The long time tail of molecular velocity correlation in a confined fluid: observation by modulated gradient spin-echo NMR

In addition to the fast correlation for local stochastic motion the molecul ar velocity correlation function in a fluid enclosed within the pore bounda ries features a slow long time tail decay [1,2]. This article presents a st udy by the NMR modulated gradient spin-echo method (MGSE) [3] on a system o f water trapped in the space between the closely packed polystyrene beads. The results prove that the obtained dependence of spin-echo attenuation on time, gradient strength and modulation frequency nicely corresponds to the recently developed NMR approach, which is able to describe the effects of a rbitrarily shaped gradient pulse sequence on the spin-echo attenuation [4,5 ]. With an MGSE pulse sequence, a repetitive train of RF pulses with inters persed gradient pulses periodically modulates the spin-phase, giving the sp in-echo attenuation proportional to a value of the velocity correlation spe ctrum at the modulation frequency. It long time tail characteristic (a low- frequency decay of the spectrum), that can be well fitted with the spectrum calculated enables to extract the low-frequency correlation spectrum of co nfined water molecules. The function exhibits a negative from the solution of the Langevin equation for restricted diffusion (which exhibits an expone ntial decay) as well as with the spectrum obtained when simulating the hydr odynamics of molecular motion constrained by capillary walls (which gives a n algebraic decay). (C) 2000 Elsevier Science B.V. All rights reserved.