J. Phys. Chem. 1995,99, 15192-15197
15192
Micrometer Size Effect upon the Viscosity of Individual Droplets Dispersed in the OiVWaterDodecyl Sulfate System: A Transient Absorption Microspectroscopic Study Seiji Funakura,*T$Kiyoharu Nakatani? Hiroaki Misawa,II Noboru Kitamura,*vD and Hiroshi Masuhara*s Microphotoconversion Project,' ERATO, Research Development Corporation of Japan, 15 Morimoto-cho, Shimogamo, Sakyo-ku, Kyoto 606, Japan Received: April 17, 1995; In Final Form: July 28, 199P
Transient absorption microspectroscopy was developed for studies on photophysical and photochemical processes in a single laser-manipulated microdroplet in water. Transient absorption spectra and their decay curves of individual tri-n-butyl phosphate (TBP) microdroplets containing zinc tetraphenylporphyrin (ZnTPP) were measured and analyzed. Owing to the intense laser excitation, the concentration of the ZnTPP triplet state was so high that the decay profiles of T, TI absorbance of ZnTPP obeyed the second-order kinetics, triplet-triplet (T-T) annihilation. Simultaneously the delayed fluorescence of ZnTPP was observed and analyzed in terms of T-T annihilation. The estimated rate constant of T-T annihilation (kn) was comparable to that obtained from T, TI absorption spectroscopy. Assuming the T-T annihilation is diffusion-controlled, the inner viscosity of a single TBP microdroplet was obtained and examined as a function of the diameter of the microdroplets. A remarkable micrometer size effect that the inner viscosity is decreased with the droplet diameter was found and discussed in terms of a swollen TBP droplet owing to the penetration of water.
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Introduction It has recently been pointed out that the micrometer dimension leads to unique photophysicallphotochemical processes in This is due to characteristic associatiodorientations of molecules and cluster formation of protic solvent^^.^ or to efficient molecular diffusion in small volume^.^^^ The microdroplet in solution is a particularly interesting system as a micrometer reaction field with a liquid-liquid interface. Among the various kinds of photochemical dynamics, the pyrene excimer formation was studied in detail for probing association and dissociation processes in individual microcapsules and The pyrene excimer formation dynamics in an individual liquid paraffin droplet dispersed in aqueous gelatin matrices were constant irrespective of the droplet size? while those in an individual microcapsule were different between the capsules.* These results clearly indicate that the photochemical dynamics is strongly dependent on the structures and properties of the microdroplet and the surrounding environmental conditions. A spectroscopic study on individual microdroplets is therefore interesting and worth elucidating as one of the topics in colloid or interface science. Their potential applications to various industries, such as photography, ink, paint, cosmetic, and food, will also receive much attention. In view of the stabilization of emulsionsI0 and their reaction rate of enzymatic catalysis,' the transport properties such as viscosity, mass diffusion, and heat transfer are important and are influenced by minute environmental conditions that differ from those exhibited in the bulk. l 2 Especially rheological
* To whom all correspondence should be addressed.
' Five-year term project; October 1988-September
1993. Present address: Chemicals Division, Dainippon Ink and Chemicals, Inc., 18, Higashifukashiba, Kamisu-machi, Ibaraki 314-02, Japan. 8 Present address: Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japan. ' I Present address: Department of Mechanical Engineering, Faculty of Engineering, The University of Tokushima, 2- 1 Minamijosanjima, Tokushima 770, Japan. Present address: Department of Applied Physics, Osaka University, Suita, Osaka 565, Japan. @Abstractpublished in Advance ACS Absrracrs, September 15, 1995.
