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Langmuir 1997, 13, 5816-5819
Lanthanide-Containing Reversed Micelles: A Structural and Luminescence Study A. Beeby,*,† I. M. Clarkson,† J. Eastoe,‡ S. Faulkner,† and B. Warne‡ Department of Chemistry, University of Durham, Durham DH1 3LE, U.K., and School of Chemistry, University of Bristol, Bristol BS8 1TS, U.K. Received April 21, 1997. In Final Form: July 25, 1997X Lanthanide surfactants of the type Ln(AOT)3 have been prepared and characterized, where Ln ) Tb, Eu, Nd, and AOT is bis(2-ethylhexyl) sulfosuccinate. Small-angle neutron scattering experiments indicate that spherical reverse micelles form in cyclohexane. The luminescence lifetimes of lanthanide ions held inside these aggregates were found to depend on the molar ratio w ) [water]/[AOT-]. This behavior is consistent with changes in the Ln3+ solvation: in “dry” micelles the ions are complexed by the surfactant polar groups, while addition of water results in progressive Ln ion hydration. The results show that reversed micelles can be used to control photophysical properties of lanthanides.
Introduction It is well-known that lanthanide ions, such as europium(III) and terbium(III), luminesce in aqueous solutions.1 The luminescence lifetime of the excited state depends on the number of water ligands, and in general the quenching becomes more efficient as the coordination increases and so lifetimes decrease.2 Reverse micelles represent an interesting controlled water environment in which to study luminescent properties of lanthanides. With ionic surfactants the micelle core consists of the polar headgroups and counterions as well as any water. A classic reversedmicelle-forming surfactant is Aerosol-OT (NaAOT, sodium bis(2-ethylhexyl) sulfosuccinate), which gives spherical aggregates of about 15 molecules in hydrocarbon solvents. These “dry” micelles do contain waters of crystallization, and in the case of NaAOT it is about one per molecule.3 Any added water swells the micelle, and the polar core radius Rc is proportional to the loading w where4
w ) [water]/[AOT-] and Rc (Å) ≈ 1.8w As w is increased, the pool water becomes progressively bulklike, but the apparent transition point depends on the technique employed. However, for NaAOT recent Fourier transform IR (FTIR) measurements indicate this occurring at w approximately 6.5 Since the water concentration and ionic strength can be readily varied, reversed micelles provide a useful medium for studying hydration interactions. For lanthanide lumophores they can be used for investigating the effects of hydration number on excited state lifetimes and hence the deactivation of the excited states. The mean hydration number of the lanthanide ion, q, can be estimated from the rate constant of nonradiative decay, and Horrocks and Sudnick have proposed that q ) A(kH - kD), where kH/D is the rate constant of nonradiative decay * Author to whom correspondence should be addressed. † University of Durham. ‡ University of Bristol. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Lanthanide Probes in Life Chemical and Earth Sciences; Bunzli, J.-C. G., Chopin, J. G., Elsevier: Amsterdam, 1989. (2) Horrocks, W. DeW.; Sudnick, D. R. Acc. Chem. Res. 1981, 14, 384. (3) Eastoe, J.; Robinson, B. H.; Fragneto, G.; Towey, T. F.; Heenan, R. K.; Leng, F. J. J. Chem. Soc., Faraday Trans. 1992, 88, 461. (4) Eastoe, J.; Robinson, B. H.; Steytler, D. C.; Thorn-Leeson, D. Adv. Colloid Interface Sci. 1991, 36, 1. (5) Moran, P. D.; Bowmaker, G.; Cooney, R. P.; Bartlett, J. R.; Woolfrey, J. L. Langmuir 1995, 11, 738.
S0743-7463(97)00404-6 CCC: $14.00
in H2O and D2O, respectively.2 A is an empirical factor obtained from a series of compounds for which q is known: AEu ) 1.05 × 10-3 s, ATb ) 4.2 × 10-3 s, and ANd ) 3.6 × 10-7 s.2,6 It is found that kr , knr; thus, knr ≈ kobs ) 1/τlum. The salts of Eu3+ and Tb3+ with AOT- have been prepared previously7-9 and luminescence measurements made of the ions in water-hydrocarbon microemulsions. However, these studies concentrated on energy transfer from an organic donor to the lanthanide acceptor as a method of detection for the organic analyte. It was also reported that the water pool size had no effect on the intensity of luminescence, but there was no discussion of luminescence lifetimes. These observations may have arisen due to the use of naphthylacetic acid as the analyte and sensitizer, which may be expected to chelate the lanthanide held within the reverse micelle. The naphthyl group makes a poor choice of analyte when using Tb3+ or Eu3+ since other processes may occur, specifically back energy transfer to the triplet state or charge transfer to the aromatic chromophore. The effects of water pool size on the quenching are better investigated by direct excitation of the metal, as is done here, since this allows more precise measurements of the photophysical properties and potential interference from other chromophores is also eliminated. Our recent investigations of luminescence from Nd3+ solutions demonstrated that it behaves in a similar fashion to Eu3+ and Tb3+, although its lifetime is much shorter due to greater susceptibility to quenching by water.6 Here, the preparation of Nd3+, Eu3+, and Tb3+ AOT- salts are described, the Nd(AOT)3 reversed micelles in cyclohexane are characterized by small-angle neutron scattering (SANS), and the luminescent lifetimes of the lanthanides ions are studied as a function of w. The results show how reversed micelles can be used to control photophysical properties of Ln3+ ions. Experimental Section Materials. Neodymium(III) nitrate hexahydrate, terbium(III) nitrate pentahydrate, europium(III) nitrate pentahydrate, Aerosol OT (Aldrich), and C6D12 and D2O (Goss Scientific Instruments) were all used as received. Diethyl ether, ethanol, (6) Beeby, A.; Faulkner, S. Chem. Phys. Lett. 1997, 266, 116. (7) Mwalupindi, A. G.; Blyshak, L. A.; Ndou, T. T.; Warner, I. M. Anal. Chem. 1991, 63, 1328. (8) Mwalupindi, A. G.; Ndou, T. T.; Warner, I. M. Anal. Chem. 1992, 64, 1840. (9) Mwalupindi, A. G.; Agbaria, R. A.; Warner, I. M. Appl. Spectrosc. 1994, 48, 1132.
