title: Electron paramagnetic resonance line shifts and line shape changes due to spin exchange of nitroxide free radicals in liquids. 7. Singly charged surfactant nitroxide.
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, <Be>, and dipolar, <Bdip>, 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<Be>/dc, is not linear with T/eta, where eta is the shear viscosity, instead following a quadratic dependence. Nevertheless, d<Be>/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<Be>/dc are decreased and d<Bdip>/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<Bdip>/dc, Omega(T), and tau(RE)(-1) are comparable for PDT and DCAT12 when compared at the same rotational diffusion rates.
authors.affiliation: Department of Chemistry, California State University at Northridge, Northridge, California 91330, USA. barney.bales@csun.edu
authors.forename: Barney L
authors.initials: BL
authors.lastname: Bales
authors.forename: Francis L
authors.initials: FL
authors.lastname: Harris
authors.forename: Mirna
authors.initials: M
authors.lastname: Peric
authors.forename: Miroslav
authors.initials: M
authors.lastname: Peric
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals / Q000737:chemistry; D008823:Micelles; D009589:Nitrogen Oxides / Q000737:chemistry*; D010880:Piperidines / Q000737:chemistry*; D011522:Protons; D000644:Quaternary Ammonium Compounds / Q000737:chemistry*; D013501:Surface-Active Agents / Q000737:chemistry*; D013696:Temperature; D014867:Water / Q000737:chemistry
chemical_list: C545386:4-(N,N-dimethyl-N-(ndodecyl) ammonium)-2,2,6,6-tetramethylpiperidinyl-N-oxy bromide; D005609:Free Radicals; D008823:Micelles; D009589:Nitrogen Oxides; D010880:Piperidines; D011522:Protons; D000644:Quaternary Ammonium Compounds; D013501:Surface-Active Agents; D014867:Water; C039900:nitroxyl