Aggregation of FeCl2 clusters in supercritical water investigated by molecular dynamics simulations

We have carried out molecular dynamics (MD) simulations of the aggregation of FeCl2 clusters in supercritical water. The particle formation in systems of 2048 water molecules (rigid SPC/E-model) and 120 Fe2+ ions and 240 Cl- ions has been investigated for 250 ps at five different state points at temperatures from 798 to 873 K and system densities from 0.18 g/cm(3) to 0.13 g/cm(3). We describe the particle growth by means of properties of the largest cluster in a system as well as cluster size averaged and time averaged observables. From preexisting or immediately formed units of Fe2+-Cl-, Fe2+-Cl-2(-), Fe2+-Cl-3(-) etc., the further growth of clusters is dominated by aggregation of such small building blocks. Clusters up to 10 ions in size with large charge imbalances are found during the growth process while a balanced positive to negative charge ratio is found on the average with time and cluster size development. Water molecules are found within the FeCl2 clusters during the whole time interval covered by the simulations, which is in agreement with the existence of crystal water in FeCl2 crystals grown from aqueous solutions. The radial distribution functions obtained from the simulation data are in good agreement with experimental results of slightly distorted FeCl2 center dot 4H(2)O crystals.