557
J. Phys. Chem. 1980, 8 4 , 557-558
(4) Bielski, B. 11.J.; Shiue, G. G. In “Oxygen Free Radicals and Tissue Damage”; Ciba Foundation Series 65 (new series); Elsevier: North-Holland, 1979; p 43. (5) Holroyd, R. A; Bielski, B. H. J. J. Am. Chem. SOC.1978, 100, 5796. (6) Kolthoff, I.hi.: Belcher, R. “Volumetric Anabsis”; Vol. 111; Interscience: New York, 1957; pp 262-268. (7) Bielski, B. 1.1. J. Phofochem. Photobiol. 1978, 28, 645. (8) Buxton, G. V.; Subhani, M. S. J. Chem. SOC.,Faraday Trans. 1 1972, 68, 947. (9) Latlmer. W. M. “Oxidation Potentials”; Prentice-Hall: New York, 1952; p 55. (10) Long, C. A,; Bielski, B. H. J., to be submitted for publication. (11) Merkel, P. El.; Kearns, D. R. J . Am. Chem. SOC.1972, 94, 7244. Charles A. Long Benon H. J. Bislskl”
Chemistry Depaifment Brookhaven National Laboratory Upton, New Yorlc 11973 Received October 4, 1979
Spin Trapping with Covalently Immobilized a-Phenyl-N-[( l-hydroxy-2-methyl)-2-propyl] Nitrone Publication costs assisted by the Natural and Engineering Science COUnCl of Canada
Sir: Recently Janzen and Wangl demonstrated the feasibility of using an immobilized nitrone for spin trapping experiments. In that work a film of poly[a(N-tert-butylnitrony1)lsty~ene(poly-PBN) was exposed to an aqueous solution of a radical source and then dissolved, and the resulting homogeneous solution was investigated by ESR. Recent use of cyanuric chloride (CC) as a linking agent,2-4 together with tbe availability of a-phenyl-N-[(l-hydroxy2-methyl)-2-propyl] nitrone (HOPBN),5,6suggested the possibility of covalently attaching this nitrone spin trap to silaceous surfaces. Thus controlled-pore glass beads7 were first treated with concentrated IlCl to enhance surface hydroxyl functionality (1h, washed with water, acetone), vacuum dried (150 “C, 4 h), and then exposed to CC in benzene (5% of dry weight, reflux 2.5 h) in order to derivatize the surface: CI
CI
The CC derivatized beads* were extracted with benzene (Soxhlet, 3 h) ‘to remove unreacted CC, then allowed to react with HOPBN (5% of dry bead weight, reflux in benzene 2.5 h), and finally extracted with benzene to remove unreacted HOPBN (Soxhlet, 24 h). The following reaction is assumed: -5
CH3
I X = C1, OH, or nitrone 0022-3654/80/2084-0557$01 .OO/O
I
C
r
10 G 7
p\\ Ill
u
l
\
r
Flgure 1. ESR spectra obtained by contacting benzene solutions of
a phenyl radical source (PAT) with HOPBN which had been covalently linked through cyanuric chloride (CC) to glass beads. (A) 5% CC-5% HOPBN loading exposed to 1 mM PAT for 21 h; (B) 5 % CC-5 % HOPBN loading exposed to 6 mM PAT for 20 h; (C) 30% CC-30% HOPBN loading exposed to 6 mM PAT for 8.5 h. See text for details of glass bead loading.
I did not give an ESR spectrum when suspended in nitrogen-purged benzene. The ability of I to function as a spin trap was evaluated by reaction with phenyl radicals produced from the room temperature thermal decomposition of phenylazotriphenylmethane (PAT). A dispersion of I in nitrogenpurged benzeneg containing 1.0 mM PAT gave rise to an ESR spectrum consisting of six sharp lines which slowly increased in intensity over a period of 48 h. The spectrum obtained after 21 h is shown in Figure 1A: a N = 14.56, upH = 2.36 G. This spectrum is assigned to the phenyl adduct of I on the basis of the agreement between these coupling constants and those observed in homogeneous solution: aN = 14.54, upH = 2.40 G (benzene). A minor component consisting of a triplet of doublets having slightly larger @-hydrogensplitting is also seen but cannot be assigned. ESR examination of the supernatant solution revealed no detectable nitroxyl radical signal indicating that the spectrum shown in Figure 1A was indeed derived from attached nitroxyls only. Elimination of either CC or HOPBN from the reaction sequence afforded systems which did not trap phenyl radicals.1° After a 48-h exposure to this solution of phenyl radical source, an additional weak, broad resonance is observed. This signal becomes the dominant feature of the spectrum when I is exposed to a higher concentration of phenyl radical source (6 mM) for 20 h (Figure 1B). This spectrum is clearly due to both “mobile” and “highly immobilized” nitroxyl radicals.ll With higher loading of the beads (30% CC-30% HOPBN of dry bead weight) exposure to 6 mM PAT gave rise to only the highly immobilized type of nitroxyl spectrum (Figure IC). 0 1980 American Chemical Society
558
J. Phys. Chem. 1980, 8 4 , 558-559
In summary, these findings indicate that nitrones like HOPBN can be covalently attached to silaceous surfaces through the CC linking agent. The immobilized nitrones are effective traps for short-lived free radicals in solution. Moreover, the resulting spin adducts can afford well-resolved ESR spectra while attached to the silaceous surface. That the spin adducts of these immobilized nitrones can give rise to relatively sharp-lined spectra indicates that these nitroxyl radicals are free to rotate about an axis which is perpendicular to the plane containing the nitroxyl z axis.12
