We present some basic criteria to obtain well-ordered Langmuir monolay
ers and of high surface densities. The fluorinated molecules such as C
10F21CH2COOH and C11F23COOH appear as good candidates to fulfil these
requirements. Independent, grazing incidence X-ray diffraction and sur
face pressure isotherm measurements give an advantage to the first mol
ecule. This may have some important practical applications in the surf
ace modification of solid substrates (e.g. silicon wafers) by thin org
anic layers. Assuming that a water film is present on the solid substr
ate prior to deposition, the above comments on Langmuir monolayers str
ongly suggest that fluorinated silanes should form optimum grafted lay
ers.