Langmuir 2002, 18, 7349-7353
7349
Influence of Temperature-Induced Aggregation on Energy Transfer from a Surfactant to Micellized Perylene Kevin Toerne and Ray von Wandruszka* Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343 Received March 19, 2002. In Final Form: June 7, 2002 Fo¨rster energy transfer from excited surfactant species was used to monitor their association with micellized perylene at different temperatures. Fluorescence from the perylene acceptor was obtained through sensitization by surfactant monomers and aggregates, the latter being identified as spectroscopic rather than physical. Surfactant monomer fluorescence was subject to a strong inner filter effect inside the micelles, indicating an abundance of spectroscopically monomeric species there. Temperature increases caused reductions in native surfactant fluorescence through thermal quenching, but sensitized fluorescence intensities generally increased with temperature. This was ascribed to a temperature-induced approach between the perylene acceptor and both monomeric and aggregated surfactant donors. Clouding of the solutions showed little evidence of organizational changes at the micellar level.
Introduction The phase behavior exhibited by solutions of nonionic surfactants heated to their cloud points has been the subject of scientific inquiry for decades.1-8 Some of the causes and mechanisms of surfactant clouding are reasonably well understood, but certain basic aspects remain unexplained. Among these are the exact nature of the aggregates formed upon clouding and the various structural modifications that surfactant assemblies undergo when the temperature is changed. Cryogenic transmission electron microscopy studies performed on surfactants of the general formula CH3(CH2)n(OCH2CH2)mOH showed structures varying from rodlike9 to globular5 in solutions of concentrations in the percentage range. The phase behavior of relatively dilute solutions of the Triton series of surfactants, which comprise a hydrocarbon portion containing a benzene ring (see Table 1), has not been as effectively characterized. A case in point is the fluctuating clouding behavior observed with micellar solutions of Triton X-114 (TX-114).10 This surfactant normally clouds at 23 °C (Table 1), but solutions can be induced to clarify (decloud) at 65 °C, only to cloud again at 70 °C. The effect was noted by Maclay11 in 1956 and again by Nishikido et al. in 1977.12 In a study by Herrmann et al., the phase diagram for a dimethylalkylphosphine oxide surfactant solution appeared to exhibit a similar effect at elevated temperatures and pressures.7 Previous work in our laboratory has shown that declouding episodes are not associated with a major (1) Ottewill, R. H.; Storer, C. C.; Walker, T. Trans. Faraday Soc. 1967, 63, 2796. (2) Balmbra, R. R.; Clunie, J. S.; Corkill, J. M.; Goodman, J. F. Trans. Faraday Soc. 1962, 58, 1661. (3) Atwood, D. J. Phys. Chem. 1969, 72, 339. (4) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977, 81, 155. (5) Danino, D.; Talmon, Y.; Zana, R. J. Colloid Interface Sci. 1997, 186, 170. (6) Robson, R. J.; Dennis, E. A. J. Phys. Chem. 1977, 81, 1075. (7) Herrmann, K. W.; Brushmiller, J. G.; Courchene, W. L. J. Phys. Chem. 1966, 70, 2909. (8) Corti, M.; Degiorgio, V. J. Phys. Chem. 1981, 85, 1442. (9) Lin, Z.; Scriven, L. E.; Davis, H. T. Langmuir 1992, 8, 2200. (10) Toerne, K.; Rogers, R.; Von Wandruszka, R. Langmuir 2000, 16, 2141. (11) Maclay, W. N. J. Colloid Sci. 1956, 11, 272. (12) Nishikido, N.; Akisada, H.; Matura, R. Mem. Fac. Sci. Kyushu Univ., Ser. C 1977, 10, 92.
Table 1. Structure and Characteristics of POE Surfactantsa
TX-114 TX-100 a
n
cmc (mM)
cloud point (°C)
8.5 9.5
0.2 0.25
23 65
Data from ref 13.
disruption of the micellar interior.14 Particle size analysis indicated that aggregates larger than normal micelless but not large enough to produce a turbid solution10s remained present throughout the declouding regimen. In this study we considered the influence of variations in surfactant aggregation on the nonradiative energy transfer from a polyoxyethylene (POE) surfactant donor to a micellized acceptor (perylene). The sequestration of these acceptor molecules in the hydrophobic interior of amphiphilic aggregates has revealed useful information about secondary surfactant structures, as well as biological membranes and organelles.15-28 The intimate contact between the components of such systems permits fluorescence excitation energy to be transferred from the donor (13) Hinze, W. L.; Pramauro, E. Crit. Rev. Anal. Chem. 1993, 24, 133. (14) Toerne, K.; Rogers, R.; Von Wandruszka, R. Langmuir 2001, 17, 6119. (15) Schillen, K.; Yekta, A.; Ni, S.; Winnik, M. A. Macromolecules 1998, 31, 210. (16) Eisenhawer, M.; Cattarinussi, S.; Kuhn, A.; Vogel, H. Biochemistry 2001, 40, 12321. (17) Zhou, J.; Yuan, X.; Jiang, M.; Zhang, Y. Macromol. Rapid Commun. 2000, 21, 579. (18) Yansheng, W.; Weijun, J.; Changsong, L.; Huiping, Z.; Hongbo, T.; Naichang, Z. Spectrochim. Acta, Part A 1997, 53, 1405. (19) Koglin, P. K. F.; Miller, D. J.; Steinwandel, J.; Hauser, M. J. Phys. Chem. 1981, 85, 2363. (20) McCarroll, M. E.; Toerne, K.; Von Wandruszka, R. Langmuir 1998, 14, 2965. (21) McCarroll, M. E.; Joly, A. G.; Wang, Z.; Friedrich, D. M.; Von Wandruszka, R. J. Colloid Interface Sci. 1999, 218, 260. (22) Honda, C.; Itagaki, M.; Takeda, R.; Endo, K. Langmuir 2002, 18, 1999. (23) Saha, D. C.; Ray, K.; Misra, T. N. Spectrochim. Acta, Part A 2000, 56, 797. (24) Sanchez, F. G.; Ruiz, C. C. J. Lumin. 1993, 55, 321. (25) Ndou, T. T.; Von Wandruszka, R. Photochem. Photobiol. 1989, 50, 547.
10.1021/la025750w CCC: $22.00 © 2002 American Chemical Society Published on Web 08/20/2002
7350
Langmuir, Vol. 18, No. 20, 2002
Toerne and von Wandruszka
to the acceptor.29 The efficiency of this process depends on the distance between the two species, their mutual orientation, and the overlap of the respective emission and excitation spectra.30-32 Fo¨rster showed that the transfer efficiency, E, decreases with the sixth power of the distance separating the donor and acceptor33
E)
R06 R06 + r6
(1)
Here R0 is the Fo¨rster radius, i.e. the critical distance at which spontaneous decay of donor excitation and energy transfer to the acceptor are equally probable, and r is the donor-acceptor separation. The energy transfer process is governed by two types of interactions: one is effective at short distances (
