J. phys. Chem. isao, 84,3339-3341
333s
Use of Pyrene Excimer Formation To Study the Effect of NaCl on the Structure of Sodium Dodecyi Sulfate Micelles P. Llanos and R. Zana' Centre de R e c k c k s sur les Mecrm&ules, In F k l Form: October 15, 1980)
C.N.R.S., 87083, Strasbowg cedex, France ( R w k e d : Augost 27, 1980;
Time-resolved fluorescence analysis of micellesolubilized pyrene confirms that the aggregation number of SDS micelles increases sharply with the NaCl concentration at C N >~0.45~ M. The range of validity of the method used in this work for the determination of micelle aggregation number is discussed.
Recently Mazer et al.' used quasi-elastic light scattering to study the effect of NaCl on the properties of SDS micelles. They found that, for an aqueous solution of SDS at a fixed concentration (C = 6.9 X M) and temperature (25 "C), an increase of ionic strength with NaCl in the concentration range CN&l = 04.6 M resulted in an increase of the surfactant aggregation number n from about 80 to 1OOO. The increase was relatively small at lower c N a(n 150 at 0.4 M NaC1) and became very large at higher cNaCl (n rv- 1000 at 0.6 M NaC1) thus suggesting a change in the micellar shape. These results were, however, contested by Turro and Yekta2 who found by using a fluorescence probing method that, for the same SDS and NaCl concentrations given above, n changes only in the range from 62 (no NaCI) to -160 ( C N ~ C=~ 0.6 M). These values of n were obtained by measuring the luminescence quenching of Ru(bpy)?+, attached electrostatically on the micelles, by 9-methylanthracene incorporated into the micelles owing to its hydrophobicity. On the contrary, another more recent work3 using classical light scattering and taking m e of eliminating all factors questioning the initial results of Mazer et al.' verified, in fact, the results of the latter. In order to try to obtain a definite answer concerning the aggregation behavior of SDS in the presence of NaC1, we have measured the aggregation number of SDS micelles using a recently reported' fluorescence probing method which involves pyrene excimer formation and time-resolved fluorescence analysis of micelle-solubilized pyrene. Our results show that the R~(bpy),~+-9-methylanthracene method does not apply in the presence of salt and confirm the formation of very large SDS micelles above 0.45 M NaC1. Moreover, they allowed us to assess the range of validity of the fluorescence probing method that we used and to obtain new information on the behavior of micelle-solubilized pyrene in the presence of NaCl. Concerning the method, we briefly recall that when the ratio p of the molar concentrations of pyrene (cp) and (), is close to 1, the fluorescence decay curve micelle C shows two decay components: a slow one corresponding to micelles having solubilized one pyrene molecule, and a fast one associated with micelles having solubilized two or more pyrene molecules (excimer formation). From the ratio of the amplitudes of these two componenta one can obtain p', and thus the aggregation number n = p ( C cmc)/cp, where C is the surfactant concentration and cmc the critical micellar concentration. The excitation of pyrene fluorescence was performed at 335 nm and the emission was monitored at 380 nm. Before going into the results we briefly discuss the assumptions inherent in this method and its limitations. It
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0022-3654/80/2084-3339$01.0010
TABLE I: Effect of NaCl on the Properties of SDS Micelles Probed by the Fluorophore Pyrene at 25 "C CNaCI, na 1,113 7 ,ns 0.0 62 (58) 1.32 333 1.25 367 0.1 90 (91) 1.21 398 0.2 111 (105) 1.21 400 0.3 120 (117) 410 0.4 135 1.18 1.17 434 0.5 210 1.17 440 0.6 400 a Values in parentheses are listed as the best presently available in ref 9.
