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pages). Ordering information is given on any current masthead page.
References and Notes (1) For previous articles in this series, see (a) N. A. Marron and J. E. Gano, Synth. Commun. 515 (1977): (b) N. A. Marron and J. E. Gano. J. Am. Chem. SOC.,98,4653 (1976). Taken in part from the Ph.D. Dissertation of D. H.-T. Chien. (2) (a) R. Givens and N. Levi in "Chemistry of Carboxylic Acids and Esters", Suppl. B. S. Patai, Ed., Wiley-Interscience, New York, 1979. (b) J. G. Calvert and J. N. Pitts, Jr. "Photochemistry", Wiley, New York, 1966. (c) Some very useful gas-phase results have been reported. See A. A. Scala, J. P. Colangelo, G. E. Hussey, and U. T. Stolle, J. Am. Chem. SOC.,96, 4069 (1974). (3) (a) J. E. Blackwood. C. L. Gladys, K. L. Loening, A. E. Petrarca, and J. E. Rush, J. Am..Chem. Soc., 90,509 (1968); (b) W. S. Johnson, A. vander Gen, and J. J. Swoboda, ibid., 89, 170 (1967); (c) R. B. Bates and D. M. Gale, ibid., 89, 5749 (1967); (d) S. F. Brady, M. A. Ikon, and W. S. Johnson, ibid., 90, 2882 (1968). (4) Trifluoroacetates are known to pyrolyze more readily than analogous acetates: J. E. Gano, unpublished results. (5) (a) J. C. Dalton and N. J. Turro, Mol. Photochem., 2, 133 (1970); (b) ibid., 2, 353 (1970). (6) (a) J. E. Gano, Mol. Photochem., 4, 527 (1972). (b) For a discussion of the sensitivity of such plots to the derived molecular parameters, see J. E. Gano and N. A. Marron, ibid.. 8. 141 (1977). (c) The derivation of these equations is provided as supplementary material. (7) (a) L. W. Johnson, H. J. Maria, and S. P. McGlynn, J. Chem. Phys., 54,3823 (1971). fb) J. G. Pacifici and J. A. Hvatt. Mol. Photochem., 3, 267 (1971). (c) The quenching rates in dodecaie (0.62 X 1O1O M-') and p e n t a n e u X 10'O M-' s-') were estimated from Table II in P. J. Wagner and I. Kochevar, J. Am. Chem. Soc., 90,2232 (1968). The dodecane value is from interpolation between decane and hexadecane. (6) (a) J. G. Gjoldback, Acta Chem. Scand., 6, 623 (1952): (b) ibid., 127
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(1963). (9) I. B. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecules", Academic Press, New York, 1965. (10) Since this assumption may give quenching rates which are too high (see, e.g., B. M. Monroe and R. P. Groff, Tetrahedron Lett., 3955 (1973)), the values for the singlet lifetimes are minimum estimates. (11) (a) L. M. Stephenson, P. R. Cavigli, and J. L. Parlett, J. Am. Chem. SOC., 93, 1985 (1971). (b) Private communication with Stephenson et al. indicated that their GLC and NMR analyses revealed no additional peak(s) attributable to the product from primary hydrogen abstaction. However, it must be nQted that they did not specifically search for this product. Consequently it was conservatively estimated that a 17% yield of this material would have easily been observed and the ketone regioselectivity limit of >50 (3 X 17) was estimated. The small fraction of singlet state reaction from ketone 6a precluded any estimate of its regioselectivity. (12) P. Ausloos and R. E. Rebbert, J. Phys. Chem., 67, 163 (1963), and references cited therein. (13) The ketone synthetic scheme seems unambiguous but the possibility of a mixup in the data has not been discounted. (14) In fact, this would not be the first report of such an observation. See N. C. Yang, M. H. Hui, and S. A. Beilard, J. Am. Chem.'Soc., 93, 4056 (1971). (15) The wording here should be clearly noted. The regioselectivity may or may not be closely related to (or identical with) the ratio of rates of hydrogenatom abstraction depending upon the reversibility of this step. (16) (a) J. A. Barltrope and J. D. Coyle, Tetrahedron Lett., 3235 (1968); (b) N. C. Yang, S. P. Elliot, and B. Fin, J. Am. Chem. SOC.,91, 7551 (1969). (17) E. L. Eliel, "Stereochemistry of Carbon Compounds", McGraw-Hill, New York, 1962, p 236. (18) (a) H. C. Brown, Org. React., 13, 30 (1963); (b) R. J. W. LeFevre and A. Sundaran. J. Chem. Soc.,' 3904 (1962); (c) F. J. Welcher. Stand. Methods Chem. Anal., 3, 242 (1966). (19) (a) A. G. Messner, D. M. Rosie, and P. A. Argabricht, Anal. Chem., 31,230 (1959); (b) "Instruments and Accessories", Varian Aerograph, Walnut Creek, Calif., 1971, p 30. (20) (a) G. M. C. Higgins, B. Saville, and M. 8. Evans, J. Chem. SOC.,702 (1965): (b) A.P.I. Research Project No. 44, 1816, April 1956.
