Pressure and temperature dependence of proton spin relaxation in liquid n-heptane

Abstract

An experimental arrangement is described which allows measurement of nuclear spin resonance parameters on liquids over the temperature range from room temperature up to 190°c and simultaneously for pressures from 0·07 to 1·9 kbar. In particular the cell containing the liquid sample allows for variation in volume of the sample up to 200 per cent and maintains it oxygen free. Measured values of proton spin lattice relaxation times, T ₁, are reported for benzene at 23°c and for n-heptane over the range 23°c to 182°c and for pressures up to 1·9 kbar. The benzene results show that T ₁ behaves rather like the self-diffusion constant, D, but is somewhat less sensitive to pressure change. The interpretation of the results for n-heptane have been considered in terms of various theories. Activation theories are unsuccessful but a partially successful interpretation has been obtained for a combination of a free volume theory (Cohen and Turnbull [22]) and an activation theory, due to Macedo and Litovitz [25] and somewhat extended. We find also that T ₁ is well represented by a simple empirical formula:

where ρ is the density of the liquid, i.e. ln T ₁ is linear in both T and ρ. Self-diffusion results on liquid ethane can be interpreted in the same way although a linear dependence of ln D on T ^-1 or T ^1/2 is almost as good.

It seems possible that the commonly encountered Arrhenius behaviour along the coexistence curve is not of fundamental significance.