Atomistic Simulation Studies of Polymers and Water Erik Johansson and Peter Ahlstr¨ om School of Engineering, University College of Bor˚ as, SE-501 90 Bor˚ as, Sweden peter.ahlstrom@hb.se Abstract. A Monte Carlo simulation study of water and hydrocarbons aiming at understanding the degradation of polyethylene cable insula- tion is presented. The equilibrium distributions and clustering of water in vapour and in hydrocarbons was investigated using Gibbs ensemble Monte-Carlo simulations. Different combinations of water and hydrocar- bon models are investigated in order to reproduce experimental densities of water and hydrocarbons in both the water phase and the hydrocarbon phase. 1 Introduction The degradation of polyethylene insulation of high voltage DC cables in the presence of water is an important yet incompletely understood problem. It is found to proceed via the formation of water trees, in which water penetrates into the cable insulation [1]. In order to separate the different possible causes for the degradation of polyethylene in real cables, a series of simulations of increasing complexity is performed. We start with simple systems like water and hydrocarbons, to fine-tune simulation parameters to experimental data before the complexity of long polymers is introduced. In an earlier study [2] we focused on the properties of liquid water in equi- librium with its own vapour since these properties will be of high relevance to the behaviour of water in hydrophobic hydrocarbons where the water properties could be expected to be similar to those of water vapour. This study gave valu- able information about the structure of water vapour. The occurence of water clusters with different sizes was monitored and some results are summarized in Fig. 1. In the present work we study the equilibrium between pure water and decane (which acts as a model hydrocarbon). In the first, preliminary study we compare the properties of pure water vapour with water in decane, in both cases in equilibrium with pure water liquid. Especially the clustering of water molecules was compared. In these studies water and decane are modelled by the well-known SPC/E [3] and TraPPE [4] potentials, respectively. Since these force fields, with a simple Lorentz-Berthelot mixing rule for water-decane interactions, did not accurately reproduce water densities in the hydrocarbon phase, the second part of this B. K˚ agstr¨om et al. (Eds.): PARA 2006, LNCS 4699, pp. 59–65, 2007. c  Springer-Verlag Berlin Heidelberg 2007