Using an ab initio electrochemical approach, we show that the water monolayer on Pd(111) undergoes a first order transition as the surface charge is tuned, with a phase equilibrium between two modified ice structures on opposite charged substrates. These findings refine the classical H-down and H-up configurations by invoking a permanent charged surface. We explore the experimental consequences of this phase behaviour on scanning tunneling microscopy (STM) with STM simulations. A strong dependence of the topographic contrast with surface charge (or equivalently STM potential) is exhibited allowing to reinterprete recent experimental observations. (c) 2007 Elsevier B.V. All rights reserved.