Octadecyltrichlorosilane? Before or after the glass has been treated with it? I don't know the answer regardless but I do know your question is so vague that no one will be able to answer it.
umm, well I was treating silicon, does that make a diffeerence? I fdound a reference that has dielectric constants for alkly-trichlorosilanes and it gives it at between 2.4 and 2.5...