Dynamical correlations in simple disorder and complex disorder liquids

Abstract

Liquids in equilibrium exhibit two types of disorder, simple and complex. Typical simple disorder liquid is liquid nitrogen, or weakly polar liquids. Complex liquids concern those who can form long lived local assemblies, and cover a large range from water to some soft matter and biological liquids. The existence of such structures leaves characteric features upon the atom-atom correlation functions, concerning both atoms which directly participate to these structures and those who do not. The question we ask here is: do these features have also characteristic dynamical aspects, which could be tracked through dynamical correlation functions? Herein, we compare the van Hove function, intermediate scattering function and the dynamical structure factor, for both types of liquids, using force field models and computer simulations. The calculations reveal the paradoxical fact that neighbouring atom correlations for simple disorder liquids relax slower than that for complex disorder liquids, while prepeak features typical of complex disorder liquids relax even slower. This is an indication of the existence of fast kinetic self-assembly processes in complex disorder liquids, while the lifetime of such assemblies itself is quite slow. This is further confirmed by the existence of a very low-k dynamical pre-peak uncovered in the case of water and ethanol.