We present an extensive ab initio molecular dynamics study on mixtures of the room temperature ionic liquid 1-ethyl-3-methylimidazolium acetate with water. To show the dependence of the properties on the concentration, we simulated four different systems: Pure IL, pure water, and two binary mixtures with different mixing ratios. We found that the imidazolium rings are stacking on top of each other in the pure ionic liquid, which is a result of the strong dispersion interactions between the cations. With increasing water content, this ordering is disturbed. While in the pure IL the anions are located almost exclusively within the cation's ring plane, they occupy both the in-plane and the on-top position in the mixture. For very high water content, the anions are mainly found on top of the imidazolium ring. The ethyl chains of the cations attract each other via dispersion force and form clusters, indicating the existence of microheterogeneity in our simulations. We also analyzed the topology of the hydrogen bond network and found that the cation is forming hydrogen bonds to multiple anions at the same time, and is therefore a bridging agent. Likewise, the anion is bridging between different cations. At moderate water content, two acetate ions are frequently coordinated to the same water molecule, leading to the formation of acetate/water clusters.
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