Polycyclic aromatic hydrocarbons (PAHs), a wide class of organic molecules with multiply fusedaromatic rings, are ubiquitous in the interstellar medium. In the condensed phase they areexpected as precursors to small graphitic grains or condensation nuclei for icy grain mantles, from which they can be transformed into more complex organic and bio-relevant molecules. However, the paths for these processes remain relatively unexplored and little is known of the dynamic behaviour around these carbonaceous, icy grains. To complete our previous studies of the dynamics of PAHs co-adsorbed with water on graphitic surfaces, we propose to measure the dynamics of water as the sole adsorbate in order to faciliate full interpretation of the data we have already obtained for this astrochemical model system.