Diffusional behavior of n-paraffins with various chain lengths in urea adduct channels by pulsed field-gradient spin-echo NMR spectroscopy

Abstract

The diffusion coefficients ( D) of n-paraffin molecules ( n-C n H 2 n+2 ) with various chain-lengths ( n = 8, 12, 21, 26, 28 and 32) in the long channels of a deuterated urea- d 4 adduct have been measured at 25 °C by means of pulsed field-gradient spin-echo 1H NMR method. The aim is to clarify diffusional behavior of the n-paraffin molecules in the urea adduct channels. From the experimental results, it is found that n-paraffin molecules are diffusing in the long channels and have two kinds of diffusion components, namely a fast ( D ≈ 10 −10 m 2/s) and a slow diffusion component ( D ≈ 10 −11 m 2/s). The diffusing-time ( Δ) dependence of the diffusion coefficients of the n-paraffins shows some likely evidence of restricted diffusion since the n-paraffin molecules are confined in the urea channel. The diffusion coefficients ( D) decrease as the carbon number increases from 8 to 28, and very slowly decreases as the carbon number increases from 28 to 32.