reflection off the front face of a quartz wedge. This was further attenuated with a divide-by-ten neutral density filter, after which the beam was directed into the pressure cell. This cell was a stainless steel monoblock with an internal volume of 13 ml and was fitted with sapphire windows at right angles to each other. Laser light entered the cell through one of the windows and the subsequent fluorescence of the probe in the micelle water cores was measured through the other window at 90 degrees to the excitation beam. A bandpass filter (600 nm to 630 nm) was used at the exit window to pass the Ru(bipy)3 fluorescence signal (centered at 614 nm) and attenuate stray laser light (532 nm) and fluorescence from the sapphire windows (691 nm). No attempt was made to use magic angle polarization, as the sapphire windows are not isotropic to polarized light. 2+
The fluorescence signal was measured using a side-on photomultiplier tube (PMT, type 948, Hamamatsu) driven at -480 V . The P M T was designed for pulsed/fast response and was terminated with a 50 ohm load. The signal from the P M T was recorded and stored on a LeCroy 7200 storage oscilloscope with a 7242 plug-in interface. The cell and P M T were housed in a black lined box to reduce room light leakage. The temperature of the cell was controlled electrically and measured using a platinum resistance temperature device (Omega). The cell was stirred magnetically. Pressure in the cell was regulated using a hand operated syringe pump (HIP) and was measured using a pressure transducer (Precise Sensors, Inc.). The spreading of the data due to the PMT response was measured by operating with an AOT/iso-octane/water microemulsion without probe or quencher in the cell. A point-successive deconvolution routine was used to remove this instrument function spreading from the data, and the canonical equation was then fit to the resulting traces using the Marquait nonlinear least-squares technique. A smoothing filter was used during the deconvolution routine to quell high frequency noise that otherwise crept in at the trace edges during iteration. This filter was triangular symmetric and ten nanoseconds in width to prevent significant distortion of the object function. Some slight ringing was noted in some of the short time data, but the fitting routine was able to converge to an acceptable fit despite the deviations. The unquenched fluorescence lifetime (τ) of the probe is required to extract exchange rate data from measurements of A2. This lifetime is temperature dependent and hence τ was first measured in a dilute aqueous solution at temperatures ranging
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from 2 5 ° C to 135°C. These data were fitted to the equation given by Caspar and Meyer (6): l/τ = ki = k' + k°' exp (-AE'/kfiT) where: ki is the unquenchedfluorescencerate, k' and k°' are equation constants, Τ is absolute temperature, kfi is Boltzmann's constant, and ΔΕ' is an effective activation energy. As shown in Figure 2, this equation fits the data well. However, the fitted constants k , k , and ΔΕ' differ significantly from those given by Caspar and Meyer, due to differences in the temperature regimes addressed (see Table I). 1
Table I. Probe lifetime
Caspar & Meyer This work
parameters
Temperature
k'
k°'
ΔΕ'
[°q
[£}]
[dl
[cm" ]
-12 to 21 25 to 135
4.1
x
\o5
4.5
1.5
x
\Q6
1.6 χ 10
x
1
i()13 1
4
3560 4222
Moreover, the unquenched fluorescence rate of the probe will differ in the micelle core from that in aqueous solution. However, this will introduce an additive systematic error in the exchange rate data at any given temperature; the trends along each isotherm will be preserved. The fluorescence quenching method and the apparatus were first tested by filling the cell with a reverse (w/o) microemulsion of AOT/iso-octane/water at a water to surfactant ratio of W=15 and at a surfactant concentration of [AOT] = 125 mM. A small quantity (25 μΐ) of the 5.35 m M probe solution was added to bring [P] to 10.3 μΜ, and the cell contents were deoxygenated by bubbling with purified propane for 15 minutes. The fluorescence decay was then measured and found to be purely exponential decay with a lifetime of 355 ns at 25°C (see Figure 3a). That this figure is lower than that in pure water confirms that the probe behaves slightly differently in the micelle core than in aqueous solution, as discussed above. However, this results in a ki 63% larger than that given by the fitted equation. Since ki in our study was significantly smaller (15% at the lower temperatures, rising to 50 % at the highest temperatures) than the measured values of A 2 (from which ki is subtracted), the error is generally under 10% andrisesto 30 % at 100°C. This error is a constant additive offset of the ke data for each isotherm, so the trends with pressure are preserved. This error does not affect the water core radius measurements. A small amount of the 107 m M quencher solution (25 μΐ) was added to bring [Qt] to 205.8 μ Μ , and the fluorescence decay was again measured (see Figure 3b). These data, when plotted on a semilog basis showed the expected exponential decay followed by straight line decay (see Figure 3c). The cell was emptied and cleaned, and (w/o) microemulsions of AOT/propane/water at W=15, [AOT]=125 m M and of DDAB/propane/water at W=23 and [DDAB]=325 m M were examined in the system. The pressure was varied from the cloud point of the systems to approximately 500 bar. For the D D AB/propane/water system the temperature was also varied from room temperature (25°C) to 100°C.
