Bilayer and trilayer crystalline formation by collapsing behenic acid monolayers at gas/aqueous interfaces

X-ray reflectivities and grazing incidence X-ray diffractions of behenic acid (BA) monolayers compressed to the collapse region reveal that the resulting structures are reproducible and exhibit a high degree of order. The structures of the collapsed monolayers depend on the subphase solution. On pure water, the collapsed monolayer forms a stable crystalline trilayer structure. For monolayers spread on Ca2+ solutions, we find that an inverted bilayer structure is formed; that is, stretched BA-Ca-BA (calcium dibehenate, with calcium ions bridging the polar headgroups) forms a monolayer with the hydrophobic tails in contact with the water surface.