Organization in pure and alternate deuterated cadmium arachidate monolayers on solid substrates and at the air/water interface studied by conventional and differential Fourier transform infrared spectroscopies

The molecular organization in monolayers of deuterated cadmium arachidate deposited on solid metallic and dielectric substrates or spread at the air/water interface is investigated using different techniques of Fourier transform infrared spectroscopy, For metals, the polarization modulated reflectivity shows that the molecules have a high molecular order and are oriented quasi perpendicularly with respect to the surface. The in-plane organization is determined by complementary transmission experiments on CaF2 plates. The CD2 bending vibration in mono- and multilayer Langmuir-Blodgett stacks reveals the hexagonal packing of a single monolayer irrespective of the hydrophilic or hydrophobic character of the substrate. Probing the stack organization using selective monolayer deuteration in hydrogenated Langmuir-Blodgett films shows that the first deposited monolayer of an odd-numbered stack keeps its hexagonal structure while additional bilayers adopt an orthorhombic packing induced by the strong polar head/head interactions, However a monolayer may have an orthorhombic arrangement either if it is deposited on an orthorhombic commensurable substrate or if it is at a low temperature in which case intermolecular interactions are strengthened. Finally, differential reflectivity allows a single monolayer spread at the air/water interface to be studied. It is shown that such a Langmuir film is arranged in a two-dimensional hexagonal packing with its alkyl chains oriented perpendicularly with respect to the surface and its COO- polar groups symmetrically anchored at the interface. (C) 1996 American Institute of Physics.