Relationships between fatty acid monolayer structure on the subphase and on solid substrates

Abstract

Docosanoic acid monolayers with known molecular packings on the water surface have been deposited on thin polymer films and then investigated using transmission electron diffraction at normal and tilted incidence. The diffraction patterns from monolayers deposited under all conditions investigated could be indexed as arising from the same conformationally disordered centred rectangular packing with molecules standing perpendicular to the substrate, although with a spread of unit cell parameters within and between each deposition condition significantly greater than the experimental error. This packing has been seen in monolayers on the water surface, but only under conditions completely different from any of those used for deposition. The patterns from CS phase samples gave spot profiles and relationships between grains distinctly different from those of L 2 and L 2 phase samples. The latter were sufficiently well defined to rule out interpretation in terms of any of the known dense crystalline packings of aliphatic chains. However, similar patterns have been reported before and appear to be related to Kitaigorodskii monoclinic subcell packing. The orientational statistics of the grain orientations provide evidence that the film morphology is influenced by that on the water surface.