2408
J. Phys. Chem. 1980, 8 4 , 2406-2408
(5) K. Kabyanasundaran and D. Duonghong, to be submitted to J. Phys. Chem . (6) G. Rothenberger, P. P. Infelta, and M. Gratzei, J . Phys. Chem., 83, 1871 (1979). (7) N. Mataga, Y. Kaifu, and M . Koizurni, Bull. Chem. SOC. Jpn., 29, 373 (1956). (8) (a) P. Mukerjee and K. Banerjee, J. Phys. Chern., 88, 3567 (1964); (b) M. S. Fernandez and P. Fromheiz, /bid.,81, 1755 (1977); (c) K. L. Mittal, Ed., "Micellization, Solubilization and Microemulsions", Plenum Press, New York, 1976.
(9) (a) E. C. Lirn, C. P. Larzara, M. Y. Yang, and G. W. Swenson, J . Chem. Phys., 43, 970 (1965); (b) E. C. Lirn and W. Y. Wen, /bid., 39, 847 (1963); (c) E. C. Lim and G. W. Swenson, lbM., 36, 118 (1963). (10) S. A. Alkaitis and M.Gratzel, J . Am. Chern. Soc., 98, 3549 (1976). (11) R. C. Kaye and H. I. Stonehiil, J. Chem. SOC.,2638 (1951). (12) S. P. McGlynn, T. Azurni, and M. Kinoshka, "Mdecular Spectroscopy of the Triplet State, Prentice-Hall, Englewood Cliffs, NJ, 1969. (13) K. Kalyanasundaran, J. Kiwi, and M. Gratzel, Helv. Chirn. Acta, 61, 2720 (1978).
Partitioning of Aromatic Alcohols between a Water Solution and Cationic Micelles Eduardo Llssl,' Elsa Abuln, and Ana M. Rocha Departamento de Qdmica. Facultad de Clencb, UnlversMsd T6cnica del Estado, Santlago, Chile (Received: December 31, 1979)
The distribution constants of alcohols of general formula HQ(CH2),CeH5(n = 0-3) between water (0.5 M in sulfate ions) and cetyltrimethylammonium micelles have been obtained from fluorescence measurements by employing nickel ions as selective quenchers of the fluorescence arising from the water phase. The results obtained show that the addition of the first and the second methylene groups decreases the fraction of probe incorporated into the micelle while the third group increases it. This anomalous behavior is not observed for the distribution of the probes between n-heptane and water (0.5 M in zinc sulfate). The results are interpreted by taking into account the tendency of both the hydroxyl and the phenyl groups to be located in the micellar surface.
The distribution of a probe molecule between the aqueous and micellar phases is a matter of current interest.*-3 Similarly, the location of the probe inside the micelle is also a matter of relevance since it can determine the accessibility of the probe to reactants present in either the aqueous or micellar phases. Fluorescence measurements have been extensively employed to determine both the probe distribution at low occupancie~~-~ and the position of the probe inside the micelle.gg In the present work we have applied this technique to determine the distribution of probes of general formula HO(CH2),,C6H5(n = 0-3) between water and cetyltrimethylammonium bromide micelles. These molecules were considered of interest since they contain a hydrophobic chain between a polar head (the OH group) and a group (the benzene ring) that in micelles is known to have a strong tendency to be located near the surface.l0
Experimental Section Materials. Cetyltrimethylammonium bromide (CTAB) (Merck, p.a.) was purified by recrystallization." The phenols (Fluka, p.a.) were purified by sublimation. Benzyl alcohol, 2-phenylethanol, and 3-phenylpropanol (Merck p.a.) were used as purchased or purified as described in the next section; nickel sulfate (Merck, p.a.) and n-heptane (Fluka p.a.) were used without further purification. Distribution Studies. The experimental method employed to determine the probe distribution between the aqueous and micellar phases was similar to that previously d e ~ c r i b e d . ~ JAll ~ measurements were carried out at (probe)/(CTAB) below 0.01 and employing a 0.5 M sulfate solution as aqueous phase. This low (probe)/(CTAB) assures a low level of occupancy (less than 20% double occupancy if the aggregation number is considered to be nearly 100). In agreement with this predominantly single occupancy, only monomeric emission was observed for all the compounds considered. When phenol or p-ethylphenol was the probe considered, the pH of the solution was kept a t 3.5 by adding concentrated HCl acid. Quenching experiments were done at fixed sulfate concentration (0.5 M) 0022-3654/80/2084-2406$0 1.OO/O
and variable amounts of Ni2+and Zn2+ions. The zinc ion was inert toward the excited probes in the aqueous and micellar phases. The distribution between the aqueous and n-heptane phases was determined spectrophotometrically by measuring the W absorption of both phases after equilibration. These measurements were carried out a t low probe cow centration (less than M) in order to obtain the partition constant in the highly diluted region. The results obtained were independent of the probe concentration. When only a small fraction of the probe is present in one of the phases, the results could be influenced by the presence of small amounts of impurities selectively extracted and with large extinction coefficients and/or high fluorescence quantum yields. In order to test this possibility some distributions were carried out employing solutions of the probe that had been previously washed several times either with water (when the probe was introduced in the n-heptane phase) or with n-heptane (when the probe was introduced in the aqueous phase). The results obtained were independent of the previous treatment of the probe solution.
