Evidence of surface reconstruction during inorganic crystal nucleation under Langmuir monolayers

When a carboxylic acid Langmuir monolayer is spread on aqueous solutions of lead and carbonate ions, an inorganic film nucleates at the organic surface. \textit{In situ} synchrotron x-ray diffraction studies show that the structure is that of hydrocerrusite (2Pb(CO$_{3})\cdot $Pb(OH)$_{2})$, oriented with the hexagonal basal planes parallel to the water surface. In addition, there are peaks from a $\surd $7 $\times \quad \surd $7 superstructure of the hydrocerrusite surface lattice. The organic monolayer unit cell contracts so as to form an epitaxial match with the superstructure. The Bragg rods of the supercell reflections reveal that the surface layer is $\sim $40 {\AA} (5-6 layers) thick, with vertical layer spacing close to that of hydrocerussite. Surface reconstruction phenomena have been observed previously only on very clean surfaces under ultrahigh vacuum