Computational study of formation dynamics and structure of amorphous ice condensates

Abstract

Condensation dynamics and structure of low temperature ≤100 K amorphous ices is studied using the classical trajectory simulation technique. Specifically, classical equations of motion are solved for a sequence of H2O molecules impinging at random on an initial nucleus of ten water molecules. The sticking probability to the cluster is found to be unity. Using this technique we accumulated three amorphous clusters of 100–300 water molecules. The analysis of the results included collision dynamics and molecular structure. Upon collision with the cluster, the impinging H2O molecule forms typically two hydrogen bonds. The two new bonds are almost always asymmetric, i.e., one bond is via an O atom of the new molecule, and the other via one of its H atoms. We identified two distinct types of collision trajectories—‘‘simple’’ trajectories and ‘‘complex’’ ones. In a simple trajectory, a new molecule is attached to the surface, without disrupting the preexisting hydrogen bond network in the cluster. In a complex trajectory, the preexisting bond network is modified significantly during the collision. Up to ∼15 hydrogen bonds can be changed (formed or destroyed) during such a trajectory; and the spatial extent of the changes can be as large as ∼20 Å. The complex trajectories comprise 60%∼70% of all the collision trajectories. Thus, the hydrogen bond network evolves continuously during the growth of the ice sample; the net trend being towards the four coordinated molecular configuration around each molecule. The average coordination number of a molecule in the final clusters is 3.5∼3.7, despite the fact that most of the water molecules are on the exposed surface of the cluster. The main features of the local molecular structure in the clusters are (a) ordering of OO and OH distances within hydrogen bonds (b) very significant disorder in O–O–O angles between adjacent hydrogen bonds; the width of the O–O–O angle distribution is ∼40°. The molecular structure seems to be closely related to that of liquid water. The experimental electron diffraction patterns of amorphous clusters are reproduced very well by our models. Comparison was also made with the radial distribution functions derived for bulk amorphous ice from x-ray and neutron diffraction experiments. Reasonable agreement was obtained within the range ≲4 Å, while at larger distances finite size effects become important.