The interaction between a lipid monolayer with positive surface charges and anionic species dissolved in the aqueous phase is investigated using the Poisson-Boltzmann equation. The monolayer and the aqueous phase are approximated by dielectric continuums whose dielectric constants are 2 and 80, respectively. It is assumed that positive charges are periodically distributed on the interface formed by the two dielectrics. For such a system, the Poisson-Boltzmann equation is analytically solved with the Debye-Huckel approximation. It is indicated that the potential on the water side is significantly modulated by the presence of the lipid phase. This effectively contributes to a strengthening of intermolecular interaction near or on the interface. In addition, the interaction depends on the surface charge density on the monolayer. Combining these findings and the results of a previous quantum chemical study (Part 1 in a series of our studies), we discuss the reason why intermolecular binding is enhanced at the air-water interface.