X-Ray Diffraction Studies of Organic Monolayers on the Surface of Water

Abstract

We have used synchrotron radiation to study organic monolayers on water ("Langmuir films"). At high monolayer pressures, lead stearate [Pb${({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{C}\mathrm{O}\mathrm{O})}_{2}$] shows a powder peak at 1.60 ${\mathrm{\AA{}}}^{\ensuremath{-}1}$, implying an area per unit cell of 17.8 ${\mathrm{\AA{}}}^{2}$ if the lattice is triangular. The correlation length is about 250 \AA{}. Lignoceric acid (${\mathrm{C}}_{23}$${\mathrm{H}}_{47}$COOH) 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.