Langmuir 1995,11, 94-100
94
Fluorescence Studies of Solute Microenvironment in Composite Clay-Surfactant Films Maryam F. Ahmadi and James F. Rusling" Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-3060 Received August 1, 1994. In Final Form: September 26, 1994@ Microenvironmental polarity and viscosity in ordered composite films made from dialkyldimethylammonium surfactants and clay were studied by using the fluorescent probes pyrene and 1,3-di(l-pyrenyl)propane (DPP).A decrease in monomer fluorescence and an increase in excimer peaks as temperature increases through the phase transition region allowed estimation of phase transition temperatures (TJ. The viscosity and polarity of probe microenvironments decreased with time of soaking the films in water, although X-ray diffraction indicated no changes in interlayer spacing. After 5 days of soaking, viscosities and polarities achieved values similar to those in the methylene chain regions of phospholipid vesicles. Results suggest that probes in freshly prepared films reside close to surfactant head groups but move toward the interior of surfactant bilayers when soaked in water for long periods. Viscosities found in these well-soaked films may be characteristic of microenvironments of nonpolar reactants when composite films are used for catalysis.
Introduction Ordered multilayer films of cationic surfactants intercalated between colloidal clay platelets can be prepared from clay and cationic surfactant.la2 When coated on graphite electrodes with cobalt phthalocyanine incorporated, composite films of clay-dialkyldimethylammonium surfactants in liquid crystal states provided catalysis for reductions of organohalide pollutant^.^^^ The same films in ordered gel states showed significantly decreased catalytic activity.2 Results of X-ray d i f f r a ~ t i o n , ~electron ,~~ microscopy, comparative phase transition and reflectance absorbance infrared spectroscopy4bsupported the proposal that surfactants are ordered in bilayer structures resembling biomembranes in these multilayer composite films. The surfactant bilayers are intercalated between clay platelets. Since our main application for these films is catalysis, questions concerning residence sites of reactants are of prime importance for understanding performance and reaction mechanisms. We recently showed that relatively nonpolar organohalides such as dibromocyclohexane and dibromobutane are preconcentrated from aqueous solutions into liquid crystal films of clay-didodecyldimethylammonium bromide (DDAB), but polar molecules like trichloroacetic acid are Thus, these films exhibit permselectivity, but the solubilization site for the nonpolar molecules has remained unclear. Fluorescence of the nonpolar solute pyrene has been used extensively as a probe for solubilization sites in aggregates such as surfactant micelles and v e s i ~ l e s . ~ Ratios of intensities of the first (lowest 1) to the third monomer vibronic peak can be used as a n estimate of the polarity of the solute's microenvironment. Ratios of excimer to monomer fluorescence intensity provide information about the relative microviscosity of the microenvironment~.~,~ ~
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~~
Abstract published in Advance A C S Abstracts, December 1,
1994. (1)Okahata, Y.;Shimizu, A. Langmuir 1989,5, 954-959. (2)Hu,N.;Rusling, J. F. Anal. Chem. 1991,63,2163-2168. (3)Zhang, H.; Rusling, J. F. Talanta 1993,40, 741-747. (4)(a)Rusling, J. F.;Ahmadi, M. F.; Hu, N.Langmuir 1992,8,24552460. (b) Suga, K.; Rusling, J . F. Langmuir 1993,9,3649-3655. ( 5 ) Kalyanasundaram, K. Photochemistry in Microheterogenous
Systems; Academic Press: Orlando, FL, 1987.
F'yrene is a reasonably good excimer-forming probe, due to its long fluorescence lifetime and formation of excimers a t low concentrations.' 1,3-Di(l-pyrenyl)propane(DPP) has been suggested as a better choice than pyrene for microviscosity studies of surfactant bilayer structures.6 Because it has two pyrene moieties in the same molecule, DPP forms intramolecular excimers at extremely low concentrations. Unlike pyrene, the amount of excimer formed for DPP does not depend on concentration. It can be used a t much lower concentrations than pyrene, with less chance of perturbing the system. The fluidity of surfactant micelles, vesicles, and biological membranes has been studied as a function oftemperature usingDPP.6,8 Zachariasse et al. showed that the excimer to monomer intensity ratio of DPP increases as the viscosity of the medium decreases.6 A low viscosity medium allows the two pyrene moieties to move more freely, which can result in a n excimer if one pyrene is photoexcited. The excimer to monomer ratios are thought to be affected only by viscosity, not by other solvent properties. Even though measurement of absolute microviscosities may be difficult by this method, relative viscosity changes are readily inve~tigated.~ In this paper, we use 1,3-di(l-pyrenyl)propane and pyrene to characterize the polarity, viscosity, and phase transitions of layered composite films of clay-DDAB and clay-DODAB (dioctadecyldimethylammonium bromide). Gel-to-liquid crystal phase transitions of the films were detected from the influence of temperature on fluorescence intensities or excimerlmonomer intensity ratios. The polarity of the solute microenvironment is relatively independent of phase state, but microviscosity is considerably smaller in the liquid crystal than in the gel state. (6)Zachariasse, K. A.; Duveneck, G.; Busse, R. J.Am. Chem. SOC. 1984,106,1045-1051. (7)Vanderkooi, J. M.;Callis, J . B. J . Biochem. 1974,13,4000-4006. (8) (a)Zachariasse, K. A.; Kuhnle, W. Z. Phys. Chem. 1976,101,267. (b)Zachariasse,K.A.; Vaz, W. L. C.; Sotomayor, C.; Kuhnle, W.Biochim. Biophys. Acta. 1982,688,323-332.(c) Almeida, L.M.; Vaz, W. L. C.; Zachariasse, K. A.; Madeira, M. C. Biochemistry 1982,21,5972-5977. (d) Turley, W. D.; Offen, H. W. J.Phys. Chem. 1986,89,3692-3964. (e) Turley, W.D.; Offen, H. W. J.Phys. Chem. 1986,90,1967-1970.