Results Distribution of the Probes between Water (0.5 M in Zinc Sulfate) and n-Heptane. The values of the distribution constant defined by eq 1 are given in Table I. K H j W = (probe)n-heptane/(probe)watar (1) These values were obtained with the concentrations expressed in mol/L. The use of concentrations instead of activities is justified by the low concentrations employed. In this table are also included the values of the molar free energies of transfer obtained by employing the diluted solutions as reference state. The trend in KHp, as well as the increment in the free energy of transfer introduced by each methylene group, is similar to those obtained in other systems and can be considered as n0rma1.l~ Distribution of the Probes between Water (0.5 M in Sulfate) and Micelles. The fluorescenceof water solutions 0 1980 American Chemical Sociefy
The Journal of Physical Chemistry, Vol. 84,No. 19, 1980 2407
Partitioning of Aromatic Alcohols in Solution
TABLE I: Distribution Constants and Free Energies of TransfeP poM
probe phenol benzyl alcohol 2-phenylethanol 3-phenylpropanol p-ethylphenol
SV, M-' 2.3 2.9 2.6 1.9 1.45
Aqueous phase 0.5 M in sulfate ions,
fw
0.33 0.47 0.53 0.13 0.13 poM
Ks, mol-' 200 100 48 320 500
M
(5)
where a is a factor that takes into account the difference in both pseudophase volumes under the experimental conditions employed. The values of KMp obtained when the concentrations are given in mol/L of aqueous solution or micellar phase are given in Table I. These values were obtained by assuming that the density of the micellar phase was 0.77 g/cm3 as in a long-chain normal h y d r o ~ a r b o n . ~Fur,~ thermore, since under our experimental conditions the critical micellar concentration was found to be considerably M because of the effect of the added less than 5 X salts, the concentration of free CTAB was neglected in the calculations. The volume of the micellar phase could differ significantly from the assumed value,15 but this difference would not modify the following discussion. In this table we have also included the values of the binding constants KS.l Under the present conditions (low cmc and large CTAB-to-probe ratio) the value of Ks can be evaluated by eq 6. Ks = [F/f, - 1I/(CTAB) (6)
Discussion A comparison of the values of KH,Wand KMp shows that micelles are considerably better solvents than nheptane for all the compounds considered. This behavior can be ascribed to the contribution of the adsorption of
-
~ cal/mol
420 210 100 670 1045
- p o w defined as equal to - R T
of all the aromatic compounds employed was quenched by nickel ions. The Stern-Volmer plots obtained were linear, and the values of their slopes (SV) are given in Table I. The fluorescence of the probes in micellar solutions arises partly from the aqueous phase and partly from the micellar phase, a distribution evidenced by the partial quenching of the fluorescence by nickel ions. The singlet lifetimes of all the compounds considered in the present work are s. Since these times are considerably of the order of shorter than those required for the interchange of probes between the water phase and the micelle^,'^ the results can be interpreted without taking into account the interchange of excited probes across the micellar boundaries. In this case the change in fluorescence emission can be related to the quencher concentration by eq 2,%12where f, gives the 1 - (4/&) = f,(SV)(Ni2+)/[1 + (SV)(Ni2+)1 (2) fraction of light emitted from the aqueous phase.12 A plot of the left-hand side of eq 2 against (SV)(Ni2+)/[l+ (SV)(Ni2+)]allows the evaluation off,. The values obM are given in Table I. These values, tained a t CTAB together with the values of F defined by eq 3, where both emission from an aqueous solution F= (3) emission from a micellar solution emissions are determined at the same probe concentration, allow an evaluation of the distribution constant KM/W= (probe),icelles/(probe),,,, (4) since it has been shown12that KMjW = a[F/fw - l1
K
- 3620 -3210 - 2760 - 3900 -4170
NOH
KH,
- WOW,
cal/mol
0.052 0.28 0.90 3.01 1.70
1770 760 63 - 660 - 320
In K M ~ .