properties at the liquid-liquid interface containing surface-active molecules are influenced by orientation of the adsorbed surfactant, formation of self-aggregated molecules, and their molecular interaction^;'^ therefore it is worth investigating the transport behavior. Recently the microviscosity in the interface and the core area of the micelles has been estimated by electron spin resonance (ESR),I4-I8 fluorescence spectro~copy,'~-~~ and absorption spectroscopy.22 For example, in aerosol OT reverse micelles the microviscosity at the interface is similar to that of the bulk.22 By an ESR study on relaxation processes of nitroxide radical, the increased amount of water led to a decrease in the interfacial microviscosity of sodium dodecyl sulfate (SDS) micelle and to an increase of its interfacial polarity.I6 Furthermore, the addition of the salt resulted in an increase of the interfacial microviscosity and a decrease of the interfacial polarity.l 5 In the cyclohexane/poly(oxyethylene)-4-nonylphenol/water system, the core becomes more polar and less viscous upon increasing the water content.I7 All the conclusions were derived on the basis of measurements of bulk dispersed solution. On the other hand, the inner viscosity in an individual droplet has never been examined. Its viscosity may differ from that in the bulk, which will be very important for controlling chemical reactions. Furthermore some specificities that may appear only in micrometer dimensions can be utilized; hence, new micrometer chemical processes are expected. We have already proposed new analysis methods to estimate solute concentration in an individual laser-trapped droplet by measuring its fluore~cence~~ and ground state absorption spectra.24 Among these studies, we reported that solute concentration in the droplet is different from that in the mother s o l ~ t i o n and ~ * often ~ ~ , ~scattered ~ between microcapsules with different diameters.8 It is strongly recommended that the photochemical process of an individual microparticle be elucidated by measuring transient absorption spectra. In the present work, we have developed a transient absorption spectroscopy system under a microscope combined with the laser-trapping techniqueZ5and have applied it to individual TJ3P droplets dispersed in the oil/water/dodecyl sulfate system. The
0022-3654/95/2099-15 192$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 41, 1995 15193
Droplet Viscosity in OiVWater/Dodecyl Sulfate
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Figure 1. Block diagram of a laser trapping-transient absorption microspectroscopy system. OL and DM are an objective lens and a dichroic mirror, respectively. HG is a harmonic generator.
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Tn TI absorption spectra of ZnTPP were measured, and the diffusion controlled T-T annihilation process was analyzed. Furthermore, the delayed fluorescence of ZnTPP was investigated, and the photochemical dynamics of the ZnTPP triplet state is considered as a function of the diameter.
Experimental Section Laser Trapping-Transient Absorption Spectroscopy System. A block diagram of the microspectroscopy system is shown in Figure 1. A 1064-nm beam (trapping beam) from a CW Nd3+:YAG laser (Spectron, SL-903U) was introduced to an optical microscope (Nikon, Optiphot-2) through dichroic mirrors (DM) and focused to a small spot (-1 pm) by an oilimmersion objective lens (OL; magnification x 100; numerical aperture, 1.3). The beam is used for trapping a Brownian oil droplet at the focal point. Trapping behavior was monitored by photographs or a CCD-TV monitor set equipped to the microscope. For transient absorption spectroscopy of a lasertrapped droplet, the third harmonics of a Q-switched Nd3+:YAG laser (Spectron SL-282G; 355 nm; pulse width, -5 ns; 3 Hz) was employed as an excitation beam. It was introduced coaxially to the optical microscope through a dichroic mirror and loosely focused to a 60-pm spot at the focal plane. A xenon flash lamp (Tokyo Instruments, XF80-60; pulse duration, -70 ps) was employed as a probe light source with the ultraviolet radiation being removed by inserting a W cut filter. The probe light was also introduced into the microscope and focused into a 6-pm spot in a sample solution. The probe light transmitting the sample solution and a condenser lens (numerical aperture, 0.6) of the microscope was collected by a lens and led to a polychromator (McPherson, 2035, 150 G/mm) and a microchannel plate/photodiode array detector (Princeton Instruments, D/SIDA-700G(B) detector, FG-100 gate pulser, and a ST-11OP controller). The xenon lamp spectra with and without excitation were corrected for removing the contribution of the delayed fluorescence, averaged over 30 pulses in the wavelength region of 400-700 nm, and analyzed by a microcomputer. The decay curves of transient absorption and delayed fluorescence were detected by a polychromator and a photomultiplier tube (Hamamatsu R666) and averaged over 32 pulses by a digital storage oscilloscope (Tektronix 2432A). Timing between the Q-switched Nd3+:YAG laser, a xenon flash lamp, and a detector was controlled by a digital delay generator (Stanford Research System Inc., Model DG-535). In these measurements,the dependence of the excitation intensity