is first assumed that all probe molecules are solubilized into the micelles. This holds well for pyrene because in our studies the pyrene concentration was always 10-100 times larger than the limiting solubility of pyrene in water. The second assumption is that the distribution of the probe among micelles follows Poisson statisticse6This is true if there is equal probability of occupation of the micelles and no limit to the number of probe molecules that can be solubilized in a given micelle. The assumption of equal probability of occupation holds well for micellar solutions of classical ionic surfactants in the absence of additives, where the micelle polydispersity is known to be smalla6Problems would arise if the investigated solutions were polydisperse or contained two widely differing types of micelles, such as rods and spheres, for instance. Also at the low p values used in this work, very few micelles contained more than two pyrene molecules and the possible existence of a maximum in the number of pyrene molecules solubilized in a given micelle did not matter. The third assumption implicit to the method is that the excimer formation in micelles containing two or more pyrene molecules is assumed to follow kinetics similar to that of intramolecular excimer formation. This is true for small micelles but, when the micelle size is sufficiently large, excimer formation might be controlled by diffusion within the micelle.' This effect results in limiting the rate and thus the extent of excimer formation and, subsequenly, in underestimating the surfactant aggregation number. Experience obtained in our laboratory after applying the above method for measuring aggregation numbers for a variety of Surfactants and factors affecting this aggregation has proven that the results are very accurate for n values up to about 200, for surfactants with 10 to 16 carbon atoms.8 We thus felt confident in applying this method to SDS in the presence of NaCl. The results are given in Table I and also shown in Figure 1. The values of n a t C N =~0, 0.1,0.2, ~ and 0.3 M are practically identical with those which are now considered as the best available This agreement is an additional indication that the 0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 25, 1980 r
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. . .
6001n c
400
'
200 '
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Figure 1. Variation of the aggregation number n of SDS mlcelles, at a surfactant concentration of 7 X IO-* M and 25 OC, upon lncreaslng NaCl concentration.
fluorescence method4used in this work is as accurate and reliable as the more classical but also much more tedious methods such as sedimentation equilibria coupled with isopiestic distillationlo or light scattering.ls The values shown in Figure 1 are all larger than those reported by Turro and Yekta? the difference becoming very large at CN&l > 0.4 M. There are two possible explanations to the failure of the Ru(bpy),2+-9-methylanthracene method to yield the correct SDS aggregation numbers in the presence of NaC1. The first one is that in the presence of a large excess of Na+ ions some Ru(bpy),2+ ions dissociate from the micelles. A simple examination of the equations2 used to calculate n from the measured fluorescence intensities shows that this effect should result in apparent n values smaller than the true ones. This explanation, however, does not appear to hold in view of the resulta of Meisel et al." who concluded from spectroscopic studies that the addition of 0.5 M NaCl does not cause any dissociation of R ~ ( b p y ) ions ~ ~ +from SDS micelles. The second explanation is that in larger micelles the quenching of Ru(bpy),2+by 9-methylanthracene is not static, contrary to what is required by the method? Recent results by Almgren and Lofroth12have actually verified this explanation by applying time-resolved fluorescence spectroscopy to the same system. The resulta in Figure 1show a clear break in the increase of n with cNAC~between 0.4 and 0.5 M, in agreement with the findings of other workers,'~~ who attributed it to the spheroid-rod transition. At CN&l = 0.5 M the value of n obtained by means of our fluoresecne method (210) is lower than those reported by Mazer et al.l (about 300) and Hayashi and Ikeda3(about 270). In spite of the shortcomings in the determination of micelle aggregation numbers by means of light scattering (see the assumptions detailed in ref 1and 3) it appears reasonable to say that SDS aggregation numbers of 200 probably represent the highest ones which can be measured by means of the excimer fluorescence method? with an error comparable to that involved in others methods. This limiting value of n beyond which the method is no longer valid for SDS micelles is close to that obtained in similar studies with other surfactantse8 Two effects may be involved to explain the failure of the method to measure large n values. The first one is the micelle polydispersity, which increases with n for a given surfactant. Mazer et al.' did report a fairly large polydispersity at high NaCl content. However, we believe that it is the large size of the SDS micelles which results, as pointed out above, in an underestimation of n. This can be shown as follows. The length of an SDS micelle of aggregation number 200, assumed to have the shape of a cylinder with hemisphere-capped e n d ~ ,is~ found ~ l ~ to be about 100 A, from the data reported by Hayashi and Ikeda.3 This length represents the maximum distance