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8 ns. pulsed Nd Y A G laser Quartz wedge
Stirred, heated right angle view cell with sapphire windows Bandpass filter Side-on photomultiplier tube Darkbox
^
Storage Oscilloscope^
Figure 1 : Schematic diagram of the apparatus used to measure quenched fluorescence lifetimes in reverse micelles
Τ 1 1 ' 1 ' I 0.0024 0.0026 0.0028 0.0030 0.0032 0.0034 0.0036
1/T [1/K] 2+
Figure 2: Ru(bipyridyl)3 fluorescence lifetime as a function of inverse temperature. Fitted equation is that given by Caspar and Meyer.(5)
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.0001
H 0
Dynamic Fluorescence Quenching in Propane
.
1
.
1
.
1
1000
500
Time
263
1500
[ns.]
Figure 3a: Unquenched probe fluorescence in the AOT/iso-octane/ water system at 25°C and 1 atm. W=15, [AOT]=125 mM.
0.03
Raw data Deconvolved data
0.02 Η
o α
S
0.01 Η
0.00 0.00e+0
1 1.00e-7
1 2.00Θ-7
1
1 3.00e-7
1 4.00Θ-7
Time [s] Figure 3b: Fluorescence decay in the AOTAso-octane/water system at 25°C and 1 atm. W=15, [AOT]=125 mM.
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> Q.
0001 H Ο.ΟΟθ+0
1
1 1.00Θ-7
•
I 2.00Θ-7
Time [s] Figure 3c: Fluorescence decay in the AOT/iso-octane/water system at 25°C (semi log plot) and 1 atm. W=15, [AOT]=125 mM.
Unfortunately, the 100°C data were of poor quality; at such temperatures the probe lifetime became sufficiently short that very little of the short time decay remained to be fitted, as is evident in Figure 4. Results and Discussion Figure 5a shows the pressure dependence of the water core radius as measured for the AOT/propane/water system (W=15, [AOT]=125 mM). For comparison, the AOT/isooctane/water system was also run at ambient conditions (25°C and 1 bar) at the same values of W and [AOT]. For the iso-octane system the measured water core radius was 2.37 nm. The micelle size found in iso-octane was similar to that found in propane, confirming other studies of these systems that solvent type has little effect on the size of the micelles. The measured size in iso-octane compares well with what has been reported by other techniques, such as quasi-elastic light scattering (QELS).(8) Figures 5a and 5b show the pressure dependence of the water core radii for the AOT/propane/water and DDAB/propane/water systems, respectively. The water core radii are independent of pressure over the pressure range studied. This independence reflects the incompressibility of water at these conditions, the insensitivity of the surfactant headgroup surface area to the density of the surrounding nonpolar medium, and the extremely low critical micelle concentrations (cmc) of these systems. Figure 5c presents the water core radius of the DDAB/propane/water system as a function of temperature. The response to temperature is consistent with the literature (9,10). A slight decrease in size with increasing temperature is noted. The density of liquid water at these conditions is relatively constant, so the size dependence is
20.
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.0001
H
265
Dynamic Fluorescence Quenching in Propane
.
1
1 1 0
0
1 20
.
Time
1
1 30
1 40
1
! 50
[ns.]
Figure 4: Quenched fluorescence decay in the DDAB/propane/water system at 100°C and 400 bar. W=23, [DDAB]=325 mM.
5
•
•
•
2000
•
•
•
•
•
4000
•
6000
•
•
8000
Pressure [psig] Figure 5a: Water core radius as a function of pressure for the AOT/propane/water system at 25°C. W=15, [AOT]=125 mM.
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10
•
•
•
•
•
•
•
•
• d
•
• Β
Β
Φ
ε s Ό
28 [C] 50 [C] 100 [C]
2000
4000
1 6000
8000
Pressure [psig] Figure 5b: Water core radius as a function of pressure for the DDAB/propane/water system. W=23, [DDAB]=325 mM.
'•3
120
Temperature [C] Figure 5c: Water core radius as a function of temperature for the DDAB/propane/water system. W=23, [DDAB]=325 mM.
