X-ray diffraction studies of organic monolayers on the surface of water

We have used synchrotron radiation to study organic monolayers on water ("Langmuir films"). At high monolayer pressures, lead stearate [Pb(C17H35COO)2] shows a powder peak at 1.60 -1, implying an area per unit cell of 17.8 2 if the lattice is triangular. The correlation length is about 250. Lignoceric acid (C23H47COOH) shows a similar peak even though no heavy ions are attached. When the pressure is reduced, the peak in lead stearate does not observably move or broaden; below the "knee" in the isotherm, however, the peak height decreases slowly with increasing area, implying a first-order melting transition.