was measured by using calibrated neutral density filters. The measurement method of trapping and excitation laser intensity was previously reported.26 All measurements were performed at ambient temperature (19-22 "C). The two-dimensional resolution (XUaxes) of this microspectroscopy system was determined as follows. A 20 pm wide black line of the objective microscale was monitored with the xenon flash lamp by shifting the microscope stage, and its transmitted intensity was plotted as a function of the position. The response had a full width at half-maximum (fwhm) of 8 pm. The depth resolution (2 axis) was estimated to be worse than 8 pm from a dependence of the absorbance of the ground state ZnTPP with the different droplet size.24 The spatial characteristicswere confirmed to be common for 440-700 nm; hence, chromatic aberration was practically not important. In this system the thickness in the direction of the microscope axis was regarded as an effective optical path length for absorption measurement; namely, the thickness of the film and the diameter of the droplet could be used as the path length, which was confirmed by measuring the ground state absorption spectra.24 Preparation of Samples. TBP (Nacalai Tesque, EP grade) was purified by the method of Y ~ s h i d a .ZnTPP ~ ~ (Wako), SDS (Nacalai Tesque, SP grade), and water (Luminasol, Dojindo Laboratory) were used as supplied. TBP and water were degassed thoroughly by a freeze-pump-thaw cycle before the emulsion was prepared. The ZnTPP concentration in TBP, abbreviated here [ZnTPP]o, was adjusted to be 4 or 8 mM, which was used as the mother solution. The 50-pL mother solution and an aqueous solution (5 mL) of SDS (9 mM) were vigorously shaken to prepare microdroplets. The sample emulsion (5- 10 pL) was then placed between a slide glass and an etched (8 mm x 8 mm with 50-150-pm depth) cover glass and was sealed by an adhesive resin to prevent evaporation of the solution, as shown in Figure 2. The mother solution of 50 pL was also placed between a slide glass and an etched (20-80pm depth) cover one, which was used as a reference for transient absorption measurement. For the preparations, all the procedures were performed in a glovebox under an atmosphere of argon, where the concentration of oxygen was less than 0.1 ppm to prevent photodecomposition of ZnTPP. This sample preparation is indispensable for studying quantitatively the photochemical dynamics.
Results and Discussion
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Tn TI Absorption Spectra of ZnTPP in Individual Droplets. The photochemical dynamics of ZnTPP in bulk solution have been studied in detai1.2*-31 The absorption maximum of the lowest triplet state (TI) in toluene is located at 465 nm, and its lifetime is 3 ms.29 A typical example of the Tn-TI absorption spectra of ZnTPP in an individual droplet is shown in Figure 3. The excitation beam was defocused so that the whole of a single individual droplet was irradiated. The spectral band shape in an individual droplet has nothing to do with the droplet diameter and agrees with that in the bulk (1cm cuvette and thin solution film). The appearanceof the broad T,-Tl absorption of ZnTPP at 465 nm was accompanied by the bleaching of the Q band at 556 nm, and the spectral change was finished in a few microsecond range. For the ground state
Funakura et al.
15194 J. Phys. Chem., Vol. 99, No. 41, 1995 l
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Wavelength (nm) Figure 3. Transient absorption spectra of ZnTPP in a single droplet excited at 355 nm (d = 27 pm; [ZnTPPIo = 8 mM). The delay times after excitation are indicated in the figure. 1.2
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Figure 5. (A) Decay profiles of the Tn T I absorption of ZnTPP at 465 nm: (a) in a droplet (d = 20 pm, [ZnTPPIo = 4 mM); (b) in a film (d = 20 pm). (B) Inverse plots of the data in A.
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Figure 4. Initial absorbance of the Tn T I absorption of ZnTPP at 465 nm as a function of the diameter. The excitation intensity is 12.8 5.7 mJ/cm2 (A), and 3.5 mJ/cm2 (0).[ZnTPPIo = 4 mM. mJ/cm2 (0).