Aclznowledgment. This work was supported in part by grants from the Natural and Engineering Science Council of Canada and NATO (Grant No. 1578). The experimental assistance of R. L. Dudley and stimulating discussions with C. A. Fyfe are gratefully acknowledged. References and Notes Janzen, E. G.;Wang, Y. Y. J. Phys. Chem. 1979, 83, 894-896. Dautartas, M. F.; Evans, J. F.; Kuwana, T. Anal. Chem. 1979, 57, 104-110. Yacynych, A. M.; Kuwana, T. Anal. Chem. 1978, 50, 640-645. Lin, A. W. C.; Yeh, P.; Yacynych, A. M.; Kuwana, T. J. Electroanal. Chem. 1977, 84, 411-419. Janzen, E. G.;Zawalski, R. C. J. Org. Chem. 1978, 43, 1900-1903. Janzen, E. G.;Dudley, R. L. Chemical Institute of Canada-American Chemical Society Joint Meeting, Montreal, Quebec, May 1977, Abstracts of Papers ORG-5 1. Nominal characteristics of these beads (CPG10, ElectroNucleonics, Inc., Fairfield, NJ) were as follows: mean bead diameter = 150 pm, mean pore diameter = 104 nm, pore volume = 1.16 mL g-I, total surface area = 29 m2 g-'. It is not known whether cross linking of CC occurs on the silica surface or whether some dimeric or telemeric CC forms as a result of hydrolysis by trace water in the benzene solvent. CC, HOPBN, and the phenyl adduct of HOPBN are all benzene soluble. Thus, the role of benzene here is both to provide a solvent for PAT and to solvate the attached spin trap and the resulting spin adduct. Under conditions of prolonged reaction in refluxing benzene, trace amounts of spin trap active HOPBN have been found to be retained on CPGlO which had not been previously exposed to CC. However, the quantity of retained nitrone is minuscule relative to that immobilized when the silaceous surface is first allowed to react with CC. The mode of retention of this trace amount of HOPBN (e.g., adsorption, entrapment in the porous structure, covalent attachment to the silaceous surface) is presently unknown. Although all observed resonances arise from nitroxyl radicals which are covalently attached to the silaceous surface, the terms "mobile" and "highly immobilized" refer to species which afford isotropic and anisotropic ESR spectra, respectively. See,for example, Griffith, 0. H.; Jost, P. C. in "Spin Labeling"; Berliner, L. G.,Ed.; Academic Press: New York, 1976; pp 466-467. Eric E. Bancroft Henry N. Blount'
Brown Chemical Laboratory The University o f Delaware Newark, Delaware 19711 Department o f Chemistry Guelph Waterloo Centre for Graduate Work in Chemistry University o f Guelph Guelph, Ontario, N I G 2 W 1 Canada
Edward G. Janzen"
Received October 15, 1979
Further Evidence for a Maximum in the Critical Micelle Concentration vs. Pressure Publication costs assisted by Fukuoka University
Sir: It has been reported by various authors'-6 that the critical micelle concentration (cmc) vs. pressure curve for ionic surfactants in aqueous solutions exhibits a maximum at a certain pressure, usually near 100 MPa. All methods used for determining the cmc under pressure have been based on electroconductivity measurements, in which a 0022-3654/80/2084-0558$0 1.OO/O
I
I
Y
11 01 0,l
,
102
,
,
200
PRESSLRE / \!Pa
Figure 1. The cmc of dodecylpyridinium bromide as a function of pressure at 303 K: (0)optical method; (0)electroconductivity method.
specific conductance vs. concentration plot shows a break at the cmc. Recently, Rodriguez and Offen7 insisted on the absence of the maximum in the cmc vs. pressure plot. They determined the differential absorbance of naphthalene AA(Amax-275 nm) as a function of pressure in two states: in saturated solution in a solubilizing system of sodium dodecyl sulfate (SDS), and in saturated solution in water. They adopted the concentration extrapolated to AA = 0 as the cmc at the particular pressure. Their cmc vs. pressure plot has no maximum but is monotonically increasing. They suggested that, since the behavior of an optical probe such as naphthalene is, in contrast to the conductivity, independent of hydrodynamic characteristics, it might be expected to offer a more reliable means of determining the cmc. We believe that it is impossible to attribute the origin of breaks in the plots of specific conductance vs. concentration to any source other than micellization, even in the presence of complicating factors arising at higher pressures. Furthermore, the volume change accompanying micelle formation, AT,,, of sodium decyl sulfate, which was determined by compressibility measurements, possessed a positive sign a t atmospheric pressure and decreased with pressure, becoming negative above approximately 100 MPa.8 This measured value of AT,,, agreed with that calculated by the following equation according to the pseudo-phase separation model for micelle formation: AVm = (1 + fi)RT(aIn cmc/aP),, where P refers to the effective degree of counterion binding by the m i ~ e l l e .In ~ other words, this agreement provides corroborating evidence that the cmc exhibits a maximum at the pressure where AVm = 0, near 100 MPa since fi -0.7 > 0. We suggest that the cmcs determined by Rodriguez and Offen is that for the formation of mixed SDS-naphthalene micelles, and thus related to the effect of pressure on the solubilization of naphthalene. In this report, we present further evidence for the cmc-pressure maximum by means of the optical method used by Rodriguez and Offen. When the cationic surfactant dodecylpyridinium bromide (DPB) associates to form micelles in water, a charge transfer (CT) band (Arna -290 nm) emerges and its absorbance increases linearly with concentration above the cmc as a result of the CT interaction of concentrated bromide anion and pyridinium cation on the micelle s u r f a ~ e . The ~ quartz cell with a rubber bellows containing an aqueous solution of DPB was placed in the pressure vessel and water was used as a pressure-transmitting fluid. With this arrangement, the absorbance of the CT band at 290 nm was measured under pressure. At high pressures, the absorbance of the CT band increased linearly with concentration above a critical
0 1980 American
Chemical Society
