Scheme of the bilayer sample: a thin layer of the studied M molecule (M being CO, HNCO, H₂CO, or NH₃) is first deposited at 15 K on the gold surface, and then a thicker layer of ASW ice is deposited on top of it at 15 K. The thickness d of the ASW layer is estimated both using H₂O IR absorption bands and by interferometry. The diffusion of the M molecule along the x direction is monitored at a fixed temperature by recording the evolution of one of its characteristic IR absorption bands as function of time using FTIR spectroscopy. A compact ASW ice favors a bulk (volume) diffusion (left part), while a porous ASW ice favors a surface diffusion along cracks and pores (surface).