Diffusional behavior of n alkanes in the rotator phase as studied by pulse field-gradient spin-echo H1 NMR method

Abstract

Self-diffusion coefficients, D, of n-C21H44, n-C23H48, n-C24H50, n-C28H58, n-C32H66, and n-C34H70 were measured in the rotator phase and the liquid phase over a wide range of temperatures by using pulse field-gradient spin–echo H1 NMR method, in order to clarify diffusional behavior of the n-alkanes in the rotator phase. From these experimental results it was found that the self-diffusion coefficients of these n-alkanes in the rotator phase and the liquid phase are decreased with a decrease in temperature with different slopes, and at the transition temperature in going from the liquid phase to the rotator phase do not change with a transitionally large decrease. From the Arrhenius plot of diffusion coefficient against temperature, the activation energies for diffusion of n-C24H50, n-C32H66, and n-C34H70 in the rotator phase were obtained to be 19.9, 33.9, and 35.9 kcal/mol, respectively, and on the other hand those in the liquid phase to be 4.8, 4.8, and 5.1 kcal/mol, respectively. Further, the corresponding values of n-C21H44, n-C23H48, and n-C28H58 in the liquid phase were obtained to be 4.7, 4.6, and 4.9 kcal/mol, respectively. It was found that the activation energy for diffusion of n-alkanes in the rotator phase is increased with an increase in the carbon number and is much larger than that in the liquid phase. Further, it was found that in the rotator phase of a n-C24H50 single crystal, the diffusion coefficients in direction perpendicular and parallel to the long chain axis were determined to be 2.70×10−6 and 1.64×10−6 cm2/s, respectively.