(0Mulders, F.;van Langen, H.; van Dangreau, H.; Joniau, M.; De Cuyper, M.; Hanssens, I. Biochemistry 1982,21, 3594-3598. (g) Zachariasse, K. A,; Kuhnle, W.; Weller, A. Chem. Phys. Lett. 1980,73,
6-11. (9)Pownall, H.J.; Smith, L. C. J.Am. Chem. SOC.1973,95,31363140.
0743-7463/95/2411-0094$09.00/0 0 1995 American Chemical Society
Langmuir, Vol. 11, No. 1, 1995 95
Microenvironment in Composite Films
Experimental Section Chemicals and Materials. 1,3-Di(l-pyrenyl)propanewas from Molecular Probes (Eugene, OR). Didodecydimethylammonium bromide (DDAB) and dioctadecyldimethylammonium bromide (DODAB) were 99+% from Eastman Kodak. The indium-doped tin oxide (InSnO2) electrodes were from Delta Technologies (Stillwater, MN, CG-901N-CUV). Bentonite clay (BentoliteH)was from Southern Clay Products and had a cation exchange capacity of 80 mequiv/100 g. AI1other chemicals were reagent grade. Apparatus and Procedures. Clay colloids and surfactant were combined in an aqueous suspension to form the composite material as described previously.2 The composite was purified, freeze-dried, and suspended in chloroform (2 or 4 mg/mL) to cast the films. Solutions of 1,3-di(l-pyrenyl)propaneand pyrene in chloroform were mixed with suspended composite and ultrasonicated for 10 min to make 10 pM 1,3-di(l-pyrenyl)propane and 0.5 mM pyrene suspensions. The higher concentration for py-rene was used due to its concentration-dependent excimer formation. Films were cast onto glass slides coated with transparent, conductive InSnO2, which also served as a working electrode. The dimension of the slides were 50 x 7 mm, which fits directly into a 1-cm quartz cuvette. The upper end of the InSnOz glass slide was connected to a copper wire with silver epoxy. The surface and sides of the electrode were then covered with insulating epoxy, leaving only a small square area (10 x 7 mm) to be coated with the composite films and excited with incident light. The epoxy-covered area was then wrapped with Teflon tape. Most experiments were done after depositing 20 pL of the composite dispersion (2 or 4 mg/mL) in chloroform onto the prepared InSnO2 slide with a micropipet. The chloroform was evaporated overnight in air, resulting in ca. 1pm thick layers of surfactant-clay film on the slide. Changes in film thickness did not change the shapes of the spectra obtained. The amount of DPP in the surfactant volume of the film was adjusted to give a DPP-surfactant mole ratio below the limiting mole ratio for self-quenching. The mole ratio of DPP to surfactant was 2 x 10-6,which is well below the molar ratios of probeflipid in other studies.8b-f Before each experiment, films were stored in pure water in a quartz cuvette for an hour while purging through a syringe needle with pure nitrogen gas saturated with water vapor. The water level was then lowered to below the square area of the slide coated with the film, to minimize the possibility of solubilizing the fluorescentprobes at higher temperatures. The cuvette which was used as the cell was sealed on top with p a r a n film. The nitrogen gas flow was continuous throughout the experiment.A small temperature probe (Fisher Quartz, digi-thermo, Model 852397) was inserted just above the square composite film area ofthe slide. The position ofthe film slide was secured by making a rectangular slot in the Teflon cap of the cuvette to hold the electrode close to the front face. Two small holes were also made in the cap for the temperature probe and for the syringe needle to introduce nitrogen. Temperature was controlled ( f O . l "C)by a thermostat bath (HAAKEA80) filled with ethylene glycoywater. Fluorescence spectroscopy was done by using SpexFluorolog-2 and Perkin-Elmer LS5OB spectrofluorometers. The optical arrangement for obtaining film spectra was described previously.10 Spectra were obtained after 1h of equilibration at each temperature. Spectra were repeated at l5-min intervals to check the temperature equilibrium. The reported intensities are the averages from all spectra taken at a specific temperature after equilibrium was reached. The excitation wavelength was 347 nm, and the excitation and emission slits were each 1.0 mm. The intensities of monomers, peak I (379 nm) and I11 (399 nm), excimer, peak IV (474 nm), and excimedmonomer ratios were plotted versus temperature. Derivatives were calculated by a Savitzky-Golay algorithm." A conventional three-electrode potentiostat was used in the controlled potential experiments. The cell was similar to that described previously,1°and used above,but employed a AglAgBr(saturated KBr) reference and Pt counter electrode. The cell was filled with 0.1 M KBr purged with nitrogen. (10)Ahmadi, M.F.;Rusling, J. F. Langmuir 1991,7 , 1529-1536. (11)Savitzky, A.;Golay, M. J. E. Anal. Chem. 1964,36,1627.
