Self-aggregated two-dimensional crystal structure of the mixed monolayer of triacontanoic acid and nanocosylamine. Evidence in favor of an ordered arrangement of ionized head groups

Grazing incidence synchrotron X-ray diffraction measurements were made on an uncompressed monolayer of 1:1 triacontanoic acid (C₂₉H₅₉COOH) and nonacosylamine (C₂₉H₅₉NH₂), spread on pure water and on aqueous ZnCl₂ solution. The measured diffraction data of the mixed monolayer on water provides clear evidence of a complete mixing of the acid and amine molecules into a two-dimensional crystal where the hydrocarbon chains pack in a herringbone motif. A model has been proposed in which the ionized acid and amine moieties are ordered and form a continuous hydrogen-bonded array.