Letters
between two pyrene molecules solubilized in such a micelle. Thus, each probe would have to diffuse over a distance of about 50 A during the lifetime of a pyrene molecule in the excited state, that is, about 400 ns (see Table I) for the formation of an excimer. The calculations show that this would have been easily achieved had the viscosity of the micellar interior been close to that of liquid hydrocarbons. This microviscosity, however, has been reported to be up to 30 cP.14 As a result the two probes cannot always form an excimer during the lifetime of the monomer pyrene excited state. This explains well the low aggregation numbers found by our method, at high NaCl content. We have listed in Table I the values of the fluorescence decay times I and of the ratio of the intensities of the first over the third peak (11/13) of the pyrene fluorescence emission spectrum obtained as part of this work. It is known that 11/13 reflects the polarity of the microenvironment of the probe16 and can thus be used to detect changes of polarity upon modification of the micellar solution. As seen from Table I, in the present case, 11/13 decreased with increasing cNacl, which indicates a decrease of the polarity of the pyrene microenvironment. When pyrene is incorporated into micelles or vesicles, it is known to lie close to the polar head groups, in the palisade layer, where it senses a highly polar envir~nment.'~Partly responsible for this may be the presence of water molecules intercalated between the head groups, in the palisade layer. In the presence of large quantities of NaCl, the repulsions between head groups are screened and the surface area per head group decreases, forcing water molecules out of the palisade layer. Naturally the polarity sensed by the probe decreases. The decrease of surface area per head group at the micelle surface upon increasing CNaCl may also force the fluorophore to be located deeper in the micelle, than in the absence of NaC1. Pyrene is knownle to interact with polar molecules and ita fluorescence decay time to be then de~reased.'~Thus by being localized deeper into the micelle, it not only senses an environment of lower polarity, but it is also expected to have a longer fluorescence decay time 7 . Indeed as shown in Table I, the 7 values for deoxygenated solutions increased with increasing CN~CI. In conclusion, the fluorescence method proposed by Atik, Nam, and Singer4is as reliable as any of the methods used so far for determining micelle aggregation numbers such as light-scattering, osmometry, and centrifugation, for n I 200. In addition to ita rapidity, it has the following advantages over these methods: (1) it permits the determination of micelle aggregation numbers at a given surfactant concentration (without requiring any extrapolation to the cmc) and in the presence of additives: (2) it is insensitive to phenomena of preferential adsorption by, and electrical interactions between, the micelle, which greatly complicate the interpretation of the results; and (3) it is applicable to any type of surfactant, whether anionic, cationic, nonionic, or amphoteric (this is not the case for the Ru(bpy),2+-9-methylanthracene method2). References and Notes (1) N. Mazer, G. Benedek, and M. C.Carey, J. Phys. Chem., 80, 1075 (1976). (2) N. J. Turro and A. Yekta, J . Am. Chem. SOC.,100,5951 (1978). (3) S. Hayashi and S. Ikeda, J. Phys. Chem., 84, 744 (1980). (4) S. Atik, M. Nam, and L. Singer, Chem. Phys. Lett., 87, 75 (1979). (5) P. Infelta and M. Griitzel, J. Chem. Phys., 70, 179 (1979). (0) E. A. G. Anlansson, S. Wall, M. Almgren, H. Hoffmann, I. Kielmann, W. Ulbrlcht, R. a n a , J. Lang, and C. Tondre, J. Phys. Chem., 80, 905 (1970); W. Baumuller, H.Hoffmann, W. Ulbricht, C.Tonbe, and R. Zana, J . Co//oidInterface Scl., 64, 418 (1978). (7) U. Gosele, U. Klein, and M. Hauser, Chem. Phys. Lett., 68, 291 (1979). (8) P. Llanos and R. Zana, manuscript In preparation. (9) J. Kratohvll. J. Colloid Interface Sd.,75, 271 (1980).
J. Phys. Chem. 1980, 84, 3341-3344 D. Doughty, J. Phys. Chem., 89, 2621 (1979). D. Meisell, M. S. Matheson, and J. Rabani, J. Am. Chem. Soc.,100, 117 (1978). M. Alrngren and J,-E. Lafroth, personal communication and paper submitted for pubilcatlon. J. Leibner and J. ,lacobus, J. Phys. Chem., 81, 130 (1977). J. Emert, C. Behrens, and M. Goklenberg, J. Am. Chem. Soc., 101,
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771 (1979); N. Turro, M. Aikawa, and A. Yekta, IbM., 101, 772 (1979). (15) K. Kalyanasimdaram, Chem. SOC. Rev., 7 , 453 (1978); J. K. Thomas, AcI:. Chem. Res., 10, 133 (1977). (16) P. Llanos and S. Georghbu, photochem. f%of&&/., 30,355 (1979); 29, 843 (197'9). (17) P. Lianos, B. Lux, and D. Gerard, J. Chim. Phys., in press.
