  The formation of interface dipoles in self-assembled monolayers (SAMs) of
--CH$_3$ and --CF$_3$ terminated short-chain alkanethiolates on Ag(111) is
studied by means of density functional theory calculations. The interface
dipoles are characterized by monitoring the change in the surface work function
upon adsorption of the SAM. We compare results obtained for SAMs in structures
with a different packing density of molecules, i.e. {$(\sqrt{7}\times\sqrt{7})
R19.1^{\circ}$}, {$(\sqrt{3}\times\sqrt{3}) R30^{\circ}$}, and {p(2$\times$2)}.
The work function of alkanethiolate SAMs on silver depends weakly on the
packing density; that of fluorinatedalkanethiolates shows a stronger
dependance. The results are analyzed in terms of two nearly independent
contributions to the interface dipole. These originate respectively from the
molecular dipoles and from a charge transfer between the metal surface and the
molecules. The charge transfer is determined by the silver--sulfur bond and it
is independent of the electronegativity of the molecules.