Photochemical and Photophysical Studies of Organized Assemblies. Interaction of Oils, Long-chain Alcohols, and Surfactants Forming Microemulsions Mats Almgren, Franz Grieser, and J. K. Thomas* Contribution from the Department of Chemistry, L'nireryitj. of ,Votre Dame, Notre Dame, Indiana 46.556. Received August 8. 1979
Abstract: The conditions necessary for forming a microemulsion system with sodium lauryl sulfate, pentanol, dodecane. and water have been established. This system was then used to influence photophysical reactions o f molecules solubili7ed in the microemulsion aggregates. The sizes of the microemulsion aggregates were also determined bq a photophysical method and bq utilizing a Poisson distribution of reactants i n the aggregates. Comments on the nature of t h e aggregate were obtained from the fluorescence spectra of pyrene carboxaldehyde which resides in the surface and pyrene which resides in the microemulsion interior. I t was concluded from studies with t h e latter probe t h a t pyrene samples a large fraction of t h e microemulsion interior during the measurements. Photoinduced reactions of excited pyrene a n d pyrene butyrate u i t h thallous ions bound to the microemulsions surface indicated that pyrene penetrates further into the aggregate than pyrene butyratc. unlike similar experiments in micelles. The local oxygen solubility in the microemulsion Mas much higher t h a n t h a t in Lvater. The results arc discussed in terms of the increased utility of microemulsions over micelles w i t h regard to promotion of certain photochemical reactions.
Introduction There are many examples of catalysis of reactions by simple micellar systems, some 1000-fold increase in rate resulting in some systems,'-4 and it is customary to compare these systems to enzymes. Although the precise nature of micellar catalysis is uncertain, it may be at least expected that micelles effectively crowd together reaction partners, by micelle-solute interaction. This is much akin to increasing the local concentration of reactants, but, although effective in practice, the rate enhancements obtained by sole consideration of this mechanism are not always sufficient to explain the observed catalytic efficiencies. Of consequence is the suggestion' that the ionic nature of the micellar surface influences the transition state 0002-7863/80/ 1502-3 188$01.OO/O
of the reaction either adversely or positively. By analogy with the above thermal systems it has been possible to design micellar systems that show significant effects on radiation-induced reactions, both photochemical and r a d i ~ l y t i c .These ~.~ systems are understood in terms of electrostatic influence of the micellar surface on the ionic nature of the reaction^.^.' Reactions are both promoted and inhibited by the correct choice of micellar structure. It is desirable at this stage to vary the parameters of the micelle as much as possible. In particular micellar size, which controls the separation of reactants, is of prime importance. One method of achieving this cffect is via microemulsions. Microemulsions are reminiscent of micelles but provide two additional unique features: (a) the possibility of using larger 1980 American Chemical Society
Almgren, Grieser, Thomas
1 Interaction of Oils, Long-chain Alcohols, and Surfactants
structures (micelles have radii m20 8, i n comparison to >IO0 A for microemulsions) allowing a greater variation in the separation of reactants, and (b) the provision of a large oil drop center for locating hydrophobic molecules. It may be noted that microemulsions are far better vehicles for solubilizing hydrophobic molecules than their smaller micellar counterparts. In some instances the dimension of a hydrophobic molecule of interest (e.g., chlorophyll) may be comparable to that of the micelle. This is rectified in the case of a microemulsion. The classical picture of a microemulsion derives much of its justification from the early work of Schulman and c o - w ~ r k e r s , ~ in particular their electron-microscope pictures of large, spherical microemulsion droplets of radii 200-500 8,. There have recently been several authoritative reviews of the nature of microemulsions;10-12some work has also been published regarding the influence of these species on chemical and photochemical reaction^.^^^'^ It is suggested that the cosurfactant (long-chain alcohol) interacts with the charged head groups of the surfactant monomers, increasing their separation and leading to a larger aggregated structure. The oil provides the stability for the larger structure by decreasing the curvature of the assembly. Light-scattering datal5 and electron microscopy indicate large (> 100 A) spherical microemulsion aggregates. There has been some modification of such a simple picture. Shah and co-workersI6 suggest that the systems formed using pentanol as cosurfactant should be looked upon as molecular solutions where the components of the system are cosolubilized. However, with hexanol true microemulsions are formed. Friberg suggests that some microemulsion systems might be looked upon as reversed micelles,I0 and Adamson’’ suggests the usage of “swollen micelles” rather than microemulsions. It is apparent that reliable experimental conditions for the formation of various microemulsion systems are available in the literature. The thermodynamic factors controlling the systems are also understood with some discussion still taking place. However, further microscopic details of the systems would be desirable: a second goal is to utilize microemulsions to control certain features of photochemical reactions that have already been studied in micellar systems. Experiments described in this paper reflect both on the nature and surroundings of probe molecules dissolved in microemulsions and on the effect of these systems on the photophysical properties of the probes.
Experimental Section Materials. Sodium lauryl sulfate (NaLS) was BDH specially pure grade, and was used as supplied. T h e cmc was measured as 8 m M , in good agreement with the literature. Related experiments with this preparation and one that had been doubly recrystallized from ethanol/water showed essentially the same results. 1 -Pentanol (9970,Aldrich) was redistilled after treatment with H z S 0 4 and 2,4-dinitrophenylhydrazine. n-Dodecane was Phillips research grade, 99.7 mol Go. Water was quadruply distilled. Pyrene and its derivatives were purified either by recrystallization from ethanol or by T L C . TINO? was Ventron ultrapure (99.9%). Oxygen was removed from aqueous solutions by bubbling with oxygen-free nitrogen (
