Nanoscale inhomogeneities in undercooled benzoic acid: A molecular dynamics study

Abstract

The dynamics of undercooled organic liquids is very complex and still far from being fully understood. A major problem is the lack of general algorithms to recognize nanoscale inhomogeneities that exploit different inner structures with respect to the bulk liquid. We show that a geometry-independent energy criterion, coupled with the requirement that the cluster must survive for times longer than thermal fluctuations, can be consistently used to individuate persistent and recognizable (meta)stable inhomogeneities or, possibly, subcritical clusters in benzoic acid. The algorithm applies to all organic liquids, regardless of their ability to form hydrogen bonds. We show that undercooled benzoic acid becomes granular with respect to the thermodynamically stable liquid, hosting medium size inhomogeneities composed of up to 17 molecules for durations on the order of hundreds of picoseconds. These correlate with the predicted increase of density and viscosity upon undercooling. Most important, the inner structure of these inhomogeneities is intermediate between the liquid and the crystal. Stacking interactions, analogue to those present in the crystal, begin to develop in the largest persistent clusters. The pros and cons of the method are discussed in the context of non-classical nucleation theories.