COLLECTIVE EFFECTS ON SINGLE-PARTICLE ORIENTATIONAL RELAXATION IN SLOW DIPOLAR LIQUIDS

Theoretical and computer simulation studies of orientational relaxatio n in dense molecular liquids are presented. The emphasis of the study is to understand the effects of collective orientational relaxation on the single-particle orientational dynamics. The theoretical analysis is based on a recently developed molecular hydrodynamic theory which a llows a self-consistent description of both the collective and the sin gle-particle orientational relaxation. The molecular hydrodynamic theo ry can be used to derive a relation between the memory function for th e collective orientational correlation function and the frequency-depe ndent dielectric function. A novel feature of the present work is the demonstration that this collective memory function is significantly di fferent from the single-particle rotational friction. However, a micro scopic expression for the single-particle rotational friction can be d erived from the molecular hydrodynamic theory where the collective mem ory function can be used to obtain the single-particle orientational f riction. This procedure allows, us to calculate the single-particle or ientational correlation function near the alpha-beta transition in the supercooled liquid. The calculated correlation function shows an inte resting bimodal decay below the bifurcation temperature as the glass t ransition is approached from above. Brownian dynamics simulations have been carried out to check the validity of the above procedure of tran slating the memory function from the dielectric relaxation data. We ha ve also investigated the following two issues important in understandi ng the orientational relaxation in slow liquids. First, we present an analysis of the ''orientational caging'' of translational motion. The value of the translational friction is found to be altered significant ly by the orientational caging. Second, we address the question of the rank dependence of the dielectric friction using both simulation and the molecular hydrodynamic theory.