370.0
400.0
450.0
500.0
s50.0
600.0
Wavelength (nm)
Figure 1. Emission spectrum of 1pM DPP in chloroform. The monomer peaks are I at 379 nm, I1 at 386 nm, and I11 at 399 nm. The excimer peak is at 474 nm. Thermal analysis of films was done by a Perkin-Elmer differential scanningcalorimeter (DSC-7),calibrated as reported previously.'Z Samples were prepared by depositing the surfactant-clay suspension with and without 1,3di(1-pyreny1)propane on a glass slide and allowing the chloroform to evaporate over 24 h in air. The films were then soaked in water for 1week. Just before analysis, the films were scraped off the slide into sample pans. The sample pan was sealed and placed in the sample holder. For DDAB-clay with and without DPP, the temperature range was from -30 to 50 "C. The equilibration time was 1min at -30 "C and 5 min at 0 "C. The phase transition of DDABclay film is close to the water phase transition, so the 5-min equilibrium time at 0 "C helped distinguish between the peaks for these two transitions. Temperature was scaned at the rate of 10 "C/min. For DODAB-clay, the temperature range was from -10 to 100 "C with 0.5-min equilibration at the lower temperature. Sample weights were about 10 mg. X-ray diffraction was done in a pulsed mode at O.OB"/step, which is 65"/min., by using a Scintag XDS 2000 powder diffractometer with a Cu Ka source at 45 kV and 40 mA. Films were prepared on glass slides as described p r e v i o ~ s l yFilms .~~ were soaked in pure water for various periods, and excess water was shaken off of the film before analysis.
Results Spectra in Organic Solvents. The fluorescence spectrum of 1,3-di(l-pyrenyl)propane(DPP) in chloroform is shown in Figure 1. The monomer peaks are I at 379 nm, I1 at 386 nm, and I11 at 399 nm. A large excimer peak is seen near 500 nm. This is due to the low viscosity of the medium, which allows facile intramolecular interaction of excited-state and ground-state pyrene moieties. Spectra of DPP were obtained in a series of solvents from which oxygen had been removed. The viscosity of each medium was estimated from the ratio of monomer to excimer peak intensities of DPP, utilizing the calibration plot published by Zachariasse6 (Table 1). Viscosities estimated by this method are in excellent agreement with the literature. These results gave us confidence that the method could be extended t o the composite films. Kalyanasundaram summarized intensity ratios of peaks VI11 of pyrene for a large range of solvents of different p ~ l a r i t i e s .For ~ low polarity solvents, the VI11 ratio is small. As the polarity of the medium increases, the Ut11 ratio also increases. As reported,12 and also reflected in ~
(12)Hu,N.;Howe, D. J.; Ahmadi, M. F.;Rusling, J. F.Anal. Chem. 1992,64,3180-3186. (13)CRC,Handbook of Chemistry and Physics, 57th ed.; CRC Press: Cleveland, OH, 1983.
96 Langmuir, Vol. 11, No. 1, 1995
Ahmadi and Rusling
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Table 1. Ratio of Excimer/Monomer and Peak Weals I11 of l,%Di(l-pyrenyl)propane in Solvents of Different Viscosities
lS5
viscosity (cP) fluorescence
moria
peak I/ peak111
method
1it.b
6.0 0.2 3.3 2.8 2.8 3.4 1.2 3.3 2.9
1.7 1.6 1.5 1.5 1.4 1.4 1.4 1.3 1.3
