We have performed grazing incidence X-ray diffraction on water-supported films of n-octadecyltrimethoxysilane, a long chain amphiphile with reactive headgroups capable of forming siloxane (Si-O-Si) intermolecular covalent bonds. We show that the structure of the polymerized monolayer remains hexagonal, as in the case of fatty acids or alcohols but that the extent of positional ordering of the aliphatic hydrocarbon chains is smaller. Moreover we observe that the positional coherence length is a strong function of the pH of the aqueous subphase onto which the film has keen formed. We interpret this behavior as a consequence of the competition between the van der Weals interactions between the aliphatic chains and the chemical cross-linking of the headgroups.