the probe on the micellar surface to its solubility.16 The results obtained for phenol and p-ethylphenol are similar to those reported by Bunton and Sepdlveda.' These authors reported values of Ks of 263 and 794 M-' for phenol and p-ethylphenol, respectively. The agreement between these values and those reported in Table I can be considered as satisfactory if the differences in experimental conditions are taken into account. The work of Bunton and Sepdveda' was carried out at rather low ionic strengths, and the micelles had only bromide ions as counterions. On the other hand, our results have been obtained at high ionic strengths and with micelles whose Stern layers comprised a mixture of bromide and sulfate ions." Both KMIW and KHIW are greater for p-ethylphenol than for phenol. A monotonic increase in KM w with the size of the alkyl groups in substituted pheno s has been previously reported by Sepdveda and Bunton.' These results also show that the increase in Ks per methylene group is less than that generally observed for the change in distribution constant. This result was explained by considering that the medium where the probes are located inside the micelles were far from hydrocarbonlike because of the penetration of water molecules. It is interesting to note that in the present work the same effect is observed in spite of the added salt.' On the other hand, the effect of increasing n from 0 to 3 upon KM when the methylene groups are introduced between the phenyl and hydroxyl groups is most unexpected and contrary to the normal trend observed for KHIw.From Table I it can be seen that KHlwalways increases when n increases, while KM,w goes through a minimum at n = 2. An explanation of this apparently anomalous effect is based on the fact that the methylene groups are introduced between two groups that are expected to be located on the micellar s ~ r f a c e . ' ~ J ~ When n is small, the methylene groups must be also located near the surface (a zone of high polarity) or they will push the phenyl group into the micellar core. Any one of these effects would increase the free energy relative to that expected if each group could be located in its optimal positions. This type of effect is obviously nonoperative when KH/Wis considered. Furthermore, when n is larger (i.e., n = 3) the aliphatic chain can be folded, leaving the extra methylene group inside the micelle while both the hydroxyl and phenyl groups remain at the micellar surface. In conclusion we consider that the anomalous trend observed can be explained in terms of the opposite tendencies of the two ends (that try to go to the surface) and the aliphatic chain (that pushes the probe toward the micellar core).
i
Acknowledgment. We thank Dr. Luis Sepdlveda for helpful discussions. References and Notes C. Bunton and L. Sepfilveda, J . Phys. Chem., 83, 680 (1979). (2) L. S. Romsted in "Micellization,Solubilizationand Microemulslons", Vol. 2, K. L. Mittal, Ed., Plenum Press, New York, 1977, p 409; M. Almgrem and R. Rydholm, J . Phys. Chem., 83, 360 (1979); C. A. (1)
2408
J. Pbys. Cbem. 1980, 8 4 , 2408-2412
Bunton, N. Carrasco, S. K. Huang, C. H. Paik, and L. S.Romsted, J . Am. Cbern. Soc., 100, 5420 (1978). (3) J. Gettings, D. Hall, P. L. Jobling, J. E. Rossing, and E. Wynjones, J. Cbem. SOC., Faraday Trans. 2 , 74, 1957 (1978). (4) R. R. Hautala, N. E. Schore, and N. J. Turro, J . Am. Cbern. Soc., 95, 5508 (1973). (5) F. Quina and V. Toscano, J. Pbys. Cbem., 81 1750 (1977). (6) M. Almgrem, F. Grieser, and J. K. Thomas, J. Am. Cbem. Soc., 101, 279 (1979). (7) M. Van Bockstaele, J. Gelan, H. Martens, J. Put, J. C. Dederen, N. Boens, and F. C. De Schrijver, Cbem. Pbys. Lett., 58, 211 (1978). (8) 0. S. Khalll and A. J. Sonnessa, Mol. Pbotocbem., 8, 399 (1977). (9) K. Kalyanasundararn, Cbem. SOC. Rev., 7, 453 (1978).
