- Electrostatic interactions between amidinium and carboxylates were used for the construction of interdigitated architectures at the air−solution interface. Spreading the water-insoluble amphiphile p-pentadecylbenzoic acid (A) on an aqueous solution of p-methylbenzamidinium (B) ions results in an intercalation of the water-soluble base between the acidic headgroups of the water-insoluble amphiphile to form an amorphous A-B-A-B monolayer according to grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity measurements. Upon compression the monolayer transforms into a crystalline film composed of three bilayers with interdigitated hydrocarbon chains, and a top layer whose chains are disordered. Water-insoluble p-heptadecylbenzamidinium spread on an aqueous solution of benzoic acid displays a surface pressure−area isotherm similar to that obtained from the above system. A mechanism that accounts for the formation of these films is presented. Deposition of p-heptadecylbenzamidinium and p-pentadecylbenzoic acid amphiphiles in a 1:1 ratio on pure water led to the formation of a crystalline monolayer phase but which is partially disordered. Over an aqueous solution containing a 1:1 mixture of benzamidinium and benzoic acid no measurable binding of these solute molecules to the polar headgroups of the 1:1 mixed monolayer could be detected by X-ray reflectivity or GIXD.