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probably a result of an increase in the effective surface area of the surfactant molecule headgroups, rather than a result of a change in water volume. (At 28°C the effective headgroup area for D D A B in propane is found to be 23.9 Â , and increases to 24.8 Â at 5 0 ° C and 29.4 Â at 100°C.) This may be due to greater solvation of the surfactant hydrocarbon tails by the nonpolar bulk fluid at the higher temperatures. 2
2
2
Figures 6a and 6b show the bimicellar water core exchange rate constant (ke) as a function of pressure in the AOT/propane/water system and the DDAB/propane/water system, respectively. For comparison, k for the AOT/isooctane/water system at 25°C was found to be 1.67 χ 10^ [1/mol-s]; approximately 6% of that in the propane-based systems. This value agrees well with the values of Lang et al (9) for the DTAB/l-pentanol/chlorobenzene/water system, the tetradecyltrimethyl ammonium bromide/1-butanol/chlorobenzene/water system, and the hexadecyltrimethyl ammonium bromide/1-butanol/chlorobenzene/water systems. They report exchange rate constants between 1x10^ and 2x10^ [1/mol-s]. e
The large differences in exchange rates between the liquid propane and liquid iso-octane systems are not due to differences in micelle size in the two solvents. Numerous studies have shown that the nature of the alkane solvent has little effect on the size of the micelles. The high rates in propane are largely due to lower viscosity of this solvent and the resulting higher micellar diffusivities. The mechanism of core exchange upon micellar collision also plays a role in determining the exchange rate, and may be affected by the nature of the continuous phase solvent. In both systems the exchange rate is largely independent of pressure, although a weak pressure dependence may be noted in the supercritical propane/DDAB system, although the data are too sparse to conclusively prove this dependence. In contrast, the exchange rate constant shows a strong increase with temperature (Figure 6c), in agreement with Jada et al. (11) This can be attributed to the temperature dependence of viscosity (12) of the bulk phase, resulting in greater diffusion rates of the micelles at higher temperatures. Conclusions Reverse micelles of A O T in iso-octane, A O T in propane, and D D A B in propane have been examined using the dynamic fluorescence quenching technique to measure micelle water core radii and bimicellar exchange rate constants. In the propane-based systems several pressures were considered and for the DDAB/propane/water system the temperature was varied between 2 5 ° C and 1 0 0 ° C to span the subcriticaVsupercritical transition of the bulk phase propane. The water core radii thus obtained agree qualitatively with those obtained by SANS or QELS, and the data show the expected trends with variance of pressure and temperature. Specifically, the water core radii of these systems are independent of pressure (reflecting the incompressibility of water at these conditions and the insensitivity of the surfactant headgroup to die bulk phase conditions), and are largely independent of temperature. The bimicellar exchange rate constants of these systems are similarly independent of pressure, reflecting the insensitivity of the micellar system to the density of the solvating fluid.. The bimicellar exchange rates are strongly temperature dependent, reflecting the sensitivity to temperature of the bulk phase transport properties. More data are required to determine if the micelle mixing is more significant at higher temperatures, i.e., if the increase in micellar core exchange rate is controlled purely by diffusional concerns.
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50 •
H
40
22
30 •
•
•
20 H
10H
τ
-r
2000
4000
Pressure
6000
8000
[psig]
Figure 6a: Bimicellar exchange rate constant as a function of pressure for the AOT/propane/water system at 25°C. W=15, [AOT>125 mM.
150
100 c
ο S 5,
m
28 [C] • 50 [C] • 100 [C]
ν
*
5 0
•
• 0
• 2000
•
•
•
•
•
•
4000
Pressure
•
•
•
6000
•
• 8000
[psig]
Figure 6b: Bimicellar exchange rate constant as a function of pressure for the DDAB/propane/water system. W=23, [DDAB]=325 mM.
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1000
100
120
Temperature [C] Figure 6c: Bimicellar exchange rate constant as a function of temperature for the DDAB/propane/water system. W=23, [DDAB]=325 mM.
Acknowledgments The authors wish to thank Don Friedrich and Wayne Hess of the Molecular Science Research Center at Battelle Pacific Northwest Laboratories (Richland, WA) for help with and the use of the laser system. This work was sponsored by the Office of Basic Energy Sciences of the U.S. Department of Energy, contract number DE-AC0676RLO 1830. T.S. Zemanian is a postdoctoral appointee at P N L through the Northwest College and University Association for Science (Washington State University, Pullman, WA).
Literature Cited 1. 2. 3. 4. 5. 6. 7.
Tingey, J.M.; Fulton, J.L.; Matson, D.W.; and Smith, R.D. J. Phys. Chem., 1991, 95, 1445-1448 Infelta, P.P; Grätzel, M . ; and Thomas, J.K. J. Phys. Chem., 1974, 78, 190-195 Atik, S.S.; and Thomas; J.K. J. Am. Chem. Soc., 1981, 103, 3550-3555 Dederen, J.C.; Van der Auweraer, M . ; and Schryver, F.C. Chem. Phys. Lett., 1979 68, 451-454 Atik, S.S., and Singer, L . A . Chem. Phys. Lett., 1978, 59, 519 Caspar, J.V., and Meyer, T.J. Inorg. Chem., 1983, 22, 2444-2453 Kotlarchyk, M ; Chen, S.; Huang, J.S.; and Kim, M.W. Phys. Rev. A, 1984, 29, 2054-2069
270 8. 9. 10. 11. 12.
SUPERCRITICAL FLUID ENGINEERING SCIENCE Jada, A; Lang, J.; Zana, R.; Makhloufi, R.; Hirsch, E.; and Candau, S.J. J. Phys. Chem., 1990, 94, 387-395 Lang, J; Jada, Α.; and Malliaris, A. J. Phys . Chem., 1988, 92, 1946-1953 Lang, J.; Lalem, N; and Zana, R. J. Phys. Chem., 1991, 95, 9533-9541 Jada, A; Lang, J; and Zana, R. J. Phys. Chem., 1989, 93, 10-12 Younglove, B.A.; and Ely, J.F. J. Phys. Chem. Ref. Data, 1987, 16, 577798
RECEIVED April 27, 1992
