Magnetic relaxation in glass-bead/water dispersions
Abstract
Proton transverse magnetisation spin-echo decays have been recorded for glass-bead dispersions in water, acetone, chloroform and butan-1,4-diol as a function of measurement frequency and π pulse separation (tcp). All the decays were non-exponential. With small beads (diameter < 60 µm), the characteristic decay time (T2(eff)) was independent of frequency (15–60 MHz) and tcp(0.1–4 ms) for water and butane-1, 4-diol, indicating that interactions with the surface provide the main T2 mechanism. For acetone and chloroform similar results were obtained with tcp < 1 ms. For longer pulse separations the values of T2(eff) decreased as tcp increased, with the rate of decrease being faster at higher measurement frequency. Here there is a contribution to T2 from diffusion through field gradients arising from bulk susceptibility effects. The behaviour of dispersions containing larger beads (diameter ca. 140 µm) was similar, except that a contribution to the water T2(eff) from bulk susceptibility effects was present with tcp > 4 ms at 60 MHz.