Effects of divalent ions on Langmuir monolayers: Synchrotron X-ray scattering studies

Thin films of organic molecules floating on aqueous substrates, when transferred onto solid substrates, can be potentially used in optoelectronics, molecular electronics, and biosensors. They can also be used as molecular templates for controlled nucleation of inorganic compounds (a model biomineralization process). These (and other) practical applications almost always involve preparation of Langmuir monolayers on salt solutions. Although structures of Langmuir monolayers on pure water have been well-characterized, only little is known about the effects of even the simplest metal salts on the crystalline structure of these monolayers. We have used grazing incidence synchrotron x-ray diffraction to study how the structure of fatty acid monolayers is affected by (1) alkaline water (no divalent ions), (2) dilute solutions of divalent salts and (3) supersaturated solutions of divalent salts.;Our results show that dilute solutions of both mono- and divalent ions change the structure of the Langmuir monolayer, but in different ways. Increasing pH in the subphase (and thus adding a monovalent ion) causes the monolayer to become more disordered. On the other hand, dissolving divalent metal salts (and raising pH) has the opposite effect: the monolayer becomes better ordered. All divalent ions effectively compress the monolaver into a tightly packed, untilted structure. Furthermore, some ions interact with the headgroups so strongly that the organic film buckles, with a periodic out-of-plane (superlattice) density modulation.;In supersaturated solutions, inorganic crystals begin to grow at the organic template (model biomineralization). Our studies reveal that barium and strontium fluoride grow preferentially oriented to the surface of the Langmuir mono layer. Barium fluoride has a contracted unit-cell at the initial stage of growth, and the organic molecules rearrange in such a way that the interfacial lattices are commensurate. Such registry at an organic-inorganic interface has long been suspected, and now has been directly confirmed. The concept of organic-inorganic structural matching plays an important precursor role in controlled biomineralization and organic-matrix-mediated materials synthesis.