Influence of headgroup cross-linking on chain packing in Langmuir monolayers of n-alkyltrialkoxysilanes

We have performed grazing incidence X-ray diffraction on water-supported fi lms of n-octadecyltrimethoxysilane, a long chain amphiphile with reactive h eadgroups capable of forming siloxane (Si-O-Si) intermolecular covalent bon ds. We show that the structure of the polymerized monolayer remains hexagon al, as in the case of fatty acids or alcohols but that the extent of positi onal ordering of the aliphatic hydrocarbon chains is smaller. Moreover we o bserve 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 t his 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.