J. Phys. Chem 1984, 88, 81-85 n - h e ~ a n e . ~Hence, ~ this recombination cannot contribute significantly to the TEA fluorescence at longer times. We also note that energy transfer from excited n-hexane, formed by electronhole recombination, to TEA may be efficient, but the high rate of electron scavenging by PPO and the short alkane excited-state lifetimes25 dictates against this explanation. We seem, then, to have ruled out the most obvious sources for TEA fluorescence, and its origins are still puzzling. However, the finding that TEA excited states are produced in significant yields, along with the fact that TEA excited states can pump scintillator excited states by energy transfer, make this phenomenon quite important in light of the number of scintillator fluorescence studies that have utilized TEA as a cation scavenger, and we are presently continuing its investigation. Summary and Conclusion We have derived the EPR spectra of a series of trialkylaminium radicals from the FDMR response of the fluorescence produced from the recombination of these radical cations with scintillator radical anions in irradiated alkane solvents. These spectra exhibit a simple splitting pattern, due to the coincidence of nitrogen triplet splittings with @ proton splittings. This has allowed the qualitative kinetic resolution of the rate of solvent hole scavenging by TEA in n-hexane and cyclohexane, and has revealed similar scavenging rates in both solvents. This suggests that the formation of TEA’. occurs via scavenging of “trapped” cyclohexane holes with a diffusion-controlled rate. The highly mobile holes, which conductivity shows to be effectively eliminated by TEA, apparently do not react to form TEA+- with a detectable efficiency. The photophysical behavior of TEA was investigated in nhexane solutions with PPO. From FDMR results we conclude that the recombination of TEA+. with PPO-. leads to significant (24) Beck, G.; Thomas, J. K. J . Chem. Phys. 1972, 57, 3649. (25) Hirayama, F.; Lipsky, S. J . Chem. Phys. 1969, 51, 3616.
81
PPO fluorescence and a reduction in the total FDMR intensity beyond that expected by an inefficiency in fluorescence production in the TEA’. PPO-- recombination. We have suggested a proton transfer reaction between TEA and TEA’. to reconcile these observations. We also note a long-lived, relatively efficient TEA fluorescence which we could not account for by invoking either ionic recombinations or solvent to amine energy transfer as its source. Our investigations into the FDMR and fluorescence behavior of the irradiated alkane solutions of trialkylamines are presently continuing. The installation of a higher power microwave source in the near future will allow a moe quantitative determination of the rate of TEA’. appearance. We are also investigating the mechanism(s) of scintillator fluorescence quenching by TEA, and the source of TEA emission. It is clear that the charge scavenging and photophysical behavior of the trialkylamines, in alkane solutions, have proven more complex than one might have expected, and that FDMR is a useful probe of these processes.
Acknowledgment. We acknowledge Dr. Myran Sauer, Jr., for making available unpublished results and his enlightening discussions. We also thank Robert Lowers and Alan Young for technical assistance and operation of the Van de Graaff accelerator. This work was performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, U S . DOE under Contract W-31-109-ENG-38. Note Added in Proof. A recent publication from the Novosibirst group (Grigoryants, V. M.; Anisimov, 0. A.; Molin, Yu., N. J. Struct. Chem. (Engl. Transl.) 1982, 23, 4) using nontime-resolved FDMR, supports our assignment of the TEA’. ESR spectrum, though their assignment was based on somewhat less conclusive data. Registry No. PPO, 92-71-7; PO-., 51741-98-1; TEA, 121-44-8; TEA’., 83097-79-4; tripropylamine, 102-69-2;tributylamine, 102-82-9; trimethylamine, 75-50-3; n-hexane, 110-54-3;cyclohexane, 110-82-7.
Ion Exchange between Monovalent and Divalent Counterions in Cationic Micellar Solution E. A. Lissi,* E. B. Abuin, Departamento de Qulmica, Universidad de Santiago de Chile, Casilla 5659-Correo
2, Santiago, Chile
L. Sepiilveda, Departamento de Quimica, Universidad de Chile, Santiago, Chile
and F. H. Quina Instituto de Quimica, Universidade de Siio Paulo, Siio Paulo, Brazil (Received: February 28, 1982; In Final Form: June 6, 1983)
Ion exchange between divalent and monovalent anions in cationic (hexadecyltrimethylammonium) micellar solution is investigated by using two different experimental techniques: fluorescence quenching of micelle-solubilized probes and ultrafiltration. Competitive ion exchange at the micelle surface between the divalent thiosulfate ion and the monovalent counterions bromide, chloride, and nitrate can be adequately described over a wide range of experimental conditions and ionic compositions of the Stern layer by using a simple “two-compartment” (“bound”/“free”) pseudophase ion-exchange formalism which neglects both the micellar surface potential and the activity coefficients of the bound counterions. Data for sulfate/bromide exchange are, however, less satisfactorily rationalized by this model due to a change in relative binding affinities at higher coverages of the micelle surface by the sulfate ion.
Introduction The binding of counterions to micelles is a matter of current interest.’-” In particular, several recent works have been devoted (1) Quina, F. H.; Chaimovich, H. J . Phys. Chem. 1979, 83, 1844-50.
0022-3654/84/2088-008 1$01.50/0
either to the evaluation of the composition of the Stern layer as a function of the total concentration of counterions in the solution (2) Chaimovich, H.; Bonilha, J. B. S.; Politi, M. J.; Quina, F. H. J . Phys. Chem. 1979, 83, 1851-4.
