Self-Aggregated Two-Dimensional Crystal Structure of the Mixed Monolayer of Triacontanoic Acid and Nonacosylamine. Evidence in Favor of an Ordered Arrangement of Ionized Head Groups

Abstract

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.