Influence of solvent chemistry on 1H NMR spectral and relaxation properties of a long-chain ionic surfactant in chloroform-d
1H NMR chemical shift, line width, indirect nuclear splitting value, peak area integration value, and spin–lattice and spin–spin relaxation times at 298 K are compared for low-concentration isotropic solutions of n-octylammonium n-octadecanoate prepared via different techniques and conditions using dried, distilled, and degassed deuterochloroform and the nontreated solvent containing tetramethylsilane. The nature of the variation of observed spectral parameters and relaxation/rotational behavior with chemical composition (presence of oxygen and other paramagnetic species, stabilizer, impurities, and degradation products) of the solvent, history of the solution, and sample containment are analyzed. Relaxation times are interpreted in terms of monomer structure and reorientation and internal rotation modes as a function of atomic position along the n-alkyl chains. Collectively, the relaxation behavior of the surfactant complies with the two-step model of fast picosecond internal rotations of different size segments containing methylene groups separated in timescale from slower large segment and overall molecular tumbling modes of the monomer. Fast motional phenomena do not appear to be appreciably influenced by the chemistry of the solvent in contrast to spectral parameters such as chemical shift and line width of the labile ammonium protons. A model is also presented to explain anomalous variation of the peak area integration value with chemical shift of the ammonium resonance peak.
Keywordsn-Octylammonium n-octadecanoate surfactant 1H NMR spin relaxation NMR spectral properties Reverse micelle Deuterochloroform chemistry
We wish to express our grateful appreciation to Professor Dr. Janez Scancar (Department of Environmental Sciences, Jožef Stefan Institute) for the preparation and analysis of a sample of deuterochloroform by ICP-MS spectrometry. M.A. Desando wishes to thank Dr. Stanley Walker (Professor Emeritus, Department of Chemistry, Lakehead University, Thunder Bay, Canada) for the permission to use for further study a sample of surfactant synthesized by the author.
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