We use the first-principles static and dynamic simulations to study the adsorption of acetic (CH3COOH) and trifluoroacetic (CF3COOH) acid on the TiO2(110) surface. The most favorable adsorption for both molecules is a dissociative process, which results in the two oxygens of the carboxylate ion bonding to in-plane titanium atoms in the surface. The remaining proton then bonds to a bridging oxygen site, forming a hydroxyl group. We further show that, by comparing the calculated dipoles of the molecules on the surface, it is possible to understand the difference in contrast over the acetate and trifluoroacetate molecules in the atomically resolved noncontact atomic force microscopy images. (C) 2004 American Institute of Physics.