absorption spectroscopy, we have already confirmed that the absorbance of ZnTPP increases linearly with the diameter of the droplet, and the Lambert-Beer equation. holds for the droplets with the diameter larger than 6 , ~ m . *Here, ~ an additional excitation pulse is introduced so that it is important and indispensable in examining similarly a linear relationship between the Tn-TI absorbance and the diameter. The initial Tn TI absorbance at 465 nm was plotted for the diameter of the individual droplet in Figure 4. The linear relationship was experimentally confirmed for different excitation intensities; hence, the Lambert-Beer law holds well for droplets with the present diameter range. Therefore, we can discuss quantitatively the photochemical dynamics of a single Brownian droplet with the present laser trapping-transient absorption spectroscopy system. T-T Annihilation of ZnTPP in Individual Droplets and Films. The T, TI absorption decay profiles of ZnTPP in a droplet and a film are shown in Figure 5A. Although the lifetime of the ZnTPP triplet state is reported to be 3 m ~ , the *~ present decay was completed in a few microseconds. The inverse plot of the Tn TI absorbance showed a linear relation with delay time up to 0.8 ps, although the SIN value was not excellent because of a weak probe light. Therefore, the decay of the triplet state obeys the second order kinetics in the time range, and the T-T annihilation process is concluded as a major deactivation channel in the present system. Using 90 OOO M-I cm-l as a molar absorption coefficient of Tn TI absorbance of ZnTPP at 465 nm?* and regarding the droplet diameter as the optical path length (20 pm), the rate constant of T-T annihilation (km) was estimated to be (2.9 f 0.7) x lo9 M-I S-I (within 10%error).34 Similarly, the solution film was analyzed, and kn was obtained. The inverse plot was also examined, as in Figure 5B,giving a linear relation with the delay time. km for a film
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Figure 6. (A) Decay profiles of the Tn T I absorption of ZnTPP at 465 nm in individual droplets. The diameter is (a) d = 67 pm, (b) d = 27 pm, (c) d = 18 pm, and (d) d = 9 pm. [ZnTPPJo= 4 mM. (B) Inverse plots of the triplet concentration calculated from the data in A.
with a thickness of 20 pm was estimated to be (1.4 f 0.3) x lo9 M-I s-I (within 10%error), which is smaller than that in the droplet. It is worth noting that kn is different between films and droplets (discussed later). Micrometer Size Effect on T-T Annihilation Rate Constant. In Figure 6A, Tn TI absorbance decay profiles in individual droplets with different diameters are shown. The ZnTPP triplet state concentration in individual droplets is calculated with the Lambert-Beer rule, and its inverse is plotted
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Droplet Viscosity in OiVWater/Dodecyl Sulfate
J. Phys. Chem., Vol. 99, No. 41, 1995 15195
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Figure 7. Size (thickness or diameter) dependence of the T-T annihilation rate constant (km) in films (0),droplets dispersed in H2O (0).and droplets dispersed in D20 (0). 0.4 -
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Figure 8. Delayed fluorescence spectra of ZnTPP in a single droplet (d = 17 pm). The delay times after the pulsed excitation are 0.1 ps (A), 0.2 ps (B), and 0.5 ps (C).
versus the delay time in Figure 6B. The linear relationship was satisfied for each droplet within experimental error, indicating that the decay of the triplet state obeys in general the second order kinetics in individual droplets with different diameters. For the droplets with diameters of 67, 27, 18, and 9 pm, kn are estimated to be (1.9 f 0.2) x lo9, (2.3 f 0.3) x lo9, (3.2 f 0.3) x lo9, and (4.5 f 0.7) x lo9 M-I s-' (within 10% error), respectively. The results are reproducible, and an excitation intensity effect upon kn was not observed below 12.8 mJ cm-*. It is extremely interesting that kn of the droplet increases with decreasing diameter. We analyzed extensively kn for individual droplets with various diameters and films with various thicknesses, and the obtained relationshipsbetween kn and size (diameter or thickness) are summarized in Figure 7. The kn value of the droplet increases with decreasing diameter, while that of the film is almost constant irrespective of its thickness. It is noticeable that the kn in individual droplets shows an interesting micrometer size effect. Delayed Fluorescence of ZnTPP due to T-T Annihilation in Individual Droplets. Upon laser excitation fluorescence spectra of ZnTPP were observed in individual droplets, which is shown in Figure 8. Those were detected even in the microsecond time range, suggesting that the T-T annihilation process results in delayed fluorescence. Indeed, ZnTPP was reported to show p-type delayed fluores~ence.~~ The peak maxima of the delayed fluorescence spectra are 605 and 655 nm, and the spectral shape in an individual droplet, irrespective of droplet diameter, agrees with those in the bulk. The kinetics of the p-type delayed fluorescence is already e~tablished?~ and two limiting cases are considered according to the relative magnitude of the unimolecular decay constant ( k ~and ) kn;(A) kT >> kn[T~],,corresponding to low excitation intensity andor large t, and (B) kT
