title: Crystalline films of interdigitated structures formed via amidinium-carboxylate interactions at the air-water interface.
abstract: 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 amorphousA-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.
authors.affiliation: Department of Materials and Interfaces, The Weizmann Institute of Science, 76100 Rehovot, Israel.
authors.forename: I
authors.initials: I
authors.lastname: Kuzmenko
authors.forename: M
authors.initials: M
authors.lastname: Kindermann
authors.forename: K
authors.initials: K
authors.lastname: Kjaer
authors.forename: P B
authors.initials: PB
authors.lastname: Howes
authors.forename: J
authors.initials: J
authors.lastname: Als-Nielsen
authors.forename: R
authors.initials: R
authors.lastname: Granek
authors.forename: G
authors.initials: G
authors.lastname: von  Kiedrowski
authors.forename: L
authors.initials: L
authors.lastname: Leiserowitz
authors.forename: M
authors.initials: M
authors.lastname: Lahav