Matched Spin Probes for the Study of the Overall Motion of Model Lubricants

✍ Scribed by M. M. Britton; S. A. Fawthrop; D. G. Gillies; L. H. Sutcliffe; X. Wu; A. I. Smirnov

• Publisher: John Wiley and Sons
• Year: 1997
• Tongue: English
• Weight: 433 KB
• Volume: 35
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/(sici)1097-458x(199708)35:8<493::aid-omr121>3.0.co;2-i

✦ Synopsis

For ESR studies of molecular rotation in liquids, it is desirable to match the spin probe as closely as possible to the solvent molecules. In this work the solvents were model lubricants whose overall motion has been characterized from carbon-13 NMR relaxation measurements. Two compounds were studied, the approximately cylindrical molecule 5-(n-octyl)naphthalene (ON) (a model low-traction Ñuid) and the approximately spherical molecule dicyclohexy-3,4-furan dicarboxylate (DCFDC) (a model high-traction Ñuid). For these two compounds the matched spin probes 5-(n-octyl)-1,1,13,3-tetrakis(trideuteriomethyl)isoindolin-2-yloxyl (OTMIOD) and 4,5bis(cyclohexyloxycarbonyl)-1,3,2-dithiazol-2-yl (BCCD), respectively, were prepared. ESR measurements were made on OTMIOD in ON and in toluene at two frequencies, namely 9 GHz (X band) and 94 GHz (W band), and the importance of making the high-frequency measurements was demonstrated for evaluation of the anisotropy parameters. The results are di †erent for the two solvents, thus proving the necessity for matching the spin probe to the solvent. BCCD was shown to be tumbling isotropically in DCFDC. Activation energies were derived for the overall motions from the ESR and NMR measurements. These activation energies were compared with those for the viscosities of the solvents. The viscosities of both ON and DCFDC were measured over a wide temperature range using an ESR method.