Diffusive dynamics: Self vs. collective behaviour

Nowadays, a wide variety of experimental techniques is used to monitor the translational diffusive dynamics in molecular liquid systems. In comparing results from different techniques attention should be paid on the se for co llective character of the relaxation mode probed [1-4]. It is a widely held view, for example, that Nuclear Magnetic Resonance (NMR) gives information on diffusive single particle correlations, whereas Dynamic Light Scatterin g (DLS), such as Photon Correlation Spectroscopy (PCS), can allow to test t he cooperative nature of molecular diffusion. PCS is a time-domain techniqu e which has been extensively employed in the study of the dynamical behavio ur of many systems including liquids, supercooled liquids, aqueous solution s to name a few. The utility of this technique has been greatly extended du ring the last few years by the introduction of logarithmic time base correl ators and crosscorrelation methods based on fiber optics. The information o n the collective diffusion coefficient, extracted from the PCS data, when c ompared with those on the self diffusion coefficient, extracted by NMR spec troscopy, can give relevant information on the repulsive or attractive natu re of the intramolecular forces between the basic units of the polymer and the intermolecular forces among the hydrated polymeric chains. (C) 2001 Els evier Science B.V. All rights reserved.