© 1997 American Chemical Society
Luminescence of Lanthanide Ions in Reverse Micelles and cyclohexane were AnalaR grade (BDH). Ln(AOT)3‚xH2O(D2O), where Ln ) Tb, Eu, or Nd, were prepared by an ion exchange technique.3 Aliquots (25 mL) of Ln(NO3)3 in either H2O or D2O (0.5 M), NaAOT in ethanol (0.3 M), and ether were thoroughly mixed resulting in a Winsor II microemulsion. Additional ether (25 mL) was then added to induce phase separation. The organic layer was extracted and washed five times with 25 mL portions of either H2O or D2O. The solvent was then removed under reduced pressure and the resulting waxy solid dried under vacuum at 40 °C for 48 h before use. The surfactants were analyzed by elemental analysis (C, H, and Na) and fast atom bombardment mass spectrometry. The latter showed peaks at m/e 995, 986, and 1002 for Eu(AOT)3, Nd(AOT)3, and Tb(AOT)3, respectively, corresponding to Eu(AOT)2+, Nd(AOT)2+, and Tb(AOT)2+.9 Elemental analyses were also consistent with the required compounds. Sodium contents were between 0.10 and 0.35%, indicating that >95% ion exchange had occurred. Water-in-cyclohexane microemulsions were prepared by adding aliquots of H2O or D2O to a cyclohexane solution of the Ln(AOT)3‚xH2O(D2O) and sonicating to give clear solutions. The concentration of AOT- was always 0.1 mol dm-3. For all of the lanthanide salts the maximum water solublization, wmax, was 12.0 ( 1.0 at 25 °C. The effects of codissolved sodium ions were investigated by sonication of aliquots of 0.1 mol dm-3 NaAOT in cyclohexane with 0.033 mol dm-3 Tb(AOT)3 solutions. Water was then added to vary w in the usual way. Photophysics. Lifetime measurements for terbium and europium were made on a luminescence spectrometer (PerkinElmer LS 50B). Excitation/emission wavelengths of 369/545nm and 394/593 were used for Tb and Eu, repectively. In both cases the bandwidths of the excitation and emission monochromators were 5 nm. Luminescence lifetimes were recorded by measuring the emission intensity as a function of delay time. All of the decays were adequately described by single expontentials, and no significant improvements were obtained by fitting a biexponential. For neodymium the lifetimes were determined using a home built ns-laser pumped fluorometer.6 Briefly, the samples were excited by a 10 Hz train of 355 nm radiation, with a typical pulse energy of 0.1-2.0 mJ per pulse and a duration of ca. 6 ns. The luminescence was monitored at 1055 nm using a liquid nitrogen cooled germanium photodiode/amplifier which has a rise time of ca. 200 ns and a full width half maximum (FWHM) response of 400 ns. The signal was captured and averaged by a digital storage oscilloscope and transferred to a PC for analysis. An instrument response function was obtained using fluorescence from a solution of a red laser dye (DCM), tf ) 2.2 ns. The lifetime of this dye is very short compared to the metal ion and is used as an alternative to a scatterer.10,11 The decays were analyzed by iterative reconvolution and nonlinear least-squares analysis of the instrument response profile with a single exponential function: more complex functions than this were not needed. The quality of the fit was judged by the residuals and reduced χ2.12 Small Angle Neutron Scattering. The LOQ time-of-flight instrument on ISIS at the Rutherford Appleton Laboratory, U.K., was used. The measurements determine the absolute scattering probability I(Q) (cm-1) as a function of momentum transfer Q (Å-1) ) (4π/λ) sin(θ/2) with λ the incident neutron wavelength (2.2 f 10 Å) and θ the scattering angle (