(10) P. Mukerjee and J. R. Cardinal, J . Pbys. Cbem., 82, 1620 (1978). (11) C. A. Bunton, F. Ramfirez, and L. SepGlveda, J . Org. Cbem., 43, 1166 (1978). (12) E. Lissi and E. Abuin, submitted for publlcatlon in J. Pbys. Cbem. (13) C. Tanford, "The Hydrophobic Effect", Wiley-Interscience, New York, 1973. (14) J. K. Thomas, F. Grieser, and M. Wong, Ber. Bunsenges. Pbys. Cbem., 82, 937 (1978). (15) A. Lo Surdo and H. E. Wirth, J. Pbys. Cbem., 76, 1333 (1972). (16) P. Mukerjee, J. R. Cardinal, and N. R. Desai in "Micellization, Solubilization and Microemulsion", Vol. 1, K. L. Mlttal, Ed., Plenum Press, New York, 1977, p 241. (17) M. Gratzel and K. Thomas, J. Am. Cbem. SOC.,95, 6885 (1973).
Transient Effects in Charge-Transfer Diffusion-Controlled Processes In Nonionic Micellest Shla M. B. Costa" and AntSnlo L. Maeanita Centro de Ou?mica Estrutural, Compiexo I, Instituto Superior Tgcnico, 1096 Lisboa Codex, Portugal (Received: July 23, 1979; In Flnal Form: April 30, 1980)
The quenching of the fluorescence of benzyl 9-anthroate (Bz-9-Ant),benzyl 1-pyrenoate (Bz-1-Pn),and benzyl 2-naphthoate (Bz-2-Np)by triethylamine (TEA) and dimethylaniline (DMA) was studied in nonionic micelles of Triton X 100. The determination of the equilibrium constants between the aqueous and micellar phase [K,= 0.8 (TEA) and K, = 30 (DMA)]allowed the evaluation of the effective quencher concentration. Fluorescence studies in steady-state and transient conditions showed the existence of transient effects which enabled calculation of reactional distances (R? and an effective diffusion coefficient (D'eff).The results are explained on the basis of a charge-transfer diffusion-controlled process consistent with the correlation of 12, and R' with parameters associated with charge-transfer phenomena.
Introduction In recent years fluorescence probes in micellar aggregates have been extensively employed, allowing studies on the behavior of excited molecules in different environments as well as the characterization of several properties of those micellar assemblies.1-2 These probes have been shown to be very useful in the estimation of the permeability of such aggregates to several species which can act as quenchers of fluorescent molecule^.^^^ In spite of the existing knowledge of qualitative aspects of the fluorescence quenching phenomena and dynamics of excited-state charge-transfer few quantitative reports have treated the special kinetic features in micellar aggregates.6-10 Recently a new model was proposed to explain quantitatively excimer formation and electron-transfer processes in micelles, taking into account the distribution of all the molecular species involved in the aqueous and micellar phases.11J2 This model, which assumes the occurrence of intramicellar events, describes the partition of the quencher molecules following a Poisson distribution into surfactant micelles and predicts a nonexponential decay of the fluorescence of the probe 'A* in the presence of a quencher (eq 1).12 In this equation, [lA*](t) = [lA*](O) exp{-t/rO + ii[exp(-k,t) - 11) (1) [lA*](O) and [lA*](t) are the concentration of excited species immediately after excitation and at time t , respectively; r0 is the lifetime of lA* in the absence of +Presented in p a r t a t the IXth International Conference on Photochemistry, 1978 (Abstract, J. Photochern., 9, 295 (1978)).
quencher; ii is the average number of quencher molecules per micelle; and k, is the intramicellar quenching rate constant. Appropriate fitting of this equation allows evaluation of ri and k,. However, this method is only applicable when the probe is long-lived (r0 > loT8s), a fact which constitutes a restriction to the choice of the probe. Indeed, it can be shown that k is limited to values of the order of 5 X lo6 s-l (because the high viscosities of the micellar media), since for shorter lifetimes [e~p(-k,t)]~~,,
