Electron Paramagnetic Resonance Line Shifts and Line Shape Changes Due to Spin Exchange of Nitroxide Free Radicals in Liquids. 7. Singly Charged Surfactant Nitroxide

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Abstract

EPR spectra of aqueous solutions of the singly charged surfactant nitroxide 4-[N,N-dimethyl-N-(ndodecyl) ammonium]-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide-d16 (DCAT12) are studied as functions of the molar concentration, c = 0.1-8 mM, and the temperature from 273 to 353 K. This concentration range is below the critical micelle concentration, cmc, at which DCAT12 forms micelles. Spin-spin broadening of the EPR lines averaged over the three lines is separated into contributions due to spin exchange, , and dipolar, , interactions yielding values of the fractional broadening by spin exchange, Omega(T), that vary from near unity at 353 K to approximately 50% at 273 K. This compares with a variation from unity to approximately 77% for a neutral spin probe perdeuterated 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (PDT) over the same range. Unlike PDT and the Stokes-Einstein prediction, the broadening constant by spin exchange, d/dc, is not linear with T/eta, where eta is the shear viscosity, instead following a quadratic dependence. Nevertheless, d/dc is remarkably close to a hydrodynamic prediction using the Stokes-Einstein equation modified to take the spin probe charge into account. Compared with PDT, values of d/dc are decreased and d/dc increased at all temperatures, while the values of the re-encounter rate, tau(RE)(-1), deduced from line shifts, are reduced. Interestingly, values of d/dc, Omega(T), and tau(RE)(-1) are comparable for PDT and DCAT12 when compared at the same rotational diffusion rates.

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