The use of 13C spin relaxation to investigate molecular motion in liquid tristearin

Abstract

Longitudinal and transverse 13C spin relaxation times have been used to investigate the molecular motion of tristearin over a range of temperatures in the melt. Overall molecular rotational diffusion rates have been obtained as well as the diffusion rates about successive bonds in the stearoyl chains. The data can be explained using an anisotropic rotor model in which the fast and slow molecular diffusion rates are ≳ 8 x 10 9 sec −1 and 0.018 (2) x 10 9 sec −1 respectively The alignment of the fast diffusion axis is close to the long chain axis of the ‘tuning fork’ model and the existence of such a configuration in the melt is supported by the observation of different relaxation times for the two chemically equivalent primary glyceryl carbons. The low flexibility gradient and high end group mobility of the acyl chain found at low temperatures in the melt is similar to that observed in lipid vesicle studies and suggest that the chains are aligned parallel. A break down of this short range order is apparent above 150°C.