0 1984 American Chemical Society
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The Journal of Physical Chemistry, Vol. 88, No. 1, 1984
or to quantitative interpretation of the behavior of micellar systems in the presence of two (or more) counter ion^.^-'^ Typically, the results have been interpreted in terms of a pseudophase ion-exchange formalism using systems in which the exchange involves only counterions of the same valence (either all monovalent or all divalent). Indeed, very few studies have been carried out employing ions of different valence over a wide range of experimental conditi~ns.~,*J~ The applicability of a simple pseudophase ion-exchange formalism to this latter type of system has been neither justified nor subjected to independent experimental verification. In the present work, we report a study of ion exchange between monovalent and divalent anions at a cationic micellar surface using two different experimental techniques, Le., fluorescence quenching'O and ultrafiltration.6 Data for a series of hexadecyltrimethylammonium salts determined over a wide range of counterion concentrations are employed to test the assumptions generally involved in the evaluation of "bound"- and "free"-counterion concentrations. Experimental Section Pyrene (Eastman Kodak) and biphenyl (Fluka) were recrystallized from ethanol. Perylene (Aldrich, Gold Label) was used as received. All inorganic salts were analytical grade (Merck) and all solutions were prepared in deionized, doubly distilled water. Hexadecyltrimethylammonium bromide, CTAB (Merck, p.a.), was used without further purification (cmc = 0.8 mM in water by surface tension). Hexadecyltrimethylammonium chloride, CTAC (Herga lndfistrias Quimicas, Rio de Janeiro), was purified as previously describedI3 (cmc = 1.3 m M in water by surface tension). Hexadecyltrimethylammonium sulfate, CTAS, was prepared from CTAB by exchange with solid Ag2S04in ethanol (30-min sonication) followed by recrystallization from acetonemethanol (conductimetric cmc = 0.22 m M in water). Hexadecyltrimethylammonium thiosulfate, CTAT, was prepared as previously described* (cmc = 0.13 mM in water from both fluorescence and conductivity measurements). Hexadecyltrimethylammonium nitrate, CTAN, was prepared from CTAB by exchange with AgNO, in ethanol; following removal of the ethanol under reduced pressure, the detergent was purified by three successive reprecipitations from acetone solution with dry ether, followed by drying under vacuum (conductimetric cmc = 0.8 mM in water). Ultrafiltration experiments were performed in an Aminco cell fitted with a PM-10 membrane. The experimental procedure was similar to that described previously.6 Fluorescence measurements were carried out on a Perkin-Elmer LS5 spectrofluorimeter, the experimental procedure being similar to that reported previously.lo All experiments were performed at 20 f 2 OC with air-equilibrated solutions.
(3) Quina, F. H.; Politi, M. J.; Cuccovia, I. M.; Baumgarten, E.; Martins-Franchetti, S. M.; Chaimovich, H. J . Phys. Chem. 1980, 84, 361-5. (4) Bartet, D.; Gamboa, C.; Sepiilveda, L. J. Phys. Chem. 1980,84,272-5. ( 5 ) Bunton, C. A,; Sepiilveda, L. J. Phys. Chem. 1979,83, 680-3. (6) Gamboa, C.; Sepiilveda, L.; Soto, R. J . Phys. Chem. 1981, 85, 1429-34. (7) Wolff, T.; Von Buenau, G. Ber. Bunsenges. Phys. Chem. 1982, 86, 225-8. (8) Cuccovia, I. M.; Aleixo, R. M. V.; Erismann, N. E., van der Zee, N. T. E.; Schreier, S.; Chaimovich, H. J . Am. Chem. SOC.1982, 104, 4554-6. (9) Abuin, E. B.; Lissi, E. A. J . Colloid Interface Sci. 1983, 93, 562-4. (10) Abuin, E.; Lissi, E.; Bianchi, N.; Miola, L.;Quina, F. H. submitted. (11) Almgren, M.; Rydholm, R. J . Phys. Chem. 1979,83, 360-4. (12) Ginani, M. F. Doctoral Thesis, Instituto de Quimica, Universidade de SHo Paulo, SHo Paulo, Brazil, 1982. Ginani, M. F.; Quina, F. H., in
preparation. (13) Bonilha, J. B. S.; Chiericato, G., Jr.; Martins-Franchetti, S. M. M.; Ribaldo, E. J.; Quina, F. H. J. Phys. Chem. 1982, 86, 4941-7. (14) Romsted, L. S. In 'Surfactants in Solution"; Lindman, B., Mittal, K.; Eds.; Plenum Press: New York, in press. (15) Schmehl, R. H.; Whitten, D. G. J . Am. Chem. SOC.1980, 102, 1938-41. (16) Ziemiecki, H.; Cherry, W. R. J. Am. Chem. SOC.1981,103,4479-83. (17) Toreman, T. K.; Sobol, W. M.; Whitten, D. G. J . Am. Chem. SOC. 1981, 103, 5333-6.
Lissi et al.
3
-. 2
1
lo
[Added Salt)
,
ImMl
20
Figure 1. Plot of the ratio of the fluorescence intensity of micelle-incorporated perylene vs. the total concentration of added salt: excitation wavelength, 400 nm; ( 0 )starting from CTAT (10 mM), sodium nitrate added; (A)starting from CTAN (10 mM), sodium thiosulfate added; (A) starting from C T A N (20 mM), sodium thiosulfate added.
Results and Discussion When at least one of the counterions present is a fluorescence quencher, ion-exchange selectivity coefficients can be obtained from fluorescence quenching experiments with micelle-incorporated probes.Io The quenching counterions employed in the present work were S2032-and Br-, with pyrene, biphenyl, and perylene as fluorescence probes. Nonquenching counterions included C1-, S042-,and NO3- and the divalent/monovalent exchanges investigated were S20>-/Br-, S2032-/Cl-, S203!-/NO
