Exchange Processes in Solutions of Nitroxide Surfact ants BY KATHARINE K. Fox Unilever Research, Port Sunlight Laboratory, Port Sunlight, Wirral, Merseyside L62 4XN Received 5th December, 1975 The temperature-dependent broadening of the electron paramagnetic resonance spectra observed in aqueous solutions of the paramagnetic surfactants 2,2,6,6-tetramethylpiperidine-oxidedecyldi- methylammonium bromide, 2,2,6,6-tetramethyIpiperidine-oxidedodecyldimethyla~onium bromide and 2,2,6,6-tetramethylpiperidine-oxidetetradecyldimethylammo~~ bromide is not caused by monomer-micelle exchange in those solutions. It is probably due to the different effectiveness of monomers and micelles in causing Heisenberg spin exchange, complicated by the temperature variation of the c.m.c. and by possible dimerization of the nitroxide head group.In a previous paper the electron paramagnetic resonance (e.p.r.) spectrum of the paramagnetic surfactant I (n = 11) was shown to have CH3 CH3 I / + I , X = O Br- 11, X = H n = 9, 11, 13 n NX / L/ I\ ' I '\ CH3 I '\ CH, CH3 temperature-dependent line broadening above the critical micelle concentration (c.m.c.) which could not be attributed to Heisenberg spin exchange. This increased line broadening was attributed to surfactant exchange between monomer and micellar environments. Further work described here 2 * indicates that this initial attribution is incorrect, and that other processes are responsible for the observed line width changes. The cationic nitroxides I (n = 9, 11 and 13) were prepared using slight modifica- tions of the procedure described previously,l* followed by recrystallisation from water.The c.m.c. values of the surfactants were determined by conductivity. The number of unpaired electrons per molecule was determined by comparing the e.p.r. spectra of the solid state nitroxide surfactants with the spectra of freshly grown CuS04. 5H20 crystal^,^ with spectral intensities determined by a nomogram method.5 The results are shown in table 1. The e.p.r. spectra were recorded with a Varian E-4 spectrometer fitted with a variable temperature control unit. Temperatures were measured to 0.1 K relative, kO.1 I< absolute, using a copper-constantan thermopile placed in the centre of the e.p.r. cavity before and after each run. Microwave power was kept at 5 mW to 220K . K .FOX 22 1 avoid saturation effects, and themodulation amplitude was kept at < 1/10 of the linewidth to avoid modulation broadening. The solutions were deoxygenated before the spectra were recorded. In the experiments described here, micellization was induced in the paramagnetic surfactant systems I (n = 9, 11, 13) not by increasing the concentration of I, but by adding enough of the appropriate diamagnetic surfactant I1 (n = 9, 11 , 13) to take the resultant system above its c.m.c. This method was adopted in order that the non-micellar system containing only I could serve as an upper limit for the Heisenberg spin exchange contribution in the I +I1 system, instead of as a lower limit as when micellisation was induced by increasing the concentration of I.The frequency with which monomeric I enters a micelle should be similar in both cases. TABLE 1 .-PROPERTIES OF NITROXIDE SURFACTANTS spins per c.m.c. X lO3lmol dm-3 compound molecule 298 K 313 K 333 K I ( n = 9) 1.00+0.02 41 51 42 f 1 43 f 1 I (n = 11) 0.9.5_+0.02 8.7f0.2 9.3f0.2 9.1f0.2 I(IZ = 13) l.OOf0.02 2.120.1 2.1f0.1 2.6k0.1 For the n = 9 compounds, two solutions each 1.8 x mol dm-3 in I(n = 9) were prepared. One solution was also 5.8 x mol dm-3 in II(n = 9), which increased the total surfactant concentration to above the combined c.m.c. for the surfactant mixture. Spectra of both solutions obtained at 349.8 K consisted of three broad lines. In the sample containing II(n = 9) the m, = 0 line was more intense relative to the mi = + 1 lines than the mi = 0 line in the other sample, a result which is consistent with the presence of an underlying micellar peak in the first sample.This effect, although diminished in magnitude, was still present at 327.9 K. Thus the sample containing II(n = 9) had micelles at both temperatures studied, although the micellar signal was narrower at the higher temperature, due to increased Heisenberg spin exchange between the surfactant molecules in the micelle. TABLE 2.