Simulations of Water Clustering in Vapour, Hydrocarbons and Polymers

Doktorsavhandling, 2007

It is commonly known that water plays a crucial role in many natural and industrial processes. One of these processes is the formation of water trees, and the subsequent breakdown of polyethylene used for high voltage cable insulation purposes. It has been shown by others that the mechanism for water molecules diffusing through amorphous polyethylene includes the formation of small water clusters. Gibbs Ensemble Monte Carlo molecular simulations have been performed to study the clustering of vapour phase water under vapour - liquid equilibrium conditions at temperatures ranging from 300 K to 600 K. The increase in vapour density with increasing temperature leads to a radical increase in the fraction of molecules belonging to clusters with two or more water molecules. It is also seen that the size of the clusters increases with temperature. The topologies of the smaller clusters, up to pentamers, have also been studied. A structural transition is observed from a large percentage with cyclic topology, which is the minimum energy configuration, at lower temperatures to predominantly linear clusters, favoured by entropic effects, at higher temperatures. Similar water properties have been obseved in simulations where the vapour phase has been replaced with a hydrocarbon rich phase (n-alkanes and polyethylene). Application of an external electric field to the polymer system reduces the water solubility and affects the water structure. A dramatic increase in water solubility in the hydrocarbon phase is observed when two oppositely charged ions are introduced in the hydrocarbon. The structure of the water changes from several small clusters, in the absence of ions, to a single large cluster with a rod-like shape. The cluster is extremely stable during the simulation. Application of an external electric field may enhance or reduce the effect of the ions depending on the direction of the field. Based on these observations is an alternative mechanism for water tree propagation proposed.