Intermolecular interactions across the air-water interface are the basis for a variety of investigations on monolayers and adsorption phenomena at the interface. We present a systematic study of the influence of different anions in the aqueous subphase on the formation and stabilization of a monolayer of the cationic amphiphile N-octadecyl-4-dimethylaminopyridinium. Some polyanions such as polystyrenesulfonate in the subphase are shown to impart strong stability to the monolayer. The monolayer stabilization through ionic complex formation at the air-water interface is modeled using semiempirical quantum chemical computations. The significance of including solvation effects in the computations is highlighted.