We have observed stages in the formation of self assembled n-octadecyltrichlorosilane monolayers on silicon wafers at different temperatures using atomic force microscopy (AFM). We use the 2D phase diagram terminology to constitute the observed phases. At low temperatures: the surface is successively covered by a gas, liquid expanded and liquid condensed phases. In this case, coexistence between the gas and liquid expanded phases is evidenced by a foam-like pattern, comparable with the liquid expanded phase of Langmuir films with inclusion of 2D gas bubbles. Both transitions are first order. At high temperatures, only the gas -->, liquid expanded transition is observed.