I R Multiphoton Pumping of Optically Selected Levels of the Thiophosgene
a 'A2 State of
D. M. Brenner Depadment of Chemlstty, Brookhaven Natlonal Laboratory. Upfon, New York 11973 (Recelvd: September 3, 1980; In Final Form: SeptembW' 30, 1980)
The first excited singlet state of thiophosgene is IR multiphoton pumped from state-selected vibronic levels of increasing vibrational energy, Ed,,. The extent of IR pumping to nonradiative (predissociating) levels is monitored from total fluorescence. The latter is observed to decrease due to IR pumping but no obvious dependence on Edb of the initial state is observed. However, at levebi requiring absorption of only one IR photon, unexpectedly little or no change in total fluorescence is observed suggesting that the absorption coefficient of thiese levels may be much smaller than at lower levels. The absorption cross section during IR multiphoton pumping leading to dissociation is postulatedl to decrease with increasing energy because the oscillator strength becomes spread oveir an increasingly larger density of vibrational states (sum rule). Experimentally, however, it has been difficult to determine the exact dependence. Double resonance experiments are required in which levels of increasing (energyare optically selected and subsequently monitored following IR multiphoton pumping.2 Monitoring the dependence by observing the change in dissociation yield ,at constant IR photon flux provides indirect evidence but is considerably less difficult to carry out. Recently, experiments involving (CH3C0)23Aand NO: which attempt to elucidate the resulting internal energy distribution following IR multiphoton absorption (IRMPA) have appeared in the literature. These experimenb involve preparation o;f specific, but uncharacterized,vibronic levels of an excited ;state foHlowed by IR multiphoton excitation. In the case of NOz, the resulting energy distribution is modeled from a comparison of the amplitude of the IRinduced, blue-shifted fluorescence spectrum with that obtained by direct visible absorption. The assumption is made that fluorescence produced under different conditions is independent of the excitation source. The validity of this assumption, however, is untested, but requires that the IR pumped state be strongly coupled to all fluorescing levels and emit on 8 similar time scale. The experiinents reported here are an initial attempt to measure the dependence of the multiphoton absorption coefficient on the initial vibrational energy content of well-defined levels of the A lA2 state of thiophosgene. Previous double-resonance,IRMPA experiments involving the ground electronic state, X lAl, have been reported elsewhere? The photophysics of thiophosgene is considerably less complex than that reported for NO2' or biacetyL8 McDonald and Bruss have shown that (1) a t excess energies < 3650 cm-l above the vibrationless level the collisionless lifetime, T~ =: 35 ps, of the A lA2 state is independent of excess vibrational energy and (2), above 3650 cm-l, emission is broken off and T~ < 150 ns, due to non0022-3654/80/2084-334 1$01.OO/O
radiative procesi3es, most likely predissociation from high vibrational levels of the ground electronic state.l0 From Herzberg-Teller theory, they conclude that unimolecular nonradiative processes do not depopulate levels below 3650 cm-l; i.e., the fluorescence quantum yield is unity. Since the total emissicin lifetime is independent of energy level below predissociation, the fluorescence amplitude must also be invariant, recalling that summation over transition probabilities from all rovibronic levels relates the Einstein coefficient A , oinly to the electronictransition probability which, in turn, are related to the lifetime.'l Pumping thiophosgene with a visible laser of narrow bandwidth (see :Figure 1) permits selection of particular rovibronic levels, of the 36C12CSisotope and produces a population of the first singlet state determined by Franck-Condon factors. The identification of these levels is made from previous spectroscopic as~ignments.'~J~ In the absence of cc~llisions,the fluorescence quantum yield will be unity. Pumping these optically prepared levels with a COz laser populates higher levels which are either nonpredissociating levels (below 3650 cm-l), which will fluoresce with the same lifetime and intensity as lower levels, or predissociating levels, which will be characterized by a decrease in fluorescence quantum yield and a shortened lifetimie. Therefore, a decrease in the fluorescence signal following IR pumping is directly related to the extent of multiphoton absorption to nonradiating levels. Since the sum of vibrational states is 2 1per cm-l at 3000 cm-' (inversion doubling12makes the actual number calculated by using the Whitten-Rabinovitch approximation14 a lower limit), these experiments probe the dependence of the absorption cross section on initial energy in a region involving discrete vibrational levels where strong mixing is present. Experimental Section The apparatus employed in these experiments has been described previously15and will be discussed only briefly. A fast flow of thbphosgene16 intersects at 90° a dye laser beam and a C02 laser beam. The background pressure produced by the flow in the detection chamber is 0.5 X 0 1980 American Chemical Soclety