-LINEWIDTH OF n = 11 SAMPLES linewidth/mT sample 318K 328K 337K 346K 357 K 366 K 7x mol dm-3 I (n = 11) 0.170 0.176 0.188 0.197 0.211 0.223 7x mol dm-3 I (n = 11)+ 8 x mol dm-3 0.168 0.174 0.182 0.193 0.210 0.226 11 (n = 11) At 327.9 K the MI = 0 line of the system which contained micelles was narrower (0.298 0.005 mT) than that of the non-micellar system (0.365 f 0.005 mT).The same pattern held at 349.8 K (0.398k0.005 mT against 0.465f0.005 mT). Thus the major effect of creating micelles is not the introduction of monomer-micelle exchange, but the removal of paramagnetic monomers from the solution with consequent reduction in Heisenberg spin exchange. It is possible to study I(n = 11) at lower nitroxide concentrations, where Heisenberg spin exchange between monomers should have a smaller effect on the observed linewidths. A solution containing 7 x mol d ~ n - ~ I(n = 11) has been compared with a solution containing 7 x mol mol dm-3 I(n = 11) and 8 x222 EXCHANGE I N NITROXIDE SURFACTANTS dm-3 II(n = 11). The e.p.r. spectra of the latter sample show one broad micellar peak at temperatures ranging between 317 and 366 K, as well as the usual three-lined monomer spectrum.The width of the MI = 0 line in each system is shown in table 2. The linewidths in the two systems are similar, with the width in the non-micellar system exceeding the width in the micellar system by - 3 times the experimental error in the region around 337 K. The major effect of the addition of micelles to this system is also the removal of paramagnetic monomers from solution, and the concomitant reduction in Heisenberg spin exchange. A similar experiment was performed for the n = 13 compounds. A solution containing 2 x mol dm-3 II(n = 13) at 343 K contained micelles whose e.p.r. spectrum consisted of one broad, exchange-narrowing line. Superimposed upon this were three monomer hyperfine structure lines, each of which exhibited proton hyperfine structure.At 298 K the micellar spectrum of this solution consisted of three broadened lines, since intra-micellar exchange was no longer great enough to cause exchange-narrowing. The three monomer lines were also present, with decreased proton hyperfine structure resolution. Monomer- micelle exchange in the solution at 298 K would cause less broadening of the monomer lines than exchange of the same magnitude in the 343 K solution, due to the different micellar signals.6 The observed effect of increased proton hyperfine structure resolution is due to the increased tumbling frequency of the monomers at the higher temperature, and the consequent increased averaging of the g- and a-tensor aniso- tropies.Monomer-micelle exchange is not fast enough to overcome this effect, and thus is <5 x lo4 s-I in this particular system. The examples given above show that this e.p.r. method cannot detect monomer- micelle exchange in the I(n = 9, 11, 13) systems when micellisation is induced by adding the diamagnetic surfactant II(n = 9, 11, 13). The effect on the linewidths previously attributed to monomer-micelle exchange is probably due to the different effectiveness of monomers and micelles in causing Heisenberg spin exchange, compli- cated by the temperature variation of the c.m.c. Dimerization of the nitroxide headgroup 2* mol dm-3 I(n = 13) and 8 x may also contribute to the observed linewidths. I would like to thank Prof. M. C . R. Symons and Mr. J. Clifford for helpful discussions during the course of this work. K. K. Fox, Trans. Faraday SOC., 1971, 67, 2802. A preliminary account of this work is published in, Chemical and Biological Applications of Relaxation Spectrometry, ed. E. Wyn-Jones (Proc. NATO Advanced Study Institute, University of Salford, 29 Aug.-12 Sept. 1974), p. 215. K. K. Fox, Ph.D. Thesis (University of Leicester, 1974). P. B. Ayscough, Electron Spin Resonance in Chemistry (Methuen, London, 1967), chap. 1. V. A. Tolkacher and A. I. Mikhailov, Probor. Tekhn. Eksper, 1963, 9, 95. J. R. Zimmerman and W. E. Brittin, J. Phys. Chem., 1957, 61, 1328. (PAPER 5/2367)