Computational studies of water vapour - Liquid equilibrium

Licentiatavhandling, 2005

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. The presence of water is also known to enhance the diffusion rate of oxygen into the polymer bulk which can lead to accelerated polymer degradation. It has been shown that the mechanism for water molecules diffusing through the amorphous phase in polyethylene includes the formation of small water clusters. Gibbs Ensemble Monte Carlo molecular simulations using the SPC/E (Single Point Charge / Extended) water model has 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, even though smaller clusters are the most abundant at all temperatures, and clusters that contains more than 20 molecules are always rare. 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. Further work will focus on simulations that include an electric field, to determine if this will influence the properties of the water vapour. Simulations will also be done where a polymer will be inserted into the vapour box to study the equilibrium distribution of water in a polymer subjected to an external electric field.