Ordering in the subphase of a langmuir monolayer: X-ray diffraction and anomalous scattering studies

The properties of amphiphilic organic monolayers ("Langmuir films") at the air-water interface are strongly affected by metal ions in the aqueous subp hase, and such monolayers can be used as templates to grow oriented crystal s from dissolved salts. Our synchrotron X-ray studies of fatty acid monolay ers spread on a dilute aqueous solution of lead ions indicate that a simila r to5 Angstrom thick ordered layer is formed in the subphase. The lattice i s commensurate with the organic lattice and has 14 times the unit cell area . It is unlikely that lead ions alone would form such a large repeat unit; indeed, using anomalous X-ray scattering, we detect no evidence of lead wit hin the ordered layer. Thus, the interfacial superlattice is not simply an array of lead ions but may consist of lead hydrolysis products and water mo lecules.