We present experimental studies of two-dimensional solids in alcohol monolayers spread on a subphase containing a soluble short fatty acid, whose surface adsorption depends on pH. Evidence of the penetration of the fatty acid into the monolayer is given by neutron reflectivity. As indicated by X-ray grazing incidence diffraction, the crystal structure of the solid phase of monolayers is not changed by the presence of these impurities. Fluorescence microscopy observations show that the morphologies of the growing solid crystals depend strongly on the pH of the subphase. They are highly branched when the pH is larger than the pK(a) of the acid dissolved in the subphase and almost compact in the opposite situation pH < pK(a). We attribute these changes to the fact that for pH > pK(a) the dissolved acid does not come to the surface, and the 2D growth is limited by the 2D diffusion of fluorescent impurities. On the contrary for pH < pK(a), the growth is controlled by fatty acid "impurities" which are rejected by the crystallization front and diffuse not only at the surface but in the subphase as well.